Method for decolorizing polyamide fiber and method for producing recycled polyamide

Abstract

To provide a polyamide fiber decolorization method employing a solvent that is relatively inexpensive, readily obtainable in large amounts, and applicable for industrial use, enabling reduction of nitrogen atom content in waste liquid from the decolorization treatment and contributing to reduction of environmental load.SOLUTION: A method for decolorizing polyamide fiber, the method comprising a step of bringing a colored polyamide fiber into contact with a metal chloride alcohol solution containing a metal chloride and an alcohol.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for decolorizing polyamide fibers and a method for producing recycled polyamide. [Background technology]

[0002] Polyamides, including typical engineering plastics such as polyamide 6 and polyamide 66, have heat resistance and good mechanical properties, are used as versatile fibers, and are one of the irreplaceable materials in modern society. Coloring of polyamide fibers and textile products (e.g., yarn, cloth, etc.) made from polyamide fibers is usually done by attaching a dye (e.g., acid dye) to the surface of the polyamide fibers or polyamide textile products.

[0003] In the recycling of polyamide fibers, if colored polyamide fibers are directly melted and recycled, the color of the dye remains, which may cause problems during recycling. In such recycling of polyamide fibers, a method for decolorizing colored polyamide fibers is needed. For example, Patent Document 1 describes a method for decolorizing a colored polymer composition containing polyamide using a non-aqueous extraction solvent containing a nitrogen-containing organic base, an ammonium salt, and an alkanol.

[0004] However, nitrogen-containing organic bases are not widely available as solvents, and therefore are not suitable for industrial use. Furthermore, the wastewater generated during the decolorization process contains a large amount of nitrogen atoms, which is considered to be an environmental burden. Therefore, there is a demand for a method for decolorizing polyamide fibers using a nitrogen-free, relatively inexpensive, and widely available solvent suitable for industrial use. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 2010-523781 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a method for decolorizing polyamide fibers, which uses a solvent that is relatively inexpensive, available in large quantities, and suitable for industrial use, thereby reducing the nitrogen atom content in the waste liquid generated in the decolorizing treatment and thereby reducing the environmental load. [Means for solving the problem]

[0007] As a result of intensive research aimed at solving the above problems, the present inventors have found that by contacting colored polyamide fibers with a metal chloride alcohol solution containing a relatively low concentration of metal chloride and alcohols, the dye adhered to the surface of the colored polyamide fibers is released, thereby making it possible to obtain decolorized polyamide fibers, and have thus completed the present invention.

[0008] That is, the present invention is as follows. [1] A method for decolorizing polyamide fibers, comprising the step of contacting colored polyamide fibers with a metal chloride alcohol solution containing a metal chloride and an alcohol. [2] The method according to [1], wherein the metal chloride is zinc chloride or calcium chloride. [3] [2] The method according to [2], wherein the metal chloride is calcium chloride. [4] The method according to any one of [1] to [3], wherein the alcohol is methanol. [5] The method according to any one of [1] to [4], wherein the metal chloride concentration of the metal chloride alcohol solution is 15 mass % or less. [6] The method according to any one of [1] to [5], wherein the polyamide fiber is a polyamide 6 fiber. [7] The method according to [6], wherein the metal chloride concentration of the metal chloride alcohol solution is 1% by mass or more and 10% by mass or less. [8] The method according to any one of [1] to [5], wherein the polyamide fiber is a polyamide 66 fiber. [9] The method according to [8], wherein the metal chloride concentration of the metal chloride alcohol solution is 1% by mass or more and 15% by mass or less.

[10] contacting the colored polyamide fibers with a first metal chloride alcohol solution comprising a metal chloride and alcohols to obtain decolorized polyamide fibers; contacting the bleached polyamide fibers with a second metal chloride alcohol solution containing a metal chloride and alcohols to obtain a polyamide dissolved solution; and A step of mixing the polyamide solution with a poor solvent for polyamide to precipitate recycled polyamide. Including, The metal chloride concentration of the second metal chloride alcohol solution is greater than the metal chloride concentration of the first metal chloride alcohol solution; A method for producing recycled polyamide. [Effects of the Invention]

[0009] According to the present invention, a method for bleaching polyamide fibers can be provided that uses a solvent that is relatively inexpensive, available in large quantities, and suitable for industrial use, thereby reducing the nitrogen atom content in the waste liquid generated in the bleaching treatment and thereby reducing the environmental load. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention will be described in detail below.

[0011] (Method for bleaching polyamide fibers) In one embodiment, the present invention provides a method for bleaching polyamide fibers. The method for bleaching polyamide fibers of the present invention includes a step of contacting colored polyamide fibers with a metal chloride alcohol solution containing a metal chloride and alcohols (hereinafter, sometimes referred to as the "contacting step"). The contacting step dissociates dyes attached to the surfaces of the colored polyamide fibers, thereby obtaining bleached polyamide fibers.

[0012] The principle of the decolorization method of the present invention is believed to be as follows. Polyamide fibers are usually colored by the attachment of a dye (e.g., an acid dye) or a dye derivative (collectively sometimes simply referred to as "dye") to the surface of the polyamide fiber. On the other hand, a metal chloride alcohol solution may have solubility in polyamide depending on various conditions, such as the composition and temperature of the solution. Therefore, it is believed that when colored polyamide fibers are brought into contact with a metal chloride alcohol solution, the surface of the polyamide fiber dissolves and the dye attached to the surface dissociates, resulting in decolorization of the polyamide fiber. The conditions for the decolorization method of the present invention are preferably conditions sufficient to decolorize the polyamide fiber without excessively dissolving the polyamide fiber; for example, a lower metal chloride concentration in the metal chloride alcohol solution is preferred.

[0013] <Polyamide> The polyamide in the present invention refers to a polymer polymerized through an amide bond, such as a polymer obtained by polycondensation of a diamine compound and a dicarboxylic acid compound, or a polymer obtained by ring-opening polymerization of a cyclic lactam.

[0014] The diamine compound is not particularly limited, but examples thereof include ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, nonanediamine, methylpentanediamine, and p-phenylenediamine.

[0015] The dicarboxylic acid compound is not particularly limited, but examples thereof include oxalic acid, succinic acid, glutaric acid, adipic acid, sebacic acid, terephthalic acid, and isophthalic acid.

[0016] The cyclic lactam is not particularly limited, but examples thereof include ε-caprolactam, undecane lactam, and lauryllactam.

[0017] The combination of diamine compounds, dicarboxylic acid compounds, and cyclic lactam compounds is not particularly limited, and multiple types of each compound may be used in combination. Polyhexamethylene adipamide, which is composed of hexamethylenediamine and adipic acid, is suitable because of its high solubility.

[0018] The polyamide is not particularly limited, but examples include those containing aliphatic polyamides such as polyamide 66 (polyhexamethylene adipamide), polyamide 6, polyamide 610, polyamide 6T, and polyamide 6I, with those containing aliphatic polyamides being preferred, and those containing polyamide 66 (polyhexamethylene adipamide) being most preferred.

[0019] <Polyamide fiber> In the present invention, "polyamide fiber" refers to a fiber containing polyamide. Examples of polyamide include those mentioned above. Polyamide fibers include fibers formed from polyamide, fibers formed from polyamide and resins containing other components, and blended fibers of these with other fibers. The form of polyamide fiber is not particularly limited, and examples include cloth, cloth products, parts of cloth products, raw yarn, twisted yarn, etc.

[0020] <Colored polyamide fiber> In the present invention, "colored polyamide fibers" refers to polyamide fibers colored with a pigment. Colored polyamide fibers are usually colored by adhering a dye (e.g., an acid dye) to the surface of the polyamide fiber. The present invention can be used with any dye. The dye is not particularly limited, and examples include natural dyes derived from plants such as madder, indigo, safflower, and purple, and synthetic dyes such as aniline black, alizarin, and indigo.

[0021] <Metal chloride alcohol solution> The term "metal chloride alcohol solution" refers to a solution containing a metal chloride and an alcohol. The metal chloride alcohol solution may further contain components other than the metal chloride and the alcohol. In the present invention, the metal chloride alcohol solution has the function of decolorizing polyamide fibers. This function is believed to be achieved by dissolving the surface of the polyamide fibers and dissociating the dye attached to the surface. Therefore, it is preferable that the metal chloride alcohol solution be sufficient to decolorize the polyamide fibers and have as low a solubility in the polyamide fibers as possible.

[0022] <<Concentration of metal chloride alcohol solution>> The total mass ratio of the metal chloride and the alcohols relative to 100% by mass of the metal chloride alcohol solution is preferably 80% by mass or more, more preferably 90% by mass or more, and even more preferably 100% by mass.

[0023] <<Alcoholic beverages>> Examples of alcohols contained in the metal chloride alcohol solution include monoalcohols such as methanol, ethanol, linear or branched propanol, linear or branched butanol, diols such as ethylene glycol, propylene diol (e.g., propylene glycol), butanediol, diethylene glycol, trihydric alcohols such as glycerin, other polyhydric alcohols, and combinations thereof. Among these, from the viewpoint of being able to sufficiently dissolve both the first polyamide and the second polyamide, methanol, ethanol, or a combination thereof is preferred, and methanol is more preferred.

[0024] <<Metal chlorides>> Examples of metal chlorides contained in the metal chloride alcohol solution include zinc chloride, magnesium chloride, and calcium chloride, with zinc chloride and calcium chloride being preferred, and calcium chloride being most preferred.

[0025] <<Concentration of metal chlorides>> The metal chloride concentration of the metal chloride alcohol solution is preferably 15% by mass or less, more preferably 12.5% ​​by mass or less, even more preferably 10% by mass or less, even more preferably 7.5% by mass or less, and even more preferably 5% by mass or less, from the viewpoint of minimizing the solubility of the metal chloride alcohol solution in polyamide fibers. Also, the metal chloride concentration of the metal chloride alcohol solution is preferably 1% by mass or more, more preferably 2% by mass or more, and even more preferably 3% by mass or more, from the viewpoint of being sufficient to decolorize polyamide fibers.

[0026] The metal chloride concentration of the metal chloride alcohol solution can be appropriately set depending on the type of polyamide. For example, when the solubility of the polyamide fiber is relatively high (e.g., polyamide 6 fiber), a relatively low metal chloride concentration (e.g., 1% by mass or more and 10% by mass or less) can be set. When the solubility of the polyamide fiber is relatively low (e.g., polyamide 66 fiber), a relatively high metal chloride concentration (e.g., 1% by mass or more and 15% by mass or less) can be set.

[0027] <<Other ingredients>> The metal chloride alcohol solution may further contain other components within the range that does not impair the effects of the present invention. Examples of other components include water. The amount of water in the metal chloride alcohol solution is not particularly limited, but it is preferable that the amount of water is 4 moles or less per mole of metal chloride in the solution.

[0028] On the other hand, from the viewpoint of reducing the nitrogen atom content in the waste liquid generated in the decolorization treatment and thereby reducing the environmental load, it is preferable that the metal chloride alcohol solution has as low a content of nitrogen-containing compounds as possible. The nitrogen atom content of the metal chloride alcohol solution is preferably 1% by mass or less, more preferably 0.1% by mass or less, and even more preferably 0.01% by mass or less.

[0029] <Contact of colored polyamide fibers with alcoholic solutions of metal chlorides> The manner in which the colored polyamide fibers are brought into contact with the metal chloride alcohol solution is not particularly limited, and examples thereof include immersing the polyamide fibers in the metal chloride alcohol solution, spraying the metal chloride alcohol solution onto the polyamide fibers, wiping the polyamide fibers with cloth, paper, sponge, etc. impregnated with the metal chloride alcohol solution, etc. Before bringing the colored polyamide fibers into contact with the metal chloride alcohol solution, the polyamide fibers may be subjected to processing such as cutting, crushing, or compression.

[0030] The temperature during contact between the polyamide fiber and the metal chloride alcohol solution is, for example, preferably 10° C. or higher, more preferably 20° C. or higher, and even more preferably 30° C. or higher, and preferably 100° C. or lower, more preferably 90° C. or lower, and even more preferably 80° C. or lower. The contact time is, for example, preferably 1 minute or longer, more preferably 5 minutes or longer, and even more preferably 60 minutes or longer.

[0031] The contact of the colored polyamide fiber with the metal chloride alcohol solution may be carried out in the presence of activated carbon (i.e., under conditions where activated carbon is present in the metal chloride alcohol solution). The presence of activated carbon in the metal chloride alcohol solution causes the dye dissociated in the metal chloride alcohol solution to be adsorbed onto the activated carbon, accelerating the decolorization of the polyamide fiber.

[0032] <Other processes> The decolorization method of the present invention may optionally further include, before or after the contacting step, a step of adding activated carbon to the metal chloride alcohol solution (hereinafter also referred to as an "activated carbon adding step"). Furthermore, the decolorization method of the present invention may optionally further include, after the contacting step, one or more of a step of removing the polyamide fiber from the metal chloride alcohol solution (hereinafter also referred to as a "solution removing step"), a step of washing the polyamide fiber with a washing liquid (hereinafter also referred to as a "washing step"), a step of removing any remaining solution such as the metal chloride alcohol solution from the polyamide fiber (hereinafter also referred to as an "adhering remaining solution removing step"), and a step of removing activated carbon from the metal chloride alcohol solution (hereinafter also referred to as an "activated carbon removing step").

[0033] When the polyamide fibers are brought into contact with the metal chloride alcohol solution by immersion, the solution removal step can be carried out, for example, by removing the polyamide fibers from the metal chloride alcohol solution.

[0034] When a washing step is performed, examples of the washing solution used in the washing step include water, alcohols, and mixtures thereof. If washing is performed multiple times, the solvent may be changed for each wash. The washing solution may contain salts, pH adjusters, and the like in amounts that do not dissolve the polyamide. Examples of salts include sodium chloride, potassium chloride, lithium chloride, calcium chloride, magnesium chloride, sodium bromide, potassium bromide, calcium bromide, magnesium bromide, sodium sulfate, potassium sulfate, sodium nitrate, potassium nitrate, calcium nitrate, and magnesium nitrate. Examples of pH adjusters include sodium acetate, potassium acetate, sodium phosphate, disodium monohydrogen phosphate, monosodium dihydrogen phosphate, potassium phosphate, dipotassium monohydrogen phosphate, monopotassium dihydrogen phosphate, acetate buffers (e.g., a mixture of acetic acid and sodium acetate), and phosphate buffers (e.g., a mixture of acetic acid and sodium acetate).

[0035] Examples of methods for carrying out the step of removing the remaining solution include drying (eg, heat drying, drying under reduced pressure, air drying, etc.), centrifugation, wiping, etc.

[0036] Examples of methods for carrying out the activated carbon addition step include directly adding activated carbon to the metal chloride alcohol solution, adding activated carbon stored in a mesh bag or basket to the metal chloride alcohol solution, adding activated carbon to a compartment separate from the compartment in which the polyamide fiber is immersed in a container with a liquid-passing partition, and passing the metal chloride alcohol solution through a columnar container filled with activated carbon. The activated carbon addition step adsorbs the dye released in the metal chloride alcohol solution onto the activated carbon. If activated carbon is present in the metal chloride alcohol solution during the contact step, the dye released in the metal chloride alcohol solution is removed by adsorption, thereby accelerating decolorization. Alternatively, if activated carbon is present in the metal chloride alcohol solution during or after the contact step, the dye released in the metal chloride alcohol solution is removed by adsorption, thereby enabling the used metal chloride alcohol solution to be obtained as a regenerated metal chloride alcohol solution with restored decolorization ability. The obtained regenerated metal chloride alcohol solution may be used as the metal chloride alcohol solution in the present invention, or may be used as a solvent for dissolving polyamide in other applications by adding a metal chloride thereto.

[0037] Methods for carrying out the activated carbon removal step include filtering the activated carbon that has been directly added, removing the mesh bag or basket containing the activated carbon, and removing the activated carbon from a container having a partition through which liquid passes.

[0038] (Method for bleaching polyamide fibers) In another embodiment, the present invention provides a method for producing recycled polyamide. The method for producing recycled polyamide of the present invention includes the steps of: a step of contacting the colored polyamide fiber with a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a bleached polyamide fiber (hereinafter, sometimes referred to as the "bleaching step"); A step of contacting the bleached polyamide fibers with a second metal chloride alcohol solution containing a metal chloride and alcohols to obtain a polyamide dissolved solution (hereinafter sometimes referred to as the "dissolving step"); and A step of mixing the polyamide solution with a poor solvent for polyamide to precipitate regenerated polyamide (hereinafter sometimes referred to as the "precipitation step"). Including, The metal chloride concentration of the second metal chloride alcohol solution is greater than the metal chloride concentration of the first metal chloride alcohol solution; A method for producing recycled polyamide.

[0039] <Decolorization process> The decolorization step in the production method of the present invention is substantially the same as the contact step in the decolorization method of the present invention.

[0040] <<Polyamide, polyamide fiber, colored polyamide fiber>> The terms "polyamide," "polyamide fiber," and "colored polyamide fiber" in the production method of the present invention are the same as those defined in the decolorization method of the present invention.

[0041] <<First metal chloride alcohol solution>> The "first metal chloride alcohol solution" in the production method of the present invention is the same as the "metal chloride alcohol solution" defined in the decolorization method of the present invention.

[0042] Contacting the colored polyamide fiber with the first metal chloride alcohol solution In the production method of the present invention, the manner and conditions for contacting the colored polyamide fibers with the first metal chloride alcohol solution are the same as the manner and conditions for contacting the colored polyamide fibers with the metal chloride alcohol solution defined in the decolorization method of the present invention.

[0043] <Other processes> The production method of the present invention may optionally further include, before or after the decolorization step, one or more of the activated carbon introduction step, the solution removal step, the washing step, the adhering residual solution removal step, and the activated carbon removal step, which are defined in the decolorization method of the present invention.

[0044] <Dissolution process> <<Second metal chloride alcohol solution>> The "second metal chloride alcohol solution" is a metal chloride alcohol solution different from the first metal chloride alcohol solution and has the function of dissolving polyamide fibers. Therefore, the second metal chloride alcohol solution is preferably sufficient to dissolve polyamide fibers. To achieve this function, the second metal chloride alcohol solution has a higher metal chloride concentration than the first metal chloride alcohol solution.

[0045] <<Concentration of metal chloride alcohol solution>> The total mass ratio of the metal chloride and alcohols relative to 100 mass% of the second metal chloride alcohol solution is preferably 80 mass% or more, more preferably 90 mass% or more, and even more preferably 100 mass%.

[0046] <<Types of alcohols and metal chlorides>> The types of "alcohols" and "metal chlorides" contained in the second metal chloride alcohol solution are the same as the types of "alcohols" and "metal chlorides" defined in the "metal chloride alcohol solution" in the decolorization method of the present invention.

[0047] <<Concentration of metal chlorides>> The second metal chloride alcohol solution has a higher metal chloride concentration than the first metal chloride alcohol solution, from the viewpoint of sufficient dissolution of the polyamide fibers. The metal chloride concentration of the second metal chloride alcohol solution is preferably 10% by mass or more, more preferably 12% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of improving the solubility of the polyamide fibers. On the other hand, the metal chloride concentration in the metal chloride alcohol solution is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less, from the viewpoint that if the concentration is too high, the metal chloride is likely to remain undissolved and become mixed in as an impurity.

[0048] The metal chloride concentration of the second metal chloride alcohol solution can be appropriately set depending on the type of polyamide. For example, when the solubility of the polyamide fiber is relatively high (e.g., polyamide 6 fiber), a relatively low metal chloride concentration (e.g., 10% by mass or more and 35% by mass or less) can be set. When the solubility of the polyamide fiber is relatively low (e.g., polyamide 66 fiber), a relatively high metal chloride concentration (e.g., 15% by mass or more and 50% by mass or less) can be set.

[0049] <<Other ingredients>> The metal chloride alcohol solution may further contain other components as long as the effects of the present invention are not impaired. Examples of other components include water. The amount of water in the metal chloride alcohol solution is not particularly limited, but it is preferable that the amount of water is 2 moles or less per mole of metal chloride in the solution.

[0050] Contacting the bleached polyamide fibers with a second alcoholic solution of a metal chloride The manner in which the bleached polyamide fibers are brought into contact with the second metal chloride alcohol solution is not particularly limited as long as the polyamide fibers can be dissolved, and examples thereof include immersing the polyamide fibers in the second metal chloride alcohol solution. Before bringing the bleached polyamide fibers into contact with the second metal chloride alcohol solution, the bleached polyamide fibers may be cut, crushed, or compressed.

[0051] The temperature during contact between the polyamide fiber and the metal chloride alcohol solution is, for example, preferably 10° C. or higher, more preferably 20° C. or higher, and even more preferably 30° C. or higher, and preferably 100° C. or lower, more preferably 90° C. or lower, and even more preferably 80° C. or lower. The contact time is, for example, preferably 1 minute or longer, more preferably 5 minutes or longer, and even more preferably 60 minutes or longer.

[0052] <Other processes> The production method of the present invention may further include, after the dissolving step, a step of removing impurities other than polyamide from the polyamide-dissolved solution (hereinafter, also referred to as an "impurity removing step"). The impurity removing step is not particularly limited, and may be carried out, for example, by removing impurities as insoluble components by filtration, centrifugation, or the like.

[0053] <Precipitation process> <<Poor solvent>> The term "poor solvent" refers to a poor solvent for polyamide, and is used to precipitate regenerated polyamide by mixing with a polyamide solution. The poor solvent is not particularly limited, as long as it is a solvent that exhibits poor solvent properties for polyamide and solubility for metal chlorides, and preferably exhibits compatibility with the alcohol solution of the second metal chloride. Examples of the poor solvent include a water-containing solution and an alcohol-containing solution. For example, the poor solvent may be a solvent that, when 100 g of the solvent is added to 1 g of polyamide (e.g., nylon 66) and stirred at 20°C for 1 hour, dissolves 0.05 g or less of polyamide in the solution.

[0054] <<Water-containing solution>> The water-containing solution is a liquid containing water or water and components other than water, such as alcohols (e.g., those exemplified as alcohols contained in the metal chloride alcohol solution), salts, pH adjusters, etc.

[0055] Examples of salts include alkali metal salts, alkaline earth metal salts, transition metal salts, and ammonium salts, and more preferably sodium salts, potassium salts, lithium salts, calcium salts, magnesium salts, barium salts, strontium salts, copper salts, and ammonium salts. Examples of anions of salts include chloride ions, fluoride ions, bromide ions, iodide ions, sulfate ions, sulfite ions, phosphate ions, nitrate ions, nitrite ions, methanesulfonate ions, benzenesulfonate ions, toluenesulfonate ions, citrate ions, oxalate ions, malate ions, tartrate ions, maleate ions, fumarate ions, and acetate ions.

[0056] Preferred salts include sodium chloride, potassium chloride, lithium chloride, calcium chloride, magnesium chloride, sodium bromide, potassium bromide, calcium bromide, magnesium bromide, sodium sulfate, potassium sulfate, sodium nitrate, potassium nitrate, calcium nitrate, magnesium nitrate, and the like.

[0057] Examples of pH adjusters include water-soluble basic salts, acid salts, and buffers (e.g., mixtures of acids and salts, mixtures of bases and salts), such as sodium acetate, potassium acetate, sodium phosphate, disodium monohydrogen phosphate, monosodium dihydrogen phosphate, potassium phosphate, dipotassium monohydrogen phosphate, monopotassium dihydrogen phosphate, acetate buffers (e.g., mixtures of acetic acid and sodium acetate), and phosphate buffers (e.g., mixtures of acetic acid and sodium acetate).

[0058] <<Alcohol-containing solution>> The water-containing solution is a liquid containing alcohols or alcohols and components other than alcohols. Examples of alcohols include those exemplified as alcohols contained in the metal chloride alcohol solution. Examples of components other than alcohols include salts. Examples of salts include those exemplified as salts that may be contained in the water-containing solution. When the salt corresponds to the metal chloride contained in the metal chloride alcohol solution described above, the concentration of the metal chloride in the alcohol-containing solution is preferably as low as possible, preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.5% by mass or less.

[0059] <<Mixing Style>> The manner in which the polyamide solution and the poor solvent are mixed in the precipitation step is not particularly limited, and examples thereof include adding the polyamide solution to the poor solvent, adding the poor solvent to the polyamide solution, and adding the polyamide solution and the poor solvent to a container. The manner of addition is not particularly limited, and examples thereof include dropwise addition, continuous pouring from above, and spraying. During mixing, the solution may be left to stand without stirring, or may be stirred.

[0060] <<Mixing ratio>> The mass of the poor solvent used in the precipitation step is preferably at least 1 time, more preferably at least 2 times, even more preferably at least 3 times, and preferably at most 100 times, more preferably at most 50 times, and even more preferably at most 20 times, the mass of the polyamide solution. If the amount of poor solvent is too small, the amount of polyamide precipitated decreases, resulting in a low yield. If the amount of poor solvent is too large, a large amount of poor solvent is used in the precipitation step, which requires a lot of work, such as a large amount of waste liquid.

[0061] <<Mixing temperature>> The temperature of the solution during mixing is preferably a temperature at which the solution does not freeze, and is, for example, preferably 10°C or higher, more preferably 15°C or higher, even more preferably 20°C or higher, and preferably 90°C or lower, more preferably 75°C or lower. The temperature of the solution may be constant throughout the precipitation step, or may be changed from the temperature at the time of mixing. For example, it is possible to add a poor solvent at 90°C to lower the temperature to 10°C.

[0062] <<Deposition time>> The time taken for precipitation is not particularly limited. By changing the time for adding the solution, the desired precipitation rate, precipitation temperature range, etc. can be achieved, and the shape and properties of the precipitated polyamide can be changed.

[0063] <Other processes> The production method of the present invention may further include, after the precipitation step, a step of washing the regenerated polyamide with a washing liquid (hereinafter also referred to as the "washing step") and a step of removing the attached solution from the regenerated polyamide (hereinafter also referred to as the "attached solution removing step"). Examples of the washing liquid used in the washing step include the solutions exemplified as the water-containing solution. Examples of methods for carrying out the attached solution removing step include drying (e.g., heat drying, vacuum drying, air drying, etc.), centrifugation, wiping, etc. [Example]

[0064] The present invention will be described in detail below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples. The measurement methods used in the examples and comparative examples are shown below.

[0065] [Absorbance measurement] Analytical equipment: Multiskan Sky (manufactured by Thermo Fischer) Cell: Quartz Optical path length: 10mm Blank: 30% calcium chloride methanol solution

[0066] [Example 1] Two grams of PA66 dyed fabric 1, cut into 5 mm square pieces, and 20 g of 10% calcium chloride methanol solution were weighed into a 50 mL round-bottom flask equipped with a stirrer, and stirring was initiated at room temperature using a magnetic stirrer. After 24 hours of stirring, the mixture was poured into a filter equipped with a 10 μm membrane filter, and the solid precipitate (2.1 g) and filtrate (19.8 g) were separated and collected. The solid precipitate was thoroughly washed with methanol and then dried in a vacuum dryer set at 50°C, yielding 1.96 g (98.0%) of solid. The resulting solid was added to a 30% calcium chloride methanol solution at a concentration of 0.2 wt% and dissolved, and the absorbance was measured. The absorbance of the dyed fabric was 54% lower than before washing.

[0067] [Example 2] 2 g of PA66 dyed fabric 2, cut into 5 mm pieces, and 20 g of 10% calcium chloride methanol solution were weighed into a 50 mL round-bottom flask equipped with a stirrer, and stirring was initiated using a magnetic stirrer. After stirring for 24 hours, the mixture was poured into a filter equipped with a 10 μm membrane filter, and the solid precipitate (2.1 g) and filtrate (19.8 g) were separated and collected. The solid precipitate was thoroughly washed with methanol and then dried in a vacuum dryer set at 50 °C, yielding 1.98 g (99.0%) of solid. The resulting solid was added to 30% calcium chloride methanol solution at a concentration of 0.2 wt%, dissolved, and the absorbance was measured. The absorbance of the dyed fabric was 26% lower than before washing.

[0068] [Example 3] 2 g of PA6 dyed yarn 3 cut into 5 mm square pieces and 20 g of 10% calcium chloride methanol solution were weighed into a 50 mL round-bottom flask equipped with a stirrer, and stirring was initiated using a magnetic stirrer. After stirring for 24 hours, the mixture was poured into a filter equipped with a 10 μm membrane filter, and the solid precipitate (1.9 g) and filtrate (20.8 g) were separated and collected. The solid precipitate was thoroughly washed with methanol and then dried in a vacuum dryer set at 50 °C, yielding 1.7 g (85.0%) of solid. The resulting solid was added to 30% calcium chloride methanol solution at a concentration of 0.2 wt%, dissolved, and the absorbance was measured. The absorbance of the dyed fabric was reduced by 72% compared to the value before washing.

[0069] [Comparative Example 1] Two grams of dyed fabric 1, cut into 5 mm square pieces, and 20 g of 10% calcium chloride aqueous solution were weighed into a 50 mL round-bottom flask equipped with a stirrer, and stirring was initiated using a magnetic stirrer. After stirring for 24 hours, the mixture was poured into a filter equipped with a 10 μm membrane filter, and the solid precipitate (2.1 g) and filtrate (19.8 g) were separated and collected. The solid precipitate was thoroughly washed with methanol and then dried in a vacuum dryer set at 50°C, yielding 1.98 g (99.0%) of solid. The resulting solid was added to a 30% calcium chloride methanol solution to a concentration of 0.2 wt%, dissolved, and the absorbance was measured. The absorbance of the dyed fabric was 2% lower than before washing. [Industrial Applicability]

[0070] According to the present invention, a method for bleaching polyamide fibers can be provided that uses a solvent that is relatively inexpensive, available in large quantities, and suitable for industrial use, thereby reducing the nitrogen atom content in the waste liquid generated in the bleaching treatment and thereby reducing the environmental load.

Claims

1. A method for decolorizing polyamide fibers, comprising the step of contacting colored polyamide fibers with a metal chloride alcohol solution containing a metal chloride and an alcohol.

2. 2. The method of claim 1, wherein the metal chloride is zinc chloride or calcium chloride.

3. 3. The method of claim 2, wherein the metal chloride is calcium chloride.

4. 3. The method according to claim 1, wherein the alcohol is methanol.

5. 3. The method according to claim 1, wherein the metal chloride concentration of the alcoholic solution of metal chloride is 15% by mass or less.

6. 3. The method according to claim 1 or 2, wherein the polyamide fibers are polyamide 6 fibers.

7. The method according to claim 6, wherein the metal chloride concentration of the metal chloride alcohol solution is 1% by mass or more and 10% by mass or less.

8. 3. The method according to claim 1 or 2, wherein the polyamide fibers are polyamide 66 fibers.

9. The method according to claim 8, wherein the metal chloride concentration of the metal chloride alcohol solution is 1% by mass or more and 15% by mass or less.

10. contacting the colored polyamide fibers with a first metal chloride alcohol solution containing a metal chloride and alcohols to obtain decolorized polyamide fibers; contacting the bleached polyamide fibers with a second metal chloride alcohol solution containing a metal chloride and alcohols to obtain a polyamide dissolved solution; and A step of mixing the polyamide solution with a poor solvent for polyamide to precipitate recycled polyamide. Including, the metal chloride concentration of the second metal chloride alcoholic solution is greater than the metal chloride concentration of the first metal chloride alcoholic solution; A method for producing recycled polyamide.