Polyamide production method, polyamide-dissolving solution production method, and polyamide-dissolving solution

Abstract

With the present invention, it is possible to provide, for the recycling of a polyamide by dissolution, a method capable of sufficiently maintaining the solubility of a polyamide-dissolving solution to be reused from a residual solution while reducing the energy required during polyamide recovery. This polyamide production method includes: a step 1 for obtaining a heated polyamide solution by heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol; a step 2 for mixing the heated polyamide solution obtained at step 1 with a poor solvent to precipitate a polyamide; a step 3 for performing solid-liquid separation of a mixture of the precipitated polyamide and solution at step 2 into a polyamide and a solution to recover the polyamide; and a step 4 for obtaining a second metal chloride alcohol solution in which the solubility of the polyamide is within a prescribed range using the solution subjected to the solid-liquid separation at step 3 as a starting material.

Description

Polyamide manufacturing method, polyamide solution manufacturing method, and polyamide solution

[0001] The present invention relates to a method for producing a polyamide, a method for producing a polyamide solution, and a polyamide solution.

[0002] Polyamides, including polyamide 6 and polyamide 66, are representative engineering plastics that have excellent heat resistance and mechanical properties. They are widely used in textiles, automobile parts, electrical appliance parts, and more, and are one of the irreplaceable materials in modern society.

[0003] In recent years, technological developments have been made in the recycling of plastics with the aim of resource conservation and carbon neutrality, and polyamides are no exception. In particular, polyamide 66 (polyhexamethylene adipamide), which is one of the polyamides produced in large quantities, is one of the polyamides that should be prioritized for recycling.

[0004] One possible method for recycling polyamides is to dissolve the polyamide in a solvent, remove insoluble impurities, and then precipitate and recover the polyamide by adjusting the polyamide concentration (e.g., concentration, dilution, etc.), or by adjusting the polyamide solubility (e.g., adding a poor solvent, cooling, etc.) (Patent Documents 1 and 2). However, many solvents used to dissolve polyamides are unsuitable for industrial use, such as strong acids like formic acid and sulfuric acid, or expensive solvents like HFIP. An example of an industrially usable solvent is the dissolution and recovery method using ethylene glycol (Patent Document 1). However, this method requires extremely high temperatures, which raises concerns about glycolysis of the polyamide. Furthermore, the solvent used must be completely removed from the sherbet-like solid and dried, which is thought to require a large amount of energy, including heating during the reaction. Therefore, a method using an alcoholic calcium chloride solution is also available, which uses low-temperature, commonly used raw materials. For example, Patent Document 2 describes a method in which a silicone-coated polyamide cloth is treated with a methanolic calcium chloride solution to dissolve the polyamide, and the polyamide is then diluted with a large amount of water or methanol to obtain the desired polyamide as a powder.

[0005] In recycling polyamide by dissolving it in an alcoholic solution of metal chloride, it is preferable to reuse the solution because discarding the solution after polyamide precipitation directly would be undesirable due to the environmental impact and increased amount of raw materials used, and a method for recycling the solution is described in Patent Document 2. This method involves drying and recovering the metal chloride, and then adding alcohol again to dissolve it, but because metal chlorides strongly interact with the solvent, drying requires a large amount of energy, which is fatal to a recycling process aimed at reducing the environmental impact.

[0006] JP 2018-172618 A Patent No. 5110704 A

[0007] An object of the present invention is to provide a method for producing polyamides that requires less energy and produces less waste when recycling polyamides by dissolving them in an alcoholic solution of a metal chloride.

[0008] As a result of intensive research into solving the above-mentioned problems, the present inventors have found that, in recycling polyamide by dissolution using a metal chloride alcohol solution, it is possible to reuse the remaining solution after polyamide recovery with less energy and reduce waste by carrying out the following steps: a step of heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; a step of mixing the heated polyamide solution obtained in the step with a poor solvent to precipitate polyamide; a step of recovering polyamide by performing solid-liquid separation of the mixture of polyamide and solution precipitated in the step into polyamide and solution; and a step of using the solution obtained through solid-liquid separation in the step as a raw material to obtain a second metal chloride alcohol solution in which polyamide solubility is within a predetermined range, thereby making it possible to reuse the remaining solution after polyamide recovery with less energy and reducing waste, which led to the completion of the present invention.

[0009] That is, the present invention is as follows: [1] A method for producing a polyamide, comprising the following steps: Step 1: heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: mixing the heated polyamide solution obtained in Step 1 with a poor solvent to precipitate a polyamide; Step 3: performing solid-liquid separation of the mixture of the polyamide precipitated in Step 2 and the solution to separate the polyamide and the solution, thereby recovering the polyamide; and Step 4: using the solution obtained from the solid-liquid separation in Step 3 as a raw material to obtain a second metal chloride alcohol solution in which the solubility of polyamide is 90% or more of the solubility of polyamide in the first metal chloride alcohol solution. [2] The method for producing a polyamide according to claim 1, wherein the poor solvent is the alcohol, and in Step 4, the second metal chloride alcohol solution is obtained by concentrating the solution obtained from the solid-liquid separation in Step 3 and / or adding a metal chloride to the solution obtained from the solid-liquid separation. [3] The method for producing a polyamide according to [1] or [2], wherein the second metal chloride alcohol solution obtained in step 4 contains a compound having an amide bond with a molecular weight of 1,000 or less at a concentration of 5% by mass or less. [4] The method for producing a polyamide according to any of [1] to [3], wherein the second metal chloride alcohol solution obtained in step 4 contains a water concentration of 20% by mass or less. [5] The method for producing a polyamide according to any of [1] to [4], wherein the difference obtained by subtracting the water concentration of the first metal chloride alcohol solution from the water concentration of the second metal chloride alcohol solution obtained in step 4 is 10% by mass or less. [6] The method for producing a polyamide according to any of [1] to [5], wherein step 4 includes a step of reducing the alcohol content by removing an alcohol component from a solution obtained from the solution obtained by solid-liquid separation in step 3 as a raw material, thereby obtaining a second metal chloride alcohol solution. [7] The method for producing a polyamide according to [6], wherein steps 1 to 4 are repeatedly carried out, and the alcohol component removed in step 4 from the solution obtained by solid-liquid separation in step 3 is used as a poor solvent in step 2 from the next step onwards.[8] The method for producing a polyamide according to any one of [1] to [7], wherein steps 1 to 4 are repeated, and the second metal chloride alcohol solution obtained in step 4 is used as the first metal chloride alcohol solution to be used in the next step 1. [9] The method for producing a polyamide according to [8], wherein the solubility of the polyamide in the second metal chloride alcohol solution obtained in step 4 each time is 90% or more of the solubility of the polyamide in the first metal chloride alcohol solution used in step 1 the first time.

[10] A method for producing a polyamide solution, comprising the following steps: Step 1: heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: mixing the heated polyamide solution obtained in Step 1 with a poor solvent to precipitate a polyamide; Step 3: performing solid-liquid separation of the mixture of the polyamide precipitated in Step 2 and the solution into polyamide and solution to recover the polyamide; Step 4: using the solution obtained through solid-liquid separation in Step 3 as a raw material, obtaining a polyamide solution as a second metal chloride alcohol solution in which the solubility of polyamide is 90% or more of the solubility of polyamide in the first metal chloride alcohol solution.

[11] A polyamide solution comprising a metal chloride, an alcohol, and a compound having an amide bond with a molecular weight of 1000 or less, wherein the concentration of the compound having an amide bond with a molecular weight of 1000 or less is 0.1% by mass or more and 5% by mass or less.

[0010] According to the present invention, a method can be provided for recycling polyamide by dissolution, which can reduce the energy required for recovering polyamide while sufficiently maintaining the solubility of the polyamide solution reused from the residual solution.

[0011] The present invention will be described in detail below.

[0012] (Method for Producing Polyamide) The method for producing polyamide according to the present embodiment is a method for producing a product polyamide (usually a powdery polyamide precipitated by crystallization) from a raw polyamide (hereinafter referred to as a "polyamide composition"). This method can be used, for example, for polyamide recycling and polyamide purification, particularly for polyamide recycling. Specifically, polyamide is produced by dissolving the polyamide composition in a metal chloride alcohol solution, which is a polyamide solution, precipitating the polyamide using a poor solvent, and recovering the polyamide by solid-liquid separation. In addition, in conjunction with the production of polyamide, a polyamide solution, which is a solvent capable of sufficiently dissolving polyamide, can also be obtained from the residual solution after polyamide recovery. This makes it possible to reduce the rate of waste of the residual solution after polyamide recovery while maintaining sufficient solubility in the polyamide solution to be reused from the residual solution.

[0013] The method for producing a polyamide of the present invention includes the following steps: Step 1: heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: mixing the heated polyamide solution obtained in Step 1 with a poor solvent to precipitate a polyamide; Step 3: subjecting the mixture of the precipitated polyamide and the solution in Step 2 to solid-liquid separation to separate the polyamide and the solution, thereby recovering the polyamide; Step 4: using the solution obtained through solid-liquid separation in Step 3 as a raw material to obtain a second metal chloride alcohol solution in which the solubility of polyamide is 90% or more of the solubility of polyamide in the first metal chloride alcohol solution. The method for producing a polyamide may further include steps other than Steps 1 to 4 (hereinafter referred to as "other steps"). In the method for producing a polyamide, Steps 1 to 4 and optionally other steps may be repeated.

[0014] <Polyamide composition> In a polyamide production method, the polyamide used as a raw material is referred to as a "polyamide composition." When the polyamide production method is used to recycle polyamide, the "polyamide composition" refers to a polyamide-containing object before recycling, such as a polyamide-containing part, a polyamide-containing member, an object obtained by removing non-polyamide-containing portions from these, and crushed material thereof. When the polyamide production method is used to purify polyamide, the "polyamide composition" refers to polyamide before purification.

[0015] The polyamide composition may consist solely of polyamide, or may further contain other components, such as a heat stabilizer. The polyamide composition may also be in the form of a composite (e.g., coating, laminate, blend, polymer alloy, etc.) of polyamide with various other polymers. The other polymers preferably have resistance to dissolution in metal chloride alcohols, such as polyolefins, nitriles, polyesters, polyimides, and silicones.

[0016] The polyamide may be a polymer formed by polymerization through an amide bond, such as a polycondensation of a diamine compound and a dicarboxylic acid compound or a ring-opening polymerization of a cyclic lactam. Examples of the diamine compound include, but are not limited to, ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, nonanediamine, methylpentanediamine, and p-phenylenediamine. Examples of the dicarboxylic acid compound include, but are not limited to, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, sebacic acid, terephthalic acid, and isophthalic acid. Examples of the cyclic lactam include, but are not limited to, ε-caprolactam, undecanelactam, and lauryllactam. The combination of the diamine compound, dicarboxylic acid compound, and cyclic lactam compound is not particularly limited, and multiple types of each compound may be used in combination. Polyhexamethylene adipamide (for example, polyhexamethylene adipamide made from hexamethylenediamine and adipic acid) has high solubility and is suitable for the manufacturing method of this embodiment.

[0017] Examples of the polyamide include polycaproamide (nylon 6), polyhexamethylene adipamide (nylon 66), polytetramethylene adipamide (nylon 46), polytetramethylene sebacamide (nylon 410), polypentamethylene adipamide (nylon 56), polypentamethylene sebacamide (nylon 510), polyhexamethylene sebacamide (nylon 610), polyhexamethylene dodecamide (nylon 612), polydecamethylene adipamide (nylon 106), polydecamethylene sebacamide (nylon 1010), poly Lidecamethylene dodecamide (Nylon 1012), polyundecaneamide (Nylon 11), polydodecanamide (Nylon 12), polycaproamide / polyhexamethylene adipamide copolymer (Nylon 6 / 66), polycaproamide / polyhexamethylene terephthalamide copolymer (Nylon 6 / 6T), polyhexamethylene adipamide / polyhexamethylene terephthalamide copolymer (Nylon 66 / 6T), polyhexamethylene adipamide / polyhexamethylene isophthalamide copolymer (Nylon 66 / 6I), polyhexamethylene Terephthalamide / Polyhexamethylene isophthalamide copolymer (Nylon 6T / 6I), Polyhexamethylene terephthalamide / Polyundecaneamide copolymer (Nylon 6T / 11), Polyhexamethylene terephthalamide / Polydodecanamide copolymer (Nylon 6T / 12), Polyhexamethylene adipamide / Polyhexamethylene terephthalamide / Polyhexamethylene isophthalamide copolymer (Nylon 66 / 6T / 6I), Polyxylylene adipamide (Nylon XD6), Polyxylylene sebacamide (Nylon XD10), Poly Examples thereof include polyhexamethylene terephthalamide / polypentamethylene terephthalamide copolymer (nylon 6T / 5T), polyhexamethylene terephthalamide / poly-2-methylpentamethylene terephthalamide copolymer (nylon 6T / M5T), polypentamethylene terephthalamide / polydecamethylene terephthalamide copolymer (nylon 5T / 10T), polynonamethylene terephthalamide (nylon 9T), polydecamethylene terephthalamide (nylon 10T), and polydodecamethylene terephthalamide (nylon 12T).The symbol " / " indicates a copolymer. These polyamides may be used singly or in combination. Among these, it is preferable to use one selected from the group consisting of polyamide 6, polyamide 66, polyamide 46, polyamide 610, and polyamide 612, with polyamide 66 being particularly preferable. Polyamide 66 itself is a commonly known polyamide resin, typically produced by polycondensation of hexamethylenediamine and adipic acid. Alternatively, polyamide 66 may be a copolymer containing less than 30% by mass of at least one monomer unit selected from the group consisting of lactams, aminocarboxylic acids, and combinations of other diamines and dicarboxylic acids, based on the total mass of all monomer units.

[0018] These polyamides may be commercially available or may be produced by known methods. Specific methods for producing polyamides are not particularly limited, but examples thereof include ring-opening polymerization of lactams, self-condensation of ω-aminocarboxylic acids, and condensation of diamines and dicarboxylic acids.

[0019] <Polyamide> In the method for producing polyamide, the polyamide to be produced may be a "polyamide" alone, or may be produced as a "polyamide composition" containing polyamide, solvent, and additives. When the method for producing polyamide is used for recycling polyamide, the "polyamide" produced refers to the polyamide after recycling. When the method for producing polyamide is used for purifying polyamide, the "polyamide" refers to the polyamide after purification.

[0020] <Metal Chloride Alcohol Solution> A "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. The metal chloride alcohol solution functions as a dissolving liquid for polyamide, i.e., a good solvent for polyamide. In a polyamide production method, the metal chloride alcohol solution used as a solvent for dissolving the polyamide composition by heating is called a "first metal chloride alcohol solution," and the metal chloride alcohol solution obtained as a polyamide dissolving liquid from the solution after polyamide recovery is called a "second metal chloride alcohol solution." The "second metal chloride alcohol solution" must be able to sufficiently dissolve polyamide. After being obtained in the polyamide production method, the second metal chloride alcohol solution can be used as a solvent for further polyamide dissolution, for example, as the first metal chloride alcohol solution in the next polyamide production method. This makes it possible to reduce the rate of waste of residual solution after polyamide recovery while maintaining a sufficient concentration of the resulting polyamide solution.

[0021] In both the first and second metal chloride alcohol solutions, from the viewpoint of the solubility of polyamide, the total mass ratio of the metal chloride and alcohols relative to 100 mass% of the metal chloride alcohol solution is preferably 70 mass% or more, more preferably 80 mass% or more, and even more preferably 90 mass% or more.

[0022] <<Alcohols>> Examples of alcohols contained in the first and second metal chloride alcohol solutions include monoalcohols such as methanol, ethanol, linear or branched propanol, and linear or branched butanol; diols such as ethylene glycol, propylene diol (e.g., propylene glycol), butanediol, and diethylene glycol; trihydric alcohols such as glycerin; other polyhydric alcohols; and combinations thereof. Among these, methanol, ethanol, or combinations thereof are preferred from the viewpoint of the solubility of polyamide, and methanol is more preferred.

[0023] <<Metal Chloride>> Examples of metal chlorides contained in the first and second metal chloride alcohol solutions include zinc chloride, magnesium chloride, and calcium chloride, with zinc chloride and calcium chloride being preferred, and calcium chloride being most preferred. The mass proportion of water in the metal chloride is not particularly limited as long as it can be adjusted so that the water content in the metal chloride alcohol solution is the desired amount, and the metal chloride may be an anhydrous, hydrated, or a mixture thereof.

[0024] <<Metal Chloride Concentration>> From the viewpoint of improving the solubility of the polyamide composition, the concentration of the metal chloride in the metal chloride alcohol solution is preferably 15% by mass or more, more preferably 18% by mass or more, and even more preferably 20% by mass or more. On the other hand, from the viewpoint of the metal chloride remaining undissolved and being easily mixed in as an impurity if the concentration of the metal chloride in the metal chloride alcohol solution is too high, the concentration is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less. Other preferred properties of the "second metal chloride alcohol solution" will be described in "Step 4" below. <<Water Concentration>> From the viewpoint of improving the solubility of the polyamide composition, the concentration of water in the metal chloride alcohol solution is preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less. On the other hand, from the viewpoint of improving the solubility of calcium chloride in methanol, the concentration of water is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more.

[0025] <Poor Solvent> The term "poor solvent" refers to a poor solvent for polyamide, and is used to precipitate polyamide by mixing with a heated polyamide solution. The poor solvent is a poor solvent for polyamide, preferably a solvent compatible with the metal chloride alcohol solution. It is not particularly limited as long as it dissolves metal chlorides. Examples include water-containing solutions and alcohol-containing solutions. However, water-free alcohol-containing solutions are preferred from the viewpoint of reducing the amount of moisture accumulated in the recycled metal chloride alcohol solution, which inhibits dissolution. When the same alcohol as the solvent for the first metal chloride alcohol solution is used as the poor solvent, it is preferable to distill off a portion of the solvent from the solution after precipitating the polyamide, thereby restoring the solubility of the metal chloride alcohol solution. Furthermore, when steps 1 to 4 are repeatedly performed, the alcohol component removed in step 4 from the solution obtained by solid-liquid separation in step 3 may be used as the poor solvent for subsequent steps 2. For example, the solvent may be such that, when 100 g of the solvent is added to 1 g of polyamide (for example, nylon 66) and the mixture is stirred at 20° C. for 1 hour, the amount of polyamide dissolved in the solution is 0.05 g or less.

[0026] <<Water-Containing Solution>> The water-containing solution is a liquid containing water or water and a component other than water. Examples of the component other than water include alcohols (e.g., those exemplified as alcohols contained in the metal chloride alcohol solution), salts, pH adjusters, etc.

[0027] 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.

[0028] 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.

[0029] Examples of pH adjusters include water-soluble basic salts, acid salts, and buffers (e.g., a mixture of an acid and a salt, or a mixture of a base and a salt), 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., a mixture of acetic acid and sodium acetate), and phosphate buffers (e.g., a mixture of acetic acid and sodium acetate).

[0030] <<Alcohol-Containing Solution>> A 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 a 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 a water-containing solution. Note that 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. A water-free alcohol-containing solution refers to an alcohol-containing solution that does not intentionally use water as a raw material, and therefore may contain a trace amount of water. For example, the water content may be 0.5% by mass or less relative to the alcohol-containing solution.

[0031] <Step 1: Heating and Dissolving Step> Step 1 is a step of heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution. In Step 1, it is preferable that the polyamide contained in the polyamide composition is sufficiently dissolved in the metal chloride alcohol solution as a solvent. Preferred conditions for Step 1 are described below.

[0032] <<Amount of Polyamide Composition Used>> As the amount of polyamide composition used in step 1, the concentration of polyamide in the heated polyamide solution is preferably 5% by mass or more, more preferably 7% by mass or more, and preferably 15% by mass or less, and more preferably 13% by mass or less. By setting the concentration at or above the lower limit, it is possible to prevent the amount of solvent from becoming too large, and by setting the concentration at or below the upper limit, it is possible to prevent the viscosity of the solution from becoming too high, optimize the dissolution time, and maintain good operability.

[0033] <<Conditions for Heat Dissolution>> The temperature for heat dissolution is not particularly limited, but is preferably 30°C or higher, more preferably 40°C or higher, and preferably 90°C or lower, and more preferably 75°C or lower. Setting the temperature to 30°C or higher prevents dissolution from taking too long, while setting the temperature to 90°C or lower not only prevents evaporation of alcohols and maintains solubility, but also contributes to suppressing corrosion of the equipment and solvolysis of the polyamide (e.g., alcoholysis and hydrolysis). During Step 1, the temperature may be constant or may be varied within the above range.

[0034] The time for heat dissolution is preferably as long as possible from the viewpoint of sufficiently dissolving the polyamide composition, for example, preferably 5 minutes or more, more preferably 30 minutes or more, and even more preferably 1 hour or more. On the other hand, the time for heat dissolution is preferably not too long from the viewpoints of preventing deterioration of the polyamide due to heating, loss of alcohols in the metal chloride alcohol solution due to volatilization, and suppressing production costs, for example, preferably 100 hours or less, more preferably 50 hours or less, and even more preferably 24 hours or less.

[0035] The dissolution may be carried out by either a batch method or a continuous method. In the case of a batch method, there is no particular limitation as to whether or not stirring is performed, but stirring is preferred. Stirring improves the dissolution rate of the polyamide solid. In the case of a continuous method, the solvent may be continuously passed through the solid, or the solution may be circulated. Circulation is preferred because it allows for a reduction in the amount of solvent used.

[0036] The shape of the container is not particularly limited, and any shape, such as a tank type or a circulation type, may be used. The shape of the container used when heating and dissolving the polyamide resin composition and the metal chloride alcohol solution is not particularly limited, and any shape, such as a tank type or a circulation type, may be used. The materials of the container and piping are also not particularly limited, and examples thereof include SUS316, SUS316L, SUS329J4L, and SUS444. The surfaces of these materials may be lined or coated, for example, with glass, fluorine-based resin, rubber, epoxy, etc., and from the viewpoint of corrosion resistance, glass and fluorine-based resin are preferred. When lining or coating is used, the materials of the container and piping themselves can be selected without considering corrosion.

[0037] <Step 2: Precipitation Step> Step 2 is a step in which the heated polyamide solution obtained in Step 1 is mixed with a poor solvent to precipitate the polyamide. In Step 2, the heated polyamide solution is placed in an environment containing a poor solvent instead of a good solvent environment containing a metal chloride alcohol solution, thereby reducing the solubility of the polyamide and causing it to precipitate. Preferred conditions for Step 2 are described below.

[0038] <<Mixing Method>> The method for mixing the heated polyamide solution and the poor solvent in step 2 is not particularly limited, and examples thereof include adding the heated polyamide solution to the poor solvent, adding the poor solvent to the heated polyamide solution, and adding the heated polyamide solution and the poor solvent to a container. The addition method is not particularly limited, and examples thereof include dropwise addition, continuous pouring from above the liquid, and spraying. During mixing, the solution may be left to stand without stirring, or may be stirred.

[0039] <<Mixing Ratio>> The mass of the poor solvent used in step 2 is preferably at least 1 time, more preferably at least 2 times, and even more preferably at least 3 times the mass of the heated polyamide solution. The upper limit is preferably no more than 10 times, more preferably no more than 5 times, and even more preferably no more than 3 times. By using an amount of poor solvent that is at least 1 time that of the heated polyamide solution, the amount of polyamide precipitated can be made to be at least a certain amount, and a decrease in yield can be suppressed. By using an amount of poor solvent that is no more than 10 times that of the heated polyamide solution, the amount of solvent removed for the recycling step does not become too large, and the energy and time required for recycling can be suppressed.

[0040] <<Mixing Temperature>> The temperature of the solution during mixing is preferably a temperature at which it 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 process, 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 and lower the temperature to 10°C. Heating and cooling may be repeated to change the state of the precipitated particles.

[0041] <<Precipitation Time>> The time required for precipitation is not particularly limited. By changing the time for adding the solution and / or poor solvent, the desired precipitation rate, precipitation temperature range, etc. can be achieved, thereby changing the shape and properties of the particles. Furthermore, by providing a mixing time after addition, it is possible to stabilize the yield and control conditions such as particle surface modification, secondary particle generation, and particle crushing.

[0042] <Step 3: Recovery> Step 3 is a step of recovering polyamide by solid-liquid separation of the mixture of polyamide precipitated in Step 2 and the solution into polyamide and solution. The recovery method by solid-liquid separation may be a commonly used solid-liquid separation method. The recovery method by solid-liquid separation may be appropriately selected depending on the shape of the polyamide, etc., and examples include filtration, centrifugation (when the polyamide is a powder, particles, or a solid object having a size that can be handled by hand), scooping using a mesh or perforated tool (when the polyamide is a particle or a solid object having a size that can be handled by hand), and lifting using a rod-shaped tool (when the polyamide is a solid object having a size that can be handled by hand). The polyamide recovered by solid-liquid separation may be washed with a solvent. Washing can remove metal chlorides and other impurities from the recovered polyamide. Washing may be performed once or multiple times. The solvent used for washing is preferably one that can dissolve metal chlorides. Examples of such a solvent include monoalcohols such as methanol, ethanol, linear or branched propanol, linear or branched butanol, and the like; diols such as ethylene glycol, propylene diol (e.g., propylene glycol), butanediol, diethylene glycol, and the like; trihydric alcohols such as glycerin, and the like; other polyhydric alcohols; combinations of these alcohols; and water. From the viewpoint of reusing the solvent after washing, it is preferable to use the same alcohol as in step 1. On the other hand, from the viewpoint of washing efficiency of metal chlorides, water is preferable as the solvent used for washing. For example, by using the same alcohol as in step 1 for the first washing and using water for the second washing and thereafter, it is possible to achieve both recovery of metal chlorides and washing efficiency.

[0043] <Step 4: Obtaining a Second Metal Chloride Alcohol Solution> Step 4 is a step of obtaining a second metal chloride alcohol solution having a polyamide solubility within the above-mentioned predetermined range using the solution obtained by solid-liquid separation in Step 3 (hereinafter, sometimes referred to as the "residual solution"). If the polyamide precipitated in Step 3 is washed with the same alcohol as in Step 1, the washing liquid may be mixed with the solid-liquid separated solution. Since the second metal chloride alcohol solution has a polyamide solubility within the above-mentioned predetermined range, it can be used as a polyamide dissolving solution. The second metal chloride alcohol solution (i.e., polyamide dissolving solution) obtained in Step 4 can be used as a solvent for further polyamide dissolution. For example, when Steps 1 to 4 are repeated, the second metal chloride alcohol solution obtained in Step 4 can be used as the first metal chloride alcohol solution to be used in the next Step 1. This reduces the amount of solution discarded as waste after polyamide recovery and makes it possible to suppress losses of metal chlorides and alcohols, which are components of the metal chloride alcohol solution used as a polyamide dissolving solution.

[0044] <<Preferred Properties of the Second Metal Chloride Alcohol Solution>> In order for the second metal chloride alcohol solution obtained in step 4 to be a solution capable of sufficiently dissolving polyamide, the solubility of polyamide must be 90% or more of the solubility of polyamide in the first metal chloride alcohol solution, preferably 95% or more, and more preferably 98% or more. Furthermore, when steps 1 to 4 are repeatedly performed, the solubility of polyamide in the second metal chloride alcohol solution obtained in each step 4 is preferably 90% or more of the solubility of polyamide in the first metal chloride alcohol solution used in the initial step 1, more preferably 95% or more, and even more preferably 98% or more. The solubility of polyamide can be measured, for example, by the method described in the Examples.

[0045] When steps 1 to 4 are repeated and the second metal chloride alcohol solution obtained in step 4 is used in the next step 1, the concentration of the metal chloride contained in the second metal chloride alcohol solution is preferably 80 to 120%, more preferably 90 to 110%, and even more preferably 95 to 105% of the concentration of the metal chloride contained in the first metal chloride alcohol solution. If the difference in concentration from the first metal chloride alcohol solution is large, the liquid properties such as density and viscosity will change, making it necessary to change the process conditions. Therefore, in order to avoid changing the process conditions, the concentration of the metal chloride contained in the second metal chloride alcohol solution relative to the concentration of the metal chloride contained in the first metal chloride alcohol solution is preferably within the above range.

[0046] Even if the solubility of polyamide is within the above range, the second metal chloride alcohol solution obtained in step 4 preferably has as low a concentration as possible of compounds having amide bonds with a molecular weight of 1,000 or less, which cause a decrease in dissolution rate. The concentration of compounds having amide bonds with a molecular weight of 1,000 or less contained in the second metal chloride alcohol solution obtained in step 4 is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 0.5% by mass or less. Furthermore, when steps 1 to 4 are repeated and the second metal chloride alcohol solution obtained in step 4 is used in the next step 1, the concentration of compounds having amide bonds with a molecular weight of 1,000 or less contained in the second metal chloride alcohol solution is preferably 200% or less, more preferably 150% or less, and even more preferably 100% or less of the concentration of compounds having amide bonds with a molecular weight of 1,000 or less contained in the first metal chloride alcohol solution.

[0047] Even if the solubility of polyamide is within the above range, the second metal chloride alcohol solution obtained in step 4 preferably has a water concentration as low as possible, which reduces the dissolution rate of polyamide. From the viewpoint of improving the solubility of the polyamide composition, the water concentration contained in the second metal chloride alcohol solution obtained in step 4 is preferably 15% by mass or less, more preferably 12% by mass or less, and even more preferably 10% by mass or less. On the other hand, from the viewpoint of improving the solubility of calcium chloride in methanol, the water concentration is preferably 0.1% by mass or more, more preferably 0.3% by mass or more, and even more preferably 0.5% by mass or more. Furthermore, when steps 1 to 4 are repeated and the second metal chloride alcohol solution obtained in step 4 is used in the next step 1, it is preferable that the water content in the second metal chloride alcohol solution is as small as possible relative to the water content of the first metal chloride alcohol solution. The difference between the water concentration in the second metal chloride alcohol solution obtained in step 4 and the water concentration in the first metal chloride alcohol solution is preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 2% by mass or less.

[0048] To achieve a polyamide solubility within the predetermined range in the second metal chloride alcohol solution, step 4 may include a restoring step (restoring step) in which a solution obtained from the solution (residual solution) obtained in step 3 after solid-liquid separation is restored so that the polyamide solubility falls within the predetermined range, thereby obtaining a second metal chloride alcohol solution. For example, the restoring step may be performed by removing alcohol components from the solution obtained from the solution (residual solution) after solid-liquid separation in step 3 to reduce the alcohol content, thereby obtaining a second metal chloride alcohol solution (i.e., obtaining the residual solution with a reduced alcohol content as the second metal chloride alcohol solution). The alcohol components may be removed by, for example, distillation of the alcohol components under heating and / or reduced pressure, membrane separation, or the like. By using the alcohol that is the solvent for the first metal chloride alcohol solution as a poor solvent, the solubility of the polyamide can be restored simply by concentrating the solution by partially distilling off the alcohol. The amount of alcohol removed by distillation may be set so that the solubility of the polyamide is 90% or more of the solubility of the polyamide in the first metal chloride alcohol solution. When the alcohol used as the poor solvent for the first metal chloride alcohol solution is used and only a portion of the alcohol is distilled off to reuse the resulting solution as a second metal chloride alcohol solution, the amount of heat required for regeneration of the solution can be reduced, which is preferable from the viewpoints of resource conservation and energy conservation. Specifically, when all of the water used as the poor solvent is distilled off and calcium chloride is dried and used, the required amount of heat is 2.0 to 3.2 kJ / g. When all of the methanol used as the poor solvent is distilled off and calcium chloride is dried and used, the required amount of heat is 1.1 to 2.1 kJ / g. On the other hand, when methanol used as the poor solvent is partially distilled off, the required amount of heat is 1.1 kJ / g or less. The required amount of heat is expressed as the value calculated from the amount of solvent distilled off and the latent heat, divided by the amount of dilution solvent. Alternatively, the restoration step may be performed by adding a metal chloride component or an alcohol solution in which a metal chloride component is dissolved to the residual solution to increase the metal chloride content, thereby obtaining the residual solution with an increased metal chloride content as the second metal chloride alcohol solution.The metal chloride may be added to the remaining solution, or the metal chloride concentration may be increased by partially distilling off the solvent and / or membrane separation, as in the restoration step. In this case, the solubility of the polyamide may be restored to 90% or more of the solubility of the first metal chloride alcohol solution by the restoration step alone, or the solubility may be stopped at less than 90% and then restored to 90% or more by adding the metal chloride component or an alcohol solution in which the metal chloride component is dissolved.

[0049] To ensure that the solubility of the polyamide in the second metal chloride alcohol solution is within the predetermined range, step 4 may include a step (confirmation step) of confirming that the solubility of the polyamide in the second metal chloride alcohol solution is within the predetermined range. The measurement of the polyamide solubility in the confirmation step can be performed, for example, by the method described in the Examples. In addition to the method of directly measuring the solubility, the solubility of the polyamide can also be calculated by clarifying the correlation between the solution composition, such as the metal chloride concentration, water content, low-molecular-weight component content, and alcohol content, and the solubility of the polyamide.

[0050] In order to reduce the accumulation of moisture and compounds having an amide bond with a molecular weight of 1,000 or less, which may be factors inhibiting dissolution of the polyamide in the second metal chloride alcohol solution, step 4 may include a step of discarding a portion of the solution (residual solution) obtained by solid-liquid separation in step 3 (partial discarding step) and a step of replenishing the residual solution with alcohol and / or metal chloride (replenishing step).

[0051] <Other Steps> The polyamide production method may further include, after step 1, a step of removing impurities other than polyamide from the heated polyamide solution (hereinafter, sometimes referred to as an "impurity removal step"). The impurity removal step is not particularly limited, and may be carried out, for example, by removing impurities as insoluble components by filtration, centrifugation, or the like. In the case of a method in which the solvent of the first metal chloride alcohol solution is used as the poor solvent and the solubility of the polyamide is restored by partial distillation of the solvent, repeated use will result in the accumulation of impurities such as water and low-molecular-weight polyamide (partly as insoluble matter) in the solution. Therefore, it is preferable to manage the solution to be used repeatedly so that the concentration of water and / or low-molecular-weight polyamide does not exceed a certain value. If the concentration of water and / or low-molecular-weight polyamide exceeds the control value as a result of repeated use, it is preferable to operate the system so that the concentration of water and / or low-molecular-weight polyamide falls below the control value, for example, by discarding part or all of the solution. When the same raw material is used regularly, and the accumulated amount of impurities such as water and low-molecular-weight polyamide is constant or fluctuates little, the amount and number of times of disposal can be determined in advance so that the amount does not exceed a certain value. For example, if it is estimated that the amount will exceed a certain value after 10 repeated uses, methods such as discarding 10% of the solution every time or discarding 20% ​​every other time can be used.

[0052] The polyamide production method may further include, after step 3, a step of washing the polyamide with a washing solution (hereinafter also referred to as the "washing step") and a step of removing the attached solution from the polyamide (hereinafter also referred to as the "attached solution removing step"). Examples of the washing solution used in the washing step include the liquids exemplified as alcohol solutions and water-containing solutions. Examples of alcohols include monoalcohols such as methanol, ethanol, linear or branched propanol, and linear or branched butanol; diols such as ethylene glycol, propylene diol (e.g., propylene glycol), butanediol, and diethylene glycol; trihydric alcohols such as glycerin; other polyhydric alcohols; and combinations thereof. It is preferable to use the alcohol used in the metal chloride alcohol solution. Multiple washing solutions may be selected. From the viewpoint of washability, it is preferable to wash with an alcohol solution followed by a water-containing solution. Examples of methods for performing the attached solution removing step include drying (e.g., heat drying, vacuum drying, air drying, etc.), centrifugation, squeezing, etc.

[0053] (Method for producing polyamide solution) The same method as described above as the "method for producing polyamide" can also be used as the "method for producing polyamide solution." That is, the method for producing a polyamide solution of the present invention produces a polyamide product from a polyamide raw material (hereinafter referred to as a "polyamide composition"), and also makes it possible to obtain a polyamide solution capable of sufficiently dissolving polyamide from the remaining solution after polyamide recovery. This method makes it possible to reduce the amount of the remaining solution after polyamide recovery that is discarded as waste liquid, and to reduce losses of metal chlorides, alcohols, and the like, which are components of the metal chloride alcohol solution used as the polyamide solution.

[0054] The method for producing a polyamide solution of the present invention comprises the following steps: Step 1: A step of heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: A step of mixing the heated polyamide solution obtained in Step 1 with a poor solvent to precipitate a polyamide; Step 3: A step of performing solid-liquid separation of the mixture of the polyamide precipitated in Step 2 and the solution into polyamide and solution to recover the polyamide; Step 4: A step of using the solution obtained through solid-liquid separation in Step 3 as a raw material to obtain a polyamide solution as a second metal chloride alcohol solution in which the solubility of polyamide is 90% or more of the solubility of polyamide in the first metal chloride alcohol solution.

[0055] The polyamide composition, polyamide to be produced, metal chloride alcohol solution, and poor solvent in the method for producing a polyamide solution are the same as those described in the method for producing a polyamide. Steps 1 to 3 in the method for producing a polyamide solution are the same as those described in the method for producing a polyamide. Step 4 in the method for producing a polyamide solution can be carried out under the same conditions as those described in the method for producing a polyamide.

[0056] (Polyamide solution) The polyamide solution contains a metal chloride, an alcohol, and a compound having an amide bond with a molecular weight of 1000 or less, and the concentration of the compound having an amide bond with a molecular weight of 1000 or less is 0.1% by mass or more and 5% by mass or less. A polyamide solution with such a composition can sufficiently dissolve polyamide. The polyamide solution can be obtained by the above-mentioned "method for producing polyamide" or "method for producing polyamide solution".

[0057] (Molding of Precipitated Polyamide) Since the precipitated polyamide is generally in a powder form, it is preferable to mold it into a polyamide solid from the viewpoint of workability in subsequent processes. For example, molding it into a polyamide solid rather than a powder form has the advantage that the raw material is less bulky and easier to feed when spinning polyamide fibers. Although powdered polyamide may be extruded and melted to form so-called pellets, from the viewpoint of obtaining a polyamide solid with little thermal history, it is preferable to mold the polyamide powder without melting it. Examples of methods for obtaining such a polyamide solid include compression molding and extrusion granulation. The polyamide solid is preferably a powder compact obtained by pressing the polyamide powder without melting it. A powder compact of polyamide powder can be molded by compression molding or extrusion granulation without heating. From the viewpoint of molding the polyamide solid into a powder compact, it is preferable that particles of 2 mm or less account for 90% or more of the total particles of the particles (granulation raw material) precipitated in step 3, and it is preferable that particles of 1 mm or less account for 90% or more of the total particles.

[0058] Examples of the production method using extrusion granulation include production by extrusion granulation, in which the granulation raw material containing the polyamide recovered in step 3 and other additives, if necessary, is fed into an extrusion granulator, and the extrudate is cut and pelletized to produce a granulated product.

[0059] The other additives include water, organic solvents, etc. Compounds having a boiling point equal to or lower than the boiling point of water are preferred from the viewpoint of being removable in the drying step of the pellets before the spinning step.

[0060] The temperature at which the polyamide recovered in step 3 is molded (for example, the temperature during extrusion granulation) is below the melting point of the polyamide recovered in step 3, from the viewpoint of suppressing the occurrence of black spots, and is preferably 200° C. or lower, more preferably 20 to 150° C. It is acceptable for the temperature to deviate from the above temperature range locally and / or for a short period of time due to frictional heat during the molding process, etc.

[0061] Examples of extrusion granulators include screw-type extrusion granulators, roll-type extrusion granulators, disc pelleter-type extrusion granulators, pellet mill-type extrusion granulators, basket-type extrusion granulators, blade-type extrusion granulators, oscillating-type extrusion granulators, gear-type extrusion granulators, and ring die-type extrusion granulators.

[0062] The size of the polyamide solids can be controlled by adjusting various parameters of the extrusion granulator. For example, in the case of a disc pelletizer, it can be controlled by adjusting the disc rotation speed, etc.

[0063] From the viewpoint of reducing the load on the extrusion granulation apparatus and enabling a high processing speed, wet granulation is preferably performed using water and / or an organic solvent. Water is preferred as the binder for wet granulation, from the viewpoint of eliminating the need for explosion-proof equipment. The amount of water added is preferably 5% or more, more preferably 7% or more, relative to the polyamide. Considering that the polyamide solid will be used as a spinning raw material, it is preferable that the polyamide does not contain a binder at the time of use, and the binder is preferably volatile and added in an amount that is easy to remove after molding. From this viewpoint, the upper limit of the amount added is preferably 25% or less, more preferably 15% or less. When wet granulation is performed, the water and / or organic solvent used in wet granulation is preferably removed after granulation. A method for removing the water and / or organic solvent after granulation is, for example, preferably drying at a temperature of 60 to 100°C at atmospheric pressure or reduced pressure, and vacuum drying may be used.

[0064] [Characteristics of Polyamide Solid] The polyamide solid is prepared by stirring 1 g of polyamide solid and 30 g of a solvent (e.g., a 20 wt % calcium chloride methanol solution) at 60°C for 12 hours, filtering the resulting solution under pressure (e.g., pressure filtration under a pressure of 0.01 MPa) using a separation material with a pore size of 10 μm (e.g., a PTFE membrane filter with a pore size of 10 μm), filtering 30 g of a solvent (e.g., a 20 wt % calcium chloride methanol solution) under pressure (e.g., pressure filtration under a pressure of 0.01 MPa), passing 100 g of methanol through the filtering material, and then vacuum drying at 50°C. Preferably, the number of insoluble particles with a major axis of 200 μm or less remaining on the separation material is 50,000 or less per 1 g of polyamide solid, and no insoluble particles with a major axis greater than 200 μm are present.

[0065] The number of the insoluble particles having a major axis of 200 μm or less remaining on the separation material is preferably 40,000 or less, more preferably 20,000 or less per gram of polyamide solid material.

[0066] It is preferable that no insoluble matter having a major axis greater than 100 μm is present on the separation material, and it is more preferable that no insoluble matter having a major axis greater than 50 μm is present on the separation material.

[0067] The insoluble matter remaining on the separation material in the above-mentioned measurement method may be insoluble matter that could not be removed in the above-mentioned step 2, or insoluble matter such as black spots that occurred during molding to obtain a polyamide solid.

[0068] In the case of polyamide solids, 1 g of polyamide solids and 30 g of a solvent (e.g., a 20 wt % calcium chloride methanol solution) are stirred at 60°C for 12 hours, and the resulting solution is subjected to pressure filtration separation (e.g., pressure filtration separation under a pressure of 0.01 MPa) using a separation material with a pore size of 10 μm (e.g., a PTFE membrane filter with a pore size of 10 μm), and 30 g of a solvent (e.g., a 20 wt % calcium chloride methanol solution) is further subjected to pressure filtration (e.g., pressure filtration separation under a pressure of 0.01 MPa), followed by passing 100 g of methanol through the solution, and then vacuum drying at 50°C. The mass of insoluble matter remaining on the separation material is preferably 0.5 ppm by mass or less, more preferably 0.4 ppm by mass or less, and even more preferably 0.2 ppm by mass or less, relative to 100% by mass of polyamide solids.

[0069] The mass of the insoluble matter remaining on the separation material can be obtained by optically measuring the volume of the insoluble matter remaining on the separation material and calculating the mass of the insoluble matter based on the specific gravity of the polyamide solid matter.

[0070] The insoluble matter remaining on the separation material in the above-mentioned measurement method may be insoluble matter that could not be removed in the above-mentioned step 2, or insoluble matter such as black spots that occurred during molding to obtain a polyamide solid.

[0071] 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 and evaluation methods used in the examples and comparative examples are shown below.

[0072] [Polyamide Solubility] The solubility of polyamide can be measured, for example, by the following method. 100 g of polyamide solution is weighed into a 250 mL glass bottle containing a stirrer. 10 g of polyamide is added, the bottle is capped, and the bottle is stirred while heated to 50°C in a water bath. After stirring for 24 hours, the temperature is returned to 25°C. If no precipitation occurs, additional polyamide is added and the same procedure is repeated. If precipitation occurs, the supernatant is recovered by filtration, and water in an amount 10 times the volume of the recovered liquid is added to precipitate the polyamide. The precipitated polyamide is recovered by filtration, thoroughly washed with water, and vacuum dried at 60°C. The recovered weight is then measured, and the solubility is calculated from the ratio of the dissolved amount to the recovered liquid volume.

[0073] [Concentration of Compounds Having Amide Bonds with Molecular Weights of 1,000 or Less] 10 g of a solution for measuring the concentration of a compound having an amide bond with a molecular weight of 1,000 or less is weighed into a 50 mL round-bottom flask, and the solvent is removed using a rotary evaporator at 40°C. 10 g of water and a stir bar are added to the resulting residue, and the flask is capped and stirred to dissolve the water-soluble components. The water-insoluble components are then separated by vacuum filtration and weighed. 10 mg of the water-insoluble components are dissolved in 5 mL of eluent under the GPC conditions below, and a sample is subjected to GPC measurement under the conditions below. The concentration of components with a molecular weight of 1,000 or less in the water-insoluble components is calculated from the ratio of the peak area of ​​a molecular weight of 1,000 or less to the peak area of ​​a molecular weight of 1,000 or more. The weight of the components with a molecular weight of 1,000 or less is determined from the weight and concentration of the water-insoluble components, and the concentration in the solution is calculated from the weight of the solution.

[0074] [Amount of heat required for concentration] The amount of heat required for concentration is calculated by dividing the sum of the following amounts of heat by the sum of the amount of poor solvent added and the amount of washing solvent used in the washing solution added to the filtrate. -Amounts of heat to be added- ・Latent heat of vaporization of the alcohol distilled off ・Latent heat of vaporization of the water distilled off ・Amount of heat required for the metal chloride to transform from a stable hydrate into the target hydrate For example, if calcium chloride exists stably as a hexahydrate and the target hydrate is a dihydrate, the amount of heat required to remove water to that point is calculated from the heat of hydration. Information from publicly known literature, etc. may be used to determine the amount of heat required for the above calculation.

[0075] - GPC conditions - Measuring instrument: HLC-8320GPC (manufactured by TOSOH Corporation) Column: TSKgel GMHHR-H(S) x 3 (4.6 mm ID x 15 cm) (manufactured by TOSOH Corporation) Eluent: HFIP (sodium trifluoroacetate - 5 mmol / L) Flow rate: 0.175 mL / min Column temperature: 40°C Detector: differential refractometry (built into the analytical instrument) Standard material: PMMA

[0076] Example 1: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The solution recovered after filtration was concentrated using an evaporator, yielding 192.5 g of methanol (GC purity 99.0%) as the fraction and 87.5 g of calcium chloride methanol solution (containing 17.5 g of calcium chloride) as the residue. The required heat was 955.5 J / g. The solubility of polyamide 66 in this solution was 15%. The water content in the calcium chloride methanol solution was 0.7% by mass due to moisture absorption. The concentration of compounds having amide bonds with molecular weights of 1000 or less was 0.2% by mass. The resulting solution was used as a polyamide dissolving solution and the same process as above was carried out to obtain powdered polyamide 66. The process was carried out in the same manner as the first run, and powdered polyamide 66 was obtained in a yield of 98.9%.

[0077] Example 2: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The solution recovered after filtration was concentrated using an evaporator, yielding 187.3 g of methanol (GC purity 99.0%) as a fraction and 92.8 g of calcium chloride methanol solution (containing 17.5 g of calcium chloride) as a residue. The required heat was 929.4 J / g. The solubility of polyamide 66 in this solution was 14%. The water content in the calcium chloride methanol solution was 0.7% by mass due to moisture absorption. The concentration of compounds having amide bonds with molecular weights of 1000 or less was 0.2% by mass. The resulting solution was used as a polyamide dissolving solution and the same process as above was carried out to obtain powdered polyamide 66. The process was carried out in the same manner as the first run, and powdered polyamide 66 was obtained in a yield of 98.7%.

[0078] Example 3: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The solution recovered after filtration was concentrated using an evaporator, yielding 182.5 g of methanol (GC purity 99.0%) as a fraction and 97.5 g of calcium chloride methanol solution (containing 17.5 g of calcium chloride) as a residue. The required heat was 905.9 J / g. The solubility of polyamide 66 in this solution was 12%. 2.5 g of calcium chloride was added to this solution and dissolved. The solubility of polyamide 66 in this solution was 15%. At this time, the water content in the calcium chloride methanol solution was 0.7% by mass due to moisture absorption. The concentration of compounds having amide bonds with molecular weights of 1000 or less was 0.2% by mass. The resulting solution was used as a polyamide dissolving solution and subjected to the same process as above to obtain powdered polyamide 66. The process was carried out in the same manner as the first run, and powdered polyamide 66 was obtained in a yield of 99.0%.

[0079] Example 4: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). 48.1 g of calcium chloride was added and dissolved in the filtered solution. The solubility of polyamide 66 in this solution was 15%. At this time, the water content in the calcium chloride methanol solution was 0.7% by mass due to moisture absorption. The concentration of the compound having an amide bond with a molecular weight of 1000 or less was 0.2% by mass. The resulting solution was used as a polyamide dissolving solution and the same process as above was carried out to obtain powdered polyamide 66. The process could be carried out in the same manner as the first time, and powdered polyamide 66 was obtained in a yield of 99.0%.

[0080] Example 5: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. 280 g of filtrate was obtained. The filtered solid precipitate was washed with 200 g of methanol on the filter, yielding 201 g of washings. The washed solid precipitate was then thoroughly washed with water. The washed solid precipitate was heated and dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The filtered solution was then added to the washings and concentrated in an evaporator to obtain 390.0 g of methanol (GC purity: 99.0%) as a fraction and 87.5 g of calcium chloride methanol solution (containing 17.5 g of calcium chloride) as a residue. The required heat was 1014 J / g. The solubility of polyamide 66 in this solution was 15%. The water content in the calcium chloride methanol solution was 0.7% by mass due to moisture absorption. The concentration of compounds having amide bonds with molecular weights of 1000 or less was 0.2% by mass. The resulting solution was used as a polyamide dissolving solution and the same process as above to obtain powdered polyamide 66. The process was carried out in the same manner as the first run, and powdered polyamide 66 was obtained in a yield of 99.0%.

[0081] Comparative Example 1: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of water was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The solution recovered after filtration was concentrated using an evaporator, and 70.0 g of methanol (GC purity 95.0%) and 192.5 g of water were recovered as distillates, but calcium chloride could not be restored to its anhydrous form. The heat required for this was 2612 J / g.

[0082] Comparative Example 2: 10 g of polyamide 66 pellets and 100 g of an 18.8 wt % calcium chloride methanol solution (water content: 6.0% by mass, polyamide 66 solubility: 13.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of water was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.8 g of powdered polyamide 66 (yield: 98.0%). The solution recovered after filtration was concentrated using an evaporator to obtain 70.0 g of methanol (GC purity 98.0%) and 186.0 g of water as fractions. 23.2 g of calcium chloride dihydrate was obtained as a residue. The calorific value required for this was 2419 J / g.

[0083] Comparative Example 3: 10 g of polyamide 66 pellets and 100 g of a 20.0 wt % calcium chloride methanol solution (water content: 0.6% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The solution recovered after filtration was concentrated using an evaporator to obtain 262.5 g of methanol (GC purity 98.0%) as a fraction. 18.6 g of calcium chloride was obtained as a residue. The calorific value required for this was 1599 J / g.

[0084] Comparative Example 4: 10 g of polyamide 66 pellets and 100 g of a 20 wt % calcium chloride methanol solution (water content: 0.5% by mass, polyamide 66 solubility: 15.0%) were added to a 300 mL glass bottle containing a stir bar. The bottle was then placed in a 60°C water bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 66, yielding a polyamide solution. The polyamide solution was transferred to a 500 mL three-neck flask equipped with a stirring blade, and 220 g of methanol was added while stirring to obtain a polyamide (polyamide 66) solid precipitate. The resulting solid precipitate was filtered and recovered using a 10 μm membrane filter. The filtrate yielded 280 g. The filtered solid precipitate was thoroughly washed with water. The washed solid precipitate was then dried in a vacuum oven at 80°C to obtain 9.9 g of powdered polyamide 66 (yield: 99.0%). The solution recovered after filtration was concentrated using an evaporator, yielding 182.5 g of methanol (GC purity 99.0%) as the fraction and 97.5 g of calcium chloride methanol solution (containing 17.5 g of calcium chloride) as the residue. The calorific value required was 906 J / g. The solubility of polyamide 66 in this solution was 12%. At this time, the water content in the calcium chloride methanol solution was 0.7% by mass due to moisture absorption. The concentration of compounds having amide bonds with molecular weights of 1000 or less was 0.2% by mass. The resulting solution was used as a polyamide dissolving solution and subjected to the same process as above to obtain powdered polyamide 66. Residuals remained during the dissolution process, making it impossible to proceed as in the first step.

[0085]

[0086] According to the present invention, a method can be provided for recycling polyamide by dissolution, which can reduce the energy required for recovering polyamide while sufficiently maintaining the solubility of the polyamide solution reused from the residual solution.

Claims

1. A method for producing a polyamide, comprising the following steps: step 1: heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; step 2: mixing the heated polyamide solution obtained in step 1 with a poor solvent to precipitate a polyamide; step 3: performing solid-liquid separation of the mixture of the precipitated polyamide and the solution in step 2 into polyamide and solution to recover the polyamide; step 4: using the solution obtained through solid-liquid separation in step 3 as a raw material to obtain a second metal chloride alcohol solution in which the solubility of polyamide is 90% or more of the solubility of polyamide in the first metal chloride alcohol solution.

2. The method for producing a polyamide according to claim 1, wherein the poor solvent is the alcohol, and in step 4, the second metal chloride alcohol solution is obtained by concentrating the solution obtained by solid-liquid separation in step 3 and / or by adding a metal chloride to the solution obtained by solid-liquid separation.

3. A method for producing a polyamide according to claim 1 or 2, wherein the concentration of the compound having an amide bond with a molecular weight of 1,000 or less contained in the second metal chloride alcohol solution obtained in step 4 is 5 mass% or less.

4. A method for producing a polyamide according to claim 1 or 2, wherein the water concentration in the second metal chloride alcohol solution obtained in step 4 is 20% by mass or less.

5. A method for producing a polyamide described in claim 1 or 2, wherein the difference between the water concentration contained in the second metal chloride alcohol solution obtained in step 4 and the water concentration contained in the first metal chloride alcohol solution is 10 mass% or less.

6. A method for producing a polyamide according to claim 1 or 2, wherein step 4 comprises a step of reducing the alcohol content by removing an alcohol component from a solution obtained from the solution obtained by solid-liquid separation in step 3, thereby obtaining a second alcohol solution of a metal chloride.

7. A method for producing a polyamide according to claim 6, comprising repeating steps 1 to 4, and using in step 4 the alcohol component removed from the solution obtained by solid-liquid separation in step 3 as a poor solvent for step 2 in the next or subsequent steps.

8. The method for producing a polyamide according to claim 1 or 2, wherein steps 1 to 4 are repeated, and the second metal chloride alcohol solution obtained in step 4 is used as the first metal chloride alcohol solution to be used in the next step 1.

9. The method for producing a polyamide according to claim 8, wherein the solubility of the polyamide in the second metal chloride alcohol solution obtained in step 4 each time is 90% or more of the solubility of the polyamide in the first metal chloride alcohol solution used in step 1 the first time.

10. A method for producing a polyamide solution, comprising the following steps: step 1: heating and dissolving a polyamide composition in a first metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; step 2: mixing the heated polyamide solution obtained in step 1 with a poor solvent to precipitate a polyamide; step 3: performing solid-liquid separation of the mixture of the polyamide precipitated in step 2 and the solution into polyamide and solution to recover the polyamide; step 4: using the solution obtained through solid-liquid separation in step 3 as a raw material, obtaining a polyamide solution as a second metal chloride alcohol solution in which the solubility of polyamide is 90% or more of the solubility of polyamide in the first metal chloride alcohol solution.

11. A polyamide solution comprising a metal chloride, an alcohol, and a compound having an amide bond with a molecular weight of 1,000 or less, wherein the concentration of the compound having an amide bond with a molecular weight of 1,000 or less is 0.1% by mass or more and 5% by mass or less.

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