Method for producing powdery polyamide

Abstract

To provide a method for producing powdery polyamide, in which the amount of liquid included in powdery polyamide that precipitates as well as the amount of washing solvent and energy of drying are reduced.SOLUTION: A method for producing powdery polyamide comprises: step 1 of obtaining a polyamide heat dissolved solution by heat-dissolving a polyamide 6 composition in a metal chloride alcohol solution including a metal chloride and an alcohol; step 2 of obtaining an alcohol diluted solution by diluting the polyamide heat dissolved solution with an alcohol; and step 3 of precipitating powdery polyamide by cooling the alcohol diluted solution. In the step 1, the mass ratio of the metal chloride relative to 100 mass% of the metal chloride alcohol solution is 10 mass% to 22 mass%,. In the step 1, the polyamide heat dissolved solution includes 0.2 mole to 2.5 moles of water relative to 1 mole of metal chloride. In the step 2, the polyamide heat dissolved solution is diluted without lowering the temperature to less than 50°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing powdered polyamide. In particular, the present invention relates to a method for producing powdered polyamide. Preferably, the present invention relates to a method for producing powdered polyamide that reduces the amount of waste liquid and the amount of energy used. [Background technology]

[0002] In relation to the present invention, polyamides, including nylon 6 and nylon 66, which are representative engineering plastics, have heat resistance and good mechanical properties and are widely used in textiles, automobile parts, electrical appliance parts, etc., and are one of the irreplaceable materials in modern society.

[0003] In recent years, technological developments have been made regarding the recycling of plastics with the aim of resource conservation and carbon neutrality, and polyamides are no exception.

[0004] Recycling methods can be broadly divided into material recycling, in which molded products are re-pelletized, and chemical recycling, in which monomers are reused through depolymerization. While material recycling raises concerns about quality instability because the polymer degradation and additives contained in the molded product remain intact in the recycled polymer, it requires fewer resources and energy because it does not involve chemical reactions and requires fewer auxiliary materials, making it the preferred method for recycling with a fixed end use. Furthermore, prior to chemical recycling, when additives and coatings are removed from processed or used polyamide recovered from factories or markets, a process to recover clean polyamide is required, just like material recycling. Therefore, material recycling technology is also useful for chemical recycling.

[0005] Physical methods for removing impurities from used polyamide and purifying it include a method of crushing the recovered material and then separating it by specific gravity (Patent Document 1). While this method requires little energy to separate the impurities, it is difficult to separate them when the polyamide and the impurities are strongly bound together by mixing, bonding, adhesion, or the like. Another method involves dissolving and removing some or all of the unwanted impurities using a solvent. However, impurities are generally added to or coated on the polyamide, and some or all of these are present inside the polyamide structure, making it difficult to dissolve all of the impurities. Furthermore, the resulting polyamide remains crushed, making it difficult to adequately control the shape, size, etc.

[0006] Another possible method is to dissolve the polyamide in a solvent, remove the impurities as insoluble matter, and then precipitate and recover the polyamide by some method. However, many of the solvents used to dissolve polyamides are unsuitable for industrial use, such as strong acids such as formic acid and sulfuric acid, or expensive solvents such as HFIP. An example of the use of a solvent that is easy to use industrially is a dissolution and recovery method using ethylene glycol (Patent Document 2). However, this method requires a very high-temperature reaction, which raises concerns about glycolysis of the polyamide. In addition, the solvent used must be completely removed from the sherbet-like solid by distillation and drying, which is thought to require a large amount of energy, including heating during the reaction. Therefore, there is also a method of dissolving polyamide in an alcoholic calcium chloride solution, which is a low-temperature method using common raw materials. For example, Patent Document 3 describes a method in which a silicone-coated polyamide cloth is treated with a methanolic calcium chloride solution to dissolve the polyamide, and the resulting solution is diluted with a large amount of water or methanol to obtain the desired polyamide as a powder.

[0007] When we tested this method of producing polyamide powder using dissolution deposition with an alcoholic solution of metal chlorides, assuming industrial production, we found that while it did indeed produce polyamide powder from which impurities such as silicone had been removed, it had significant problems when implemented industrially. The polyamide powder precipitated from the solvent contained a large amount of solvent during precipitation, and even filtration or centrifugation could not easily remove the solvent that had penetrated into the solid. As a result, multiple washes with large amounts of washing solvent were required to remove impurities such as metal chlorides from the solid. Another problem was that drying the washed powder required a large amount of solvent, which required a lot of energy to remove the large amount of solvent. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 5841598 [Patent Document 2] Japanese Patent Application Publication No. 2018-172618 [Patent Document 3] Patent No. 5110704 [Patent Document 4] Japanese Patent Application Publication No. 60-233129 Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to efficiently produce a polyamide powder.

[0010] After extensive investigation into the above-mentioned problematic phenomenon, we discovered that the reason polyamide powder contains a large amount of solvent is due to the porosity of the powder particles. A method for controlling the porosity of polyamide is described in Patent Document 4. Patent Document 4 describes an invention in which porous particles are obtained by dissolving polyamide in a metal chloride methanol solution, adding an aqueous methanol solution with a limited water content to the heated polyamide solution, and then cooling and precipitating the solution. Patent Document 4 also cites a method of diluting the solution with methanol as an example of suppressing porosity, and the invention emphasizes that the solvent conditions for precipitation are important. Using this invention as a reference, we conducted an investigation into suppressing porosity, and although there was some suppression effect, a porous solid was still obtained, and no significant effect was observed.

[0011] Further investigation revealed that the degree of porosity differs depending on the type of polyamide, and the invention was completed by examining the conditions for polyamide 6, which has a particularly high social need for recycling.

[0012] Therefore, an object of the present invention is to provide a method for producing a powdery polyamide that suppresses the porosity of polyamide 6 and reduces the amount of solvent contained therein, thereby reducing the amount of waste liquid and the energy required for drying. [Means for solving the problem]

[0013] In order to achieve this invention, we have conducted extensive research using an approach different from that of the prior art and have surprisingly discovered that the solution composition at the time of dissolution has a greater impact on porosity than the composition at the time of precipitation.We have discovered that by adjusting the solution composition at the time of dissolution to a specific range, it is possible to suppress the porosity of the polyamide powder and significantly reduce the amount of solvent contained during precipitation, which has led to the completion of the invention.

[0014] That is, the present invention is as follows. [1] Step 1: a step of dissolving a polyamide 6 composition by heating in a metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: Diluting the heated polyamide solution with alcohol to obtain an alcohol-diluted solution; and Step 3: Cooling the alcohol dilution to precipitate the powdered polyamide Including, In the step 1, the mass ratio of the metal chloride to 100 mass% of the metal chloride alcohol solution is 10 mass% or more and 22 mass% or less, In the step 1, the heated polyamide solution contains 0.2 mol or more and 2.5 mol or less of water per 1 mol of metal chloride, In step 2, the polyamide heated solution is diluted without lowering the temperature below 50°C. 1. A method for producing powdered polyamide comprising the steps of: [2] Step 1: a step of dissolving a polyamide 6 composition by heating in a metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: Diluting the heated polyamide solution with alcohol to obtain an alcohol-diluted solution; and Step 3: Cooling the alcohol dilution to precipitate the powdered polyamide Including, In the step 1, the mass ratio of the metal chloride to 100 mass% of the metal chloride alcohol solution is 10 mass% or more and 22 mass% or less, In the step 1, the heated polyamide solution contains 0.2 mol or more and 2.5 mol or less of water per 1 mol of metal chloride, The mass of the polyamide precipitated in the step 2 is 1 mass % or less based on the total mass of the polyamide contained in the heated polyamide solution. 1. A method for producing powdered polyamide comprising the steps of: [3] The method for producing a powdery polyamide according to [1] or [2], further comprising step 4: washing the powdery polyamide obtained in step 3 with a solvent at least once. [4] The method for producing a powdery polyamide according to [3], wherein the solvent used for the first washing in step 4 is the same alcohol as the alcohol used in step 1. [5] The method for producing a powdery polyamide according to any one of [1] to [4], wherein the heating and dissolving temperature in the step 1 is 60°C or higher and 80°C or lower. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention (hereinafter simply referred to as "the present embodiment") will be described in detail.

[0016] The method for producing powdered polyamide of this embodiment includes step 1: heating and dissolving a polyamide 6 composition in a metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a polyamide heated-dissolved solution, step 2: diluting the polyamide heated-dissolved solution with alcohol to obtain an alcohol-diluted solution, and step 3: cooling the alcohol-diluted solution to precipitate a powdered polyamide, wherein in step 1 the mass ratio of metal chloride relative to 100 mass% of the metal chloride alcohol solution is 10 mass% or more and 22 mass% or less, in step 1 the polyamide heated-dissolved solution contains 0.2 mol or more and 2.5 mol or less of water per 1 mol of metal chloride, and in step 2 the polyamide heated-dissolved solution is diluted at a temperature not lower than 50°C.

[0017] Another method for producing a powdered polyamide according to the present embodiment includes step 1: heating and dissolving a polyamide 6 composition in a metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a polyamide heated-solution, step 2: diluting the polyamide heated-solution with alcohol to obtain an alcohol-diluted solution, and step 3: cooling the alcohol-diluted solution to precipitate a powdered polyamide, wherein in step 1 the mass ratio of the metal chloride relative to 100 mass% of the metal chloride alcohol solution is 10 mass% or more and 22 mass% or less, the polyamide heated-solution in step 1 contains 0.2 mol or more and 2.5 mol or less of water per 1 mol of metal chloride, and the mass of polyamide precipitated in step 2 is 1 mass% or less relative to the total mass of polyamide contained in the polyamide heated-solution.

[0018] The manufacturing method of this embodiment may be a method consisting of only steps 1 to 3, or may further include other steps.

[0019] The compounds used in the production method of this embodiment will be described below.

[0020] <Polyamide> The polyamide 6 may be, for example, a polymer obtained by ring-opening polymerization of ε-caprolactam, which is a cyclic lactam.

[0021] <Polyamide 6 composition> The polyamide 6 composition may contain additives intended to provide effects during final use and processing. Additives include, but are not limited to, heat stabilizers, pigments, dyes, and plasticizers. Each additive may be either an inorganic salt or an organic compound. The polyamide 6 composition may contain polyamide coated with a resin other than polyamide. The coating resin may include, but is not limited to, polyolefin resin, polyurethane, acrylic resin, silicone resin, etc. The coating resin may be a single resin or multiple resins. In particular, from the viewpoint of production efficiency of the powdered polyamide, the mass proportion of the polyamide coated with a resin other than polyamide is preferably 30 to 100 mass% relative to 100 mass% of the polyamide resin composition, more preferably 60 mass% or more, even more preferably 70 mass% or more, and particularly preferably 80 mass% or more. The resin-coated polyamide may be a recycled polyamide raw material, such as fibers made from polyamide, or process offcuts or waste from molded products such as automobile parts and electrical appliance parts. Specific examples include process offcuts or waste from clothing, airbags, tire cords, engine compartment and intake system parts, fuel system parts, connectors, fishing nets, UD tape, etc. Examples of polyamide coated with polyurethane or acrylic resin include base fabrics used in clothing, etc., and the type of polyurethane or acrylic resin is not particularly limited. Resins other than polyamide that coat the polyamide 6 composition may be removed before step 1. The method for removing the coating resin is not particularly limited, but examples include peeling by friction, peeling by tension, and peeling by dissolution or swelling due to contact with an aqueous solution or an organic solvent.

[0022] The polyamide 6 composition may contain sewing thread. The material of the sewing thread is not particularly limited, but it is preferable that the sewing thread is made of the same polyamide as the base fabric, since it can be recovered as recycled polyamide.

[0023] The polyamide 6 composition may consist solely of the polyamide, or may contain the polyamide and other components, such as other resins, metals, or other impurities mixed, attached, or coated on the polyamide. From the viewpoint of the recovery rate of the polyamide, the mass proportion of the polyamide relative to 100 mass% of the polyamide 6 composition is preferably 30 to 100 mass%, more preferably 70 mass% or more, even more preferably 80 mass% or more, even more preferably 85 mass% or more, and particularly preferably 100 mass%. If impurities other than polyamide are contained, a step of separating the impurities from the polyamide may be included. The separation method is not particularly limited, but if the impurities are insoluble in the dissolved polyamide 6 composition, they can be separated by methods such as filtration, centrifugation, and sedimentation. If the impurities dissolve in the solvent together with the polyamide, possible methods include extraction separation in the dissolved state, membrane separation, and electrodialysis, or washing after precipitating the polyamide in the precipitation step described below.

[0024] <Metal chloride alcohol solution> The metal chloride alcohol solution contains a metal chloride and an alcohol, and may further contain hydroxides of the same metals as the metal chlorides, and other components. In particular, from the viewpoint of the solubility of polyamide, the total mass ratio of the metal chloride and the alcohol to 100% by mass of the metal chloride alcohol solution is preferably 80% by mass or more, and more preferably 90% by mass or more.

[0025] Examples of the first alcohol include methanol, ethanol, linear or branched propanol, linear or branched butanol, ethylene glycol, propylene diol, butane diol, and other diols, and combinations thereof. Among these, from the viewpoint of the solubility of polyamide, methanol, ethanol, or combinations thereof are preferred, and methanol is more preferred. The first alcohol may be one type or a combination of multiple types.

[0026] The mass ratio of the metal chloride to 100% by mass of the metal chloride alcohol solution is preferably 10% by mass or more, more preferably 12% by mass or more, particularly preferably 14% by mass or more, and preferably 22% by mass or less, more preferably 21% by mass or less, particularly preferably 20% by mass or less. If the mass ratio is less than 10% by mass, the amount of polyamide dissolved is small, requiring a large amount of solvent. On the other hand, if the mass ratio exceeds 22% by mass, a large amount of solvent is required for precipitation.

[0027] Examples of the metal chloride include zinc chloride, magnesium chloride, and calcium chloride, with zinc chloride and calcium chloride being preferred, and calcium chloride being most preferred. The metal chlorides can be added as anhydrous or hydrated forms. The mass proportion of water in the metal chloride alcohol solution is preferably 30 mass % or less, more preferably 15 mass % or less, and even more preferably 10 mass % or less, and it is particularly preferable that the solution contains no water.

[0028] Each step in the manufacturing method of this embodiment will be described.

[0029] <Step 1: Step of obtaining a heated polyamide solution> The polyamide 6 composition is subjected to a dissolution treatment using an alcohol solution of a metal chloride. In the step 1, the polyamide 6 composition and the metal chloride alcohol solution are mixed and heated to dissolve.

[0030] The temperature for heating and dissolving is not particularly limited, but is preferably 30° C. or higher, more preferably 60° C. or higher, and preferably 90° C. or lower, more preferably 80° C. or lower. Alternatively, the temperature may be 40 to 60° C. If the temperature is too low, dissolution will be slow, and if the temperature exceeds 90° C., the temperature will be higher than the boiling point, which is undesirable from the standpoint of corrosiveness and decomposition. During the above step 1, the temperature may be constant or may be varied within the above range.

[0031] The dissolution may be carried out by either a batch method or a continuous method. In the case of a batch method, stirring is not particularly limited, but stirring is preferred, as stirring improves the dissolution rate of the polyamide. In the case of a continuous system, 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.

[0032] The shape of the container used when heating and dissolving the polyamide 6 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 include SUS316, SUS316L, SUS329J4L, and SUS444. The surfaces of these materials may be lined or coated, for example, with glass, fluorine-based resin, rubber, and epoxy being preferred from the viewpoint of corrosion resistance. When lined or coated, the container and piping themselves can be made of any material without considering corrosion.

[0033] The heating and dissolving time is not particularly limited, but is preferably 5 minutes to 100 hours.

[0034] The mass ratio of the polyamide 6 composition to the metal chloride alcohol solution used in step 1 is not particularly limited, but is preferably 5 mass% or more, more preferably 7 mass% or more, and preferably 15 mass% or less, more preferably 13 mass%. If it is less than 5 mass%, too much solvent will be required, and if it exceeds 15 mass%, the viscosity will increase, prolonging the dissolution time and worsening operability. Furthermore, for the same reasons as above, the mass proportion of polyamide in the above-mentioned heated polyamide melt is preferably 5 mass% or more, more preferably 7 mass% or more, and is preferably 15 mass% or less, more preferably 13 mass% or less. In order to suppress the liquid content of the precipitate obtained by crystallization, the molar ratio of water to 1 mole of metal chloride in the polyamide heated solution is preferably 0.2 moles or more, more preferably 0.5 moles or more, and even more preferably 1 mole or more, and is preferably 2.5 moles or less, more preferably 2.0 moles or less, and even more preferably 1.5 moles or less. Although the mechanism is unclear, it is believed that the presence of water during dissolution affects the state of hydrogen bonding of polyamide in the metal chloride alcohol solution, thereby regulating the microscopic solubility and dispersibility that cannot be evaluated in the macroscopic dissolved state. It is believed that a ratio of less than 0.2 moles reduces the dispersibility of calcium chloride in alcohol and makes it difficult for it to interact with polyamide. On the other hand, a ratio of more than 2.5 moles makes it difficult for polyamide to be uniformly dispersed in the solution due to its low solubility in water.

[0035] The heated polyamide solution obtained in step 1 is preferably used in step 2 immediately after the step. If necessary, fine foreign matter may be removed from the heated polyamide solution obtained in step 1 by solid-liquid separation. Examples of solid-liquid separation include centrifugation, filtration, and sedimentation, and the solid-liquid separation may be carried out continuously or batchwise.

[0036] <Step 2: Step of obtaining alcohol diluted solution> Step 2 is a step of diluting the heated polyamide solution obtained in step 1 with alcohol. Examples of the second alcohol used in step 2 include methanol, ethanol, linear or branched propanol, linear or branched butanol, diols such as ethylene glycol, propylene diol, and butanediol, and combinations thereof. Among these, from the viewpoint of the solubility of polyamide, methanol, ethanol, or combinations thereof are preferred, and methanol is more preferred. The second alcohol may be one type or a combination of two or more types.

[0037] The method for diluting the heated polyamide solution will now be described in detail. The dilution ratio with alcohol is preferably 1.5 to 5 times. In this specification, the dilution ratio is defined as the value obtained by dividing the mass of the polyamide heat-dissolved solution at the time of dissolution by the mass of the diluted alcohol solution. In this case, even if precipitates are present in the diluted alcohol solution, the mass of the diluted alcohol solution is defined as the weight including the precipitates. If the dilution ratio is less than 1.5 times, the amount of polyamide precipitated is small, resulting in a low yield. If the dilution ratio is more than 5 times, precipitation is likely to occur during dilution.

[0038] In step 2, from the viewpoint of suppressing the liquid content of the precipitate obtained by crystallization, it is preferable to dilute the heated polyamide solution so that the temperature does not fall below 50° C. Although the mechanism behind this is not clear, it is thought that by maintaining a high temperature during the dilution stage, rapid precipitation can be avoided and the properties of the precipitate can be made uniform from the initial stage to the end of precipitation. The temperature at which the solution is diluted with alcohol is not particularly limited, but is preferably 30 to 90°C. If the solution is diluted at a temperature lower than 30°C, the particle size will become smaller due to precipitation during dilution. If the solution is diluted at a temperature higher than 90°C, uneven precipitation is likely to occur due to evaporation and condensation of the alcohol near the liquid surface. The temperature from the production of the polyamide heated solution to the preparation of the alcohol dilution is preferably within ±10°C of the heating and dissolving temperature in step 1, more preferably within ±5°C, from the viewpoint of obtaining a powdered polyamide having a larger particle size and a narrower particle size distribution in a short time.

[0039] The dilution rate of the alcohol is not particularly limited, but it is preferable to add the alcohol at a rate that does not cause a sudden change in concentration or temperature during dilution, which would result in precipitation of the polyamide.

[0040] The amount of polyamide precipitated during dilution (i.e., the mass of polyamide precipitated in step 2) is preferably 1% by mass or less, more preferably less than 1% by mass, of the total mass of polyamide contained in the heated polyamide solution obtained in step 1. The polyamide precipitated during dilution and the polyamide precipitated during cooling have different microscopic separation mechanisms when precipitated from the solvent, and as the amount of polyamide precipitated during dilution increases, the liquid content of the precipitate increases. Furthermore, since the particles are formed due to a sudden change in composition, their particle size cannot be controlled, and if there are many of them, it becomes difficult to control the particle size during the cooling and precipitation process described below.

[0041] The temperature of the alcohol used for dilution is not particularly limited, but is preferably 15 to 90° C. If the temperature is lower than 15° C., localized precipitation is likely to occur during dilution, and if the temperature is higher than 90° C., the temperature is higher or close to the boiling point of the alcohol, so treatment under pressure is required.

[0042] The water content of the alcohol used for dilution is not particularly limited, but is preferably 0.005% by mass or more, more preferably 0.005% by mass or more, and is preferably 50% by mass or less, and more preferably 1% by mass or less. If the water content is high, polyamides will precipitate rapidly because they have low solubility in aqueous solutions. Since the precipitation properties differ depending on the type of polyamide, the amount of water can be changed within the above range depending on the polyamide.

[0043] The dilution method is not particularly limited, and may be either a batch method or a continuous method. In the case of a batch method, stirring is not particularly limited, but stirring is preferred, as stirring makes the temperature and concentration uniform.

[0044] The shape of the container in which the heated polyamide solution is diluted with the alcohol is not particularly limited, and any shape, such as a tank or circulation type, may be used. The same container as in step 1 may be used. The materials for the containers and piping are also not particularly limited, and examples include SUS316, SUS316L, SUS329J4L, and SUS444. The surfaces of these materials may be lined or coated, for example, with glass, fluorine-based resin, rubber, and epoxy being preferred from the viewpoint of corrosion resistance. When lined or coated, the container and piping materials themselves can be trimmed without considering corrosion.

[0045] <Step 3: Precipitating powdered polyamide> Step 3 is a step of cooling the alcohol diluted solution obtained in step 2 to precipitate polyamide.

[0046] It is preferable to stir the alcohol diluted solution when cooling it. Stirring makes the temperature and concentration uniform, making it easier to control the particle size. Depending on the equipment and method used for stirring, it is preferable to stir under conditions that do not easily cause particle destruction or shearing due to stirring.

[0047] The cooling rate is not particularly limited, but is preferably 10°C / Hr or more, more preferably 20°C / Hr or more, and even more preferably 40°C / Hr or more, and is preferably 100°C / Hr or less, more preferably 70°C / Hr or less, and even more preferably 68°C / Hr or less. A cooling rate of less than 10°C / Hr takes a long time, and a cooling rate of more than 100°C / Hr results in small particle size due to rapid precipitation. The particle size can be controlled by changing the cooling rate.

[0048] The temperature after cooling is not particularly limited, but is preferably at least 10° C. lower than the temperature at the time of dilution. If the temperature difference is less than 10° C., there is little precipitation and particles are less likely to grow.

[0049] The precipitated solid is preferably recovered by solid-liquid separation. The method of solid-liquid separation is not particularly limited, but examples include filtration, centrifugation, sedimentation, etc. Either method may be a batch method or a continuous method.

[0050] The solid obtained by solid-liquid separation is preferably washed with a solvent. The washing solvent is not particularly limited, but it is preferable to use a solution having the same composition as the liquid portion at the time of precipitation, a good solvent, or a solvent capable of dissolving metal chlorides. Alcohols such as methanol and ethanol, and water are preferred, and combinations of these may also be used.

[0051] The washing method is not particularly limited, and examples thereof include a batch washing method, a continuous washing method in which a washing solvent is passed through a solid-liquid separator such as a filter or a centrifugal separator, and a combination thereof.

[0052] After washing, the polyamide can be dried and solidified by distilling off the washing solvent by heating and / or reducing the pressure, thereby obtaining a powdery polyamide.

[0053] The production method of this embodiment preferably includes step 4 of washing the powdery polyamide obtained in step 3 with a solvent at least once. The washing may be carried out once or multiple times. Of the above washings, the solvent used in the first washing is preferably the same alcohol as that used in step 1, from the viewpoints of washing efficiency and reuse of raw materials. That is, it is preferably the same alcohol as that contained in the metal chloride alcohol solution in step 1. The solution contained in the polyamide after crystallization contains the metal chloride used during dissolution. By using the same alcohol as that used in step 1, the filtrate from the filtration after crystallization and the washing liquid can be combined and concentrated to reuse the solution as a dissolution liquid, thereby reducing the amount of metal chloride waste. Furthermore, using a different solvent significantly changes the affinity between the polyamide and the solvent, reducing washing efficiency.

[0054] The manufacturing method of this embodiment may further include a step of concentrating the metal chloride alcohol solution from the viewpoint of reusing the metal chloride alcohol solution. For example, the metal alcohol solution containing metal chlorides and alcohol obtained in the polyamide recovery step may be concentrated and reused. The concentration method may be, for example, concentration by heating.

[0055] As described above, according to the production method of this embodiment, when polyamide useful as an engineering plastic is dissolved and precipitated to produce powdered polyamide, the metal chloride and water contents of the metal chloride alcohol solution used to dissolve the polyamide are set within specific ranges, and the temperature during dilution or the amount of precipitation during dilution is set within specific ranges, thereby reducing the liquid content of the precipitated polyamide and providing a method for producing powdered polyamide that reduces the amount of washing solvent and the energy required for drying. [Example]

[0056] The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

[0057] [Example 1] A 300 mL three-neck flask containing a stirrer was charged with 10 g of polyamide 6 pellets and 80.1 g of a 20 wt% calcium chloride methanol solution (16.0 g calcium chloride, 0.8 g water (molar water / molar calcium chloride = 0.3)) as a metal chloride alcohol solution. The flask was then placed in an 80°C oil bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 6, yielding a polyamide solution. The polyamide solution was diluted with 250.2 g of methanol while stirring. The internal temperature was maintained above 60°C during the addition. After the addition of methanol, the flask was removed from the oil bath and allowed to cool. After 1 hour, stirring was stopped when the temperature reached 30°C, and the solid precipitate was recovered by pressure filtration using a 10 μm membrane filter. The wet filter cake weighed 32.9 g. 50 g of methanol was added to the filter, and pressure filtration was repeated. The amount of calcium chloride in the filtrate was analyzed by ion chromatography, and the remaining amount of calcium chloride in the polyamide was calculated. The process from adding methanol to calculating the remaining amount of calcium chloride was repeated, and washing was stopped when the remaining amount was 1000 ppm or less. Washing was performed five times, with a total amount of washing solution of 250 g. The mass (g) of washing solution relative to the mass (g) of raw material polyamide 6 pellets was 25. The solid precipitate after washing was heated and dried in a vacuum dryer at 80°C, yielding 9.9 g of powdered polyamide 6 (yield 99.0%). The energy required for drying, calculated from the amount of solvent contained, was 2.5 kJ per 1 g of polyamide 6.

[0058] [Comparative Example 1] A 300 mL three-neck flask containing a stirrer was charged with 10 g of polyamide 6 pellets and 100 g of a 25 wt% calcium chloride methanol solution (25.0 g calcium chloride, 8.1 g water (molar water / molar calcium chloride = 2)) as a metal chloride alcohol solution. The flask was then placed in an 80°C oil bath and stirred with a magnetic stirrer for 12 hours to dissolve the polyamide 6, yielding a polyamide solution. The polyamide solution was diluted with 200.2 g of methanol while stirring. The internal temperature was maintained above 60°C during the addition. After the addition of the aqueous methanol solution, the flask was removed from the oil bath and allowed to cool. After 1 hour, stirring was stopped when the temperature reached 30°C, and the solid precipitate was recovered by pressure filtration using a 10 μm membrane filter. The wet filter cake weighed 55.0 g. 50 g of methanol was added to the filter, and pressure filtration was repeated. The amount of calcium chloride in the filtrate was analyzed by ion chromatography, and the remaining amount of calcium chloride in the polyamide was calculated. The process from adding methanol to calculating the remaining amount of calcium chloride was repeated, and washing was stopped when the remaining amount was 1000 ppm or less. Washing was performed seven times, with a total amount of washing solution of 350 g. The solid precipitate after washing was heated and dried in a vacuum dryer at 80 °C, yielding 9.8 g of powdered polyamide 6 (yield 98.0%). The energy required for drying, calculated from the amount of solvent contained, was 5.0 kJ per 1 g of polyamide 6. [Industrial Applicability]

[0059] The production method of this embodiment can provide a method for producing a powdery polyamide that reduces the liquid content of the precipitated powdery polyamide, and reduces the amount of washing solvent and the energy required for drying.

Claims

1. Step 1: a step of heating and dissolving a polyamide 6 composition in a metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: Diluting the heated polyamide solution with alcohol to obtain an alcohol-diluted solution; and Step 3: Cooling the alcohol dilution to precipitate the powdered polyamide Including, In the step 1, the mass ratio of the metal chloride relative to 100 mass% of the metal chloride alcohol solution is 10 mass% or more and 22 mass% or less, In the step 1, the heated polyamide solution contains 0.2 mol to 2.5 mol of water per 1 mol of metal chloride, In step 2, the polyamide heated solution is diluted without lowering the temperature below 50°C.

1. A method for producing powdered polyamide comprising the steps of:

2. Step 1: a step of heating and dissolving a polyamide 6 composition in a metal chloride alcohol solution containing a metal chloride and an alcohol to obtain a heated polyamide solution; Step 2: Diluting the heated polyamide solution with alcohol to obtain an alcohol-diluted solution; and Step 3: Cooling the alcohol dilution to precipitate the powdered polyamide Including, In the step 1, the mass ratio of the metal chloride relative to 100 mass% of the metal chloride alcohol solution is 10 mass% or more and 22 mass% or less, In the step 1, the heated polyamide solution contains 0.2 mol to 2.5 mol of water per 1 mol of metal chloride, The mass of the polyamide precipitated in the step 2 is 1 mass % or less based on the total mass of the polyamide contained in the heated polyamide melt.

1. A method for producing powdered polyamide comprising the steps of:

3. 3. The method for producing a powdery polyamide according to claim 1 or 2, further comprising step 4: washing the powdery polyamide obtained in step 3 with a solvent one or more times.

4. 4. The method for producing a powdery polyamide according to claim 3, wherein the solvent used for the first washing in step 4 is the same alcohol as that used in step 1.

5. 3. The method for producing a powdery polyamide according to claim 1, wherein the heating and melting temperature in step 1 is 60°C or higher and 80°C or lower.