Polyamide recycling method

Abstract

A polyamide recycling method, comprising: step (A), providing a plastic-containing material containing polyamide; step (B), using a reaction solvent to dissolve the polyamide contained in the plastic-containing material, to obtain a first mixture, wherein the reaction solvent contains formic acid and an ester solvent; and step (C), separating the reaction solvent from the first mixture to obtain a polyamide-containing recycled material, wherein based on the total weight of the polyamide-containing recycled material, the content of the polyamide is 90 wt% or above. In addition to exhibiting good selectivity and a high recycle rate, the polyamide recycling method reduces costs and avoids associated environmental issues because the reaction solvent can be recycled.

Description

Process for recycling polyamides TECHNICAL FIELD

[0001] The present invention relates to a plastic recycling method, in particular, a method for recycling polyamides. BACKGROUND

[0002] The invention of plastics began in the 19th century, and because of its low cost and versatility, it has become a common raw material for various products. Among them, polyamide (polyamide, PA, also commonly known as "Nylon") has been widely used in clothing raw materials, daily necessities, fishing nets or industrial products, etc. fields, not only because it is widely used in the manufacture of stockings, but also because of its light weight, soft and delicate touch, high elasticity, excellent wear resistance and other advantages. In recent years, functional textiles (functional textiles or performance textiles) have taken advantage of the commercial market applications of medical care, sports, fitness, etc. Nylon has also occupied an important market share with the popularity of functional textiles.

[0003] Based on the fact that plastics are not easily decomposed in nature, plastic products are everywhere in daily life, and gradually form a serious environmental problem in the process of replacement over time. Even though governments and enterprises are committed to promoting waste recycling, plastic recycling is more complex and not cost-effective than metal, paper and glass recycling, so the annual plastic recycling amount is less than 10% of the annual plastic production amount. The main reason is that plastic products usually contain multiple chemical components that are difficult to separate, and some chemical components may not be handled by recycling facilities, so plastic products are often eventually disposed of by incineration or burial.

[0004] The current plastic recycling method mainly includes: (1) mechanical recycling: recycling plastics through mechanical processes (grinding, washing, separation, drying, regranulation and compounding), and the recycled plastics obtained can replace virgin plastics as production raw materials; (2) chemical recycling: usually using chemical reagents to chemically depolymerize plastic waste into monomers or various valuable chemical substances, and then reusing the aforementioned monomers and other substances in the process of producing plastics. However, in the mechanical recycling method, the pulverized waste plastics not only become smaller in size, but also often undergo degradation reactions, resulting in a decrease in the performance of recycled plastics, limiting their application fields and the number of recycling times. Although chemical recycling can theoretically recycle plastics indefinitely, the use of chemical reagents often has environmental concerns due to their toxicity or danger.

[0005] In addition, fishing nets (main material: nylon) accounting for about 10% of marine litter are usually recycled through steps such as crushing, extruding, and pelletizing, but waste fishing nets are not easy to classify and clean at the beginning, and the reclaimed nylon obtained through mechanical recycling has poor toughness and weavability, and can only be used to make articles with low quality requirements, so that the commercial value of nylon cannot be fully realized. In addition, the dissolution reaction of nylon can only occur in a strongly polar solution, so the industry usually selects a solution containing sulfuric acid for the dissolution reaction of nylon, but sulfuric acid not only has a very high boiling point (337°C), but also has strong corrosiveness and toxicity, so it is not suitable for large-scale recycling operations. Therefore, there is still an urgent need to develop a new nylon recycling method.

[0006] SUMMARY

[0007] Due to the defects of the prior art described above, the purpose of the present application is to provide a polyamide recycling method which not only has the advantages of simplicity and safety, but also can reduce the production cost of the reclaimed polyamide prepared therefrom.

[0008] Another purpose of the present application is to provide a polyamide recycling method which has a high recovery rate and the performance of the reclaimed polyamide prepared therefrom is not significantly degraded, so it can be applied to multiple fields and thus achieve the goal of sustainable development of circular economy.

[0009] To achieve the above-mentioned purposes, the present application provides a polyamide recycling method, which comprises:

[0010] Step (A): providing a plastic-containing material, wherein the plastic-containing material comprises polyamide;

[0011] Step (B): dissolving the polyamide contained in the plastic-containing material with a reaction solvent to obtain a first mixture; wherein the reaction solvent comprises formic acid and an ester solvent; and

[0012] Step (C): separating the reaction solvent from the first mixture to obtain polyamide-containing reclaimed materials; wherein the content of polyamide is 90 weight percent (wt%) or more based on the total weight of the polyamide-containing reclaimed materials.

[0013] The present application uses a specific solvent combination (i.e. formic acid and ester solvent) to dissolve polyamide as the target of recycling, which can achieve high selective recycling effect, and thus the present application can be applied to plastic products containing composite materials, and the recovery rate of polyamide is high. Moreover, since the target of recycling does not need to be crushed, and the mixing of different chemical components can be avoided, the performance of the obtained recycled material containing polyamide will not be significantly degraded. In addition, since the specific solvent combination used in the present application does not cause the polyamide to be cracked into monomers and needs to be re-injected into the polyamide production process, the present application is simpler and more cost-effective than traditional chemical recycling methods, and thus has more commercial value. In addition, the boiling point of formic acid is much lower than that of sulfuric acid, and the use of formic acid is reduced by the combination of formic acid and ester solvent, which not only improves the safety of the operation during the dissolution process, but also greatly reduces the cost of purchasing solvents due to the lower cost of ester solvents compared to formic acid.

[0014] According to the present application, the plastic-containing material can be polyamide, or the plastic-containing material is a mixture containing polyamide, but is not limited thereto. In the present specification, "polyamide" refers to a polymer synthesized by condensation polymerization of dicarboxylic acid and diamine, or ring-opening polymerization of lactam, and its structural feature includes amide group (-C(=O)NH-). Specifically, the polyamide can be polyamide 6 (PA6, made from ε-caprolactam), polyamide 66 (PA66, made from hexamethylenediamine and adipic acid), polyamide 12 (PA12, made from 12-aminododecanoic acid), polyamide 11 (PA11, made from 11-aminoundecanoic acid), but is not limited thereto.

[0015] In some embodiments, when the plastic-containing material contains other components in addition to polyamide, the first mixture contains not only a mixed solution, but also a portion that is insoluble in the reaction solvent, such as a precipitate or a suspension, but is not limited thereto. The aforementioned mixed solution contains the polyamide and the reaction solvent. Preferably, the content of polyamide is 50 wt% or more based on the total weight of the plastic-containing material, but is not limited thereto.

[0016] Specifically, the other components of the plastic-containing material can include polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyurethane (PU), or a combination thereof, but are not limited thereto.

[0017] In some embodiments, the plastic-containing material is in the form of particles, but is not limited thereto. Preferably, the average particle size of the plastic-containing material can be greater than 0 millimeter (mm) and less than or equal to 10 mm, but is not limited thereto. More preferably, the average particle size of the plastic-containing material can be greater than 1 mm and less than or equal to 5 mm, for example, 0.5 mm, 1 mm, 1.5 mm, 2 mm, or 3 mm.

[0018] In some embodiments, before the step (B), the present application can further comprise a preliminary classification step of configuring a classification solution with different specific gravities to float out plastic-containing materials with different specific gravities, so as to screen out some non-polyamide components, thereby improving the recovery efficiency of polyamide. The preliminary classification step can be between the step (A) and the step (B), but is not limited thereto. Generally, the specific gravity of PE and PP is less than or equal to 1; the specific gravity of PET can be greater than 1.35 or greater than 1.38; the specific gravity of PVC can be greater than 1.3; the specific gravity of nylon 6 or nylon 66 is about 1.13 to 1.15, but is not limited thereto.

[0019] According to the present application, the formic acid can be an aqueous solution with a weight percentage concentration of 88% or more, but is not limited thereto; preferably, the formic acid can be an aqueous solution with a weight percentage concentration of 93% or more, but is not limited thereto; more preferably, the formic acid can be an aqueous solution with a weight percentage concentration of 99% or more. The weight percentage concentration of the formic acid refers to the concentration before mixing with the ester solvent.

[0020] Preferably, the ester-based solvent includes an ester-based compound having a carbon number of 2 to 12, but is not limited thereto. For example, the ester-based solvent can include methyl formate, ethyl formate, methyl acetate, ethyl acetate (EA), n-propyl acetate, n-butyl acetate, iso-butyl acetate, isoamyl acetate (IAA), ethyl 3-ethoxypropionate (EEP), ethyl ethoxyacetate, 1-methoxy-2-propanol acetate (PGMEA), ethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, diethylene glycol monoethyl ether acetate, ethyl-3-ethoxypropionate, 3-methoxybutyl acetate, ethyl lactate, butyl lactate, propyl lactate, or a combination thereof, but is not limited thereto. More preferably, the ester-based solvent includes ethyl acetate, n-butyl acetate, isoamyl acetate, or a combination thereof.

[0021] Preferably, the content of formic acid is greater than or equal to 35 vol% and less than 100 vol% based on the total volume of the reaction solvent, but is not limited thereto. More preferably, the content of formic acid is greater than 35 vol% and less than or equal to 80 vol% based on the total volume of the reaction solvent.

[0022] Preferably, the content of the ester-based solvent is greater than 0 vol% and less than or equal to 65 vol% based on the total volume of the reaction solvent, but is not limited thereto. More preferably, the content of the ester-based solvent is greater than or equal to 20 vol% and less than 65 vol% based on the total volume of the reaction solvent.

[0023] Preferably, the operation temperature of the step (B) is 18 °C to 100 °C, but not limited thereto. For example, the operation temperature of the step (B) can be 20 °C, 23 °C, 25 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, or 95 °C.

[0024] In some embodiments, since the "stirring" can avoid the plastic-containing materials from sticking to each other or adhering to the surface of the reaction vessel containing the first mixture, i.e., can maintain the plastic-containing materials in a continuous suspension in the reaction solvent and help the plastic-containing materials to dissolve, thereby improving the dissolution efficiency and recovery rate, the step (B) can comprise a first stirring process, the revolution(s) per minute (RPM) of which can be 100 revolutions per minute to 200 revolutions per minute, but not limited thereto. Wherein, the aforementioned first stirring process can comprise a mechanical stirring in the reaction vessel, and the aforementioned mechanical stirring can be achieved by a rotating paddle and / or a static fin, but not limited thereto. Preferably, the revolution(s) per minute of the first stirring process can be 150 rpm to 200 rpm, for example, 150 rpm, 160 rpm, 170 rpm, 180 rpm, 190 rpm, or 200 rpm. In addition, the duration of the step (B) can be 20 minutes (minute, min) to 120 min, but not limited thereto; the "duration" refers to the time for the reaction solvent to dissolve the polyamide contained in the plastic-containing materials. In some embodiments, the duration can be 30 min, 45 min, 55 min, 60 min, 70 min, 90 min, or 110 min.

[0025] According to the present application, the volume and weight of the plastic-containing materials are not particularly limited, as long as the plastic-containing materials as a whole can be in contact with the reaction solvent; preferably, the reaction solvent can flood the whole of the plastic-containing materials. In an embodiment, based on 100 milliliters (mL) of the reaction solvent, the weight of the plastic-containing materials is greater than 0 grams (g) and less than or equal to 40 g, for example, 0.05 g, 0.1 g, 0.5 g, 1 g, 2 g, 3 g, 4 g, 5 g, 6 g, 7 g, 8 g, 10 g, 12 g, 14 g, 15 g, 20 g, 25 g, 30 g, 33 g, 35 g, or 38 g. Preferably, when more than 80 wt% of the plastic-containing materials is polyamide, based on 100 mL of the solvent, the weight of the plastic-containing materials is greater than 0 g and less than or equal to 11 g, but not limited thereto. According to the present application, the ratio of the volume of the solvent to the weight of the plastic-containing materials can avoid the problem that the first mixture is difficult to stir due to too high consistency during the dissolution process.

[0026] In some embodiments, the step (C) further comprises a standing cooling process before the separating step. That is, in the step (C), the first mixture is first allowed to stand to obtain a standing first mixture; then, the standing first mixture is separated from the first mixture to obtain the polyamide-containing recycled material; wherein the temperature of the standing first mixture is lower than the temperature of the first mixture. Preferably, the standing time is 2 hours to 3 hours, but not limited thereto.

[0027] By evaporating the reaction solvent in a negative pressure environment, in addition to effectively separating the reaction solvent and the polyamide as the recycling target, the reaction solvent or the plastic-containing material can also be prevented from deteriorating due to excessive heating. Furthermore, in the step (C), the separated reaction solvent can be further recycled (i.e., recycled reaction solvent), not only avoiding the subsequent processing cost of discharging chemical solvents, but also reducing the raw material cost in the process, thereby further reducing the overall manufacturing cost. In addition, the potential environmental problems caused by the discharge of chemical solvents can also be avoided, thereby further reducing the burden on the environment. In addition, the polyamide recycling method of the present application can not use a precipitant to separate the polyamide-containing recycled material.

[0028] Preferably, in the step (C), the reaction solvent can be separated from the first mixture by using a reduced pressure concentration method; wherein the pressure of the reduced pressure concentration method is greater than or equal to 0 mbar (equivalent to 100 Pa) and less than or equal to 50 mbar, but not limited thereto. For example, the pressure can be 1 mbar, 5 mbar, 10 mbar, 15 mbar, 20 mbar, 25 mbar, 30 mbar or 40 mbar. More preferably, the pressure can be greater than or equal to 0 mbar and less than or equal to 20 mbar. In some embodiments, the operation time of the step (C) can be 5 minutes to 1 hour, but not limited thereto. Preferably, when the pressure of the reduced pressure concentration method is greater than or equal to 0 mbar and less than or equal to 20 mbar, the operation time can be 5 minutes to 20 minutes.

[0029] Preferably, in the step (C), the separated reaction solvent can be further collected to obtain a recycled reaction solvent, which can be used as the reaction solvent of step (B); wherein the collection method can be achieved by a conduit and a condensing device, but not limited thereto.

[0030] According to the present application, the recovered reaction solvent comprises formic acid and the ester solvent. In some embodiments, the recovered reaction solvent can be directly reused in the present application, thus simplifying the process, saving production cost, and helping to reduce the generation of waste liquid. In other embodiments, the recovered reaction solvent can be further subjected to a solvent separation step: placing the recovered reaction solvent in a reduced pressure environment, first boiling the recovered reaction solvent at a first solvent recovery temperature, and obtaining a first recovery liquid and a first recovery residual liquid by condensation; wherein the first recovery liquid comprises the ester solvent or formic acid with a lower boiling point, and the first recovery residual liquid comprises the ester solvent or formic acid with a higher boiling point. In addition, the first recovery residual liquid can be further boiled at a second solvent recovery temperature, and a second recovery liquid can be obtained by condensation; wherein the first solvent recovery temperature is lower than the second solvent recovery temperature. Preferably, the first recovery liquid and / or the second recovery liquid can be subjected to multiple aforementioned solvent separation steps to improve the purity of the first recovery liquid and / or the second recovery liquid.

[0031] In some embodiments, especially but not limited to when the plastic-containing material comprises other components in addition to polyamide, the step (C) can comprise the step (C1) and step (C2). Step (C1): subjecting the first mixture to a filtration process to obtain a filtrate and a precipitate; and step (C2): separating the reaction solvent from the filtrate to obtain the polyamide-containing recovered material. Wherein the precipitate can comprise the aforementioned other components in the plastic-containing material.

[0032] Further, in step (C1), the filtration process can use a filter to filter the above-mentioned first mixture that generates a precipitate; the filter only needs to be able to separate the precipitate and is not particularly limited, for example, a filter with an average pore size of 100 microns or less can be used; the filtration process can also use vacuum filtration (air suction filtration) to shorten the filtration time. At the same time, step (C1) can also remove impurities in the plastic-containing material to improve the purity of the polyamide in the polyamide-containing recovered material.

[0033] In some embodiments, the step (C1) can further comprise a settling process before the filtration process; that is, in the step (C1), the first mixture is first settled to obtain a settled first mixture; then, the settled first mixture is subjected to the filtration process to obtain the filtrate and the precipitate; wherein the temperature of the settled first mixture is lower than that of the first mixture. Preferably, the settling time is 2 to 3 hours, but not limited thereto. By settling the first mixture at room temperature, the filtration process is performed after the settled first mixture has obvious precipitation, which can shorten the number of times of filtration and reduce the amount of impurities contained in the filtrate. In short, the present application can remove undissolved components, suspensions and precipitates by settling precipitation, that is, settling the first mixture without heating, in combination with a filter.

[0034] Preferably, in the step (C2), the reaction solvent contained in the filtrate can also be separated by the aforementioned reduced pressure concentration method, but not limited thereto.

[0035] Preferably, in the step (C), especially but not limited to the step (C2), the temperature can be 40 to 100°C, for example: 45°C, 50°C, 60°C, 70°C, 80°C, 90°C or 100°C. In this way, the reaction solvent can be continuously maintained in a boiling state under reduced pressure, so as to accelerate and stabilize the recovery of the separated reaction solvent.

[0036] In some embodiments, the step (C) can comprise a second stirring process; for example, the speed per minute of the second stirring process can be greater than 0 rpm and less than or equal to 300 rpm, but not limited thereto. Wherein, the aforementioned second stirring process can be performed in the same way as the first stirring process, but not limited thereto. Preferably, the speed per minute of the second stirring process can be 10 to 100 rpm, for example: 15 rpm, 17 rpm, 19 rpm, 22 rpm, 25 rpm, 30 rpm, 50 rpm, 70 rpm, 80 rpm or 90 rpm.

[0037] Preferably, the recovered reaction solvent obtained in the step (C2) can also be subjected to the aforementioned solvent separation step to further obtain the first recovery liquid and / or the second recovery liquid, but not limited thereto. Preferably, the step (C2) can comprise the aforementioned second stirring process.

[0038] In some embodiments, in the step (C), especially but not limited to the step (C2), the polyamide-containing recovery material can be obtained after a drying process; wherein the drying temperature can be 70 to 90°C, for example: 70°C, 75°C, 80°C, 85°C or 90°C, but not limited thereto.

[0039] In some embodiments, when the plastic-containing material contains other components in addition to the polyamide, the polyamide-containing recycled material obtained by the present application through the dissolution, extraction and separation of the plastic-containing material containing the polyamide using the reaction solvent can be enriched in the polyamide component compared to the plastic-containing material as the starting material; that is, the weight proportion of the polyamide in the polyamide-containing recycled material can be greater than the weight proportion of the polyamide in the plastic-containing material.

[0040] Preferably, the content of the polyamide is 95 wt% or more, based on the total weight of the polyamide-containing recycled material. More preferably, the content of the polyamide is 99 wt% or more, based on the total weight of the polyamide-containing recycled material.

[0041] In some embodiments, the polyamide-containing recycled material can be in the form of particles (e.g., with an average particle size of 3 mm to 5 mm) or irregular sheets, but is not limited thereto.

[0042] Preferably, in the polyamide recycling method, the polyamide recovery rate can be 90% or more; more preferably, the polyamide recovery rate can be 95% or more; and even more preferably, the polyamide recovery rate can be 97% or more. The aforementioned recovery rate is defined as the quotient obtained by dividing the weight of the polyamide in the polyamide-containing recycled material by the weight of the polyamide in the plastic-containing material.

[0043] In the present specification, a range represented by "a small value to a large value" means a range greater than or equal to the small value and less than or equal to the large value, unless otherwise specified. For example, the operating temperature of step (B) is 18°C to 100°C, which means that the operating temperature is "greater than or equal to 18°C and less than or equal to 100°C". BRIEF DESCRIPTION OF DRAWINGS

[0044] FIG. 1 is a flowchart of the polyamide recycling method of the present application.

[0045] FIG. 2 is an infrared spectrum of the polyamide-containing recycled material obtained in Examples 18 and 19. DETAILED DESCRIPTION

[0046] The following examples illustrate the embodiments of the polyamide recycling method of the present application, and those skilled in the art can easily understand the advantages and effects achieved by the present application through the content of the following examples. It should be understood that the examples listed in the present specification are only used to illustrate the embodiments of the present application, and are not intended to limit the scope of the present application. Those skilled in the art can make various modifications and changes based on their general knowledge without departing from the spirit of the present application, in order to implement or apply the content of the present application.

[0047] Examples 1 to 17

[0048] As shown in FIG. 1, the recycling method of the polyamide of the present application first proceeds step S1: Step (A): providing a plastic-containing material, which comprises a polyamide. Specifically, the plastic-containing material used in Examples 1 to 17 is a commercially available nylon raw pellet (source: Chuanju Enterprise Co., Ltd., model: NYLON6 master batch), which has a pellet form, and each pellet has a volume of about 0.5 cubic centimeters; wherein the weight of the plastic-containing material of each group of examples is 8g.

[0049] Next, step S2: Step (B): dissolving the polyamide contained in the plastic-containing material with a reaction solvent to obtain a first mixture; wherein the reaction solvent comprises formic acid and an ester solvent. Specifically, the type of ester solvent contained in the reaction solvent of each group of examples is shown in Table 1, and the reaction solvent is mixed according to the volume ratio required in Table 1, and the volume of the reaction solvent is quantitatively measured as 100mL. Then, pour the reaction solvent into a 1-liter vacuum concentration bottle, and then pour the above-mentioned plastic-containing material into the vacuum concentration bottle to obtain an initial mixture. Subsequently, connect the vacuum concentration device (maintain normal atmospheric pressure) to the vacuum concentration bottle, and use a water bath to heat the initial mixture according to the dissolution temperature shown in Table 1, and at the same time, perform a first stirring process on the initial mixture, and after the plastic-containing material is completely dissolved, a first mixture is obtained, and the dissolution time of each group (i.e. the duration required to obtain the first mixture) is recorded. Wherein the first stirring process is at a speed of 170rpm per minute, and the duration of step (B) of each group of examples is shown in Table 1.

[0050] It should be noted that the concentration of the first mixture set by each group of examples is about 8g / 100mL (i.e. 80g / L), the main reason is that under this condition, the flowability of the first mixture is good, the overall operation can have better convenience, and it is sufficient to represent the properties and performance under this experimental condition, and can avoid the increase of operation error caused by too long dissolving plastic-containing material time.

[0051] Finally, step S3: Step (C) is performed to separate the reaction solvent from the first mixture to obtain a polyamide-containing recovered material; wherein the content of polyamide is 90 wt% or more based on the total weight of the polyamide-containing recovered material. Specifically, first, the first mixture of each group of examples is left to stand at room temperature (25°C) for a period of time to obtain a first mixture that has been left to stand; then, the reduced pressure concentration device system is depressurized using an air suction pump to perform the separation step, i.e., to separate the reaction solvent from the first mixture; wherein the air suction pump is continuously started for about 1 to 2 minutes to reduce the pressure in the reduced pressure concentration device system to 0 to 20 mbar. Then, the aforementioned reduced pressure concentration flask is placed in a water bath for heating while a second stirring process is performed to continuously form a thin film of the crude product on the surface of the aforementioned reduced pressure concentration flask and to accelerate the evaporation of the reaction solvent; wherein the rotation speed of the second stirring process is set to 20 rpm. After the boiling of the first mixture is observed, the temperature is maintained for a period of time, and the temperature range at which boiling occurs is recorded in Table 1 (i.e., the temperature of step (C)). Subsequently, the air suction pump is turned off. On the one hand, the vapor of the reaction solvent is condensed by a condenser tube to recover the reaction solvent to obtain recovered reaction solvent, while the pressure in the system is maintained from rising. After no reaction solvent is observed in the reduced pressure concentration flask and the vapor of the reaction solvent is completely evaporated and condensed, the volume of the recovered reaction solvent collected is measured to calculate the recovery rate of the reaction solvent and is recorded in Table 2. On the other hand, if the thin film of the crude product still contains too much reaction solution, it will stick to the surface of the glass container and be difficult to completely remove, thereby affecting the recovery rate of the polyamide and the reaction solvent, so the thin film of the crude product in the reduced pressure concentration flask needs to be dried before being removed. Then, the aforementioned thin film of the crude product is placed in an oven with a temperature setting of 80°C for about 1 hour to remove the reaction solvent that may remain in the aforementioned thin film of the crude product; then, the dried thin film is cooled at room temperature until its weight no longer changes, thereby obtaining a polyamide-containing recovered material, and the weight of the polyamide-containing recovered material is precisely weighed by an electronic balance to calculate the recovery rate of the polyamide and is recorded in Table 2.

[0052] Reference Example 1

[0053] The recovery method of the polyamide of Reference Example 1 is substantially the same as the recovery method of the polyamide of Example 1, and the main difference is that: (1) the reaction solvent used in this reference example is only formic acid and does not contain ester solvents; in addition, the detailed parameters of Reference Example 1, and the recovery rate of the polyamide of the polyamide-containing recovered material and the recovery rate of the recovered reaction solvent are sequentially recorded in Tables 1 and 2.

[0054] Example 18

[0055] The recovery method of the polyamide of Example 18 is substantially the same as that of Example 4, with the main differences being that (1) the plastic-containing material used in this example is a mixture of six components, namely polyamide, PET, PE, PP, PVC, and PU; and (2) the first mixture needs to be filtered using a stainless steel filter screen.

[0056] Specifically, the plastic-containing material used in this example is a mixture of six commercially available plastic particles, each having a volume of about 0.5 cubic centimeters. The total weight of the plastic-containing material is 10.088 grams, which is obtained by weighing the plastic-containing material on a balance. The plastic-containing material is placed in a clean beaker. The six commercially available plastic particles are polyamide (5.008 grams), PET, PP, PE, PVC, and PU, and the weight ratio of the six components is about 5:1:1:1:1:1.

[0057] Next, 100 mL of a reaction solvent having a weight ratio of formic acid to ethyl acetate of 4:1 is prepared. First, 60 mL of the reaction solvent is placed in the beaker to obtain an initial mixture. Then, the beaker is sealed and the initial mixture is heated to 70°C using a hot plate while being stirred at a speed of 170 rpm per minute. After the polyamide in the plastic-containing material is completely dissolved (about 60 minutes), a first mixture is obtained, and the dissolution time is recorded in Table 1. The remaining 40 mL of the reaction solvent is preheated and stored for subsequent use.

[0058] Next, the first mixture is filtered using a stainless steel filter screen having an average pore size of 100 microns. The precipitate remaining on the stainless steel filter screen is then rinsed with the aforementioned 40 mL of the reaction solvent. Finally, the filtrate having a total volume of 100 mL is placed in a 1-liter vacuum concentration bottle, and the vacuum concentration device is connected to the vacuum concentration bottle (maintaining normal atmospheric pressure) to perform vacuum concentration. In addition, the precipitate remaining on the stainless steel filter screen is air-dried and then placed in an oven set to a temperature of 80°C for about 2 hours. After cooling, the weight of the precipitate is measured on an electronic balance and found to be 5.17 grams.

[0059] The reaction solvent was then poured into a rotary evaporator flask, and the plastic-containing material was then poured into the rotary evaporator flask. Subsequently, the rotary evaporator flask was connected to a rotary evaporator system, and the rotary evaporator system was depressurized by a rotary evaporator pump to perform a separation step, i.e., to separate the reaction solvent from the first mixture; wherein the rotary evaporator pump was continuously started for about 1 minute to 2 minutes, so that the pressure in the rotary evaporator system was reduced to 0 mbar to 20 mbar. Then, the aforementioned rotary evaporator flask was placed in a water bath for heating while a second stirring process was performed to continuously form a thin film of the crude product on the surface of the aforementioned rotary evaporator flask, and to accelerate the evaporation of the reaction solvent; wherein the rotation speed of the second stirring process was set to 20 rpm. After the first mixture was observed to boil at about 70°C for a period of time, the temperature was maintained, and the rotary evaporator pump was turned off. On the one hand, the vapor of the reaction solvent was condensed by a condenser tube to recover the reaction solvent, and the recovered reaction solvent was obtained, while the pressure in the system was not increased. After the reaction solvent in the rotary evaporator flask was not observed, and the vapor of the reaction solvent was completely evaporated and condensed, the volume of the recovered reaction solvent collected was measured to calculate the recovery rate of the reaction solvent and record it in Table 2. On the other hand, the thin film of the crude product in the rotary evaporator flask was removed when it was in a dry state, and the aforementioned thin film of the crude product was placed in an oven with a temperature setting of 80°C for about 2 hours to remove the reaction solvent that might remain in the aforementioned thin film of the crude product; then, the dried thin film was cooled at room temperature, and when its weight no longer changed, the polyamide-containing recovered material was obtained, and its weight (4.999 g) was precisely weighed by an electronic balance to calculate the recovery rate of the polyamide and record it in Table 2. Among them, the appearance of the polyamide-containing recovered material obtained in this example was indeed a nearly white film; in addition, the appearance of the precipitate (in the form of particles) obtained by the aforementioned filtration process in this example could clearly show that it was almost the same as the appearance of PET, PP, PE, PVC, and PU contained in the original plastic-containing material.

[0060] Example 19

[0061] The recovery method of the polyamide of Example 19 was substantially the same as that of Example 18, and the main difference was that: (1) the reaction solvent used in this example was a mixed solvent of formic acid and isoamyl acetate, and the weight ratio of formic acid to isoamyl acetate was 3:2; (2) the boiling temperature in the stage of rotary evaporation was 90°C.

[0062] The plastic-containing material used in this embodiment is a mixture of six types of commercially available plastic particles, each having a volume of about 0.5 cubic centimeter. The total weight of the plastic-containing material is 10.037 grams, which is measured using a balance. The six types of commercially available plastic particles are polyamide (5.007 grams), PET, PP, PE, PVC, and PU, and the weight ratio of the six types of particles is about 5:1:1:1:1:1.

[0063] Next, 100 mL of a reaction solvent having a weight ratio of formic acid to isoamyl acetate of 3:2 is prepared. First, 60 mL of the reaction solvent is placed in the beaker to obtain an initial mixture. Then, the beaker is sealed and the initial mixture is heated to 70°C using an electric heating plate while being stirred at a speed of 170 rpm per minute for a first stirring process. After the polyamide in the plastic-containing material is completely dissolved (about 60 minutes), a first mixture is obtained, and the dissolution time is recorded in Table 1. The remaining 40 mL of the reaction solvent is preheated and stored for subsequent rinsing.

[0064] Next, the first mixture is filtered using a stainless steel filter screen having an average pore size of 100 microns. The residue on the stainless steel filter screen is rinsed with the 40 mL of the reaction solvent. Finally, the filtrate having a total volume of 100 mL is placed in a 1-liter vacuum concentration bottle, and the vacuum concentration bottle is connected to a vacuum concentration device (maintaining normal atmospheric pressure) to perform vacuum concentration. In addition, the residue on the stainless steel filter screen is air-dried and then placed in an oven set to a temperature of 80°C for about 2 hours. After cooling, the weight of the residue is measured using an electronic balance and is found to be 5.06 grams.

[0065] Next, the reaction solvent was poured into a vacuum concentration flask, and then the plastic material was poured into the vacuum concentration flask. Subsequently, the vacuum concentration system was depressurized by connecting the vacuum concentration flask to a vacuum pump, and the first mixture was separated from the reaction solvent. The vacuum pump was continuously operated for about 1 to 2 minutes to reduce the pressure in the vacuum concentration system to 0 to 20 mbar. Next, the vacuum concentration flask was placed in a water bath and heated while performing a second stirring process to continuously form a thin film of the crude product on the surface of the vacuum concentration flask and accelerate the evaporation of the reaction solvent. The rotation speed of the second stirring process was set to 20 rpm. After the first mixture was observed to boil at about 90°C for a period of time, the temperature was maintained, and the vacuum pump was turned off. On one hand, the vapor of the reaction solvent was condensed by a condenser tube to recover the reaction solvent, and the pressure in the system was maintained. After the reaction solvent in the vacuum concentration flask was completely evaporated and condensed, the volume of the recovered reaction solvent was measured to calculate the recovery rate of the reaction solvent and record it in Table 2. On the other hand, the thin film of the crude product in the vacuum concentration flask was removed when it was in a dry state, and the thin film of the crude product was placed in an oven set to 80°C for about 2 hours to remove the reaction solvent that may remain in the thin film of the crude product. Next, the dried thin film was cooled to room temperature, and when its weight no longer changed, a polyamide-containing recycled material was obtained, and its weight was precisely measured by an electronic balance (5.002 g) to calculate the recovery rate of the polyamide and record it in Table 2.

[0066] Test 1: Fourier-transform infrared spectroscopy (FTIR)

[0067] To determine whether the polyamide-containing recycled material mixed with other plastic components during the recycling process or was deteriorated and contaminated during the process of the polyamide-containing recycled material, the Fourier-transform infrared spectrometer (manufacturer: PerkinElmer Inc., model: SPECTRUM TWO FT-IR SPECTROSCOPY) was used to analyze the polyamide-containing recycled materials obtained in Examples 1 to 17 to confirm the purity of the obtained polyamide; wherein the analysis wave number range of the infrared spectrometer was 600 cm -1 to 4000 cm -1 . After comparing the FTIR spectra obtained from each group of examples with the standard polyamide, it was determined that the characteristic peak signals of the two were completely consistent, so it was determined that it was indeed polyamide. Since there was no commercially available standard, the aforementioned nylon pellets were used as the standard, and the analysis spectrum was consistent with the literature.

[0068] Similarly, the polyamide-containing recycled materials obtained from Examples 18 and 19 were analyzed under the same measurement conditions as described above. As shown in FIG. 2, the FTIR spectra of each of the polyamide-containing recycled materials were identical to that of the standard polyamide, indicating that the polyamide-containing recycled materials were indeed polyamide. The three sets of analysis results in FIG. 2, from top to bottom, are the FTIR spectrum of Example 18, the FTIR spectrum of Example 19, and the standard polyamide.

[0069] Table 1: Ester solvent used in Reference Example 1 (R1) and Examples 1-19 (E1-E19), volume of ester solvent, volume ratio of formic acid to ester solvent, operating temperature of Step (B) (hereinafter referred to as dissolution temperature), duration of Step (B) (hereinafter referred to as dissolution time), and temperature of Step (C)

[0070] Table 2: Polyamide recovery rate and reaction solvent recovery rate of Reference Example 1, Examples 1-19

[0071] Discussion of experimental results

[0072] According to the results in Tables 1 and 2, the use of a mixture of formic acid and ester solvent according to the present application produced the same results as the use of formic acid alone according to Reference Example 1 and Example 1, in terms of both dissolution time and the subsequent polyamide recovery rate and reaction solvent recovery rate. This demonstrates that the use of a combination of formic acid and ester solvent according to the present application can reduce the amount of formic acid used, thereby improving the safety of the overall process and reducing the manufacturing cost due to the lower cost of ester solvents compared to formic acid.

[0073] In addition, according to Table 2, the polyamide recovery rate of each of the examples was greater than 99%, and the FTIR spectra had no noise, indicating that the recovered material (i.e., the polyamide film) had high purity. According to the experimental results of Examples 18 and 19, even when the polyamide was recovered from a plastic-containing material containing only 50 wt% polyamide, the polyamide recovery rate was greater than 99.8%. This demonstrates that the polyamide recovery method according to the present application has high selectivity, can to some extent solve the problem of not being able to separate a plurality of chemical components at the same time, and improves the defect that mechanical recycling methods cannot have selectivity. In addition, since the present application does not require a preliminary crushing step, the problem of degradation of the properties of recycled plastics can be avoided.

[0074] In summary, the polyamide recycling method of the present application can obtain high-purity regenerated polyamide through a simple device without using complex reaction equipment or preparation process, so the present application can indeed be carried out in a simple and efficient manner, and even if applied to large-scale preparation requirements, it is a very safe method. Furthermore, the present application can indeed have excellent polyamide recovery rate, and the reaction solvent can be recycled and reused, not only can reduce the production cost of the regenerated polyamide prepared therefrom, avoid the cost increase caused by the emission of organic solvents, but also can reduce the burden on the environment. Therefore, more people in this field should be attracted to invest in the plastic recycling industry, thereby achieving the goal of sustainable recycling economy.

Claims

1. A method for recycling polyamide, comprising: Step (A) : providing a plastic-containing material, wherein the plastic-containing material comprises polyamide; Step (B): dissolving the polyamide contained in the plastic-containing material with a reaction solvent to obtain a first mixture; wherein, the reaction solvent comprises formic acid and an ester solvent; and Step (C) : separating the reaction solvent from the first mixture to obtain a polyamide-containing recycled material; wherein the content of polyamide is 90 wt% or more, based on the total weight of the polyamide-containing recycled material.

2. The polyamide recovery method according to claim 1, wherein, the ester solvent comprises methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, (2-methylpropyl) acetate, isoamyl acetate, 3-ethoxypropyl acetate, 2-ethoxyethyl acetate, 1,2-propylene glycol methyl ether acetate, or a combination thereof.

3. The polyamide recovery method according to claim 2, wherein, the ester solvent comprises ethyl acetate, n-butyl acetate, isoamyl acetate, or a combination thereof.

4. The polyamide recovery method according to claim 1, wherein, the content of the formic acid is greater than or equal to 35 vol% and less than 100 vol%, based on the total volume of the reaction solvent.

5. The polyamide recovery method according to claim 3, wherein, the content of the ester solvent is greater than or equal to 20 vol% and less than 65 vol%, based on the total volume of the reaction solvent.

6. The polyamide recovery method according to any one of claims 1 to 5, wherein, the operating temperature of Step (B) is 18°C to 100°C.

7. The polyamide recovery method according to claim 6, wherein, Step (B) comprises a first stirring process, wherein the rotation speed per minute of the first stirring process is 100 rounds per minute to 200 rounds per minute, and the duration of Step (B) is 20 minutes to 120 minutes.

8. The polyamide recovery method of any one of claims 1 to 5, wherein, in Step (C), the reaction solvent is separated from the first mixture by using a reduced pressure concentration method; wherein the pressure of the reduced pressure concentration method is greater than or equal to 0 mbar and less than or equal to 50 mbar.

9. The polyamide recovery method of claim 8, wherein, in Step (C), the separated reaction solvent is collected as the reaction solvent of Step (B).

10. The polyamide recovery method of any one of claims 1 to 5, wherein, the weight of the plastic-containing material is greater than 0 g and less than or equal to 40 g, based on 100 mL of the reaction solvent.

Images