Method for recycling waste polyamide 6

Abstract

The application relates to the technical field of waste nylon recycling, and particularly discloses a method for recycling waste polyamide 6. The method comprises the following steps: preparing a prepolymer through acidolysis-grafting reaction of waste polyamide 6, a dibasic acid and a bifunctional stabilizer, preparing a melt through esterification end-capping and vacuum polycondensation of the purified prepolymer, a dihydric alcohol and a catalyst, and spinning the melt after adding a light-aging-resistant additive to obtain regenerated polyamide 6 fibers. The regenerated polyamide 6 fibers prepared by the method have excellent light-aging resistance, can effectively inhibit yellowing and fracture strength attenuation, and are suitable for high-end fields such as outdoor textiles; the recycling method is simple and easy to operate, has mild conditions, realizes high-value closed-loop recycling of waste polyamide 6, and the prepared regenerated polyamide 6 fibers have stable performance.

Description

Technical Field

[0001] This application relates to the field of waste nylon recycling technology, and more specifically, it relates to a method for recycling waste polyamide 6. Background Technology

[0002] Polyamide 6 (commonly known as Nylon 6) is the world's second most produced synthetic polymer material. With its excellent tensile strength, abrasion resistance, elastic recovery, and chemical resistance, it is widely used in textiles and apparel (outdoor jackets, sun-protective clothing, sportswear), industrial textiles (tire cord, industrial conveyor belts, fishing nets), and engineering plastics. With the continuous growth in consumption, millions of tons of waste polyamide 6 are generated annually. Its chemical properties are stable, and its natural degradation cycle can take decades. Traditional landfill disposal consumes a large amount of land resources, while incineration produces toxic and harmful gases, causing secondary pollution. While physical recycling methods are simple, multiple melting cycles lead to molecular chain degradation, resulting in a significant decrease in the performance of recycled products, which can only be used for low-end products. Against this backdrop, chemical recycling methods that can achieve closed-loop, high-value regeneration of polymer materials have become a research hotspot and development direction in the field of waste polyamide 6 recycling.

[0003] In existing technologies, such as the patent application with publication number CN118812843A, a method for the acid hydrolysis and recycling of waste polyamide 6 is provided. This method breaks through the traditional chemical recycling approach, which requires complete depolymerization into caprolactam monomers. It innovatively uses a diacid to perform controlled acid hydrolysis of waste polyamide 6 under normal pressure to prepare a prepolymer of polyamide 6 with dual-carboxyl-terminated groups. After purification, a diol is added for esterification and end-capping, followed by directional chain growth through transesterification vacuum polycondensation. Finally, recycled polyamide 6 fibers are obtained through melt spinning. This technology features mild reaction conditions, low energy consumption, and simple equipment requirements. Furthermore, the initial mechanical properties of the prepared recycled fibers can reach the level of petroleum-based virgin products, providing important technical support for the efficient and low-cost recycling of waste polyamide 6 and possessing good potential for industrial application.

[0004] However, the recycled polyamide 6 fibers prepared by this technology still suffer from insufficient resistance to photoaging, severely limiting their application in high-end fields such as outdoor textiles. The reasons for this are twofold. First, during acid hydrolysis, the amide bonds in the polyamide 6 molecular chain undergo homolytic cleavage, generating a large number of terminal free radicals. Although subsequent esterification and polycondensation reactions consume most of these active free radicals, some remain trapped within the polymer matrix, unable to be completely quenched, forming potential photo-oxidation initiation sites. Second, trace amounts of metal ions are inevitably introduced during the production, processing, and use of waste polyamide 6 raw materials. These include copper and iron ions from dye fixing agents and antistatic agents used in textile dyeing and finishing, as well as iron, chromium, and nickel ions introduced by wear and tear on production equipment. Existing technologies using vacuum distillation combined with activated carbon decolorization and purification processes can only remove polar small molecule impurities, offering almost no removal effect on these trace metal ions existing in complexed or adsorbed states. Residual active free radicals can directly trigger the oxidative breakage of amide bonds under ultraviolet radiation, while trace metal ions can act as catalysts to accelerate the decomposition of hydroperoxides, generating more free radicals and forming a chain degradation reaction. Ultimately, this leads to a significant decrease in the tensile strength and easy yellowing of regenerated fibers after short-term light exposure, making it impossible to meet the long-term weather resistance requirements of outdoor products. Summary of the Invention

[0005] To enhance the light aging resistance of recycled polyamide 6 fibers prepared from waste polyamide 6, this application provides a method for recycling waste polyamide 6.

[0006] The method for recycling waste polyamide 6 provided in this application adopts the following technical solution: A method for recycling waste polyamide 6 includes the following steps: S1. Waste polyamide 6, diacid, and bifunctional stabilizer are heated and stirred under nitrogen protection to carry out an integrated acid hydrolysis-grafting reaction to obtain a grafted double-terminated carboxyl polyamide 6 prepolymer; the bifunctional stabilizer is obtained by nucleophilic substitution reaction of hindered phenol-active chlorine intermediate and acyl hydrazine metal chelating agent. S2. The branched double-ended carboxyl polyamide 6 prepolymer was purified to obtain high-purity double-ended carboxyl polyamide 6 prepolymer. S3. High-purity double-ended carboxyl polyamide 6 prepolymer is heated in a reactor with diol and esterification catalyst to carry out esterification end-capping reaction to obtain esterification intermediate. Then, ester exchange catalyst is added to carry out vacuum polycondensation reaction to obtain recycled polyamide 6 melt. S4. Add a light-resistant aging agent to the recycled polyamide 6 melt, stir evenly, and then spin, cool, stretch, and wind to obtain recycled polyamide 6 fiber.

[0007] By employing the above technical solution, the bifunctional stabilizer is prepared by a nucleophilic substitution reaction between a hindered phenol-active chlorine intermediate and an acylhydrazine-based metal chelating agent. Its molecular structure simultaneously contains both hindered phenolic antioxidant functional groups and acylhydrazine-based metal chelating functional groups. In the integrated acid hydrolysis-grafting reaction, it can be in-situ grafted onto the prepolymer molecular chain of double-terminated carboxyl polyamide 6 via chemical bonding, achieving a strong bond between the stabilizer and the polymer matrix. This avoids the defects of traditional physically added stabilizers, such as easy migration and uneven dispersion, ensuring the long-lasting free radical quenching and metal ion passivation functions. Compared to existing technologies that only physically add stabilizers and cannot achieve functional synergy, this solution, through an integrated grafting design, ensures that the bifunctional stabilizer is uniformly distributed in the polymer molecular chain. This allows for timely capture of terminal free radicals generated during acid hydrolysis, while simultaneously passivating trace metal ions remaining in the waste raw materials, thus blocking the photo-oxidative chain degradation reaction from two levels.

[0008] The coordinated steps of this process route further ensure improved light aging resistance: the purification process in S2 removes unreacted small molecule impurities and some residual metal ions, providing high-purity prepolymers for subsequent reactions and reducing the impact of impurities on fiber performance; the esterification and vacuum polycondensation reaction in S3 achieves directional growth of molecular chains, ensuring the mechanical properties of the recycled melt while avoiding the introduction of additional impurities; the addition of light aging resistant additives in S4, together with bifunctional stabilizers, forms a synergistic protection, further enhancing the long-term light aging resistance of the fiber. The resulting recycled polyamide 6 fiber can effectively overcome the defects of severe yellowing and rapid attenuation of breaking strength in existing technologies, meeting the long-term use requirements of high-end fields such as outdoor textiles.

[0009] Optionally, the bifunctional stabilizer is prepared using the following method: (1) Mix 2,6-di-tert-butyl-4-hydroxymethylphenol, chloroacetyl chloride and solvent, and stir at 200-300 r / min at 20-40℃ for 4-8 h to obtain the hindered phenol-active chlorine intermediate; (2) The hindered phenol-active chlorine intermediate, acylhydrazine metal chelating agent and triethylamine are mixed and stirred at 300-500 r / min at 40-80℃ for 8-12 h to obtain a bifunctional stabilizer.

[0010] By employing the above-mentioned technical solution, a bifunctional molecule with both free radical quenching and metal ion passivation functions was precisely constructed through a two-step reaction, solving the problems of complex preparation processes, single function, or poor functional synergy in existing stabilizers. This preparation method utilizes mild reaction conditions. The first step involves the reaction of 2,6-di-tert-butyl-4-hydroxymethylphenol with chloroacetyl chloride to introduce an active chlorine terminal group at the hindered phenolic group, achieving reactive activation of the antioxidant functional group and laying the foundation for subsequent linkage with acylhydrazine-based metal chelating agents. The second step involves a nucleophilic substitution reaction to link the acylhydrazine-based metal chelating agent to the intermediate, forming a stable bifunctional molecular structure. The entire preparation process is characterized by easily controllable reaction conditions and high product purity, ensuring the structural stability and functional effectiveness of the bifunctional stabilizer. It avoids problems such as functional group failure and impurity introduction caused by unreasonable preparation processes, providing a stable and reliable functional component for the implementation of the recycling process in this application and ensuring the stability of the photoaging resistance of regenerated fibers.

[0011] Optionally, in step (1), the mass ratio of 2,6-di-tert-butyl-4-hydroxymethylphenol, chloroacetyl chloride and solvent is 1:(0.8-1.2):(5-8); the solvent is anhydrous dichloromethane or anhydrous ethyl acetate.

[0012] By adopting the above technical solution and using anhydrous dichloromethane or anhydrous ethyl acetate as solvent, the hydrolysis of chloroacetyl chloride can be effectively avoided, ensuring the directional progress of the reaction, improving the purity and activity of the intermediate, providing a guarantee for the efficient preparation of the subsequent bifunctional stabilizer, and thus ensuring that it can fully exert its free radical quenching and metal ion passivation functions in the acid hydrolysis-grafting integrated reaction.

[0013] Optionally, in step (2), the mass ratio of the hindered phenol-active chlorine intermediate, the acylhydrazine metal chelating agent, and triethylamine is 1:(0.6-0.9):(0.5-0.7); the acylhydrazine metal chelating agent is 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionylhydrazine or oxaloyl dihydrazine.

[0014] By adopting the above technical solution, triethylamine, as an acid-binding agent, can neutralize the hydrogen chloride generated in the reaction, promoting the reaction forward and improving the yield and purity of the bifunctional stabilizer. 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionylhydrazide or oxaloyl dihydrazide are selected as hydrazide-based metal chelating agents. Their molecular structure contains abundant coordinating groups, which can form stable chelate rings with trace metal ions in waste raw materials, significantly improving the passivation effect of metal ions. Simultaneously, they form a synergistic effect with hindered phenolic functional groups, further enhancing the free radical quenching ability, providing key support for improving the photoaging resistance of regenerated fibers.

[0015] Optionally, in step S1, the temperature of the integrated acid hydrolysis-grafting reaction is 230-260℃, the reaction time is 2-4h, and the stirring speed is 200-300r / min. The amount of the bifunctional stabilizer added is 0.5%-5.0% of the mass of waste polyamide 6; The dicarboxylic acid is one of adipic acid, sebacic acid, or terephthalic acid, and the mass ratio of the dicarboxylic acid to waste polyamide 6 is (0.1-0.3):1.

[0016] Optionally, in step S2, the specific method for purifying the branched double-terminated carboxyl polyamide 6 prepolymer is as follows: The grafted bi-terminated carboxyl polyamide 6 prepolymer was dissolved in an organic solvent to prepare a prepolymer solution with a mass concentration of 10%-25%. The solution was heated to 60-80℃ and stirred until completely dissolved. An adsorbent was added, and the solution was stirred at 200-300 r / min at 60-80℃ for 0.5-1.5 h for adsorption. After adsorption, the adsorbent was removed by filtration, and the filtrate was collected. The filtrate was subjected to vacuum distillation to remove the organic solvent, and dried to obtain high-purity bi-terminated carboxyl polyamide 6 prepolymer. The adsorbent is a mixture of activated carbon, mercaptosilica gel and lanthanum stearate in a mass ratio of 10:(1-4):(0.1-0.3), and the total amount of adsorbent added is 1%-3% of the mass of the grafted double-ended carboxyl polyamide 6 prepolymer.

[0017] By employing the above-mentioned technical solution, this purification process utilizes solution adsorption. The prepolymer is dissolved in an organic solvent to form a low-viscosity solution, and adsorption is carried out under mild conditions of 60-80℃. Within this temperature range, activated carbon can effectively adsorb organic pigments and polar small molecule impurities. The chelating functional groups of mercaptosilicone are stable and can selectively adsorb residual free and weakly complexed trace metal ions. The adsorption mass transfer rate in the solution system is fast and efficient, and the filtration operation is simple and easy. Lanthanum stearate, as a rare earth chelating component, can synergistically remove complexed copper, iron, chromium, and nickel trace metal ions from waste polyamide 6 with mercaptosilicone, while simultaneously assisting activated carbon in removing organic pigments and small molecule impurities. The filtrate after adsorption is recovered by vacuum distillation to recover the organic solvent, making it environmentally friendly. This purification method, combined with the metal ion passivation function of the bifunctional stabilizer, synergistically reduces photo-oxidation catalytic sites from the source, providing a guarantee for the subsequent preparation of high-performance regenerated fibers.

[0018] Optionally, in step S3, the reaction temperature of the esterification capping reaction is 180-200℃, the reaction time is 2-4h, and the stirring speed is 200-250r / min; The vacuum polycondensation reaction is carried out at a temperature of 250-260℃, a pressure of 10-200Pa, a reaction time of 2-4h, and a stirring speed of 150-200r / min.

[0019] Optionally, in step S3, the diol is ethylene glycol or propylene glycol; the esterification catalyst is stannous octoate or tetrabutyl titanate; and the transesterification catalyst is tetrabutyl titanate or antimony trioxide.

[0020] Optionally, in step S3, the mass ratio of the high-purity double-ended carboxyl polyamide 6 prepolymer, the diol, and the esterification catalyst is 100:(10-15):(0.1-0.3); the amount of the transesterification catalyst added is 0.05%-0.2% of the mass of the high-purity double-ended carboxyl polyamide 6 prepolymer.

[0021] Optionally, in step S4, the light aging resistant additive is obtained by compounding UV-350 and UV-3346 in a mass ratio of (2-2.5):1, and the amount of light aging resistant additive added is 0.8%-1.5% of the total mass of the recycled polyamide 6 melt.

[0022] By adopting the above technical solution, the UV-350 and UV-3346 compound system has a synergistic protective effect, effectively absorbing ultraviolet rays and capturing free radicals generated during use. Together with the bifunctional stabilizer, it forms a multi-layered protective system, further improving the fiber's resistance to light aging and delaying yellowing and weakening of breaking strength during outdoor use. A reasonable compounding ratio and dosage ensure that the light aging auxiliaries are fully compatible and uniformly dispersed with the recycled polyamide 6 melt, avoiding uneven protective effects caused by auxiliaries agglomeration, and preventing a decrease in fiber mechanical properties due to excessive dosage. Ultimately, this enables the recycled polyamide 6 fiber to meet the long-term weather resistance requirements of outdoor textiles.

[0023] In summary, this application has the following beneficial effects: 1. This application employs a synergistic design combining a bifunctional stabilizer and an integrated acid hydrolysis-grafting process, effectively addressing the core pain point of insufficient light aging resistance in existing technologies for recycled polyamide 6 fibers. Addressing the issues of yellowing and decreased tensile strength caused by residual free radicals and trace metal ion catalytic degradation in existing technologies, the bifunctional stabilizer in this application is prepared via a nucleophilic substitution reaction. Its molecular structure simultaneously contains hindered phenolic antioxidant functional groups and hydrazide-based metal chelating functional groups, allowing for in-situ grafting onto the polyamide 6 prepolymer molecular chain during the acid hydrolysis reaction. This achieves a strong bond with the polymer matrix, completely avoiding the migration and uneven dispersion defects of traditional physically additive stabilizers. This design can promptly capture terminal free radicals generated during acid hydrolysis while passivating residual trace metal ions in waste materials, blocking the photo-oxidative chain degradation reaction from two key levels. This significantly improves the light aging resistance of recycled fibers, enabling them to meet the long-term use requirements of high-end fields such as outdoor textiles.

[0024] 2. This application employs a precise bifunctional stabilizer preparation process and raw material ratio, providing a reliable guarantee for the stable implementation of the regeneration process and the improvement of photoaging resistance. Addressing the problems of complex existing stabilizer preparation processes, poor functional synergy, and insufficient product purity, this application prepares the bifunctional stabilizer through a two-step mild reaction. By rationally selecting reaction raw materials and ratios, and choosing suitable solvents and acid-binding agents, the reaction is ensured to proceed in a directional and efficient manner, improving product purity and structural stability, and avoiding functional group failure or impurity introduction. Simultaneously, by optimizing the ratio parameters of each raw material, the antioxidant and metal chelating functions of the bifunctional stabilizer achieve the best synergistic effect, ensuring its full role in the integrated acid hydrolysis-grafting reaction. This lays the foundation for the smooth implementation of the subsequent regeneration process and further guarantees the stability and consistency of the photoaging resistance of the regenerated fibers.

[0025] 3. This application addresses the limitations of existing purification processes in effectively removing trace metal ions and providing limited light protection. Step S2 utilizes a composite adsorbent composed of activated carbon and mercapto-silica gel. These two components complement each other, specifically adsorbing organic pigments, small molecule impurities, and residual free and weakly complexed trace metal ions. This reduces photo-oxidation catalytic sites at the source and complements the metal passivation function of the bifunctional stabilizer. Step S4 employs a specifically formulated light-aging resistant agent that effectively absorbs ultraviolet light and captures free radicals generated during fiber use. This agent, combined with the bifunctional stabilizer grafted onto the molecular chain, forms a multi-layered protective system, delaying fiber yellowing and breaking strength degradation. This further enhances the long-term light-aging resistance, ensuring the regenerated fiber can withstand complex outdoor environments for extended periods and meet the needs of high-end applications. Detailed Implementation

[0026] The present application will be further described in detail below with reference to the embodiments.

[0027] Preparation example of bifunctional stabilizers Preparation Example 1 The bifunctional stabilizer was prepared using the following method: (1) Take 10.0 g of 2,6-di-tert-butyl-4-hydroxymethylphenol, 8.0 g of chloroacetyl chloride and 50.0 g of anhydrous dichloromethane in a mass ratio of 1:0.8:5 and add them to a three-necked flask. Stir the mixture at 200 r / min at 20 °C for 8 h. After the reaction is complete, remove the solvent by vacuum distillation to obtain the hindered phenol-active chlorine intermediate. (2) Take 10.0 g of the hindered phenol-active chlorine intermediate, 6.0 g of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionylhydrazine and 5.0 g of triethylamine prepared above in a mass ratio of 1:0.6:0.5 and add them to a three-necked flask. Stir the mixture at 300 r / min at 40 °C for 12 h. After the reaction is completed, filter, wash and vacuum dry to obtain the bifunctional stabilizer.

[0028] Preparation Example 2 The bifunctional stabilizer was prepared using the following method: (1) Take 10.0 g of 2,6-di-tert-butyl-4-hydroxymethylphenol, 10.0 g of chloroacetyl chloride and 65.0 g of anhydrous ethyl acetate in a mass ratio of 1:1.0:6.5, add them to a three-necked flask, stir at 250 r / min at 30 °C for 6 h, and after the reaction is completed, remove the solvent by vacuum distillation to obtain the hindered phenol-active chlorine intermediate; (2) Take 10.0 g of the hindered phenol-active chlorine intermediate, 7.5 g of oxaloyl dihydrazide and 6.0 g of triethylamine prepared above in a mass ratio of 1:0.75:0.6 and add them to a three-necked flask. Stir the mixture at 400 r / min at 60 °C for 10 h. After the reaction is completed, filter, wash and vacuum dry to obtain the bifunctional stabilizer.

[0029] Preparation Example 3 The bifunctional stabilizer was prepared using the following method: (1) Take 10.0 g of 2,6-di-tert-butyl-4-hydroxymethylphenol, 12.0 g of chloroacetyl chloride and 80.0 g of anhydrous dichloromethane in a mass ratio of 1:1.2:8 and add them to a three-necked flask. Stir the mixture at 300 r / min at 40 °C for 4 h. After the reaction is complete, remove the solvent by vacuum distillation to obtain the hindered phenol-active chlorine intermediate. (2) Take 10.0 g of the hindered phenol-active chlorine intermediate, 9.0 g of 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionylhydrazine and 7.0 g of triethylamine prepared above in a mass ratio of 1:0.9:0.7 and add them to a three-necked flask. Stir the mixture at 500 r / min at 80 °C for 8 h. After the reaction is completed, filter, wash and vacuum dry to obtain the bifunctional stabilizer.

[0030] Example The waste polyamide 6 used in this application embodiment is waste polyamide 6 fiber scraps from textile mills, which are crushed, washed and dried for later use. Its particle size is 80-120 mesh and its moisture content is ≤0.5%.

[0031] Example 1 A method for recycling waste polyamide 6 includes the following steps: S1. Take 100.0g of waste polyamide 6, 10.0g of adipic acid and 0.5g of the bifunctional stabilizer prepared in Preparation Example 1, add them to the reaction vessel, introduce nitrogen gas at a flow rate of 50mL / min for protection, heat to 230℃, stir at a speed of 200r / min for 4h, and after the reaction is completed, obtain the grafted double-ended carboxyl polyamide 6 prepolymer; S2. The grafted double-ended carboxyl polyamide 6 prepolymer was dissolved in ethanol to prepare a 15% (w / w) prepolymer solution. The solution was heated to 70°C and stirred until completely dissolved. 2.0 g of adsorbent was added. The adsorbent consisted of activated carbon, mercaptosilica gel, and lanthanum stearate mixed in a mass ratio of 10:1:0.1. The solution was stirred at 200 r / min for 1 h at 70°C for adsorption. After adsorption, the adsorbent was removed by filtration, and the filtrate was collected. The filtrate was subjected to vacuum distillation (pressure 0.08 MPa, temperature 70°C) to remove the ethanol. After drying, high-purity double-ended carboxyl polyamide 6 prepolymer was obtained. S3. 100.0 g of high-purity double-ended carboxyl polyamide 6 prepolymer, 10.0 g of ethylene glycol, and 0.1 g of stannous octoate were added to a reactor, heated to 180°C, and stirred at 200 r / min for 4 h to carry out an esterification end-capping reaction to obtain an esterification intermediate; then 0.05 g of tetrabutyl titanate was added, the reactor temperature was adjusted to 250°C, the pressure was reduced to 10 Pa, and a vacuum polycondensation reaction was carried out at 150 r / min for 4 h to obtain recycled polyamide 6 melt; S4. Add a light-resistant aging agent to the recycled polyamide 6 melt. The light-resistant aging agent is obtained by compounding UV-350 and UV-3346 in a mass ratio of 2:1. The amount of light-resistant aging agent added is 0.8% of the total mass of the recycled polyamide 6 melt. Then stir at a speed of 200 r / min for 15 min to ensure uniform mixing. Subsequently, the melt is spun through a spinneret with a spinneret orifice diameter of 0.2 mm and a spinneret speed of 8 m / min. It is then cooled by cooling air at a temperature of 25℃ and a wind speed of 1.5 m / s. Then it is stretched by a stretching machine with a stretch ratio of 3.0 times and a stretching temperature of 80℃. Finally, it is wound by a winding machine at a winding speed of 25 m / min to obtain recycled polyamide 6 fiber.

[0032] Example 2 A method for recycling waste polyamide 6 includes the following steps: S1. Take 100.0g of waste polyamide 6, 20.0g of sebacic acid and 2.75g of the bifunctional stabilizer prepared in Preparation Example 2, add them to the reaction vessel, introduce nitrogen gas at a flow rate of 80mL / min for protection, heat to 245℃, stir at a speed of 250r / min for 3h, and after the reaction is completed, obtain the grafted double-ended carboxyl polyamide 6 prepolymer; S2. The grafted double-ended carboxyl polyamide 6 prepolymer was dissolved in ethanol to prepare a 20% (w / w) prepolymer solution. The solution was heated to 70°C and stirred until completely dissolved. 2.0 g of adsorbent was added. The adsorbent consisted of activated carbon, mercaptosilica gel, and lanthanum stearate in a (w / w) mass ratio of 10:2.5:0.2. The solution was stirred at 70°C and 250 r / min for 0.75 h for adsorption. After adsorption, the adsorbent was removed by filtration, and the filtrate was collected. The filtrate was subjected to vacuum distillation (0.08 MPa, 70°C) to remove the ethanol. After drying, high-purity double-ended carboxyl polyamide 6 prepolymer was obtained. S3. 100.0 g of high-purity double-ended carboxyl polyamide 6 prepolymer, 12.5 g of propylene glycol, and 0.2 g of tetrabutyl titanate were added to a reactor, heated to 190°C, and stirred at 225 r / min for 3 h to carry out an esterification end-capping reaction to obtain an esterification intermediate; then 0.125 g of antimony trioxide was added, the reactor temperature was adjusted to 255°C, the pressure was reduced to 105 Pa, and a vacuum polycondensation reaction was carried out at 175 r / min for 3 h to obtain recycled polyamide 6 melt; S4. Add a light-resistant aging agent to the recycled polyamide 6 melt. The light-resistant aging agent is obtained by compounding UV-350 and UV-3346 at a mass ratio of 2.25:1. The amount of light-resistant aging agent added is 1.15% of the total mass of the recycled polyamide 6 melt. Then stir at a speed of 220 r / min for 18 min to ensure uniform mixing. Subsequently, the melt is spun through a spinneret with a spinneret orifice diameter of 0.2 mm and a spinneret speed of 10 m / min. It is then cooled by cooling air at a temperature of 28℃ and a wind speed of 1.8 m / s. After that, it is stretched by a stretching machine with a stretch ratio of 3.5 times and a stretching temperature of 85℃. Finally, it is wound by a winding machine at a winding speed of 30 m / min to obtain recycled polyamide 6 fiber.

[0033] Example 3 A method for recycling waste polyamide 6 includes the following steps: S1. Take 100.0g of waste polyamide 6, 30.0g of terephthalic acid and 5.0g of the bifunctional stabilizer prepared in Preparation Example 3, add them to the reaction vessel, introduce nitrogen gas at a flow rate of 100mL / min for protection, heat to 260℃, stir at a speed of 300r / min for 2h, and after the reaction is completed, obtain the grafted double-ended carboxyl polyamide 6 prepolymer; S2. The grafted double-ended carboxyl polyamide 6 prepolymer was dissolved in isopropanol to prepare a prepolymer solution with a mass concentration of 25%. The solution was heated to 80°C and stirred until completely dissolved. 3.0 g of adsorbent was added. The adsorbent consisted of activated carbon, mercaptosilica gel, and lanthanum stearate in a mass ratio of 10:4:0.3. The solution was stirred at 300 r / min for 0.5 h at 80°C for adsorption. After adsorption, the adsorbent was removed by filtration, and the filtrate was collected. The filtrate was subjected to vacuum distillation (pressure 0.08 MPa, temperature 80°C) to remove isopropanol. After drying, high-purity double-ended carboxyl polyamide 6 prepolymer was obtained. S3. 100.0 g of high-purity double-ended carboxyl polyamide 6 prepolymer, 15.0 g of ethylene glycol, and 0.3 g of stannous octoate were added to a reactor, heated to 200 °C, and stirred at 250 r / min for 2 h to carry out an esterification end-capping reaction to obtain an esterification intermediate; then 0.2 g of tetrabutyl titanate was added, the reactor temperature was adjusted to 260 °C, the pressure was reduced to 200 Pa, and a vacuum polycondensation reaction was carried out at 200 r / min for 2 h to obtain recycled polyamide 6 melt; S4. Add a light-resistant aging agent to the recycled polyamide 6 melt. The light-resistant aging agent is obtained by compounding UV-350 and UV-3346 at a mass ratio of 2.5:1. The amount of light-resistant aging agent added is 1.5% of the total mass of the recycled polyamide 6 melt. Then stir at a speed of 250 r / min for 20 min to ensure uniform mixing. Subsequently, the melt is spun through a spinneret with a spinneret orifice diameter of 0.2 mm and a spinneret speed of 12 m / min. It is then cooled by cooling air at a temperature of 30℃ and a wind speed of 2.0 m / s. Then it is stretched by a stretching machine with a stretch ratio of 4.0 times and a stretching temperature of 90℃. Finally, it is wound by a winding machine at a winding speed of 35 m / min to obtain recycled polyamide 6 fiber.

[0034] Example 4 A method for recycling waste polyamide 6 differs from Example 2 in that, in this example, the bifunctional stabilizer in step S1 is the bifunctional stabilizer prepared in Preparation Example 3.

[0035] Example 5 A method for recycling waste polyamide 6 differs from Example 2 in that, in this example, the light aging resistant additive in step S4 is obtained by compounding UV-350 and UV-3346 in a mass ratio of 1:1.

[0036] Example 6 A method for recycling waste polyamide 6 differs from Example 2 in that, in this example, the temperature of the integrated acid hydrolysis-grafting reaction in step S1 is 200°C.

[0037] Example 7 A method for recycling waste polyamide 6 differs from Example 2 in that activated carbon is used as the adsorbent in step S2 of this example.

[0038] Comparative Example Comparative Example 1 Regenerated polyamide 6 fibers were prepared according to Example 1 in the patent application document with publication number CN118812843A entitled "A Method for Acid Dissolution and Recycling of Waste Polyamide 6".

[0039] Comparative Example 2 A method for recycling waste polyamide 6 differs from Example 2 in that, in this comparative example, no bifunctional stabilizer is added in step S1.

[0040] Comparative Example 3 A method for recycling waste polyamide 6 differs from Example 2 in that, in step S1 of this comparative example, a mixture of 2,6-di-tert-butyl-4-hydroxymethylphenol, chloroacetyl chloride, and oxaloyl dihydrazine in a mass ratio of 1:1:1 is used to replace the bifunctional stabilizer.

[0041] Comparative Example 4 A method for recycling waste polyamide 6 differs from Example 2 in that the purification process in S2 was not performed in this comparative example, and the grafted double-ended carboxyl polyamide 6 prepolymer obtained in S1 was directly used in the esterification and end-capping reaction in S3.

[0042] Comparative Example 5 A method for recycling waste polyamide 6 differs from Example 2 in that, in this comparative example, no light aging resistant agent is added in step S4, and recycled polyamide 6 fibers are directly obtained by spinning recycled polyamide 6 melt.

[0043] Performance testing 1. Initial fracture strength The tensile properties of short chemical fibers were tested using an electronic universal testing machine, in accordance with GB / T14344-2015 "Test Method for Tensile Properties of Short Chemical Fibers". The two ends of the sample were fixed to the machine clamps, with a clamping distance of 20 cm and a tensile speed of 50 mm / min. The sample was stretched until it broke, and the maximum load at break was recorded. The breaking strength was calculated based on the sample cross-sectional area using the following formula: Breaking strength (MPa) = Maximum load (N) / Sample cross-sectional area (mm²). 2 ).

[0044] 2. Loss rate of fracture strength after photoaging Referring to GB / T16422.2-2014 "Laboratory Light Source Exposure Test Methods for Plastics - Part 2: Xenon Arc Lamp", the samples were placed in a xenon arc lamp aging test chamber, and the test conditions were set as follows: irradiation power 500W, blackboard temperature 60℃, relative humidity 50%, and irradiation time 1000h (simulating long-term outdoor use environment). After irradiation, the breaking strength of the samples was measured according to the "initial breaking strength" test method, and the breaking strength loss rate was calculated. The calculation formula is: breaking strength loss rate (%) = (initial breaking strength - breaking strength after photoaging) / initial breaking strength × 100%. The lower the loss rate, the better the fiber's resistance to photoaging.

[0045] 3. Color fastness to light yellowing Referring to GB / T30669-2014 "Textiles - Tests for Color Fastness - Assessment of Color Change - Gray Scale", a xenon arc lamp aging test chamber and a gray scale for assessing color change were used for testing. The samples were first divided into two groups: one group served as a blank control group (without photoaging treatment), and the other group was placed in the xenon arc lamp aging test chamber. The test conditions were set as follows: irradiation power 500W, blackboard temperature 60℃, relative humidity 50%, and irradiation time 1000h. After irradiation, the photoaged samples and the blank control group were placed side-by-side. Under standard light source D65 conditions, the yellowing level was assessed using GB / T250-2008 "Textiles - Tests for Color Fastness - Assessment of Color Change - Gray Scale", with levels ranging from 1 to 5, where level 5 is the best (no yellowing) and level 1 is the worst (severe yellowing). The higher the level, the stronger the fiber's resistance to photo-yellowing and the better its resistance to photoaging.

[0046] Table 1 Test Results

[0047] As can be seen from Table 1, the regenerated polyamide 6 fibers in Examples 1-4 showed a breakage strength loss rate of only 6.2%-8.5% after photoaging, and the light yellowing fastness reached level 5 (no obvious yellowing). In contrast, the breakage strength loss rate of Comparative Example 1 was as high as 25.7%, and the light yellowing fastness was only level 1.5. The difference between the two is significant, which fully demonstrates the superiority of the technical solution of this application.

[0048] Comparative Example 2, without the addition of a bifunctional stabilizer, showed inferior fracture strength loss (28.9%) and light yellowing fastness (grade 1.5) compared to all examples, indicating that the bifunctional stabilizer is key to improving light aging resistance. Comparative Example 3, using a physically mixed stabilizer instead of the grafted bifunctional stabilizer of this application, showed better fracture strength loss (20.3%) and light yellowing fastness (grade 2) than Comparative Examples 1-2, but still far inferior to the examples. This confirms that the bifunctional stabilizer of this application, through chemical bonding grafting, can avoid the defects of easy migration and uneven dispersion of traditional physically added stabilizers, achieving a long-term synergistic effect of free radical quenching and metal ion passivation.

[0049] The comparison between Examples 5-7 and Example 2 further verifies the rationality of the process parameters and raw material ratios of this application. Example 5 deviates from the specified ratio of light-aging accelerators in this application, resulting in a breakage strength loss rate of 10.8%. Although the light yellowing fastness remains at grade 5, the long-term protective effect is reduced. Example 7 does not use the composite adsorbent of this application, but only activated carbon adsorption, leading to an increase in the residual amount of metal ions, which causes the breakage strength loss rate to rise to 11.5% and the light yellowing fastness to drop to grade 4.5. In Example 6, the acid hydrolysis-grafting reaction temperature deviates from the specified range, and the grafting of the bifunctional stabilizer is insufficient. Not only does the breakage strength loss rate rise to 12.3% and the light yellowing fastness to drop to grade 4.5, but the initial breakage strength also drops to 45.6 MPa. This indicates that the process parameters and raw material system specified in this application are important supports for ensuring the comprehensive performance of the fiber.

[0050] Comparative Example 4, which did not undergo S2 purification and directly used the unpurified prepolymer in the reaction, had the worst breaking strength loss rate (30.2%) and light yellowing fastness (grade 1.5) among all samples. This was because the small molecule impurities and trace metal ions that were not removed increased the photo-oxidation catalytic sites, accelerating fiber degradation and yellowing. Comparative Example 5, which did not add light aging accelerators, saw its breaking strength loss rate rise to 15.7% and its light yellowing fastness drop to grade 3. This confirmed the synergistic protective effect of light aging accelerators and bifunctional stabilizers. The combination of the two can construct a multi-level protection system, further improving the long-term light aging resistance of fibers.

[0051] In summary, this application, through its integrated design of dual-functional stabilizer grafting, purification with composite adsorbents, formulation of light-aging resistant additives, and precise control of process parameters, effectively blocks the photo-oxidative chain degradation reaction at its source, thereby inhibiting fiber yellowing and the decline in breaking strength. The overall performance of the embodiments is superior to the comparative examples, especially in terms of light yellowing fastness and breaking strength stability, fully demonstrating that the technical solution of this application can effectively solve the defects of existing technologies, enabling recycled polyamide 6 fibers to meet the long-term use requirements of high-end fields such as outdoor textiles.

[0052] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for recycling waste polyamide 6, characterized in that, The steps include the following: S1. Waste polyamide 6, diacid, and bifunctional stabilizer are heated and stirred under nitrogen protection to carry out an integrated acid hydrolysis-grafting reaction to obtain a grafted double-terminated carboxyl polyamide 6 prepolymer; the bifunctional stabilizer is obtained by nucleophilic substitution reaction of hindered phenol-active chlorine intermediate and acyl hydrazine metal chelating agent. S2. Dissolve the grafted double-ended carboxyl polyamide 6 prepolymer in an organic solvent to prepare a prepolymer solution with a mass concentration of 10%-25%. Heat the solution to 60-80℃ and stir until completely dissolved. Add the adsorbent and stir at 200-300 r / min at 60-80℃ for 0.5-1.5 h. After adsorption, filter to remove the adsorbent and collect the filtrate. Distill the filtrate under reduced pressure and dry to obtain high-purity double-ended carboxyl polyamide 6 prepolymer. The adsorbent is a mixture of activated carbon, mercaptosilica gel, and lanthanum stearate in a mass ratio of 10:(1-4):(0.1-0.3), and the total amount of adsorbent added is 1%-3% of the mass of the grafted double-ended carboxyl polyamide 6 prepolymer. S3. High-purity double-ended carboxyl polyamide 6 prepolymer is heated in a reactor with diol and esterification catalyst to carry out esterification end-capping reaction to obtain esterification intermediate. Then, ester exchange catalyst is added to carry out vacuum polycondensation reaction to obtain recycled polyamide 6 melt. S4. Add a light aging resistant agent to the recycled polyamide 6 melt, stir evenly, and then spin, cool, stretch and wind to obtain recycled polyamide 6 fiber. In step S1, the bifunctional stabilizer is prepared using the following method: (1) Mix 2,6-di-tert-butyl-4-hydroxymethylphenol, chloroacetyl chloride and solvent, and stir at 200-300 r / min at 20-40℃ for 4-8 h to obtain the hindered phenol-active chlorine intermediate; (2) The hindered phenol-active chlorine intermediate, acylhydrazine metal chelating agent and triethylamine are mixed and stirred at 300-500 r / min at 40-80℃ for 8-12 h to obtain a bifunctional stabilizer.

2. The method for recycling waste polyamide 6 according to claim 1, characterized in that: In step (1), the mass ratio of 2,6-di-tert-butyl-4-hydroxymethylphenol, chloroacetyl chloride and solvent is 1:(0.8-1.2):(5-8); the solvent is anhydrous dichloromethane or anhydrous ethyl acetate.

3. The method for recycling waste polyamide 6 according to claim 1, characterized in that: In step (2), the mass ratio of the hindered phenol-active chlorine intermediate, the acylhydrazine metal chelating agent and triethylamine is 1:(0.6-0.9):(0.5-0.7); the acylhydrazine metal chelating agent is 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionylhydrazine or oxaloyl dihydrazine.

4. The method for recycling waste polyamide 6 according to claim 1, characterized in that: In step S1, the temperature of the integrated acid hydrolysis-grafting reaction is 230-260℃, the reaction time is 2-4h, and the stirring speed is 200-300r / min; The amount of the bifunctional stabilizer added is 0.5%-5.0% of the mass of waste polyamide 6; The dicarboxylic acid is one of adipic acid, sebacic acid, or terephthalic acid, and the mass ratio of the dicarboxylic acid to waste polyamide 6 is (0.1-0.3):

1.

5. The method for recycling waste polyamide 6 according to claim 1, characterized in that: In step S3, the reaction temperature of the esterification capping reaction is 180-200℃, the reaction time is 2-4h, and the stirring speed is 200-250r / min; The vacuum polycondensation reaction is carried out at a temperature of 250-260℃, a pressure of 10-200Pa, a reaction time of 2-4h, and a stirring speed of 150-200r / min.

6. The method for recycling waste polyamide 6 according to claim 1, characterized in that, In step S3, the diol is ethylene glycol or propylene glycol; the esterification catalyst is stannous octoate or tetrabutyl titanate; and the transesterification catalyst is tetrabutyl titanate or antimony trioxide.

7. The method for recycling waste polyamide 6 according to claim 1, characterized in that: In step S3, the mass ratio of the high-purity double-ended carboxyl polyamide 6 prepolymer, the diol, and the esterification catalyst is 100:(10-15):(0.1-0.3); the amount of the transesterification catalyst added is 0.05%-0.2% of the mass of the high-purity double-ended carboxyl polyamide 6 prepolymer.

8. The method for recycling waste polyamide 6 according to claim 1, characterized in that: In step S4, the light aging resistant additives are obtained by compounding UV-350 and UV-3346 in a mass ratio of (2-2.5):1, and the amount of light aging resistant additives added is 0.8%-1.5% of the total mass of the recycled polyamide 6 melt.

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