A method for recycling ultra-high molecular weight polyethylene fibers based on depolymerization with green solvents

Abstract

This invention discloses a method for recycling ultra-high molecular weight polyethylene (UHMWPE) fibers based on green solvent depolymerization, belonging to the field of polymer material recycling and regeneration technology. The method includes: preparing a eutectic solvent by mixing DL-menthol and thymol; cutting UHMWPE fiber waste into segments, washing with alkali, washing with water, and drying; heating and stirring the pretreated fibers and the eutectic solvent under nitrogen protection to induce swelling and unwinding; adding 0.1–1.0 wt% L-ascorbic acid (by weight of UHMWPE fibers) and heating the reaction until the system exhibits a uniform flow dynamic; recovering the eutectic solvent by vacuum flash evaporation; and obtaining recycled microparticles by underwater pelletizing the solvent-free UHMWPE melt. This invention utilizes the synergistic effect of a green eutectic solvent and L-ascorbic acid to efficiently depolymerize UHMWPE fibers under mild conditions. The solvent can be recycled, the process is short, there is no secondary pollution, and the resulting recycled microparticles are of high quality.

Description

Technical Field

[0001] This invention relates to the field of polymer material recycling and regeneration technology, specifically to a method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization. Background Technology

[0002] Ultra-high molecular weight polyethylene (UHMWPE) fiber is widely used in high-end fields such as bulletproof armor, marine cables, medical implants and sports equipment due to its extremely high specific strength, excellent wear resistance and good chemical inertness. According to statistics, the global annual output of UHMWPE fiber has exceeded 80,000 tons and continues to grow at an average annual rate of 12%. However, the recycling and reuse of post-consumer waste and production scraps of UHMWPE fiber has always been an industry problem. The core technical obstacles to UHMWPE fiber recycling are: (1) Ultra-high molecular weight gives the material extremely high melt viscosity, and it can hardly flow at conventional thermoplastic processing temperatures; (2) Fibrous UHMWPE has a supramolecular structure with highly oriented extended chains and alternating folded chain lamellar crystals. Simple melting will lead to severe thermal degradation of molecular chains, with a molecular weight loss of 30% to 60%; (3) Traditional solvent methods use high-boiling-point organic solvents such as decahydronaphthalene and mineral oil, which not only pose VOC emissions and environmental safety hazards, but also have high energy consumption and large residues in solvent recycling, making it difficult to guarantee the quality of recycled products.

[0003] Existing recycling technologies mainly include the following categories: Mechanical recycling: This involves multi-stage crushing followed by high-temperature molding or RAM extrusion. While simple, the extremely high melt viscosity of UHMWPE results in uneven product density and mechanical properties only 40%–60% of the raw material. Furthermore, prolonged high-temperature processing leads to severe oxidative degradation and discoloration. Solvent dissolution-reprecipitation: Using decahydronaphthalene, xylene, or mineral oil as solvents, UHMWPE is dissolved and then precipitated at 130–180 °C. This method achieves good molecular weight retention, but the high toxicity and boiling point of the solvents make recycling difficult (recovery rate is typically <85%), and requires large amounts of antioxidants. Supercritical fluid method: Using supercritical CO2 or supercritical propane as the medium. While environmentally friendly, this method requires extremely high operating pressures, large equipment investments, low penetration efficiency into UHMWPE fibers, and processing times of several hours. Pyrolysis: UHMWPE is thermally decomposed into low-molecular-weight products such as wax and fuel oil under an inert atmosphere. This method is essentially a downgraded recycling process, where the products cannot be reused in the manufacture of high-performance fibers, resulting in a significant reduction in the value of resource utilization.

[0004] In summary, the common problem with existing technologies is that they cannot achieve low-temperature, low-energy, and non-toxic fluidization processing while maintaining the ultra-high molecular weight of UHMWPE. The industry urgently needs a novel recycling technology that can selectively unravel the supramolecular structure of fibers, avoid main chain breakage and degradation, and utilize green and recyclable solvents. Summary of the Invention

[0005] The purpose of this invention is to provide a method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization, so as to solve the technical problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization includes the following steps: S1. Preparation of eutectic solvent: DL-menthol and thymol are mixed, heated and stirred to form eutectic solvent (DES). S2. UHMWPE fiber pretreatment: The UHMWPE fiber waste is sequentially cut into sections, soaked in alkaline solution to remove surface oil, washed with water and dried. S3. Swelling and unwinding reaction: The pretreated fiber and the eutectic solvent are put into a sealed reaction vessel and heated and stirred under nitrogen protection; S4. L-Ascorbic Acid Catalytic Depolymerization: 0.1-1.0 wt% L-ascorbic acid (by weight of UHMWPE fiber) is added to the reaction system in step S3, and the system is heated until it becomes a homogeneous flow state to obtain a mixture; S5. Recovering the eutectic solvent by vacuum flash evaporation: The mixture obtained in step S4 is subjected to vacuum flash evaporation under absolute pressure and temperature to recover the eutectic solvent; S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is pelletized underwater to obtain regenerated UHMWPE microparticles.

[0007] The technical principle of this invention is as follows: (1) Molecular design principle of eutectic solvent: DL-menthol acts as a hydrogen bond acceptor (HBA), and thymol acts as a hydrogen bond donor (HBD). The two form a eutectic system through intermolecular OH•••O hydrogen bond association, and the freezing point of the mixture is much lower than that of the two pure components. The DES system has the following characteristics: (a) Both components are food-grade additives, non-toxic and biodegradable; (b) The vapor pressure of DES is about 0.5 to 2 kPa at 100°C, which is suitable for efficient recovery by vacuum flash evaporation; (c) The phenolic hydroxyl group and benzene ring structure of thymol provide the necessary aryl π electron cloud density for "CH•••π" complexation.

[0008] (2) Selective untangling mechanism of "CH•••π" transient complexation: In UHMWPE fibers, the molecular chains in the straight-chain crystalline region are tightly arranged and highly crystalline, making it difficult for DES molecules to penetrate; while the surface layer and amorphous region of the folded chain lamellar crystals have a large free volume and chain segment mobility, allowing DES to preferentially penetrate these regions. The π electron cloud of the thymol benzene ring forms a directional "CH•••π" weak interaction with the CH bond of the -CH2 group on the UHMWPE chain. This interaction is similar to a molecular lubricant, reducing the sliding resistance between entangled chain segments and gradually untangling the physical entanglement points of the folded chain lamellar crystals. Since the energy of this complexation interaction is much lower than that of the covalent bond energy, it will not trigger homolytic or heterolytic cleavage of the main chain CC bond, thus ensuring a high molecular weight retention rate.

[0009] (3) Bifunctional catalytic mechanism of L-ascorbic acid: The enediol structure of L-ascorbic acid (vitamin C) has extremely strong reducing properties, which can rapidly donate hydrogen to carbon free radicals and peroxy free radicals, quenching them into stable non-free radical products and inhibiting the propagation of free radical chain degradation reactions. At the same time, the four hydroxyl groups on the L-ascorbic acid molecule can form multi-point hydrogen bond bridges with the hydroxyl groups of menthol in DES, enhancing the wetting and spreading ability of DES in the amorphous region of UHMWPE and increasing the swelling and unwinding rate. This dual function of protection and promotion allows L-ascorbic acid to produce significant effects at extremely low addition levels.

[0010] Preferably, in step S1, the molar ratio of DL-menthol to thymol is (1-3):1.

[0011] Preferably, in step S1, the heating and stirring temperature is 45–65°C, and the time is 20–40 min.

[0012] Preferably, in step S3, the mass ratio of the pretreated fiber to the eutectic solvent is 1:(3-8).

[0013] Preferably, in step S3, the heating and stirring reaction temperature is 100-110°C and the time is 40-50 min.

[0014] Preferably, in step S4, the amount of L-ascorbic acid added is 0.3 to 0.5 wt% of the mass of UHMWPE fiber.

[0015] Preferably, in step S4, the heating reaction temperature is 90–120°C and the heating reaction time is 20–50 min.

[0016] Preferably, in step S4, the L-ascorbic acid is added in the form of a eutectic solvent pre-dispersion, specifically by ultrasonically dispersing the L-ascorbic acid powder with the eutectic solvent at room temperature for 10-15 minutes until a uniform suspension is formed.

[0017] Preferably, in step S5, the absolute pressure of the reduced pressure flash evaporation is 1-3 kPa and the temperature is 110-130°C.

[0018] Preferably, in step S6, the cooling water temperature for underwater pelletizing is 20–25°C, and the die hole diameter is 0.8–1.5 mm.

[0019] Compared with the prior art, the beneficial effects of the present invention are: (1) The two components of the solvent DES, DL-menthol and thymol, are food-grade natural substances that are non-toxic, non-irritating, and biodegradable. This fundamentally avoids the occupational health risks and environmental emission pressures caused by the use of toxic organic solvents such as decahydronaphthalene and xylene in traditional recycling processes. The catalyst L-ascorbic acid is a bio-based vitamin C that is completely harmless. The entire process has no VOC emissions and no wastewater generation.

[0020] (2) The selective untangling mechanism achieved by the transient complexation of “CH•••π”, combined with the free radical inhibition protection of L-ascorbic acid, can completely preserve the covalent main chain while untangling the supramolecular physical entanglement of the fiber. The viscosity-average molecular weight retention rate of the recycled product is ≥95%, which is far superior to mechanical recycling method and traditional solvent method.

[0021] (3) The depolymerization reaction temperature is only 90-120℃ (far lower than the melting point of UHMWPE of about 136℃ and the traditional melt processing temperature of 200-250℃), the operating pressure is slightly positive pressure nitrogen protection, the equipment requirements are low, the energy consumption is reduced by more than 50% compared with the traditional method, and the safety is significantly improved.

[0022] (4) High-efficiency closed-loop solvent circulation: DES can be quickly recovered by vacuum flash evaporation, with a single recovery rate of ≥98%. After being recycled ≥10 times, its performance can be restored by vacuum distillation. The comprehensive utilization rate of solvent is >99.5%, which greatly reduces production costs.

[0023] (5) Excellent product quality: The MFI of recycled UHMWPE microparticles is 0.1~0.5g / 10min (190℃ / 21.6kg), which has good processing fluidity; the thermal decomposition temperature is ≥420℃, and the thermal stability is excellent; the residual DES is <200ppm, which meets the purity requirements of food contact grade and medical grade UHMWPE; it can be directly used for gel spinning to prepare high-strength fibers or injection molding to prepare wear-resistant profiles.

[0024] (6) Strong adaptability to continuous process: The underwater microsphere pelletizing process is mature and has high capacity. The resulting microspheres have regular shapes and narrow particle size distribution, which facilitates automated metering and subsequent processing and is suitable for industrial continuous production. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Example 1 A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization includes the following steps: S1. Preparation of eutectic solvent: 2 mol DL-menthol and 0.75 mol thymol were placed in a sealed stainless steel reactor and stirred at 300 r / min at 50℃ for 30 min to form a homogeneous and transparent eutectic liquid through hydrogen bonding. The mixture was then naturally cooled to room temperature to obtain the eutectic solvent DES.

[0027] S2. UHMWPE fiber pretreatment: Take 500g of UHMWPE fiber waste (viscosity average molecular weight 3.2 million g / mol), and cut the fiber into segments of 5-8mm in length using a rotary blade shredder; immerse the cut fiber segments in a 3wt% sodium carbonate aqueous solution at 50℃ for 15min to remove surface oil and impurities; rinse with deionized water until the pH of the washing solution is 7.0; dry in a 70℃ forced-air drying oven for 2h until the moisture content is 0.05wt%.

[0028] S3. Swelling and untangling reaction: 500g of pretreated fiber and 3000g of eutectic solvent obtained in step S1 were put into a 5L sealed pressure-resistant reactor with an anchor-type stirrer. Under the protection of nitrogen gas with a slight positive pressure of 0.05MPa, the temperature was increased to 105℃ at a rate of 3℃ / min, and the stirring was maintained at a stirring speed of 150r / min for 45min. This allowed the eutectic solvent molecules to penetrate into the amorphous region of the fiber and the surface defect region of the folded chain lamellar crystals. The aromatic ring π electron cloud of thymol formed a transient "CH•••π" non-covalent complex with the CH bond on the UHMWPE main chain, selectively untangling the physical entanglement points of the folded chain lamellar crystals of the fiber.

[0029] S4. L-Ascorbic Acid Catalytic Depolymerization: Maintaining the reaction system temperature of step S3 at 105℃, 2.3g of L-ascorbic acid powder and 16g of eutectic solvent were ultrasonically dispersed at room temperature for 13min to prepare a uniform suspension. The pre-dispersed solution was added to the reaction vessel and stirred at 100℃ and 150r / min for 30min until the system became a uniform flow state, thus obtaining a mixture.

[0030] S5. Reduced pressure flash evaporation to recover eutectic solvent: The mixture obtained in step S4 is transferred to a thin film evaporator and subjected to reduced pressure flash evaporation at an absolute pressure of 2 kPa and a temperature of 120°C. The evaporated eutectic solvent vapor is condensed and recovered at 20°C by a tube condenser.

[0031] S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is metered and conveyed to the die head of an underwater pelletizer at 145°C via a gear pump. After being extruded through a die with a diameter of 1.0 mm, the melt is instantly quenched and pelletized by high-speed circulating cooling water at 23°C to obtain recycled UHMWPE microparticles. Example 2 A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization includes the following steps: S1. Preparation of eutectic solvent: 1 mol DL-menthol and 0.75 mol thymol were placed in a sealed stainless steel reactor and stirred at 300 r / min at 50℃ for 30 min to form a homogeneous and transparent eutectic liquid through hydrogen bonding. The mixture was then naturally cooled to room temperature to obtain the eutectic solvent DES.

[0032] S2. UHMWPE fiber pretreatment: Take 500g of UHMWPE fiber waste (viscosity average molecular weight 3.2 million g / mol), and cut the fiber into segments of 5-8mm in length using a rotary blade shredder; immerse the cut fiber segments in a 3wt% sodium carbonate aqueous solution at 50℃ for 15min to remove surface oil and impurities; rinse with deionized water until the pH of the washing solution is 7.0; dry in a 70℃ forced-air drying oven for 2h until the moisture content is 0.05wt%.

[0033] S3. Swelling and untangling reaction: 500g of pretreated fiber and 2000g of eutectic solvent obtained in step S1 were put into a 5L sealed pressure-resistant reactor with an anchor-type stirrer. Under the protection of nitrogen gas with a slight positive pressure of 0.05MPa, the temperature was increased to 105℃ at a rate of 3℃ / min, and the stirring was maintained at a stirring speed of 150r / min for 45min. This allowed the eutectic solvent molecules to penetrate into the amorphous region of the fiber and the surface defect region of the folded chain lamellar crystals. The aromatic ring π electron cloud of thymol formed a transient "CH•••π" non-covalent complex with the CH bond on the UHMWPE main chain, selectively untangling the physical entanglement points of the folded chain lamellar crystals of the fiber.

[0034] S4. L-Ascorbic Acid Catalytic Depolymerization: Maintaining the reaction system temperature of step S3 at 105℃, 1.8g of L-ascorbic acid powder and 16g of eutectic solvent were ultrasonically dispersed at room temperature for 13min to prepare a uniform suspension. The pre-dispersed solution was added to the reaction vessel and stirred at 100℃ and 150r / min for 30min until the system became a uniform flow state, thus obtaining a mixture.

[0035] S5. Reduced pressure flash evaporation to recover eutectic solvent: The mixture obtained in step S4 is transferred to a thin film evaporator and subjected to reduced pressure flash evaporation at an absolute pressure of 2 kPa and a temperature of 120°C. The evaporated eutectic solvent vapor is condensed and recovered at 20°C by a tube condenser.

[0036] S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is metered and conveyed to the die head of an underwater pelletizer at 145°C via a gear pump. After being extruded through a die with a diameter of 1.0 mm, the melt is instantly quenched and pelletized by high-speed circulating cooling water at 23°C to obtain recycled UHMWPE microparticles. Example 3 A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization includes the following steps: S1. Preparation of eutectic solvent: 1.5 mol DL-menthol and 0.75 mol thymol were placed in a sealed stainless steel reactor and stirred at 300 r / min at 50 °C for 30 min to form a homogeneous and transparent eutectic liquid through hydrogen bonding. The mixture was then naturally cooled to room temperature to obtain the eutectic solvent DES.

[0037] S2. UHMWPE fiber pretreatment: Take 500g of UHMWPE fiber waste (viscosity average molecular weight 3.2 million g / mol), and cut the fiber into segments of 5-8mm in length using a rotary blade shredder; immerse the cut fiber segments in a 3wt% sodium carbonate aqueous solution at 50℃ for 15min to remove surface oil and impurities; rinse with deionized water until the pH of the washing solution is 7.0; dry in a 70℃ forced-air drying oven for 2h until the moisture content is 0.05wt%.

[0038] S3. Swelling and untangling reaction: 500g of pretreated fiber and 2500g of eutectic solvent obtained in step S1 were put into a 5L sealed pressure-resistant reactor with an anchor-type stirrer. Under the protection of nitrogen gas with a slight positive pressure of 0.05MPa, the temperature was increased to 105℃ at a rate of 3℃ / min, and the stirring was maintained at a stirring speed of 150r / min for 45min. This allowed the eutectic solvent molecules to penetrate into the amorphous region of the fiber and the surface defect region of the folded chain lamellar crystals. The aromatic ring π electron cloud of thymol formed a transient "CH•••π" non-covalent complex with the CH bond on the UHMWPE main chain, selectively untangling the physical entanglement points of the folded chain lamellar crystals of the fiber.

[0039] S4. L-Ascorbic Acid Catalytic Depolymerization: Maintaining the reaction system temperature of step S3 at 105℃, 2.0g of L-ascorbic acid powder and 16g of eutectic solvent were ultrasonically dispersed at room temperature for 13min to prepare a uniform suspension. The pre-dispersed solution was added to the reaction vessel and stirred at 100℃ and 150r / min for 30min until the system became a uniform flow state, thus obtaining a mixture.

[0040] S5. Reduced pressure flash evaporation to recover eutectic solvent: The mixture obtained in step S4 is transferred to a thin film evaporator and subjected to reduced pressure flash evaporation at an absolute pressure of 2 kPa and a temperature of 120°C. The evaporated eutectic solvent vapor is condensed and recovered at 20°C by a tube condenser.

[0041] S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is metered and conveyed to the die head of an underwater pelletizer at 145°C via a gear pump. After being extruded through a die with a diameter of 1.0 mm, the melt is instantly quenched and pelletized by high-speed circulating cooling water at 23°C to obtain recycled UHMWPE microparticles. Example 4 A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization includes the following steps: S1. Preparation of eutectic solvent: 2.25 mol DL-menthol and 0.75 mol thymol were placed in a sealed stainless steel reactor and stirred at 300 r / min at 65 °C for 40 min to form a homogeneous and transparent eutectic liquid through hydrogen bonding. The mixture was then naturally cooled to room temperature to obtain the eutectic solvent DES.

[0042] S2. UHMWPE fiber pretreatment: Take 500g of UHMWPE fiber waste (viscosity average molecular weight 3.2 million g / mol), and cut the fiber into segments of 5-8mm in length using a rotary blade shredder; immerse the cut fiber segments in a 3wt% sodium carbonate aqueous solution at 50℃ for 15min to remove surface oil and impurities; rinse with deionized water until the pH of the washing solution is 7.0; dry in a 70℃ forced-air drying oven for 2h until the moisture content is 0.05wt%.

[0043] S3. Swelling and untangling reaction: 500g of pretreated fiber and 4000g of eutectic solvent obtained in step S1 were put into a 5L sealed pressure-resistant reactor with an anchor-type stirrer. Under the protection of nitrogen gas with a slight positive pressure of 0.05MPa, the temperature was increased to 110℃ at a rate of 3℃ / min, and the stirring was maintained at a stirring speed of 150r / min for 50min. This allowed the eutectic solvent molecules to penetrate into the amorphous region of the fiber and the surface defect region of the folded chain lamellar crystals. The π electron cloud of the aromatic ring of thymol formed a transient "CH•••π" non-covalent complex with the CH bond on the UHMWPE main chain, selectively untangling the physical entanglement points of the folded chain lamellar crystals of the fiber.

[0044] S4. L-Ascorbic Acid Catalytic Depolymerization: Maintaining the reaction system temperature of step S3 at 105℃, 2.5g of L-ascorbic acid powder and 16g of eutectic solvent were ultrasonically dispersed at room temperature for 15min to prepare a uniform suspension. The pre-dispersed solution was added to the reaction vessel and stirred at 120℃ and 150r / min for 50min until the system became a uniform flow state, thus obtaining a mixture.

[0045] S5. Reduced pressure flash evaporation to recover eutectic solvent: The mixture obtained in step S4 is transferred to a thin film evaporator and subjected to reduced pressure flash evaporation at an absolute pressure of 3 kPa and a temperature of 130°C. The evaporated eutectic solvent vapor is condensed and recovered at 20°C by a tube condenser.

[0046] S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is metered and pumped to the die head of an underwater pelletizer at 145°C via a gear pump. After being extruded through a die orifice with a diameter of 1.5 mm, the melt is instantly quenched and pelletized by high-speed circulating cooling water at 25°C to obtain recycled UHMWPE microparticles. Example 5 A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization includes the following steps: S1. Preparation of eutectic solvent: 0.75 mol DL-menthol and 0.75 mol thymol were placed in a sealed stainless steel reactor and stirred at 300 r / min at 45 °C for 20 min to form a homogeneous and transparent eutectic liquid through hydrogen bonding. The mixture was then naturally cooled to room temperature to obtain the eutectic solvent DES.

[0047] S2. UHMWPE fiber pretreatment: Take 500g of UHMWPE fiber waste (viscosity average molecular weight 3.2 million g / mol), and cut the fiber into segments of 5-8mm in length using a rotary blade shredder; immerse the cut fiber segments in a 3wt% sodium carbonate aqueous solution at 50℃ for 15min to remove surface oil and impurities; rinse with deionized water until the pH of the washing solution is 7.0; dry in a 70℃ forced-air drying oven for 2h until the moisture content is 0.05wt%.

[0048] S3. Swelling and untangling reaction: 500g of pretreated fiber and 1500g of eutectic solvent obtained in step S1 were put into a 5L sealed pressure-resistant reactor with an anchor-type stirrer. Under the protection of nitrogen gas with a slight positive pressure of 0.05MPa, the temperature was increased to 100℃ at a rate of 3℃ / min, and the stirring was maintained at a stirring speed of 150r / min for 40min. This allowed the eutectic solvent molecules to penetrate into the amorphous region of the fiber and the surface defect region of the folded chain lamellar crystals. The π electron cloud of the aromatic ring of thymol formed a transient "CH•••π" non-covalent complex with the CH bond on the UHMWPE main chain, selectively untangling the physical entanglement points of the folded chain lamellar crystals of the fiber.

[0049] S4. L-Ascorbic Acid Catalytic Depolymerization: Maintaining the reaction system temperature of step S3 at 105℃, 1.5g of L-ascorbic acid powder and 16g of eutectic solvent were ultrasonically dispersed at room temperature for 10min to prepare a uniform suspension. The pre-dispersed solution was added to the reaction vessel and stirred at 90℃ and 150r / min for 20min until the system became a uniform flow state, thus obtaining a mixture.

[0050] S5. Reduced pressure flash evaporation to recover eutectic solvent: The mixture obtained in step S4 is transferred to a thin film evaporator and subjected to reduced pressure flash evaporation at an absolute pressure of 1 kPa and a temperature of 110°C. The evaporated eutectic solvent vapor is condensed and recovered at 20°C by a tube condenser.

[0051] S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is metered and conveyed to the die head of an underwater pelletizer at 145°C via a gear pump. After being extruded through a die orifice with a diameter of 0.8 mm, the melt is instantly quenched and pelletized by high-speed circulating cooling water at 20°C to obtain recycled UHMWPE microparticles. Comparative Example 1 (without DES, conventional hot melt method): The difference from Example 1 is that 500g of pretreated UHMWPE fiber was directly fed into a twin-screw extruder and melt-extruded at 230°C and 100r / min. The die head pressure was as high as 45MPa, barely producing material. After water cooling and pelletizing, the material was tested.

[0052] Comparative Example 2 (using decahydronaphthalene instead of DES): The difference from Example 1 is that decahydronaphthalene is used as a solvent to replace DES in the polyethylene fiber recycling process.

[0053] Comparative Example 3 (without L-ascorbic acid): The difference from Example 1 is that L-ascorbic acid is not added during the polyethylene fiber recycling process.

[0054] Performance testing: (1) Determination of viscosity-average molecular weight and molecular weight retention rate: The intrinsic viscosity of the dilute solution of regenerated UHMWPE microparticles was determined by the Ubbelohde capillary viscosity method with decahydronaphthalene as solvent in a constant temperature water bath at 135℃. The viscosity-average molecular weight Mv was calculated by the Mark-Houwink equation. Molecular weight retention rate = (viscosity-average molecular weight of regenerated microparticles / viscosity-average molecular weight of raw material fiber 3.2 million g / mol) × 100%.

[0055] (2) Melt flow index (MFI) determination: The melt flow rate meter was used to determine the melt flow index of the regenerated UHMWPE microparticles at 190℃ and 21.6kg load. The unit was g / 10min. Each sample was measured in parallel 3 times and the arithmetic mean was taken.

[0056] (3) Determination of thermal decomposition temperature (Td,5%): Thermogravimetric analyzer (TGA) was used to heat the sample from room temperature to 700℃ at a rate of 10℃ / min under nitrogen atmosphere (flow rate 50mL / min). The temperature corresponding to the loss of 5% of sample mass was recorded as Td,5%, which was used to evaluate the thermal stability of the regenerated microparticles.

[0057] (4) Differential Scanning Calorimetry (DSC) Analysis: A differential scanning calorimeter was used to perform a second temperature scan under a nitrogen atmosphere, with the temperature increasing from 30℃ to 200℃ at a rate of 10℃ / min. The melting peak temperature (Tm) was recorded. The crystallinity Xc was calculated by the ratio of the melting enthalpy ΔHm to the theoretical melting enthalpy ΔH0 (293J / g) of fully crystalline polyethylene, i.e., Xc = / ΔH0×100%.

[0058] (5) Determination of residual solvent content: Headspace-gas chromatography (HS-GC) was used. Approximately 0.5 g of regenerated UHMWPE microparticles were weighed and placed in a 20 mL headspace vial and sealed. After headspace equilibration at 120 °C for 30 min, the sample was injected. The residual content of DES components (menthol and thymol) was detected by FID detector and quantified by external standard method. The results were expressed as ppm (μg / g).

[0059] (6) Determination of eutectic solvent recovery rate: The mass of the eutectic solvent recovered by vacuum flash condensation in step S5 is accurately weighed (accuracy 0.1g). The ratio of the mass of the eutectic solvent recovered by vacuum flash condensation in step S3 to the initial mass of the eutectic solvent added is the recovery rate. DES recovery rate = (mass of recovered DES / mass of added DES) × 100%.

[0060] (7) Particle yield determination: The mass of the regenerated UHMWPE particles obtained after underwater pelletizing, centrifugation and dehydration and vacuum drying at 60°C for 2 hours in step S6 is accurately weighed by mass weighing method. The ratio of the mass of the regenerated UHMWPE particles obtained after underwater pelletizing, centrifugation and dehydration and vacuum drying at 60°C for 2 hours to the dry basis mass of the pretreated fiber in step S2 is the particle yield. Particle yield = (mass of regenerated particles / mass of pretreated fiber) × 100%.

[0061] (8) Particle size and sphericity determination: The particle size distribution of regenerated UHMWPE particles was determined by laser particle size analyzer, and the median particle size D50 and particle size span (D90-D10) / D50 were recorded. At the same time, 50 particles were randomly selected and their major and minor axis dimensions were measured under a stereomicroscope. The sphericity ψ = minor axis / major axis was calculated and the average value was taken.

[0062] Table 1:

[0063] Note: Comparative Example 1 uses the conventional hot melting method without solvent, so the residual solvent and solvent recovery rate are indicated by "—"; in Comparative Example 2, the residual solvent is the amount of decahydronaphthalene residue.

[0064] As can be seen from Table 1: (1) Regarding the molecular weight retention rate, the molecular weight retention rates of Examples 1 to 5 were all above 93%, with Example 3 (DL-menthol to thymol molar ratio 2:1, fiber to DES mass ratio 1:5) showing the best performance at 96.3%. In contrast, the molecular weight retention rate of Comparative Example 1 (conventional hot melt method) was only 61.9%, indicating that the high-temperature melting processing at 230℃ caused severe thermal oxidative degradation and mechanochemical chain scission of the UHMWPE main chain; Comparative Example 2 (decahydronaphthalene replacing DES) had a retention rate of 85.9%, indicating that decahydronaphthalene, due to the lack of aromatic ring π electron cloud, could not form CH•••π transient complexes, resulting in poor unentanglement selectivity, and the high-temperature distillation recovery process exacerbated the thermal degradation; Comparative Example 3 (without L-ascorbic acid) had a retention rate of 87.5%, proving that in the absence of free radical scavenger protection, the free radicals generated by trace dissolved oxygen and shear stress during processing would trigger significant oxidative chain scission reactions. This invention utilizes the synergistic effect of selective unwrapping of DES (CH•••π) and free radical inhibition by L-ascorbic acid to control molecular weight loss to within 7%, which is significantly better than existing technologies.

[0065] (2) Regarding thermal stability, the Td,5% of Examples 1-5 were all ≥420℃, the DSC melting point was all ≥135.3℃, and the crystallinity was all ≥63.5%, which were close to the thermal properties of the raw material UHMWPE, indicating that the polymer chain integrity and crystalline structure of the recycled microparticles were well maintained. The Td,5% of Comparative Example 1 was only 395℃, the melting point was 133.5℃, and the crystallinity was 55.2%, which was significantly lower than that of the other examples, confirming that high-temperature thermal degradation not only led to a decrease in molecular weight, but also destroyed the ordered crystalline structure and thermal stability of the polymer.

[0066] (3) Regarding solvent recovery and residue, the DES recovery rate of Examples 1-5 was ≥97.8%, and the residual solvent content was <200ppm, meeting the purity requirements of food contact grade and medical grade UHMWPE materials. In Comparative Example 2, the decahydronaphthalene recovery rate was only 82.3%, and the residual solvent was as high as 1850ppm. This is because the boiling point of decahydronaphthalene (187℃) is much higher than that of the DES component, making it difficult to recover by atmospheric distillation. Higher temperatures and longer times are required, and the high temperature, in turn, exacerbates the thermal degradation of UHMWPE, forming a vicious cycle.

[0067] (4) Regarding particle morphology, the sphericity of the particles obtained in Examples 1-5 is ≥0.90, and the particle size distribution is concentrated, which is beneficial for accurate metering and uniform feeding in subsequent gel spinning or injection molding processes. Comparative Example 1 uses conventional extrusion pelletizing. Due to the extremely high melt viscosity of UHMWPE, the output is unstable, resulting in irregular particle shapes (sphericity only 0.65) and large particle size (D50=3.20mm), which will seriously affect the uniformity of subsequent processing and product quality. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the essence and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for recycling ultra-high molecular weight polyethylene fibers based on green solvent depolymerization, characterized in that, Includes the following steps: S1. Preparation of eutectic solvent: DL-menthol and thymol are mixed, heated and stirred to form a eutectic solvent; S2. UHMWPE fiber pretreatment: The UHMWPE fiber waste is sequentially cut into sections, soaked in alkaline solution to remove surface oil, washed with water and dried. S3. Swelling and unwinding reaction: The pretreated fiber and the eutectic solvent are put into a sealed reaction vessel and heated and stirred under nitrogen protection; S4. L-Ascorbic Acid Catalytic Depolymerization: 0.1-1.0 wt% L-ascorbic acid (by weight of UHMWPE fiber) is added to the reaction system in step S3, and the system is heated until it becomes a homogeneous flow state to obtain a mixture; S5. Recovering the eutectic solvent by vacuum flash evaporation: The mixture obtained in step S4 is subjected to vacuum flash evaporation under absolute pressure and temperature to recover the eutectic solvent; S6. Underwater microsphere pelletizing: The UHMWPE melt after desolventizing in step S5 is pelletized underwater to obtain regenerated UHMWPE microparticles.

2. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S1, the molar ratio of DL-menthol to thymol is (1-3):

1.

3. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S1, the heating and stirring temperature is 45-65℃, and the time is 20-40 minutes.

4. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S3, the mass ratio of the pretreated fiber to the eutectic solvent is 1:(3-8).

5. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S3, the heating and stirring reaction temperature is 100-110℃, and the time is 40-50 min.

6. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S4, the amount of L-ascorbic acid added is 0.3 to 0.5 wt% of the mass of UHMWPE fiber.

7. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S4, the heating reaction temperature is 90–120°C, and the heating reaction time is 20–50 min.

8. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S4, the L-ascorbic acid is added in the form of a eutectic solvent pre-dispersion, specifically by ultrasonically dispersing the L-ascorbic acid powder with the eutectic solvent at room temperature for 10-15 minutes until a uniform suspension is formed.

9. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S5, the absolute pressure of the reduced pressure flash evaporation is 1-3 kPa and the temperature is 110-130°C.

10. The method for recycling ultra-high molecular weight polyethylene fiber based on green solvent depolymerization according to claim 1, characterized in that, In step S6, the cooling water temperature for underwater pelletizing is 20-25℃, and the die hole diameter is 0.8-1.5mm.

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