Method for recycling ultra-high molecular weight polyethylene fibers by low-temperature cryogenic shearing and pulverization

Abstract

This invention discloses a method for recovering ultra-high molecular weight polyethylene (UHMWPE) fibers through cryogenic shearing and pulverization, relating to the field of material recycling technology. The method includes: cutting UHMWPE fiber scraps into short segments, immersing them in an ionic liquid for ultrasonic treatment to swell and peel off surface adhesives and resins, washing and drying to obtain clean fibers; continuously feeding the clean fibers into a liquid nitrogen spiral cryogenic tunnel, instantly freezing them below the glass transition temperature to obtain brittle fiber material; then feeding it into a supersonic shear pulverizer equipped with a tungsten carbide tilting cutter head, where a high-speed rotating cutter head with a front angle of 10-15° performs impact shearing, causing the fibers to break into thin flake-like micro-powder; finally, through negative pressure turbine classification, particles with a diameter ≥150μm are returned for further pulverization, while micro-powder <150μm is conveyed by cryogenic pulsed pneumatic conveying to a homogenization chamber for stirring and homogenization, thus obtaining UHMWPE micro-powder. This method has high recovery efficiency and produces high-quality micro-powder, enabling resource utilization.

Description

Technical Field

[0001] This invention relates to the field of polymer material recycling technology, specifically a method for recycling ultra-high molecular weight polyethylene fiber by cryogenic shearing and pulverization. Background Technology

[0002] Ultra-high molecular weight polyethylene (UHMWPE, typically with a molecular weight ≥ 1.5 million g / mol) fiber is a high-performance specialty fiber with extremely high specific strength, specific modulus, abrasion resistance, and chemical stability. It is widely used in bulletproof protective equipment, ropes and cables, medical implant materials, and engineering composite materials. During the production and processing of UHMWPE fiber products, a large amount of scrap, waste, and defective products are inevitably generated. Statistics show that the generation rate of UHMWPE fiber scrap in the fiber composite materials industry is as high as 15-25%. These waste materials usually contain matrix materials such as epoxy resin and polyurethane adhesives, and are currently mostly disposed of through landfill or incineration. Due to its extremely high molecular weight and highly crystalline structure, UHMWPE material has extremely strong solvent resistance and resistance to chemical degradation, making conventional dissolution, regeneration, and chemical recycling methods almost ineffective. At the same time, the extremely high melt viscosity of UHMWPE (due to severe molecular chain entanglement, its flowability is extremely poor) prevents it from being reprocessed using conventional thermoplastic melt extrusion or injection molding processes. Therefore, crushing UHMWPE waste into fine powder and then molding it through cold pressing sintering or solid-phase extrusion is currently recognized as the most industrially feasible recycling route. However, UHMWPE fibers have extremely high toughness and impact resistance at room temperature, with a tensile strength exceeding 3.0 GPa and an elongation at break of 3–4%. When subjected to mechanical forces at room temperature, the fibers' enormous elastic deformation capacity makes them extremely difficult to cut or crush, resulting in severe fiber pulling, entanglement, and springback, rendering conventional crushing equipment ineffective.

[0003] In the existing technology, low-temperature pulverization has been applied to the pulverization of some thermoplastic polymers, but there are the following prominent problems: (1) Insufficient cooling: The traditional liquid nitrogen spraying and soaking method is not efficient enough for cooling high-crystallinity UHMWPE fibers. The core area of ​​the fiber often cannot be cooled to below the glass transition temperature (about -120℃) in a very short time, resulting in the fiber still maintaining a tough state inside, and the pulverization efficiency is extremely low. (2) Incompatible pulverization mechanism: Traditional low-temperature pulverization mostly uses hammer mills or ball mills, which mainly rely on impact and grinding for pulverization. For UHMWPE fibers, even at the embrittlement temperature, the fibers still have a certain interlayer slippage ability. The impact-grinding mechanism is difficult to effectively cut the highly oriented molecular chains, which easily produces irregular large particles. The fineness and uniformity of the pulverization are difficult to meet the requirements of subsequent molding. (3) Degradation caused by thermal effect: The frictional heat in the high-speed pulverization process can cause the local temperature to rise instantly by 80 to 150℃ or more (hot spot effect), which leads to severe thermal oxidation degradation and molecular chain breakage of UHMWPE. The molecular weight retention rate is usually less than 80%, and the core performance advantages of UHMWPE materials are lost. (4) Powder agglomeration problem: During the temperature recovery process after crushing, the surface molecular chain activity of UHMWPE micro powder is restored, and the micro powder particles are very easy to re-adhere and agglomerate, resulting in a significant increase in effective particle size, which seriously affects the uniformity of subsequent molding processes and product performance. (5) Difficulty in removing surface adhesive layer: If the epoxy resin and other adhesives attached to the surface of fiber scraps are not thoroughly removed, impurity phases will be introduced into the subsequent crushing and molding process, reducing the purity and reprocessing performance of the recycled powder.

[0004] In summary, existing technologies urgently need a systematic solution that can achieve efficient degumming and purification, deep embrittlement and freezing, fine shearing and crushing, and low-temperature closed-loop conveying and homogenization of UHMWPE fiber scraps, while ensuring molecular weight retention and powder fineness, and achieve high-quality, low-energy recycling and reuse of UHMWPE fiber waste. Summary of the Invention

[0005] The purpose of this invention is to provide a method for recovering ultra-high molecular weight polyethylene fibers by cryogenic shearing and pulverization, thereby solving the technical problems mentioned in the background section.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for recycling ultra-high molecular weight polyethylene fibers by cryogenic shear pulverization includes the following steps: (1) After cutting the scraps of ultra-high molecular weight polyethylene fiber into short fiber segments, immerse them in an ionic liquid and perform ultrasonic treatment at 80-90°C to swell and peel off the adhesive and resin matrix attached to the fiber surface. After treatment, wash and dry to obtain clean fiber material. (2) The clean fiber material obtained in step (1) is continuously fed into a liquid nitrogen spiral cryogenic tunnel and instantly frozen at -90 to -150°C to reduce the fiber temperature to below its glass transition temperature and obtain brittle fiber material. (3) The brittle fiber material obtained in step (2) is directly fed into a supersonic shearing pulverizer equipped with a tungsten steel tilting cutter disc for shearing and pulverizing. Liquid nitrogen cooling gas is introduced into the pulverizing chamber of the supersonic shearing pulverizer to maintain the temperature below -80℃. The pulverizing chamber is in a nitrogen protective atmosphere. The tilting cutter disc rotates at high speed and the cutting edge front angle of the cutter disc is 10-15°. The fiber is brittlely broken into thin sheet-like micro powder under instantaneous impact shearing action. (4) The thin-film micro powder is sucked into the grading chamber by the negative pressure turbine built into the pulverizer for particle size classification. Particles with a particle size ≥150μm are automatically returned to the pulverizer for further shearing. Qualified micro powder with a particle size <150μm is transported to the homogenization chamber by low-temperature pulse pneumatic conveying and then discharged after stirring and homogenization to obtain ultra-high molecular weight polyethylene micro powder.

[0007] Preferably, in step (1), the ionic liquid is selected from one or more of 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium acetate, and 1-ethyl-3-methylimidazolium acetate.

[0008] Preferably, in step (1), the liquid-to-solid ratio of the ionic liquid to the short fiber is (8-15):1 mL / g.

[0009] Preferably, in step (2), the spiral conveying rate of the liquid nitrogen spiral cryogenic tunnel is 0.5 to 3.0 m / min, and the tunnel length is 3 to 8 m.

[0010] Preferably, in step (2), the liquid nitrogen spray flow rate is 50-200 L / h.

[0011] Preferably, in step (3), the rotational speed of the tilting cutter head is 8000 to 18000 r / min.

[0012] Preferably, in step (3), the blade material in the cutter head is tungsten carbide-based cemented carbide with a hardness HRA≥89.

[0013] Preferably, in step (4), the working negative pressure of the negative pressure turbine is -2.0 to -5.0 kPa, and the turbine speed is 3000 to 6000 r / min.

[0014] Preferably, in step (4), the conveying medium of the low-temperature pulse pneumatic conveying is low-temperature nitrogen, and the temperature of the conveying pipeline is maintained below -60°C.

[0015] Compared with the prior art, the beneficial effects of the present invention are: (1) The whole process is a low temperature closed design with high molecular weight retention: from cryogenic freezing, shearing and crushing, graded conveying to homogenization and collection, the whole process maintains a low temperature nitrogen protective atmosphere, which effectively avoids thermal oxidation and mechanical degradation of UHMWPE during processing. The molecular weight retention is ≥96%, which is much higher than <80% of the existing low temperature crushing technology.

[0016] (2) Supersonic shearing mechanism, high and uniform fineness: The innovative use of tungsten steel tilting disc supersonic shearing method to replace the traditional impact-grinding method utilizes concentrated shear stress to precisely break brittle fibers, obtaining thin flake-shaped micro powder with a particle size of 2-8μm and a narrow particle size distribution (D90 / D10≤5), which is far superior to the 50-200μm particle size level of the traditional method.

[0017] (3) Uniform and thorough freezing in spiral deep cryogenic tunnel: The Archimedes spiral structure combined with multi-point liquid nitrogen atomization spray ensures that the fiber achieves uniform deep cryogenic embrittlement from the surface to the core during continuous conveying, and the liquid nitrogen utilization rate is increased to more than 80%, effectively eliminating the phenomenon of cold shell and hot core, and ensuring the consistency of subsequent crushing.

[0018] (4) Ionic liquid ultrasonic adhesive removal is green and efficient: Ionic liquid is used to replace traditional organic solvents. Combined with ultrasonic cavitation effect, the adhesive removal rate is ≥98% under mild conditions. The ionic liquid can be recycled (recovery rate ≥95%), with no VOCs emission, making it green and environmentally friendly.

[0019] (5) Negative pressure classification and pulse conveying eliminate agglomeration: Built-in turbine negative pressure online classification ensures the qualified rate of powder particle size. The embolism flow mode of low temperature pulse pneumatic conveying and the dual protection of low temperature nitrogen cooling fundamentally solve the problem of re-agglomeration of ultrafine UHMWPE powder.

[0020] (6) High powder activity, can be directly molded: The obtained micro powder retains the original ultra-high molecular weight characteristics and high crystallinity of UHMWPE, with a large specific surface area (1.5~6.0m²). 2 ( / g), with high surface activity, can be directly used for cold pressing sintering or solid-phase extrusion molding without the need for processing aids, simplifying the reprocessing process.

[0021] (7) Low energy consumption and high efficiency: The specific energy consumption of supersonic shearing is only 30-50% of that of traditional impact crushing. Combined with efficient liquid nitrogen utilization and continuous operation process, the overall energy consumption is significantly reduced, and it has good industrial economics. Detailed Implementation

[0022] 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.

[0023] Example 1 A method for recycling ultra-high molecular weight polyethylene fibers by cryogenic shear pulverization includes the following steps: (1) Take 100g of UHMWPE fiber scrap (derived from bulletproof vest cutting waste, fiber molecular weight 3.8 million g / mol, surface contains epoxy resin adhesive), and cut it into short pieces about 20mm in length. Add the short pieces to 1300mL of 1-butyl-3-methylimidazolium chloride (already heated to liquid), heat to 85℃, and turn on the ultrasonic generator (frequency 35kHz, power density 1.5W / cm³). 2 The fiber material was ultrasonically treated for 30 minutes. After treatment, the fiber material was removed, washed three times with deionized water, and dried in a vacuum drying oven at 80℃ for 2 hours to obtain clean fiber material.

[0024] (2) The clean fiber material is continuously fed into the liquid nitrogen spiral cryogenic tunnel (tunnel length 6m, spiral blade diameter 300mm, pitch 200mm) through a sealed spiral feeder. The spiral conveying speed is set to 2.5m / min. There are 8 liquid nitrogen atomizing nozzles in the tunnel, the liquid nitrogen spray flow rate is 150L / h, and the temperature in the tunnel is stably controlled at -120℃. The material temperature at the outlet is measured by an infrared thermometer and is -125℃, which is lower than the glass transition temperature of UHMWPE (about -120℃), and the fiber is completely brittle.

[0025] (3) The brittle fiber material is directly fed into the supersonic shear shredder through a sealed and insulated conveying pipe (the outer layer of the pipe is covered with a 50mm thick polyurethane insulation layer, and low-temperature nitrogen is introduced into the pipe). Cutter head parameters: cutter head diameter 400mm, number of blades 8, WC-Co cemented carbide (Co content 8wt%, HRA 90), cutting edge rake angle 13°, blade tilt angle 2°, cutter head rotation speed 15000r / min. Liquid nitrogen cooling gas is introduced into the shredding chamber, and the temperature inside the chamber is maintained at -95℃ under nitrogen atmosphere protection.

[0026] (4) The pulverizer's built-in negative pressure turbine (working negative pressure -4kPa, turbine speed 5000r / min) draws the pulverized product into the classification chamber. Particles with a diameter ≥150μm are returned to the pulverizing chamber for further shearing. Qualified micro powder is conveyed to the homogenization chamber through a low-temperature pulse pneumatic conveying system (conveying medium: -70℃ low-temperature nitrogen, pulse frequency 20 times / min). The stirring rate in the homogenization chamber is 50r / min, the temperature is -45℃, and the material is discharged after homogenization for 20min to obtain UHMWPE micro powder product.

[0027] Example 2 A method for recycling ultra-high molecular weight polyethylene fibers by cryogenic shear pulverization includes the following steps: (1) Take 100g of UHMWPE fiber scrap (derived from bulletproof vest cutting waste, fiber molecular weight 3.8 million g / mol, surface contains epoxy resin adhesive), and cut it into short pieces about 20mm in length. Add the short pieces to 900mL of 1-butyl-3-methylimidazolium acetate, heat to 85℃, and turn on the ultrasonic generator (frequency 30kHz, power density 0.8W / cm). 2 The fiber material was ultrasonically treated for 30 minutes. After treatment, the fiber material was removed, washed three times with deionized water, and dried in a vacuum drying oven at 80℃ for 2 hours to obtain clean fiber material.

[0028] (2) The clean fiber material is continuously fed into the liquid nitrogen spiral cryogenic tunnel (tunnel length 6m, spiral blade diameter 300mm, pitch 200mm) through a sealed spiral feeder. The spiral conveying speed is set to 1.0m / min. There are 8 liquid nitrogen atomizing nozzles in the tunnel, the liquid nitrogen spray flow rate is 80L / h, and the temperature in the tunnel is stably controlled at -120℃. The material temperature at the outlet is measured by an infrared thermometer and is -125℃, which is lower than the glass transition temperature of UHMWPE (about -120℃), and the fiber is completely brittle.

[0029] (3) The brittle fiber material is directly fed into the supersonic shear shredder through a sealed and insulated conveying pipe (the outer layer of the pipe is covered with a 50mm thick polyurethane insulation layer, and low-temperature nitrogen is introduced into the pipe). Cutter head parameters: cutter head diameter 400mm, number of blades 8, WC-Co cemented carbide (Co content 8wt%, HRA 90), cutting edge rake angle 12°, blade tilt angle 2°, cutter head rotation speed 10000r / min. Liquid nitrogen cooling gas is introduced into the shredding chamber, and the temperature inside the chamber is maintained at -95℃ under nitrogen atmosphere protection.

[0030] (4) The pulverizer's built-in negative pressure turbine (working negative pressure -3kPa, turbine speed 4000r / min) draws the pulverized product into the classification chamber. Particles with a diameter ≥150μm are returned to the pulverizing chamber for further shearing. Qualified micro powder is conveyed to the homogenization chamber through a low-temperature pulse pneumatic conveying system (conveying medium: -70℃ low-temperature nitrogen, pulse frequency 20 times / min). The stirring rate in the homogenization chamber is 50r / min, the temperature is -45℃, and the material is discharged after homogenization for 20min to obtain UHMWPE micro powder product.

[0031] Example 3 A method for recycling ultra-high molecular weight polyethylene fibers by cryogenic shear pulverization includes the following steps: (1) Take 100g of UHMWPE fiber scrap (derived from bulletproof vest cutting waste, fiber molecular weight 3.8 million g / mol, surface contains epoxy resin adhesive), and cut it into short pieces about 20mm in length. Add the short pieces to 1100mL of 1-ethyl-3-methylimidazolium acetate, heat to 85℃, and turn on the ultrasonic generator (frequency 30kHz, power density 1.2W / cm). 2 The fiber material was ultrasonically treated for 30 minutes. After treatment, the fiber material was removed, washed three times with deionized water, and dried in a vacuum drying oven at 80℃ for 2 hours to obtain clean fiber material.

[0032] (2) The clean fiber material is continuously fed into the liquid nitrogen spiral cryogenic tunnel (tunnel length 6m, spiral blade diameter 300mm, pitch 200mm) through a sealed spiral feeder. The spiral conveying speed is set to 2.0m / min. There are 8 liquid nitrogen atomizing nozzles in the tunnel, the liquid nitrogen spray flow rate is 120L / h, and the temperature in the tunnel is stably controlled at -120℃. The material temperature at the outlet is measured by an infrared thermometer and is -125℃, which is lower than the glass transition temperature of UHMWPE (about -120℃), and the fiber is completely brittle.

[0033] (3) The brittle fiber material is directly fed into the supersonic shear shredder through a sealed and insulated conveying pipe (the outer layer of the pipe is covered with a 50mm thick polyurethane insulation layer, and low-temperature nitrogen is introduced into the pipe). Cutter head parameters: cutter head diameter 400mm, number of blades 8, WC-Co cemented carbide (Co content 8wt%, HRA 90), cutting edge rake angle 12°, blade tilt angle 2°, cutter head rotation speed 12000r / min. Liquid nitrogen cooling gas is introduced into the shredding chamber, and the temperature inside the chamber is maintained at -95℃ under nitrogen atmosphere protection.

[0034] (4) The pulverizer's built-in negative pressure turbine (working negative pressure -3kPa, turbine speed 4500r / min) draws the pulverized product into the classification chamber. Particles with a diameter ≥150μm are returned to the pulverizing chamber for further shearing. Qualified micro powder is conveyed to the homogenization chamber through a low-temperature pulse pneumatic conveying system (conveying medium: -70℃ low-temperature nitrogen, pulse frequency 20 times / min). The stirring rate in the homogenization chamber is 50r / min, the temperature is -45℃, and the material is discharged after homogenization for 20min to obtain UHMWPE micro powder product.

[0035] Example 4 A method for recycling ultra-high molecular weight polyethylene fibers by cryogenic shear pulverization includes the following steps: (1) Take 100g of UHMWPE fiber scrap (derived from bulletproof vest cutting waste, fiber molecular weight 3.8 million g / mol, surface contains epoxy resin adhesive), and cut it into short pieces about 20mm in length. Add the short pieces to 1500mL of 1-butyl-3-methylimidazolium acetate, heat to 90℃, and turn on the ultrasonic generator (frequency 40kHz, power density 2.0W / cm³). 2 The fiber material was ultrasonically treated for 30 minutes. After treatment, the fiber material was removed, washed three times with deionized water, and dried in a vacuum drying oven at 80℃ for 2 hours to obtain clean fiber material.

[0036] (2) The clean fiber material is continuously fed into the liquid nitrogen spiral cryogenic tunnel (tunnel length 8m, spiral blade diameter 300mm, pitch 200mm) through a sealed spiral feeder. The spiral conveying speed is set to 3.0m / min. There are 8 liquid nitrogen atomizing nozzles in the tunnel, the liquid nitrogen spray flow rate is 200L / h, and the temperature in the tunnel is stably controlled at -150℃. The material temperature at the outlet is measured by an infrared thermometer and is -125℃, which is lower than the glass transition temperature of UHMWPE (about -120℃), and the fiber is completely brittle.

[0037] (3) The brittle fiber material is directly fed into the supersonic shear shredder through a sealed and insulated conveying pipe (the outer layer of the pipe is covered with a 50mm thick polyurethane insulation layer, and low-temperature nitrogen is introduced into the pipe). Cutter head parameters: cutter head diameter 400mm, number of blades 8, WC-Co cemented carbide (Co content 8wt%, HRA 90), cutting edge rake angle 15°, blade tilt angle 2°, cutter head rotation speed 18000r / min. Liquid nitrogen cooling gas is introduced into the shredding chamber, and the temperature inside the chamber is maintained at -95℃ under nitrogen atmosphere protection.

[0038] (4) The pulverizer has a built-in negative pressure turbine (working negative pressure -5kPa, turbine speed 6000r / min) to suck the pulverized product into the classification chamber. Particles with a diameter ≥150μm are returned to the pulverizing chamber for further shearing. Qualified micro powder is conveyed to the homogenization chamber through a low-temperature pulse pneumatic conveying system (conveying medium: -70℃ low-temperature nitrogen, pulse frequency 20 times / min). The stirring rate in the homogenization chamber is 50r / min, the temperature is -45℃, and the material is discharged after homogenization for 20min to obtain UHMWPE micro powder product.

[0039] Example 5 A method for recycling ultra-high molecular weight polyethylene fibers by cryogenic shear pulverization includes the following steps: (1) Take 100g of UHMWPE fiber scrap (derived from bulletproof vest cutting waste, fiber molecular weight 3.8 million g / mol, surface contains epoxy resin adhesive), and cut it into short pieces about 20mm in length. Add the short pieces to 800mL of 1-butyl-3-methylimidazolium acetate, heat to 80℃, and turn on the ultrasonic generator (frequency 20kHz, power density 0.5W / cm³). 2 The fiber material was ultrasonically treated for 30 minutes. After treatment, the fiber material was removed, washed three times with deionized water, and dried in a vacuum drying oven at 80℃ for 2 hours to obtain clean fiber material.

[0040] (2) The clean fiber material is continuously fed into the liquid nitrogen spiral cryogenic tunnel (tunnel length 3m, spiral blade diameter 300mm, pitch 200mm) through a sealed spiral feeder. The spiral conveying speed is set to 0.5m / min. There are 8 liquid nitrogen atomizing nozzles in the tunnel, the liquid nitrogen spray flow rate is 50L / h, and the temperature in the tunnel is stably controlled at -90℃. The material temperature at the outlet is measured by an infrared thermometer and is -125℃, which is lower than the glass transition temperature of UHMWPE (about -120℃), and the fiber is completely brittle.

[0041] (3) The brittle fiber material is directly fed into the supersonic shear shredder through a sealed and insulated conveying pipe (the outer layer of the pipe is covered with a 50mm thick polyurethane insulation layer, and low-temperature nitrogen is introduced into the pipe). Cutter head parameters: cutter head diameter 400mm, number of blades 8, WC-Co cemented carbide (Co content 8wt%, HRA 90), cutting edge rake angle 10°, blade tilt angle 2°, cutter head rotation speed 8000r / min. Liquid nitrogen cooling gas is introduced into the shredding chamber, and the temperature inside the chamber is maintained at -95℃ under nitrogen atmosphere protection.

[0042] (4) The pulverizer's built-in negative pressure turbine (working negative pressure -2kPa, turbine speed 3000r / min) draws the pulverized product into the classification chamber. Particles with a diameter ≥150μm are returned to the pulverizing chamber for further shearing. Qualified micro powder is conveyed to the homogenization chamber through a low-temperature pulse pneumatic conveying system (conveying medium: -70℃ low-temperature nitrogen, pulse frequency 20 times / min). The stirring rate in the homogenization chamber is 50r / min, the temperature is -45℃, and the material is discharged after homogenization for 20min to obtain UHMWPE micro powder product.

[0043] Comparative Example 1 (Traditional organic solvent degumming + liquid nitrogen immersion + hammer mill) 100g of UHMWPE fiber scrap (same as in Example 1, derived from bulletproof vest cutting waste, fiber molecular weight 3.8 million g / mol, surface containing epoxy resin adhesive) was taken and cut into short segments approximately 20mm in length. The short segments were immersed in acetone at room temperature for 60 minutes to remove the adhesive. After removal, they were washed three times with deionized water and dried in an 80℃ vacuum drying oven for 2 hours. The de-adhesive fiber material was then completely immersed in liquid nitrogen (-196℃) for 5 minutes. Immediately after removal, it was fed into a hammer mill (hammer speed 3000 r / min) for pulverization. Pulverization was carried out under normal atmospheric conditions without nitrogen protection. The pulverized product was classified using a vibrating sieve, collecting powder with a particle size <150μm.

[0044] Comparative Example 2 (room temperature high-speed shearing and pulverizing, without cryogenic treatment) Take 100g of UHMWPE fiber scrap (same as in Example 1) and process it using the same ionic liquid ultrasonic degumming process as in step (1) of Example 1 to obtain clean fiber material. The clean fiber material is not subjected to any low-temperature freezing treatment and is directly fed into an ultrasonic shear pulverizer at room temperature (25℃) for pulverization (the blade parameters are exactly the same as in step (3) of Example 1: blade diameter 400mm, number of blades 8, WC-Co hard alloy, cutting edge rake angle 12°, blade rotation speed 12000r / min). There is no liquid nitrogen cooling or nitrogen protection in the pulverization chamber.

[0045] Comparative Example 3 (Ionic liquid degumming + liquid nitrogen spiral cryogenic + ball milling) 100g of UHMWPE fiber scrap (same as in Example 1) was processed using the same ionic liquid ultrasonic degumming process as step (1) in Example 1 to obtain clean fiber material. The clean fiber material was then cryogenically treated using the same liquid nitrogen spiral cryogenic tunnel process as step (2) in Example 1 to obtain brittle fiber material. The brittle fiber material was not processed by an ultrasonic shear mill, but was rapidly transferred to a low-temperature planetary ball mill for pulverization. The ball milling parameters were: 500mL ball mill jar volume, 20 stainless steel grinding balls (φ10mm balls, 50 φ5mm balls), ball-to-material ratio 10:1, rotation speed 400r / min, and pulverization time 30min. No liquid nitrogen was added for cooling during the ball milling process, and no nitrogen protection was introduced. The initial temperature was approximately -90℃, and the temperature inside the jar rose to approximately -15℃ at the end of pulverization. The pulverized product was graded using a vibrating sieve, and powder with a particle size <150μm was collected.

[0046] Comparative Example 4 (ionic liquid debinding + liquid nitrogen spiral cryogenic + supersonic shear pulverization, but the pulverization chamber was not protected by cryogenic nitrogen gas) 100g of UHMWPE fiber scrap (same as in Example 1) was processed using the same ionic liquid ultrasonic degumming process as in step (1) of Example 1 to obtain clean fiber material. The fiber material was then cryogenically treated using the same liquid nitrogen spiral cryogenic tunnel process as in step (2) of Example 1 to obtain brittle fiber material. The brittle fiber material was fed into a supersonic shear pulverizer for pulverization. The cutter head parameters were exactly the same as in step (3) of Example 1 (cutter head diameter 400mm, number of blades 8, WC-Co hard alloy, cutting edge rake angle 12°, cutter head speed 12000r / min), but liquid nitrogen cooling gas was not introduced into the pulverization chamber, and a low-temperature nitrogen protective atmosphere was not maintained; pulverization was carried out under normal atmospheric conditions. The pulverized product was classified using a vibrating screen, and powder with a particle size <150μm was collected and pneumatically conveyed to the collection bin at room temperature.

[0047] Comparative Example 5 (ionic liquid degumming + liquid nitrogen spiral cryogenics + supersonic shear pulverization + ambient temperature conveying, negative pressure-free grading and low-temperature pulse conveying) 100g of UHMWPE fiber scrap (same as in Example 1) was processed using the same ionic liquid ultrasonic degumming process as step (1) in Example 1 to obtain clean fiber material. The fiber material was then cryogenically treated using the same liquid nitrogen spiral cryogenic tunnel process as step (2) in Example 1 to obtain brittle fiber material. The brittle fiber material was fed into a supersonic shear pulverizer for pulverization. The cutter head parameters and pulverization chamber conditions were exactly the same as in step (3) of Example 1 (including liquid nitrogen cooling gas protection) to complete the shear pulverization. The pulverized product was not graded by a built-in negative pressure turbine, nor was a low-temperature pulse pneumatic conveying system used. Instead, it was directly collected from the pulverizer outlet by natural discharge and graded by a vibrating screen (passing through a 150μm sieve) under ambient atmospheric conditions. Qualified powder was spirally conveyed to the collection bin at ambient temperature without homogenization treatment.

[0048] Comparative Example 6 (direct cryogenic shearing and pulverization without removing adhesive) Take 100g of UHMWPE fiber scrap (same as in Example 1, with epoxy resin adhesive on the surface), cut it into short pieces about 20mm in length, and do not perform any adhesive removal treatment. Directly subject the adhesive-coated fiber material to liquid nitrogen spiral cryogenic freezing, supersonic shear pulverization, negative pressure classification and low temperature pulse conveying homogenization in the same manner as steps (2), (3) and (4) of Example 1 to obtain a powder product.

[0049] Performance testing: 1. Median Particle Size (D50) and Particle Size Distribution (D90 / D10): The particle size distribution of the micronized powder was determined using a laser particle size analyzer (Malvern Mastersizer 3000). Approximately 0.5 g of sample was dispersed in anhydrous ethanol and ultrasonically dispersed for 5 min to break up any possible soft agglomerates. The sample was then added to the wet dispersion unit of the laser particle size analyzer for measurement. The light-blocking degree was controlled at 10–20%, and the stirring speed was 2000 r / min. Each sample was measured in triplicate, and the average value was taken. The instrument automatically outputs the median particle size D50, D10, and D90 values. The particle size distribution width index D90 / D10 was calculated; a smaller ratio indicates a narrower particle size distribution and more uniform particle size.

[0050] 2. Viscosity-average molecular weight and molecular weight retention rate: The viscosity-average molecular weight of UHMWPE micropowder was determined using the Ubbelohde viscometer method. Using decalin as a solvent, the micropowder sample was dissolved in a constant-temperature oil bath at 135℃ to prepare dilute solutions of different concentrations (concentration gradients of 0.01, 0.02, 0.03, and 0.05 g / dL). The outflow time of each concentration solution was measured using an Ubbelohde viscometer, and the specific viscosity and relative viscosity were calculated. Extrapolation to zero concentration yielded the intrinsic viscosity [η]. The viscosity-average molecular weight Mη was then calculated using the Mark-Houwink equation. The molecular weight retention rate was calculated as follows: Molecular weight retention rate (%) = (Viscosity-average molecular weight of recycled micropowder / Viscosity-average molecular weight of original fiber) × 100%. This indicator reflects the integrity of the UHMWPE molecular chain during the recycling process.

[0051] 3. Bulk Density: Allow the dried powder sample to fall freely through a 25mm diameter funnel into a 100mL standard measuring cup. After the powder naturally accumulates and overflows the cup, use a scraper to horizontally scrape off the excess powder along the rim. Weigh the powder in the measuring cup and calculate the bulk density ρ = m / V (g / cm³). 3 Each sample was measured in triplicate, and the average value was taken. Bulk density reflects the morphology, particle size, and degree of agglomeration of micro powder particles. Thin-film micro powders usually have lower bulk density due to the large gaps between particles.

[0052] 4. Specific Surface Area (BET Method): A fully automated specific surface area and pore size analyzer was used, with high-purity nitrogen as the adsorbate, and nitrogen adsorption-desorption experiments were conducted at liquid nitrogen temperature (-196℃). Before testing, the sample was degassed under vacuum at 80℃ for 12 hours to remove surface-adsorbed water and impurity gases. Approximately 0.5–1.0 g of sample was placed in a test tube, and adsorption data were collected at multiple points within a relative pressure range of P / P0 = 0.05–0.30. The specific surface area was calculated according to the BET multilayer adsorption theory. This indicator directly reflects the fineness and surface roughness of the microparticles. A larger specific surface area indicates finer particles and a more irregular surface, which is beneficial for particle fusion during subsequent cold pressing and sintering.

[0053] 5. Residual Adhesive Content: The residual adhesive content in the micronized powder was determined using thermogravimetric analysis (TGA). Approximately 10 mg of sample was placed in a platinum crucible of the thermogravimetric analyzer, and the temperature was increased from room temperature to 700℃ at a rate of 10℃ / min under a nitrogen atmosphere (flow rate 50 mL / min). The mass-temperature curve (TG curve) and the differential thermogravimetric curve (DTG curve) were recorded. The thermal decomposition of UHMWPE mainly occurred in the range of 430–500℃, while the thermal decomposition temperature of epoxy resin adhesives was mainly concentrated in the range of 300–420℃. The residual adhesive content was calculated based on the mass loss ratio between the two decomposition temperature ranges. Each sample was measured twice in parallel, and the average value was taken.

[0054] 6. Particle Morphology: The morphology of the micropowder particles was observed using field emission scanning electron microscopy. A small amount of dried micropowder sample was uniformly dispersed on a conductive tape and sputtered with gold (approximately 10 nm thick) to enhance conductivity and secondary electron signal. Secondary electron imaging images were acquired at different magnifications (500×, 2000×, 5000×, 10000×) under accelerating voltages of 3–5 kV and working distances of 8–10 mm. The overall shape of the particles (flake-like / block-like / irregular), surface smoothness, edge regularity, and the presence of residual fibers or agglomerates were observed and recorded.

[0055] 7. Tensile Strength of Cold-Pressed Sintered Products: The tensile strength of standard samples prepared from recycled UHMWPE micropowder using a cold-pressing sintering process was determined. The specific method was as follows: Sufficient micropowder was placed into a metal mold and pre-pressed into a blank at 10 MPa pressure at room temperature. Then, it was sintered at 200℃ and 5 MPa pressure for 2 hours. After cooling to room temperature in the furnace, it was demolded to obtain sintered plates. The plates were processed into dumbbell-shaped 1A samples (gauge length 80 mm, width 10 mm, thickness 4 mm). Tensile tests were conducted using a universal testing machine (such as an Instron 5967) at (23±2)℃ and a tensile rate of 50 mm / min. The maximum tensile stress was recorded as the tensile strength. At least 5 samples were tested in each group, and the average value was taken.

[0056] Table 1: 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 recovering ultra-high molecular weight polyethylene fibers by cryogenic shearing and pulverization, characterized in that, Includes the following steps: (1) After cutting the scraps of ultra-high molecular weight polyethylene fiber into short fiber segments, they are immersed in an ionic liquid. The liquid-to-solid ratio of the ionic liquid to the short fiber segments is (8-15):1mL / g. The fiber segments are subjected to ultrasonic treatment at 80-90℃ to swell and peel off the adhesive and resin matrix attached to the fiber surface. After treatment, the fiber is washed with water and dried to obtain clean fiber material. The ionic liquid is selected from one or more of 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium acetate, and 1-ethyl-3-methylimidazolium acetate. (2) The clean fiber material obtained in step (1) is continuously fed into a liquid nitrogen spiral cryogenic tunnel and instantly frozen at -90 to -150°C to reduce the fiber temperature to below its glass transition temperature and obtain brittle fiber material. (3) The brittle fiber material obtained in step (2) is directly fed into a supersonic shearing pulverizer equipped with a tungsten steel tilting cutter disc for shearing and pulverizing. Liquid nitrogen cooling gas is introduced into the pulverizing chamber of the supersonic shearing pulverizer to maintain the temperature below -80℃. The pulverizing chamber is in a nitrogen protective atmosphere. The tilting cutter disc rotates at high speed and the cutting edge front angle of the cutter disc is 10-15°. The fiber is brittlely broken into thin sheet-like micro powder under instantaneous impact shearing action. (4) The thin-film micro powder is sucked into the grading chamber for particle size classification by the built-in negative pressure turbine of the pulverizer. The working negative pressure of the negative pressure turbine is -2.0 to -5.0 kPa, and the turbine speed is 3000 to 6000 r / min. Particles with a particle size ≥150 μm are automatically returned to the pulverizer for further shearing. Qualified micro powder with a particle size <150 μm is transported to the homogenization chamber for collection by low-temperature pulse pneumatic conveying. After being stirred and homogenized, the material is discharged to obtain ultra-high molecular weight polyethylene micro powder.

2. The method for recovering ultra-high molecular weight polyethylene fiber by cryogenic shearing and pulverization according to claim 1, characterized in that, In step (2), the spiral conveying rate of the liquid nitrogen spiral cryogenic tunnel is 0.5 to 3.0 m / min, and the tunnel length is 3 to 8 m.

3. The method for recovering ultra-high molecular weight polyethylene fiber by cryogenic shearing and pulverization according to claim 1, characterized in that, In step (2), the liquid nitrogen spray flow rate is 50-200 L / h.

4. The method for recovering ultra-high molecular weight polyethylene fiber by cryogenic shearing and pulverization according to claim 1, characterized in that, In step (3), the rotational speed of the tilting cutter head is 8000 to 18000 r / min.

5. The method for recovering ultra-high molecular weight polyethylene fiber by cryogenic shearing and pulverization according to claim 1, characterized in that, In step (3), the blade material in the tilting cutter head is tungsten carbide-based cemented carbide with a hardness HRA≥89.

6. The method for recovering ultra-high molecular weight polyethylene fiber by cryogenic shearing and pulverization according to claim 1, characterized in that, In step (4), the conveying medium for the cryogenic pulse pneumatic conveying is cryogenic nitrogen.

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