A method for preparing a recycled EVA footwear midsole

Abstract

This invention relates to the field of EVA plastic recycling technology, and discloses a method for preparing recycled EVA shoe midsoles. By weight, the raw materials include EVA waste, antioxidants, aromatic disulfides, plasticizers, nano-microwave sensitizers, stearic acid, foaming agents, foaming aids, crosslinking agents, and crosslinking accelerators. The EVA waste is EVA scraps or waste products without glue. The EVA waste, antioxidants, fillers, and naphthenic oil are mixed and then sequentially subjected to intensive mixing, extrusion, calendering, microwave reaction, and secondary extrusion. Subsequently, foaming agents, foaming aids, crosslinking agents, and crosslinking accelerators are added, followed by secondary intensive mixing, rolling, and then cutting or granulation to obtain recycled EVA material. This material is then added to a midsole mold and heated to foam, yielding the recycled EVA shoe midsole. De-crosslinking and recycling of crosslinked EVA results in lower recycling costs and retains most of the properties of the rubber components in the EVA, solving the problem of high costs associated with pyrolysis recycling, which previously made the recycling of EVA waste economically unprofitable.

Description

Technical Field

[0001] This invention relates to the field of EVA plastic recycling technology, and in particular to a method for preparing recycled EVA shoe midsoles. Background Technology

[0002] As a core functional component of footwear, the midsole is currently primarily based on ethylene-vinyl acetate copolymer (EVA). To meet the stringent high-performance requirements of various sports activities, the EVA system is typically modified with multiple rubber components. For example, natural rubber is used to increase toughness, EPDM rubber to improve weather resistance and aging resistance, and butadiene rubber or nitrile rubber is introduced to enhance abrasion and oil resistance. This multi-component, highly polar polymer system forms the basis of footwear material production, but it also introduces significant chemical complexity to its subsequent recycling and reuse.

[0003] In the footwear material production process, approximately 20% to 30% of scrap materials are inevitably generated. Because these scrap materials have undergone deep chemical cross-linking (vulcanization) and physical foaming processes, they form a stable three-dimensional network structure, causing their physical form to change from thermoplastic to thermosetting, completely losing their plastic characteristics that allow for heating, melting, and remanufacturing. This "spatially locked" molecular network prevents waste EVA midsoles from being directly reused as raw materials in the production line without special chemical treatment, becoming a bottleneck in the greening process of the footwear material industry.

[0004] Currently, the conventional treatment method for such cross-linked waste is mostly thermal pyrolysis. However, pyrolysis processes have drawbacks such as extremely high energy consumption and difficulty in precisely controlling process parameters, often leading to disordered breakage of the polymer backbone. This not only results in low purity and poor sensory properties of the recycled products, but also a significant decrease in mechanical properties compared to virgin materials, making their recycling value extremely low. Because the cost of recycling has long exceeded the market value of recycled materials, companies lack the economic incentive and practical feasibility for recycling. Therefore, developing a low-cost decrosslinking technology that preserves the integrity of the molecular backbone has become an urgent need for the industry to achieve a high-value closed-loop recycling system. Summary of the Invention

[0005] To address the aforementioned shortcomings, the present invention aims to propose a method for preparing recycled EVA shoe midsoles, thereby solving the problem that existing technologies must employ thermal pyrolysis even for EVA waste materials without adhesive, resulting in high recycling costs.

[0006] To achieve this objective, the present invention adopts the following technical solution: A method for preparing a recycled EVA shoe midsole, comprising, by weight, 100 parts of EVA waste, 0.3-0.5 parts of antioxidant, 2-3.5 parts of aromatic disulfide, 2-4 parts of plasticizer, 0.3-0.8 parts of nano-microwave sensitizer, 0.6-1 parts of stearic acid, 3-5 parts of foaming agent, 0.7-1.2 parts of foaming aid, 0.6-1.3 parts of crosslinking agent, and 0.02-0.05 parts of crosslinking accelerator; EVA waste refers to cross-linked foamed EVA scraps or waste products that are discarded during the production of EVA midsoles for footwear. The steps include: Add all the EVA waste, antioxidants, fillers and naphthenic oil to the first internal mixer in proportion, at a temperature of 80~105℃, mix for 5~8 minutes, and then send it to the first screw extruder. In the first screw extruder, the temperature is 115~130℃, and after mixing, the mixture is extruded to the calender. The material is pressed into a thickness of 3-5 mm in a calender and then transferred to a microwave reactor. In the microwave reactor, the temperature is first continuously irradiated to 150~155℃, and then pulse irradiation is used to control the temperature at 155~160℃, and then it is transmitted to the second screw extruder. In the second screw extruder, the temperature is 110~140℃. After cooling and mixing, the mixture is extruded into the second internal mixer. Add foaming agent, foaming aid, crosslinking agent and crosslinking accelerator to the second internal mixer, mix at 80~100℃ for 2~8 minutes, and send the mixture to the roller press to roll into a large plate. The large plate is then cut or granulated to obtain recycled EVA material. Recycled EVA material is added to the midsole mold and heated to foam, thus producing recycled EVA footwear midsoles.

[0007] Preferably, the aromatic disulfide is an alkylphenol disulfide or a compound of it and a diphenyl disulfide.

[0008] Preferably, the microwave sensitizer includes an electrical loss material and a dielectric loss material, wherein the ratio of electrical loss material to dielectric loss material is 1:1~10, the electrical loss material is nano-carbon black or graphene, and the dielectric loss material is nano-silicon carbide or aluminum-doped zinc oxide.

[0009] Preferably, before internal mixing, there is also a waste crushing step and a premixing step. The waste crushing step involves crushing the EVA waste to 5-10 mesh before feeding the crushed EVA waste into the first internal mixer. The premixing step is as follows: First, the naphthenic oil, stearic acid, antioxidant and aromatic disulfide are heated to 75~85℃ and stirred until well mixed. Then, the nano microwave sensitizer is added and dispersed evenly. Finally, the evenly dispersed material is fed into the first internal mixer.

[0010] Preferably, the viscosity of the naphthenic oil is 16~22 mm. 2 / s.

[0011] Preferably, the irradiation process of the microwave reactor is as follows: the microwave reactor frequency is 2.45 GHz, first irradiation is carried out continuously at 400~800W for 35~60s, followed by pulse irradiation at 600~1000W, with a single pulse duration of 25~50s, the ratio of on time to off time is 1:2~5, and the cycle is repeated 8~20 times.

[0012] Preferably, the temperatures of each section of the first screw extruder from the feed end to the discharge end are set to 115~125℃, 130~135℃, and 125~130℃, respectively.

[0013] Preferably, the process also includes an exhaust step. After the material is irradiated by microwaves, it is cooled and vacuum-exhausted in the second screw extruder. The cooling rate is 15~25℃ / min, and the vacuum degree of the vacuum exhaust is -0.08~-0.095MPa. The temperatures of each section of the second screw extruder from the feed end to the discharge end are set to 130~140℃, 120~125℃, 115~120℃, and 110~115℃, respectively, and vacuum degassing is performed at the front end of the fourth section at 110~115℃.

[0014] Preferably, the crosslinking agent is dicumyl peroxide and sulfur, with the ratio of dicumyl peroxide to sulfur being 1:4~20, and the crosslinking accelerator is dibenzothiazole disulfide.

[0015] Preferably, the foaming agent is azodicarbonamide, and the foaming aids are nano zinc oxide and urea, with the ratio of nano zinc oxide to urea being 2~3:1.

[0016] The technical solution provided by this invention may include the following beneficial effects: 1. De-crosslinking and recycling of crosslinked EVA is a more cost-effective method than pyrolysis recycling, and it retains most of the properties of the rubber components in EVA. This solves the problem that the high cost of pyrolysis recycling makes the recycling of EVA waste uneconomical.

[0017] 2. By employing a gradient process of pre-swelling, high-shear, thin-sheet microwave pulse decrosslinking, and low-temperature reinforcing crosslinking, the problem of significant performance degradation in EVA waste during recycling is solved. Utilizing the synergy of microwave sensitizers and microwave technology, precise disassembly of the crosslinked network is achieved within the range of 155~160℃. While restoring the plasticity of EVA material, over 70% of the main properties of virgin EVA material, such as resilience, compression, and abrasion resistance, are retained. Production can be initiated by adding foaming agents, crosslinking agents, and related additives, resulting in significant cost advantages and environmental benefits, making the recycling of EVA materials truly feasible.

[0018] 3. Alkylphenol disulfide and diphenyl disulfide combined with naphthenic oil is an effective combination for treating various rubber blends, compatible with both polar and non-polar rubber components in waste materials. When diphenyl disulfide is used alone as an aromatic disulfide, it can easily lead to a strong odor in the product due to reaction residues.

[0019] 4. By adjusting the ratio of electrically depleting materials and dielectric materials, a composite microwave sensitization network is constructed using electrically depleting materials such as graphene and nano-carbon black, and dielectric materials such as nano-silicon carbide and aluminum-doped zinc oxide. The extremely fast conductivity loss response of nano-carbon black or graphene enables instantaneous temperature rise. Combined with the excellent microwave penetration capability and polarization loss effect of nano-silicon carbide or nano-aluminum-doped zinc oxide, an absorbing network covering the surface to the depths is constructed, ensuring synchronous temperature rise between the center and surface of the 3-5 mm thick sheet, fundamentally solving the technical problem of uneven microwave heating.

[0020] 5. The waste crushing and premixing steps synergistically improve the mixing and swelling efficiency of the internal mixing process. By mixing easily mixable naphthenic oil, stearic acid, antioxidants and aromatic disulfides, and then mixing and dispersing them with a nano-microwave catalyst, the self-aggregation of nano-components is effectively suppressed, and a physical coating layer is imparted to them.

[0021] 6. Select a viscosity of 16~22mm. 2 This grade of naphthenic oil, with its good fluidity, achieves deep swelling of cross-linked waste materials during the internal mixing stage, synergistically allowing the de-crosslinking agent to effectively diffuse within the cross-linked network, thereby precisely locating and weakening sulfur bonds. The appropriate viscosity ensures efficient mixing of materials under strong shear while effectively preventing increased processing resistance due to excessively high viscosity after melting. Simultaneously, this grade of naphthenic oil has a higher flash point, typically greater than 210℃, significantly reducing the risk of flashover during processing.

[0022] 7. First, irradiate continuously at 400W for 20 seconds. The low-power irradiation at 400W allows energy to slowly penetrate into the particles, avoiding local overheating and scorching of highly polar components such as nitrile rubber. Then, irradiate continuously at 800W for 30 seconds to rapidly raise the overall temperature to 150~155℃ to initiate the decrosslinking reaction. Finally, perform 8~20 cycles of pulse irradiation to control the temperature within the optimal decrosslinking reaction temperature range of 155~160℃, ensuring that the crosslinking network has sufficient time to fully open.

[0023] 8. By installing an indirect water-cooled sleeve and a vacuum degassing device in the second screw extruder, a precise transition from high-temperature decrosslinking to low-temperature stable feeding is achieved. Rapid cooling prevents the decrosslinking reaction from continuing and causing monomer residue, while also preventing excessively rapid cooling from causing a sudden and significant increase in melt viscosity, resulting in excessive screw torque or material overflow from the venting port. Vacuum degassing ensures the removal of thiol-like odor molecules generated by the decrosslinking reaction, low-volatile components in the naphthenic oil, and the small amount of acetic acid released by EVA upon heating.

[0024] Temperature control is achieved through a gradient cooling process from 140°C to 115°C. At this temperature, the viscosity of the EVA melt is moderate, sufficient to separate the powder and ensure uniform mixing of the added materials. This results in recycled EVA compound with excellent stability and superior secondary foaming and crosslinking properties. Subsequently, efficient vacuum degassing is performed at the front end of the fourth section to remove volatile small molecules generated by the decrosslinking reaction, significantly reducing the odor of the finished product and ensuring the density of the EVA melt. Detailed Implementation

[0025] To facilitate understanding of the present invention, a more complete description is provided below. The present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0026] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the specification of this invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0028] A method for preparing a recycled EVA shoe midsole, comprising, by weight, 100 parts of EVA waste, 0.3-0.5 parts of antioxidant, 2-3.5 parts of aromatic disulfide, 2-4 parts of plasticizer, 0.3-0.8 parts of nano-microwave sensitizer, 0.6-1 parts of stearic acid, 3-5 parts of foaming agent, 0.7-1.2 parts of foaming aid, 0.6-1.3 parts of crosslinking agent, and 0.02-0.05 parts of crosslinking accelerator; EVA waste refers to cross-linked foamed EVA scraps or waste products that are discarded during the production of EVA midsoles for footwear. The steps include: Add all the EVA waste, antioxidants, fillers and naphthenic oil to the first internal mixer in proportion, at a temperature of 80~105℃, mix for 5~8 minutes, and then send it to the first screw extruder. In the first screw extruder, the temperature is 115~130℃, and after mixing, the mixture is extruded to the calender. The material is pressed into a thickness of 3-5 mm in a calender and then transferred to a microwave reactor. In the microwave reactor, the temperature is first continuously irradiated to 150~155℃, and then pulse irradiation is used to control the temperature at 155~160℃, and then it is transmitted to the second screw extruder. In the second screw extruder, the temperature is 110~140℃. After cooling and mixing, the mixture is extruded into the second internal mixer. Add foaming agent, foaming aid, crosslinking agent and crosslinking accelerator to the second internal mixer, mix at 80~100℃ for 2~8 minutes, and send the mixture to the roller press to roll into a large plate. The large plate is then cut or granulated to obtain recycled EVA material. Recycled EVA material is added to the midsole mold and heated to foam, thus producing recycled EVA footwear midsoles.

[0029] In the production of footwear midsoles, the amount of waste material that does not contain glue accounts for 60-70% of the total waste material after cross-linking. Its composition is relatively stable. After de-cross-linking treatment, it can be directly recycled in the production of midsoles, which has important economic and environmental value.

[0030] De-crosslinking and recycling of crosslinked EVA is a more cost-effective method than pyrolysis recycling, and it retains most of the properties of the rubber components in EVA. This solves the problem that the high cost of pyrolysis recycling makes the recycling of EVA waste economically unprofitable.

[0031] Aromatic disulfides, acting as de-crosslinking agents, break down under microwave hot spots. The resulting sulfur free radicals efficiently attack the SS bonds of rubber and the weak crosslinking points of EVA, capturing hydrogen from the EVA crosslinking points, thus destroying the network and reforming a thermoplastic blended EVA compound.

[0032] Naphthenic oil, as a carrier for plasticizers and decrosslinking agents, exhibits excellent compatibility with EVA and its rubber components. Its light color and low volatility at high temperatures allow for rapid swelling of waste materials and provide overall lubrication and balance, enabling the decrosslinking agent to precisely locate sulfur bonds. Furthermore, naphthenic oil can encapsulate microwave sensitizers, preventing localized dry burning and ignition within the EVA that could lead to charring black spots, thus improving yield and the safety of microwave processes. With a flash point greater than 170℃, the entire production process is integrated with a high-sensitivity infrared thermometer and a closed-loop feedback system to ensure the material melt temperature remains below 160℃ in real-time, and the overall process temperature is controlled to not exceed 160℃. This prevents a significant degradation in the performance of recycled EVA materials and ensures safe processing.

[0033] Nanoscale microwave sensitizers ensure that microwave energy is evenly distributed among different components, forming uniformly distributed microwave hot spots.

[0034] The long carbon chains of stearic acid exhibit strong polarity, allowing them to melt and coat the surface of nanoparticles during the intensive mixing and heating process, thereby improving the dispersion efficiency of the microwave sensitizer in the EVA melt. The polar, long carbon chains of stearic acid melt and coat the nanoparticles during intensive mixing and heating, improving sensitizer dispersion while their weakly acidic environment effectively regulates the kinetic balance of the decrosslinking reaction, inhibiting early-stage localized overheating and coking.

[0035] The first internal mixing process utilizes the heat generated by the friction between the materials to raise the system temperature to 80-100℃. At this point, the naphthenic oil penetrates the surface of the particles, causing them to expand slightly. Temperature control is employed to prevent localized overheating, melting, and clumping that could lead to blockages. After internal mixing, the materials are further melted and homogenized using a first screw extruder.

[0036] Continuous irradiation in the early stages of microwave irradiation utilizes the efficient microwave absorption properties of the microwave sensitizer to rapidly heat the sheet, breaking through the softening point of EVA and reaching the decrosslinking temperature of 150°C. This triggers the desulfurization reaction, causing aromatic disulfides to break down under the action of the microwave hot spot. The generated sulfur free radicals efficiently attack the SS bonds of the rubber and the weak crosslinking points of EVA, capturing hydrogen from the EVA crosslinking points, thus disrupting the network and reforming a thermoplastic blended EVA compound. Subsequent pulse irradiation allows for thermal diffusion time, while controlling the temperature at the optimal decrosslinking temperature of 155-160°C to improve reaction efficiency. This avoids excessive degradation of the molecular chains due to temperatures exceeding 160°C and prevents localized overheating and charring caused by uneven absorption capabilities of the components during prolonged continuous irradiation.

[0037] Finally, the material is cooled in the second screw extruder, and foaming agent, foaming aid, crosslinking agent and crosslinking accelerator are added in the second mixing. It is then rolled into a large plate and cut or granulated to obtain recycled EVA material. After the recycled EVA material is put into the midsole mold, it is foamed according to the conventional process to obtain the midsole.

[0038] This invention addresses the significant performance degradation of EVA waste during recycling through a gradient process involving pre-swelling, strong shearing, thin-sheet microwave pulse decrosslinking, and low-temperature reinforcing crosslinking. Utilizing the synergy of microwave sensitizers and microwave technology, precise disassembly of the crosslinked network is achieved within the 155-160℃ range. While restoring the plasticity of EVA material, it retains over 70% of the main properties of virgin EVA, such as resilience, compression, and abrasion resistance. Production can be initiated simply by adding foaming agents, crosslinking agents, and related additives, resulting in significant cost advantages and environmental benefits, making the recycling of EVA materials truly feasible.

[0039] Antioxidants construct a comprehensive thermo-oxidative protection barrier within the system. Particularly during the microwave decrosslinking stage, they can instantly capture free radicals generated by microwave hot spots and sulfur bond breakage, effectively inhibiting the thermo-oxidative degradation of naphthenic oil and EVA molecular chains. This addresses the issues of yellowing, physical property degradation, and scorching that easily occur in recycled materials. Through synergistic effects with nano-aluminum-doped zinc oxide, the material maintains a molecular weight distribution and color stability similar to virgin EVA even after undergoing high-temperature decrosslinking at 160℃, laying the foundation for subsequent high-quality foaming.

[0040] Preferably, the antioxidant is a combination of 1010 or 1076 and 168, where 1010 / 1076:168 = 1:1~2.

[0041] 1010 and 1076 have good thermal stability, providing long-term antioxidant protection for materials. 168 belongs to the phosphite antioxidant class, and its core advantage lies in its ability to decompose harmful hydroperoxides into harmless substances. In the high-energy environment of microwave pulses, the synergistic effect of 168 with 1010 or 1076 greatly extends the processing window of materials and prevents materials from being oxidized during processing.

[0042] Preferably, it also includes 0.5 to 5 parts of other functional adjustment components, which are added in the second internal mixer to adjust the processing properties of the recycled EVA compound or the properties of the resulting midsole.

[0043] Add 1-3 parts of ultrafine active aluminum silicate or nano light calcium as a reinforcing filler to improve the tear strength of the midsole.

[0044] Adding 0.5 to 1.5 parts of polyethylene wax improves fluidity and adjusts the processing properties of recycled EVA compounds.

[0045] Adding 0.5 to 1.5 parts of monoglyceride improves the surface smoothness of the midsole and prevents orange peel texture.

[0046] Adding 2-5 parts of polyolefin elastomer reduces the shrinkage rate of the midsole.

[0047] Add 0.5-3 parts titanium dioxide and 0.02-0.05 parts ultramarine blue to improve the whiteness of the midsole.

[0048] It should be noted that the second internal mixer uses hot feed, and the required mixing time is typically 2-5 minutes. Choosing a lower temperature of 80-90°C and a shorter mixing time of 2-5 minutes can better maintain the performance of the recycled EVA. While adding functional modifiers during the second internal mixer can avoid their interference with the microwave decrosslinking reaction (e.g., the reflection of microwaves by inorganic fillers) or the denaturation and performance degradation of the functional modifiers due to microwave heating (e.g., the thermal decomposition of organic components), it is necessary to appropriately increase the temperature of the second internal mixer to 90-100°C and extend the mixing time to 4-8 minutes to ensure uniform mixing of the materials.

[0049] As a simple improvement, the preparation method of the present invention can also include the addition of new material. After the new material is mixed with recycled EVA material, it is adjusted accordingly using conventional additives, and then added to the midsole mold for foaming to prepare the midsole.

[0050] Preferably, the aromatic disulfide is an alkylphenol disulfide or a compound of it and a diphenyl disulfide.

[0051] Alkylphenol disulfide and diphenyl disulfide combined with naphthenic oil is an effective combination for treating various rubber blends, compatible with both polar and non-polar rubber components in waste materials. However, when diphenyl disulfide is used alone as an aromatic disulfide, it can easily lead to a strong odor in the product due to reaction residues.

[0052] Preferably, the microwave sensitizer includes an electrical loss material and a dielectric loss material, wherein the ratio of electrical loss material to dielectric loss material is 1:1~10, the electrical loss material is nano-carbon black or graphene, and the dielectric loss material is nano-silicon carbide or aluminum-doped zinc oxide.

[0053] Nano-silicon carbide or aluminum-doped zinc oxide are dielectric loss materials with excellent microwave penetration and polarization loss effects, ensuring that the center of even thick sheets can heat up synchronously. Nano-carbon black and graphene are conductive microwave absorbing materials that generate heat through conductivity loss and have an extremely fast response to microwaves, raising the local temperature within seconds.

[0054] By adjusting the ratio of electrically depleting materials and dielectric materials, a composite microwave sensitization network is constructed using electrically depleting materials such as graphene and nano-carbon black, and dielectric materials such as nano-silicon carbide and aluminum-doped zinc oxide. The extremely fast conductivity loss response of nano-carbon black or graphene enables instantaneous heating, which, combined with the excellent microwave penetration and polarization loss effect of nano-silicon carbide or nano-aluminum-doped zinc oxide, constructs an absorbing network covering the surface to the depths. This ensures synchronous temperature rise between the center and surface of a 3-5 mm thick sheet, fundamentally solving the technical problem of uneven microwave heating.

[0055] Although nano-aluminum-doped zinc oxide is expensive to use, its lighter color prevents recycled materials from turning black, making it suitable for light-colored midsoles. Furthermore, nano-aluminum-doped zinc oxide, through its semiconductor properties, can impedance-matched couple with graphene and nano-carbon black, effectively suppressing the risk of tip discharge in carbon-based materials in microwave fields, further improving processing safety. In addition, nano-aluminum-doped zinc oxide not only absorbs energy to promote decrosslinking during the microwave stage but also acts as a vulcanization accelerator during the midsole foaming and molding stage, promoting vulcanization crosslinking.

[0056] In the light-colored system, the amount of graphene can be controlled at the percolation threshold of 0.05 to 0.1 parts, and 0.5 to 0.8 parts of nano-aluminum-doped zinc oxide is mainly used to carry energy, ensuring that sufficient thermal energy is obtained without compromising whiteness.

[0057] In dark-colored systems, the amount of nano-carbon black can be increased, reducing the cost of microwave sensitizers, increasing overall microwave absorption performance, shortening microwave irradiation time, and improving production efficiency.

[0058] Preferably, before internal mixing, there is also a waste crushing step and a premixing step. The waste crushing step involves crushing the EVA waste to 5-10 mesh before feeding the crushed EVA waste into the first internal mixer. The premixing step is as follows: First, the naphthenic oil, stearic acid, antioxidant and aromatic disulfide are heated to 75~85℃ and stirred until well mixed. Then, the nano microwave sensitizer is added and dispersed evenly. Finally, the evenly dispersed material is fed into the first internal mixer.

[0059] The waste crushing and premixing steps work synergistically to improve the mixing and swelling efficiency of the internal mixer. By mixing easily mixable naphthenic oil, stearic acid, antioxidants and aromatic disulfides, and then mixing and dispersing them with a nano-microwave catalyst, the self-aggregation of nano-components is effectively suppressed and a physical coating layer is imparted to them.

[0060] Preferably, the viscosity of the naphthenic oil is 16~22 mm. 2 / s.

[0061] Select a viscosity of 16~22mm 2 This grade of naphthenic oil, with its good fluidity, achieves deep swelling of cross-linked waste materials during the internal mixing stage, synergistically allowing the de-crosslinking agent to effectively diffuse within the cross-linked network, thereby precisely locating and weakening sulfur bonds. The appropriate viscosity ensures efficient mixing of materials under strong shear while effectively preventing increased processing resistance due to excessively high viscosity after melting. Simultaneously, this grade of naphthenic oil has a higher flash point, typically greater than 210℃, significantly reducing the risk of flashover during processing.

[0062] Preferably, the irradiation process of the microwave reactor is as follows: the microwave reactor frequency is 2.45 GHz, first irradiation is carried out continuously at 400~800W for 35~60s, followed by pulse irradiation at 600~1000W, with a single pulse duration of 25~50s, the ratio of on time to off time is 1:2~5, and the cycle is repeated 8~20 times.

[0063] In a specific embodiment, the particles are first continuously irradiated with 400W for 20 seconds, followed by low-power irradiation with 400W to allow energy to slowly penetrate into the particles, avoiding local overheating and scorching of highly polar components such as nitrile rubber. Then, they are continuously irradiated with 800W for 30 seconds to rapidly raise the overall temperature to 150~155℃ to initiate the decrosslinking reaction. Finally, 8~20 cycles of pulse irradiation are performed to control the temperature within the optimal decrosslinking reaction temperature range of 155~160℃, ensuring that the crosslinked network has sufficient time to fully open.

[0064] Preferably, the temperatures of each section of the first screw extruder from the feed end to the discharge end are set to 115~125℃, 130~135℃, and 125~130℃, respectively.

[0065] By performing temperature-controlled high-viscosity high-shear mixing at 115~135℃ in the first screw extruder, the uniform dispersion of the material is ensured, guaranteeing the compactness and stability of subsequent rolling, and providing the necessary preparation for the uniform decrosslinking of the subsequent microwave segment.

[0066] Preferably, the process also includes an exhaust step. After the material is irradiated by microwaves, it is cooled and vacuum-exhausted in the second screw extruder. The cooling rate is 15~25℃ / min, and the vacuum degree of the vacuum exhaust is -0.08~-0.095MPa. The temperatures of each section of the second screw extruder from the feed end to the discharge end are set to 130~140℃, 120~125℃, 115~120℃, and 110~115℃, respectively, and vacuum degassing is performed at the front end of the fourth section at 110~115℃.

[0067] By installing an indirect water-cooled sleeve and a vacuum degassing device in the second screw extruder, a precise transition from high-temperature decrosslinking to low-temperature stable feeding is achieved. Rapid cooling prevents the decrosslinking reaction from continuing and causing monomer residue, while also preventing excessively rapid cooling from causing a sudden and significant increase in melt viscosity, resulting in excessive screw torque or material overflow from the venting port. Vacuum degassing ensures the removal of thiol-like odor molecules generated by the decrosslinking reaction, low-volatile components in the naphthenic oil, and the small amount of acetic acid released by EVA upon heating.

[0068] First, the temperature is controlled by a gradient cooling process from 140 to 115°C. At this temperature, the viscosity of the EVA melt is moderate enough to separate the powder and ensure uniform mixing of the added materials. This results in a recycled EVA compound with excellent stability and superior secondary foaming and crosslinking properties. Then, efficient vacuum degassing is performed at the front end of the fourth section to remove volatile small molecules generated by the de-crosslinking reaction, significantly reducing the odor of the finished product and ensuring the density of the EVA melt.

[0069] Preferably, the crosslinking agent is dicumyl peroxide and sulfur, with the ratio of dicumyl peroxide to sulfur being 1:4~20, and the crosslinking accelerator is dibenzothiazole disulfide.

[0070] A composite crosslinking system of dicumyl peroxide and sulfur is employed, leveraging the complementary advantages of the high thermal stability of carbon-carbon bonds and the high toughness of polysulfide bonds. This significantly enhances the tear resistance and resilience of recycled shoe soles while imparting excellent flexural strength. Furthermore, the precise regulation of vulcanization kinetics using dibenzothiazole disulfide effectively balances the foaming and crosslinking rates, ensuring a uniform and fine cell structure. The activation effect of nano-zinc oxide and stearic acid further ensures a perfect match between the crosslinking temperature and the EVA melt processing temperature.

[0071] Preferably, the foaming agent is azodicarbonamide, and the foaming aids are nano zinc oxide and urea, with the ratio of nano zinc oxide to urea being 2~3:1.

[0072] Using azodicarbonamide as the core, the decomposition temperature is lowered to match the melting range of EVA through the activation effect of nano-zinc oxide and urea, ensuring the synchronization of foaming and cross-linking. Nano-zinc oxide catalyzes foaming while neutralizing acid and preventing yellowing. Combined with urea's precise control over bubble growth rate, this induces a fine and uniform closed-cell structure. This significantly reduces the density of the shoe sole while effectively solving the problems of collapse and large pores that are common in recycled material products, improving appearance quality and rebound feel.

[0073] Example 1 By weight, the raw materials include: 100 portions of EVA waste; Antioxidant 0.4 parts, 1076:168=1:1; 3 parts of alkylphenol disulfide Plasticizer 3 parts, naphthenic oil, viscosity 16mm 2 / s; Nanowave sensitizer, consisting of 0.6 parts of nano-aluminum-doped zinc oxide and 0.05 parts of graphene; Stearic acid 0.8 parts; Foaming agent, 4 parts azodicarbonamide; 1 part foaming agent, nano zinc oxide: urea = 2:1; 1 part crosslinking agent, dicumyl peroxide: sulfur = 1:4; Crosslinking accelerator, 0.03 parts of dibenzothiazole disulfide; The preparation steps include: The EVA waste was pre-crushed using a blade shredder, passed through a 5-mesh sieve, and 100 portions were taken. Naphthenic oil, stearic acid, antioxidant and aromatic disulfide are heated to 85°C and stirred until well mixed. Then, nano-microwave sensitizer is added and dispersed evenly. The above materials are added to the first internal mixer at a temperature of 80°C and mixed for 8 minutes before being sent to the first screw extruder. The temperatures of each section of the first screw extruder are set to 125℃, 135℃, and 130℃ from the feed end to the discharge end, respectively. After being mixed in the first screw extruder, the mixture is extruded to the calender. The material is pressed into a thickness of 5mm using a three-roll calender and then transferred to a microwave reactor. In the microwave reactor, the microwave is first continuously irradiated with 400W for 20s, then continuously irradiated with 800W for 30s, and then irradiated with 800W pulses. The duration of each pulse is 35s, the on time is 10s, the off time is 28s, and the cycle is repeated 12 times. Then it is sent to the second screw extruder. In the second screw extruder, the indirect water-cooled circulating sleeve is turned on and the cooling rate is controlled at 20℃ / min. The vacuum pump at the exhaust port at the front end of the fourth section is turned on to maintain the vacuum at -0.09MPa. The temperatures of each section of the second screw extruder from the feed end to the discharge end are set to 140℃, 125℃, 120℃ and 115℃ respectively. After cooling and mixing, the mixture is extruded into the second internal mixer. In the second internal mixer, foaming agent, foaming aid, crosslinking agent and crosslinking accelerator are added in proportion, the temperature is 80℃, and the mixture is mixed for 5 minutes. The mixture is then sent to the roller press to be rolled into a large plate. The large plate is cut to obtain recycled EVA material. Recycled EVA material is added to the midsole mold and heated at 170℃ for 15 minutes to foam and obtain recycled EVA footwear midsole.

[0074] Example 2 The difference from Example 1 is that, according to parts by weight, the raw materials are: The nano-microwave sensitizer was changed to 0.4 parts of nano-aluminum-doped zinc oxide and 0.08 parts of graphene; Plasticizer changed to 5 parts, naphthenic oil, viscosity 16mm 2 / s; In the preparation steps: The microwave continuous irradiation parameters were changed to: first irradiate with 400W for 20 seconds, then irradiate with 700W for 40 seconds. The vacuum level of the second screw extruder was changed to -0.092 MPa.

[0075] Example 3 The difference from Example 1 is that, according to parts by weight, the raw materials are: The nano-microwave sensitizer was changed to 0.3 parts nano-silicon carbide and 0.5 parts nano-carbon black; Plasticizer changed to 2 parts, naphthenic oil, viscosity 22mm 2 / s; In the preparation steps: The microwave continuous irradiation parameters were changed to: first 500W irradiation for 15s, then 800W irradiation for 20s, followed by 1000W pulse irradiation, with a single pulse duration of 30s, an on time of 8s, an off time of 20s, and a cycle of 18 times. The vacuum level of the second screw extruder was changed to -0.085 MPa.

[0076] Comparative Example The midsole material is made from virgin EVA, and the composition of the midsole material is the same as that of the EVA waste material in Example 1.

[0077] Table 1 Performance Test Results

[0078] The test results in Table 1 show that: Example 1 achieves a relatively balanced performance retention.

[0079] Example 2, by increasing the proportion of graphene, utilizes its high aspect ratio to form a denser conductive / thermal network within the adhesive compound. Combined with a lower power (700W) but longer (40s) continuous irradiation, a gentler temperature rise environment is provided, maximizing the protection of the long branches of EVA from thermal degradation. Increasing the amount of naphthenic oil to 5 parts, along with a higher vacuum level, not only allows for more complete swelling of the waste material to facilitate the penetration of the decrosslinking agent, but also effectively removes excess low-molecular-weight volatiles in the later stages, ensuring the final midsole achieves a high resilience of 60.2%, close to that of virgin material, resulting in a soft and bouncy feel.

[0080] Example 3 utilizes a combination of nano-silicon carbide and nano-carbon black, coupled with a 1000W high-power pulse to generate extremely strong instantaneous energy release. This mode offers higher decrosslinking efficiency for the rubber components. Subsequently, a short-duration, high-frequency, multi-cycle pulse strategy induces sulfur bond breakage multiple times through a high-energy electromagnetic field without increasing overall heat accumulation. Due to the more compact reconstructed crosslinking density, with the same amount of foaming agent and crosslinking agent, the resulting midsole exhibits abrasion resistance even surpassing that of a midsole made from virgin EVA, reducing wear to 122mg.

[0081] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0082] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for preparing a recycled EVA footwear midsole, characterized in that: By weight, the raw materials include 100 parts of EVA waste, 0.3-0.5 parts of antioxidant, 2-3.5 parts of aromatic disulfide, 2-4 parts of plasticizer, 0.3-0.8 parts of nano-microwave sensitizer, 0.6-1 parts of stearic acid, 3-5 parts of foaming agent, 0.7-1.2 parts of foaming aid, 0.6-1.3 parts of crosslinking agent, and 0.02-0.05 parts of crosslinking accelerator; EVA waste refers to cross-linked foamed EVA scraps or waste products that are discarded during the production of EVA midsoles for footwear. The steps include: Add all the EVA waste, antioxidants, fillers and naphthenic oil to the first internal mixer in proportion, at a temperature of 80~105℃, mix for 5~8 minutes, and then send it to the first screw extruder. In the first screw extruder, the temperature is 115~130℃, and after mixing, the mixture is extruded to the calender. The material is pressed into a thickness of 3-5 mm in a calender and then transferred to a microwave reactor. In the microwave reactor, the temperature is first continuously irradiated to 150~155℃, and then pulse irradiation is used to control the temperature at 155~160℃, and then it is transmitted to the second screw extruder. In the second screw extruder, the temperature is 110~140℃. After cooling and mixing, the mixture is extruded into the second internal mixer. Add foaming agent, foaming aid, crosslinking agent and crosslinking accelerator to the second internal mixer, mix at 80~100℃ for 2~8 minutes, and send the mixture to the roller press to roll into a large plate. The large plate is then cut or granulated to obtain recycled EVA material. Recycled EVA material is added to the midsole mold and heated to foam, thus producing recycled EVA footwear midsoles.

2. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: Aromatic disulfides are alkylphenol disulfides or mixtures thereof with diphenyl disulfides.

3. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: The microwave sensitizer includes an electrical loss material and a dielectric loss material, with an electrical loss material to dielectric loss material ratio of 1:1~10. The electrical loss material is nano-carbon black or graphene, and the dielectric loss material is nano-silicon carbide or aluminum-doped zinc oxide.

4. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: Before internal mixing, there are also waste crushing and premixing steps. The waste crushing step involves crushing the EVA waste to 5-10 mesh before feeding the crushed EVA waste into the first internal mixer. The premixing step is as follows: First, the naphthenic oil, stearic acid, antioxidant and aromatic disulfide are heated to 75~85℃ and stirred until well mixed. Then, the nano microwave sensitizer is added and dispersed evenly. Finally, the evenly dispersed material is fed into the first internal mixer.

5. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: The viscosity of naphthenic oil is 16~22 mm. 2 / s.

6. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: The irradiation process of the microwave reactor is as follows: the microwave reactor frequency is 2.45GHz, first irradiation is carried out continuously at 400~800W for 35~60s, followed by pulse irradiation at 600~1000W, with a single pulse duration of 25~50s, and the ratio of on time to off time is 1:2~5, and the cycle is repeated 8~20 times.

7. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: The temperatures of each section of the first screw extruder are set to 115~125℃, 130~135℃, and 125~130℃ from the feed end to the discharge end, respectively.

8. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: It also includes an exhaust step. After the material is irradiated by microwave, it is cooled and vacuumed in the second screw extruder. The cooling rate is 15~25℃ / min, and the vacuum degree of the vacuum exhaust is -0.08~-0.095MPa. The temperature of each section of the second screw extruder from the feed end to the discharge end is set to 130~140℃, 120~125℃, 115~120℃, and 110~115℃ respectively, and vacuum degassing is performed at the front end of the fourth section of 110~115℃.

9. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: The crosslinking agents are dicumyl peroxide and sulfur, with a ratio of dicumyl peroxide to sulfur of 1:4~20. The crosslinking accelerator is dibenzothiazole disulfide.

10. The method for preparing a recycled EVA footwear midsole according to claim 1, characterized in that: The foaming agent is azodicarbonamide, and the foaming aids are nano zinc oxide and urea, with a ratio of nano zinc oxide to urea of ​​2~3:1.

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