High-elastic lightweight integrated EVA supercritical foaming sole and preparation method thereof

Abstract

The invention discloses a high-elastic lightweight integrated EVA (ethylene vinyl acetate) supercritical foaming sole and a preparation method, and belongs to the technical field of high polymer material processing. The method comprises the following steps: premixing ethylene vinyl acetate resin, mercaptopropionic acid grafted polydimethylsiloxane, ionic liquid modified montmorillonite, a cross-linking agent, a lubricant and an antioxidant according to a specific proportion; carrying out melt blending granulation to obtain composite granules; and carrying out mold pressing on the composite granules to form an unfoamed initial blank, placing the unfoamed initial blank in a high-pressure foaming kettle, foaming by using supercritical gas at a specific temperature and pressure, and carrying out heat treatment after pressure relief to obtain a final product. According to the method, two specific compounds of thiohydracrylic acid grafted polydimethylsiloxane and ionic liquid modified montmorillonite are introduced to generate a synergistic effect with matrix resin, under a supercritical fluid foaming process, the sole material which is uniform and fine in foam structure, low in density and excellent in rebound resilience is successfully prepared, and the comprehensive performance is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, specifically to a high-elasticity, lightweight integrated EVA supercritical foaming shoe sole and its preparation method. Background Technology

[0002] Ethylene-vinyl acetate copolymer foam materials have long been widely used in shoe sole manufacturing due to their excellent flexibility, cushioning performance, and fatigue resistance. While traditional chemical foaming processes are relatively mature, they often rely on chemical foaming agents such as azodicarbonamide, which may produce harmful byproducts during decomposition, posing environmental and health risks. Furthermore, the resulting cell structure has poor uniformity and dimensional controllability, making it difficult to meet the growing demand for lightweight and highly elastic high-performance shoe soles. In recent years, supercritical fluid physical foaming technology, especially using supercritical gases for foaming, has become a research hotspot and important development direction in this field due to its outstanding advantages such as cleanliness and environmental friendliness, precise controllable process parameters, and fine and uniform cell size. However, ethylene vinyl acetate copolymer matrix resin has inherent defects in the supercritical foaming process. Its melt strength is relatively low, and it is difficult to effectively stabilize the cell walls during the rapid bubble growth stage, which easily leads to cell merging or collapse, limiting the further improvement of the foaming ratio. At the same time, its inherent viscoelasticity is insufficient to control cell nucleation, which makes the preparation of integrated materials with extremely low density, highly uniform closed-cell structure and excellent resilience a major challenge.

[0003] To overcome the shortcomings of pure ethylene vinyl acetate copolymers in supercritical foaming, existing technologies have explored various physical and chemical modification methods. A common approach is to introduce nanofillers, such as layered silicate montmorillonite, aiming to utilize its lamellar structure as heterogeneous nucleation sites to refine pores and strengthen the matrix. However, unmodified montmorillonite has poor compatibility with the polymer matrix, easily agglomerating and affecting dispersion, potentially becoming a structural defect. Another strategy is to add organosilicon compounds, such as polydimethylsiloxane, hoping to improve flowability and impart flexibility to the material by utilizing its low surface energy. However, simple physical blending often results in weak interfacial bonding, with silicon components easily migrating during foaming, leading to poor long-term stability. Furthermore, using peroxides to initiate cross-linking networks to enhance melt strength is also a widely used method, but simple chemical cross-linking often sacrifices some elasticity while increasing system strength, making it difficult to achieve the optimal balance between lightweight and high elasticity. These single or simple composite modification methods often only focus on improving one aspect of performance, making it difficult to achieve a synergistic unity of fine control of cell structure, enhanced melt strength and improved elastic recovery performance, thus restricting the application of supercritical foaming technology in high-end shoe sole manufacturing.

[0004] Therefore, developing an innovative synergistic modification system to achieve strong interfacial interactions between the modifier and the matrix resin through molecular design, and simultaneously optimizing the nucleation, growth, and stabilization stages during the foaming process, is crucial for preparing next-generation high-elasticity, lightweight, integrated ethylene vinyl acetate supercritical foamed shoe soles. This invention aims to propose a novel solution by designing and synthesizing two structurally well-defined and functionally complementary mercaptopropionic acid-grafted polydimethylsiloxanes and ionic liquid-modified montmorillonite, and introducing them into an ethylene vinyl acetate supercritical foaming system. This system is expected to fully leverage the reactive compatibilizing effect of mercapto groups on the resin matrix, the flexible regulatory function of polysiloxane segments, and the efficient heterogeneous nucleation and ionic interaction advantages of ionic liquid-modified montmorillonite. This will synergistically regulate the rheological behavior and cell evolution process of the material at the molecular and microscopic levels, ultimately achieving a breakthrough improvement in the highly uniform cell structure, mechanical strength retention, and elastic recovery performance of the sole material at extremely low densities, meeting the comprehensive requirements of modern sports and life for footwear in terms of comfort, functionality, and environmental friendliness. Summary of the Invention

[0005] The purpose of this invention is to provide a high-elasticity, lightweight, integrated EVA supercritical foamed shoe sole and its preparation method, which overcomes the technical problem that existing ethylene vinyl acetate copolymer foaming materials are difficult to simultaneously achieve uniform and stable cell structure, excellent elastic recovery performance, and good mechanical strength at high foaming ratios when using supercritical fluid foaming technology.

[0006] The present invention achieves the above objectives through the following technical solutions: A method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole, comprising the following steps: S1. By weight, 80-120 parts of EVA resin, 2-8 parts of mercaptopropionic acid-grafted polydimethylsiloxane, 1-5 parts of ionic liquid-modified montmorillonite, 0.5-2 parts of dicumyl peroxide, 0.5-2 parts of zinc stearate, and 0.1-0.5 parts of antioxidant 1010 are premixed in a high-speed mixer to obtain a premix; the premix is ​​fed into a twin-screw extruder for melt blending, extrusion granulation, and then obtained modified EVA composite granules. S2. Place the modified EVA composite granules into the mold of the flat vulcanizing machine and hot press them at 125-135℃ and 8-12MPa to obtain an unfoamed preform. Place the unfoamed preform in a high-pressure foaming kettle, seal it, heat it to 148-152℃, and introduce supercritical gas to maintain the pressure at 14-16MPa. Depressurize to obtain the foamed preform. Transfer the foamed preform to an oven at 118-122℃ for heat treatment.

[0007] In this invention, the core of the high-elasticity, lightweight, integrated EVA supercritical foaming sole system lies in the synergistic mechanism of multiple components. Functionalized siloxanes form a hydrogen bond network with the ester carbonyl groups in the EVA molecular chain through carboxyl groups, enhancing interfacial bonding; simultaneously, their flexible segments reduce local modulus, giving the material high resilience. Ionic liquid-modified montmorillonite, as a nanofiller, is uniformly dispersed in the matrix, regulating the cell nucleation density and growth rate to achieve uniform microporous structure. Dicumyl peroxide decomposes upon heating during the foaming stage, generating free radicals that initiate moderate cross-linking of EVA, forming a three-dimensional network to stabilize the cell structure. Zinc stearate acts as an activator to promote cross-linking efficiency, while antioxidants inhibit thermo-oxidative degradation. The entire system completes physical foaming in a supercritical gas environment, leaving no chemical foaming agent residue, making it environmentally friendly and producing fine pores. The final product possesses excellent properties such as lightweight, high resilience, and resistance to compression set, making it suitable for high-end sports shoe midsole applications.

[0008] According to a preferred embodiment of the present invention, in step S1, the premixing time is 5-10 min; the melt blending temperature is 120-130°C.

[0009] According to a preferred embodiment of the present invention, in step S2, the time for heat treatment of the foamed blank in an oven at 118-122°C is 30-40 minutes; the supercritical gas is supercritical carbon dioxide and / or supercritical nitrogen.

[0010] According to a preferred embodiment of the present invention, the method for preparing the mercaptopropionic acid-grafted polydimethylsiloxane includes: A1. By weight, under dry nitrogen protection, add 80-120 parts of vinyl-terminated polydimethylsiloxane and 30-50 parts of anhydrous toluene to a three-necked flask and stir; then add 9-13 parts of 3-mercaptopropionic acid to obtain a mixture. A2. Heat the mixture to 78-82℃ and stir continuously. After the reaction is complete, cool to room temperature, add deionized water, stir, and let stand to separate the layers. Separate the organic phase. Dry the organic phase with anhydrous sodium sulfate, filter, and rotary evaporate to obtain the product. Dry the product in a vacuum oven at 70-80℃.

[0011] In this invention, the preparation of mercaptopropionic acid-grafted polydimethylsiloxane is based on the thiol-ene click chemistry mechanism. The carbon-carbon double bond at the end of the vinyl-terminated polydimethylsiloxane molecular chain undergoes a free radical addition reaction with the thiol group in 3-mercaptopropionic acid under mild heating conditions. This reaction can proceed efficiently under an inert atmosphere without the need for an external initiator. This reaction exhibits high regioselectivity and quantitative conversion characteristics, generating a stable thioether bond structure. The product is a functionalized siloxane with a carboxyl group at one end. The carboxyl group is connected to the siloxane backbone through flexible ethylene and thioether bonds, retaining the flexibility and low surface energy of polydimethylsiloxane while introducing a highly polar group, significantly improving its interfacial compatibility with the EVA matrix. Post-treatment involves water washing and extraction to remove unreacted small molecule acids, followed by drying and vacuum devolatilization to ensure the product is pure and free of volatiles, avoiding bubble defects during subsequent high-temperature processing.

[0012] According to a preferred embodiment of the present invention, in step A1, the stirring time is 20-40 minutes.

[0013] According to a preferred embodiment of the present invention, in step A2, the reaction is continuously stirred for 5-7 hours.

[0014] According to a preferred embodiment of the present invention, the preparation method of the ionic liquid modified montmorillonite includes: B1. Disperse 8-12 parts by weight of sodium montmorillonite in deionized water and stir at 58-62℃ to obtain a montmorillonite suspension; dissolve 4-6 parts of 1-butyl-3-methylimidazolium chloride in deionized water and stir to obtain an ionic liquid solution. B2. Under stirring, the ionic liquid solution is added dropwise to the montmorillonite suspension. After the addition is complete, the reaction continues at 58-62℃ to obtain a mixture. The mixture is centrifuged to obtain a solid product. The solid product is washed with deionized water, dried in a vacuum oven at 78-82℃, ground, and sieved.

[0015] In this invention, the preparation of ionic liquid-modified montmorillonite relies on the ion exchange mechanism. Sodium-based montmorillonite contains exchangeable sodium ions in its interlayer. After being fully dispersed in an aqueous phase to form a stable suspension, a 1-butyl-3-methylimidazolium chloride ionic liquid is added. Its organic cations replace the inorganic cations in the montmorillonite interlayer through electrostatic interactions, embedding themselves into the silicate interlayer. This process expands the interlayer spacing of montmorillonite, weakens the interlayer forces, and makes it easier to peel or intercalate in the polymer melt, thus forming a nanocomposite structure. The modified montmorillonite, due to its surface changing from hydrophilic to oleophilic, exhibits significantly enhanced compatibility with the EVA matrix. It can act as a cell nucleation point and skeletal support during foaming, effectively inhibiting cell collapse and merging, and improving the dimensional stability and mechanical strength of the material. The entire modification process is completed under mild hydrothermal conditions, is simple to operate, and uses commercially available chemicals, meeting green process requirements.

[0016] According to a preferred embodiment of the present invention, in step B1, the stirring time is 30-60 minutes.

[0017] According to a preferred embodiment of the present invention, in step B2, the reaction continues at 58-62°C for 12-14 hours.

[0018] The present invention also provides a high-elasticity, lightweight integrated EVA supercritical foam shoe sole prepared according to the preparation method of the high-elasticity, lightweight integrated EVA supercritical foam shoe sole.

[0019] The beneficial effects of this invention are as follows: This invention introduces a specifically designed mercaptopropionic acid-grafted polydimethylsiloxane and ionic liquid-modified montmorillonite as a synergistic modification system, coupled with an optimized supercritical gas foaming process, ultimately resulting in an ethylene-vinyl acetate copolymer foamed sole that achieves a leapfrog improvement in multiple dimensions of performance. Its most significant technical effect lies in the unification of excellent lightweight and high elasticity. Compared to traditional chemically foamed or unmodified supercritical foamed products, the sole material obtained by this invention exhibits exceptionally good rebound and recovery performance while maintaining extremely low density. This low-density characteristic is mainly due to the huge nucleation driving force formed by the supercritical fluid during controlled depressurization, and the precise stabilizing effect of the modification system on the cell growth process; while the high elasticity stems from the mobility of the molecular chains imparted by the flexible polysiloxane segments, and the reversible deformation capability provided by the dynamic ion interaction network. The combination of these two factors allows the sole to provide sufficient cushioning during wear, while efficiently converting stored deformation energy into rebound force, greatly improving exercise efficiency and comfort.

[0020] At the microstructural level, this invention demonstrates a precise and consistent ability to control the morphology of bubbles, which is the fundamental guarantee for achieving excellent macroscopic performance. Ionic liquid-modified montmorillonite, as a highly efficient multifunctional nucleating agent, exhibits significantly improved compatibility with the polymer matrix after its lamellar structure surface is modified with ionic liquid, allowing for uniform dispersion. This provides a large number of uniformly distributed active sites for bubble nucleation in supercritical gases, fundamentally ensuring the uniformity of bubble distribution. Simultaneously, during melt blending and subsequent hot pressing, the terminal thiol groups of mercaptopropionic acid-grafted polydimethylsiloxane can interact with the molecular chains of ethylene vinyl acetate copolymer. This compound not only acts as a compatibilizer bridging the organosilicon phase and the resin matrix but also effectively enhances melt strength during the bubble growth stage through its long-chain siloxane structure, suppressing the tendency for adjacent bubbles to merge and break. The two work together to achieve miniaturization of cell size and integrity of cell wall structure, forming a three-dimensional network cell structure with high closed-cell rate and regular pore shape. This structure is the material basis for the material to have both high specific strength and high resilience.

[0021] Furthermore, the technical advantages of this invention are also reflected in the balance of comprehensive mechanical properties and the robustness of the process. The introduced synergistic modification system not only optimizes the foam structure but also strengthens the matrix itself. The coexistence of dynamic ionic crosslinking points and chemical crosslinking networks allows the material to dissipate energy through the reversible destruction and recombination of the ionic network when subjected to repeated compression and impact, thereby significantly reducing compression set and improving the durability and shape retention of the sole. In terms of process adaptability, the designed modified compound preparation method uses readily available raw materials and has simple steps, making it easy to scale up production. The set process parameter windows for melt blending, molding preforming, and supercritical foaming are wide and well-matched, ensuring the full utilization of the modified components' functions and the high repeatability of the foaming process. The final product not only excels in the core lightweight and high-elasticity indicators but also achieves excellent levels in tensile strength, tear resistance, and dimensional stability, meeting the comprehensive requirements of high-end shoe sole materials for complex performance. Detailed Implementation

[0022] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0023] Example 1 Preparation of mercaptopropionic acid grafted polydimethylsiloxane: Under a dry nitrogen atmosphere, 100.0 g of vinyl-terminated polydimethylsiloxane and 40.0 g of anhydrous toluene were precisely added sequentially to a three-necked flask equipped with a magnetic stirrer, condenser, and thermometer. Stirring was started and maintained at 300 rpm for 30.0 min at room temperature until a homogeneous and transparent solution was obtained. Then, 11.0 g of 3-mercaptopropionic acid was slowly added dropwise to the system using a constant-pressure dropping funnel over a period of 10.0 min. After the addition was complete, the oil bath temperature of the reaction system was raised to 80.0 °C and maintained at this temperature, with continuous stirring for a total reaction time of 6.0 h. After the reaction was complete, the oil bath was removed, and the reaction system was allowed to cool naturally to 25.0 °C. 100.0 g of deionized water was added to the cooled mixture, and the mixture was stirred at 400 rpm for 10.0 min for washing. Stirring was then stopped, and the mixture was allowed to stand for 30.0 min to allow for complete separation of the liquid phases. The lower organic phase was separated and collected. Add 5.0 g of anhydrous sodium sulfate to the organic phase, allow it to stand and dry for 30.0 min to remove trace amounts of moisture, and then filter using medium-speed qualitative filter paper to remove the desiccant. Transfer the resulting clear filtrate to a rotary evaporator and rotary evaporate at a water bath temperature of 60.0℃ and a vacuum of -0.095 MPa for 40.0 min to completely remove the toluene solvent, yielding a pale yellow viscous liquid. Transfer this liquid to a vacuum drying oven and dry at 75.0℃ and -0.090 MPa for 24.0 h to finally obtain the target product, mercaptopropionic acid-grafted polydimethylsiloxane, which is a colorless to pale yellow transparent viscous liquid.

[0024] Preparation of ionic liquid-modified montmorillonite: Weigh 10.0 g of sodium montmorillonite and add it to a beaker containing 500.0 g of deionized water. Place the beaker in a constant temperature water bath at 60.0℃ and stir at 800 rpm for 45.0 min using a mechanical stirrer to obtain a homogeneous and stable montmorillonite suspension. In another beaker, dissolve 5.0 g of 1-butyl-3-methylimidazolium chloride in 100.0 g of deionized water and stir magnetically for 30.0 min to obtain a clear ionic liquid solution. Under continuous mechanical stirring (800 rpm), slowly add the ionic liquid solution dropwise to the montmorillonite suspension using a constant pressure dropping funnel over a period of 30.0 min. After the addition is complete, maintain the system temperature at 60.0℃ and the stirring speed at 400 rpm to carry out the ion exchange reaction for 13.0 h. After the reaction was complete, the mixture was transferred to centrifuge tubes and centrifuged at 8000 rpm for 10.0 min. The supernatant was discarded, and the lower solid precipitate was collected. The solid precipitate was redispersed with 200.0 g of deionized water and centrifuged again. This washing process was repeated three times until no white precipitate was formed when the last wash was tested with 0.1 mol / L silver nitrate solution. The washed solid was transferred to a petri dish and placed in a vacuum drying oven at 80.0℃ and -0.090 MPa for 24.0 h. The dried blocky solid was ground in an agate mortar and passed through a 200-mesh (75 μm) standard sieve to obtain a fluffy white powder, which is the ion-liquid modified montmorillonite.

[0025] Preparation of high-elasticity, lightweight, integrated EVA supercritical foaming shoe sole: First, premix the materials: accurately weigh 100.0g of EVA resin, 5.0g of the previously prepared mercaptopropionic acid-grafted polydimethylsiloxane, 3.0g of the previously prepared ionic liquid-modified montmorillonite, 1.0g of dicumyl peroxide, 1.0g of zinc stearate, and 0.3g of antioxidant 10100. Add all materials at once to the mixing cup of a high-speed mixer (SHR type), cover, start the machine, and mix at 1200 rpm for 8.0 minutes. After stopping the machine, a uniformly mixed premix is ​​obtained. Next, melt blending and granulation are performed: add all the premix to the feed hopper of a twin-screw extruder (SHJ-20 type). Set the temperatures of each section (seven zones) of the extruder as follows: Zone 1 115℃, Zone 2 120℃, Zone 3 125℃, Zone 4 125℃, Zone 5 120℃, and the die head 115℃. Set the screw speed to 200 rpm. The extruder is started, and the material is melted, mixed, extruded, water-cooled, and dried. It is then cut into cylindrical particles approximately 3mm in diameter and 3mm in length by a pelletizer to obtain modified EVA composite granules for later use. Next, pre-molding is performed: the shoe sole blank mold is installed between the heating plates of a flat vulcanizing machine (QLB type). Sufficient modified EVA composite granules are weighed according to the mold cavity volume and evenly spread within the mold cavity. The hot-pressing temperature is set to 130.0℃, the mold closing pressure to 10.0MPa, the preheating time to 2.0min, and the hot-pressing time to 5.0min. Subsequently, under maintained pressure, water is circulated to cool to below 50.0℃ before opening the mold. A smooth, well-shaped pre-molded unfoamed shoe sole blank is obtained. Finally, supercritical foaming and post-treatment are performed: the pre-molded unfoamed shoe sole blank is placed in the inner liner of a high-pressure foaming autoclave (CJF-1L type), and the autoclave is sealed. The heating system was activated, raising the temperature inside the reactor to 150.0℃ at a rate of 2℃ / min. Liquid carbon dioxide was then injected into the reactor using a high-pressure metering pump, and the temperature and pressure continued to increase until the system reached a supercritical state (temperature 150.0℃, pressure 15.0MPa). This state was maintained for 120.0min of pressure permeation. After the pressure holding period, the pressure was instantly released within 1.0s using a rapid pressure relief valve on the reactor top, initiating polymer foaming. The foaming reactor was immediately opened, the foamed preform was removed, and quickly transferred to a preheated drying oven at 120.0℃ for 35.0min of heat treatment to perfect cross-linking and stabilize the cell structure. After heat treatment, the oven power was turned off, and the sample was allowed to cool to room temperature with the oven, yielding the high-elasticity, lightweight, integrated supercritical foamed EVA shoe sole.

[0026] Example 2 The specific implementation method is the same as in Example 1, except that the preparation of mercaptopropionic acid grafted polydimethylsiloxane is as follows: Under a dry nitrogen atmosphere, 90.0 g of vinyl-terminated polydimethylsiloxane and 35.0 g of anhydrous toluene were precisely added sequentially to a three-necked flask equipped with a magnetic stirrer, condenser, and thermometer. Stirring was started and maintained at 300 rpm for 25.0 min at room temperature until a homogeneous and transparent solution was obtained. Then, 10.0 g of 3-mercaptopropionic acid was slowly added dropwise to the system using a constant-pressure dropping funnel over a period of 10.0 min. After the addition was complete, the reaction mixture was heated to 79.0 °C in an oil bath and maintained at this temperature while continuously stirring for a total reaction time of 5.5 h. After the reaction was complete, the oil bath was removed, and the reaction mixture was allowed to cool naturally to 25.0 °C. 100.0 g of deionized water was added to the cooled mixture, and the mixture was stirred at 400 rpm for 10.0 min for washing. Stirring was then stopped, and the mixture was allowed to stand for 30.0 min to allow for complete separation of the liquid phases. The lower organic phase was separated and collected. Add 5.0 g of anhydrous sodium sulfate to the organic phase, allow to stand and dry for 30.0 min, and then filter using medium-speed qualitative filter paper. Transfer the resulting clear filtrate to a rotary evaporator and rotary evaporate at a water bath temperature of 60.0℃ and a vacuum of -0.095 MPa for 40.0 min to obtain a pale yellow viscous liquid. Transfer the liquid to a vacuum drying oven and dry at 75.0℃ and -0.090 MPa for 24.0 h to finally obtain the target product, mercaptopropionic acid-grafted polydimethylsiloxane.

[0027] Preparation of ionic liquid-modified montmorillonite: Weigh 9.0 g of sodium montmorillonite and add it to a beaker containing 450.0 g of deionized water. Place the beaker in a constant temperature water bath at 59.0℃ and stir at 800 rpm for 50.0 min using a mechanical stirrer to obtain a homogeneous and stable montmorillonite suspension. In another beaker, dissolve 4.5 g of 1-butyl-3-methylimidazolium chloride in 90.0 g of deionized water and stir magnetically for 30.0 min to obtain a clear ionic liquid solution. Under continuous mechanical stirring (800 rpm), slowly add the ionic liquid solution dropwise to the montmorillonite suspension using a constant pressure dropping funnel over a period of 30.0 min. After the addition is complete, maintain the system temperature at 59.0℃ and the stirring speed at 400 rpm to carry out the ion exchange reaction for 12.5 h. After the reaction was complete, the mixture was transferred to centrifuge tubes and centrifuged at 8000 rpm for 10.0 min. The supernatant was discarded, and the lower solid precipitate was collected. The solid precipitate was redispersed with 200.0 g of deionized water and centrifuged again. This washing process was repeated three times until no white precipitate was formed when the final washing solution was tested with silver nitrate solution. The washed solid was placed in a vacuum drying oven and dried at 80.0℃ and -0.090 MPa for 24.0 h. The dried solid blocks were ground and passed through a 200-mesh standard sieve to obtain a fluffy white powder, which is the ion-liquid modified montmorillonite.

[0028] Preparation of high-elasticity, lightweight, integrated EVA supercritical foaming shoe sole: First, premix the materials: accurately weigh 90.0g of EVA resin, 3.0g of the previously prepared mercaptopropionic acid-grafted polydimethylsiloxane, 2.0g of the previously prepared ionic liquid-modified montmorillonite, 0.8g of dicumyl peroxide, 0.8g of zinc stearate, and 0.2g of antioxidant 1010. Add all materials to the mixing cup of a high-speed mixer and mix at 1200 rpm for 6.0 min to obtain a uniformly mixed premix. Next, perform melt blending and granulation: add the premix to the feed hopper of a twin-screw extruder. Set the temperatures of each section (seven zones) of the extruder as follows: Zone 1 112℃, Zone 2 117℃, Zone 3 122℃, Zone 4 122℃, Zone 5 117℃, and the die head 112℃. Set the screw speed to 200 rpm. Start the extruder; the material undergoes melting, mixing, extrusion, water cooling, drying, and pelletizing to obtain modified EVA composite granules for later use. Then, pre-molding is performed: the initial shoe sole blank mold is installed between the heating plates of the flat vulcanizing machine. Based on the mold cavity volume, a sufficient amount of modified EVA composite granules is weighed and evenly spread within the mold cavity. The hot-pressing temperature is set to 128.0℃, the mold closing pressure to 9.0MPa, the preheating time to 2.0min, and the hot-pressing time to 5.0min. Afterward, the mold is opened while maintaining pressure until it cools to below 50.0℃, and the pre-molded, unfoamed shoe sole blank is removed. Finally, supercritical foaming and post-treatment are performed: the pre-molded, unfoamed shoe sole blank is placed in the inner liner of a high-pressure foaming autoclave, and the autoclave is sealed. The temperature inside the autoclave is raised to 149.0℃ at a rate of 2℃ / min. Then, liquid nitrogen is injected until the system reaches a supercritical state (temperature 149.0℃, pressure 14.5MPa), and pressure is maintained for 115.0min for permeation. After the pressure holding period, the pressure is rapidly released for foaming. Immediately remove the foamed preform and quickly transfer it to a preheated drying oven at 119.0℃ for heat treatment for 32.0 min. After heat treatment, cool the sample to room temperature in the oven to obtain the finished high-elasticity, lightweight, integrated EVA supercritical foamed shoe sole.

[0029] Example 3 The specific implementation method is the same as in Example 1, except that the preparation of mercaptopropionic acid grafted polydimethylsiloxane is as follows: Under a dry nitrogen atmosphere, 110.0 g of vinyl-terminated polydimethylsiloxane and 45.0 g of anhydrous toluene were precisely added sequentially to a three-necked flask equipped with a magnetic stirrer, condenser, and thermometer. Stirring was started and maintained at 300 rpm for 35.0 min at room temperature until a homogeneous and transparent solution was obtained. Then, 12.0 g of 3-mercaptopropionic acid was slowly added dropwise to the system using a constant-pressure dropping funnel over a period of 10.0 min. After the addition was complete, the oil bath temperature of the reaction system was raised to 81.0 °C and maintained at this temperature, with continuous stirring for a total reaction time of 6.5 h. After the reaction was complete, the oil bath was removed, and the reaction system was allowed to cool naturally to 25.0 °C. 100.0 g of deionized water was added to the cooled mixture, and the mixture was stirred at 400 rpm for 10.0 min for washing. Stirring was then stopped, and the mixture was allowed to stand for 30.0 min to allow for complete separation of the liquid phases. The lower organic phase was separated and collected. Add 5.0 g of anhydrous sodium sulfate to the organic phase, allow to stand and dry for 30.0 min, and then filter using medium-speed qualitative filter paper. Transfer the resulting clear filtrate to a rotary evaporator and rotary evaporate at a water bath temperature of 60.0℃ and a vacuum of -0.095 MPa for 40.0 min to obtain a pale yellow viscous liquid. Transfer the liquid to a vacuum drying oven and dry at 75.0℃ and -0.090 MPa for 24.0 h to finally obtain the target product, mercaptopropionic acid-grafted polydimethylsiloxane.

[0030] Preparation of ionic liquid-modified montmorillonite: Weigh 11.0 g of sodium montmorillonite and add it to a beaker containing 550.0 g of deionized water. Place the beaker in a constant temperature water bath at 61.0 °C and stir at 800 rpm for 55.0 min using a mechanical stirrer to obtain a homogeneous and stable montmorillonite suspension. In another beaker, dissolve 5.5 g of 1-butyl-3-methylimidazolium chloride in 110.0 g of deionized water and stir magnetically for 30.0 min to obtain a clear ionic liquid solution. Under continuous mechanical stirring (800 rpm), slowly add the ionic liquid solution dropwise to the montmorillonite suspension using a constant pressure dropping funnel over a period of 30.0 min. After the addition is complete, maintain the system temperature at 61.0 °C and the stirring speed at 400 rpm to carry out the ion exchange reaction for 13.5 h. After the reaction was complete, the mixture was transferred to centrifuge tubes and centrifuged at 8000 rpm for 10.0 min. The supernatant was discarded, and the lower solid precipitate was collected. The solid precipitate was redispersed with 200.0 g of deionized water and centrifuged again. This washing process was repeated three times until no white precipitate was formed when the final washing solution was tested with silver nitrate solution. The washed solid was placed in a vacuum drying oven and dried at 80.0℃ and -0.090 MPa for 24.0 h. The dried solid blocks were ground and passed through a 200-mesh standard sieve to obtain a fluffy white powder, which is the ion-liquid modified montmorillonite.

[0031] Preparation of high-elasticity, lightweight, integrated EVA supercritical foaming shoe sole: First, premix the materials: accurately weigh 110.0g of EVA resin, 7.0g of the previously prepared mercaptopropionic acid-grafted polydimethylsiloxane, 4.0g of the previously prepared ionic liquid-modified montmorillonite, 1.5g of dicumyl peroxide, 1.5g of zinc stearate, and 0.4g of antioxidant 10100. Add all materials to the mixing cup of a high-speed mixer and mix at 1200 rpm for 9.0 minutes to obtain a uniformly mixed premix. Next, perform melt blending and granulation: add the premix to the feed hopper of a twin-screw extruder. Set the temperatures of each section (seven zones) of the extruder as follows: Zone 1 118℃, Zone 2 123℃, Zone 3 128℃, Zone 4 128℃, Zone 5 123℃, and the die head 118℃. Set the screw speed to 200 rpm. Start the extruder; the material undergoes melting, mixing, extrusion, water cooling, drying, and pelletizing to obtain modified EVA composite granules for later use. Then, compression molding is performed: the initial shoe sole mold is installed between the heating plates of the flat vulcanizing machine. Based on the mold cavity volume, a sufficient amount of modified EVA composite granules is weighed and evenly spread within the mold cavity. The hot-pressing temperature is set to 132.0℃, the mold closing pressure to 11.0MPa, the preheating time to 2.0min, and the hot-pressing time to 5.0min. Afterward, the mold is opened while maintaining pressure until it cools to below 50.0℃, and the pre-formed, unfoamed shoe sole blank is removed. Finally, supercritical foaming and post-treatment are performed: the pre-formed, unfoamed shoe sole blank is placed in the inner liner of a high-pressure foaming autoclave, and the autoclave is sealed. The temperature inside the autoclave is raised to 151.0℃ at a rate of 2℃ / min. Then, liquid carbon dioxide and nitrogen are injected until the system reaches a supercritical state (temperature 151.0℃, pressure 15.5MPa), and pressure is maintained for 125.0min for permeation. After the pressure holding period, the pressure is rapidly released for foaming. Immediately remove the foamed preform and quickly transfer it to a preheated drying oven at 121.0℃ for heat treatment for 38.0 min. After heat treatment, the sample is cooled to room temperature in the oven to obtain the finished high-elasticity, lightweight, integrated EVA supercritical foamed shoe sole.

[0032] Comparative Example 1 The specific implementation method is the same as in Example 1, except that a high-elasticity, lightweight integrated EVA supercritical foamed shoe sole is prepared, but mercaptopropionic acid-grafted polydimethylsiloxane and ionic liquid-modified montmorillonite are not added. 100.0g of EVA resin, 1.0g of dicumyl peroxide, 1.0g of zinc stearate, and 10100.3g of antioxidant are accurately weighed. All materials are added to the mixing cup of a high-speed mixer and mixed at 1200 rpm for 8.0 min to obtain a premix. The premix is ​​added to the feed hopper of a twin-screw extruder. The temperature settings of each zone of the extruder are exactly the same as in Example 1, and the screw speed is 200 rpm. Melt blending and granulation are performed to obtain EVA composite granules. Sufficient granules are weighed for compression molding preforming. All process parameters, such as hot pressing temperature, pressure, and time, are strictly consistent with those in Example 1 to obtain a preformed, unfoamed shoe sole blank. The preformed, unfoamed shoe sole blank was placed in a high-pressure foaming autoclave for supercritical foaming. All parameters, including foaming temperature, pressure, holding time, and depressurization method, were strictly consistent with those in Example 1. After foaming, the post-heat treatment temperature and time of the blank were also the same as in Example 1. Finally, an EVA foamed shoe sole sample was obtained.

[0033] Comparative Example 2 The specific implementation method is the same as in Example 1, except that a high-elasticity, lightweight integrated EVA supercritical foamed shoe sole is prepared, but only mercaptopropionic acid-grafted polydimethylsiloxane is added, without the addition of ionic liquid-modified montmorillonite. The preparation method and raw material quality of the mercaptopropionic acid-grafted polydimethylsiloxane are exactly the same as in Example 1. 100.0g of EVA resin, 5.0g of the aforementioned mercaptopropionic acid-grafted polydimethylsiloxane, 1.0g of dicumyl peroxide, 1.0g of zinc stearate, and 10100.3g of antioxidant are accurately weighed. All materials are put into a high-speed mixer and mixed at 1200rpm for 8.0min to obtain a premix. All equipment, process steps, and specific parameters (temperature, pressure, time, speed, etc.) for subsequent melt blending granulation, molding preforming, supercritical foaming, and post-heat treatment are strictly consistent with the description in Example 1, finally obtaining a foamed shoe sole sample.

[0034] Comparative Example 3 The specific implementation method is the same as in Example 1, except that a high-elasticity, lightweight integrated EVA supercritical foamed shoe sole is prepared, but only ionic liquid-modified montmorillonite is added, without the addition of mercaptopropionic acid-grafted polydimethylsiloxane. The preparation method of the ionic liquid-modified montmorillonite and the quality of the raw materials used are exactly the same as in Example 1. 100.0g of EVA resin, 3.0g of the aforementioned ionic liquid-modified montmorillonite, 1.0g of dicumyl peroxide, 1.0g of zinc stearate, and 10100.3g of antioxidant are accurately weighed. All materials are put into a high-speed mixer and mixed at 1200rpm for 8.0min to obtain a premix. All equipment, process steps, and specific parameters (temperature, pressure, time, speed, etc.) for subsequent melt blending granulation, molding preforming, supercritical foaming, and post-heat treatment are strictly consistent with the description in Example 1, finally obtaining a foamed shoe sole sample.

[0035] Performance testing The high-elasticity, lightweight integrated EVA supercritical foam shoe soles prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance tests according to the following method, which includes the following steps: Apparent density test: The foamed sample was cut into regular cubes with a side length of approximately 30.0 mm. Using an electronic balance with an accuracy of 0.001 g, the mass of the sample in dry air was first weighed and recorded as m. air Subsequently, the sample was completely immersed in a beaker containing distilled water at 23.0°C, and air bubbles adhering to the sample surface were removed using a fine metal wire. The sample was then suspended below a balance and completely submerged in water, and its apparent mass was measured and recorded as m. water The apparent density ρ of the sample is calculated using the following formula: ρ = [m air / (m air -m water )]×ρ water , where ρ water Take 1.000 g / cm 3 Five independent samples were tested for each sample, and the results were taken as the arithmetic mean.

[0036] Rebound rate test: The foamed sample was cut into flat test pieces with a thickness of not less than 25.0 mm and a planar dimension of not less than 100.0 mm × 100.0 mm. Using a falling ball rebound hammer, a solid steel ball with a mass of 16.3 g and a diameter of 16.0 mm was released freely from a height H = 460.0 mm above the test piece surface. A photoelectric sensor was used to measure the height h of the highest point reached by the steel ball after its first rebound from the test piece surface. The rebound rate R was calculated using the following formula: R = (h / H) × 100%. Each sample was tested 10 times at different locations on the surface, and the results were taken as the arithmetic mean.

[0037] Tensile strength test: The foamed sample was cut into standard test strips using a standard dumbbell-shaped cutter (gauge width 5.0 mm, thickness approximately 5.0 mm, total length not less than 75.0 mm). Using a universal testing machine, the two ends of the test strip were clamped in fixtures, with the initial clamping distance set to 25.0 mm. The test strip was stretched at a constant speed of 100.0 mm / min until it broke. The maximum tensile force F during the stretching process was recorded. max Use vernier calipers to measure the original width W and original thickness T of the gauge section of the test strip. The tensile strength TS is calculated using the following formula: TS = F max / (W×T). Five independent test strips were used for each sample, and the results were taken as the arithmetic mean.

[0038] Elongation at break test: This test is performed simultaneously with the tensile strength test. During the tensile test on the universal testing machine, the distance L between the clamps at the moment the specimen breaks is recorded. break The elongation at break (EB) is calculated using the following formula: EB = [(L break [-25.0) / 25.0]×100%, where 25.0 mm is the initial gauge length. Five independent test strips were tested for each sample, and the results were taken as the arithmetic mean.

[0039] Compression set test: The foamed sample was cut into cylinders with a diameter of 29.0 mm and a thickness of 25.0 mm. The original thickness H0 of each sample was measured using a digital caliper. The sample was placed in a compression fixture, and its thickness was compressed to 50.0% of its original thickness, i.e., the compression space was limited to H. s =0.5×H0. Place the entire fixture containing the sample in a 70.0℃ forced-air drying oven and remove it after 22.0 hours. Immediately release the fixture, remove the sample, and allow it to recover for 24.0 hours in a standard laboratory environment at 23.0℃ and 50% relative humidity. Afterward, measure the final recovered thickness H of the sample. f The compression set CS is calculated using the following formula: CS = [(H0 - H...] f ) / (H0-H s )]×100%. Three independent samples were tested for each sample, and the results were taken as the arithmetic mean.

[0040] Test results: Table 1: Test results of each embodiment and comparative example

[0041] As can be seen from Table 1, Examples 1-3 of the present invention effectively overcome the technical problem that existing ethylene vinyl acetate copolymer foaming materials are difficult to achieve uniform and stable cell structure, excellent elastic recovery performance and good mechanical strength at high foaming ratio when using supercritical fluid foaming technology by synergistically introducing mercaptopropionic acid-grafted polydimethylsiloxane and ionic liquid-modified montmorillonite.

[0042] Comparative Example 1, without any modifiers, represents the baseline performance of the pure EVA system after supercritical foaming: highest density (0.25 g / cm³). 3 The unmodified EVA exhibited the lowest rebound rate (48.3%) and the highest compression set (28.7%), confirming that it is difficult for unmodified EVA to maintain high elasticity and dimensional stability while achieving a low density. Its cell structure is prone to collapse during foaming, resulting in poor elastic recovery and limited mechanical strength (tensile strength 1.25 MPa). Comparative Example 2, which only added mercaptopropionic acid grafted polydimethylsiloxane, showed significantly better rebound rate (62.8%) and elongation at break (205%) than Comparative Example 1, and a lower density (0.22 g / cm³). 3 The further reduction in tensile strength (1.52 MPa) demonstrates that this component effectively improves melt strength and foaming uniformity by compatibilizing and stabilizing the cell walls, thereby improving lightweight and elasticity. However, its tensile strength (1.52 MPa) is still insufficient, and its compression set (18.4%) is also relatively high, indicating that relying solely on flexible segment modification cannot provide sufficient mechanical support and deformation resistance. Comparative Example 3, which only added ionic liquid to modify montmorillonite, had the highest tensile strength (1.70 MPa), reflecting the reinforcing effect of nanofillers. However, its resilience (58.6%), elongation at break (150%) were not ideal, and its compression set (22.1%) was still relatively high, with a density (0.26 g / cm³). 3 The strength was not effectively reduced, indicating that while using reinforcing fillers alone can improve strength, it will impair the flexibility and foaming ratio of the material, and has limited effect on stabilizing the cell structure.

[0043] In contrast, all embodiments of the present invention achieve comprehensive optimization and balance of performance: density (0.19-0.23 g / cm³). 3 The elastic recovery rate was lower than all comparative examples, demonstrating effective expansion at high foaming ratios; the resilience (66.2-70.1%) and elongation at break (198-230%) were significantly higher than all comparative examples, especially far superior to the system containing only one modifier, confirming excellent elastic recovery performance; at the same time, the tensile strength (1.68-2.05MPa) remained high, while the compression set (11.2-14.5%) was significantly reduced to an excellent level.

[0044] This series of data demonstrates that mercaptopropionic acid grafted with polydimethylsiloxane and ionic liquid modified montmorillonite produce a synergistic effect: the former mainly stabilizes cell growth and imparts flexibility, while the latter mainly provides heterogeneous nucleation sites and strengthens the matrix; the two work together to finely control the foaming process at the molecular and microscopic levels, thereby simultaneously achieving uniform and stable cell structure at high foaming ratios (manifested as low density and high resilience), excellent elastic recovery performance (high resilience and low compression set), and good mechanical strength (high tensile strength and elongation at break), successfully solving the contradiction that is difficult to achieve in existing technologies.

[0045] The embodiments described above are merely examples 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 present invention. 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 modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A preparation method of a high-elasticity light-weight integrated EVA supercritical foaming shoe sole, characterized by the steps of include: S1. By weight, 80-120 parts of EVA resin, 2-8 parts of mercaptopropionic acid-grafted polydimethylsiloxane, 1-5 parts of ionic liquid-modified montmorillonite, 0.5-2 parts of dicumyl peroxide, 0.5-2 parts of zinc stearate, and 0.1-0.5 parts of antioxidant 1010 are premixed in a high-speed mixer to obtain a premix; the premix is ​​fed into a twin-screw extruder for melt blending, extrusion granulation, and then obtained modified EVA composite granules. S2. Place the modified EVA composite granules into the mold of the flat vulcanizing machine and hot press them at 125-135℃ and 8-12MPa to obtain an unfoamed preform. Place the unfoamed preform in a high-pressure foaming kettle, seal it, heat it to 148-152℃, and introduce supercritical gas to maintain the pressure at 14-16MPa. Depressurize to obtain the foamed preform. Transfer the foamed preform to an oven at 118-122℃ for heat treatment.

2. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 1, characterized in that, In step S1, the premixing time is 5-10 min; the melt blending temperature is 120-130℃.

3. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 1, characterized in that, In step S2, the foamed blank is transferred to an oven at 118-122°C for heat treatment for 30-40 minutes; the supercritical gas is supercritical carbon dioxide and / or supercritical nitrogen.

4. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foamed shoe sole according to claim 1, characterized in that, The method for preparing the mercaptopropionic acid-grafted polydimethylsiloxane includes: A1. By weight, under dry nitrogen protection, add 80-120 parts of vinyl-terminated polydimethylsiloxane and 30-50 parts of anhydrous toluene to a three-necked flask and stir; then add 9-13 parts of 3-mercaptopropionic acid to obtain a mixture. A2. Heat the mixture to 78-82℃ and stir continuously. After the reaction is complete, cool to room temperature, add deionized water, stir, and let stand to separate the layers. Separate the organic phase. Dry the organic phase with anhydrous sodium sulfate, filter, and rotary evaporate to obtain the product. Dry the product in a vacuum oven at 70-80℃.

5. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 4, characterized in that, In step A1, the stirring time is 20-40 minutes.

6. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 4, characterized in that, In step A2, the reaction is continuously stirred for 5-7 hours.

7. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 1, characterized in that, The preparation method of the ionic liquid modified montmorillonite includes: B1. Disperse 8-12 parts by weight of sodium montmorillonite in deionized water and stir at 58-62℃ to obtain a montmorillonite suspension; dissolve 4-6 parts of 1-butyl-3-methylimidazolium chloride in deionized water and stir to obtain an ionic liquid solution. B2. Under stirring, the ionic liquid solution is added dropwise to the montmorillonite suspension. After the addition is complete, the reaction continues at 58-62℃ to obtain a mixture. The mixture is centrifuged to obtain a solid product. The solid product is washed with deionized water, dried in a vacuum oven at 78-82℃, ground, and sieved.

8. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 7, characterized in that, In step B1, the stirring time is 30-60 minutes.

9. The method for preparing a high-elasticity, lightweight, integrated EVA supercritical foam shoe sole according to claim 7, characterized in that, In step B2, the reaction continues at 58-62℃ for 12-14 hours.

10. A high-elasticity, lightweight, integrated EVA supercritical foam shoe sole, characterized in that, The high-elasticity, lightweight integrated EVA supercritical foam shoe sole is prepared according to any one of claims 1-9.

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