Preparation method of high-elastic shock-absorbing EVA (Ethylene Vinyl Acetate) composite insole

Abstract

The invention provides a preparation method of a high-elastic damping EVA composite insole, which comprises the following steps: S10: providing raw material components including an ethylene-vinyl acetate copolymer, an ethylene-octylene copolymer, an olefin block copolymer, a high-damping styrene elastomer, ethylene propylene diene monomer and an auxiliary agent; s20, the raw material components are sequentially subjected to high-speed shearing mixing, screw extrusion mixing and closed mixing, and an unfoamed rubber material is prepared; and S30, the non-foamed rubber material is placed in a mold cavity, and different heating temperatures are applied to different areas of the rubber material in the pressurizing and foaming process. The EVA composite insole prepared by the preparation method can be balanced between high rebound resilience and high shock absorption performance.

Description

Technical Field

[0001] This invention relates to the field of EVA composite midsole technology, and specifically to a method for preparing a high-elasticity, shock-absorbing EVA composite midsole. Background Technology

[0002] Ethylene-vinyl acetate copolymer (EVA) foam has long been a primary material for midsoles in athletic and casual shoes due to its lightweight, softness, and ease of processing. However, existing EVA composite midsoles present a trade-off between shock absorption and rebound performance. To reduce impact during exercise, current technologies typically add high-damping materials (such as conventional styrene-based elastomers) to EVA. While this method improves energy absorption, the high-damping material converts most of the mechanical energy into heat dissipation, leading to a decrease in the material's energy storage modulus and a significant reduction in rebound, resulting in insufficient energy return during exercise. Furthermore, under prolonged and repeated compression, high-damping formulations are prone to molecular chain slippage, causing permanent compression deformation (i.e., collapse) and affecting the material's lifespan. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned defects or problems in the prior art and provide a method for preparing a high-elasticity and shock-absorbing EVA composite midsole. The EVA composite midsole prepared by this method can achieve a balance between high resilience and high shock absorption performance.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: Technical Solution 1: A method for preparing a high-elasticity, shock-absorbing EVA composite midsole, comprising the following steps: S10: Providing raw material components, wherein the raw material components include: ethylene-vinyl acetate copolymer, ethylene-octene copolymer, olefin block copolymer, high-damping styrene elastomer, EPDM rubber, and additives; wherein the glass transition temperature of the high-damping styrene elastomer is -37℃ to -34℃, and the loss factor at a test frequency of 1Hz is 0.94-0.99; S20: Sequentially subjecting the raw material components to high-speed shear mixing, screw extrusion mixing, and closed-loop mixing to obtain an unfoamed rubber compound; S30: The unfoamed rubber material is placed into the mold cavity, and different heating temperatures are applied to different areas of the rubber material during the pressure foaming process. The mold cavity is divided into a first region and a second region along the longitudinal direction. The first region corresponds to the forefoot part of the midsole, and the second region corresponds to the heel part of the midsole. The heating temperature applied to the first region is lower than the heating temperature applied to the second region, so that the foaming rate of the rubber material in the first region is lower than that in the second region, forming a gradient structure in which the density of the first region is higher than that of the second region in one molding process.

[0005] Technical Solution Two based on Technical Solution One: The raw material components, by mass parts, include: 14-26 parts of ethylene-vinyl acetate copolymer, 19-27 parts of ethylene-octene copolymer, 14-23 parts of olefin block copolymer, 13-21 parts of high-damping styrene elastomer, 3-9 parts of ethylene propylene diene monomer (EPDM) rubber, 2.5-6.5 parts of inorganic filler, 0.18-0.48 parts of lubricant, 0.18-0.48 parts of vulcanization activator, 0.09-0.23 parts of masking agent, 0.17-0.47 parts of crosslinking agent, and 3.7-4.5 parts of high-temperature foaming agent; the molecular chain of the high-damping styrene elastomer contains styrene-hydrogenated butadiene random segments and polystyrene microblocks.

[0006] Technical Solution 3 based on Technical Solution 1: In step S10, the Shore hardness of the olefin block copolymer is 70-85, and the elongation at break is above 650%; the ethylene content of the EPDM rubber is 48%-62%, and the Mooney viscosity ML 1+4 125℃ is 35-65; the initial decomposition temperature of the high-temperature foaming agent in the additives is 160℃-165℃ after activation.

[0007] Technical Solution 4 based on Technical Solution 1: In step S20, the high-speed shear mixing process is as follows: at a temperature of 20℃-30℃, the mixture is mixed at a speed of 1800rpm-2200rpm for 3-5 minutes.

[0008] Technical Solution 5 based on Technical Solution 1: In step S20, the screw extrusion mixing process is as follows: the material after high-speed shear mixing is mixed with crosslinking agent and high-temperature foaming agent, and extruded at a temperature of 90℃-110℃ and a screw speed of 120rpm-160rpm.

[0009] Technical Solution Six Based on Technical Solution One: In step S20, the closed mixing process is as follows: the material after screw extrusion and mixing is mixed at a temperature of 125℃-145℃. When the coefficient of variation of the mixing uniformity of the material reaches less than 2% and the decomposition rate of the crosslinking agent is less than 5%, the mixing is terminated and the material is discharged.

[0010] Technical solution seven based on technical solution one: Between step S20 and step S30, step S25 is also included: pressing the sealed mixed rubber compound into a sheet with a thickness of 2.1mm-2.5mm, and then performing air cooling and water cooling in sequence to reduce the temperature of the rubber compound to below 85℃.

[0011] Technical solution eight based on technical solution one: In step S30, the heating temperature applied to the first region is 165℃-170℃, and the heating temperature applied to the second region is 175℃-180℃; the pressure for pressurized foaming is 6MPa-8MPa, and the holding time is 120 seconds-140 seconds.

[0012] Technical Solution Nine based on Technical Solution Eight: In step S30, after completing the foaming process with differentiated heating temperature, the heating temperature applied to the entire mold cavity is increased to 185℃-195℃, the pressure is increased to 7MPa-9MPa, and the holding time is 60 seconds-80 seconds. Technical solution ten based on technical solution one: After step S30, step S40 is also included: the foamed rubber material is set at a temperature of 2℃-8℃ and a pressure of 21kN-25kN for a setting time of 80 seconds-100 seconds.

[0013] As can be seen from the above description of the present invention, compared with the prior art, the present invention has the following beneficial effects: In existing technologies, conventional styrene-based elastomers are typically added to reduce impact force. However, this disrupts the continuous phase structure of the matrix material, leading to a decrease in the material's storage modulus, resulting in reduced resilience and increased susceptibility to compression set. In the first technical solution of this invention, a high-damping styrene-based elastomer with a glass transition temperature of -37°C to -34°C and a loss factor of 0.94-0.99 is preferably used as the energy-absorbing unit. This specific temperature range allows the material to operate in a high-energy-dissipation state under normal operating conditions, efficiently converting impact kinetic energy into heat energy. Simultaneously, the olefin block copolymer introduced into the formulation utilizes its crystalline hard segments as physical anchors to form an interpenetrating network structure with the high-damping phase at the microscopic level. This structure utilizes the elastic recovery force of the olefin block copolymer to restrict the irreversible slippage of the high-damping molecular chains after compression, thereby maintaining a high level of impact absorption capacity while preserving the material's rapid rebound and anti-collapse performance, overcoming the shortcomings of single modification methods that cannot simultaneously achieve high shock absorption and high resilience. However, during the molding process, the above-mentioned formulation, due to the introduction of highly crystalline olefin block copolymers and high-viscosity styrene-based elastomers, results in extremely high melt strength and elastic recovery tendency of the rubber compound during foaming, causing the material to tend to maintain the uniformity of overall density. Current midsoles require a density difference structure with a harder forefoot and softer heel. To address this, this solution utilizes the temperature-sensitive chemical property of the foaming agent's decomposition rate. By applying specific temperature differences in different areas of the mold, the rubber compound in the low-temperature zone (first zone) is in the foaming induction period, where the cross-linking reaction rate dominates, thus locking the polymer chains before the bubbles expand significantly, forcing the formation of a high-density support structure. Meanwhile, the rubber compound in the high-temperature zone (second zone) is in the foaming explosion period, where the expansion pressure generated by the gas breaks through the constraints of the high-viscosity matrix, forming a low-density buffer structure. This achieves a gradient distribution of macroscopic density without sacrificing the material's microscopic mechanical properties.

[0014] In technical solution two, the specific proportions of raw material components and the microstructure of the styrene-based elastomer are defined to ensure that the continuous phase matrix composed of ethylene-vinyl acetate copolymer and polyolefin elastomer has a sufficient volume ratio. This avoids excessive reduction in resilience performance due to phase inversion caused by excessive high-damping phase content. Simultaneously, the specifically defined high-damping styrene-based elastomer molecular chain contains styrene-hydrogenated butadiene random segments and polystyrene microblocks. This specific block structure improves its interfacial compatibility with nonpolar olefin block copolymers and ethylene-octene copolymers, enhances the gripping force of physical anchors on the energy-absorbing molecular chains, prevents damping performance degradation due to phase separation during long-term dynamic compression, and ensures the durability of high-elasticity shock absorption performance.

[0015] In technical solution three, the relevant parameters of the olefin block copolymer and EPDM rubber are defined to ensure a balance between the rheological properties and final mechanical properties of the material during processing. Simultaneously, the limitation on the initial decomposition temperature of the high-temperature foaming agent enables the density gradient distribution in this solution. If the decomposition characteristics of the foaming agent do not match the set mold temperature, for example, if the decomposition temperature is too high, neither the low-temperature nor the high-temperature zone can effectively foam, leading to gradient construction failure. This solution adjusts the initial decomposition temperature of the high-temperature foaming agent to 160℃-165℃ using an activator, matching the heating temperature setting of the low-temperature zone in gradient molding. This allows the rubber compound in the first region to be in the initial induction period of foaming agent decomposition, with a slow and controllable reaction rate, achieving a dense structure lock-in in conjunction with the crosslinking reaction; while the rubber compound in the second region can foam rapidly, thus achieving density gradient changes in different regions of the forefoot and heel.

[0016] In technical solution four, a low-temperature, high-speed shear mixing process is employed to solve the problem of dispersing high-viscosity rubber components. Because the formulation contains high-viscosity EPDM rubber and styrene-based elastomers, conventional mixing methods struggle to disperse them to the micron level. This solution utilizes the strong shear field generated by high-speed rotation to break up large rubber particles, while simultaneously controlling the mixing temperature at 20℃-30℃. This effectively removes the frictional heat generated by high shear, ensuring uniform dispersion of high-viscosity components while preventing heat accumulation in the rubber compound during the premixing stage and thus preventing early activation of heat-sensitive additives.

[0017] In technical solution five, the temperature range for screw extrusion mixing is limited, solving the problem of early decomposition of heat-sensitive crosslinking agents and foaming agents when mixed into high-viscosity matrices. After physical dispersion is completed by high-speed shearing, the crosslinking agent and foaming agent need to be uniformly mixed into the matrix. However, the matrix viscosity is still high at this time. If the extrusion temperature is too low, plasticization will be poor; if the temperature is too high, scorching will occur. The temperature range set in this solution is slightly higher than the melting point of the matrix resin to ensure good flow and coating properties, while being lower than the critical decomposition temperature of the crosslinking agent. This ensures that the additives are homogenized and dispersed, and that the rubber compound does not undergo pre-crosslinking reaction before entering the internal mixing process, thus preserving a complete processing window.

[0018] In technical solution six, the termination criteria for closed mixing are defined, solving the problem of a narrow processing window in the final mixing stage for high-performance formulations. Due to the complexity of the components and the large viscosity differences, a large amount of shear heat is generated during the homogenization process of closed mixing, causing the dispersion endpoint and the scorching start point to coincide in time. Therefore, this solution does not control the termination of closed mixing through time or temperature, but rather uses detection methods to measure the coefficient of variation of mixing uniformity and the decomposition rate of crosslinking agents. By using more precise indicators, the moment when the material reaches the optimal dispersion state without scorching occurs is determined, thereby eliminating performance fluctuations between batches.

[0019] In technical solution seven, a sheeting and compounding cooling step is added between mixing and foaming to eliminate the interference of residual heat inside the rubber compound on the foaming process. The mixed rubber compound usually carries a high level of residual heat. If directly piled up or subjected to subsequent operations, the accumulated heat will cause the crosslinking agent in the central area to slowly decompose (latent scorching), disrupting the gradient formation. This solution presses the rubber compound into thin sheets to increase the heat dissipation surface area and combines air cooling and water cooling to rapidly cool it to below 85°C. By uniformly restoring the temperature of the rubber compound to a lower level, it ensures that all areas of the rubber compound are heated from the same initial temperature when subsequently placed into the mold.

[0020] In technical solution eight, the specific heating temperature ranges of the first and second regions are defined. The temperature of the first region is set at 165℃-170℃, which is within the decomposition induction period of the activated foaming agent. Foaming is slow but cross-linking is fast, which is conducive to forming a high-density forefoot. The temperature of the second region is set at 175℃-180℃, which is within the peak period of foaming agent decomposition. Gas production is rapid, which is conducive to forming a low-density heel.

[0021] In technical solution nine, an overall high-temperature and high-pressure curing step is introduced after differential heating, solving the problem of incomplete shaping caused by the gradient molding stage. To form a high-density structure, the first region is at a lower temperature, which, while inhibiting foaming, also leads to incomplete cross-linking reactions, making the product prone to dimensional shrinkage or deformation after demolding. This solution, after the initial formation of the gradient structure, raises the overall temperature to 185℃-195℃ and increases the pressure, promoting the complete decomposition of the cross-linking agent in all regions, perfecting the three-dimensional cross-linking network, and simultaneously using high pressure to further compact the cell walls, thereby significantly improving the dimensional stability and mechanical strength of the material.

[0022] In technical solution ten, a low-temperature, high-pressure cold-pressing process is employed to solve the shrinkage and deformation problem of high-resilience foamed materials after demolding. Due to the high resilience and closed-cell structure of this composite material, the high-pressure gas inside the cells during hot demolding can easily cause unexpected expansion or cooling shrinkage in the product. This solution rapidly places the high-temperature product into a low-temperature environment of 2℃-8℃ and applies high pressure, fixing the movement of the polymer chain segments in a very short time. This causes the cell walls to harden and solidify quickly, effectively counteracting pressure changes in the internal gas and ensuring the dimensional stability of the final product. Detailed Implementation

[0023] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are preferred embodiments of the present invention and should not be considered as excluding other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0024] In the claims and description of this invention, the terms "comprising," "having," and variations thereof are used to mean "including but not limited to."

[0025] This invention relates to a method for preparing a high-elasticity, shock-absorbing EVA composite material, which mainly includes the following steps: S10: Provide raw material components, which include: ethylene-vinyl acetate copolymer, ethylene-octene copolymer, olefin block copolymer, high-damping styrene elastomer, EPDM rubber and additives; wherein the high-damping styrene elastomer has a glass transition temperature of -37°C to -34°C and a loss factor of 0.94-0.99 at a test frequency of 1Hz; S20: The raw material components are sequentially subjected to high-speed shear mixing, screw extrusion mixing and closed mixing to obtain an unfoamed rubber compound; S30: The unfoamed rubber material is placed into the mold cavity, and different heating temperatures are applied to different areas of the rubber material during the pressure foaming process; wherein, the mold cavity is divided into a first area and a second area along the longitudinal direction, the first area corresponds to the forefoot part of the midsole, and the second area corresponds to the heel part of the midsole; the heating temperature applied to the first area is lower than the heating temperature applied to the second area, so that the foaming rate of the rubber material in the first area is lower than the foaming rate of the rubber material in the second area, forming a gradient structure in which the density of the first area is higher than that of the second area in one molding.

[0026] The following will provide a detailed explanation of each of the above steps.

[0027] In step S10, the required raw material components are first prepared. Specifically, in this step, the required raw material components are weighed and prepared. The raw material components, by mass parts, include resin and elastomer matrix, functional modifier, and chemical additives. Specifically, 14 to 26 parts of ethylene-vinyl acetate copolymer, 19 to 27 parts of ethylene-octene copolymer, 14 to 23 parts of olefin block copolymer, 13 to 21 parts of high-damping styrene elastomer, and 3 to 9 parts of ethylene propylene diene monomer (EPDM) rubber are weighed.

[0028] For the aforementioned polymer components, the selected olefin block copolymer must meet the following physical properties: Shore hardness of 70 to 85 and elongation at break of over 650%. The selected high-damping styrene elastomer must contain styrene-hydrogenated butadiene random segments and polystyrene microblocks in its molecular chain structure, and its physical properties must meet the following requirements: glass transition temperature of -37°C to -34°C and loss factor of 0.94 to 0.99 at a test frequency of 1 Hz. The selected ethylene propylene diene monomer (EPDM) rubber must have an ethylene content between 48% and 62%, and a Mooney viscosity (ML 1+4) between 35 and 65 at 125°C.

[0029] In addition to the aforementioned polymer matrix material, various additives need to be weighed according to their mass percentages, including: 2.5 to 6.5 parts inorganic filler, 0.18 to 0.48 parts lubricant, 0.18 to 0.48 parts vulcanization activator, 0.09 to 0.23 parts covering agent, 0.17 to 0.47 parts crosslinking agent, and 3.7 to 4.5 parts high-temperature foaming agent. The selection or treatment of the high-temperature foaming agent must ensure that its initial decomposition temperature after contact and activation with the aforementioned vulcanization activator and other additives is within the range of 160°C to 165°C. All the weighed components should be prepared for use in subsequent mixing processes.

[0030] The purpose of step S20 is to convert the raw material components provided in step S10 into a uniformly mixed and non-pre-crosslinked rubber compound. This process includes three stages in sequence: high-speed shear mixing, screw extrusion mixing, and closed mixing.

[0031] The first step is a high-speed shear mixing stage. The ethylene-vinyl acetate copolymer, ethylene-octene copolymer, olefin block copolymer, high-damping styrene elastomer, EPDM rubber, and other powder additives (including inorganic fillers, lubricants, vulcanization activators, and masking agents) weighed in step S10 are added to a high-speed mixer. During mixing, the temperature inside the mixing chamber is controlled between 20°C and 30°C, and the stirring blade speed is set to 1800 rpm to 2200 rpm. Mixing continues for 3 to 5 minutes to obtain a premixed material with preliminary physical mixing.

[0032] The subsequent screw extrusion mixing stage is then carried out. The premixed material after high-speed shear mixing is fed into the feed port of a screw extruder (e.g., a twin-screw extruder), and the crosslinking agent and high-temperature foaming agent weighed in step S10 are added to the extruder during this stage. The barrel temperature range of the screw extruder is set to 90°C to 110°C, and the screw speed is set to 120 rpm to 160 rpm. Under the conveying and shearing action of the screw, the material undergoes plasticization and further dispersion of the additives within the specified temperature range, and is discharged from the extruder die.

[0033] Next, a closed mixing stage is performed. The material discharged from the screw extruder is directly fed into the mixing chamber of the internal mixer. The mixing temperature of the internal mixer is set to 125℃ to 145℃. In this stage, the mixing and chemical states of the material are monitored in real time using an in-situ spectral detection system (e.g., a near-infrared spectral probe). The mixing state of the material is determined by analyzing the spectral variance of the characteristic absorption peaks of the polymer matrix (e.g., CH bond overtone peaks), and the coefficient of variation of mixing uniformity is calculated accordingly. The chemical state of the material is determined by monitoring the change in the area of ​​the characteristic absorption peaks of specific chemical bonds of the crosslinking agent (e.g., peroxide bonds), and the decomposition rate of the crosslinking agent is calculated accordingly. When the real-time monitoring data shows that the coefficient of variation of mixing uniformity of the material reaches below 2%, and simultaneously shows that the decomposition rate of the crosslinking agent is less than 5%, the mixing operation is immediately stopped and the rubber compound is discharged from the internal mixer.

[0034] After completing the closed mixing and discharge process described above, the rubber compound needs to be sheeted and cooled before entering the mold for foaming (i.e., step S25). The lumpy rubber compound discharged from the internal mixer is fed into a sheeting machine (such as a two-roll open mill) and pressed into continuous sheets with a thickness of 2.1 mm to 2.5 mm. Subsequently, the rubber sheet is cooled by passing it through an air-cooling device and a water-cooling device in sequence, controlling the cooling process until the temperature of the rubber compound drops below 85°C, finally obtaining a cooled and shaped unfoamed rubber sheet for subsequent cutting and use.

[0035] The purpose of step S30 is to place the unfoamed rubber material processed in steps S20 and S25 into a specific mold and transform it into a foamed midsole with a gradient structure of hard front and soft back through a hot-pressing process. First, the unfoamed rubber material sheet after cooling and setting is cut according to the size of the mold cavity, and the cut rubber material is filled into the mold cavity. The mold cavity is divided into a first region and a second region in the longitudinal length, wherein the first region corresponds to the forefoot part of the sole and the second region corresponds to the heel part of the sole.

[0036] In the initial stage of pressurized foaming, differentiated heating control is implemented on the rubber compound within the mold cavity. Specifically, the heating temperature applied to the first region of the mold is set to 165°C to 170°C, while the heating temperature applied to the second region of the mold is set to 175°C to 180°C, ensuring that the heating temperature applied to the first region is always lower than that applied to the second region. Under these temperature settings, the mold is closed and a pressure of 6MPa to 8MPa is applied, maintaining this pressure and differentiated temperature state for 120 to 140 seconds. During this process, because the temperature in the first region is lower, the foaming rate of the rubber compound in this region is lower than that in the second region, thereby initially constructing a gradient structure within the rubber compound where the density of the first region is higher than that of the second region.

[0037] After completing the foaming process at the differentiated heating temperatures, in order to fully mature the material and stabilize its structure, the entire mold cavity is then subjected to a uniform high-temperature and high-pressure treatment. The heating temperature applied to the entire mold cavity (including the first and second regions) is increased to 185°C to 195°C, while the mold closing pressure is increased to 7MPa to 9MPa, and maintained at this state for 60 to 80 seconds to ensure that the foaming and cross-linking reactions are complete.

[0038] After the hot-pressing foaming process is completed, the mold is quickly opened and the high-temperature foamed molding compound is removed, and it immediately enters the cold-pressing and shaping process. The removed foamed compound is placed into a cold-pressing mold, and a pressure of 21kN to 25kN is applied at a low temperature of 2℃ to 8℃ for shaping. This cold-pressing state is maintained for 80 to 100 seconds to allow the foamed material to cool and solidify, ultimately producing a high-elasticity, shock-absorbing EVA composite midsole with stable geometric dimensions and a gradient density structure.

[0039] In existing technologies, conventional styrene-based elastomers are typically added to reduce impact force. However, this disrupts the continuous phase structure of the matrix material, leading to a decrease in the material's storage modulus, resulting in reduced resilience and increased susceptibility to compression set. In the first technical solution of this invention, a high-damping styrene-based elastomer with a glass transition temperature of -37°C to -34°C and a loss factor of 0.94-0.99 is preferably used as the energy-absorbing unit. This specific temperature range allows the material to operate in a high-energy-dissipation state under normal operating conditions, efficiently converting impact kinetic energy into heat energy. Simultaneously, the olefin block copolymer introduced into the formulation utilizes its crystalline hard segments as physical anchors to form an interpenetrating network structure with the high-damping phase at the microscopic level. This structure utilizes the elastic recovery force of the olefin block copolymer to restrict the irreversible slippage of the high-damping molecular chains after compression, thereby maintaining a high level of impact absorption capacity while preserving the material's rapid rebound and anti-collapse performance, overcoming the shortcomings of single modification methods that cannot simultaneously achieve high shock absorption and high resilience. However, during the molding process, the above-mentioned formulation, due to the introduction of highly crystalline olefin block copolymers and high-viscosity styrene-based elastomers, results in extremely high melt strength and elastic recovery tendency of the rubber compound during foaming, causing the material to tend to maintain the uniformity of overall density. Current midsoles require a density difference structure with a harder forefoot and softer heel. To address this, this solution utilizes the temperature-sensitive chemical property of the foaming agent's decomposition rate. By applying specific temperature differences in different areas of the mold, the rubber compound in the low-temperature zone (first zone) is in the foaming induction period, where the cross-linking reaction rate dominates, thus locking the polymer chains before the bubbles expand significantly, forcing the formation of a high-density support structure. Meanwhile, the rubber compound in the high-temperature zone (second zone) is in the foaming explosion period, where the expansion pressure generated by the gas breaks through the constraints of the high-viscosity matrix, forming a low-density buffer structure. This achieves a gradient distribution of macroscopic density without sacrificing the material's microscopic mechanical properties.

[0040] To further illustrate the technical effects of the present invention, the following embodiments and comparative examples are provided.

[0041] To ensure the feasibility of this invention and the accuracy of the test results, the raw material specifications used in the examples and comparative examples are as follows (unless otherwise specified, all components are expressed in parts by mass): Ethylene-vinyl acetate copolymer (EVA): Grade 7470, purchased from Formosa Plastics Industrial Co., Ltd. Vinyl acetate content (VA%) is 18%, melt flow rate (MFI) is 2.5 g / 10 min.

[0042] Ethylene-octene copolymer (POE): Grade 8150, purchased from Dow Chemical. Density 0.868 g / cm³, melt flow rate 0.5 g / 10 min.

[0043] Olefin block copolymer (OBC): Grade 9107, purchased from Dow Chemical. Shore A hardness is 83, elongation at break is >700%, and melting point is 119℃.

[0044] High-damping styrene-based elastomer (core component of this invention): grade YH530, purchased from Sinopec. The molecular chain of this elastomer contains styrene-hydrogenated butadiene random segments and polystyrene microblocks, with a glass transition temperature (Tg) of -35.5℃ and a loss factor (Tanδ) of 0.96 at 1Hz.

[0045] Common styrene-based elastomer (for comparison): Grade G1651, purchased from Kraton Polymers. Standard SEBS, not modified for high damping.

[0046] Ethylene propylene diene monomer (EPDM) rubber: Grade 3092, purchased from Mitsui Chemicals. Ethylene content 55%, Mooney viscosity ML 1+4 125℃ is 50.

[0047] Inorganic filler: Talc powder, brand name BH818, purchased from Haicheng, Liaoning.

[0048] Lubricants: stearic acid (STE) and zinc stearate (ZNST), purchased from Hangzhou Oil & Fat Chemical Co., Ltd.

[0049] Sulfidation activator: Nano zinc oxide, purchased from Dalian Chemical.

[0050] Opacifier: Rutile titanium dioxide R103, purchased from DuPont (Chemours).

[0051] Crosslinking agent: dicumyl peroxide (DCP), purchased from AkzoNobel.

[0052] High-temperature foaming agent: Modified azodicarbonamide (AC-3000F), purchased from Ginlong Technologies. After activation treatment, the initial decomposition temperature was adjusted to 163℃.

[0053] Example 1 First, proceed to step S10 and weigh the raw material components. By mass, weigh 14 parts of ethylene-vinyl acetate copolymer, 19 parts of ethylene-octene copolymer, 14 parts of olefin block copolymer, 13 parts of high-damping styrene elastomer, 3 parts of EPDM rubber, 2.5 parts of inorganic filler, 0.18 parts of lubricant, 0.18 parts of vulcanization activator, 0.09 parts of masking agent, 0.17 parts of crosslinking agent, and 3.7 parts of high-temperature foaming agent.

[0054] Next, proceed to step S20 to prepare the unfoamed compound. All raw materials except the crosslinking agent and high-temperature foaming agent are fed into a high-speed mixer. Under temperature control at 20°C, the speed is set to 1800 rpm for high-speed shear mixing for 3 minutes. Subsequently, the mixture, along with the crosslinking agent and high-temperature foaming agent, is fed into a twin-screw extruder. The barrel temperature is set to 90°C and the screw speed to 120 rpm for low-temperature extrusion mixing. The extruded material is immediately fed into an internal mixer, with the mixing temperature set to 125°C. During the closed mixing process, when the coefficient of variation of the material's mixing uniformity drops to 1.9% and the crosslinking agent decomposition rate reaches 4.8%, mixing is immediately stopped and the material is discharged.

[0055] Then, step S25 is performed for sheeting and cooling. The discharged rubber compound is pressed into sheets with a thickness of 2.1 mm using an open mill, and then treated by a combined air-cooling and water-cooling system to rapidly reduce the temperature of the rubber compound to below 85°C.

[0056] Next, step S30 is performed for gradient foaming molding. The cooled rubber material is cut and placed into the mold cavity. The heating temperature of the first area corresponding to the forefoot is set to 165°C, and the heating temperature of the second area corresponding to the heel is set to 175°C. After the mold is closed, a pressure of 6MPa is applied and held for 120 seconds, using the temperature difference to induce a difference in foaming rate between the forefoot and heel. Then, the curing process in step S30 is performed, and the overall temperature of the mold is rapidly increased to 185°C and the pressure is increased to 7MPa, held for 60 seconds to complete the cross-linking and shaping.

[0057] Finally, proceed to step S40 for cold pressing and shaping. Quickly transfer the foamed product into the cold press mold, hold it at a low temperature of 2°C and a pressure of 21kN for 80 seconds, then remove it and trim the edges to obtain the finished product.

[0058] Example 2 First, proceed to step S10, weighing the raw material components. By mass, weigh 26 parts of ethylene-vinyl acetate copolymer, 27 parts of ethylene-octene copolymer, 23 parts of olefin block copolymer, 21 parts of high-damping styrene elastomer, 9 parts of EPDM rubber, 6.5 parts of inorganic filler, 0.48 parts of lubricant, 0.48 parts of vulcanization activator, 0.23 parts of masking agent, 0.47 parts of crosslinking agent, and 4.5 parts of high-temperature foaming agent.

[0059] Next, step S20 is performed to prepare the unfoamed compound. All raw materials except the crosslinking agent and high-temperature foaming agent are fed into a high-speed mixer. Under temperature control at 30°C, the speed is set to 2200 rpm, and high-speed shear mixing is carried out for 5 minutes to ensure sufficient dispersion of the high-content elastomer components. Subsequently, the mixture, along with the crosslinking agent and high-temperature foaming agent, is fed into a twin-screw extruder. The barrel temperature is set to 110°C, and the screw speed to 160 rpm for extrusion mixing. The extruded material is then fed into an internal mixer, with the mixing temperature set to 145°C. During the closed mixing process, when the coefficient of variation of the material mixing uniformity drops to 1.8% and the crosslinking agent decomposition rate reaches 4.5%, mixing is immediately stopped and the material is discharged.

[0060] Then, step S25 is performed for sheeting and cooling. The discharged rubber compound is pressed into sheets with a thickness of 2.5 mm using an open mill, and then treated by a combined air-cooling and water-cooling system to reduce the temperature of the rubber compound to below 85°C.

[0061] Next, step S30 is performed for gradient foaming molding. The cooled rubber material is cut and placed into the mold cavity. The heating temperature of the first area corresponding to the forefoot is set to 170°C, and the heating temperature of the second area corresponding to the heel is set to 180°C. After the mold is closed, a pressure of 8MPa is applied and held for 140 seconds. Then, the curing process in step S30 is performed, raising the overall mold temperature to 195°C and the pressure to 9MPa, and holding for 80 seconds.

[0062] Finally, proceed to step S40 for cold pressing and shaping. Quickly transfer the foamed product into the cold press mold, hold it at 8°C and 25kN pressure for 100 seconds, then remove it and trim the edges to obtain the finished product.

[0063] Example 3 First, proceed to step S10 and weigh the raw material components. By mass, weigh 20 parts of ethylene-vinyl acetate copolymer, 23 parts of ethylene-octene copolymer, 18 parts of olefin block copolymer, 17 parts of high-damping styrene elastomer, 6 parts of EPDM rubber, 4.5 parts of inorganic filler, 0.33 parts of lubricant, 0.33 parts of vulcanization activator, 0.16 parts of masking agent, 0.32 parts of crosslinking agent, and 4.1 parts of high-temperature foaming agent.

[0064] Next, proceed to step S20 to prepare the unfoamed compound. All raw materials except the crosslinking agent and high-temperature foaming agent are fed into a high-speed mixer. Under temperature control at 25°C, the speed is set to 2000 rpm, and high-speed shear mixing is performed for 4 minutes. Subsequently, the mixture, along with the crosslinking agent and high-temperature foaming agent, is fed into a twin-screw extruder, with the barrel temperature set at 100°C and the screw speed at 140 rpm. The extruded material is then fed into an internal mixer, with the mixing temperature set at 135°C. During the closed mixing process, when the coefficient of variation of the material's mixing uniformity drops to 1.5% and the crosslinking agent decomposition rate reaches 3.0%, mixing is immediately stopped and the material is discharged.

[0065] Then, step S25 is performed for sheeting and cooling. The discharged rubber compound is pressed into sheets with a thickness of 2.3 mm using an open mill, and then treated by a combined air-cooling and water-cooling system to reduce the temperature of the rubber compound to below 85°C.

[0066] Next, step S30 is performed for gradient foaming molding. The cooled rubber material is cut and placed into the mold cavity. The heating temperature of the first area corresponding to the forefoot is set to 168°C, and the heating temperature of the second area corresponding to the heel is set to 178°C. After the mold is closed, a pressure of 7MPa is applied and held for 130 seconds. Then, the curing process in step S30 is performed, raising the overall mold temperature to 190°C and the pressure to 8MPa, and holding for 70 seconds.

[0067] Finally, proceed to step S40 for cold pressing and shaping. Quickly transfer the foamed product into the cold press mold, hold it at 5°C and 23kN for 90 seconds, then remove it and trim the edges to obtain the finished product.

[0068] Based on Examples 1 to 3 above (corresponding to the lower limit, upper limit, and optimal value of the parameter range, respectively), the following comparative examples are provided to further demonstrate the inventiveness of the technical solution of the present invention. These comparative examples verify the necessity of the synergistic effect of the core components, the gradient molding process, and the multi-stage mixing process in the formulation of the present invention.

[0069] Comparative Example 1 The preparation method is exactly the same as in Example 3, except for the selection of raw material components in step S10: In Example 3, 17 parts of "high-damping styrene elastomer (YH530) with a glass transition temperature of -35.5°C and a loss factor of 0.96" were replaced in equal amounts with 17 parts of "conventional styrene-ethylene / butene-styrene block copolymer (grade G1651, with a glass transition temperature of approximately -60°C and a loss factor of approximately 0.1)". The remaining components and their amounts remained unchanged.

[0070] Comparative Example 2 The preparation method is exactly the same as in Example 3, except for the selection of raw material components in step S10: Remove 18 parts of "olefin block copolymer (9107)" from Example 3 and replace these 18 parts with an equal amount of "ethylene-octene copolymer (POE 8150)". That is, the total amount of POE in the formulation increases to 41 parts, and it does not contain OBC. The remaining components and their amounts remain unchanged.

[0071] Comparative Example 3 The formula and steps S10, S20, and S25 are exactly the same as in Example 3, except for the foaming process in step S30: In step S30, the heating temperatures of the first and second regions are not differentiated; instead, a uniform heating temperature of 175°C is applied to the entire mold cavity. The pressure foaming pressure is 7 MPa, and the holding time is 130 seconds. The subsequent curing and cold pressing shaping processes in step S40 are consistent with those in Example 3.

[0072] Comparative Example 4 The formula is exactly the same as in Example 3, except for the mixing process in step S20: A conventional single-stage internal mixing process was adopted. All raw materials (including crosslinking agent and high-temperature foaming agent) from step S10 were fed into the internal mixer at once. The mixing temperature was set to 115°C. High-speed shearing and screw extrusion were not performed, nor was real-time monitoring of mixing uniformity and decomposition rate conducted. Instead, the mixing time was set to 10 minutes (600 seconds) based on conventional experience before discharge. Subsequent steps S25, S30, and S40 were consistent with those in Example 3.

[0073] The performance of the EVA composite midsoles prepared in Examples 1-3 and Comparative Examples 1-4 was tested. The test standards are as follows: Apparent density: Tested according to GB / T 6343-2009 "Determination of apparent density of foamed plastics and rubber". Samples were taken from the forefoot and heel of the finished product midsole for measurement.

[0074] Resilience (falling ball method): Tested according to GB / T 1681-2009 "Determination of resilience of vulcanized rubber". The higher the test result, the better the resilience performance.

[0075] Compression set: Tested according to GB / T 7759.1-2015 "Determination of compression set of vulcanized rubber or thermoplastic rubber - Part 1: At room temperature and high temperature". Test conditions were set as follows: compression ratio 50%, test temperature 50℃, compression time 22 hours, recovery time 30 minutes. This index is used to evaluate the material's resistance to collapse; the lower the value, the better.

[0076] Hardness: Tested according to GB / T 531.1-2008 "Test method for indentation hardness of vulcanized rubber or thermoplastic rubber" using a Shore C hardness tester.

[0077] Appearance quality: Visually inspect the uniformity of the foam cells, whether the surface is collapsed or has scorched particles.

[0078] The test results are as follows:

[0079] A method for preparing a high-elasticity, shock-absorbing EVA composite midsole includes the following steps: S10: Providing raw material components, wherein the raw material components include: ethylene-vinyl acetate copolymer, ethylene-octene copolymer, olefin block copolymer, high-damping styrene elastomer, EPDM rubber, and additives; wherein the glass transition temperature of the high-damping styrene elastomer is -37℃ to -34℃, and the loss factor at a test frequency of 1Hz is 0.94-0.99; S20: Sequentially subjecting the raw material components to high-speed shear mixing, screw extrusion mixing, and closed-loop mixing to obtain an unfoamed rubber compound; S30: The unfoamed rubber material is placed into the mold cavity, and different heating temperatures are applied to different areas of the rubber material during the pressure foaming process. The mold cavity is divided into a first region and a second region along the longitudinal direction. The first region corresponds to the forefoot part of the midsole, and the second region corresponds to the heel part of the midsole. The heating temperature applied to the first region is lower than the heating temperature applied to the second region, so that the foaming rate of the rubber material in the first region is lower than that in the second region, forming a gradient structure in which the density of the first region is higher than that of the second region in one molding process.

[0080] Compared to Example 3, Comparative Example 1, which used a conventional styrene-based elastomer instead of the high-damping elastomer specific to this invention, could still be molded, but its resilience was significantly reduced to 48%, and its compression set increased. This indicates that conventional elastomers cannot form an effective interpenetrating anchoring network with olefin block copolymers, and under pressure, they only exhibit simple physical filling and energy dissipation, resulting in weak resilience.

[0081] Comparative Example 2, lacking the olefin block copolymer, showed a decrease in compression set to 45%, demonstrating that the crystalline hard segments of the olefin block copolymer played a crucial physical anchoring role in the system, locking the high-damping molecular chains and preventing structural collapse of the material under long-term stress.

[0082] In Comparative Example 3, after eliminating the differentiated heating in step S30, the density of the forefoot and heel of the resulting midsole tended to be consistent, failing to form a gradient structure. This confirms that for the rubber compound of this invention, which has high melt strength and high resilience, the material itself tends to expand uniformly and cannot naturally form a significant density difference like ordinary low-end EVA. Only through the means provided by this invention can this problem be overcome, and a stable density gradient be constructed.

[0083] Comparative Example 4, produced using a conventional internal mixing process, resulted in severe scorching, abnormally high density, a rough appearance, and inability to foam properly. This is because the high-performance formulation of this invention has complex components and high viscosity; the shear heat accumulated during prolonged conventional mixing can cause heat-sensitive additives to fail. This demonstrates that the method provided by this invention is a necessary prerequisite for the industrial-scale preparation of this high-performance formulation.

[0084] The foregoing description of the specifications and embodiments is intended to explain the scope of protection of this invention, but does not constitute a limitation on the scope of protection of this invention. Modifications, equivalent substitutions, or other improvements to the embodiments of this invention or a portion thereof that can be obtained by those skilled in the art through logical analysis, reasoning, or limited experimentation, based on the teachings of this invention or the foregoing embodiments, in conjunction with common knowledge, general technical knowledge, and / or existing technology, should all be included within the scope of protection of this invention.

Claims

1. A method for preparing a high-elasticity shock-absorbing EVA composite midsole, characterized in that, The method comprises the following steps: S10: providing raw material components, the raw material components comprising: ethylene-vinyl acetate copolymer, ethylene-octene copolymer, olefin block copolymer, high-damping styrene-based elastomer, ethylene-propylene-diene rubber and additives; wherein the high-damping styrene-based elastomer has a glass transition temperature of-37℃ to-34℃ and a loss factor of 0.94-0.99 at a test frequency of 1Hz; S20: sequentially performing high-speed shearing mixing, screw extrusion mixing and closed mixing on the raw material components to obtain unfoamed rubber compound; S30: placing the unfoamed rubber compound into a mold cavity, and applying different heating temperatures to different areas of the rubber compound during the pressurized foaming process; wherein the mold cavity is divided into a first area and a second area along the longitudinal direction, the first area corresponds to the forefoot part of the midsole, and the second area corresponds to the heel part of the midsole; the heating temperature applied to the first area is lower than the heating temperature applied to the second area, so that the foaming rate of the rubber compound in the first area is lower than the foaming rate of the rubber compound in the second area, and a gradient structure with the density of the first area being higher than the density of the second area is formed in one-time molding.

2. The method for preparing high-elastic damping EVA composite material according to claim 1, characterized in that, In step S10, the raw material components comprise, by mass fraction: ethylene-vinyl acetate copolymer 14-26 parts, ethylene-octene copolymer 19-27 parts, olefin block copolymer 14-23 parts, high-damping styrene-based elastomer 13-21 parts, ethylene-propylene-diene rubber 3-9 parts, inorganic filler 2.5-6.5 parts, lubricant 0.18-0.48 parts, vulcanization activator 0.18-0.48 parts, covering agent 0.09-0.23 parts, crosslinking agent 0.17-0.47 parts, and high-temperature foaming agent 3.7-4.5 parts; the molecular chain of the high-damping styrene-based elastomer comprises styrene-hydrogenated butadiene random segments and polystyrene micro-blocks.

3. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, In step S10, the olefin block copolymer has a Shore hardness of 70-85 and an elongation at break of more than 650%; the ethylene-propylene-diene rubber has an ethylene content of 48%-62% and a Mooney viscosity ML 1+4 125℃ of 35-65; and the high-temperature foaming agent in the additives has an initial decomposition temperature of 160℃-165℃ after activation.

4. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, In step S20, the high-speed shearing mixing process is as follows: mixing at a temperature of 20℃-30℃ and a speed of 1800rpm-2200rpm for 3-5 minutes.

5. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, In step S20, the screw extrusion mixing process is as follows: mixing the material after high-speed shearing mixing with the crosslinking agent and the high-temperature foaming agent, and extruding at a temperature of 90℃-110℃ and a screw speed of 120rpm-160rpm.

6. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, In step S20, the closed mixing process is as follows: mixing the material after screw extrusion mixing at a temperature of 125℃-145℃; when the uniformity coefficient of the material is less than or equal to 2% and the crosslinking agent decomposition rate is less than 5%, the mixing is terminated and the material is discharged.

7. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, Between step S20 and step S30, step S25 is further included: the closed mixed rubber is pressed into a sheet with a thickness of 2.1-2.5 mm, and then air cooling and water cooling are sequentially performed to reduce the temperature of the rubber to below 85℃.

8. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, In step S30, the heating temperature applied to the first area is 165-170℃, the heating temperature applied to the second area is 175-180℃, the pressure for pressure foaming is 6-8 MPa, and the holding time is 120-140 seconds.

9. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 8, characterized in that, In step S30, after the foaming process with differential heating temperature is completed, the heating temperature applied to the whole of the mold cavity is increased to 185-195℃, the pressure is increased to 7-9 MPa, and the holding time is 60-80 seconds.

10. The method for preparing a high-elasticity, shock-absorbing EVA composite material according to claim 1, characterized in that, After step S30, step S40 is further included: the foamed and molded rubber is shaped at a temperature of 2-8℃ and a pressure of 21-25 kN, and the shaping time is 80-100 seconds.

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