Light EVA (Ethylene Vinyl Acetate) foaming material based on intermittent foaming heating and preparation method thereof

Abstract

The invention relates to the technical field of polymer foam materials, and discloses a light EVA foam material based on intermittent foaming heating and a preparation method thereof. The ethylene-vinyl acetate copolymer adhesive is prepared from the following raw materials: an ethylene-vinyl acetate copolymer, a maleic anhydride grafted ethylene-vinyl acetate copolymer, azodicarbonamide, basic zinc carbonate, triallyl isocyanurate, stearic acid and dicumyl peroxide. During preparation, a double-machine-position stepped heating and mechanical mode locking transfer process is adopted. The first-stage heating is initiated by dicumyl peroxide to construct a covalent cross-linked skeleton; in the second-stage foaming heating process, basic zinc carbonate is heated and decomposed, and anhydride groups are promoted to be hydrolyzed and combined with released zinc oxide in situ to form ionic crosslinking. A double-order cross-linked network formed by covalent bonds and ionic bonds can absorb bubble expansion stress and stabilize an internal microstructure. According to the invention, the occurrence rate of cell merging and cracking under the condition of low density is reduced, and the foaming material with low apparent density and small compression set rate is obtained.

Description

Technical Field

[0001] This invention relates to the field of polymer foaming materials technology, specifically to a lightweight EVA foaming material based on intermittent foaming heating and its preparation method. Background Technology

[0002] Ethylene-vinyl acetate copolymer (EVA) foam materials are widely used in footwear, packaging, and sporting goods due to their excellent cushioning and shock absorption properties and ease of processing. With the increasing demand for lightweight materials, the preparation of lightweight EVA foams with lower apparent density has become a key research focus in this field.

[0003] In conventional manufacturing processes, to reduce material density, it is usually necessary to increase the amount of chemical foaming agent to improve the foaming ratio. However, a large amount of foaming agent releases a large amount of gas instantaneously upon thermal decomposition, generating extremely high internal gas pressure. Traditional single covalent crosslinking systems provide limited melt strength during the high-temperature foaming stage and cannot withstand the intense gas expansion and stretching. When the gas expansion rate exceeds the deformation limit of the polymer matrix melt, the cell walls are prone to rupture, leading to cell merging and collapse of the material's internal structure.

[0004] Meanwhile, commonly used chemical foaming agents such as azodicarbonamide release a large amount of heat during decomposition. This concentrated exothermic phenomenon causes excessively high local temperatures in the polymer matrix, further reducing the melt viscosity of the matrix and making it difficult for the expanded cell structure to solidify and set. In traditional single-stage hot-press foaming processes, the cross-linking reaction and the extensive decomposition of the foaming agent are difficult to separate effectively, and the cross-linking network inside the material is subjected to the impact of high-pressure gas before it has fully developed. This limitation in the process not only results in uneven cell size distribution in lightweight EVA foam materials but also leads to high shrinkage and high compression set in the final molded product, making it difficult for existing foam materials to achieve both ultra-low density and good mechanical and dimensional stability. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a lightweight EVA foam material based on intermittent foaming heating and its preparation method. This solves the problem that existing ethylene-vinyl acetate copolymer foam materials often suffer from poor matching between the melt strength of the polymer matrix and the gas release rate of the foaming agent when preparing low-density lightweight foams, leading to easy cell merging, collapse, and rupture, making it difficult to obtain foam materials that combine low density and high structural stability.

[0006] In a first aspect, the present invention provides a lightweight EVA foam material based on intermittent foaming heating, employing the following technical solution: The lightweight EVA foam material based on intermittent foaming heating is made from raw materials comprising the following parts by weight: 90-95 parts of ethylene-vinyl acetate copolymer; 5-10 parts of maleic anhydride-grafted ethylene-vinyl acetate copolymer; 10-15 parts of azodicarbonamide; 2-4 parts of basic zinc carbonate; 0.5-1.0 parts of triallyl isocyanurate; Stearic acid 0.5-1.0 parts; 0.8-1.2 parts of dicumyl peroxide.

[0007] By employing the above technical solution, the raw material system of this invention can spontaneously construct a two-stage cross-linked network composed of covalent and ionic bonds during processing. Specifically, dicumyl peroxide decomposes upon heating to generate free radicals, which, with the assistance of triallyl isocyanurate, abstract hydrogen atoms from the ethylene-vinyl acetate copolymer macromolecular chain. This process triggers a free radical coupling reaction between molecular chains, thereby forming a preliminary carbon-carbon covalent cross-linked framework, which enhances the initial melt strength of the polymer system.

[0008] As the system temperature rises to the foaming stage, azodicarbonamide decomposes in large quantities, producing nitrogen and carbon monoxide. During this period, basic zinc carbonate undergoes a decomposition reaction upon heating, generating nascent zinc oxide, carbon dioxide, and water vapor.

[0009] It is worth noting that the water vapor released from these decompositions, in a high-temperature and high-pressure closed environment, will attack the anhydride groups on the maleic anhydride-grafted ethylene-vinyl acetate copolymer segments, causing them to undergo a hydrolysis ring-opening reaction to generate dicarboxylic acids. The generated dicarboxylic acids will then undergo an in-situ neutralization reaction with the nascent zinc oxide released in the same region, forming physical cross-linking points for zinc carboxylic acid ion clusters.

[0010] This in-situ generation process of dynamic ionic crosslinking introduces a large number of ionic bonds into the polymer matrix, which, together with the previously formed covalent bonds, constitute a two-stage crosslinking network. Since the formation of ionic bonds is an exothermic reaction, it effectively balances the intense exothermic reaction generated by the decomposition of azodicarbonamide, thus helping to stabilize the microscopic temperature distribution. Furthermore, ionic clusters can undergo reversible sliding and recombination at the high-shear tensile cell walls to absorb the expansion stress of the bubbles, thereby preventing cell rupture and fixing the microstructure formed during foaming.

[0011] Preferably, the foaming material is made from raw materials comprising the following parts by weight: 92 parts of ethylene-vinyl acetate copolymer; 8 parts of maleic anhydride-grafted ethylene-vinyl acetate copolymer; 12 parts of azodicarbonamide; 3 parts basic zinc carbonate; 0.8 parts of triallyl isocyanurate; Stearic acid 0.8 parts; 1.0 part of dicumyl peroxide.

[0012] By adopting the above technical solution, the stoichiometric ratio of each component is in an optimal state, which makes the covalent crosslinking density and ionic crosslinking density reasonably distributed. This is beneficial for the final foamed material to have a low compression set while maintaining a low apparent density.

[0013] Preferably, the maleic anhydride-grafted ethylene-vinyl acetate copolymer is obtained by reactive extrusion of a raw material comprising the following parts by weight: 100 parts of ethylene-vinyl acetate copolymer; Maleic anhydride 1.5-2.5 parts; 0.05-0.10 parts of 2,5-dimethyl-2,5-bis-tert-butylperoxyhexane; The grafting rate of the maleic anhydride-grafted ethylene-vinyl acetate copolymer is 0.82%-1.46%.

[0014] Preferably, the method for preparing the maleic anhydride-grafted ethylene-vinyl acetate copolymer includes the following steps: Weigh the raw materials according to the weight percentages and mix them in a high-speed mixer at room temperature for 3-5 minutes to obtain a mixture. Add the mixture to the main feed port of a co-rotating twin-screw extruder for reactive extrusion. Set the feeding section temperature to 140-150℃, the reaction section temperature to 170-190℃, the metering and die head section temperature to 160-170℃, and the screw speed to 150-250 rpm. Open the vacuum exhaust port at the end of the extruder and control the vacuum degree to -0.08MPa to -0.09MPa to remove volatile substances. After water cooling, pelletizing, and drying, the extruded polymer is obtained as a maleic anhydride-grafted ethylene-vinyl acetate copolymer.

[0015] By adopting the above technical solution and controlling the grafting rate within the range of 0.82%-1.46%, the main purpose is to provide sufficient anhydride reaction sites for the foaming system to support the ionic crosslinking density in the later stages of the reaction. Additionally, this avoids the problem of excessive rigidity of polymer macromolecular chains due to an excessively high grafting rate, which would limit the foaming expansion ratio.

[0016] Preferably, in the foaming material: The sum of the weight parts of the ethylene-vinyl acetate copolymer and the maleic anhydride-grafted ethylene-vinyl acetate copolymer is 100 parts. The weight ratio of the triallyl isocyanurate to the stearic acid is 1:1.

[0017] By adopting the above technical solution, maintaining a constant total amount of matrix resin and a proportional combination of crosslinking agent and lubricating dispersion system, the shear heat generation during the mixing process can be controlled below the safe critical point, thereby avoiding early decomposition of crosslinking agent and foaming agent.

[0018] Secondly, the present invention provides a method for preparing lightweight EVA foam material based on intermittent foaming heating, employing the following technical solution: A method for preparing lightweight EVA foam material based on intermittent foaming heating, used to prepare the aforementioned lightweight EVA foam material based on intermittent foaming heating, includes the following steps: The ethylene-vinyl acetate copolymer and the maleic anhydride-grafted ethylene-vinyl acetate copolymer were put into a pressure mixer for mixing, and then azodicarbonamide, basic zinc carbonate, triallyl isocyanurate and stearic acid were added and the mixing continued. Turn on the internal mixer to cool it down. After the material cools down, add dicumyl peroxide and mix. Discharge the material onto a two-roll mill to produce thin sheets and cut them into masterbatch sheets. Let them stand to mature. The matured masterbatch is placed in a mold cavity equipped with a mechanical mold locking device, pushed into the first flat vulcanizing machine, the mold is closed and pressurized, and the temperature and pressure are kept constant at the first temperature. After the pressure holding is completed, the upper and lower plates of the mold are rigidly locked by the mechanical mold locking device to maintain the pressure inside the mold cavity, the pressure of the first vulcanizing machine is unloaded, the locked mold is moved out and directly pushed into the second flat vulcanizing machine that has been preheated to the second temperature, which is higher than the first temperature. Apply the mold closing pressure again on the second flat vulcanizing machine and maintain the pressure at a constant temperature. After the timer ends, unload the pressure instantly and simultaneously release the mechanical mold locking device to allow the polymer matrix to expand and eject from the mold. Cool and solidify to obtain the final product.

[0019] By adopting the above technical solution, the dual-machine step-heating constant-pressure foaming control and mechanical mold-locking transfer technology provide a matching thermodynamic environment for the formation of the aforementioned two-stage cross-linked network. During the constant-temperature pressure holding period at the first temperature, the thermal decomposition of dicumyl peroxide triggers the formation of a carbon-carbon covalent cross-linked network. At this time, the ambient temperature is lower than the large-scale decomposition temperature of the foaming agent and basic zinc carbonate, which ensures that the polymer matrix establishes initial melt strength under static pressure without gas interference.

[0020] During the transfer phase between the two independent vulcanizing units, a mechanical mold-locking device is used to rigidly lock the mold, forcibly maintaining a high-pressure state inside the mold cavity. This operation eliminates the early, phased expansion of the material caused by the unloading of pressure from the external equipment's hydraulic cylinders, thus avoiding the series formation and coarseness of bubbles.

[0021] When the locked mold is pushed into the second, higher-temperature flat vulcanizing machine, the system heats up rapidly. Azodicarbonamide and basic zinc carbonate decompose simultaneously at the higher temperature, and the resulting mixed gas causes a sharp increase in pressure within the mold cavity. The nascent zinc oxide cross-links with the hydrolyzed anhydride groups to form an ionic bond network. After the pressure holding time ends, the external pressure and the mold-locking device are instantly removed, and the matrix undergoes instantaneous three-dimensional expansion and ejection under the extreme internal and external pressure difference, completing the foaming molding process.

[0022] Preferably, the temperature for mixing the ethylene-vinyl acetate copolymer and the maleic anhydride-grafted ethylene-vinyl acetate copolymer in a pressure mixer is 90-100°C, and the mixing time is 3-5 minutes; after adding the azodicarbonamide, basic zinc carbonate, triallyl isocyanurate and stearic acid, the mixing temperature is further increased to 95-105°C, and the mixing time is 3-5 minutes.

[0023] Preferably, the addition of dicumyl peroxide is carried out when the temperature of the material body drops to 85-90°C, and the mixing time after adding dicumyl peroxide is 1-2 minutes; when the material is discharged onto the two-roll mill, the two-roll mill is preheated to 75-80°C; the conditions for standing and maturing are: standing and maturing at room temperature for 12-24 hours.

[0024] By employing the above technical solution, the timing of adding the crosslinking initiator is controlled through staged cooling. Combined with the subsequent long-term static curing process at room temperature, residual internal stresses applied to the polymer chains during the mixing process can be eliminated. This stress relief promotes the dispersion of reactants within the polymer matrix, thereby improving the surface smoothness and foaming uniformity of the foamed products.

[0025] Preferably, the filling rate of the master film placed in the mold cavity is 95%-100%; the mold closing pressure of the first flat vulcanizing machine is 10-15MPa, the first temperature is set to 130-145℃, and the constant temperature and pressure holding time at the first temperature is 8-12 minutes.

[0026] By adopting the above technical solution, the covalent cross-linking reaction of the polymer matrix can achieve a certain conversion rate at this time, and the generated gel network modulus is sufficient to encapsulate and support the high-pressure gas generated by the subsequent high-temperature dissociation reaction.

[0027] Preferably, the reapplied mold closing pressure of the second flat vulcanizing machine is 10-15 MPa, the second temperature is set to 160-180℃, and the constant temperature and pressure holding time at the second temperature is 5-10 minutes; the time for instantaneous pressure unloading and simultaneous release of the mechanical mold locking device is controlled within 2 seconds.

[0028] By adopting the above technical solution, the high-temperature environment of 160-180℃ ensures the progress of the basic zinc carbonate phase change reaction and the azodicarbonamide vaporization reaction. Combined with a demolding speed controlled within 2 seconds, the internal melt can expand rapidly, which not only helps reduce the apparent density of the material but also provides conditions for forming a closed-cell foam material with a uniform pore size distribution.

[0029] This invention provides a lightweight EVA foam material based on intermittent foaming heating and its preparation method. It has the following beneficial effects: 1. This invention introduces maleic anhydride grafts into an ethylene-vinyl acetate copolymer matrix, and combines them with dicumyl peroxide and basic zinc carbonate to construct a two-stage crosslinked network of covalent and ionic bonds within the system. The covalent bonds initiated by dicumyl peroxide provide the basic melt strength of the system, while the dynamic ionic bonds induced by the decomposition of basic zinc carbonate can slip and recombine during cell wall stretching to absorb stress. This dual-network structure enables the polymer to withstand higher internal gas pressure during the foaming expansion stage, reducing the probability of cell rupture or merging, thereby reducing the overall apparent density of the material while maintaining a low compression set.

[0030] 2. This invention utilizes the dual role of basic zinc carbonate in promoting foaming and initiating phase change in the system. During the high-temperature foaming stage, the water vapor released by the thermal decomposition of basic zinc carbonate will promote the hydrolysis and ring opening of maleic anhydride groups. The generated carboxyl groups will then combine with the simultaneously released nascent zinc oxide to form ionic crosslinking points. This reaction process occurs simultaneously with the gas release of azodicarbonamide, and the ionic bond formation process can balance the violent heat fluctuations caused by the large-scale decomposition of the foaming agent, thereby stabilizing the micro-temperature field and improving the cell collapse phenomenon caused by local overheating.

[0031] 3. This invention employs a preparation process combining dual-machine stepped heating with mechanical mold-locking transfer. After the material undergoes low-temperature constant-pressure pre-crosslinking in the first vulcanizing machine, the pressure inside the mold cavity is maintained by a mechanical mold-locking device and the material is directly transferred to the second vulcanizing machine at a higher temperature. The intervention of mechanical mold-locking prevents the early disordered expansion of the material due to pressure unloading when it leaves the first machine, ensuring that azodicarbonamide and basic zinc carbonate can accumulate sufficient gas pressure in the second high-temperature reaction. Combined with the subsequent instantaneous pressure relief and demolding operation, this provides the physical molding conditions for forming a closed-cell foam with small and uniform pore size. Attached Figure Description

[0032] Figure 1 This is a graph showing the change of the center temperature of the lightweight EVA foam material of the present invention over time during the high-temperature foaming stage. Figure 2 This is a partial Fourier transform infrared spectrum of the foaming material and related base resin of the present invention; Figure 3 The graph shows the evolution curves of energy storage torque of different foaming material systems in the step-heating rheological test of the present invention. Figure 4 The graph shows the test results of apparent density and foaming ratio of various foaming material samples of the present invention. Figure 5 This is a comparison chart of the test results of the 50°C isothermal compression set and tear strength of the foamed material sample of the present invention. Figure 6 This is a comparison chart of the dynamic rebound and high-temperature dimensional stability of various foaming material samples of the present invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing maleic anhydride-grafted ethylene-vinyl acetate copolymer, including the following steps: Weigh out 100 parts by weight of ethylene-vinyl acetate copolymer, 2.0 parts by weight of maleic anhydride, and 0.08 parts by weight of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane. Mix them in a high-speed mixer at room temperature for 4 minutes. Add the mixture to the main feed port of a co-rotating twin-screw extruder for reactive extrusion. Set the extruder feeding section temperature to 145℃, the reaction section temperature to 180℃, the metering and die head section temperature to 165℃, and the screw speed to 200 rpm. Open the vacuum exhaust port at the end of the extruder and control the vacuum degree to -0.08 MPa to remove unreacted maleic anhydride monomers and volatile substances remaining from initiator decomposition. After water cooling and shaping, the extruded polymer strip is sliced ​​by a pelletizer and dried at 60℃ to obtain maleic anhydride-grafted ethylene-vinyl acetate copolymer. The grafting rate was determined to be 1.15%.

[0035] Preparation Example 2: This preparation example provides a method for preparing maleic anhydride-grafted ethylene-vinyl acetate copolymer, including the following steps: Weigh out 100 parts by weight of ethylene-vinyl acetate copolymer, 1.5 parts by weight of maleic anhydride, and 0.05 parts by weight of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, and mix them in a high-speed mixer at room temperature for 3 minutes. Add the mixture to the main feed port of a co-rotating twin-screw extruder for reactive extrusion. Set the extruder feeding section temperature to 140℃, the reaction section temperature to 170℃, the metering and die head section temperature to 160℃, and the screw speed to 150 rpm. Open the vacuum exhaust port at the end of the extruder and control the vacuum degree to -0.08 MPa to remove unreacted maleic anhydride monomers and volatile substances remaining from initiator decomposition. After water cooling and shaping, the extruded polymer strip is sliced ​​by a pelletizer and dried at 60℃ to obtain maleic anhydride-grafted ethylene-vinyl acetate copolymer, with a grafting rate of 0.82%.

[0036] Preparation Example 3: This preparation example provides a method for preparing maleic anhydride-grafted ethylene-vinyl acetate copolymer, including the following steps: Weigh out 100 parts by weight of ethylene-vinyl acetate copolymer, 2.5 parts by weight of maleic anhydride, and 0.10 parts by weight of 2,5-dimethyl-2,5-bis(tert-butylperoxy)hexane, and mix them in a high-speed mixer at room temperature for 5 minutes. Add the mixture to the main feed port of a co-rotating twin-screw extruder for reactive extrusion. Set the extruder feeding section temperature to 150℃, the reaction section temperature to 190℃, the metering and die head section temperature to 170℃, and the screw speed to 250 rpm. Open the vacuum exhaust port at the end of the extruder and control the vacuum degree to -0.09 MPa to remove unreacted maleic anhydride monomers and volatile substances remaining from initiator decomposition. After water cooling and shaping, the extruded polymer strip is sliced ​​by a pelletizer and dried at 60℃ to obtain maleic anhydride-grafted ethylene-vinyl acetate copolymer, with a grafting rate of 1.46%.

[0037] Examples 1-5: Example 1: This embodiment provides a method for preparing lightweight EVA foam material based on intermittent foaming heating, including the following steps: Weigh 92 parts by weight of ethylene-vinyl acetate copolymer and 8 parts by weight of maleic anhydride-grafted ethylene-vinyl acetate copolymer obtained in Preparation Example 1, and put them into a pressure mixer. Mix at 95°C for 4 minutes. Then add 12 parts of azodicarbonamide, 3 parts of basic zinc carbonate, 0.8 parts of triallyl isocyanurate and 0.8 parts of stearic acid. Maintain the rotor speed at 100°C and continue mixing for 4 minutes. Turn on the cooling water circulation of the mixer. When the temperature of the material body drops to 85°C, add 1.0 part of dicumyl peroxide and mix for 2 minutes. Discharge the mixed rubber mass onto a two-roll mill preheated to 80°C, thinly sheet it and cut it into master rubber sheets. Let it stand and cure at room temperature for 12 hours. Place the cured master rubber sheets into a mold cavity equipped with a mechanical locking and fastening device. The rubber filling rate is 98%. The mold is pushed into the first flat vulcanizing machine, the mold is closed and pressurized to 12MPa, the mold temperature is set to 135℃, and the temperature and pressure are maintained for 10 minutes. After the stage, the upper and lower plates of the mold are rigidly locked by mechanical mold locking buckles to maintain the pressure inside the mold cavity. The hydraulic cylinder pressure of the first vulcanizing machine is unloaded, the locked mold is moved out and directly pushed into the second adjacent flat vulcanizing machine that has been preheated to 165℃. The mold closing pressure of 12MPa is reapplied on the second vulcanizing machine and the temperature and pressure are maintained for 7 minutes. After the timer ends, the hydraulic cylinder pressure is instantly unloaded within 2 seconds and the mechanical mold locking device is released simultaneously. The polymer matrix expands instantly and ejects from the mold. It is then naturally cooled and shaped at room temperature to obtain the lightweight EVA foam material.

[0038] Example 2: This embodiment provides a method for preparing lightweight EVA foam material based on intermittent foaming heating, including the following steps: Weigh 95 parts by weight of ethylene-vinyl acetate copolymer and 5 parts by weight of maleic anhydride-grafted ethylene-vinyl acetate copolymer obtained in Preparation Example 2, and add them to a pressure mixer. Mix at 90°C for 5 minutes, then add 10 parts of azodicarbonamide, 2 parts of basic zinc carbonate, 0.5 parts of triallyl isocyanurate, and 0.5 parts of stearic acid. Maintain the rotor speed at 95°C and continue mixing for 5 minutes. Turn on the cooling water circulation of the mixer. When the material temperature drops to 85°C, add 0.8 parts of dicumyl peroxide and mix for 2 minutes. Discharge the mixed rubber mass onto a two-roll mill preheated to 75°C, thinly sheet it, and cut it into masterbatch sheets. Let it stand at room temperature for 24 hours to cure. Place the cured masterbatch sheets into a mold cavity equipped with a mechanical clamping device, with a filling rate of 95%. The mold is pushed into the first flat vulcanizing machine, the mold is closed and pressurized to 10 MPa, the mold temperature is set to 130℃, and the pressure is maintained at this temperature for 12 minutes. After this stage, the upper and lower plates of the mold are rigidly locked using mechanical locking clips to maintain the pressure inside the mold cavity. The hydraulic cylinder pressure of the first vulcanizing machine is unloaded, the locked mold is removed, and directly pushed into the second adjacent flat vulcanizing machine, which has been preheated to 160℃. The mold closing pressure of 10 MPa is reapplied on the second vulcanizing machine, and the pressure is maintained at this temperature for 10 minutes. After the timer expires, the hydraulic cylinder pressure is instantly unloaded within 2 seconds, and the mechanical locking device is simultaneously released. The polymer matrix instantly expands and ejects from the mold, and is naturally cooled and solidified at room temperature to obtain the lightweight EVA foam material.

[0039] Example 3: This embodiment provides a method for preparing lightweight EVA foam material based on intermittent foaming heating, including the following steps: Weigh 90 parts by weight of ethylene-vinyl acetate copolymer and 10 parts by weight of maleic anhydride-grafted ethylene-vinyl acetate copolymer obtained in Preparation Example 3, and add them to a pressure mixer. Mix at 100°C for 3 minutes. Then add 15 parts of azodicarbonamide, 4 parts of basic zinc carbonate, 1.0 part of triallyl isocyanurate, and 1.0 part of stearic acid, and continue mixing at 105°C while maintaining the rotor speed. Turn on the cooling water circulation of the mixer, and when the material temperature drops to 90°C, add 1.2 parts of dicumyl peroxide and mix for 1 minute. Discharge the mixed rubber mass onto a two-roll mill preheated to 80°C, thinly sheet it, and cut it into masterbatch sheets. Let it stand at room temperature for 16 hours to cure. Place the cured masterbatch sheets into a mold cavity equipped with a mechanical clamping device, with a filling rate of 100%. The mold is pushed into the first flat vulcanizing machine, the mold is closed and pressurized to 15 MPa, the mold temperature is set to 145℃, and the pressure is maintained at this temperature for 8 minutes. After this stage, the upper and lower plates of the mold are rigidly locked using mechanical locking clips to maintain the pressure inside the mold cavity. The hydraulic cylinder pressure of the first vulcanizing machine is unloaded, the locked mold is removed, and directly pushed into the second adjacent flat vulcanizing machine, which has been preheated to 180℃. The mold closing pressure of 15 MPa is reapplied on the second vulcanizing machine, and the pressure is maintained at this temperature for 5 minutes. After the timing ends, the hydraulic cylinder pressure is instantly unloaded within 2 seconds, and the mechanical locking device is simultaneously released. The polymer matrix instantly expands and ejects from the mold, and is naturally cooled and solidified at room temperature to obtain the lightweight EVA foam material.

[0040] Example 4: This embodiment provides a method for preparing lightweight EVA foam material based on intermittent foaming heating, including the following steps: The formulation and mixing / sheet extrusion steps are exactly the same as in Example 1. The cured masterbatch sheet is placed in a mold cavity equipped with a mechanical locking device, with a filling rate of 98%. The mold is pushed into the first flat vulcanizing machine, the mold is closed and pressurized to 12 MPa, the mold temperature is set to 130°C, and the pressure is maintained at a constant temperature for 12 minutes. After this stage, the upper and lower plates of the mold are rigidly locked using mechanical locking buckles to maintain the pressure inside the mold cavity. The hydraulic cylinder pressure of the first vulcanizing machine is unloaded, and the locked mold is removed and directly pushed into the second adjacent flat vulcanizing machine, which has been preheated to 180°C. A mold closing pressure of 12 MPa is reapplied on the second vulcanizing machine, and the pressure is maintained at a constant temperature for 5 minutes. After the timing ends, the hydraulic cylinder pressure is instantly unloaded within 2 seconds, and the mechanical locking device is simultaneously released. The polymer matrix instantly expands and ejects from the mold, and is naturally cooled and solidified at room temperature to obtain the lightweight EVA foam material.

[0041] Example 5: This embodiment provides a method for preparing lightweight EVA foam material based on intermittent foaming heating, including the following steps: The formulation and mixing / sheet extrusion steps are exactly the same as in Example 1. The cured masterbatch sheet is placed in a mold cavity equipped with a mechanical locking device, with a filling rate of 98%. The mold is pushed into the first flat vulcanizing machine, the mold is closed and pressurized to 12 MPa, the mold temperature is set to 145°C, and the temperature and pressure are maintained for 8 minutes. After the stage, the upper and lower plates of the mold are rigidly locked using mechanical locking buckles to maintain the pressure inside the mold cavity. The hydraulic cylinder pressure of the first vulcanizing machine is unloaded, the locked mold is removed and directly pushed into the second adjacent flat vulcanizing machine preheated to 160°C. The mold closing pressure of 12 MPa is reapplied on the second vulcanizing machine and the temperature and pressure are maintained for 10 minutes. After the timer expires, the hydraulic cylinder pressure is instantly unloaded within 2 seconds and the mechanical locking device is simultaneously released. The polymer matrix instantly expands and ejects from the mold, and is naturally cooled and shaped at room temperature to obtain the lightweight EVA foam material.

[0042] Comparative Examples 1-5: Comparative Example 1: Compared with Example 1, the difference is that the preparation process does not use the intermittent heating process of dual-position mold transfer, but adopts the traditional single-stage constant temperature foaming process: the mold containing the master film is directly pushed into the flat vulcanizing machine that has been preheated to 165°C, the mold is closed and pressurized to 12MPa, and the constant temperature and pressure are maintained for 17 minutes. After the timer ends, the hydraulic cylinder pressure is instantly unloaded and the mold is demolded. The rest is the same.

[0043] Comparative Example 2: Compared with Example 1, the difference is that maleic anhydride-grafted ethylene-vinyl acetate copolymer is not added to the composition formulation. Instead, it is replaced with an equal part by weight of ethylene-vinyl acetate copolymer, that is, the total amount of ethylene-vinyl acetate copolymer added is 100 parts by weight. All other aspects are the same.

[0044] Comparative Example 3: Compared with Example 1, the difference is that basic zinc carbonate is not added to the composition formulation, but is replaced by an equal part by weight of ordinary industrial-grade zinc oxide powder with an average particle size D50 of 1 μm to 3 μm, and the rest are the same.

[0045] Comparative Example 4: Compared with Example 1, the difference is that basic zinc carbonate is not added to the composition formulation, but is replaced with 3 parts by weight of ordinary industrial-grade zinc oxide powder, and 0.5 parts by weight of deionized water is added in the mixing stage. All other aspects are the same.

[0046] Comparative Example 5: The difference from Example 1 is that triallyl isocyanurate is not added to the composition formulation, but all other aspects are the same.

[0047] Test Examples 1-6: Test Example 1: Experimental description: This test case mainly focuses on the thermodynamic objective characterization of the central heat accumulation phenomenon of lightweight EVA foam material during the molding process. The test objects include the unfoamed master films prepared by Examples 1 to 5 and Comparative Examples 1 to 5.

[0048] Experimental steps: The master rubber sheets produced and cured by the two-roll mill in each embodiment and comparative example are stacked and assembled according to the size of the test mold cavity. A K-type armored thermocouple probe is pre-embedded at the geometric center of the stacked master rubber sheets, and the lead-out end is connected to a multi-channel temperature data recorder. The rubber material with thermocouples is placed in the mold and pushed into the flat vulcanizing machine for molding and foaming operation according to the process conditions set in each embodiment and comparative example. The data recorder is started and continuously collects the internal temperature of the rubber material during the entire heating, pressure holding and foaming process with a period of 1 second. The highest peak temperature reached at the center of the rubber material during the high-temperature foaming stage is extracted, and the corresponding template set temperature is recorded. The difference between the highest peak temperature at the center and the template set temperature is calculated to evaluate the heat release inside the system.

[0049] Experimental data: Table 1. Test data of peak central heating temperature for each embodiment and comparative example. in conclusion: Based on the data in Table 1 and Figure 1 The temperature change curve at the center during the high-temperature foaming stage shown in the figure highly matches the thermodynamic evolution of the foaming system within the confined space with the previously designed chemical mechanism. In the actual industrial production of conventional thick-film molding foam, the large amount of reaction heat released by the concentrated decomposition of azodicarbonamide foaming agent often cannot be conducted to the outside of the mold in time through the polymer melt. The internal thermo-oxidative degradation of the material caused by the heat accumulation at the center is the core reason for the batch scrapping of products.

[0050] observe Figure 1 The curves of Comparative Example 3, marked with dotted lines and hollow triangles, and the corresponding test data show that when conventional zinc oxide is used as a foaming accelerator in the system, zinc oxide only plays a catalytic role in reducing the decomposition activation energy of the foaming agent, without providing any thermodynamic buffer. This causes the core temperature of the rubber compound to run out of control very quickly after entering the high-temperature foaming stage, deviating severely from the 165℃ reference temperature marked with a dotted line in the figure, rapidly soaring to 208.7℃ and producing a vicious temperature rise of up to 43.7℃. Comparative Example 1, which directly uses a single-stage heating process, in... Figure 1 The middle part is represented by a dotted line with a square mark. Due to the lack of buffering and heat dissipation capacity of the network structure in the early pre-crosslinking stage, the material has a significant lag in the early temperature rise, followed by a concentrated exothermic mutation, resulting in a temperature rise and heat accumulation phenomenon of 21.4℃.

[0051] In contrast, after introducing basic zinc carbonate into the formulation system, the maximum central temperature rise in Examples 1 to 5 was consistently suppressed to below 6°C. Specifically, as... Figure 1 As shown in the curve of Example 1, marked with solid lines and solid dots, the actual internal temperature remained close to the set reference temperature and fluctuated smoothly throughout the entire foaming cycle, without any destructive temperature spikes. Basic zinc carbonate undergoes rapid dehydration and decarbonation decomposition within the foaming range of 160°C to 180°C. This typical endothermic phase change process precisely offsets the exothermic peak generated by the violent decomposition of azodicarbonamide at the microscopic scale.

[0052] The thermodynamic adaptive regulation mechanism based on the in-situ physicochemical transformation of the material completely cuts off the heat source that causes core burning defects inside the material. Combined with the stepped temperature rise time difference established by the dual-machine mold-locking transfer process, this foaming system effectively suppresses the damage to the early covalent network structure caused by drastic heating. This allows the subsequently generated in-situ water vapor and nascent zinc oxide to smoothly trigger the hydrolysis and dynamic ionic cross-linking reaction of maleic anhydride macromolecular chains in an ideal rheological environment with highly uniform temperature and thermodynamic stability.

[0053] Test Example 2: Experimental description: This test example mainly uses Fourier transform infrared spectroscopy (FTIR) to characterize the evolution of chemical bonds within the cross-linked foaming system. The test objects include the pure maleic anhydride-grafted ethylene-vinyl acetate copolymer resin obtained in Preparation Example 1, and samples taken from the central region of the foaming materials prepared in Examples 1 to 5 and Comparative Examples 2 to 4.

[0054] Experimental steps: The outer skin of each foamed sample was peeled off, and a thin slice approximately 2 mm thick was cut from the geometric center of the material. To eliminate interference from unreacted foaming agent and other polar small molecule additives, the slice was placed in a Soxhlet extractor and extracted with anhydrous ethanol under reflux for 12 hours. It was then dried to constant weight in a vacuum drying oven at 60°C. A Fourier transform infrared spectrometer equipped with an attenuated total reflectance (ATR) accessory was then turned on, using a diamond crystal. Before testing, the crystal surface was cleaned with anhydrous ethanol, and the air background spectrum was collected. The dried sample was then flattened and adhered to the ATR crystal surface, and uniform mechanical pressure was applied to ensure good light coupling contact. The spectrometer resolution was set to 4 cm⁻¹. -1 The number of scans was 32, at 4000cm -1 Up to 600cm -1 Infrared absorption spectra of samples were collected within the wavenumber range.

[0055] The obtained spectral data were extracted, and baseline calibration was performed using spectral processing software, with a focus on reading the 1780 cm⁻¹.-1 (Asymmetric stretching vibration of carbonyl group in cyclic anhydrides), 1585 cm⁻¹ -1 (Asymmetric stretching vibration of zinc carboxylate) and 1735cm -1 (Absorbance value at the carbonyl stretching vibration of the acetate group in EVA, used as an internal standard reference).

[0056] Experimental data: Table 2 Comparison of infrared absorption peak absorbance of characteristic functional groups of various foaming materials and matrix resins in conclusion: Based on the data in Table 2 and Figure 2 A comparison of local FTIR spectra of different foaming material systems reveals that quantitative analysis using infrared spectroscopy directly confirms the complex interphase reaction kinetics within the systems. The transformation of microscopic chemical bonds directly determines the direction of the material's macroscopic rheological properties. The polymer melt exhibits extremely high viscosity in a closed high-pressure mold cavity, making it extremely difficult to achieve uniform dispersion of added polar small molecules at the molecular scale. Based on the test results and... Figure 2 The pure resin shown by the solid line at the bottom in Preparation Example 1 serves as a reference standard, and its spectrum is at 1780 cm⁻¹. -1 The auxiliary line retains a very strong characteristic peak of cyclic anhydrides at 1735 cm⁻¹. -1 The ester group shows an absorption peak at 1585 cm⁻¹, which is used as an internal standard for the system. -1 There is virtually no absorption signal at this point. If the catalytic system relies solely on ordinary zinc oxide solid particles in a conventional formulation, such as... Figure 2 As shown in the spectrum of Comparative Example 3, displayed by the dashed line in the middle, the absorbance of the anhydride peak of this sample is still as high as 0.176, while at 1585 cm⁻¹... -1 The absorption peak of zinc salt, representing ionic bonds, is negligible. Experimental phenomena and data clearly demonstrate that, in the absence of water molecules as an initiator, solid-phase zinc oxide has extremely difficulty overcoming the significant steric hindrance to directly open the five-membered ring of maleic anhydride. Even when liquid water was artificially introduced during the preparation of Comparative Example 4 in an attempt to trigger hydrolysis, the reaction conversion rate remained low due to the large interfacial tension and aggregation effect of the droplets. The forcibly introduced free water even caused the formation of large local bubbles, leading to the collapse of the foamed structure.

[0057] In the synergistic network constructed in each embodiment, the reaction process is fundamentally reversed. As the system heats up with each process stage and crosses the decomposition threshold of basic zinc carbonate, the zinc carbonate, uniformly distributed in the matrix, decomposes in situ, releasing water vapor in a molecular state and nascent, highly reactive zinc oxide. This in-situ generated mixed gas can freely diffuse between high-viscosity polymer segments, triggering the hydrolysis of anhydride groups on adjacent macromolecular chains with extremely precise stoichiometry. The two highly reactive carboxyl groups generated in the reaction immediately undergo acid-base neutralization with the nascent zinc oxide, reflected in... Figure 2 In the spectrum of Example 1 depicted at the top by a dashed line, 1780 cm⁻¹ -1 The anhydride peak at that position has almost completely disappeared, replaced by a peak at 1585 cm⁻¹. -1 The strong asymmetric stretching vibration peak of zinc carboxylate salt that emerged at the indicator line, accompanied by a jump in the calculated relative ionization index to 0.154, cleverly bypassed the mass transfer barrier in the high-viscosity melt by relying on thermally induced phase change and in-situ induced spontaneous reaction. At the same moment when the azo blowing agent generates a large amount of nitrogen gas, a dense dynamic ionic bond node is forcibly riveted onto the polymer matrix. The high-strength ionic cluster network effectively restrains the polymer molecular chains that attempt to slide violently due to high temperature, giving the melt extremely high structural strength to encapsulate the expanding gas, thereby macroscopically and firmly locking the microscopic framework of the ultra-lightweight foam.

[0058] Test Example 3: Experimental description: This test example aims to characterize the network crosslinking kinetics of the foamed material system at different process stages through a step-heating test procedure using a rotorless vulcanizer (MDR). To eliminate the interference of gas expansion on the internal pressure and torque signals of the rheological chamber, the experimental subject was the masterbatch sheet prepared according to the formulations of the examples and comparative examples (azodicarbonamide foaming agent was removed during the preparation process).

[0059] Experimental steps: Obtain the foam-free curing masterbatch sheets corresponding to the formulations of each embodiment and comparative example. Cut them into circular samples with diameters and thicknesses that meet the requirements of the test chamber volume of the rotorless vulcanizer using a standard cutter at room temperature. Turn on the rotorless vulcanizer and program a non-isothermal rheological test program in the control software: set the initial test temperature to the first-stage isothermal holding temperature of the corresponding sample (e.g., 135°C) and maintain this temperature for 10 minutes; then start the forced heating program to rapidly heat the mold cavity to the second-stage foaming temperature (e.g., 165°C) within 1 minute, and continue isothermal scanning for 10 minutes. Then, fill the mold with the foamed masterbatch sheets. The sample was placed between the upper and lower mold cavities of the vulcanizer, which had been preheated to the initial set temperature. The mold was closed and pressurized, and the test was started at an oscillation frequency of 1.67 Hz and a standard strain amplitude. The instrument automatically and continuously recorded the change curve of the energy storage torque (S') characterizing the dynamic modulus of the system over time. After the test, the key feature point data on the curve were extracted, including the pre-crosslinking torque value (S'1) at the end of the first stage of isothermal treatment and the final torque value (S'2) at the end of the second stage of isothermal treatment. The crosslinking contribution of the secondary reaction in the high-temperature stage was objectively evaluated by calculating the absolute difference (ΔS') between the two stages of torque.

[0060] Experimental data: Table 3 Torque characteristic data of each foaming material system in stepped temperature rise rheological test. in conclusion: Figure 3 This is a step-temperature rheological kinetic curve of the foaming agent-free system of this invention. The horizontal axis represents the test time, and the vertical axis represents the energy storage torque characterizing the crosslinking network density. Vertical auxiliary lines clearly divide the entire test process into a 135°C pre-crosslinking stage, a transient heating zone, and a 165°C high-temperature scanning stage. In the curves, the dotted-dash line indicates Comparative Example 2 without maleic anhydride grafts, the dashed line indicates Comparative Example 3 using ordinary zinc oxide, and the thick solid line indicates the system of this invention represented by Example 1.

[0061] Combining the data in Table 3 with Figure 3The evolutionary trajectory presented objectively demonstrates the dynamic regulation of polymer melt viscoelasticity by the two-stage crosslinking mechanism. In actual thick-plate molding foaming R&D and pilot production, operators often observe that single crosslinked systems are prone to internal bubble formation and external collapse during the high-temperature foaming stage. The underlying cause of this macroscopic defect is directly related to the precipitous drop in viscosity of the polymer matrix caused by the intensified thermal motion of molecular chains at high temperatures. From the test data and the trend of the first stage of the curve, it can be seen that during the isothermal period of 135℃, dicumyl peroxide, under the action of the auxiliary crosslinking agent, initiated the basic free radical covalent crosslinking reaction. Including all systems containing crosslinking agents, including Example 1, the torque steadily increased and gradually converged, eventually forming an initial modulus plateau of about 4.0 dNm. This pre-crosslinked network provides a preliminary anti-deformation skeleton for the polymer matrix.

[0062] Maintaining sufficient melt confinement during subsequent high-temperature shocks is crucial to the success of foaming. As the rheometer program forcibly raised the temperature to 165°C, the rheological behavior of each system showed significant differentiation. Comparative Examples 2 and 3, lacking reactive grafts or using only ordinary zinc oxide, exhibited relatively flat storage torque curves after entering the second stage, showing only slight increases of 0.79 dNm and 0.97 dNm, respectively. This extremely limited increase primarily stemmed from the continued decomposition of residual peroxides within the matrix. At this temperature, the thermal softening effect significantly weakened the binding force of the covalent network on the melt. If the material were in a true aerated foam state at this point, such a weak modulus would inevitably be unable to withstand the tearing force of the outward expansion of high-pressure nitrogen gas, making bubble wall rupture unavoidable.

[0063] observe Figure 3The response state of Example 1, represented by the thick solid line, shows a fundamental reversal. After crossing the heating transition zone and reaching 165°C, the torque of this sample not only showed no signs of thermal decay but also initiated a steep secondary increase in a very short time, generating an additional torque increment of up to 4.42 dNm. This ultimately doubled the overall rheological modulus of the system to 8.54 dNm. Since the volume interference of bubbles generated by the decomposition of azo foaming agent was excluded in this rheological test, this surge in rheological resistance objectively and purely reflects the dramatic change in the internal chemical phase, namely, the decomposition of basic zinc carbonate. Moisture and nascent zinc oxide rapidly trigger the ring-opening and in-situ ionic crosslinking of maleic anhydride upon reaching their specific decomposition temperature. Two types of chemical bonds with drastically different properties achieve deep kinetic decoupling on a unified rheological time axis. The early covalent network is responsible for capturing small bubbles in the early nucleation stage in the lower temperature region, while the later explosively growing ionic bond clusters act as highly dense high-temperature resistant physical crosslinking nodes. This spontaneous time difference reaction effectively compensates for the deficiency in melt strength at high temperatures, thereby completely locking the microscopic cavity morphology of the product when it is demolded at high temperature at the macroscopic process level.

[0064] Test Example 4: Experimental description: This test case aims to objectively quantify the macroscopic foaming performance of each foaming system after molding, demolding, and cooling. The test objects are the final foamed molding materials prepared in Examples 1 to 5 and Comparative Examples 1 to 5.

[0065] Experimental steps: After each foamed sample was molded and naturally cooled to room temperature, the outer skin layer containing a dense crust was removed using a band saw. A regular rectangular test block with dimensions of approximately 50mm × 50mm × 25mm was cut from the core and placed in a standard experimental environment at 23℃ and 50% relative humidity for 24 hours to eliminate internal residual stress. The dimensions of the sample block in the length, width, and thickness directions were measured using a digital vernier caliper with an accuracy of 0.01mm. Three measurements were taken at different locations for each dimension, and the arithmetic mean was calculated to determine the accurate apparent volume of the sample block. The measured sample block was then weighed on an analytical balance with an accuracy of 0.001g. The apparent density of the material was obtained by dividing the sample mass by its apparent volume. Combined with the theoretical density constant of the unfoamed masterbatch of this formulation system, which is approximately 0.95g / cm³, this density was determined. 3 Calculate its actual foaming ratio.

[0066] Experimental data: Table 4. Apparent density and foaming ratio test data of foamed materials in each embodiment and comparative example. in conclusion: Figure 4 This is a comparison chart of the apparent density and expansion ratio test results of various foaming material samples of the present invention. The horizontal axis of the chart shows the numbers of each test sample in sequence. The left vertical axis and the corresponding black solid line marked with a solid square show the apparent density value of each foaming sample. The right vertical axis and the corresponding black dashed line marked with a hollow circle show its actual expansion ratio. The test objects in the chart cover Examples 1 to 5, which adopt the two-stage crosslinking mechanism of the present invention, as well as Comparative Examples 1 to 5, which lack the corresponding crosslinking or thermodynamic regulation mechanism.

[0067] Combining the data in Table 4 with Figure 4 The curve trend and the measurement results of macroscopic density directly reflect the ability of the foaming system to maintain the bubble structure during the instant of high-temperature expansion. In the actual research and development testing of lightweight materials, researchers often find that simply increasing the amount of foaming agent often cannot achieve a lower density. The collapse of the cells caused by the high-pressure gas breaking through the melt and escaping is the core bottleneck restricting the improvement of the foaming ratio. Figure 4 At the test node of Comparative Example 3, due to the use of ordinary zinc oxide, which lacks thermodynamic buffering capacity, thermal runaway occurred internally. The rapid temperature rise caused the polymer melt viscosity to drop below the critical value, and the large amount of nitrogen gas generated easily tore through the matrix and escaped outwards. The apparent density of the final molded sample was as high as 0.1570 g / cm³. 3 The foaming ratio hovered only around 6.1 times. When these samples were dissected, large areas of bubbles and interconnected airflow channels could usually be observed directly. Comparative Example 5, lacking the support of a pre-crosslinked network, exhibited a higher apparent density of 0.1830 g / cm³. 3 This further confirms that if there is no covalent bond to provide basic constraint in the early stage of bubble nucleation, the gas is very likely to undergo phase separation at the microscale and cannot be uniformly retained in the matrix.

[0068] How can this rheological defect caused by high temperature be overcome through chemical network design? In Comparative Example 2, which deviates from the secondary reinforcement of ionic crosslinking, although the system successfully passed the pre-crosslinking stage, at the moment of demolding at 165℃, the single covalent bond alone was insufficient to resist the tensile stress generated by the matrix's expansion of more than ten times its volume. The material underwent significant macroscopic shrinkage after demolding, causing the density to rise back to 0.0960 g / cm³. 3 On the contrary Figure 4 Examples 1 to 5, which are located below the curve, have an apparent density that remains consistently between 0.0410 and 0.0520 g / cm³. 3In the low-density range, the foaming ratio generally exceeds 20 times. This leap in physical properties is essentially due to the in-situ hydrolysis and ionic cross-linking reaction triggered by the decomposition of basic zinc carbonate. In the later stage of rapid bubble expansion, the high-density dynamic ion clusters provide structural reinforcement for the stretched and thinned polymer pore walls. The high-strength two-dimensional network not only resists the erosion and wall breakage by gas, but also provides the material with the necessary dimensional support force at the moment of demolding and pressure relief, ensuring that the gas volume generated by the decomposition of azo foaming agent can be effectively converted into a reduction in the macroscopic density of the material.

[0069] Test Example 5: Experimental description: This test case aims to evaluate the end-side mechanical behavior of a polymer foam network constructed through two-stage crosslinking, focusing on the material's creep recovery ability under long-term load and its structural toughness against external crack propagation. The test objects are the core layer materials of the foamed molded samples prepared in Examples 1 to 5 and Comparative Examples 1 to 5.

[0070] Experimental steps: The molded skin layer of each foamed sample was removed. A cylindrical specimen with a diameter of 29.0 mm and a thickness of approximately 12.5 mm was cut from the geometric center of the material using a precision punching machine for compression set testing. Simultaneously, a standard right-angle tear test piece with a thickness of 5.0 mm was cut, with a 1.0 mm slit pre-cut at the right-angle vertex for tear strength testing. All specimens were conditioned for 24 hours in a constant temperature and humidity environment of 23℃ and 50% relative humidity. The initial center thickness of the cylindrical specimens was measured using a digital thickness gauge. The specimens were then placed between two parallel metal plates on the upper and lower sides of a constant temperature compression setter. The limiting pads were adjusted, and a mechanical load was applied to compress the specimens to 50% of their initial thickness. The compressed specimens were then... The test fixture was moved into a preheated drying oven at 50°C and kept at a constant temperature for 6 hours to simulate the damp heat load conditions of cushioning components such as shoe materials in actual wearing environments. After the specified time, the test fixture was quickly removed from the drying oven and the compressive load was released. The sample was transferred to a room temperature insulated wooden board and allowed to recover freely for 30 minutes. The center thickness was measured again, and the compression permanent deformation rate was calculated by the difference between the initial thickness and the recovered thickness. The right-angle tear specimen was clamped in the upper and lower clamps of the universal testing machine. The tensile gauge length was set and peeling tension was performed at a constant rate of 50 mm / min. The highest load peak value that occurred during the complete tear fracture was recorded and divided by the actual thickness of the specimen to obtain the tear strength.

[0071] Experimental data: Table 5. Test data of compression set and tear strength of various foaming materials at 50℃ isothermal temperature. in conclusion: Figure 5 This is a comparative graph of the mechanical and recovery properties of various foamed material samples of the present invention. The horizontal axis of the graph shows the sample numbers of the samples tested in sequence, covering Examples 1 to 5 based on the construction of a two-stage crosslinking network, and Comparative Examples 1 to 5 using conventional formulations or lacking corresponding control components. The left vertical axis and the corresponding black solid line marked with a solid dot visually show the trend of compression set of each sample at 50°C. The right vertical axis and the corresponding black dashed line marked with a hollow triangle indicate the tear strength level of each sample.

[0072] Combining the data in Table 5 with Figure 5 The curve distribution shows that the macroscopic mechanical response of foamed materials under complex stress conditions is essentially a direct mapping of their microscopic multi-linked network topology. In practical applications of lightweight cushioning materials, structural creep under long-term pressure and cell wall rupture caused by external tearing are the main factors leading to the decline in product lifespan. Evaluating these properties often requires more than just focusing on the intrinsic strength of the matrix resin; it also necessitates examining whether the cured cell skeleton during foaming possesses a sufficient mechanism for stress dissipation. Figure 5 The curve fluctuations in the right half of the comparative examples reveal that the foaming system lacking effective network support exhibits significant fragility in mechanical performance. Comparative Example 3, due to thermal runaway in the early stages of molding, resulted in the collapse of the cell structure and large-area co-occurrence of bubbles. The material retained dense structural defects, leading to irreversible viscous slippage of polymer chains under 50°C isothermal compression. The solid dots representing this sample in the figure climbed to a high level of 72.4%, and it exhibited extremely low fracture resistance of 6.93 N / mm in the tear test. This confirms, at the testing level, the disruptive effect of macroscopic defects on the mechanical transmission path. Even in Comparative Example 2, with its relatively intact cell morphology, the rigid covalent bonds could not rearrange themselves to release localized stress concentration under prolonged high-temperature compressive stress due to the reliance on a single covalent cross-linked network. Ultimately, buckling fatigue of the cell skeleton occurred, with its deformation rate remaining at a relatively high level of 45.4%.

[0073] When dealing with such sustained destructive loads, the two-stage cross-linked network constructed in this scheme exhibits corresponding structural advantages, from Figure 5The data strips for Examples 1 to 5 in the left half show that the solid line at the bottom indicates that the material generally controls the compression deformation rate below 34% at 50°C, while the dotted line at the top shows an overall increase in its tear strength index. This change in mechanical response is directly attributed to the role of the in-situ generated zinc carboxylate ion clusters as dynamic physical cross-linking nodes in the polymer matrix. When the pores are torn by external force, these ionic bonds can act as sacrificial bonds, absorbing the fracture energy at the crack propagation front through their own breakage and reversible recombination, giving the thin pore walls a certain structural toughness, and hindering the rapid penetration of microcracks. Under the isothermal compression condition of 50°C, since this temperature is far from the thermodynamic critical point of ionic bond dissociation, the high-density ion clusters and the previously formed covalent network intertwine and interlock, restricting the large-scale slippage of polymer chain segments in the amorphous region. When researchers use cutting tools to prepare samples of such materials in the laboratory, they can intuitively feel that the cut surface of the sample exhibits a dense rebound resistance similar to that of an elastomer. This network reinforcement driven by chemical mechanisms enables the material to restore its shape by means of the elastic recovery force accumulated inside the network after long-term strain, thus avoiding to some extent the engineering pain points of traditional ultralight foamed materials, such as easy collapse and low tear strength.

[0074] Test Example 6: Experimental description: This test case aims to evaluate the energy feedback characteristics of foamed materials under dynamic impact conditions and their dimensional stability in high-temperature processing environments. The test subjects are the core layer materials of the foamed molded samples prepared in Examples 1 to 5 and Comparative Examples 1 to 5.

[0075] Experimental steps: The molded outer layer of each foamed sample was removed. Using a band saw and precision slicer, the core material was cut into 100mm × 100mm × 20mm cubes for the drop ball rebound test. Simultaneously, strips measuring 150mm × 25mm × 10mm were cut for the heat shrinkage rate test. All samples were conditioned for 24 hours at 23℃ and 50% relative humidity. The cube samples were placed horizontally on the rigid base of the drop ball rebound tester, using a standard steel ball weighing 16g and with a diameter of 16mm. The electromagnetic release device was adjusted so that the bottom of the steel ball was 500mm above the sample surface. The steel ball was released to allow free fall and impact the sample surface. The highest point of the rebound was recorded. The average value of five different locations on the same sample surface was taken, and the percentage of the rebound height to the initial fall height was calculated as the rebound rate. The initial longitudinal length between the marks in the center area of ​​the strip sample was measured using a vernier caliper with an accuracy of 0.02mm. The sample with a talcum powder anti-stick layer on the bottom was placed flat on a horizontal pallet and transferred as a whole into a forced convection drying oven preheated to 120°C. It was kept at a constant temperature of 120°C for 40 minutes to simulate the subsequent processing conditions of secondary bonding or hot pressing of shoe soles. After the heating time was reached, the sample was removed and allowed to cool and shrink naturally at standard room temperature for 24 hours. The longitudinal length between the markings was measured again, and the linear thermal shrinkage rate of the material was calculated by dividing the difference in longitudinal length by the initial length.

[0076] Experimental data: Table 6. Test data of ball rebound rate and linear heat shrinkage rate at 120℃ for each foamed material sample. in conclusion: Figure 6 This is a comparison chart of the dynamic rebound and high-temperature dimensional stability of various foaming material samples of this invention. The horizontal axis of the chart shows the numbers of each test sample in sequence, and the chart is presented in the form of a double Y-axis bar chart. The left vertical axis and the corresponding dark gray bars show the drop ball rebound rate of each material; the right vertical axis and the adjacent light gray bars reflect the linear thermal shrinkage rate of each sample at 120℃.

[0077] Combining the data in Table 6 with Figure 6The intuitive columnar distribution and the differences in topological response of the macromolecular network under extreme boundary conditions of transient impact and sustained thermal field confirm the influence of chemical phase regulation on the macroscopic properties of materials. In the engineering practice of polymer foam materials, high elasticity and high-temperature dimensional stability often have an inverse relationship. Reducing the crosslinking density can usually give the molecular chains higher conformational freedom, thereby improving the elastic feedback at room temperature. However, in secondary heat processing or high-temperature use environments, the sparse network is unable to restrain the slippage of molecular chains excited by pyrolysis, leading to irreversible macroscopic shrinkage of the material. This phenomenon is intuitively reflected in the test results of Comparative Example 2. Since no ionic crosslinking mechanism was introduced, the system only maintained the covalent network of the initial stage. In the drop ball test, the rebound rate corresponding to its dark gray columns reached 52.2%, indicating that the material has a certain elastic basis. However, after being heated in an oven at 120°C, the longitudinal dimension of the test strip shrank significantly, and the light gray columns representing the thermal shrinkage rate became abnormally prominent, with the value climbing to 6.33%. When measuring the dimensions of samples after cooling on the sample preparation stage, obvious concave deformation is often found at the edges of the samples. This indicates that single covalent crosslinking is completely insufficient to counteract the release process of residual internal stress at high temperatures. Comparative Examples 3 to 5 are limited by phase separation or thermal runaway in the early stage of pore formation. The severe damage to the pore structure not only cuts off the stress transmission network, causing most of the impact energy of the falling ball to be dissipated by internal friction, resulting in a rebound rate of about 40%, but also the irregular skeleton cannot provide sufficient thermodynamic support, and the shrinkage rate is generally high.

[0078] To break free from the mutually exclusive constraints of mechanical and thermal properties, we turn our attention to the examples on the left side of the chart that constructed a two-stage crosslinking network through the decomposition of basic zinc carbonate. This clearly demonstrates a deep decoupling of various performance indicators. Examples 1 to 5 not only increased the rebound rate of the dropped ball to a high-elasticity range of over 57% (some exceeding 65%), but also steadily suppressed the thermal shrinkage rate at 120°C to below 3%, with Example 3 achieving a low shrinkage level of 1.66%. This physical characterization, which simultaneously achieves high elasticity and resistance to thermal shrinkage, directly points to the synergistic distribution mechanism of covalent and ionic bonds within the system. When subjected to an instantaneous impact from a steel ball at room temperature, the dynamic ionic clusters, acting as reversible physical crosslinking points in the polymer matrix, can coordinate the conformational transformation of molecular chain segments within a microsecond-level deformation cycle, storing the impact kinetic energy in the crosslinking network and rapidly releasing it during the subsequent rebound phase, reducing heat dissipation caused by molecular chain friction. When the material is placed in a high-temperature field of 120°C, although the thermal motion of polymer chain segments in some amorphous regions intensifies, the high-density zinc carboxylate ionic bonds, due to their thermal dissociation temperature being much higher than the test temperature, act as rigid anchoring nodes within the matrix. The dense ionic cluster network, combined with the underlying covalent cross-linking, forms a physical barrier against thermodynamic relaxation, locking the polymer chains' attempts to coil and contract at the molecular scale. In shoe sole secondary molding production lines, this type of material can often withstand prolonged oven heating without dimensional deviations, providing solid feasibility support for the large-scale application of ultralightweight foamed materials under complex working conditions.

Claims

1. A lightweight EVA foam material based on intermittent foaming and heating, characterized in that, Made from the following ingredients in parts by weight: Ethylene-vinyl acetate copolymer: 90-95 parts; Maleic anhydride-grafted ethylene-vinyl acetate copolymer: 5-10 parts; Azodicarbonamide: 10-15 parts; Basic zinc carbonate: 2-4 parts; Triallyl isocyanurate: 0.5-1.0 parts; Stearic acid: 0.5-1.0 parts; Dicumyl peroxide: 0.8-1.2 parts.

2. The lightweight EVA foam material based on intermittent foaming and heating according to claim 1, characterized in that, The foaming material is made from raw materials comprising the following parts by weight: Ethylene-vinyl acetate copolymer: 92 parts; Maleic anhydride-grafted ethylene-vinyl acetate copolymer: 8 parts; Azodicarbonamide: 12 parts; Basic zinc carbonate: 3 parts; Triallyl isocyanurate: 0.8 parts; Stearic acid: 0.8 parts; Dicumyl peroxide: 1.0 part.

3. The lightweight EVA foam material based on intermittent foaming and heating according to claim 1, characterized in that, The maleic anhydride-grafted ethylene-vinyl acetate copolymer is obtained by reactive extrusion of raw materials comprising the following parts by weight: Ethylene-vinyl acetate copolymer: 100 parts; Maleic anhydride: 1.5-2.5 parts; 2,5-Dimethyl-2,5-bis(tert-butylperoxy)hexane: 0.05-0.10 parts; The grafting rate of the maleic anhydride-grafted ethylene-vinyl acetate copolymer is 0.82%-1.46%.

4. The lightweight EVA foam material based on intermittent foaming and heating according to claim 3, characterized in that, The method for preparing the maleic anhydride-grafted ethylene-vinyl acetate copolymer, Includes the following steps: Weigh the raw materials according to the weight percentages, and mix them in a high-speed mixer at room temperature for 3-5 minutes to obtain a mixture. The mixture is fed into the main feed port of a co-rotating twin-screw extruder for reactive extrusion. The feeding section temperature is set to 140-150℃, the reaction section temperature to 170-190℃, the metering and die head section temperature to 160-170℃, and the screw speed to 150-250 rpm. The vacuum exhaust port is opened at the end of the extruder, and the vacuum degree is controlled at -0.08MPa to -0.09MPa to remove volatile substances. The extruded polymer is then water-cooled, shaped, pelletized, and dried to obtain maleic anhydride-grafted ethylene-vinyl acetate copolymer.

5. The lightweight EVA foam material based on intermittent foaming and heating according to claim 1, characterized in that, In the foamed material: The sum of the weight parts of the ethylene-vinyl acetate copolymer and the maleic anhydride-grafted ethylene-vinyl acetate copolymer is 100 parts. The weight ratio of the triallyl isocyanurate to the stearic acid is 1:

1.

6. A method for preparing lightweight EVA foam material based on intermittent foaming heating, characterized in that, The method for preparing the lightweight EVA foam material based on intermittent foaming heating as described in any one of claims 1-5 comprises the following steps: The ethylene-vinyl acetate copolymer and the maleic anhydride-grafted ethylene-vinyl acetate copolymer were put into a pressure mixer for mixing, and then azodicarbonamide, basic zinc carbonate, triallyl isocyanurate and stearic acid were added and the mixing continued. Turn on the internal mixer to cool it down. After the material cools down, add dicumyl peroxide and mix. Discharge the material onto a two-roll mill to produce thin sheets and cut them into masterbatch sheets. Let them stand to mature. The matured masterbatch is placed in a mold cavity equipped with a mechanical mold locking device, pushed into the first flat vulcanizing machine, the mold is closed and pressurized, and the temperature and pressure are kept constant at the first temperature. After the pressure holding is completed, the upper and lower plates of the mold are rigidly locked by the mechanical mold locking device to maintain the pressure inside the mold cavity, the pressure of the first vulcanizing machine is unloaded, the locked mold is moved out and directly pushed into the second flat vulcanizing machine that has been preheated to the second temperature, which is higher than the first temperature. Apply the mold closing pressure again on the second flat vulcanizing machine and maintain the pressure at a constant temperature. After the timer ends, unload the pressure instantly and simultaneously release the mechanical mold locking device to allow the polymer matrix to expand and eject from the mold. Cool and solidify to obtain the final product.

7. The method for preparing lightweight EVA foam material based on intermittent foaming heating according to claim 6, characterized in that, The ethylene-vinyl acetate copolymer and the maleic anhydride-grafted ethylene-vinyl acetate copolymer are mixed in a pressure mixer at a temperature of 90-100°C for 3-5 minutes. After adding the azodicarbonamide, basic zinc carbonate, triallyl isocyanurate and stearic acid, the mixing temperature is continued at 95-105℃ for 3-5 minutes.

8. The method for preparing lightweight EVA foam material based on intermittent foaming heating according to claim 6, characterized in that, The addition of dicumyl peroxide is carried out when the temperature of the material body drops to 85-90℃, and the mixing time after adding dicumyl peroxide is 1-2 minutes. When the material is discharged onto the two-roll mill, the two-roll mill is preheated to 75-80°C; The conditions for static curing are: static curing at room temperature for 12-24 hours.

9. The method for preparing lightweight EVA foam material based on intermittent foaming heating according to claim 6, characterized in that, The filling rate of the masterbatch sheet placed in the mold cavity is 95%-100%; The mold closing pressure of the first flat vulcanizing machine is 10-15MPa, the first temperature is set to 130-145℃, and the constant temperature and pressure holding time at the first temperature is 8-12 minutes.

10. The method for preparing lightweight EVA foam material based on intermittent foaming heating according to claim 6, characterized in that, The second flat vulcanizing machine reapplies a mold closing pressure of 10-15 MPa, the second temperature is set to 160-180℃, and the constant temperature and pressure holding time at the second temperature is 5-10 minutes. The time for instantly unloading the pressure and simultaneously releasing the mechanical locking device is controlled within 2 seconds.

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