Breathable and heat-conducting EVA foam material and preparation method thereof

Abstract

The invention discloses a breathable and heat-conducting EVA foam material and a preparation method thereof. The three-dimensional hybrid composite phase change material, namely Si3N4 / PEG (at) GNPs, is formed by connecting the three heat-conducting fillers with different morphologies. The graphene nanosheet (GNPs) aerogel is prepared by taking bio-based sodium alginate as a carrier, and a hydrogen bond is formed between sodium alginate and PEG, so that the graphene nanosheet (GNPs) aerogel has good compatibility with PEG. Si3N4 has high thermal conductivity, excellent insulativity and a small expansion coefficient, and due to a covalent bond and a hexagonal structure, the introduction of Si3N4 makes the distribution of PEG in the GNPs aerogel more uniform. Si3N4 / PEG (at) GNPs and EVA resin are subjected to melt blending and mold pressing foaming to prepare the heat-conducting EVA foaming material, and the heat-conducting EVA foaming material has excellent mechanical property, heat stability and heat-conducting property and is wide in application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of materials, and particularly relates to a breathable and heat-conductive EVA foaming material and a preparation method thereof. Background Art

[0002] The characteristics of EVA are good flexibility, rubber-like elasticity, still having good flexibility at -50 °C, transparency and surface glossiness, good chemical stability, good anti-aging and ozone resistance, and non-toxicity. It has good blending property with fillers, coloring property and molding processability. EVA is widely used in fields such as foamed shoe materials, films, packaging films, hot melt adhesives, wire and cable, toys, etc. In the EVA resin used in shoe materials, the vinyl acetate content is generally 15% - 22%. Since the EVA resin blend foamed products have properties such as softness, good elasticity, and chemical corrosion resistance, they are widely used in the soles and interior materials of mid-to-high-end tourist shoes, hiking shoes, slippers, and sandals. In addition, this material is also used in the fields of sound insulation boards, gymnastics mats and sealing materials. In the fields of shoe materials, sealing materials, sports protectors, etc., due to the very low thermal conductivity of EVA (0.035 to 0.045 W m -1 k -1 ). It is mostly used in fields such as construction and packaging, but its low thermal conductivity is not sufficient to meet the heat dissipation requirements in its practical applications in shoe materials, sports protectors, seat cushions, gymnastics mats, etc.

[0003] To improve the low thermal conductivity caused by the disordered arrangement of polymer molecular chains and increase its thermal conductivity. In the past few decades, a large number of studies have been devoted to preparing thermally conductive polymer composites by compounding with dispersed filler particles. However, in practical applications, due to the poor compatibility between the thermally conductive filler and the matrix, it is often necessary to achieve a high thermal conductivity through a high filling amount, resulting in higher costs, heavier weights and poorer processability. By optimizing the surface characteristics of the filler, the interfacial thermal resistance between the filler and the matrix can be improved, thereby enhancing the thermal conductivity of the polymer composite. This method effectively reduces the thermal resistance at the interface, enabling heat to be transferred more efficiently in the composite material.

[0004] In recent years, due to the existence of a considerable number of contact interfaces between the single dispersed filler and the polymer matrix and the large load severely reducing the performance of the polymer, these adverse factors limit the potential of the thermally conductive filler in enhancing the heat conduction of the polymer, making it difficult to fully play its role.

[0005] Based on this, the present invention proposes a breathable and heat-conductive EVA foaming material and a preparation method thereof. Summary of the Invention

[0006] The present invention prepares a thermally conductive EVA foamed material by melt blending a modified thermally conductive filler and an EVA resin and then performing compression molding and foaming. The thermally conductive filler is a three-dimensional hybrid composite phase change material formed by chemically connecting three different morphologies of thermally conductive fillers, namely Si 3 N 4 / PEG@GNPs. Prepare graphene nanoplate (GNPs) aerogel using biobased sodium alginate as the carrier, and use silicon nitride (Si 3 N 4 ) as the thermal conductivity enhancer for the PEG and aerogel-based composite phase change material. Since hydrogen bonds are formed between sodium alginate and PEG, it has good compatibility with PEG. Si 3 N 4 has high thermal conductivity, excellent insulation, and a small expansion coefficient. Due to covalent bonds and a hexagonal structure, the introduction of Si 3 N 4 fills the gaps on the surface of PEG, making the distribution of PEG in the GNPs aerogel more uniform.

[0007] The purpose of the present invention is to provide a highly thermally conductive EVA foamed material and its preparation method, which have good thermal conductivity and mechanical properties.

[0008] To achieve the above object, the present invention adopts the following technical solutions: A breathable and thermally conductive EVA foamed material, the raw materials used in parts by weight include: 70 parts of EVA (ethylene-vinyl acetate), 20 parts of LDPE (low density polyethylene), 10 parts of POE-g-MAH (maleic anhydride grafted polyolefin elastomer), 5 - 15 parts of thermally conductive filler, 2.5 parts of AC foaming agent, 1.5 parts of crosslinking agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent.

[0009] Further, the thermally conductive filler is a Si 3 N 4 / PEG@GNPs composite. The preparation steps of the POE-g-MAH include: weighing 0.4 parts by weight of maleic anhydride MAH, dissolving MAH in acetone to prepare a solution. Weigh 4 parts by weight of polyolefin elastomer POE particles, spray the acetone solution of MAH into the POE particles, and mix evenly in a high-speed mixer. Let it stand to volatilize the acetone. The mixed material is melt blended in a torque rheometer for 10 min, the rotor speed is 30 r / min, and the torque temperature is 165 °C. After the blending is completed, the melt is rolled into sheets on a two-roll mill.

[0010] Among them, the preparation of the thermally conductive filler includes the following steps: (1) Using natural flake graphite as raw material, GO was prepared by the improved Hummers method. 1.5 g of natural flake graphite, 1 g of NaNO 3 and 100 mL of concentrated H 2 SO 4 (above 70wt%) were added into a 500 mL round-bottom flask. The flask was placed in an ice-water bath to keep the temperature of the mixture below 10 °C. Meanwhile, under stirring, 5 g of KMnO 4 was slowly added to the reaction mixture in five portions. The mixture was heated to 35 °C and stirred for 24 h, then 150 mL of deionized water was slowly added, and the temperature was raised to 90 °C and continuously stirred for about 4 h. Subsequently, 100 mL of deionized water and 5 mL of 30wt% H 2 O 2 aqueous solution were added, and the mixture was stirred for another 1 h. The resulting mixture was filtered and washed repeatedly with diluted HCl solution until sulfate ions could not be detected by BaCl 2 . Finally, the sample was freeze-dried to obtain GO; (2) First, 200 mg of sodium alginate was weighed and dissolved in deionized water, and stirred at a rate of 1500 r / min. Then, after stirring evenly, 100 mg of GO prepared in step (1) was added to the mixture and frozen in the refrigerator. Finally, the sample was placed in a freeze dryer for 48 hours to finally obtain GNPs.

[0011] (3) 2 g of polyethylene glycol PEG was weighed and placed in a beaker, and heated to dissolve in a water bath at 85 °C. Then 1 g of Si 3 N 4 was added to anhydrous ethanol and ultrasonicated for 40 min. After that, the ultrasonically treated mixture was put into the melted PEG, and mechanically stirred with a mixer at a speed of 1700 r / pm for 90 minutes until the ethanol and absolute water were completely volatilized.

[0012] (4) The GNPs obtained in step (2) were added to the Si 3 N 4 / PEG solution and stirred at 1500 r / min for 1 h, then the product was placed in a vacuum drying oven and vacuum dried at 75 °C for 12 h to obtain the Si 3 N 4 / PEG@GNPs composite.

[0013] Further, the filler is: one or more compositions of nano-zinc oxide, zinc stearate; the crosslinking agent is: dicumyl peroxide (DCP); the lubricant is: one or more of stearic acid, butyl stearate, oleamide; the demolding agent is paraffin.

[0014] Furthermore, the EVA foam material has an open-cell structure, and the preparation steps include: melt-blending two materials with significantly different crystallization temperatures, namely EVA and LDPE. There are two blended polymers with different crystallization temperature values in the melt structure. The polymer with a higher crystallization temperature will crystallize before the polymer with a lower crystallization temperature, thereby generating pressure on the cell structure to rupture and connect the cells to form an open-cell structure. At the same time, the secondary foaming method is used as the molding foaming method to make the EVA foam material into an open-cell structure.

[0015] Furthermore, the preparation method of the breathable and heat-conducting EVA foam material includes: mixing 70 parts of EVA (ethylene-vinyl acetate), 20 parts of LDPE (low-density polyethylene), 10 parts of POE-g-MAH (maleic anhydride-modified polyolefin elastomer), 5 - 15 parts of heat-conducting filler, 2.5 parts of AC foaming agent, 1.5 parts of cross-linking agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent in a mixer, and then subjecting the blended product to hot and cold molding; specifically including the following steps: (1) Put EVA, POE-g-MAN, LDPE, and cross-linking agent into a mixer preheated to 130 °C and blend for 10 min. The rotation speed of the mixer is 30 rpm; (2) Add the heat-conducting filler, AC foaming agent, filler, calcium-zinc stabilizer, lubricant, and demolding agent to the blend obtained in step (1), and continue to mix in the mixer at 130 °C for 10 min; (3) Press the blend obtained in step (2) into a thin sheet with an open mill and then transfer it to a flat vulcanizing machine for molding and foaming. The temperatures of the upper and lower templates are both set at 150 °C, the pressure is 10 MPa, and the molding time is 600 s for primary pre-foaming. Place the pre-foamed semi-finished product in a blast drying oven at 60 °C for 12 h, and then adjust the temperatures of the upper and lower templates to 175 °C, the pressure to 20 MPa, and the molding time to 380 s for secondary foaming. After opening the mold, the breathable and heat-conducting EVA foam material will automatically pop out.

[0016] Preferably, the filler is nano-zinc oxide; the lubricant is stearic acid; the demolding agent is paraffin.

[0017] The beneficial effects of the present invention are as follows: The present invention uses the crystallization temperature difference method to melt-blend two materials with significantly different crystallization temperatures, namely EVA (crystallization temperature 10 °C) and LDPE (crystallization temperature 90 °C). The two semi-crystalline polymers with different crystallization temperatures are melt-blended to maximize the stiffness contrast between the hard and soft regions in the polymer matrix. Since the open-cell process occurs after the foam extrudate leaves the mold, the foam structure will naturally be cooled by the surrounding air. If the melt structure has two different T cFor the blend polymer of values, the high T c polymer will crystallize before the low T c polymer. This means that between the two T c values, the soft section (i.e., the low T c polymer) is almost like a liquid, while the hard section (i.e., the high T c polymer) is almost like a solid, resulting in a large stiffness contrast. If the cell opening process occurs when the temperature of the foam structure is between the two T c values, then due to the large stiffness difference, the chance of cell opening will be greatly increased. In the melt structure, there are blend polymers with two different crystallization temperature values. The high crystallization temperature polymer will crystallize before the low crystallization temperature polymer, thereby exerting pressure on the cell structure to cause the cells to rupture and connect to form an open-cell structure. At the same time, the secondary foaming method is used as the molding foaming method to make the EVA foaming material into an open-cell foaming structure. Since the open-cell foaming structure is internally interconnected, the air circulation efficiency is improved, and its air permeability is enhanced.

[0018] The present invention uses a 3D hybrid structure material formed by covalent reaction between three different materials, namely spherical nano-silicon nitride particles, flaky polyethylene glycol, and lamellar graphene oxide, as the thermal conductive filler. Doping hybrid fillers in polymers is an effective method to achieve high-performance composite materials. The synergistic effect between different-shaped thermal conductive fillers can greatly improve the thermal conductivity of the composite material. The particulate filler provides short-distance heat transfer and more contact possibilities; it endows the filler with the formation of a three-dimensional (3D) structure in the composite material. On the one hand, it can establish multiple heat conduction paths to form an efficient heat conduction network, thereby improving the heat conduction efficiency; on the other hand, it can make the filler evenly distributed in the matrix, thereby improving the mechanical properties of the composite material. Both can effectively reduce the negative impact brought by the interfacial thermal resistance between the filler and the filler interface. At the same time, with the help of this multi-dimensional 3D structure, only a relatively small doping amount is needed to maximize the improvement of the thermal conductivity. The 3D framework network provides rich paths and interfacial contact points for phonon transfer, which helps to reduce the scattering effect of phonons at the interface.

[0019] The three-dimensional hybrid filler Si 3 N 4 / PEG@GNPs prepared by the present invention can achieve high thermal conductivity of the composite material at a relatively low doping amount. This benefits from the three-dimensional thermal conduction network built by the hybrid filler inside the composite material, which reduces the scattering of phonons and constructs multi-level heat conduction paths, thereby greatly improving the thermal conductivity of the EVA foaming material. While ensuring the thermal conductivity, it reduces costs and improves the mechanical properties of the composite material. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is the Fourier infrared diagram of the composite thermal conductive filler prepared by the present invention.

[0021] Figure 2 Scanning electron micrograph of the composite thermal conductive filler prepared for the present invention.

[0022] Figure 3 Cell structure diagram of the EVA foam material prepared for the present invention. Detailed implementation manners

[0023] To make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below in conjunction with specific implementation manners. However, the present invention is not limited thereto.

[0024] In the following examples and comparative examples, the parts by weight of the raw materials are used; the crystallization temperatures of the EVA and LDPE used are 10°C and 90°C respectively, and the molecular weight of the PEG used is 2000.

[0025] Example 1 (1) Preparation of the polymer matrix: 70 parts of EVA, 20 parts of LDPE, 10 parts of POE-g-MAH, and 1.5 parts of cross-linking agent were put into a preheated internal mixer at 130°C and blended for 10 min. The rotation speed of the internal mixer was 30 rpm; (2) Preparation steps of the composite thermal conductive filler Si 3 N 4 / PEG@GNPs: (2-1) Using natural flake graphite as the raw material, GO was prepared by an improved Hummers method. 1.5 g of natural flake graphite, 1 g of NaNO 3 and 100 mL of concentrated H 2 SO 4 (75 wt%) were added to a 500 mL round-bottom flask. The flask was placed in an ice-water bath to keep the temperature of the mixture below 10°C. Meanwhile, under stirring, 5 g of KMnO 4 was slowly added to the reaction mixture in five portions. The mixture was heated to 35°C and stirred for 24 h, then 150 mL of deionized water was slowly added, and the temperature was raised to 90°C and stirred continuously for about 4 h. Subsequently, 100 mL of deionized water and 5 mL of 30 wt% H 2 O 2 aqueous solution were added, and the mixture was stirred for 1 h. After the obtained mixture was filtered, it was washed with diluted HCl solution multiple times until sulfate ions could not be detected by BaCl 2 . Finally, the sample was freeze-dried. GO was obtained; (2-2) First, weigh 200 mg of sodium alginate and dissolve it in 100 ml of deionized water, and stir at a rate of 1500 r / min. Then, after stirring evenly, add 100 mg of GO prepared in step (2-1) to the mixture and freeze it in the refrigerator. Finally, place the sample in a freeze dryer for 48 hours to finally obtain GNPs.

[0026] (2-3) Weigh 2 g of PEG and put it into a beaker, and heat it in a water bath at 85 °C until it dissolves. Then weigh 1 g of Si 3 N 4 and add it to 50 ml of absolute ethanol and perform ultrasonic treatment for 40 min. After that, put the ultrasonically treated mixture into the melted PEG and mechanically stir it with a mixer at a speed of 1700 r / pm for 90 minutes until the absolute ethanol and water are completely volatilized to obtain Si 3 N 4 / PEG.

[0027] (2-4) Add the GNPs obtained in step (2-2) to the Si 3 N 4 / PEG solution and stir at 1500 r / min for 1 h, then place the product in a vacuum drying oven and vacuum dry it at 75 °C for 12 h to obtain Si 3 N 4 / PEG@GNPs composite.

[0028] (3) Preparation of open-cell breathable and heat-conducting EVA foam material: (3-1) Add 5 parts of composite heat-conducting filler, 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent to a kneader and continue to knead with the molten blend obtained in step (1) in the kneader at 130 °C for 10 min; (3-2) Press the blend obtained in step (3-1) into a thin sheet with a two-roll mill and then transfer it to a flat vulcanizer for mold pressing and foaming. The temperatures of the upper and lower templates are both set at 150 °C, the pressure is 10 MPa, and the mold pressing time is 600 s for a primary pre-foaming. Place the pre-foamed semi-finished product in a blast drying oven at 60 °C for 12 h, then adjust the temperatures of the upper and lower templates to 175 °C, the pressure to 15 MPa, and the mold pressing time to 380 s for a secondary foaming. After opening the mold, the EVA foam material pops out automatically.

[0029] Example 2 (1) Preparation of polymer matrix: The same as Example 1 (2) Preparation of Si 3 N 4 / PEG@GNPs: The same as Example 1 (3) Preparation of the porous breathable and thermally conductive EVA foam material: (3-1) Add 10 parts of the composite thermal conductive filler, 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of mold release agent into a mixer and continue to mix with the molten blend obtained in step (1) in the mixer at 130 °C for 10 min; (3-2) Press the blend obtained in step (3-1) into a thin sheet using an open mill and then transfer it to a flat vulcanizer for mold pressing and foaming. Set the temperatures of the upper and lower templates to 150 °C, the pressure to 10 MPa, and the mold pressing time to 600 s for the first pre-foaming. Place the pre-foamed semi-finished product in a blast drying oven at 60 °C for 12 h. Then adjust the temperatures of the upper and lower templates to 175 °C, the pressure to 15 MPa, and the mold pressing time to 380 s for the second foaming. After opening the mold, the EVA foam material pops out automatically.

[0030] Example 3 (1) Preparation of the polymer matrix: The same as in Example 1 (2) Preparation of Si 3 N 4 / PEG@GNPs: The same as in Example 1 (3) Preparation of the porous breathable and thermally conductive EVA foam material: (3-1) Add 15 parts of the composite thermal conductive filler, 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of mold release agent into a mixer and continue to mix with the molten blend obtained in step (1) in the mixer at 130 °C for 10 min; (3-2) Press the blend obtained in step (3-1) into a thin sheet using an open mill and then transfer it to a flat vulcanizer for mold pressing and foaming. Set the temperatures of the upper and lower templates to 150 °C, the pressure to 10 MPa, and the mold pressing time to 600 s for the first pre-foaming. Place the pre-foamed semi-finished product in a blast drying oven at 60 °C for 12 h. Then adjust the temperatures of the upper and lower templates to 175 °C, the pressure to 15 MPa, and the mold pressing time to 380 s for the second foaming. After opening the mold, the EVA foam material pops out automatically.

[0031] Comparative Example 1 (without Si 3 N 4 / PEG@GNPs thermal conductive filler) (1) Preparation of the polymer matrix: The same as in Example 1 (2) Preparation of the porous breathable EVA foam material: (2-1) Add 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of mold release agent into a mixer and continue to mix with the molten blend obtained in step (1) in the mixer at 130 °C for 10 min; (2-2) The blend obtained in step (2-1) is pressed into a thin sheet using a two-roll mill and then transferred to a flat vulcanizing machine for mold pressing and foaming. The temperatures of the upper and lower templates are both set to 150 °C, the pressure is 10 MPa, the mold pressing time is 600 s for primary foaming. The semi-finished product after primary foaming is placed in a blast drying oven at 60 °C for 12 h. Then, the temperatures of the upper and lower templates are adjusted to 175 °C, the pressure is 15 MPa, and the mold pressing time is 380 s for secondary foaming. After the mold is opened, the EVA foam material pops out automatically.

[0032] Comparative Example 2 (Si 3 N 4 , PEG, and GNPs are not subjected to any compounding treatment and are separately added to the system) (1) Preparation of the polymer matrix: The same as in Example 1 (2) Preparation of the porous breathable and heat-conductive EVA foam material: (2-1) 5 parts of Si 3 N 4 , PEG, and GNPs directly mixed in a specific ratio (mass ratio 1:2:0.3), 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent are added to a kneader and kneaded with the molten blend obtained in step (1) in the kneader at 130 °C for 10 min; (2-2) The blend obtained in step (2-1) is pressed into a thin sheet using a two-roll mill and then transferred to a flat vulcanizing machine for mold pressing and foaming. The temperatures of the upper and lower templates are both set to 150 °C, the pressure is 10 MPa, the mold pressing time is 600 s for primary foaming. The semi-finished product after primary foaming is placed in a blast drying oven at 60 °C for 12 h. Then, the temperatures of the upper and lower templates are adjusted to 175 °C, the pressure is 15 MPa, and the mold pressing time is 380 s for secondary foaming. After the mold is opened, the EVA foam material pops out automatically.

[0033] Comparative Example 3 (Adding Si 3 N 4 / PEG, but not adding GNPs) (1) Preparation of the polymer matrix: The same as in Example 1 (2) Preparation of Si 3 N 4 / PEG: The same as in Example 1 (3) Preparation of the porous breathable and heat-conductive EVA foam material: (3-1) 5 parts of Si 3 N 42.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium-zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent are added to the masticator and kneaded with the molten blend obtained in step (1) for another 10 min in the masticator at 130 °C. (3-2) The blend obtained in step (3-1) is pressed into a thin sheet by an open mill and then transferred to a flat vulcanizer for mold pressing and foaming. The temperatures of the upper and lower templates are both set at 150 °C, the pressure is 10 MPa, and the mold pressing time is 600 s for primary pre-foaming. The pre-foamed semi-finished product is placed in a blast drying oven at 60 °C for drying for 12 h. Then, the temperatures of the upper and lower templates are both adjusted to 175 °C, the pressure is 15 MPa, and the mold pressing time is 380 s for secondary foaming. After the mold is opened, the EVA foamed material pops out automatically.

[0034] Comparative Example 4 (adding PEG@GNPs, but not adding Si 3 N 4 ) (1) Preparation of polymer matrix: The same as in Example 1 (2) Preparation of PEG@GNPs: (2-1) Using natural flake graphite as the raw material, GO is prepared by the improved Hummers method. 1.5 g of natural flake graphite, 1 g of NaNO 3 and 100 mL of concentrated H 2 SO 4 (75 wt%) are added to a 500 mL round-bottom flask. The flask is placed in an ice-water bath to keep the temperature of the mixture below 10 °C. Meanwhile, under stirring, 5 g of KMnO 4 is slowly added to the reaction mixture in five portions. The mixture is heated to 35 °C and stirred for 24 h, then 150 mL of deionized water is slowly added, and the temperature is raised to 90 °C and continuously stirred for about 4 h. Subsequently, 100 mL of deionized water and 5 mL of 30 wt% H 2 O 2 aqueous solution are added, and the mixture is stirred for another 1 h. The resulting mixture is filtered and washed with diluted HCl solution multiple times until sulfate ions cannot be detected by BaCl 2 . Finally, the sample is freeze-dried to obtain GO; (2-2) First, 200 mg of sodium alginate is weighed and dissolved in 100 ml of deionized water and stirred at a rate of 1500 r / min. Then, after stirring evenly, 100 mg of GO prepared in step (2-1) is added to the mixture and frozen in a refrigerator. Finally, the sample is placed in a freeze-dryer for 48 h, and finally GNPs are obtained.

[0035] (2 - 3) Weigh 2 g of PEG and put it into a beaker, then heat it in a water bath at 85 °C until it dissolves. After that, add the GNPs obtained in step (2 - 2) to the melted PEG, and mechanically stir it with a mixer at a speed of 1700 r / pm for 90 minutes until the absolute ethanol and water are completely volatilized. Then place the product in a vacuum drying oven and vacuum dry it at 75 °C for 12 h to obtain the PEG@GNPs composite.

[0036] (3) Preparation of open - cell breathable and heat - conductive EVA foam material: (3 - 1) Add 5 parts of PEG@GNPs, 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium - zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent into a mixer and continue to mix with the molten blend obtained in step (1) in a mixer at 130 °C for 10 min; (3 - 2) Press the blend obtained in step (3 - 1) into a thin sheet with a two - roll mill and then transfer it to a flat vulcanizing machine for mold - pressing and foaming. Set the upper and lower template temperatures to 150 °C, the pressure to 10 MPa, and the mold - pressing time to 600 s for a primary pre - foaming. Place the pre - foamed semi - finished product in a blast drying oven at 60 °C for 12 h. Then adjust the upper and lower template temperatures to 175 °C, the pressure to 15 MPa, and the mold - pressing time to 380 s for a secondary foaming. After opening the mold, the EVA foam material pops out automatically.

[0037] Comparative Example 5 (without using the crystallization temperature difference method and the secondary foaming method) (1) Preparation of polymer matrix: Mix 90 parts of EVA, 10 parts of POE - g - MAH, and 1.5 parts of cross - linker in a pre - heated mixer at 130 °C for 10 min. The mixer speed is 30 rpm; (2) Si 3 N 4 / PEG@GNPs preparation: The same as in Example 1 (3) Preparation of open - cell breathable and heat - conductive EVA foam material: (3 - 1) Add 5 parts of heat - conductive filler, 2.5 parts of AC blowing agent, 2.5 parts of filler, 3 parts of calcium - zinc stabilizer, 1 part of lubricant, and 0.2 part of demolding agent into a mixer and continue to mix with the molten blend obtained in step (1) in a mixer at 130 °C for 10 min; (3 - 2) Press the blend obtained in step (3 - 1) into a thin sheet with a two - roll mill and then transfer it to a flat vulcanizing machine for mold - pressing and foaming. Set the upper and lower template temperatures to 175 °C, the pressure to 15 MPa, and the mold - pressing time to 380 s for foaming. After opening the mold, the EVA foam material pops out automatically.

[0038] Perform performance tests on the samples obtained in the examples and comparative examples.

[0039] Table 1: Performance test From the test results in Table 1, in Example 1 and Comparative Example 5, after the EVA foam is not made into an open-cell structure, the mechanical properties of the product show a slightly increasing trend. This is because the closed-cell structure enables the cell walls to be interconnected, which can improve the mechanical properties of the foam. However, the open-cell rate shows a significant decreasing trend, indicating that the open-cell rate is significantly increased after the material is made into an open-cell structure, enhancing the air circulation rate and air permeability of the product. In Examples 1-3, as the content of the thermal conductive filler increases, the tensile strength, cell density, hardness, and elongation at break of the product all show an increasing trend. Due to the increase in hardness, the resilience and compression deformation of the material slightly decrease. Except for the slight decrease in resilience and compression deformation due to the increase in hardness and the slight decrease in the open-cell rate, other properties are better than those of Comparative Example 1 without the addition of the thermal conductive filler, indicating that Si 3 N 4 / PEG@GNPs thermal conductive filler has a good modification effect on the mechanical properties of the foaming material. After doping with Si 3 N 4 / PEG@GNPs, the thermal conductivity coefficient of the composite material is improved and increases with the increase in the doping amount. This benefits from the excellent thermal conductivity of Si 3 N 4 / PEG@GNPs. In Comparative Example 2, when Si 3 N 4 , PEG, and GNPs are directly added, except for hardness and thermal conductivity coefficient, the physical properties of the foaming material all decrease compared with those of Comparative Example 1. This is because the separate addition is prone to agglomeration. Compared with Comparative Examples 3 and 4, in Example 1, PEG can endow the composite material with good crystallization performance. GNPs aerogel forms a continuous body as the matrix, and GNPs in different layers are connected to each other, effectively interacting to form a 3D heat transfer channel. The synergistic effect between Si 3 N 4 and GNPs aerogel makes the heat transfer system more complete.

[0040] Figure 1 is the Fourier infrared spectrum of the powder of Si 3 N 4 / PEG@GNPs thermal conductive filler. The typical absorption peak at 3450 cm -1 is the -OH vibration peak, and the absorption peak at 2890 cm -1 is the stretching vibration peak of -CH. In addition, the absorption peaks at 1637 cm -1 and 1110 cm -1 belong to the stretching vibration and asymmetric stretching vibration peaks of C=O and C-O. In the Si 3 N 4 spectrum, at 900 cm-1 -1100 cm -1 The broad absorption peak appearing at this position is the vibration peak of Si-N. It can be clearly observed that Si 3 N 4 All the characteristic peaks of the Si 3 N 4 / PEG@GNPs composite phase change material are consistent with those of pure PEG and Si -1 and no obvious new peaks appear. In addition, compared with pure PEG, the intensity of the crystallization peak of the composite material at 960 cm 3 N 4 shows almost no change, and no new characteristic peaks appear, indicating that there is no obvious chemical reaction between PEG, GNPs and Si

[0041] Figure 2 For Si 3 N 4 / PEG@GNPs composite phase change material, the SEM image shows that there is no obvious and serious interfacial separation or cracking on the fracture surface of the composite phase change material. The porous structure in the GNPs aerogel is covered by PEG, showing good compatibility. At the same time, it shows a smoother surface and a denser structure, and the introduction of Si 3 N 4 is uniformly distributed on the surface of the composite phase change material.

[0042] Figure 3 The SEM image shows open-cell pores. It can be seen that most of the cell structures are open-cell structures. First, when the soft cross-section of the low-Tc polymer (LDPE) forms smaller dispersed phases, and the hard cross-section of the high-Tc polymer (EVA) forms the main melt matrix, the cell openings can be initiated and propagated through well-dispersed low-Tc polymer domains, which are surrounded between the growing adjacent cells. These soft domains can extend as the cells grow, that is, the cell walls become thinner until they rupture to form an open-cell structure.

[0043] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. A breathable and heat-conductive EVA foam material, characterized in that: The raw materials of the EVA foaming material include, by weight: 70 parts of EVA, 20 parts of LDPE, 10 parts of POE-g-MAH, 5-15 parts of thermal conductive filler, 2.5 parts of AC foaming agent, 1.5 parts of cross-linking agent, 2.5 parts of filler, 3 parts of calcium zinc stabilizer, 1 part of lubricant, and 0.2 parts of release agent.

2. The breathable and heat-conductive EVA foam material according to claim 1, characterized in that: The thermal conductive filler is Si3N4 / PEG@GNPs, and the preparation steps include: (1) Preparation of GO: 1.5 g of flake graphite, 1 g of NaNO3, and 100 mL of concentrated sulfuric acid were added to a 500 mL round-bottom flask. The flask was placed in an ice-water bath to keep the temperature of the mixture below 10 °C. While stirring, 5 g of KMnO4 was slowly added to the reaction mixture in five portions. The mixture was heated to 35 °C and stirred for 24 h. Then, 150 mL of deionized water was slowly added. The temperature was raised to 90 °C and stirred for 4 h. Subsequently, 100 mL of deionized water and 5 mL of 30 wt% H2O2 aqueous solution were added and stirred for another 1 h. The resulting mixture was filtered and washed with a diluted HCl solution several times until sulfate ions could no longer be detected in BaCl2. Finally, the sample was freeze-dried to obtain GO. (2) First, weigh 200 mg of sodium alginate and dissolve it in deionized water, and stir it at a rate of 1500 r / min. After stirring evenly, add 100 mg of GO prepared in step (1) and freeze it in a refrigerator. Finally, place the sample in a freeze dryer for 48 hours to obtain GNPs. (3) Weigh 2 g of polyethylene glycol (PEG) into a beaker and heat it in a water bath at 85°C until dissolved. Then weigh 1 g of Si3N4 and add it to anhydrous ethanol and perform ultrasonic treatment for 40 min. Then, place the ultrasonically treated mixture into the molten PEG and stir it mechanically at a speed of 1700 rpm for 90 min with a mixer until the ethanol and absolute water are completely evaporated. (4) The GNPs obtained in step (2) were added to the Si3N4 / PEG solution obtained in step (3) and stirred at 1500 r / min for 1 h. The product was then placed in a vacuum drying oven and vacuum dried at 75 °C for 12 h to obtain a Si3N4 / PEG@GNPs composite.

3. The breathable and heat-conductive EVA foam material according to claim 1, characterized in that: The filler is one or more of nano zinc oxide and zinc stearate.

4. The breathable and heat-conductive EVA foam material according to claim 1, characterized in that: The cross-linking agent is dicumyl peroxide.

5. The breathable and heat-conductive EVA foam material according to claim 1, characterized in that: The lubricant is one or more of stearic acid, butyl stearate and oleamide.

6. The breathable and heat-conductive EVA foam material according to claim 1, characterized in that: The release agent is paraffin.

7. The breathable and heat-conductive EVA foam material according to claim 1, characterized in that: The preparation of POE-g-MAH comprises: weighing 0.4 parts by weight of maleic anhydride, dissolving the maleic anhydride in acetone to prepare a solution; weighing 4 parts by weight of polyolefin elastomer POE particles, spraying the maleic anhydride acetone solution into the POE particles, mixing them evenly in a high-speed mixer, and standing to volatilize the acetone; the mixed materials are melt-blended for 10 minutes using a torque rheometer, the rotor speed is 30 r / min, the torque temperature is 165°C, and after the blending is completed, the melt is melted into sheets in an open mill.

8. The method for preparing a breathable and heat-conductive EVA foam material according to any one of claims 1 to 7, characterized in that: The EVA foam material is an open-cell structure, and the preparation steps include: melt-blending EVA and LDPE, two materials with a large difference in crystallization temperature, wherein the melt structure contains two blended polymers with different crystallization temperature values, and the high crystallization temperature polymer will crystallize before the low crystallization temperature polymer, thereby generating pressure on the pore structure to cause the pores to rupture and connect to generate an open-cell structure, and at the same time, using a secondary foaming method as a compression foaming method to prepare the EVA foam material into an open-cell pore structure.

9. The method for preparing the breathable and heat-conductive EVA foam material according to claim 8, characterized in that: 70 parts of EVA, 20 parts of LDPE, 10 parts of POE-g-MAH, 5-15 parts of thermal conductive filler, 2.5 parts of AC foaming agent, 1.5 parts of cross-linking agent, 2.5 parts of filler, 3 parts of calcium zinc stabilizer, 1 part of lubricant and 0.2 parts of release agent are put into an internal mixer for blending, and then the blended product is subjected to cold and hot compression molding.

10. The method for preparing the breathable and heat-conductive EVA foam material according to claim 9, characterized in that: The following steps are involved: (1) Put EVA, LDPE, POE-g-MAH and cross-linking agent into an internal mixer preheated at 130°C and blend for 10 min to obtain blend 1. The speed of the internal mixer is 30 rpm; (2) Adding thermal conductive filler, AC foaming agent, filler, calcium zinc stabilizer, lubricant and release agent to blend 1, and continuing to mix in an internal mixer at 130° C. for 10 min to obtain blend 2; (3) After blend 2 is pressed into a thin sheet using an open mixing mill, it is transferred into a flat vulcanizer for compression foaming. The upper and lower mold temperatures are both set to 150°C, the pressure is 10 MPa, and the molding time is 600 s for a pre-foaming process. The pre-foamed semi-finished product is placed in a 60°C forced air drying oven and dried for 12 h. Then, the upper and lower mold temperatures are adjusted to 175°C, the pressure is 15 MPa, and the molding time is 380 s for a secondary foaming process. After the mold is opened, the EVA foam material automatically pops out.

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