A method for producing an ethylene-vinyl acetate copolymer foam

Abstract

This invention discloses a method for preparing an ethylene-vinyl acetate copolymer foam material, comprising the following steps: (1) Tea stem pretreatment: The tea stems are pretreated using a physical-chemical pretreatment method. The tea stems are pulverized to 100-300 mesh using a pulverizer. The tea stem powder is then added to a NaOH aqueous solution, allowed to stand, and washed with pure water until pH=7 to remove residual NaOH. The powder is then added to an VTMO aqueous solution, heated and stirred, filtered, and dried to obtain modified tea stem powder for later use; (2) Intensive mixing; (3) Thin-pass milling; (4) Foaming. This invention uses a physical-chemical dual pretreatment method to modify the tea stems, which effectively improves the interfacial compatibility between biomass fibers and the EVA / POE polymer matrix, avoids problems such as fiber agglomeration and uneven cell size during foaming, and ensures the uniformity and stability of the foam structure.

Description

Technical Field

[0001] This invention belongs to the field of polymer material processing technology, specifically relating to a method for preparing an ethylene-vinyl acetate copolymer foam material. Background Technology

[0002] EVA (ethylene-vinyl acetate copolymer) is a high-performance thermoplastic polymer. The vinyl acetate (VA) content in its molecular chain can be precisely controlled to adjust the material's hardness, flexibility, and adhesion, thus meeting diverse needs in various fields. EVA not only possesses excellent flexibility, high elasticity, and impact resistance, but also good cushioning properties, chemical corrosion resistance, weather resistance, and ease of processing, making it an ideal material for cross-disciplinary applications. Specifically, this material exhibits excellent mechanical properties, with good flexibility, impact resistance, and abrasion resistance, and its mechanical properties are further optimized with increasing vinyl acetate content. In terms of thermal properties, it has a low melting point, typically between 110℃ and 130℃, facilitating thermoforming. Regarding chemical corrosion resistance, it can withstand various chemicals such as acids, alkalis, and salts. In terms of weather resistance, it can effectively resist the effects of external environmental factors such as ultraviolet radiation and temperature changes.

[0003] EVA foam materials are increasingly widely used due to their lightweight, softness, and impact resistance. Current research mainly focuses on two areas: foaming technology and process optimization, and material modification. By optimizing process parameters and introducing blended components or fillers, the overall performance of the material is continuously improved.

[0004] In the footwear materials industry, EVA (ethylene-vinyl acetate copolymer) has become one of the core materials in modern footwear manufacturing due to its excellent properties. It is mainly used in shoe midsoles, linings, and insoles. Made through a chemical foaming process, this lightweight, highly elastic closed-cell foam provides excellent cushioning, rebound, and wearing comfort. By adjusting the vinyl acetate (VA) content, the material's hardness and toughness can be flexibly controlled to meet the functional requirements of different shoe styles, including casual shoes, athletic shoes, and even professional running shoes. Furthermore, EVA is easily compounded with other materials, often combined with rubber and TPU to further enhance its abrasion resistance, stability, and aesthetic design. Under the trend of sustainable development, EVA's recyclability and its potential for using recycled materials make it an important material driving the green transformation of the footwear industry, continuously leading the design direction towards lightweighting, functionality, and environmental friendliness.

[0005] In Fujian, a province that ranks among the top in the country in both tea and footwear industries, discarded tea stems, which account for nearly 30% of the weight of tea, are becoming an important resource for "turning waste into treasure." As a byproduct of tea processing, tea stems are characterized by low procurement costs, stable and widespread sources, making them ideal biomass fillers. After appropriate modification, their interfacial compatibility with polymer matrices can be significantly optimized, fully leveraging their reinforcing and functional properties, and promoting the development of polymer materials towards a green and sustainable direction. Integrating tea stems into shoe sole materials through foaming technology can not only reduce the use of petrochemical raw materials and solve the problem of odor easily generated by traditional shoe materials, but also introduce the natural antibacterial properties of tea polyphenols into footwear products. Cross-industry innovation practices represented by Fujian enterprises such as Anta have made the tea stem particles in shoe soles a flowing symbol of Fujian tea culture—from Wuyi rock tea to Anxi Tieguanyin, each pair of shoes, while practicing the "dual carbon" goal, also carries the extension of the tea industry chain and the connotation of the coordinated development of "three teas," achieving an organic unity of environmental benefits, functional value, and cultural heritage.

[0006] Chemical foaming is one of the classic methods for preparing polymer foams. This method utilizes the gas-generating effect of foaming agents such as azodicarbonamide, p-toluenesulfonyl hydrazine, and sodium bicarbonate / zinc carbonate to initiate cell nucleation within the system, ultimately forming the desired cell structure.

[0007] Existing pure chemical foaming technology has several significant drawbacks in both its technology and application. In terms of cost, it relies entirely on high-priced virgin EVA granules, lacking low-cost alternatives, resulting in high raw material costs and difficulty in effectively controlling unit production costs, thus weakening its competitiveness in the low-end market. Regarding resources and environmental protection, this process uses only virgin EVA, consuming large amounts of petroleum-derived resources and failing to achieve resource utilization of solid waste, resulting in a high carbon footprint. This aligns poorly with current "dual-carbon" policies and the concept of green circular development, making it difficult to meet downstream brands' requirements for green supply chains. In terms of functional adaptability, the rigidity, dimensional stability, and vibration damping and sound absorption performance of its products are relatively limited, making it difficult to meet the application scenarios of functional components with specific requirements for rigidity, dimensional accuracy, and cushioning and noise reduction. Regarding production flexibility, the density and hardness of the products mainly rely on process parameter adjustments, which have a limited adjustment range and high costs, resulting in low raw material utilization and failing to meet the trend of efficient resource utilization. Furthermore, this process lacks green labels such as bio-based or recycled resources, making it difficult to adapt to the current promotional needs of some downstream markets for green products, placing it in a passive position in differentiated competition. In summary, the main drawbacks of traditional pure virgin EVA chemical foaming are high cost, low resource utilization efficiency, insufficient environmental compatibility, and poor flexibility in functional adjustment. These shortcomings limit its ability to occupy the mid-to-high-end market based solely on its basic performance advantages, and its applicability in low-cost, green resource-oriented application scenarios is limited. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects of the prior art and provide a method for preparing ethylene-vinyl acetate copolymer foam material.

[0009] The technical solution of the present invention is as follows:

[0010] A method for preparing an ethylene-vinyl acetate copolymer foam material includes the following steps:

[0011] (1) Tea stem pretreatment: The tea stems were pretreated by physical-chemical pretreatment method. The tea stems were crushed to 100-300 mesh using a pulverizer. Then the tea stem powder was added to NaOH aqueous solution. After standing, it was washed with pure water until pH=7 to remove residual NaOH. Then it was added to VTMO aqueous solution, heated and stirred, filtered and dried to obtain modified tea stem powder for later use.

[0012] (2) Internal mixing: After preheating the internal mixer, add EVA, EVA recycled material and POE, and stir at low speed; then add the above modified tea stem powder, zinc oxide, stearic acid, talc and white carbon black, and stir at high speed; finally add azodicarbonamide and DCP, and stir until the material is free of stratification and has a loose aggregate to obtain the internal mixing material.

[0013] (3) Open mill thin pass: The intensively mixed material is turned over and cut several times on the open mill, and then thinned to 1-1.5 mm. After turning over and cutting several times, the material is discharged to obtain the composite material;

[0014] (4) Foaming: Place the composite material obtained in step (3) into a preheated mold on a flat vulcanizing apparatus and apply pressure at the same temperature to foam.

[0015] In a preferred embodiment of the present invention, in step (1), the concentration of the NaOH aqueous solution is 10-20 wt%.

[0016] In a preferred embodiment of the present invention, in step (1), the solid-liquid ratio of tea stem powder to NaOH aqueous solution is 1:5-3:5, and the standing time is 24-48 h.

[0017] In a preferred embodiment of the present invention, in step (1), the settling time is 24-48 h.

[0018] In a preferred embodiment of the present invention, in step (1), the amount of VTMO used is 1-2 wt% of the total mass of the tea stem powder.

[0019] In a preferred embodiment of the present invention, in step (1), the stirring temperature is 60-80°C and the stirring time is 3-5 h.

[0020] In a preferred embodiment of the present invention, in step (2), the mass ratio of EVA, recycled EVA, POE, modified tea stem powder, zinc oxide, stearic acid, talc, silica, azodicarbonamide and DCP is 70-100:5-10:5-10:1-5:2-4:0.5-1.5:3-8:1-3:2-4:0.6-1.4.

[0021] In a preferred embodiment of the present invention, the preheating temperature of the internal mixer in step (2) is 100-120 °C.

[0022] In a preferred embodiment of the present invention, the preheating temperature of the mold in step (4) is 170-180°C.

[0023] In a preferred embodiment of the present invention, in step (4), the pressure is 10-15 MPa and the foaming time is 5-10 min.

[0024] The beneficial effects of this invention are:

[0025] 1. By adding recycled EVA, this invention effectively reduces raw material costs, reduces the consumption of petroleum-based virgin materials, and significantly reduces carbon emissions, which aligns with the "dual carbon" goals and green manufacturing requirements. At the same time, it directly crushes and reuses scraps and defective products from the production process, essentially achieving zero-waste production and improving resource utilization efficiency.

[0026] 2. This invention uses a physical-chemical dual pretreatment method to modify tea stems, which effectively improves the interfacial compatibility between biomass fibers and EVA / POE polymer matrix, avoids problems such as fiber agglomeration and uneven cell structure during foaming, and ensures the uniformity and stability of foam structure.

[0027] 3. This invention uses tea stems as biomass filler, which further reduces production costs, realizes high-value utilization of agricultural waste, reduces the environmental pressure caused by traditional landfill or incineration, and at the same time improves the overall biodegradation rate of the material, reducing the pollution load on the environment.

[0028] 4. The active ingredients such as tea polyphenols naturally contained in the tea stems of this invention endow EVA foaming materials with excellent antibacterial, anti-mildew, and deodorizing functions, and give the products a fresh tea aroma, which meets the increasing demand for functionalization and differentiation of downstream products such as shoe materials, while enhancing the market competitiveness of the products.

[0029] 5. The process route of this invention is simple and the parameters are controllable. It is compatible with existing industrial production lines and can be flexibly expanded to other biomass fillers, different recycled materials and various foaming molding methods to achieve large-scale production. While maintaining good foaming performance, it comprehensively improves the environmental protection attributes and added value of the material. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of the tea stem pretreatment in Embodiments 1 and 2 of the present invention.

[0031] Figure 2 This is a schematic diagram of steps (2) to (4) in embodiments 1 and 2 of the present invention.

[0032] Figure 3 The image shows the XRD patterns of tea stem powder before and after the addition of VTOM aqueous solution in Example 1 of this invention.

[0033] Figure 4 The images show SEM images of tea stem powder before and after the addition of VTOM aqueous solution in Example 1 of this invention. Detailed Implementation

[0034] The technical solution of the present invention will be further explained and described below with reference to specific embodiments and accompanying drawings.

[0035] The process of the following embodiments is as follows: Figure 1 and Figure 2 As shown.

[0036] Example 1

[0037] (1) Pretreatment of tea stems: The tea stems were pretreated using a physical-chemical pretreatment method. The tea stems were pulverized to 300 mesh using a pulverizer. Then, the tea stem powder was added to a 10 wt% NaOH aqueous solution and allowed to stand for 24 h at a solid-liquid ratio of 1:5. After standing, the solution was washed with pure water until pH = 7 to remove residual NaOH and dried for later use. Then, VTMO (vinyltrimethoxysilane) was added at a ratio of 1.5 wt% of the total mass of the tea stem powder to prepare an aqueous solution. The tea stem powder was placed in this solution and heated and stirred at 80 ℃ for 4 h. After filtration, the solution was dried at 110 ℃ to obtain the desired product. Figure 3 and Figure 4 The modified tea stem powder shown is ready for use.

[0038] (2) Internal mixing: Preheat the internal mixer to 110 °C, add 85.71 parts by weight of EVA, 4.76 parts by weight of recycled EVA and 9.52 parts by weight of POE, and stir at 20 rpm for 3 min; then add 0.95 parts by weight of the modified tea stem powder prepared in step (1), 2.67 parts by weight of zinc oxide, 0.76 parts by weight of stearic acid, 3.81 parts by weight of talc and 2.86 parts by weight of silica, and stir at 40 rpm for 5 min; finally add 2.48 parts by weight of azodicarbonamide and 0.95 parts by weight of DCP, and stir at 40 rpm until the material is free of stratification and has a loose, clumped shape to obtain the internally mixed material;

[0039] (3) Open mill thin pass: The intensively mixed material is turned over twice on an open mill at 70 ℃, and then thinned to 1-1.5 mm. After turning over 5 times, the material is discharged to obtain the composite material;

[0040] (4) Foaming: Place the composite material obtained in step (3) into a mold preheated to 170 °C on a flat vulcanizing apparatus, apply a pressure of 15 MPa at the same temperature, and foam for 7 min to obtain the final product.

[0041] The performance test results of the product obtained in this embodiment are as follows: tensile strength is 3.24 MPa, and density is 0.16 g / cm³. 3 The elongation at break was 335.17%.

[0042] Example 2

[0043] (1) Pretreatment of tea stems: The tea stems were pretreated by physical-chemical pretreatment method. The tea stems were crushed to 300 mesh using a pulverizer. Then, the tea stem powder was added to a 10 wt% NaOH aqueous solution and allowed to stand for 24 h at a solid-liquid ratio of 1:5. After that, the solution was washed with pure water until pH = 7 to remove residual NaOH and dried for later use. VTMO was added at a ratio of 1.5 wt% of the total mass of the tea stem powder to prepare an aqueous solution. The tea stem powder was placed in the solution and heated and stirred at 80 ℃ for 4 h. After filtration, the solution was dried at 110 ℃ to obtain modified tea stem powder for later use.

[0044] (2) Internal mixing: Preheat the internal mixer to 110 °C, add 90 parts by weight of EVA, 5 parts by weight of recycled EVA and 5 parts by weight of POE, and stir at 20 rpm for 3 min; then add 2 parts by weight of the modified tea stem powder prepared in step (1), 2.0 parts by weight of zinc oxide, 0.8 parts by weight of stearic acid, 6 parts by weight of talc and 3 parts by weight of silica, and stir at 40 rpm for 5 min; finally add 2.48 parts by weight of azodicarbonamide and 0.95 parts by weight of DCP, and stir at 40 rpm until the material is free of stratification and has a loose, clumped shape to obtain the internally mixed material;

[0045] (3) Open mill thin pass: The intensively mixed material is turned over twice on an open mill at 70 ℃, and then thinned to 1-1.5 mm. After turning over 5 times, the material is discharged to obtain the composite material;

[0046] (4) Foaming: Place the composite material obtained in step (3) into a mold preheated to 170 °C on a flat vulcanizing apparatus, apply a pressure of 15 MPa at the same temperature, and foam for 7 min to obtain the final product.

[0047] The performance test results of the product obtained in this embodiment are as follows: tensile strength is 2.25 MPa, and density is 0.22 g / cm³. 3 The elongation at break is 455.53%.

[0048] Comparative Example 1 (only EVA recycled material added)

[0049] (1) No tea stem pretreatment was performed in this comparative example, nor were any modified tea stems added.

[0050] (2) Open milling: 72.73 parts by weight of EVA, 18.18 parts by weight of recycled EVA and 9.09 parts by weight of POE are added to an open mill at 100℃-120℃ and mixed thoroughly. Then the open milling temperature is reduced to 100℃. 2.67 parts by weight of zinc oxide, 0.52 parts by weight of stearic acid and 4.55 parts by weight of talc are pre-mixed physically and evenly. They are then evenly dispersed into the polymer matrix on an open mill. 2.21 parts by weight of azodicarbonamide are added and the open milling continues. Finally, 0.91 parts by weight of DCP are added to obtain the refined composite material.

[0051] (3) Foaming: Place the composite material obtained in step (2) into a mold preheated to 175 °C on a flat vulcanizing apparatus, apply a pressure of 15 MPa at the same temperature, and foam for 7 min to obtain the final product.

[0052] The performance test results of the product obtained in this comparative example are as follows: tensile strength of 2.29 MPa and density of 0.19 g / cm³. 3 The elongation at break was 174.81%.

[0053] Comparative Example 2 (Pure EVA)

[0054] (1) No tea stem pretreatment was performed in this comparative example, nor were modified tea stems, recycled EVA materials and POE added.

[0055] (2) Open milling: 100 parts by weight of EVA are added to an open mill at 100 ℃-120 ℃ and mixed evenly. Then the open milling temperature is reduced to 100 ℃. 1.43 parts by weight of zinc oxide, 0.57 parts by weight of stearic acid and 4 parts by weight of talc are pre-mixed evenly and evenly dispersed into the polymer matrix on an open mill. Then 2.71 parts by weight of azodicarbonamide are added and the open milling continues. Finally, 0.86 parts by weight of DCP are added to obtain the refined composite material.

[0056] (3) Foaming: Place the composite material obtained in step (2) into a mold preheated to 175 °C on a flat vulcanizing apparatus, apply a pressure of 15 MPa at the same temperature, and foam for 7 min to obtain the final product.

[0057] The performance test results of the product obtained in this comparative example are as follows: tensile strength of 2.53 MPa and density of 0.14 g / cm³. 3 The elongation at break was 126.81%.

[0058] The above description is merely a preferred embodiment of the present invention, and therefore should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of the patent and the contents of the specification should still fall within the scope of the present invention.

Claims

1. A method for preparing an ethylene-vinyl acetate copolymer foam material, characterized in that: Includes the following steps: (1) Tea stem pretreatment: The tea stems were pretreated by physical-chemical pretreatment method. The tea stems were crushed to 100-300 mesh using a pulverizer. Then the tea stem powder was added to NaOH aqueous solution. After standing, it was washed with pure water until pH=7 to remove residual NaOH. Then it was added to VTMO aqueous solution, heated and stirred, filtered and dried to obtain modified tea stem powder for later use. (2) Internal mixing: After preheating the internal mixer, add EVA, EVA recycled material and POE, and stir at low speed; then add the above modified tea stem powder, zinc oxide, stearic acid, talc and white carbon black, and stir at high speed; finally add azodicarbonamide and DCP, and stir until the material is free of stratification and has a loose aggregate to obtain the internal mixing material. (3) Open mill thin pass: The intensively mixed material is turned over and cut several times on the open mill, and then thinned to 1-1.5 mm. After turning over and cutting several times, the material is discharged to obtain the composite material; (4) Foaming: Place the composite material obtained in step (3) into a preheated mold on a flat vulcanizing apparatus and apply pressure at the same temperature to foam.

2. The preparation method according to claim 1, characterized in that: In step (1), the concentration of the NaOH aqueous solution is 10-20 wt%.

3. The preparation method according to claim 1, characterized in that: In step (1), the solid-liquid ratio of tea stem powder to NaOH aqueous solution is 1:5-3:5, and the standing time is 24-48 h.

4. The preparation method according to claim 1, characterized in that: In step (1), the settling time is 24-48 h.

5. The preparation method according to claim 1, characterized in that: In step (1), the amount of VTMO used is 1-2 wt% of the total mass of the tea stem powder.

6. The preparation method according to claim 1, characterized in that: In step (1), the stirring temperature is 60-80℃ and the stirring time is 3-5 h.

7. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of EVA, recycled EVA, POE, modified tea stem powder, zinc oxide, stearic acid, talc, silica, azodicarbonamide and DCP is 70-100:5-10:5-10:1-5:2-4:0.5-1.5:3-8:1-3:2-4:0.6-1.

4.

8. The preparation method according to claim 1, characterized in that: The preheating temperature of the internal mixer in step (2) is 100-120 ℃.

9. The preparation method according to claim 1, characterized in that: The preheating temperature of the mold in step (4) is 170-180 ℃.

10. The preparation method according to claim 1, characterized in that: In step (4), the pressure is 10-15 MPa and the foaming time is 5-10 min.

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