Lightweight eva composite chemical foaming sole material with bimodal cell structure and preparation method thereof

Abstract

The application provides a light EVA composite chemical foaming sole material with a bimodal cell structure and a preparation method thereof, and the method comprises the following steps: S1, 50-70 parts of ethylene-vinyl acetate, 10-25 parts of polyolefin thermoplastic elastomer, 20-40 parts of low-molecular-weight thermoplastic elastomer, wear-resistant agent and active agent are mixed, melt homogenization granulation is carried out, and a first material is obtained; the low-molecular-weight thermoplastic elastomer is selected from one or more of polyurethane elastomer, polyester elastomer and polyamide elastomer; S2, the first material, a chemical foaming agent and a crosslinking agent are mixed and then granulated to obtain a second material; S3, the second material is subjected to injection pressure holding and temperature reduction foaming; and S4, the small foaming semi-finished product obtained is subjected to secondary mold pressing to obtain the light EVA composite chemical foaming sole material. The foaming sole prepared by the application has the characteristics of low specific gravity, high resilience and small shrinkage, the physical properties are controllable, and the market prospect is good.

Description

Technical Field

[0001] This invention relates to the field of foamed shoe sole technology, specifically to a lightweight EVA composite chemical foamed shoe sole material with a bimodal cell structure and its preparation method. The EVA composite chemical foamed shoe sole material has the characteristics of being lightweight, highly resilient, and shrinkage resistant. Background Technology

[0002] In the market, EVA composite chemical foaming for athletic shoes uses EVA (ethylene-vinyl acetate copolymer) as the main raw material, and is made into foamed soles through a chemical foaming process. This foaming process results in either open-cell or closed-cell structures. Currently, the specific gravity of the sole material is generally reduced by increasing the foaming ratio. However, a high foaming ratio makes shrinkage difficult to control, often resulting in inconsistent sole lengths and uncontrollable sole properties. Consequently, the length of the left and right shoes worn by consumers typically differs by 3-4mm. Furthermore, the rebound, compression deformation, and specific gravity of the soles show significant differences in test data between the left and right feet. The shorter shoe is more likely to feel cramped, resulting in poor foot comfort during exercise and potentially causing injury to the consumer's feet. Summary of the Invention

[0003] In view of this, the present invention provides a lightweight EVA composite chemical foaming sole material with a bimodal pore structure and its preparation method. The foaming sole prepared by the present invention has a low specific gravity, high resilience and low shrinkage, controllable physical properties, and good market prospects.

[0004] This invention provides a method for preparing a lightweight EVA composite chemical foaming shoe sole material with a bimodal pore structure, comprising the following steps:

[0005] S1. According to the mass parts, 50-70 parts of ethylene-vinyl acetate, 10-25 parts of polyolefin thermoplastic elastomer, 20-40 parts of low molecular weight thermoplastic elastomer, wear-resistant agent and activator are mixed, and then melt-homogenized and granulated to obtain the first material; the low molecular weight thermoplastic elastomer is selected from one or more of polyurethane elastomer, polyester elastomer and polyamide elastomer.

[0006] S2. The first material, chemical foaming agent and crosslinking agent are mixed and then granulated to obtain the second material;

[0007] S3. Inject the second material under pressure, cool down and foam to obtain a small foamed semi-finished product;

[0008] S4. The small foamed semi-finished product is subjected to secondary molding to obtain a lightweight EVA composite chemical foamed shoe sole material with a bimodal cell structure.

[0009] Preferably, the vinyl acetate unit in the ethylene-vinyl acetate ester has a mass content of 18% to 33%, and the polyolefin thermoplastic elastomer is selected from one or more of ethylene-butene / octene copolymers, styrene-butadiene block copolymers, and ethylene propylene diene monomer (EPDM) elastomers.

[0010] Preferably, the wear-resistant agent has a mass fraction of 1-10 parts; the wear-resistant agent is a pre-dispersed silicone masterbatch wear-resistant agent, with ethylene-vinyl acetate copolymer as the carrier of the wear-resistant agent.

[0011] Preferably, the activator comprises 0.2-0.5 parts stearic acid, 0.5-1 parts zinc stearate, and 1.2-2 parts zinc oxide.

[0012] Preferably, the Vicat softening point temperature of the low molecular weight thermoplastic elastomer is 110℃~160℃.

[0013] Preferably, the melt homogenization and granulation are carried out using a twin-screw extruder at a temperature of 140℃~200℃.

[0014] Preferably, the chemical foaming agent is 2.8-6 parts by weight and is selected from one or more of azodicarbonamide and 4,4-oxodibenzenesulfonylhydrazine; the crosslinking agent is 1,4-di-tert-butylperoxyisopropylbenzene, and is 0.1-0.5 parts by weight.

[0015] Preferably, the injection pressure holding and cooling foaming specifically includes: placing the second material in an injection molding extrusion molding equipment, with a clamping pressure of 120 Bar, a molding die temperature of 160℃~170℃, holding the temperature and pressure for crosslinking for 600 seconds~700 seconds, cooling to 110℃~140℃ and then opening the mold for foaming to obtain a small foamed semi-finished product.

[0016] Preferably, the temperature of the secondary molding is 130℃~140℃, and the hot pressing time is 260 seconds~300 seconds.

[0017] This invention provides a lightweight EVA composite chemical foaming sole material with a bimodal pore structure obtained by the preparation method described above. It exhibits both regular millimeter-scale and micrometer-scale pore structures and has a density of less than 0.2 g / cm³. 3 .

[0018] The concept of bimodal cell structure has been proposed in the field of polymer materials science. Bimodal cell foam materials refer to foam materials with two different cell sizes. Typically, the larger cell size is between 0.1 and 1.2 mm, and the smaller cell size is 5% to 50% of the larger cell size. Compared to unimodal cell structures, foam materials with bimodal cell structures are often used in packaging materials, sound-absorbing materials, and thermal insulation materials, exhibiting superior performance.

[0019] The concept of bimodal foam cells is rarely seen in foamed shoe sole materials, and their preparation is more commonly seen in physical foaming than chemical foaming. The physical properties of foamed shoe sole materials are usually adjusted through the material's inherent properties, and rarely through the effect of the foam cell structure.

[0020] To achieve performance equal to or even better than traditional single-peak open-cell or closed-cell foam materials in terms of reducing the specific gravity, rebound, shrinkage, compression deformation, and cushioning of shoe sole foam materials at a relatively low foaming ratio, this invention adjusts the physical properties from the foaming structure, developing a bimodal composite chemical foam shoe sole material. This primarily involves introducing bimodal pores (millimeter-scale and micrometer-scale) into the EVA midsole foam material through a secondary foaming process. This results in a special pore structure with both regular millimeter-scale and micrometer-scale pores. Small pores are formed within the EVA matrix, while large pores can be formed in novel low-molecular-weight thermoplastic elastomer components (TPU, TPEE, PEBAX nylon elastomers, etc.) and then dispersed within the small-pore EVA foam matrix. In this invention, the presence of large pores effectively reduces the density of the foam material. Because the large pores are formed within high-performance elastomers, combined with this high proportion of elastomer composite, the fatigue resistance, support performance, and dimensional stability of the foam material are effectively improved, mitigating the problem of significant differences in physical energy between the left and right feet caused by sole shrinkage. The EVA composite chemical foaming sole material prepared by this invention has low specific gravity, high resilience, and low shrinkage. When used in shoe soles, it can meet the needs of consumers, bring them a better experience, and prevent them from feeling uncomfortable or injured. It has good market prospects.

[0021] Furthermore, the preparation of this foamed material can be carried out under the equipment conditions of traditional EVA foaming processes, avoiding the expensive and complex physical foaming process. This application broadens the approach to controlling the cell structure of EVA composite foamed shoe sole materials and preparing high-performance shoes. Attached Figure Description

[0022] Figure 1 This is an image of the sample from Example 1.

[0023] Figure 2 Here is a SEM X35 image of the sample from Example 1;

[0024] Figure 3 Here is a SEM X45 image of the sample from Example 1;

[0025] Figure 4 SEM X35 image of sample 1 (Comparative Example 1);

[0026] Figure 5 The image shows a SEM X45 image of the sample from Comparative Example 1. Detailed Implementation

[0027] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be described in detail below with reference to specific embodiments. The description in this part is only exemplary and explanatory, and should not be used to limit the scope of protection of the present invention in any way.

[0028] This invention provides a method for preparing a lightweight EVA composite chemical foaming shoe sole material with a bimodal pore structure, comprising the following steps:

[0029] S1. According to the mass parts, 50-70 parts of ethylene-vinyl acetate, 10-25 parts of polyolefin thermoplastic elastomer, 20-40 parts of low molecular weight thermoplastic elastomer, wear-resistant agent and activator are mixed, and then melt-homogenized and granulated to obtain the first material; the low molecular weight thermoplastic elastomer is selected from one or more of polyurethane elastomer, polyester elastomer and polyamide elastomer.

[0030] S2. The first material, chemical foaming agent and crosslinking agent are mixed and then granulated to obtain the second material;

[0031] S3. Inject the second material under pressure, cool down and foam to obtain a small foamed semi-finished product;

[0032] S4. The small foamed semi-finished product is subjected to secondary molding to obtain a lightweight EVA composite chemical foamed shoe sole material with a bimodal cell structure.

[0033] The foaming material provided in this application has both regular millimeter-scale and micrometer-scale cell structures, making it a chemical foaming material with a bimodal cell structure. The preparation of this material can be carried out under the equipment conditions of traditional EVA foaming process, avoiding the expensive and complex physical foaming process. Moreover, the obtained EVA composite foaming material exhibits comprehensive properties such as light weight, fatigue resistance, and excellent shrinkage resistance, which is beneficial for its application in the preparation of sports shoe soles.

[0034] The chemically foamed material with a bimodal pore structure described in this application can be achieved through the following technical solution: it is prepared using a specific ethylene-vinyl acetate composite material via a secondary foaming process. Preferably, the ethylene-vinyl acetate composite material comprises the following components in parts by weight: 50-70 parts ethylene-vinyl acetate, 10-25 parts polyolefin thermoplastic elastomer, 20-40 parts low molecular weight thermoplastic elastomer, 1-10 parts abrasion-resistant agent, 0.2-0.5 parts stearic acid, 0.5-1 part zinc stearate, 1.2-2 parts zinc oxide, 0.1-0.5 parts crosslinking agent, and 2.8-6 parts foaming agent.

[0035] The chemically foamed material with a bimodal pore structure provided in this invention is prepared by a chemical foaming process from a composite mainly composed of ethylene-vinyl acetate (EVA), specifically comprising 50-70 parts by weight of ethylene-vinyl acetate copolymer. The ethylene-vinyl acetate copolymer, also known as ethylene-vinyl acetate copolymer, is a thermoplastic resin obtained by copolymerizing ethylene and vinyl acetate (VA), abbreviated as EVA. EVA copolymer has the characteristics of high resilience and good flexibility; specifically, in the embodiments of this application, the VA content in the EVA can be 18%-33% by weight. Preferably, the ethylene-vinyl acetate copolymer includes one or more of the following: EVA 7470M, EVA VJ33121, and EVA 7360M (all from Formosa Plastics Corporation, Taiwan).

[0036] This invention introduces a certain proportion of thermoplastic polyolefin elastomers into the EVA foaming formulation to improve the performance of the foaming material, enabling it to achieve high resilience, softness, and comfort. The aforementioned thermoplastic polyolefin elastomers mainly play a role in regulating basic physical properties in the formulation, including one or more of ethylene-butene / octene random copolymers, ethylene-butene / octene block copolymers, styrene-butadiene block copolymers (SEBS), and ethylene-propylene diene monomer (EPDM) elastomers, preferably ethylene-butene / octene copolymers, such as olefin block copolymers (OBC).

[0037] In the formulation of this invention, the preferred mass fraction of the polyolefin thermoplastic elastomer is 15-24 parts. As an olefin block copolymer (OBC) described in this invention, the basic physical properties are mainly adjusted in the formulation. This type of elastomer monomer includes 1-octene, ethylene, etc., and the ratio of ethylene to octene is controlled through polymerization to prepare an olefin copolymer with alternating "soft segments" and "hard segments". Due to its multi-block structure, OBC exhibits superior performance, with higher melting temperature, lower glass transition temperature, high elasticity, ease of processing, fast crystallization rate, and high tensile strength, making it an elastic material with broad application prospects. SEBS is saturated SBS, or hydrogenated SBS, obtained by hydrogenating special linear SBS to saturate the double bonds. With appropriate directional hydrogenation of SBS in the presence of a catalyst, the polybutadiene segments are hydrogenated into polyethylene (E) and polybutene (B) segments, hence the name SEBS. Ethylene propylene diene monomer (EPDM) is a terpolymer of ethylene, propylene, and a non-conjugated diene, exhibiting good weather resistance. Specifically, the polyolefin thermoplastic elastomers (olefin block copolymers) mentioned include the following grades: Infuse 9107, Infuse 9007, Infuse 9500, Infuse 9530, etc., and these grade products are all manufactured by Dow Company.

[0038] The EVA foam material described in this invention is mainly produced by chemical foaming. In a mold cavity sealed under a certain pressure (the clamping pressure is usually 120 Bar), the chemical foaming agent decomposes at high temperature to produce nitrogen and a small amount of carbon dioxide, forming a high-pressure gas in the material. It mainly exists in the form of gas nuclei. The growth of gas nuclei forms pores slowly. At the same time as foaming, the peroxide crosslinking agent decomposes to produce crosslinking, which makes the material form a network structure in the microstructure. At the moment of mold opening, the pressure binding the material decreases, the gas nuclei grow to form pores, the material expands and foams, and the crosslinking effect locks the pores inside the material, thereby forming a foam material.

[0039] In this invention, the low molecular weight thermoplastic elastomers mainly refer to TPU (thermoplastic polyurethane elastomer), TPEE (thermoplastic polyester elastomer), and PEBAX (thermoplastic nylon elastomer or polyamide elastomer). TPU, TPEE, and PEBAX nylon elastomers have relatively low molecular weight, low high-temperature melt strength, and low peroxide crosslinking degree. When these elastomers are compounded with EVA formulations, at the same foaming temperature, TPU, TPEE, and PEBAX nylon elastomers tend to form large cells, achieving the purpose of forming a bimodal cell structure. However, when these elastomers are compounded with EVA formulations in a certain proportion, it is difficult to obtain foamed materials using EVA shoe material foaming processes.

[0040] Furthermore, the Vicat softening point temperature (T) of this type of elastomer is between 110-160℃ (test method GB / T8802-2001). The chemical foaming temperature of the EVA formulation is 175±5℃, and the Vicat softening point temperature of the low molecular weight thermoplastic elastomer is between 110-155℃. This ensures that during the pressure holding and cooling process, the temperature is not too low, preventing the composite material from failing to expand and foam. Simultaneously, it prevents the low molecular weight thermoplastic elastomer component in the matrix from insufficiently cooling due to excessively high temperatures, resulting in low melt strength. In such cases, the melt would be completely broken down by the rapidly expanding gas nuclei during mold opening and foaming, preventing the formation of foamed material. In addition, this Vicat softening point temperature is much higher than that of EVA and thermoplastic polyolefin elastomers, significantly improving the thermal deformability of the composite material. When the low molecular weight thermoplastic elastomer constitutes a small proportion of the composite matrix, it is mostly dispersed in the matrix as an "island" phase. During the foaming process, due to the obstruction of the matrix material, the pores in the low molecular weight thermoplastic elastomer phase are difficult to fuse and form macropores. At the same time, due to the binding effect of the matrix material, the pores in the low molecular weight thermoplastic elastomer phase are difficult to grow towards macropores. However, when its proportion in the composite matrix reaches a certain level, the melt strength of the composite material is too low, and the melt will be completely broken down by the rapidly expanding gas nuclei, resulting in no foamed material.

[0041] By weight, the ethylene-vinyl acetate composite material of the present invention comprises 20-40 parts of the low molecular weight thermoplastic elastomer. The low molecular weight thermoplastic elastomer can be commercially available, and preferably includes: DuPont Hytrel 4056 and Hytrel 4556, Arkema Pebax Clear300, Pebax4033, and Pebax4533, and BASF Elastollan 1190A and Elastollan 1195A.

[0042] In this embodiment of the invention, 5-10 parts of a wear-resistant agent are preferably used, with 0.25-0.5 parts of stearic acid, 0.5-1 parts of zinc stearate, and 1.2-2 parts of zinc oxide as activators. This invention does not impose any special restrictions on the source of each raw material component; commercially available products in the art can be used. The wear-resistant agent is preferably a pre-dispersed silicone masterbatch wear-resistant agent, with ethylene-vinyl acetate copolymer as the carrier.

[0043] In an embodiment of the present invention, 50-70 parts by weight of ethylene-vinyl acetate, 10-25 parts by weight of polyolefin thermoplastic elastomer, 20-40 parts by weight of low molecular weight thermoplastic elastomer, wear-resistant agent, and activator are mixed and melt-homogenized and granulated to obtain the first material. In this step of the embodiment of the present invention, other raw materials except for foaming agent and crosslinking agent are weighed and added together to a twin-screw extruder for melt-homogenization and granulation. The key is the temperature setting of the twin-screw extruder, which is mainly based on the melt temperature of the non-polyolefin thermoplastic elastomer with the higher melt temperature among the components, within ±20°C of its melt temperature. When the extrusion temperature is lower than the melt temperature, and the material is in a highly elastic state above the softening point, the materials can also be homogenized under the strong shear of the screw, while the high melt temperature component is dispersed among the low melt temperature component, thus reducing the dispersion processing temperature of the material.

[0044] Specifically, the temperature of the twin-screw extruder can be set to 140-200℃. This processing temperature will cause the foaming agent and crosslinking agent to decompose, resulting in a "dead material" phenomenon in subsequent processing. Therefore, the foaming agent and crosslinking agent should be removed and added back in the subsequent dispersion process.

[0045] This invention employs a chemical foaming method to obtain a foamed midsole material. The foaming formulation includes 0.15-0.35 parts of crosslinking agent and 2.8-4.2 parts of foaming agent. For the chemical foaming of EVA composites, the degree of crosslinking significantly affects foaming. This is typically adjusted by the composition of the crosslinking agent. If the degree of crosslinking is too low, the pores cannot effectively trap gas and are broken, forming localized air pockets. If the degree of crosslinking is too high, the pores are difficult to grow, making material expansion and foaming difficult, resulting in a material with an unsuitable foaming ratio. Furthermore, excessively high crosslinking hinders molecular chain movement, reduces material flowability, and easily leads to production problems such as poor patterns and edge bursting. In this invention, the amount of crosslinking agent ensures a suitable degree of crosslinking in the composite material, guaranteeing that the composite material's strength is sufficient to trap gas without affecting the pore fusion growth of the low melt strength phase. Preferably, the crosslinking agent is 1,4-di-tert-butylperoxyisopropylbenzene. The foaming agent is preferably selected from one or more of azodicarbonamide and 4,4-oxodibenzenesulfonylhydrazine; preferred varieties include AC6000H and OBSH 130A, both of which can foam normally at a temperature of 160-185℃.

[0046] According to the material composition, in this embodiment of the invention, the first material, chemical foaming agent, and crosslinking agent from the above steps are taken and mixed in an internal mixer. After being mixed evenly, they are granulated to obtain the second material. In this embodiment of the invention, the second material is subjected to injection molding with pressure holding and cooling to obtain a small foamed semi-finished product. The obtained small foamed semi-finished product can be placed to cool for 24 hours, and then subjected to secondary molding to obtain a chemical foamed material with a bimodal cell structure, which is the EVA composite chemical foamed shoe sole material.

[0047] In an embodiment of the present invention, the mixing temperature can be 120℃~130℃, the mixing time is 10min~15min, the material is turned 4-5 times, and then the mixed material is poured into a granulator for granulation. This step is a routine operation for EVA mixing and granulation, and can be carried out under reasonable operating conditions.

[0048] Furthermore, the injection molding with pressure holding and cooling foaming specifically includes: pouring the prepared second material into the injection molding machine, feeding and extruding it into the mold according to the mold material quantity, the temperatures of the first, second, third and fourth zones of the extruder can be adjusted to 80℃, 80℃, 85℃ and 90℃ respectively, and the clamping pressure is 120 Bar; the temperature of the upper and lower mold plates can be adjusted to 160-170℃ and 160-170℃ respectively, and after holding the temperature and pressure for vulcanization for 600-700 seconds, the cooling equipment is turned on to hold the pressure and cool down. When the temperature drops to 110-140℃, the mold is opened and foaming is performed to obtain a small foamed semi-finished product.

[0049] It should be noted that the clamping force of injection molding machines involved in production is usually adjusted by a pressure gauge. The pressure gauge bar is also a unit of atmospheric pressure, and pressure (P) equals force (F) divided by area (S); while most other clamping forces are in T (tons), which is force (F). The relationship between the two is: P=F / S.

[0050] This invention utilizes a single injection molding process to produce small-scale foamed semi-finished products from EVA through a secondary process. It leverages the high clamping pressure and superior mold sealing of the single injection molding process. Under clamping pressure, the material foams at high temperature within the mold. Due to the limited space within the mold, high-pressure gas nuclei formed inside the material are difficult to grow, and low-melt-strength gas nuclei do not quickly penetrate the gas walls. However, during the cooling and pressure-holding period, these low-melt-strength gas nuclei slowly grow, break down, and fuse into large pores. When the temperature reaches a constant level, the melt strength increases. At this point, the mold is opened, and the pores continue to grow. The melt strength of the material can lock in the gas, causing the material to expand and foam, thus obtaining the small-scale foamed semi-finished product.

[0051] In an embodiment of the present invention, the obtained small foamed semi-finished product is cooled for 24 hours and then subjected to secondary molding: through secondary molding, foamed shoe midsole and test piece materials are obtained. The hot pressing temperature is 130-140℃; the hot pressing time is 260-300 seconds; the cooling water temperature can be 15℃, and the cooling time is preferably 420 seconds. Compared with the secondary molding temperature of conventional EVA, the hot pressing temperature and the hot pressing time of the embodiment of the present invention are lower, ensuring that the formed pores will not rupture again. At the same time, the degree of cross-linking of the material is low, the fluidity is good, and the hot pressing moldability of the material is guaranteed.

[0052] This invention provides a lightweight EVA composite chemical foaming sole material with a bimodal pore structure obtained by the preparation method described above. It exhibits both regular millimeter-scale and micrometer-scale pore structures and has a density of less than 0.2 g / cm³. 3 For example, a density of 0.13-0.18 g / cm³ 3 With an elasticity of over 54% and a heat shrinkage of 0.2-0.5%, it is a bimodal foam material with high resilience, shrinkage resistance, and lightweight structure, which can be used to make sports shoe soles to meet consumer demand.

[0053] To further understand this application, the following detailed description, in conjunction with specific embodiments, provides the chemical foaming material with a bimodal pore structure and its preparation method.

[0054] Example 1

[0055] A chemical foaming material with a bimodal cell structure, comprising the following raw materials in parts by weight:

[0056] 52 parts of ethylene-vinyl acetate copolymer, 17 parts of olefin block copolymer, 35 parts of low molecular weight thermoplastic elastomer, 10 parts of abrasion resistant agent, 0.5 parts of stearic acid, 0.6 parts of zinc stearate, 1.5 parts of zinc oxide, 0.5 parts of crosslinking agent, and 4.8 parts of foaming agent.

[0057] The ethylene-vinyl acetate copolymer is model 7360M, with a VA content of 21%, and is produced by Formosa Plastics Corporation in Taiwan, China.

[0058] The olefin block copolymer, model OBC Infuse 9107, is manufactured by Dow Chemical Company.

[0059] The low molecular weight thermoplastic elastomer is a polyester elastomer (TPEE) with a Vicat softening point of 108°C, model Hytrel 4056, manufactured by DuPont.

[0060] The wear-resistant agent is a pre-dispersed silicone masterbatch wear-resistant agent, with ethylene-vinyl acetate copolymer as the carrier of the wear-resistant agent.

[0061] The crosslinking agent is 1,4-bis-tert-butylperoxyisopropylbenzene, model BIBP PERKADOX 14S-FL, manufactured by AkzoNobel.

[0062] The foaming agent is azodicarbonamide, model AC6000, produced by Hangzhou Haihong Fine Chemical Co., Ltd. The stearic acid, zinc stearate, and zinc oxide are commercially available.

[0063] This invention also provides a method for preparing a chemically foamed material with a bimodal pore structure, comprising the following steps:

[0064] (1) According to the above material composition, take all materials except foaming agent and crosslinking agent and add them to the twin-screw extruder for melt homogenization and granulation. The temperature of the twin-screw extruder is set to 160±5℃.

[0065] (2) According to the above material composition, take the material, foaming agent and crosslinking agent in step (1), and carry out internal mixing in an internal mixer. The internal mixing temperature is 120~130℃, the internal mixing time is 10~15min, the material is turned over 4-5 times, and after mixing evenly, it is granulated.

[0066] (3) Add the material from (2) into the hopper of a specific injection machine, and inject the material into the closed mold through the material gun. The mold clamping pressure is 120 Bar, and the upper and lower mold temperatures are set to 165-170℃ respectively. After holding the pressure and vulcanizing for 700 seconds, open the upper and lower mold heating plate cooling device for cooling. When the temperature inside the mold cools to 110-120℃, open the mold to obtain the small foamed semi-finished product.

[0067] (4) After the small foamed semi-finished product obtained in (3) is placed and cooled for 24 hours, it is molded to obtain a chemical foamed material with a bimodal cell structure; hot pressing temperature: 135-140℃; hot pressing time: 300 seconds; cooling water temperature: 15℃; cooling time: 420 seconds.

[0068] Example 2

[0069] A chemical foaming material with a bimodal cell structure, comprising the following raw materials in parts by weight:

[0070] 60 parts of ethylene-vinyl acetate copolymer, 15 parts of olefin block copolymer, 25 parts of low molecular weight thermoplastic elastomer, 10 parts of abrasion resistant agent, 0.5 parts of stearic acid, 0.6 parts of zinc stearate, 1.5 parts of zinc oxide, 0.25 parts of crosslinking agent, and 5 parts of foaming agent.

[0071] The ethylene-vinyl acetate copolymer, model 7470M, is manufactured by Formosa Plastics Corporation in Taiwan, China.

[0072] The olefin block copolymer, model OBC Infuse 9107, is manufactured by Dow Chemical Company.

[0073] The low molecular weight thermoplastic elastomer is a polyester elastomer (TPEE), model Hytrel 4556, manufactured by DuPont.

[0074] The wear-resistant agent is a pre-dispersed silicone masterbatch wear-resistant agent, with ethylene-vinyl acetate copolymer as the carrier of the wear-resistant agent.

[0075] The crosslinking agent is 1,4-bis-tert-butylperoxyisopropylbenzene, model BIBP PERKADOX 14S-FL, manufactured by AkzoNobel.

[0076] The foaming agent is azodicarbonamide, model AC6000, produced by Hangzhou Haihong Fine Chemical Co., Ltd.; the stearic acid, zinc stearate and zinc oxide are commercially available.

[0077] This invention also provides a method for preparing a chemically foamed material with a bimodal pore structure, comprising the following steps:

[0078] (1) According to the above material composition, take all materials except foaming agent and crosslinking agent and add them to the twin-screw extruder for melt homogenization and granulation. The temperature of the twin-screw extruder is set to 195-200℃.

[0079] (2) According to the above material composition, take the material, foaming agent and crosslinking agent in step (1), and carry out internal mixing in an internal mixer. The internal mixing temperature is 120~130℃, the internal mixing time is 10~15min, the material is turned over 4-5 times, and after mixing evenly, it is granulated.

[0080] (3) Add the material from (2) into the hopper of a specific injection machine, and inject the material into the closed mold through the material gun. The mold clamping pressure is 120 Bar, and the upper and lower mold temperatures are set to 165-170℃ respectively. After holding the pressure and vulcanizing for 700 seconds, open the upper and lower mold heating plate cooling device for cooling. When the temperature inside the mold cools to 110-120℃, open the mold to obtain the small foamed semi-finished product.

[0081] (4) After the small foamed semi-finished product obtained in (3) is placed and cooled for 24 hours, it is molded to obtain a chemical foamed material with a bimodal cell structure; hot pressing temperature: 130-140℃; hot pressing time: 300 seconds; cooling water temperature: 15℃; cooling time: 420 seconds.

[0082] Example 3

[0083] A chemical foaming material with a bimodal cell structure, comprising the following parts by weight:

[0084] 55 parts of ethylene-vinyl acetate copolymer, 23 parts of olefin block copolymer, 22 parts of low molecular weight thermoplastic elastomer, 10 parts of abrasion resistant agent, 0.5 parts of stearic acid, 0.6 parts of zinc stearate, 1.5 parts of zinc oxide, 0.5 parts of crosslinking agent, and 5 parts of foaming agent.

[0085] The ethylene-vinyl acetate copolymer, model 7360M, was manufactured by Formosa Plastics Corporation in Taiwan, China.

[0086] The polyolefin thermoplastic elastomer, model OBC Infuse 9107, is manufactured by Dow Chemical Company.

[0087] The novel thermoplastic elastomer is a nylon elastomer, model Pebax 4533, manufactured by Arkema.

[0088] The wear-resistant agent is a pre-dispersed silicone masterbatch wear-resistant agent, with ethylene-vinyl acetate copolymer as the carrier of the wear-resistant agent.

[0089] The crosslinking agent is 1,4-bis-tert-butylperoxyisopropylbenzene, model BIBP PERKADOX 14S-FL, manufactured by AkzoNobel.

[0090] The foaming agent is azodicarbonamide, model AC6000, produced by Hangzhou Haihong Fine Chemical Co., Ltd.; the stearic acid, zinc stearate and zinc oxide are commercially available.

[0091] This invention also provides a method for preparing a chemically foamed material with a bimodal pore structure, comprising the following steps:

[0092] (1) According to the above material composition, take all materials except foaming agent and crosslinking agent and add them to the twin-screw extruder for melt homogenization and granulation. The temperature of the twin-screw extruder is set to 160±5℃.

[0093] (2) According to the above material composition, take the material, foaming agent and crosslinking agent in step (1), and carry out internal mixing in an internal mixer. The internal mixing temperature is 120~130℃, the internal mixing time is 10~15min, the material is turned over 4-5 times, and after mixing evenly, it is granulated.

[0094] (3) Add the material from (2) into the hopper of a specific injection machine, and inject the material into the closed mold through the material gun. The mold clamping pressure is 120 Bar, and the upper and lower mold temperatures are set to 165-170℃ respectively. After holding the pressure and vulcanizing for 700 seconds, open the upper and lower mold heating plate cooling device for cooling. When the temperature inside the mold cools to 110-120℃, open the mold to obtain the small foamed semi-finished product.

[0095] (4) After the small foamed semi-finished product obtained in (3) is placed and cooled for 24 hours, it is molded to obtain a chemical foamed material with a bimodal cell structure; hot pressing temperature: 135-140℃; hot pressing time: 300 seconds; cooling water temperature: 15℃; cooling time: 420 seconds.

[0096] Table 1 lists the formulations of Examples 1-3.

[0097]

[0098] The table below shows the performance test results of the sample examples, all obtained according to national standard test methods. Furthermore, Figure 1 This is a photograph of the sample appearance in Example 1. Figure 2 , Figure 3 These are scanning electron microscope (SEM) images at different scales. The dissection of the epidermis clearly reveals two different types of pores: one large pore, on the order of millimeters, clearly visible to the naked eye; and the other small pore, on the order of micrometers, barely visible to the naked eye and requiring a microscope.

[0099] Table 2 Performance of Samples 1-3 in Examples 1-3

[0100]

[0101] As can be seen from the appearance and microstructure of Example 1, by controlling the difference in melt strength, under the traditional EVA chemical foaming process and equipment conditions, this embodiment of the invention prepares a special foam material with both regular millimeter-scale and micrometer-scale pores. The millimeter-scale pores encapsulate air, allowing the sole to remain relatively lightweight at a lower foaming ratio. The micrometer-scale pores are uniform and delicate, which is beneficial for the material to maintain high resilience. The interaction between the millimeter- and micrometer-sized pores is equivalent to placing countless air springs within the shoe sole. When subjected to external force, the air within the millimeter-sized pore cavities is compressed, forming compressed air columns. As the intensity of the exercise load increases, the height of the millimeter-sized pores decreases, the volume of the pore cavities decreases, the stiffness of the millimeter-sized pore walls increases, and the effective load-bearing area of ​​the air columns within the millimeter-sized pore cavities increases. At this point, the load-bearing capacity of the shoe sole increases, and the millimeter-sized pores provide high rebound feedback, ensuring that consumers do not experience a loss of elasticity and a reduced user experience. When the external force load decreases or disappears, the height of the millimeter-sized pores increases, the volume of the pore cavities increases, the stiffness of the millimeter-sized pore walls decreases, the effective load-bearing area of ​​the air columns within the millimeter-sized pore cavities decreases, and the material thickness returns to its original state. The numerous air springs in the prepared foam material give it excellent properties such as high resilience, low compression, lightweight, and low shrinkage.

[0102] Comparative Example 1

[0103] A conventional EVA composite chemical foaming material comprises the following raw materials in parts by weight:

[0104] 52 parts of ethylene-vinyl acetate copolymer, 17 parts of olefin block copolymer, 35 parts of polyester elastomer, 10 parts of abrasion resistant agent, 0.5 parts of stearic acid, 0.6 parts of zinc stearate, 1.5 parts of zinc oxide, 0.5 parts of crosslinking agent, and 4.2 parts of foaming agent.

[0105] Step 1) Weigh the raw materials (excluding crosslinking agent and foaming agent) according to the proportions by weight: ethylene-vinyl acetate copolymer, polyester elastomer, olefin block copolymer, abrasion resistant agent, stearic acid, zinc stearate, and zinc oxide.

[0106] Step 2) Pour the weighed raw materials from Step 1) into the internal mixer for internal mixing, turning the material 4-5 times, controlling the mixing temperature at 115-120℃, and the time at 12-15 minutes.

[0107] Step 3) Add the crosslinking agent and foaming agent to the material in Step 2) and continue to mix. The mixing temperature is controlled at 110-115℃ and the time is about 3-5 minutes.

[0108] Step 4) The material that was mixed evenly in the open mill in step 3) is transferred to the granulator for granulation, and the granulated particles are cooled to room temperature.

[0109] Step 5) Take the particles from Step 4) and perform small foaming. The mold temperature is about 155-160℃ and the vulcanization time is about 500-600 seconds to obtain the small foamed preform.

[0110] Step 6) Hot-press and cool the small foamed blank from Step 5) to obtain the EVA foam material shoe sole. The hot-pressing temperature is 150℃ and the hot-pressing and cooling time is 420 seconds.

[0111] Figure 4 , Figure 5 The images shown are SEM images of the Comparative Example 1 sample at different scales. Observation of both appearance and SEM images reveals that it cannot achieve the bimodal bubble effect of the Example; further testing showed that its springback, compression set, thermal shrinkage, and rebound properties all failed to match those of the Example.

[0112] Comparative Example 2

[0113] A conventional EVA composite chemical foaming material comprises the following parts by weight:

[0114] 55 parts of ethylene-vinyl acetate copolymer, 23 parts of olefin block copolymer, and polyolefin elastomer (hardness 70°A, specific gravity 0.889 g / cm³). 3 22 parts of (melt index 0.5, melting point 55℃), 10 parts of abrasion resistant agent, 0.5 parts of stearic acid, 0.6 parts of zinc stearate, 1.5 parts of zinc oxide, 5 parts of crosslinking agent, and 3.5 parts of foaming agent.

[0115] Step 1) Weigh the raw materials (excluding crosslinking agent and foaming agent) according to the proportions by weight: ethylene-vinyl acetate copolymer, polyolefin elastomer, olefin block copolymer, wear-resistant agent, stearic acid, zinc stearate, and zinc oxide.

[0116] Step 2) Pour the weighed material from Step 1) into the internal mixer for internal mixing, turning the material 4-5 times, controlling the mixing temperature at 115-120℃, and the time at 12-15 minutes.

[0117] Step 3) Add the crosslinking agent and foaming agent to the material in Step 2) and continue to mix. The mixing temperature is controlled at 110-115℃ and the time is about 3-5 minutes.

[0118] Step 4) The material that was mixed evenly in the open mill in step 3) is transferred to the granulator for granulation, and the granulated particles are cooled to room temperature.

[0119] Step 5) Take the particles from Step 4) and perform small foaming. The mold temperature is about 155-160℃ and the vulcanization time is about 500-600 seconds to obtain the small foamed preform.

[0120] Step 6) The small foamed preform from Step 5) is hot-pressed and cooled for shaping. The hot-pressing temperature is 150°C, and the hot-pressing and cooling time are both 420 seconds, thus obtaining the EVA foamed material shoe sole. Observation through appearance and electron microscopy shows that it cannot achieve the bimodal cell effect of the embodiment; after testing, the rebound, compression deformation, heat shrinkage, and elasticity all fail to reach the effect of the embodiment.

[0121] Table 3 Formulations for Comparative Examples 1-2

[0122]

[0123] The table below shows the performance test results of the comparative samples, all of which were obtained according to the national standard test methods:

[0124] Table 4 Performance of Comparative Examples 1-2

[0125]

[0126] As can be seen from the above embodiments, the present invention is simpler to operate compared to physical foaming methods. Furthermore, the present invention introduces a relatively large proportion of high-hardness, high-strength elastomers (TPEE, PEBA, etc.) into the formulation system, resulting in an EVA composite foam material with good hardness (e.g., 51-56C) and lightweight (down to 0.13 g / cm³). 3 It features low thermal shrinkage, low compression set, and high resilience (rebound rate up to 63%). This chemically foamed material with a bimodal cell structure can be used in the manufacture of athletic shoe soles and has promising market prospects.

[0127] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a lightweight EVA composite chemically foamed shoe sole material with a bimodal pore structure, characterized in that, Includes the following steps: S1. According to the mass parts, 50-70 parts of ethylene-vinyl acetate, 10-25 parts of polyolefin thermoplastic elastomer, 20-40 parts of low molecular weight thermoplastic elastomer, wear-resistant agent and activator are mixed, and then melt-homogenized and granulated to obtain the first material; the low molecular weight thermoplastic elastomer is selected from one or more of polyurethane elastomer, polyester elastomer and polyamide elastomer; the Vicat softening point temperature of the low molecular weight thermoplastic elastomer is 110℃~160℃. S2. The first material, chemical foaming agent and crosslinking agent are mixed and then granulated to obtain the second material; S3. The second material is subjected to injection molding, pressure holding, cooling and foaming. The injection molding, pressure holding and cooling foaming specifically includes: placing the second material in an injection molding equipment, with the mold temperature at 160℃~170℃, maintaining the temperature and pressure for crosslinking for 600 seconds~700 seconds, cooling to 110℃~140℃ and then opening the mold to foam, thereby obtaining a small foamed semi-finished product. S4. The small foamed semi-finished product is subjected to secondary molding to obtain a lightweight EVA composite chemical foamed shoe sole material with a bimodal cell structure; the temperature of the secondary molding is 130℃~140℃ and the hot pressing time is 260 seconds~300 seconds.

2. The preparation method according to claim 1, characterized in that, The ethylene-vinyl acetate ester contains 18% to 33% vinyl acetate units by mass, and the polyolefin thermoplastic elastomer is selected from one or more of ethylene-butene / octene copolymers, styrene-butadiene block copolymers, and ethylene propylene diene monomer (EPDM) elastomers.

3. The preparation method according to claim 1, characterized in that, The wear-resistant agent has a mass fraction of 1-10 parts; the wear-resistant agent is a pre-dispersed silicone masterbatch wear-resistant agent, with ethylene-vinyl acetate copolymer as the carrier of the wear-resistant agent.

4. The preparation method according to claim 1, characterized in that, The activator comprises 0.2-0.5 parts stearic acid, 0.5-1 parts zinc stearate, and 1.2-2 parts zinc oxide.

5. The preparation method according to any one of claims 1-4, characterized in that, The melt homogenization and granulation are carried out using a twin-screw extruder at a temperature of 140℃~200℃.

6. The preparation method according to any one of claims 1-4, characterized in that, The chemical foaming agent is 2.8-6 parts by weight and is selected from one or more of azodicarbonamide and 4,4-oxodibenzenesulfonylhydrazine; the crosslinking agent is 1,4-di-tert-butylperoxyisopropylbenzene, and is 0.1-0.5 parts by weight.

7. The preparation method according to claim 6, characterized in that, The clamping pressure for the injection molding process with pressure holding and cooling is 120 Bar.

8. The lightweight EVA composite chemical foaming sole material with a bimodal pore structure obtained by the preparation method according to any one of claims 1-7, simultaneously possessing both regular millimeter-scale and micrometer-scale pore structures, and having a density of less than 0.2 g / cm³. 3 .

Images