Modified core-shell structure glue powder, functional master batch, foaming material and manufacturing method

Abstract

The application relates to the technical field of high polymer materials, in particular to a modified core-shell structure glue powder, functional master batch, foaming material and manufacturing method. The glue powder comprises a core layer and a shell layer coated on the surface of the core layer. The core layer comprises a vinyl acetate-carboxylic acid copolymer, carboxymethyl cellulose and a crosslinking agent. The shell layer comprises polyvinyl alcohol, inorganic nanoparticles and an anti-caking agent.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically to a modified core-shell structured adhesive powder, functional masterbatch, foaming material, and manufacturing method. Background Technology

[0002] Redispersible latex powder is a type of powdered polymer material produced by spray drying of polymer emulsions. It can redisperse upon the addition of water to form a stable emulsion and is widely used in building mortars, coatings, adhesives, and other fields. Most existing redispersible latex powders use vinyl acetate-ethylene copolymers (VAE) and tertiary acetate copolymers as the matrix, obtained through emulsion polymerization followed by spray drying. To improve the performance of the powder, researchers have attempted to introduce core-shell structure designs, placing polymers of different compositions in the core and shell layers respectively to achieve functional partitioning. For example, by copolymerizing tertiary acetate emulsions with carboxyl monomers, carboxylated core-shell structured redispersible latex powders can be obtained. The carboxyl functional groups in these powders can undergo crosslinking reactions with cementitious gel materials, thereby improving the bonding strength and water resistance in cement-based systems. Applications are mainly concentrated in building materials.

[0003] To address the challenge of recycling EVA foam scraps, researchers have attempted to achieve EVA carboxylation through chemical modification. For example, Chinese invention patent CN117343239A discloses a chemically grafted modified EVA method. This involves a free radical grafting reaction between EVA and a polar monomer during melt blending, grafting the polar monomer onto the EVA macromolecular chain to obtain carboxylated EVA. This carboxylated EVA can be used to prepare modified EVA foam materials; the carboxyl-induced hydrogen bonding crosslinking strengthens the cell wall of the foam material and improves its mechanical properties. Furthermore, Chinese invention patent CN118085437B discloses a method for preparing foam materials based on the decrosslinking and recycling of EVA waste. This method uses dithiobenzoic acid as a decrosslinking agent, expanding the three-dimensional crosslinked network of EVA waste through a solvent expansion method, allowing the decrosslinking agent to uniformly penetrate the network structure, and then achieving decrosslinking through high-temperature shearing.

[0004] While the aforementioned patents address the relevant technical issues, they still have certain shortcomings: the carboxylated EVA preparation method disclosed in CN117343239A requires melt grafting chemical modification of EVA, resulting in a complex process. Furthermore, the direct grafting of carboxyl groups onto the EVA backbone leads to irreversible covalent or hydrogen bond crosslinking, making it difficult to form a controllable core-shell structure and a reversible crosslinking network. The EVA waste recycling method disclosed in CN118085437B requires organic solvent-assisted expansion, which is not only complex and costly but also leaves solvent residues, hindering environmental protection and large-scale production. More importantly, none of the above technologies solve the problem of severe mechanical property degradation during EVA scrap recycling, particularly the significant decrease in tear strength and resilience, nor do they achieve effective optimization of the cell structure. Summary of the Invention

[0005] The purpose of this invention is to address the deficiencies and shortcomings of existing technologies by providing a modified core-shell structured adhesive powder, functional masterbatch, foaming material, and manufacturing method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a modified core-shell structured adhesive powder, characterized in that: the adhesive powder comprises a core layer and a shell layer covering the surface of the core layer; the core layer comprises vinyl acetate-carboxylic acid copolymer, carboxymethyl cellulose, and a crosslinking agent; the shell layer comprises polyvinyl alcohol, inorganic nanoparticles, and an anti-caking agent; the mass content of inorganic nanoparticles in the shell layer is 2%–5%; the bulk density of the adhesive powder is 400–600 g / L; the average particle size is 80–100 μm; the fineness (≥150 μm) is ≤8.0%; the ash content (calcined at 1000℃) is 10%–14%; the degree of alcoholysis of the polyvinyl alcohol is 85%–90%; and the mass ratio of polyvinyl alcohol to inorganic nanoparticles in the shell layer is (4–6):1.

[0007] A further improvement is that the vinyl acetate-carboxylic acid copolymer contains 25%–40% carboxylic acid monomers and 40%–50% solids. The degree of substitution of the carboxymethyl cellulose is 0.8 to 1.2; The crosslinking agent is selected from ammonium persulfate or triethyl citrate; The inorganic nanoparticles are nano-silica with a particle size of 20-50 nm and a specific surface area of ​​≥150 m² / g. The anti-caking agent is magnesium stearate.

[0008] To achieve the above objectives, the present invention can also employ the following technical solution: a method for preparing modified core-shell structured adhesive powder, comprising the following steps: Step 1: Provide a core layer emulsion, wherein the core layer emulsion comprises vinyl acetate-carboxylic acid copolymer, carboxymethyl cellulose and a crosslinking agent; Step 2: Provide a shell solution, wherein the shell solution contains polyvinyl alcohol, inorganic nanoparticles and an anti-caking agent, the solid content of the shell solution is 10% to 15%, and the mass ratio of the core layer emulsion to the shell solution is (3.5 to 4.5): 1; Step 3: The core layer emulsion and the shell layer solution are simultaneously atomized through a dual-channel nozzle and then spray-dried to obtain core-shell structured powder; Step 4: The powder is sieved and dried to obtain the modified core-shell structured adhesive powder.

[0009] A further improvement is that, when providing the core layer emulsion, the vinyl acetate-carboxylic acid copolymer emulsion is mixed with carboxymethyl cellulose and the pH is adjusted to 6.5-7.5, then a crosslinking agent is added and pre-reacted at 55-65°C for 25-40 minutes.

[0010] To achieve the above objectives, the present invention may also employ the following technical solution: a functional masterbatch comprising a carrier resin and a modified core-shell structured adhesive powder dispersed in the carrier resin, wherein the modified core-shell structured adhesive powder is the modified core-shell structured adhesive powder as described above.

[0011] A further improvement is that, by weight, the functional masterbatch comprises: 15-35 parts of the modified core-shell structured adhesive powder, 45-70 parts of the carrier resin, 1-12 parts of the anti-sticking agent, 2-5 parts of the dispersant, and 0.5-1.5 parts of the antioxidant.

[0012] A further improvement is that the carrier resin is one or more of thermoplastic elastomers and rubbers, wherein the thermoplastic elastomer is one or a mixture of several of ethylene-vinyl acetate copolymers, polyolefin thermoplastic elastomers, hydrogenated styrene-butadiene block copolymers, and olefin block copolymers; the anti-sticking agent is talc, calcium carbonate powder, or zinc stearate; and the dispersant is glyceryl stearate, polyethylene wax, or a mixture of both.

[0013] To achieve the above objectives, the present invention can also employ the following technical solution: a method for preparing a functional masterbatch, comprising the following steps: S1: Dry the modified core-shell structured adhesive powder as defined in any one of claims 1 to 4; S2: The dried adhesive powder is mixed with carrier resin, anti-sticking agent, dispersant and antioxidant and then melt-blended; S3: Extruding and granulating the compounded product to obtain functional masterbatch. This process offers technical advantages such as good granule flowability, suitability for injection molding foaming, and accurate metering.

[0014] To achieve the above objectives, the present invention may also employ the following technical solution: a foamed material comprising EVA matrix resin, EVA scrap or post-consumer recycled material, and functional masterbatch, wherein the functional masterbatch is as described above, comprising, by weight: 35-75 parts of EVA matrix resin, 10-50 parts of EVA scrap or post-consumer recycled material, 5-15 parts of the functional masterbatch, 5-15 parts of POE, 5-15 parts of EPDM, 2.5-4.5 parts of foaming agent, 0.6-1.2 parts of crosslinking agent, 0.8-1.5 parts of foaming accelerator, 0.3-0.8 parts of lubricant, and 0.5-1.0 parts of dispersant; wherein the foaming accelerator is zinc oxide, the crosslinking agent is di-tert-butyl peroxide diisopropylbenzene (BIPB) or diisopropylbenzene peroxide (DCP); and the foaming agent is azodicarbonamide.

[0015] To achieve the above objectives, the present invention may also employ the following technical solution: a method for preparing EVA foamed products, characterized by comprising the following steps: Q1: Perform the first stage of intensive mixing and kneading of each component in the foam as described above; Q2: Add foaming agent, crosslinking agent, foaming accelerator, lubricant and dispersant to the product of the first stage of intensive mixing and carry out the second stage of intensive mixing; Q3: The product from the second stage of intensive mixing is then subjected to open-milling and sheeting. Q4: Further granulate the product from open milling to obtain foamed granules; Q5: Place the foamed granules into a mold for compression molding or into an injection molding machine for injection molding. Q6: After foaming, heat setting is performed to obtain EVA foamed products.

[0016] After adopting the above technical solution, the beneficial effects of the present invention are as follows: This invention utilizes a dual-channel simultaneous atomization spray drying process involving a core emulsion and a shell solution to form a complete core-shell coating structure. Polyvinyl alcohol in the shell acts as a protective colloid, imparting excellent redispersibility to the powder. Nano-inorganic particles adhere to the shell surface, providing isolation. Combined with an anti-caking agent, these components prevent agglomeration and adhesion of the powder particles during storage and transportation. This ensures the powder maintains good flowability and redispersibility even after long-term storage, with a stable bulk density of 400–600 g / L and a moisture content ≤0.5%, facilitating industrial packaging, transportation, and... In addition, the outer shell of the adhesive powder is mainly composed of polyvinyl alcohol, which has similar compatibility with the vinyl acetate segments in the EVA molecular chain. During the melt blending process, it can be uniformly dispersed in the EVA matrix, avoiding macroscopic phase separation. The average particle size and fineness distribution of the adhesive powder are precisely controlled by airflow classification, which can not only ensure uniform dispersion in the EVA matrix, but also avoid the loss of excessively fine powder during the mixing process. The ash content index reflects the effective introduction of inorganic nanoparticles, ensuring the reinforcing effect without impairing the melt flowability of the foaming system.

[0017] This invention is the first to introduce a core-shell structured redispersible polymer powder into an EVA foaming and recycling system. The core innovation lies in the carboxyl functional groups contained in the vinyl acetate-carboxylic acid copolymer in the core layer of the polymer powder. During the foaming and crosslinking process, these groups undergo a coordination crosslinking reaction with zinc oxide (ZnO) in the formulation, generating thermally reversible ionic crosslinks in situ. This ionic crosslinking network coexists with the covalent crosslinking network initiated by di-tert-butyl peroxide (BIPB) or di-propyl peroxide (DCP), forming a double-crosslinked reinforced structure. The presence of ionic crosslinks endows the crosslinking network with a certain degree of dynamic reversibility, allowing it to dissipate energy through the breaking and recombination of ionic bonds under external force, thereby significantly improving the material's tear resistance. This invention, through a core-shell structure and ionic crosslinking, ensures dispersion in the shell layer and provides reactivity in the core layer; both are indispensable, unlike existing technologies that rely solely on carboxylated EVA or simple core-shell polymer powders. Experiments have shown that, with the addition of the functional masterbatch of this invention, the rebound rate of foamed midsoles containing more than 30% EVA scrap or post-consumer recycled materials can reach over 54%, and the tear strength is improved by 36% to 43% compared to the system without the addition. This solves the problem of severe mechanical property degradation during the recycling process of EVA scrap or post-consumer recycled materials. Simultaneously, the rubber powder particles are uniformly dispersed in the EVA foam matrix, and their microsphere morphology acts as a heterogeneous nucleating agent during the foaming process, reducing the free energy barrier required for the decomposition of gas molecules by the foaming agent to form bubble nuclei, thus promoting the simultaneous and uniform generation of a large number of bubble nuclei. Compared with the traditional recycling system without added rubber powder, the cell size is refined from >200μm to 80-150μm, the cell density is significantly increased, and the cell size distribution is more uniform. Furthermore, the fine and uniform cell structure increases the closed-cell rate from 75% to >90%, giving the foamed material better resilience, lower water absorption, and superior thermal insulation performance. Optimization of the cell structure also reduces density fluctuations in foamed products, improving dimensional stability and appearance quality.

[0018] This invention, from the preparation of the adhesive powder to the manufacturing of functional masterbatch and the molding of foamed products, utilizes a dry or aqueous system throughout the entire process, without involving the introduction or use of any organic solvents. Compared to existing technologies that require organic solvents to assist in expansion for the decrosslinking and recycling of EVA waste, this invention fundamentally avoids the use of organic solvents, eliminates the safety hazards of solvent residues to foamed products, and simplifies the process by eliminating solvent recovery and equipment corrosion prevention steps, thereby reducing equipment investment and production costs, and aligning with green and environmentally friendly industrial policies. In the preparation method of the adhesive powder, the spray drying technology itself directly obtains the dry powder product from the aqueous system, achieving zero organic solvent emissions throughout the entire production chain.

[0019] This invention adds POE and EPDM as toughening components simultaneously. POE imparts excellent resilience and low-temperature toughness to the foamed material, while EPDM further improves melt strength and heat resistance. The two work synergistically to ensure that the foaming system containing a high proportion of EVA scraps maintains sufficient melt strength during the foaming process, avoiding cell merging and collapse, thereby obtaining a uniform and fine cell structure. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to embodiments and comparative examples, but the present invention is not limited to the following embodiments. The raw materials used in the embodiments are all commercially available industrial products, wherein: The EVA matrix resin is an ethylene-vinyl acetate copolymer with a VA content of 18%–26% and a melt index of 1.5–150 g / 10 min; EVA scraps are waste materials generated during the production of sports shoe midsoles or recycled sports shoe or slipper materials, which are mixed in an internal mixer and extruded and stretched in an open mill to prepare thin fabric with a thickness of ≤0.18±0.02 mm, or powder with a particle size of ≤200 μm after pulverization; PO E is selected from polyolefin elastomers with a specific gravity ≤0.868g / cm³ and a hardness of approximately 63°A; EPDM rubber is selected from EPDM rubber with a Mooney viscosity (ML1₊4, 125℃) of 40~60; foaming agent AC is azodicarbonamide with a gas evolution ≥200mL / g; crosslinking agent BIPB is di-tert-butyl peroxide diisopropylbenzene or DCP is diisopropylbenzene peroxide; ZnO particle size ≤1μm; stearic acid, zinc stearate and polyethylene wax are all industrial grade.

[0021] In the preparation of the adhesive powder, the solid content of the vinyl acetate-carboxylic acid copolymer emulsion is 40%–50%, and the content of carboxylic acid monomer is 25%–40%; the degree of substitution of sodium carboxymethyl cellulose is 0.8–1.2, and the viscosity is 300–800 mPa·s; the degree of alcoholysis of PVA1788 is 85%–90%; the particle size of nano silica is 20–50 nm, and the specific surface area is ≥150 m² / g; the anti-caking agent is magnesium stearate.

[0022] In all embodiments, "parts" refers to parts by mass. Example 1:

[0023] I. Preparation of Modified Core-Shell Redispersible Powder (1) Core layer emulsion pretreatment: Take 65 parts of vinyl acetate-carboxylic acid copolymer emulsion (calculated by solid content) and 5 parts of sodium carboxymethyl cellulose, put them into an emulsification kettle and stir at 1000 r / min for 30 min. Adjust the pH to 7.0 with ammonia water, add 1.0 part of crosslinking agent ammonium persulfate, heat to 60℃, and pre-react for 30 min to obtain crosslinked modified core layer emulsion.

[0024] (2) Preparation of shell solution: Take 18 parts of PVA1788, 3.5 parts of nano silica and 2 parts of magnesium stearate, add them to deionized water to prepare a solution with a solid content of 12%, stir at 80°C until completely dissolved to obtain shell solution.

[0025] (3) Synchronous atomization spray drying: A centrifugal spray drying tower is used to synchronously atomize the core layer emulsion and the shell layer solution at a mass ratio of 4:1 through a dual-channel nozzle. The inlet air temperature is 190℃, the outlet air temperature is 85℃, and the atomizing disc speed is 18000r / min for spray drying.

[0026] (4) Post-processing: The powder obtained by spray drying is classified by airflow classification and sieve, and fractions with a particle size of 50-300μm are collected and dynamically dried to a moisture content of ≤0.5% to obtain modified core-shell structure redispersible adhesive powder (denoted as adhesive powder A).

[0027] Tests showed that the bulk density of adhesive powder A was 500 g / L, the fineness (≥150 μm) was 5.5%, and the ash content (calcined at 1000℃) was 12.1%.

[0028] II. Preparation of Functional Masterbatch (1) Dry the adhesive powder A under vacuum at 70°C for 3 hours.

[0029] (2) Add the following ingredients in the following proportions by weight: 33 parts of adhesive powder A, 57 parts of carrier EVA (VA content 18%, melt index 5g / 10min), 5 parts of talc powder (1250 mesh), 4 parts of dispersant (glyceryl stearate and polyethylene wax mixed in a mass ratio of 1:2), and 1.0 part of antioxidant 1010.

[0030] (3) Put the above materials into a mixer and mix them at 105°C and 50 r / min for 8 minutes until they are uniformly plasticized, and then discharge them.

[0031] (4) The molten material discharged from the internal mixing is transported to a water-cooled single-screw extruder (length-to-diameter ratio 14:1), the temperature of each zone is set to 85℃, the screw speed is 80r / min, and the material is extruded and pelletized to obtain functional masterbatch (referred to as masterbatch A).

[0032] III. Preparation of EVA Foamed Midsole (1) Weigh each component according to the following mass ratio: 53 parts of EVA matrix resin, 30 parts of EVA scrap or post-consumer recycled material thin fabric, 5 parts of functional masterbatch A, 6 parts of POE, 6 parts of EPDM, 3.5 parts of foaming agent AC, 1.2 parts of crosslinking agent BIPB, 1.2 parts of ZnO, 0.5 parts of stearic acid, 0.8 parts of zinc stearate, and 2 parts of polyethylene wax.

[0033] (2) Put EVA matrix resin, EVA scrap or post-consumer recycled material, functional masterbatch A, POE and EPDM into a mixer and mix at 105°C for 6 minutes. Add foaming agent AC, crosslinking agent DCP, ZnO, stearic acid, zinc stearate and polyethylene wax and continue mixing for 2 minutes.

[0034] (3) Transfer the internally mixed material to the open mill, pass it through the mill 4 times at a roller temperature of 100°C and a roller gap of 1.5 mm, and then produce the sheet.

[0035] (4) The material after sheeting is conveyed to a water-cooled single-screw extruder (length-to-diameter ratio 14:1), with a temperature of 85℃ in each zone and a screw speed of 80r / min. The material is extruded and granulated to obtain foamed granules.

[0036] (5) Place the foamed granules into the mold and compress and foam them for 12 minutes at 180°C and 12MPa.

[0037] (6) After foaming, heat set at 70℃ for 15 minutes to obtain EVA foamed midsole (referred to as sample A). Example 2:

[0038] I. Preparation of Modified Core-Shell Redispersible Powder (1) Core layer emulsion pretreatment: Take 55 parts of vinyl acetate-carboxylic acid copolymer emulsion and 8 parts of sodium carboxymethyl cellulose, put them into an emulsification kettle and mix at 1200 r / min for 20 min. Adjust the pH to 6.5 with ammonia water, add 1.5 parts of crosslinking agent triethyl citrate, heat to 55℃, and pre-react for 40 min to obtain crosslinked modified core layer emulsion.

[0039] (2) Preparation of shell solution: Take 25 parts of PVA1788, 5 parts of nano silica and 3 parts of magnesium stearate, add them to deionized water to prepare a solution with a solid content of 15%, stir at 75°C until completely dissolved to obtain shell solution.

[0040] (3) Synchronous atomization spray drying: A centrifugal spray drying tower is used to synchronously atomize the core layer emulsion and the shell layer solution at a mass ratio of 3.5:1 through a dual-channel nozzle. The inlet air temperature is 175℃, the outlet air temperature is 75℃, and the atomizing disc speed is 15000r / min for spray drying.

[0041] (4) The post-processing is the same as in Example 1, to obtain the modified core-shell structure redispersible powder (denoted as powder B).

[0042] Tests showed that the bulk density of adhesive powder B was 420 g / L, the fineness (≥150 μm) was 7.2%, and the ash content (calcined at 1000℃) was 13.5%.

[0043] II. Preparation of Functional Masterbatch The following ingredients were added in the indicated weight proportions: 35 parts of rubber powder B, 36.5 parts of carrier EVA (VA content 26%, melt index 4 g / 10 min), 10 parts of carrier EVA (VA content 28%, melt index 150 g / 10 min), 12 parts of calcium carbonate powder (1250 mesh), 5 parts of dispersant (polyethylene wax), and 1.5 parts of antioxidant 1076. The remaining preparation conditions were the same as in Example 1, yielding the functional masterbatch (denoted as Masterbatch B).

[0044] III. Preparation of EVA Foamed Midsole The following components were weighed according to the following mass ratio: 39 parts EVA matrix resin, 40 parts EVA scrap fabric, 7 parts functional masterbatch B, 10 parts POE, 10 parts EPDM, 4.5 parts foaming agent AC, 1.2 parts crosslinking agent DCP, 0.8 parts ZnO, 0.8 parts stearic acid, and 1.0 part zinc stearate. The remaining preparation conditions were the same as in Example 1, yielding an EVA foamed midsole (denoted as Sample B). Example 3:

[0045] I. Preparation of Modified Core-Shell Redispersible Powder (1) Core layer emulsion pretreatment: Take 75 parts of vinyl acetate-carboxylic acid copolymer emulsion and 3 parts of sodium carboxymethyl cellulose, put them into an emulsification kettle and mix at 800 r / min for 40 min. Adjust the pH to 7.5 with ammonia water, add 0.5 parts of crosslinking agent ammonium persulfate, heat to 65℃, and pre-react for 25 min to obtain crosslinked modified core layer emulsion.

[0046] (2) Preparation of shell solution: Take 12 parts of PVA1788, 2 parts of nano silica and 1 part of magnesium stearate, add them to deionized water to prepare a solution with a solid content of 10%, stir at 85°C until completely dissolved to obtain shell solution.

[0047] (3) Synchronous atomization spray drying: A centrifugal spray drying tower is used to synchronously atomize the core layer emulsion and the shell layer solution at a mass ratio of 4.5:1 through a dual-channel nozzle. The inlet air temperature is 205℃, the outlet air temperature is 95℃, and the atomizing disc speed is 20000r / min for spray drying.

[0048] (4) The post-processing is the same as in Example 1, to obtain the modified core-shell structure redispersible powder (denoted as powder C).

[0049] The test results showed that the bulk density of adhesive powder C was 580 g / L, the fineness (≥150 μm) was 4.8%, and the ash content (calcined at 1000℃) was 10.5%.

[0050] II. Preparation of Functional Masterbatch The following ingredients were added in the indicated weight proportions: 25 parts of adhesive powder C, 73 parts of carrier EVA (VA content 18%, melt index 2g / 10min), 2 parts of talc powder (2000 mesh), 2 parts of dispersant (glyceryl stearate), and 0.5 parts of antioxidant 168. The remaining preparation conditions were the same as in Example 1, yielding a functional masterbatch (denoted as Masterbatch C).

[0051] III. Preparation of EVA Foamed Midsole The following components were weighed according to the following mass ratio: 63 parts EVA matrix resin, 20 parts EVA scrap powder, 12 parts functional masterbatch C, 6 parts POE, 6 parts EPDM, 2.5 parts foaming agent AC, 0.6 parts crosslinking agent DCP, 1.5 parts ZnO, 0.3 parts stearic acid, and 0.5 parts zinc stearate. The remaining preparation conditions were the same as in Example 1, yielding an EVA foamed midsole (denoted as Sample C). Example 4:

[0052] I. Preparation of Modified Core-Shell Redispersible Powder

[0053] The preparation conditions for the adhesive powder were basically the same as in Example 1, except that the mass ratio of the core layer emulsion to the shell layer solution was adjusted to 4.5:1, the inlet air temperature was adjusted to 200℃, the outlet air temperature was adjusted to 90℃, and the atomizing disc rotation speed was adjusted to 16000 r / min. The resulting adhesive powder was denoted as adhesive powder D.

[0054] The test results showed that the bulk density of the adhesive powder D was 550 g / L, the fineness (≥150 μm) was 5.0%, and the ash content (calcined at 1000℃) was 11.8%.

[0055] II. Preparation of Functional Masterbatch The preparation conditions for the functional masterbatch were basically the same as in Example 1, except that: the amount of adhesive powder D added was 20 parts, the carrier EVA was 66.2 parts, the anti-blocking agent was 10 parts of calcium carbonate powder (1500 mesh), the dispersant was 3 parts, and the antioxidant was 0.8 parts. The resulting functional masterbatch was denoted as Masterbatch D.

[0056] III. Preparation of EVA Foamed Midsole The preparation conditions for the EVA foamed midsole were basically the same as in Example 1, except that: the EVA matrix resin was 58 parts, the EVA scrap or post-consumer recycled material thin fabric was 25 parts, the functional masterbatch D was 5 parts, POE was 6 parts, EPDM was 6 parts, the foaming agent AC was 3.0 parts, the crosslinking agent DCP was 0.9 parts, ZnO was 1.0 part, stearic acid was 0.6 parts, and zinc stearate was 0.7 parts. The resulting EVA foamed midsole was designated as Sample D. Comparative Example 1:

[0057] The only difference between Comparative Example 1 and Example 1 is that no functional masterbatch is added, and the amount of EVA is increased accordingly to maintain the total mass of the formulation. The specific formulation is as follows: 73 parts EVA matrix resin, 15 parts EVA scrap fabric, 6 parts POE, 6 parts EPDM, 3.5 parts foaming agent AC, 0.9 parts crosslinking agent DCP, 1.2 parts ZnO, 0.5 parts stearic acid, and 0.8 parts zinc stearate. The preparation method is the same as in Example 1. The resulting EVA foamed substrate is designated as Sample E1. Comparative Example 2:

[0058] The difference between Comparative Example 2 and Example 1 is that no modified core-shell redispersible polymer powder is added to the functional masterbatch, while the remaining components and their amounts remain unchanged. Specifically, the functional masterbatch formulation is: 87 parts EVA carrier, 8 parts talc, 4 parts dispersant, and 1.0 part antioxidant 1010 (the 25 parts of polymer powder A are made up by an equal amount of carrier EVA). The preparation method is the same as in Example 1. The resulting EVA foaming substrate is designated as Sample E2. Comparative Example 3:

[0059] The difference between Comparative Example 3 and Example 1 is that: commercially available ordinary VAE redispersible latex powder (non-core-shell structure, without carboxyl functional monomers) was used instead of the modified core-shell structure redispersible powder of the present invention, and the amount added to the functional masterbatch was the same at 25 parts. The remaining formulation and preparation method were the same as in Example 1. The resulting EVA foaming substrate was designated as Sample E3. Performance testing:

[0060] The following performance tests were conducted on the EVA foamed midsole samples obtained in Examples 1-4 and Comparative Examples 1-3, and the results are shown in Table 1.

[0061] Test method: (1) Rebound rate: Refer to GB / T 1681—2009 "Determination of resilience of vulcanized rubber" and use a falling ball rebound tester.

[0062] (2) C-type tear strength: Refer to GB / T 529—2008 "Determination of tear strength of vulcanized rubber or thermoplastic rubber (trouser-shaped, right-angled and crescent-shaped specimens)" and use crescent-shaped (C-type) specimens for testing.

[0063] (3) Cell size and distribution: The cross-section of the foamed sample was observed using a scanning electron microscope (SEM), and the cell size was statistically analyzed using Image-Pro Plus image analysis software.

[0064] (4) Closed cell ratio: The true density meter was used to test the ratio of open and closed cell volume in rigid foam plastics, referring to GB / T 10799—2008.

[0065] (5) Density: Tested according to GB / T 6343—2009 "Determination of apparent density of foamed plastics and rubber".

[0066] Table 1. Performance test results of EVA foamed midsoles in each embodiment and comparative example.

[0067] Results Analysis As shown in Table 1: (1) The EVA foam midsoles in Examples 1-4, which added the functional masterbatch of the present invention, all achieved a rebound rate of over 54% (58.0%-60.5%) and a C-type tear strength of 21.1-21.8 kg / cm. Compared with Comparative Example 1 (without functional masterbatch, rebound rate 49.8%, tear strength 6.3 kg / cm), the rebound rate increased by approximately 16.47%-21.49%, and the C-type tear strength increased by approximately 235%-246%. Compared with Comparative Example 2, which added blank masterbatch without adhesive powder, the high amount of recycled EVA (40%) in the formulation of Comparative Example 2 directly led to severe stratification and bubbles during foaming, making it impossible to obtain qualified foaming material. Therefore, compared with Comparative Example 2, the performance improvement of Examples 1-4 was more significant. Compared to Comparative Example 3 (52.2% resilience and 7.5 kg / cm tear strength) which added commercially available VAE adhesive powder, the present invention has significant advantages in both resilience and tear strength, indicating that the ionic cross-linking network formed by the carboxyl groups in the core layer of the adhesive powder and ZnO in the system plays a key role in improving the mechanical properties.

[0068] (2) The average cell size of Examples 1-4 was 98-135 μm, and the closed-cell rate was 90.3%-93.5%, while the average cell size of Comparative Example 1 was 215 μm and the closed-cell rate was only 75.0%. This indicates that the microspheres of the present invention played a good heterogeneous nucleation role in the foaming process, significantly refining the cell structure and improving the closed-cell rate. The average cell size of Comparative Example 3 was 185 μm and the closed-cell rate was 80.0%, which was better than Comparative Example 1, but still not as good as the examples of the present invention. This indicates that although ordinary adhesive powder has a certain nucleation effect, it lacks the combined effect of ionic cross-linking networks, and the degree of optimization of the cell structure is limited.

[0069] (3) The density of the foamed substrate in Examples 1 to 4 is 0.180 to 0.203 g·cm³. -3 The value was lower than that of Comparative Example 1 (0.215 g·cm⁻¹). -3 This indicates that the refinement of the pore structure and the increase in closed-cell ratio effectively reduced the material density. The density of Comparative Example 3 was 0.200 g·cm³. -3 This is somewhere between the embodiments of the present invention and Comparative Example 1.

[0070] (4) Among the various embodiments, the comprehensive performance of Example 1 is the best (rebound rate 60.5%, tear strength 21.7Kg / cm, average cell size 105μm, closed cell rate 92.5%), indicating that when the amount of functional masterbatch added, the carboxyl content in the adhesive powder and the process parameters are in the optimal range, the formation efficiency of the ion crosslinking network and the heterogeneous nucleation effect reach the best balance.

[0071] The above embodiments demonstrate that the modified core-shell structure redispersible adhesive powder, functional masterbatch, and EVA foaming composition provided by the present invention can generate a thermally reversible ionic crosslinking network through the reaction of the carboxyl groups of the adhesive powder core layer with ZnO in the foaming system. Even in foaming formulations containing more than 30% EVA scrap or post-consumer recycled materials, a high-performance foamed midsole with a resilience of ≥54% and a tear strength of ≥20.0Kg / cm can still be obtained, realizing the high-performance recycling and reuse of EVA scrap.

[0072] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions above are merely illustrative of the principles of the invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents. Any aspects not detailed in the present invention are well-known to those skilled in the art.

Claims

1. A modified core-shell structured adhesive powder, characterized in that: The adhesive powder comprises a core layer and a shell layer covering the surface of the core layer. The core layer comprises vinyl acetate-carboxylic acid copolymer, carboxymethyl cellulose, and a crosslinking agent. The shell layer comprises polyvinyl alcohol, inorganic nanoparticles, and an anti-caking agent. The mass content of inorganic nanoparticles in the shell layer is 2% to 5%. The bulk density of the adhesive powder is 400 to 600 g / L, the average particle size is 10-500 μm, the fineness (≥150 μm) is ≤10.0%, the ash content (calcined at 1000℃) is 10% to 14%, the degree of alcoholysis of the polyvinyl alcohol is 85% to 90%, and the mass ratio of polyvinyl alcohol to inorganic nanoparticles in the shell layer is (4 to 6):

1. The core layer contains carboxyl groups, which can react with zinc oxide at the foaming temperature to form thermally reversible ionic crosslinking bonds.

2. The modified core-shell structure adhesive powder according to claim 1, characterized in that: The vinyl acetate-carboxylic acid copolymer contains 25%–40% carboxylic acid monomers and 40%–50% solids. The degree of substitution of the carboxymethyl cellulose is 0.8 to 1.2; The crosslinking agent is selected from ammonium persulfate or triethyl citrate; The average particle size of the adhesive powder is 80-100μm; The inorganic nanoparticles are nano-silica with a particle size of 20-50 nm and a specific surface area of ​​≥150 m² / g. The anti-caking agent is magnesium stearate.

3. A method for preparing a modified core-shell structured adhesive powder, characterized in that: Includes the following steps: Step 1: Provide a core layer emulsion, wherein the core layer emulsion comprises vinyl acetate-carboxylic acid copolymer, carboxymethyl cellulose and a crosslinking agent; Step 2: Provide a shell solution, wherein the shell solution contains polyvinyl alcohol, inorganic nanoparticles and an anti-caking agent, the solid content of the shell solution is 10% to 15%, and the mass ratio of the core layer emulsion to the shell solution is (3.5 to 4.5): 1; Step 3: The core layer emulsion and the shell layer solution are simultaneously atomized through a dual-channel nozzle and then spray-dried to obtain core-shell structured powder; Step 4: The powder is sieved and dried to obtain the modified core-shell structured adhesive powder.

4. The method for preparing a modified core-shell structured adhesive powder according to claim 3, characterized in that: It also includes, when providing the core layer emulsion, mixing the vinyl acetate-carboxylic acid copolymer emulsion with carboxymethyl cellulose, adjusting the pH to 6.5-7.5, adding a crosslinking agent, and pre-reacting at 55-65°C for 25-40 minutes.

5. A functional masterbatch, characterized in that: It includes a carrier resin and a modified core-shell structured adhesive powder dispersed in the carrier resin, wherein the modified core-shell structured adhesive powder is the modified core-shell structured adhesive powder according to any one of claims 1 to 2.

6. A functional masterbatch according to claim 5, characterized in that: By weight, the functional masterbatch comprises: 15-35 parts of the modified core-shell structured adhesive powder, 45-70 parts of the carrier resin, 1-12 parts of the anti-sticking agent, 2-5 parts of the dispersant, and 0.5-1.5 parts of the antioxidant.

7. A functional masterbatch according to claim 5, characterized in that: The carrier resin is one or more of thermoplastic elastomers and rubbers, wherein the thermoplastic elastomer is one or a mixture of several of ethylene-vinyl acetate copolymers, polyolefin thermoplastic elastomers, hydrogenated styrene-butadiene block copolymers, and olefin block copolymers; the anti-sticking agent is talc, calcium carbonate powder, or zinc stearate; and the dispersant is glyceryl stearate, polyethylene wax, or a mixture of both.

8. A method for preparing a functional masterbatch, characterized in that: Includes the following steps: S1: Dry the modified core-shell structured adhesive powder as defined in any one of claims 1 to 4; S2: The dried adhesive powder is mixed with carrier resin, anti-sticking agent, dispersant and antioxidant and then melt-blended; S3: Extrude the compounded product into granules to obtain functional masterbatch.

9. A foamed material, characterized in that: The product comprises EVA matrix resin, EVA scrap or post-consumer recycled material, and functional masterbatch. The functional masterbatch is the functional masterbatch described in any one of claims 5 to 7, and by weight comprises: 35 to 75 parts of EVA matrix resin, 10 to 50 parts of EVA scrap or post-consumer recycled material, 5 to 15 parts of the functional masterbatch, 5 to 15 parts of POE, 5 to 15 parts of EPDM, 2.5 to 4.5 parts of foaming agent, 0.6 to 1.2 parts of crosslinking agent, 0.8 to 1.5 parts of foaming accelerator, 0.3 to 0.8 parts of lubricant, and 0.5 to 1.0 parts of dispersant. The foaming accelerator is zinc oxide, the crosslinking agent is di-tert-butyl peroxide (BIPB) or di-propyl peroxide (DCP), and the foaming agent is azodicarbonamide.

10. A method for preparing EVA foamed products, characterized in that: Includes the following steps: Q1: The components in the foam as described in claim 9 are subjected to a first stage of intensive mixing; Q2: Add foaming agent, crosslinking agent, foaming accelerator, lubricant and dispersant to the product of the first stage of intensive mixing and carry out the second stage of intensive mixing; Q3: The product from the second stage of intensive mixing is then subjected to open-milling and sheeting. Q4: Further granulate the product from open milling to obtain foamed granules; Q5: Place the foamed granules into a mold for compression molding or into an injection molding machine for injection molding. Q6: After foaming, heat setting is performed to obtain EVA foamed products.

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