Preparation method and application of EVA material with high foaming ratio

Abstract

The invention relates to a preparation method and application of a high-foaming-ratio EVA (Ethylene Vinyl Acetate) material. The method comprises the following steps: sequentially adding an EVA mixture, a functional aid, a calcium carbonate nucleating agent, stearic acid, an AC foaming agent and zinc oxide into an internal mixer, mixing at 110-130 DEG C, carrying out hot press molding, carrying out sealed standing in an inert gas atmosphere, and carrying out electron beam irradiation treatment; and then pressurizing, foaming and depressurizing and cooling at 140-160 DEG C to prepare the foaming material. By optimizing the proportion of the irradiation sensitizer and the antioxidant and cooperatively regulating and controlling the adding amount of zinc oxide and the irradiation dose, the heat treatment temperature is remarkably reduced, the foaming process can be carried out at the temperature lower than that of a traditional process, energy consumption and by-product emission are reduced, and meanwhile high foaming ratio, excellent rebound resilience and cushioning performance are obtained. In addition, the prepared EVA cross-linked foaming material has no odor residue, is lower in cost, is suitable for the fields of shoe insoles, sound insulation materials, automotive interiors and the like, and has the characteristics of excellent light weight, low cost and environmental protection.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material foaming, and more particularly to a preparation method and application of an EVA material with a high foaming ratio. Background Art

[0002] Currently, ethylene vinyl acetate copolymer (EVA) foam is widely used in shoe midsoles, sports equipment cushioning pads, and other applications due to its excellent resilience, softness, and lightweight properties. Traditional EVA foaming processes typically utilize physical foaming agents (such as AC foaming agents) that directly decompose at high temperatures to generate gas, enabling expansion molding of the substrate. This process bypasses the cross-linking step and relies primarily on the material's inherent viscoelasticity to maintain a stable cell structure.

[0003] However, traditional non-cross-linked EVA foaming has the following shortcomings:

[0004] (1) The foaming temperature is high (usually 200-220°C), which consumes a lot of energy and easily causes thermal degradation of the material;

[0005] (2) The cell size is uneven, which is prone to cell collapse and cell opening, affecting the mechanical properties and appearance quality of the material;

[0006] (3) The foaming ratio is limited, making it difficult to achieve ultra-lightweight requirements;

[0007] (4) The high-temperature foaming process is prone to produce harmful gases, increasing environmental pollution and safety risks.

[0008] To overcome the above problems, some studies have attempted to introduce chemical cross-linking processes (such as using peroxide cross-linking), but there are defects such as complex processes, difficulty in accurately controlling the degree of cross-linking, and residual initiators leading to unstable material properties.

[0009] Therefore, how to reduce foaming energy consumption, optimize the foam structure, and achieve green and environmentally friendly production while ensuring the foaming ratio and material properties has become a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0010] The present invention aims to overcome the problems of high-temperature energy consumption, unstable cell structure, limited foaming ratio and environmental pollution in traditional EVA foaming process, and provides a preparation method of high-foaming ratio EVA material and its application.

[0011] The present invention achieves efficient foaming under low energy consumption conditions by introducing electron beam irradiation cross-linking technology, combining optimization of the proportion of functional additives and regulation of the decomposition temperature of the foaming agent by zinc oxide, while obtaining excellent mechanical properties and shock-absorbing properties.

[0012] To achieve the above object, the present invention provides the following technical solutions:

[0013] A method for preparing a high-expansion-ratio EVA material comprises the following steps:

[0014] (1) Adding the EVA mixture, functional additives, calcium carbonate nucleating agent, stearic acid, AC foaming agent and zinc oxide to an internal mixer in sequence, and mixing at 110-130° C. for 10-20 minutes to obtain a uniform mixture;

[0015] (2) placing the mixed material in a flat plate vulcanizer and hot pressing it at 120-130°C and 10-15 MPa for 20-30 minutes to form a sheet with a thickness of 1-2 mm;

[0016] (3) Place the hot-pressed sheet into a plastic bag, seal it, and let it stand in an inert gas atmosphere for 6-72 hours;

[0017] (4) using an electron beam to irradiate and crosslink the sealing sheet, with an irradiation voltage of 5-20 MeV, a beam current of 5-15 mA, and a total irradiation dose of 5-25 kGy;

[0018] (5) Place the irradiated sheet into a foaming mold, preheat to 140-160°C and 5-10 MPa, maintain for 2-3 minutes, quickly release the pressure, open the mold, and cool to shape.

[0019] More specifically, the EVA mixture in step (1) is composed of EVA and high-pressure polyethylene (Low-Density Polyethylene, LDPE), with the weight parts being 60-70 parts and 25-35 parts respectively;

[0020] Preferably, the melt index of EVA is 3-20 g / 10 min, and the melt index of LDPE is 1.5-15 g / 10 min;

[0021] More preferably, the melt index of EVA is 3-6 g / 10 min, and the melt index of LDPE is 1.5-2.5 g / 10 min.

[0022] More specifically, the raw materials used in the preparation method of step (1) further include the following auxiliary components in parts by weight:

[0023] 1.5-3 parts of calcium carbonate nucleating agent,

[0024] Functional additives 0.8-1.5 parts,

[0025] 0.5-0.6 parts of stearic acid,

[0026] AC foaming agent 5-10 parts,

[0027] 0.5-1 part of zinc oxide.

[0028] More specifically, the functional auxiliary agent in step (1) is a combination of a radiation sensitizer and an antioxidant.

[0029] Preferably, the radiation sensitizer is any one of triallyl isocyanurate (TAIC), trimethylolpropane triacrylate (TMPTMA), and triallyl cyanurate (TAC), or a combination of at least two thereof; and the antioxidant is any one of antioxidant 1010, antioxidant 1076, and antioxidant 168, or a combination of at least two thereof;

[0030] More preferably, the radiation sensitizer is triallyl isocyanurate (TAIC), and the antioxidant is antioxidant 1010.

[0031] More specifically, the inert gas in step (3) is any one of one or more mixed gases of nitrogen, helium or argon, or a combination of at least two of them, and the standing time is 1-72 hours;

[0032] Preferably, the standing time in step (3) is 48-72 hours.

[0033] More specifically, the electron beam irradiation in step (4) has an irradiation dose of 15-25 kGy;

[0034] And the radiation sensitizer, antioxidant and radiation dose D (kGy) satisfy the following relationship:

[0035]

[0036] When D≤15kGy, a=0.006-0.008, b=0.1-0.15;

[0037] When D>15kGy, a=0.01-0.015, b=0.2-0.25.

[0038] More specifically, the foaming temperature T (°C) in step (5) satisfies the following relationship with the irradiation dose D (kGy) and the zinc oxide addition amount Z (%):

[0039] T=210-a×Db×Z

[0040] Wherein, a = 2-3°C / kGy, b = 45-55°C / %;

[0041] Preferably, when D is 5-25 kGy and Z is 0.5-1.0%, T is controlled between 140-165°C.

[0042] More specifically, the foaming process in step (5) adopts a staged pressure reduction, that is, first maintaining a pressure of 5-8 MPa for 30-60 seconds, then reducing it to 2-4 MPa and maintaining it for 1-2 minutes, and then quickly releasing the pressure to open the mold and cool it down to shape.

[0043] Through the above-mentioned process optimization, the present invention significantly reduces the temperature and energy consumption required for foaming, improves the uniformity of the pore structure and the foaming ratio, and at the same time gives the product excellent rebound and shock-absorbing properties, which is particularly suitable for shoe midsoles, sound insulation materials, automotive interiors and other fields.

[0044] Compared with the existing non-cross-linked EVA thermal foaming technology, the advantages of the technical solution of this application are at least:

[0045] (1) Reducing foaming energy consumption: The present invention significantly reduces the decomposition temperature of the AC foaming agent by introducing an electron beam irradiation cross-linking process and precisely controlling the ratio of functional additives to irradiation dose, so that the foaming process is completed at a low temperature of 140-160°C, which is about 30%-40% lower than the traditional process (200-220°C), significantly reducing energy consumption and equipment heat load.

[0046] (2) Improvement of pore structure and foaming ratio: Due to the introduction of radiation cross-linking, the EVA matrix forms a moderate cross-linking network, which effectively inhibits the rupture and collapse of pores during the foaming process, significantly improves the pore uniformity and closed cell ratio, and achieves a high foaming ratio and excellent dimensional stability.

[0047] (3) Improve mechanical properties and shock absorption performance: The radiation cross-linked EVA foam material has higher resilience, compression recovery and durability, and is particularly suitable for application scenarios requiring high shock absorption performance, such as shoe midsoles, sound insulation materials, and automotive interiors.

[0048] (4) Green and environmentally friendly and reduces pollution emissions: The process of the present invention uses electron beam irradiation, and there is no need to introduce traditional chemical cross-linking agents (such as organic peroxides), which avoids the problem of initiator decomposition residues and reduces harmful gas emissions during the foaming process, meeting the requirements of green manufacturing and environmentally friendly production.

[0049] (5) The process is simple and highly controllable: The present invention adopts conventional mixing, hot pressing, irradiation, foaming and other steps. The process flow is simple, the conditions are controllable, it is easy to promote and apply industrially, and it has strong adaptability and can flexibly adjust the crosslinking degree and foaming ratio according to different application requirements.

[0050] Other features and advantages of the present invention will be described in detail in the following specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] Figure 1This is a scanning electron microscope (SEM) comparison of the conventional chemical foaming EVA material (left) and the radiation cross-linked EVA foam material (right) of the present invention. DETAILED DESCRIPTION

[0052] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0053] The following are specific embodiments:

[0054] Group A Examples

[0055] Used to illustrate the effect of the ratio of EVA mixture to functional additives on foaming properties.

[0056] Example A1 group

[0057] This group of examples is carried out with reference to the method of claim 1, using the following specific process steps, Example A1a:

[0058] (1) 65 parts of EVA (melt index 3-6 g / 10 min) and 30 parts of LDPE (melt index 2.77 g / 10 min) were mixed, and 1.2 parts of functional additives were added (the radiation sensitizer was TAIC, the antioxidant was antioxidant 1010, satisfying the antioxidant / radiation sensitizer = 0.012 × D + 0.2 = 0.44, D is the target radiation dose kGy), and at the same time, 2 parts of calcium carbonate nucleating agent, 0.5 parts of stearic acid, 7 parts of AC foaming agent, and 0.8 parts of zinc oxide were added, and the materials were added in sequence;

[0059] Mix in an internal mixer at 120°C for 15 min until the material is evenly dispersed;

[0060] (2) The mixture was placed in a flat plate vulcanizer at a temperature of 125°C and a pressure of 12 MPa for 25 min to obtain a sheet with a thickness of about 1.5 mm;

[0061] (3) The sheet was placed in a plastic bag, sealed after being filled and exhausted with nitrogen several times, and allowed to stand in a nitrogen atmosphere for 48 hours;

[0062] (4) irradiation cross-linking was performed using a 10 MeV electron beam, a beam current of 10 mA, and a total irradiation dose of 20 kGy;

[0063] (5) The irradiated material was placed in a foaming mold preheated at 150°C, and a pressure of 6 MPa was applied and maintained for 45 seconds. The pressure was then reduced to 3 MPa and maintained for 1 minute. After that, the pressure was quickly released and the mold was opened for cooling to obtain an irradiated cross-linked EVA foam material.

[0064] The obtained foamed material was observed by scanning electron microscopy (SEM) to evaluate the cell structure.

[0065] Example A1b:

[0066] The same as Example A1a, except that the irradiation dose is 15 kGy, and the ratio of the functional additives is adjusted to antioxidant / irradiation sensitizer = 0.006×D+0.1 = 0.19.

[0067] A2 Group Examples

[0068] This group of examples is used to illustrate the effect of the irradiation dose and the ratio of functional additives on the foaming performance.

[0069] Referring to the method of Example A1a, the ratio of irradiation dose and functional additive was changed, and the specific settings were as follows:

[0070] In Example A2a, the irradiation dose was 20 kGy, and the ratio of the functional additives was adjusted to antioxidant / irradiation sensitizer = 0.007 × D + 0.1 = 0.24;

[0071] Example A2b: The irradiation dose was 10 kGy, and the ratio of the functional additives was adjusted to antioxidant / irradiation sensitizer = 0.012 × D + 0.2 = 0.32;

[0072] Example A2c: The irradiation dose was 10 kGy, and the ratio of the functional additives was adjusted to antioxidant / irradiation sensitizer = 0.002×D+0.1=0.12.

[0073] Other process parameters are the same as those in Example A1a.

[0074] A3 Group Examples

[0075] This group of examples is used to illustrate the effect of changing the EVA / LDPE ratio on foaming performance.

[0076] Referring to the method of Example A1a, the EVA mixture ratio was changed as follows:

[0077] Example A3a: EVA 80 parts, LDPE 15 parts;

[0078] Example A3b: 50 parts of EVA, 45 parts of LDPE.

[0079] Other formulations and process parameters were consistent with those in Example A1a.

[0080] A4 Group Examples

[0081] This group of examples is used to illustrate the effects of different radiation sensitizers and antioxidants on performance.

[0082] Based on the process of Example A1a, the types of functional additives were adjusted respectively:

[0083] Example A4a: The radiation sensitizer is replaced with TMPTMA;

[0084] Example A4b: The antioxidant was replaced with antioxidant 1076;

[0085] Example A4c: The antioxidant was replaced with antioxidant 168.

[0086] The remaining conditions were the same as in Example A1a.

[0087] A5 Group Examples

[0088] This group of examples is used to illustrate the effect of inert atmosphere treatment on foaming performance.

[0089] Referring to the method of Example A1a, the inert gas treatment method was changed:

[0090] Example A5a: No nitrogen filling or exhaust was performed, and the sample was left to stand in air for 48 hours;

[0091] Example A5b: The standing time in the inert gas atmosphere is shortened to 2 h.

[0092] The other steps were the same as those in Example A1a.

[0093] A6 Group Example

[0094] This group of examples is used to illustrate the effects of irradiation dose, zinc oxide content and foaming temperature setting on foaming performance.

[0095] This group of examples was carried out with reference to the process flow of Example A1a, except that the irradiation dose (D), the amount of zinc oxide added (Z), and the setting method of the foaming temperature (T) were adjusted respectively, as follows:

[0096] Example A6a: The irradiation dose was 20 kGy, the zinc oxide addition amount was 0.5%, and the foaming temperature was set to T = 210-4 × D-45 × Z = 107.5°C;

[0097] Example A6b: The irradiation dose was 20 kGy, the zinc oxide addition amount was 1.0%, and the foaming temperature was set to T = 210-4 × D-55 × Z = 75°C;

[0098] Example A6c: The irradiation dose is 20 kGy, the zinc oxide addition amount is 1.0%, and the foaming temperature is set to T = 210-1×D-20×Z = 170°C.

[0099] Wherein, T is the mold setting temperature during the foaming stage (°C), D is the irradiation dose (kGy), and Z is the zinc oxide addition amount (%).

[0100] The other process steps are the same as those in Example A1a.

[0101] Group B comparative example

[0102] This group of comparative examples is used to illustrate the impact on the foaming effect when the specific process parameters of the present invention are not adopted.

[0103] This comparative example was carried out with reference to Example A1a, except for the following differences, the other steps were the same:

[0104] Comparative Example B1: In step (4), the electron beam irradiation treatment is not performed and the foaming is performed directly;

[0105] Comparative Example B2: The irradiation dose in step (4) was set to 2 kGy;

[0106] Comparative Example B3: The irradiation dose in step (4) was 30 kGy;

[0107] Comparative Example B4: No functional additive was added in step (1);

[0108] Comparative Example B5: In step (1), only a radiation sensitizer is added without adding an antioxidant;

[0109] Comparative Example B6: No zinc oxide was added in step (1);

[0110] Comparative Example B7: In step (1), the amount of zinc oxide added was 2%;

[0111] Comparative Example B8: In step (1), 100 parts of a single EVA raw material was used without adding LDPE.

[0112] Group C Comparative Example

[0113] Used to illustrate the difference between the traditional conventional EVA high-temperature foaming process and the process of the present invention.

[0114] Comparative Example C1

[0115] This comparative example C1 adopts the following specific process steps:

[0116] (1) 65 parts of EVA (melt index 3-6 g / 10 min) and 30 parts of LDPE (melt index 2.77 g / 10 min) were mixed, and 0.8 parts of chemically cross-linked dibenzoyl peroxide (DCP) and 0.5 parts of antioxidant 1010 were added. At the same time, 2 parts of calcium carbonate nucleating agent, 0.5 parts of stearic acid, 7 parts of AC foaming agent, and 0.8 parts of zinc oxide were added, and the materials were added in sequence.

[0117] Mix in an internal mixer at 120°C for 10 min until the material is evenly dispersed.

[0118] (2) The mixture was placed in a flat plate vulcanizer at a temperature of 125°C and a pressure of 12 MPa for 25 min to obtain a sheet with a thickness of about 1.5 mm.

[0119] (3) After the sheet is hot-pressed, it is placed in an oven at 120°C for heat treatment for 2 h.

[0120] (4) The heat-treated sheet was placed in a foaming mold preheated at 220°C, a pressure of 6 MPa was applied and maintained for 8 minutes, and then the pressure was quickly released and the mold was opened for cooling to obtain a traditional chemically cross-linked EVA foam material.

[0121] The performance tests of the radiation foaming materials in the embodiments and comparative examples include three indicators: foaming ratio, rebound rate and compression deformation rate:

[0122] Foaming ratio

[0123] The expansion ratio test was performed by setting the side length (50 mm) of the square mold cavity as the reference L. The length L1 and width L2 of the foamed sample after standing for 24 hours were measured using a vernier caliper. The expansion ratio of the sample was calculated using the formula (L1 + L2) / 2L.

[0124] Rebound performance

[0125] According to HG / T4993-2016 "Test Method for Rebound Elasticity of Microporous Materials for Footwear", an elasticity testing machine is used to measure the rebound rate of falling balls, which reflects the elastic recovery ability of the material;

[0126] Compression deformation rate

[0127] According to HG / T2876-2009 “Test Method for Compression Deformation of Microporous Materials of Rubber and Plastic Shoes”, the samples were compressed at 50°C for 6 hours and their permanent deformation was measured.

[0128] All test samples were cut from the middle of each sheet, using a standard-sized specimen. The results were averaged over three measurements. The relevant data are summarized in Table 1.

[0129] Table 1

[0130]

[0131]

[0132] As shown in Table 1, Standard Example A1 exhibits the best overall performance, with a foaming ratio of 5.80-5.84, a rebound rate of 68.1-68.5%, and a compression set as low as 26.4-26.6%. Group A2 demonstrates that matching the irradiation dose with the additive ratio significantly improves foaming performance; either too high or too low a ratio reduces effectiveness. Group A3 results indicate that the EVA / LDPE ratio significantly influences the foam structure; an inappropriate combination can impair elasticity and foaming ratio. Group A4 demonstrates that different sensitizer and antioxidant combinations have a certain impact on performance, with TAIC + 1010 achieving the best results. Group A5 demonstrates that inert gas treatment is crucial for cell formation; failure to apply nitrogen or insufficient rest time can degrade performance. Group A6 validates the effectiveness of the temperature control model; appropriately setting the foaming temperature can significantly optimize structure and cushioning properties. In the comparative examples, the lack of irradiation, additives, inappropriate dosage, or lack of atmosphere control all resulted in significant performance degradation. While the traditional high-temperature chemical cross-linking foaming method (C1) slightly outperformed some of the comparative examples, it was still significantly inferior to the process of the present invention overall. In summary, the present invention significantly improves the foaming ratio, structural uniformity and cushioning performance of EVA materials by optimizing radiation cross-linking, auxiliary agent ratio and temperature control strategy, and has obvious technical advantages.

[0133] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A method for preparing a high foaming ratio EVA material, characterized in that: include: (1) Add the EVA mixture, functional additives, calcium carbonate nucleating agent, stearic acid, AC foaming agent and zinc oxide to an internal mixer in sequence and mix at 110-130° C. for 10-20 minutes; (2) placing the mixed material in a flat plate vulcanizer and hot pressing at 120-130°C and 10-15 MPa for 20-30 minutes to form a sheet with a thickness of 1-2 mm; (3) Place the hot-pressed sheet into a plastic bag, seal it, and let it stand in an inert gas atmosphere; (4) irradiating the sealing sheet with an electron beam, wherein the irradiation dose, the irradiation sensitizer, and the antioxidant need to satisfy a specific numerical relationship; (5) The irradiated sheet is placed in a foaming mold and foamed under pressure at high temperature, and then cooled to form a foamed material.

2. The preparation method according to claim 1, characterized in that The EVA mixture consists of EVA and LDPE; Preferably, the weight proportions of EVA and LDPE are 60-70 parts and 25-35 parts respectively; Preferably, the melt index of EVA is 3-20 g / 10 min; The melt index of LDPE is 3-15g / 10min.

3. The preparation method according to claim 1, characterized in that The raw materials used in the preparation method also include the following auxiliary components in parts by weight: 1.5-3 parts of calcium carbonate nucleating agent, Functional additives 0.8-1.5 parts, 0.5-0.6 parts of stearic acid, AC foaming agent 5-10 parts, 0.5-1 part of zinc oxide.

4. The preparation method according to claim 1 or 3, characterized in that The functional additive is a mixture of a radiation sensitizer and an antioxidant; Preferably, the radiation sensitizer is any one of triallyl isocyanurate (TAIC), trimethylolpropane trimethacrylate (TMPTMA), and triallyl cyanurate (TAC), or a combination of at least two thereof; The antioxidant is any one of antioxidant 1010, antioxidant 1076, and antioxidant 168, or a combination of at least two of them.

5. The preparation method according to claim 1, characterized in that The inert gas in step (3) is any one of nitrogen, helium or argon, or a combination of at least two of them, and the standing time is 1-72 hours; preferably, the standing time is 48-72 hours.

6. The preparation method according to claim 1, characterized in that Step (4) electron beam irradiation with an irradiation dose of 5-25 kGy; Preferably, the radiation dose D (kGy) and the radiation sensitizer and antioxidant satisfy the following relationship: When D≤15kGy, a=0.006-0.008, b=0.1-0.15, When D>15kGy, a=0.01-0.015, b=0.2-0.

25.

7. The preparation method according to claim 1, characterized in that The foaming temperature T (°C) in step (5) satisfies the following relationship with the irradiation dose D (kGy) and the zinc oxide addition amount Z (%): T=210-a×Db×Z Wherein, a = 2-3°C / kGy, b = 45-55°C / %; Preferably, when D is 5-25 kGy and Z is 0.5-1.0%, T is controlled between 140-165°C.

8. The preparation method according to claim 1, characterized in that Step (5) comprises: placing the irradiated sheet into a foaming mold preheated to 140-160° C. and 5-10 MPa, maintaining the pressure for 2-3 minutes, quickly releasing the pressure, opening the mold, and cooling the mold to set the shape; Preferably, step (5) adopts a staged pressure reduction treatment, that is, the sheet is kept at 140-160°C and 5-10 MPa pressure for 30-60 seconds, then reduced to 2-4 MPa and maintained for 1-2 minutes, and then quickly released the pressure to open the mold and cool and shape.

9. Use of the highly foamed EVA material according to any one of claims 1 to 8 in the preparation of shoe midsoles, sound insulation materials or automobile interiors using radiation foaming technology.

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