Low-hardness, high-fatigue-resistance EVA composite foaming material and preparation method thereof

Abstract

The application relates to the technical field of high-molecular foaming materials, in particular to a low-hardness high-fatigue-resistance EVA composite foaming material and a preparation method thereof, which comprises the following components in parts by weight: ethylene-vinyl acetate polymer 35-45 parts, thermoplastic polyurethane elastomer 17-23 parts, butadiene and styrene copolymer 13-17 parts, zinc oxide 3-5 parts, lubricant 7-8 parts, azodicarbonamide 11-12 parts, filler 4-6 parts, carboxylated nano-silicon dioxide 2-5 parts, soluble zinc salt 0.5-2 parts and dicumyl peroxide 0.5-1.5 parts. 2+ The application constructs a Zn-carboxyl coordination crosslinking network, so that the dynamic fatigue resistance of the material is revolutionarily improved; the moderate dynamic reversibility of the coordination bond can efficiently dissipate energy and quickly recover in long-term compression cycles, and fundamentally solves the problem that the low-hardness foaming material is prone to fatigue collapse.

Description

Technical Field

[0001] This invention relates to the field of polymer foaming materials technology, specifically to an ethylene-vinyl acetate copolymer (EVA)-based composite foaming material suitable for heavy-duty products (such as backpack shoulder straps, back panels, shoe materials, etc.), and particularly to an EVA / thermoplastic polyurethane (TPU) / butadiene-styrene copolymer ternary composite foaming material with low hardness, high resilience, excellent fatigue resistance and dimensional stability, and its molding foaming preparation method. Background Technology

[0002] Ethylene-vinyl acetate copolymer (EVA) foam is widely used in footwear, sports protective gear, toys, packaging, and bag padding due to its excellent flexibility, cushioning, chemical resistance, and processing properties. Especially in products such as heavy-duty backpacks and outdoor equipment, EVA foam is often used as the core cushioning layer for load-bearing components such as shoulder straps and back panels. Its performance directly determines the product's comfort, durability, and load-bearing capacity.

[0003] Traditional EVA molding foam materials are typically prepared using EVA as the base resin, combined with azodicarbonamide (AC) foaming agent, dicumyl peroxide (DCP) crosslinking agent, zinc oxide (ZnO) activator, stearic acid lubricant, and calcium carbonate filler. However, this traditional formulation has significant performance bottlenecks. Specifically, these bottlenecks manifest in the following aspects:

[0004] The contradiction between hardness and comfort: General-purpose EVA foam materials (with a hardness of 38±5 degrees) often require high hardness and compressive strength to ensure a certain load-bearing capacity and resistance to deformation.

[0005] Insufficient fatigue resistance and long-term durability: Under dynamic loading conditions, traditional EVA foam materials are prone to permanent compression deformation, resulting in high thickness loss and reduced cushioning performance. According to industry standards (such as the DS-247 fatigue test, 20,000 compressions under 750N pressure), although the thickness loss rate of traditional materials may be lower than the 12% threshold, their fatigue stability is poor, and abnormal thickness changes may even occur after the test (such as negative loss rate, indicating uncontrollable creep or failure of the structure). After long-term use, problems such as fracture and collapse are likely to occur.

[0006] Low hardness and high resilience / fatigue resistance are difficult to achieve simultaneously: To obtain a softer feel, existing technologies typically reduce material hardness by increasing the amount of foaming agent or plasticizer. However, this often leads to insufficient melt strength, large and uneven cell structure, resulting in poor resilience, a sharp increase in compression set, and deterioration of fatigue resistance. Therefore, developing an EVA foam material that combines low hardness, high resilience, excellent compression fatigue resistance, and good dimensional stability is a pressing technical challenge in the current bag and footwear materials industry. Summary of the Invention

[0007] The purpose of this invention is to provide a low-hardness, high-fatigue-resistance EVA composite foam material and its preparation method. This invention breaks through the traditional reinforcement approach relying on covalent crosslinking or hydrogen bonding, and for the first time proposes and successfully constructs a "metal-carboxylic acid coordination crosslinking network." Specifically, this invention introduces carboxylated nano-silica and soluble zinc salts (such as zinc acetate) into the existing EVA / TPU / SBS ternary composite system, and synergistically optimizes the amount of zinc oxide used, so that zinc ions (Zn... 2+ The metal undergoes a coordination complexation reaction with the carboxyl groups (-COOH) on the surface of nano-silica, forming stable metal-carboxylic acid coordination bonds. These coordination bonds construct a high-density ionic cross-linked region around the nanoparticles, while the nanoparticles connect with each other via the "nanoparticle → Zn" pathway. 2+ →The bridging effect of nanoparticles forms a three-dimensional coordination cross-linking network.

[0008] To achieve the above objectives, this invention provides a low-hardness, high-fatigue-resistance EVA composite foam material, comprising the following components by weight: 35-45 parts ethylene-vinyl acetate polymer, 17-23 parts thermoplastic polyurethane elastomer, 13-17 parts butadiene-styrene copolymer, 3-5 parts zinc oxide, 7-8 parts lubricant, 11-12 parts azodicarbonamide, 4-6 parts filler, 2-5 parts carboxylated nano-silica, 0.5-2 parts soluble zinc salt, and 0.5-1.5 parts dicumyl peroxide.

[0009] Optionally, the lubricant includes one or more of stearic acid, glyceryl monostearate, and calcium stearate.

[0010] Optionally, the filler includes one or more of calcium carbonate, talc, mica powder, and kaolin.

[0011] Optionally, the soluble zinc salt includes one or more of zinc acetate, zinc sulfate, or zinc chloride.

[0012] Optionally, the soluble zinc salt is zinc acetate, and the amount added is 0.5-1.5 parts.

[0013] Optionally, the thermoplastic polyurethane elastomer is a polyester-type or polyether-type TPU.

[0014] Optionally, the butadiene-styrene copolymer is a styrene-butadiene-styrene block copolymer (SBS) or styrene-butadiene rubber (SBR).

[0015] Optionally, the particle size of the carboxylated nano-silica is 20-80 nm.

[0016] This invention also provides a method for preparing a low-hardness, high-fatigue-resistance EVA composite foam material, comprising the following steps:

[0017] S1. Mixing: Weigh all raw materials according to the stated weight proportions and put them into a two-roll mill or internal mixer for uniform mixing; control the mixing temperature at 100-120℃ for 10-20 minutes until a homogeneous, particle-free colloidal mixture is formed, and then sheet it out; during this process, soluble zinc salts (such as zinc acetate) may partially decompose or disperse in the matrix upon heating, but zinc ions (Zn) will remain. 2+ It can be retained and coordinate with the carboxyl groups formed subsequently; key point: the mixing temperature should not exceed 130℃ to prevent the AC foaming agent from decomposing prematurely, and at the same time avoid excessive decomposition of zinc acetate;

[0018] S2. Molding and preheating: Cut the mixed sheet material and place it into the mold. Put the mold into the flat vulcanizing machine, close the mold, apply a pressure of 5-10 MPa, and start heating to raise the mold temperature to 150℃-160℃.

[0019] S3. Crosslinking and Coordination Network Formation: When the temperature reaches the decomposition temperature range of the crosslinking agent (150℃-160℃), chemical crosslinking occurs between the EVA, TPU, and SBS molecular chains, forming a preliminary three-dimensional network structure; simultaneously, Zn... 2+ Ions undergo coordination complexation reactions with carboxyl groups (-COOH) on the surface of nano-silica and other oxygen-containing functional groups in the system (such as urethane groups of TPU and carboxyl groups of stearic acid) to construct a metal-carboxylic acid coordination cross-linking network in situ. This coordination network interpenetrates with the covalent cross-linking network to form an interpenetrating double network structure.

[0020] S4. Foaming: Under the strong activation of zinc oxide, the decomposition temperature of azodicarbonamide is reduced to about 160℃, and it decomposes rapidly to produce a large amount of nitrogen gas. At this time, since the polymer melt has sufficient strength (contributed by covalent crosslinking and coordination crosslinking), and nano-silica provides a large number of heterogeneous nucleation sites, the gas is effectively encapsulated, nucleated and expanded to form a uniform and dense closed-cell structure. The temperature is maintained at 160℃-165℃ for 25-35 minutes during this stage.

[0021] S5. Mold opening, pressure release, and shaping: Quickly open the flat vulcanizing machine to release the pressure inside the mold; the foam undergoes secondary expansion under the instantaneous pressure drop, and then cools and shapes to obtain the final low-hardness, ultra-high fatigue-resistant EVA composite foam material product.

[0022] The low-hardness, high-fatigue-resistance EVA composite foam material provided in this invention has at least one of the following technical effects:

[0023] 1. Excellent fatigue resistance: This invention constructs Zn 2+ The carboxyl-coordinated crosslinking network fundamentally improves the dynamic fatigue resistance of the material. The moderate dynamic reversibility of the coordination bonds allows for efficient energy dissipation and rapid recovery during long-term compression cycles, fundamentally solving the problem of fatigue collapse in low-hardness foamed materials. In the DS-247 fatigue test (20,000 repeated compressions under 750N pressure), the thickness loss rate of the material of this invention is as low as 0.35%, far below the 12% standard line for traditional materials, and significantly superior to existing materials using hydrogen bond crosslinking technology (typically with a thickness loss rate of 0.5%-0.8%).

[0024] 2. Ideal low hardness and high comfort: The hardness of the material of this invention can still be controlled at 22±5 degrees (Shore C or ASKER C type hardness), and the CC40 compression test value is precisely controlled within the ideal comfort range of 80-90KPa. Even in the presence of a coordination cross-linking network, by optimizing the amount of nano-silica and zinc ions, excessive increase in material rigidity is avoided, achieving a perfect balance between soft touch and excellent support.

[0025] 3. Uniform and fine cell structure: Carboxylated nano-silica, as a heterogeneous nucleating agent, provides a large number of uniformly dispersed nucleation sites during the foaming process, which significantly refines the cell size (the average pore diameter is reduced from 80-120μm in traditional materials to 40-60μm) and increases the cell density by 2-3 times. The fine and uniform cell structure further enhances the material's resistance to compression deformation and resilience.

[0026] 4. Synergistic Reinforcement Through Multiple Crosslinking: In this invention, covalent crosslinking provides basic structural strength and dimensional stability, while coordination crosslinking (Zn) provides additional strength and stability. 2+ -COOH provides dynamically reversible energy dissipation and rapid recovery capabilities, while nano-silica provides physical reinforcement and nucleation; the three reinforcement mechanisms work synergistically, enabling the material to achieve high performance at low densities. Under the premise of low hardness, it achieves comprehensive mechanical properties far exceeding those of existing technologies. Detailed Implementation

[0027] The embodiments of the present invention are described in detail below, and are intended to explain the embodiments of the present invention, but should not be construed as limiting the present invention.

[0028] Performance testing methods:

[0029] Hardness: According to ASTM D2240 standard, ASKER C type hardness tester was used to test after being placed in an environment of 23±2°C for 24 hours.

[0030] Density: Tested using an electronic densitometer in accordance with ASTM D297 standard.

[0031] Compression test (CC40): According to the DS-0452 standard, compress the sample to 40% of its original thickness and record the pressure (kPa) required at this time.

[0032] Fatigue test: According to the DS-247 standard, a dynamic fatigue testing machine was used to compress the specimen 20,000 times with a cyclic pressure of 750N and a frequency of 5Hz. After the test and 24 hours of recovery, the thickness of the central area of ​​the specimen was measured and the thickness loss rate was calculated. Thickness loss rate (%) = (initial thickness - thickness after test) / initial thickness × 100%.

[0033] Tensile elongation at break: Tested using a universal tensile testing machine at a tensile speed of 500 mm / min, in accordance with GB / T 6344-2008 standard.

[0034] Example 1

[0035] An EVA composite foam material based on metal ion coordination crosslinking has the following formulation by weight parts:

[0036] EVA: 40 servings

[0037] Polyester-type TPU: 20 parts

[0038] SBS: 15 copies

[0039] Zinc oxide: 4.0 parts

[0040] Stearic acid: 7.5 parts

[0041] Zinc acetate: 1.0 part

[0042] Carboxylated nano-silica with a particle size of 20-50 nm: 3.0 parts

[0043] AC foaming agent: 11.5 parts

[0044] Light calcium carbonate: 5 parts

[0045] DCP: 1.0 copy.

[0046] The method for preparing the composite foamed material:

[0047] S1. Mixing: EVA, TPU, SBS, zinc oxide, stearic acid, calcium carbonate, and DCP are added to a Banbury mixer and mixed at 110°C for 8 minutes. Then, carboxyl-functionalized nano-silica and zinc acetate are added, and mixing continues for 5 minutes to ensure uniform dispersion of all components, forming a gel-like mass. This mass is then pressed into sheets approximately 8 mm thick using a two-roll mill. It should be noted that zinc acetate is added in the final stage to reduce heat history.

[0048] S2. Molding: After cutting the sheet material, place it into a mold with dimensions of 300mm×300mm×15mm, place it on a flat vulcanizing machine preheated to 155°C, apply a pressure of 8 MPa, and raise the temperature.

[0049] S3. Crosslinking and Coordination Network Formation: After the mold temperature stabilizes at 160°C, maintain the temperature and pressure for 30 minutes. During this period, DCP decomposes, initiating covalent crosslinking, while Zn... 2+ It undergoes a coordination reaction with the carboxyl groups on the surface of nano-silica to form a coordination cross-linking network.

[0050] S4. Foaming: The AC foaming agent decomposes under the activation of zinc oxide and stearic acid, generating gas and forming uniform cells in the double-network reinforced melt.

[0051] S5. Mold Opening: After the heat preservation is completed, the mold is quickly removed from the vulcanizing machine and opened quickly. The foam expands instantly, resulting in a soft foam board with a thickness of about 25mm.

[0052] Example 2

[0053] The formula is basically the same as in Example 1, except that the amount of carboxylated nano-silica is adjusted. The specific formula is as follows:

[0054] EVA: 40 servings

[0055] Polyester-type TPU: 20 parts

[0056] SBS: 15 copies

[0057] Zinc oxide: 4.0 parts

[0058] Stearic acid: 7.5 parts

[0059] Zinc acetate: 1.0 part

[0060] Carboxylated nano-silica with a particle size of 20-50 nm: 2.0 parts

[0061] AC foaming agent: 11.5 parts

[0062] Light calcium carbonate: 5 parts

[0063] DCP: 1.0 copy.

[0064] The preparation method is the same as in Example 1.

[0065] Example 3

[0066] The formula is basically the same as in Example 1, except that the amount of carboxylated nano-silica is adjusted. The specific formula is as follows:

[0067] EVA: 40 servings

[0068] Polyester-type TPU: 20 parts

[0069] SBS: 15 copies

[0070] Zinc oxide: 4.0 parts

[0071] Stearic acid: 7.5 parts

[0072] Zinc acetate: 1.0 part

[0073] Carboxylated nano-silica with a particle size of 20-50 nm: 5.0 parts

[0074] AC foaming agent: 11.5 parts

[0075] Light calcium carbonate: 5 parts

[0076] DCP: 1.0 copy.

[0077] The preparation method is the same as in Example 1.

[0078] Example 4 (Different soluble zinc salts)

[0079] This example uses zinc sulfate. The zinc acetate was substituted, and the rest of the formulation was the same as in Example 1.

[0080] Preparation method: Same as in Example 1.

[0081] Comparative Example 1 (no DCP, only coordination crosslinking)

[0082] To verify the individual contribution of the coordination crosslinking network, no DCP crosslinking agent was added in this embodiment, and the rest of the formulation was the same as in Example 1.

[0083] Preparation method: Same as in Example 1, except that DCP is omitted.

[0084] Comparative Example 2 (simulating the hydrogen bond crosslinking scheme of CN202311324653.0)

[0085] To compare the effects of this invention with existing hydrogen bonding crosslinking technologies, this comparative example references the core idea of ​​CN202311324653.0, adding carboxylated EVA (EVA-COOH) to the basic formulation and restoring the zinc oxide content to 6.5 parts, without adding the carboxylated functionalized nano-silica and soluble zinc salt of this invention. The formulation is as follows:

[0086] EVA: 30 servings

[0087] EVA-COOH: 10 portions

[0088] Polyester-type TPU: 20 parts

[0089] SBS: 15 copies

[0090] Zinc oxide: 6.5 parts

[0091] Stearic acid: 7.5 parts

[0092] AC foaming agent: 11.5 parts

[0093] Light calcium carbonate: 5 parts

[0094] DCP: 1.0 copy

[0095] Preparation method: Same as in Example 1.

[0096] Comparative Example 3 (no soluble zinc salt added, only carboxyl-functionalized nano-silica added)

[0097] To demonstrate the necessity of soluble zinc salts, the formulation of this comparative example is essentially the same as that of Example 1, but zinc acetate is not added, and the amount of zinc oxide is restored to 6.5 parts (although the total zinc content is not low, zinc oxide is insoluble in the polymer matrix, making it difficult to effectively release Zn). 2+ (Coordination with carboxyl groups). Formulation:

[0098] EVA: 40 servings

[0099] Polyester-type TPU: 20 parts

[0100] SBS: 15 copies

[0101] Zinc oxide: 6.5 parts

[0102] Stearic acid: 7.5 parts

[0103] Carboxylated nano-silica with a particle size of 20-50 nm: 3.0 parts

[0104] AC foaming agent: 11.5 parts

[0105] Light calcium carbonate: 5 parts

[0106] DCP: 1.0 copy.

[0107] Comparative Example 4 (no carboxyl-functionalized nano-silica added, only zinc acetate added)

[0108] To demonstrate the necessity of carboxyl-functionalized nano-silica, the formulation of this comparative example is essentially the same as that of Example 1, but without the addition of carboxyl-functionalized nano-silica. Formulation:

[0109] EVA: 40 servings

[0110] Polyester-type TPU: 20 parts

[0111] SBS: 15 copies

[0112] Zinc oxide: 6.5 parts

[0113] Stearic acid: 7.5 parts

[0114] Zinc acetate: 1.0 part

[0115] AC foaming agent: 11.5 parts

[0116] Light calcium carbonate: 5 parts

[0117] DCP: 1.0 copy.

[0118] Table 1 shows the performance comparison of each embodiment and each comparative example.

[0119]

[0120] Examples 1-4 of this invention all achieved low hardness (22-24 degrees) and an ideal CC40 value (84-90 kPa), while exhibiting significantly lower fatigue thickness loss rates than Comparative Example 2 (hydrogen bond crosslinking technology). The thickness loss rates of Examples 1 and 3 were as low as 0.28%-0.32%, only about half that of Comparative Example 2. A comparison of Examples 1 and 4 shows that zinc sulfate can also provide Zn. 2+ It forms coordination crosslinks, but the effect is slightly inferior to that of zinc acetate. Zinc acetate is the preferred option due to its better dispersibility in organic systems and moderate thermal stability.

[0121] Compared to Example 1, Comparative Example 1 had no DCP, only coordination crosslinking, no covalent crosslinking agent was added, and it relied solely on Zn. 2+ - The carboxyl coordination crosslinking network reduces the fatigue resistance of the material, which fully demonstrates that the present invention effectively improves the comprehensive performance of the composite foam material by forming a dual network through covalent crosslinking and coordination crosslinking.

[0122] Comparative Example 3 lacked soluble zinc salt, and Comparative Example 4 lacked carboxylated nano-silica. The fatigue resistance of both decreased significantly, which proves the synergistic necessity of the combination of "carboxylated nano-silica + soluble zinc salt" in this invention.

[0123] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-hardness, high-fatigue-resistance EVA composite foam material, characterized in that, The product comprises the following components by weight: 35-45 parts ethylene-vinyl acetate polymer, 17-23 parts thermoplastic polyurethane elastomer, 13-17 parts butadiene-styrene copolymer, 3-5 parts zinc oxide, 7-8 parts lubricant, 11-12 parts azodicarbonamide, 4-6 parts filler, 2-5 parts carboxylated nano silica, 0.5-2 parts soluble zinc salt, and 0.5-1.5 parts dicumyl peroxide.

2. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 1, characterized in that, The lubricant includes one or more of stearic acid, glyceryl monostearate, and calcium stearate.

3. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 1, characterized in that, The filler includes one or more of calcium carbonate, talc, mica powder, and kaolin.

4. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 1, characterized in that, The soluble zinc salt includes one or more of zinc acetate, zinc sulfate, or zinc chloride.

5. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 4, characterized in that, The soluble zinc salt is zinc acetate, and its addition amount is 0.5-1.5 parts.

6. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 1, characterized in that, The thermoplastic polyurethane elastomer is a polyester-type or polyether-type TPU.

7. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 1, characterized in that, The butadiene-styrene copolymer includes one or more of styrene-butadiene-styrene block copolymers or styrene-butadiene rubber.

8. The low-hardness, high-fatigue-resistance EVA composite foam material according to claim 1, characterized in that, The particle size of the carboxylated nano-silica is 20-80 nm.

9. A method for preparing a low-hardness, high-fatigue-resistance EVA composite foam material according to any one of claims 1-8, characterized in that, Includes the following steps: S1. Mixing: Weigh all raw materials according to the stated weight proportions and put them into an open mill or internal mixer for uniform mixing; control the mixing temperature at 100℃-120℃ for 10-20 minutes until a uniform, particle-free colloidal mixture is formed and then sheeted out. S2. Molding and preheating: Cut the mixed sheet material and place it into the mold. Put the mold into the flat vulcanizing machine, close the mold, apply a pressure of 5-10 MPa, and start heating to raise the mold temperature to 150℃-160℃. S3. Crosslinking and Coordination Network Formation: When the temperature reaches the decomposition temperature range of the crosslinking agent (150℃-160℃), chemical crosslinking occurs between the EVA, TPU, and SBS molecular chains, forming a preliminary three-dimensional network structure; simultaneously, Zn... 2+ Ions undergo coordination complexation reactions with carboxyl groups on the surface of nano-silica and other oxygen-containing functional groups in the system, thereby constructing a metal-carboxylic acid coordination crosslinking network in situ. S4. Foaming: Under the strong activation of zinc oxide, the decomposition temperature of azodicarbonamide is reduced to about 160℃, and it decomposes rapidly to produce a large amount of nitrogen gas; the temperature is maintained at 160℃-165℃ for 25-35 minutes during this stage. S5. Mold opening, pressure release, and shaping: Quickly open the flat vulcanizing machine to release the pressure inside the mold; the foam undergoes secondary expansion under the instantaneous pressure drop, and then cools and shapes to obtain the final low-hardness, ultra-high fatigue-resistant EVA composite foam material product.

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