Recyclable PVA / biomass thermoplastic foam material

Abstract

The application discloses a recyclable PVA / biomass thermoplastic foaming material and belongs to the technical field of high polymer foaming materials. The material is made of the following components: 100 parts of polyvinyl alcohol, 10-30 parts of modified straw powder, 5-20 parts of modified shell powder, 20-40 parts of plasticizer, 2-5 parts of thermal decomposition inhibitor and 0.5-3 parts of coordination metal salt. The coordination metal salt is used to build a coordination bond between PVA and straw lignin, improve the interface compatibility and increase the melt strength. The supercritical CO2 foaming is used to obtain foaming beads with fine and dense cells. The in-situ polymerization is used to introduce a polyacrylic second network into the cell skeleton to form an interpenetrating network structure. Finally, the high-temperature welding is used to realize the integrated forming. The foaming material prepared by the application can be widely used in cold chain transportation packaging field and has remarkable environmental and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of polymer foaming material technology, specifically relating to a recyclable PVA / biomass thermoplastic foaming material. Background Technology

[0002] Cold chain logistics is a crucial link in ensuring the quality and safety of temperature-sensitive products such as fresh food and pharmaceuticals. Among these, cold storage materials are a core element of cold chain transportation, and their performance directly affects the efficiency of the entire cold chain system. Currently, commonly used cold storage materials for short-distance cold chain transportation are ice packs, ice cubes, or traditional gel cold storage agents. However, ice packs and ice cubes have drawbacks such as being inconvenient to carry, taking up a lot of space, and the water flowing out after melting can easily contaminate products; while traditional hydrogels have good water retention, they have poor mechanical properties, require special packaging, are difficult to recycle, and are mostly non-biodegradable.

[0003] Polyvinyl alcohol (PVA), a water-soluble and biodegradable polymer, possesses excellent film-forming properties, gas barrier properties, and biocompatibility, making it widely used in packaging, biomedicine, and other fields. However, due to the strong intramolecular and intermolecular hydrogen bonds, PVA's melting temperature (approximately 220-240℃) is close to its decomposition temperature (approximately 200-250℃), resulting in an extremely narrow thermoplastic processing window and making it difficult to foam through traditional extrusion or injection molding. To address the processing challenges of PVA, researchers have attempted solution foaming and chemical crosslinking modifications, but these methods suffer from drawbacks such as complex processes, long production cycles, and difficulty in continuous production. Supercritical carbon dioxide foaming technology is widely used in polyolefins, but in polar PVA matrices, CO2 adsorption capacity is poor, making foaming behavior difficult to control.

[0004] Patent application CN107353425A discloses a method for manufacturing a whole-straw foamed material, but this technology uses steam as a foaming agent, resulting in coarse cell structures and limited mechanical properties. Patent CN117343378A discloses a method for enhancing the foaming performance of polyvinyl alcohol (PVA) with cellulose nanofibers using supercritical carbon dioxide. While this method achieves cell refinement, it does not solve the interfacial bonding problem between PVA and the filler, and it does not address the construction of a multi-level pore structure. Furthermore, the molding of PVA foam beads typically uses steam welding, but due to the low temperature of the steam, only the PVA surface swells slightly, resulting in weak interfacial bonding between beads and poor overall mechanical properties of the product, making it difficult to meet the requirements for cold chain transport containers.

[0005] In summary, existing technologies generally suffer from poor interfacial compatibility, coarse cell structure, and insufficient recyclability, making it difficult to achieve a balance between the overall performance of the material. There is an urgent need to develop a thermoplastic foam material that can resolve the contradiction between high water absorption, high strength, and recyclability. Summary of the Invention

[0006] The purpose of this invention is to provide a recyclable PVA / biomass thermoplastic foam material to solve the problems of difficult thermoplastic processing, poor mechanical properties, insufficient water absorption and retention capacity, and difficulty in recycling of existing PVA foam materials.

[0007] The objective of this invention can be achieved through the following technical solutions: A recyclable PVA / biomass thermoplastic foam material, comprising the following components by weight: 100 parts of polyvinyl alcohol (PVA); 10-30 parts modified straw powder; 5-20 parts of modified shell powder; Plasticizer 20-40 parts; 2-5 parts of thermal decomposition inhibitor; 0.5-3 parts of coordination metal salt; The modified straw powder is straw powder that has been treated with alkali and surface-modified with a silane coupling agent, with a particle size of 500-800 mesh; The modified seashell powder is seashell powder that has been calcined and ball-milled, with a particle size of 800-1000 mesh; The coordination metal salt is selected from Mg²⁺. + Ca² + or Zn² + Salts.

[0008] By constructing a system of "coordination bond enhancement-biomass synergistic nucleation," the synergistic optimization of interfacial chemical reinforcement and cell structure regulation is achieved in the PVA matrix, thereby solving the problems of poor interfacial compatibility, insufficient mechanical properties, and weak water absorption and retention capacity of traditional PVA / biomass composites. The aforementioned proportion range ensures that the PVA matrix possesses both thermoplastic processability and hydrophilicity, providing suitable melt strength and reaction sites for subsequent foaming. The cellulose and lignin in modified straw powder contain abundant hydroxyl groups, which can form hydrogen bonds with PVA molecular chains, playing a physical cross-linking and reinforcing role. Modified shell powder, after calcination and activation, forms a porous structure with surface activity, acting as a heterogeneous nucleating agent to refine cell pores, and its microporous structure significantly improves the material's water absorption and retention capacity through capillary action. Coordination metal salts utilize the phenolic hydroxyl groups in straw lignin and the hydroxyl groups on PVA molecular chains to form coordination bonds with metal ions, constructing a dynamic physical cross-linking network, fundamentally solving the problem of poor interfacial compatibility between PVA and biomass fillers.

[0009] Furthermore, the degree of hydrolysis of the polyvinyl alcohol is 88%-92%, and the degree of polymerization is 1500-2000. PVA with excessively high hydrolysis has high crystallinity, making thermoplastic processing difficult; while PVA with excessively low hydrolysis has insufficient hydrophilicity, affecting its water absorption and retention properties.

[0010] Furthermore, the modified straw powder is prepared by the following steps: Straw powder is soaked in a 3-8 wt% alkaline solution for 1-3 hours, washed until neutral, dried, and then placed in an ethanol solution with 1-3% (w / w) of silane coupling agent KH550 or KH560 added. The mixture is reacted at 50-70℃ for 0.5-2 hours, filtered, and dried to obtain the final product. Alkali treatment removes the waxy layer and some lignin from the straw surface, exposing more hydroxyl groups; the silane coupling agent modification introduces functional groups that can react with PVA onto the straw surface, further improving interfacial bonding.

[0011] Furthermore, the modified seashell powder is prepared by the following steps: Waste seashells are cleaned and crushed, then calcined at 700-900℃ for 1-3 hours, followed by wet ball milling for 6-10 hours. After drying, micro- and nano-sized modified seashell powder is obtained. The calcination process decomposes CaCO3 into CaO, generating a large number of microporous structures and significantly increasing the specific surface area. The ball milling process further refines the particle size to the submicron level, which is beneficial for uniform dispersion in the PVA matrix.

[0012] Furthermore, the plasticizer is at least one of glycerol, sorbitol, and polyethylene glycol; the thermal decomposition inhibitor is at least one of calcium stearate, zinc stearate, or pentaerythritol; and the coordinating metal salt is at least one of magnesium chloride, calcium chloride, or zinc acetate.

[0013] Furthermore, the preparation method of the recyclable PVA / biomass thermoplastic foam material includes the following steps: S1. PVA, modified straw powder, modified shell powder, plasticizer, thermal decomposition inhibitor and coordination metal salt are mixed evenly according to the ratio, and then melt-blended and extruded to granulate to obtain PVA / biomass composite material particles. S2. Place the composite material particles obtained in step S1 in a high-pressure reactor, introduce CO2, heat to 120-150℃, pressurize to 12-20MPa, so that CO2 reaches a supercritical state, maintain pressure for 2-5 hours for infiltration, then quickly release pressure to foam, and cool to obtain PVA / biomass composite foamed beads. S3. Immerse the foamed beads obtained in step S2 into a solution containing acrylic monomers, crosslinking agents and initiators. After vacuum-assisted impregnation, heat up to carry out in-situ polymerization reaction to form a second network in the foam skeleton. After washing and drying, composite foam beads are obtained. S4. Fill the composite foam beads obtained in step S3 into the mold, heat to 170-230℃ to fuse the bead interface, hold pressure, cool and demold to obtain recyclable PVA / biomass thermoplastic foam material.

[0014] In the composite material preparation stage, PVA is combined with modified straw powder and modified shell powder in Mg... 2+Under the influence of various factors, the mixture is melt-blended through coordination bonds, improving interfacial compatibility and increasing melt strength; calcined shell powder acts as a heterogeneous nucleating agent for uniform dispersion. During the supercritical CO2 foaming stage (120-150℃, 12-20MPa), the dynamic reversibility of coordination bonds allows for moderate molecular chain movement while maintaining melt strength. Upon depressurization, the shell powder induces cell refinement, forming a uniform microporous structure. In the in-situ polymerization stage, vacuum impregnation allows acrylic monomers to enter the cell framework, polymerizing to form a second polyacrylic acid network, which, together with PVA, constitutes an interpenetrating network, compensating for the decrease in water absorption during recycling. In the high-temperature welding stage (170-230℃), the thermal dissociation of coordination bonds promotes interfacial molecular chain diffusion, and upon cooling, rebonding achieves chemical-physical dual interfacial healing, with a weld strength exceeding 85% of the matrix.

[0015] Furthermore, in step S2, the foaming temperature is 125-135℃, the foaming pressure is 14-18MPa, and the pressure holding and permeation time is 2.5-3.5h. This parameter range ensures optimal solubility and diffusion rate of CO2 in the PVA matrix, while the coordination crosslinking network provides sufficient melt strength, resulting in finely pore-rich and uniformly distributed foamed beads.

[0016] Furthermore, in step S3, the acrylic monomer is at least one of acrylic acid, methacrylic acid, or acrylamide; the crosslinking agent is N,N'-methylenebisacrylamide; the initiator is potassium persulfate or ammonium persulfate; and the in-situ polymerization reaction temperature is 50-70°C, and the reaction time is 1-3 hours.

[0017] Furthermore, the heating method described in step S4 is high-frequency electromagnetic heating or hot air heating, the heating time is 10-30 minutes, and the welding pressure is 0.2-0.5 MPa.

[0018] Furthermore, the recyclable PVA / biomass thermoplastic foam material is used in cold chain transportation packaging.

[0019] The beneficial effects of this invention are: (1) This invention improves the performance of PVA / biomass foam materials by precisely designing the component ratio and material structure. In terms of component composition, 100 parts of PVA are used as the core skeleton. The combination of parameters such as the degree of hydrolysis of 88%-92% and the degree of polymerization of 1500-2000 makes the molecular chain have both suitable thermoplastic processability and hydrophilicity, providing a molecular basis for subsequent foaming. The ratio of 10-30 parts of modified straw powder and 5-20 parts of modified shell powder ensures the uniform dispersion and synergistic enhancement of biomass filler in PVA matrix. Combined with the plasticizing effect of 20-40 parts of plasticizer, the melt flowability and processing stability of composite materials are optimized. The introduction of 0.5-3 parts of coordination metal salt constructs a dynamic physical cross-linking network, which fundamentally solves the interfacial compatibility problem between PVA and biomass filler.

[0020] (2) This invention achieves multiple effects through the introduction of coordination metal salts. Mg 2+ The coordination bonds formed with the hydroxyl groups of PVA and the phenolic hydroxyl groups of straw lignin not only significantly improve the melt strength and thermal stability of PVA, enabling the composite material to be stably extruded and granulated within the temperature range of 160-195℃, but also enhance the composite material's CO2 adsorption capacity, providing more nucleation sites for foaming, refining the cell diameter, and increasing the cell density. The dynamic reversibility of this coordination structure allows for dissociation during high-temperature welding, promoting molecular chain diffusion, and reformation upon cooling, achieving interfacial chemical healing and maintaining weld strength.

[0021] (3) The ratio of 10-30 parts modified straw powder to 5-20 parts modified shell powder used in this invention ensures that the biomass filler exerts a synergistic nucleation effect. After alkali treatment and modification with silane coupling agent, the surface active hydroxyl density of the straw powder increases, forming strong hydrogen bonds with PVA. After calcination and activation, the shell powder forms a porous CaO structure with a high specific surface area. Its microporous structure significantly enhances the water absorption and retention capacity of the material through capillary action, and works synergistically with the hydration of the PVA skeleton to improve the water absorption rate. This multi-dimensional synergistic effect results in the formation of a composite network structure with both physical entanglement and chemical cross-linking after film formation, which not only significantly improves the balance of the material's mechanical properties, but also greatly enhances its structural stability during repeated use. Detailed Implementation

[0022] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0023] Throughout the preparation process, the order of addition of each component, the temperature range, the stirring rate, and the reaction time were strictly controlled. The coordination metal salt must be added simultaneously with PVA during the melt blending stage to ensure the presence of Mg. 2+ To ensure sufficient contact with hydroxyl groups and the formation of coordination bonds, the supercritical foaming temperature must be controlled between 120-150℃. This ensures that CO2 reaches a supercritical state while avoiding excessive dissociation of coordination bonds and a decrease in melt strength due to excessive temperature. In-situ polymerization must be carried out after foaming to ensure that the second network is uniformly distributed within the foam skeleton. The high-temperature welding temperature must be controlled between 170-230℃ to ensure sufficient diffusion and welding of PVA molecular chains while avoiding thermal decomposition of the material due to excessive temperature. Furthermore, the calcination temperature of the modified shell powder determines its pore structure and surface activity. If the calcination temperature is too low (<700℃), CaCO3 decomposes incompletely, resulting in limited increase in specific surface area. If the calcination temperature is too high (>900℃), CaO grains coarsen, and surface activity decreases.

[0024] The obtained PVA / biomass foam material has a cross-linked structure consisting of three parts: first, a physical cross-linked network formed by hydrogen bonds of the PVA main chain; second, Mg... 2+ The material exhibits excellent structural stability during repeated water absorption-drying cycles due to the reversibility of the coordination bonds and the uniform distribution of the second network, which are formed by coordination bonds with hydroxyl groups and the in-situ polymerization of polyacrylic acid-based second networks.

[0025] In the technical solution described in this invention, all operational steps, material ratios, and process parameters are essential technical features for achieving the desired technical effect, and none can be omitted. If the coordination metal salt is omitted, the interfacial bonding between PVA and biomass filler is weak, resulting in a significant decrease in mechanical properties; if modified shell powder is omitted, the number of cell nucleation points decreases, leading to larger cells and reduced water absorption; if in-situ polymerization to construct a double network is omitted, the material's water retention rate rapidly decreases during recycling; if the high-temperature welding temperature is below 170°C, the bead interface welding is insufficient, resulting in poor mechanical properties of the product. Therefore, this invention, through the synergy of coordination bond enhancement design, biomass synergistic nucleation, double-network water retention construction, and high-temperature welding technology, achieves improvements in the mechanical properties, water absorption and retention properties, and recyclability of PVA / biomass foam materials, resolving long-standing technical contradictions.

[0026] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0027] Example 1

[0028] This embodiment provides a recyclable PVA / biomass thermoplastic foam material, prepared through the following steps: S1. Pretreatment of biomass materials: (1) Preparation of modified straw powder: Take 100g of straw powder (500 mesh), soak it in 5wt% NaOH solution for 2h, wash it with deionized water until neutral, and dry it at 60℃ for 12h. Place the alkali-treated straw powder in 500mL of ethanol, add 2g of silane coupling agent KH560, react at 60℃ for 1h, filter, and dry at 60℃ to obtain modified straw powder (particle size 500 mesh).

[0029] (2) Preparation of modified shell powder: Take 200g of waste shells, clean and crush them, and place them in a muffle furnace. Calcinate at 800℃ for 2h and cool naturally. Place the calcined shell powder in a planetary ball mill, add deionized water and wet ball mill for 8h (ball-to-material ratio 10:1, speed 300rpm). Take out the slurry, dry at 100℃, grind and sieve to obtain modified shell powder (particle size 900 mesh).

[0030] S2. Add 100 parts PVA (PVA-1788, degree of hydrolysis 88%, degree of polymerization 1700), 20 parts modified straw powder, 10 parts modified shell powder, 30 parts plasticizer (glycerol: sorbitol = 2:1), 3 parts thermal decomposition inhibitor calcium stearate, and 1.5 parts magnesium chloride to a high-speed mixer and mix for 10 minutes. Then, feed the mixture into a twin-screw extruder for melt blending and extrusion. The screw speed is 150 rpm, and the temperatures of each section of the extruder are set as follows: Zone 1 165℃, Zone 2 175℃, Zone 3 185℃, Zone 4 190℃, and the die head 185℃. Extrusion granulation is performed to obtain PVA / biomass composite particles, which are then dried at 60℃ for 4 hours for later use.

[0031] S3. Place 50g of composite material particles in a 500mL high-pressure reactor and purge the air in the reactor three times with CO2. Heat to 130℃ and pressurize to 16MPa to bring the CO2 to a supercritical state, and maintain the pressure for 3 hours. Then, rapidly depressurize to atmospheric pressure within 2 seconds. The foamed particles foam during the depressurization process. After cooling, remove the particles to obtain PVA / biomass composite foamed beads.

[0032] S4. Preparation of polymerization solution: Dissolve 10g acrylic acid, 0.2g N,N'-methylenebisacrylamide, and 0.1g potassium persulfate in 100mL deionized water, and adjust the pH to 5.0 with NaOH. Immerse 20g of the foamed beads obtained in step S3 into the polymerization solution, and impregnate under vacuum (-0.08MPa) for 20min. Remove the beads, drain the surface solution, and place them in a 60℃ oven for 2h to carry out in-situ polymerization. After the reaction is complete, wash three times with deionized water and dry at 60℃ to constant weight to obtain composite foam beads.

[0033] S5. High-Temperature Fusion Molding: Composite foam beads are filled into a rectangular mold (200mm×150mm×50mm), heated to 200℃ using a high-frequency electromagnetic heating device, and held at that temperature for 15 minutes. Simultaneously, a pressure of 0.3MPa is applied to fuse the beads. After holding the pressure and cooling to room temperature, the material is demolded to obtain a recyclable PVA / biomass thermoplastic foam material (wall thickness 25mm).

[0034] Example 2

[0035] Compared with Example 1, the difference is that the amount of modified straw powder is 30 parts and the amount of modified shell powder is 5 parts, the rest are the same.

[0036] Example 3

[0037] Compared with Example 1, the difference is that the amount of modified straw powder is 10 parts and the amount of modified shell powder is 20 parts, the rest are the same.

[0038] Example 4

[0039] Compared with Example 1, the difference is that PVA-1792 (92% degree of alcoholysis) was used, and the rest are the same.

[0040] Example 5

[0041] Compared with Example 1, the difference is that the amount of magnesium chloride used is 0.5 parts, and the rest are the same.

[0042] Example 6

[0043] Compared with Example 1, the difference is that the amount of magnesium chloride used is 3.0 parts, and the rest are the same.

[0044] Example 7

[0045] Compared with Example 1, the difference is that the foaming temperature in step S3 is 125°C, and the rest are the same.

[0046] Example 8

[0047] Compared with Example 1, the difference is that the high-temperature welding temperature in step S5 is 180°C, and the rest are the same.

[0048] Comparative Example 1

[0049] The difference from Example 1 is that magnesium chloride is not added, but the rest are the same.

[0050] Comparative Example 2

[0051] Compared with Example 1, the difference is that modified shell powder is not added, the amount of modified straw powder is adjusted to 30 parts, and the rest are the same.

[0052] Comparative Example 3

[0053] Compared with Example 1, the difference is that the in-situ polymerization in step S4 is omitted, while the rest are the same.

[0054] Comparative Example 4

[0055] Compared with Example 1, the difference is that PVA-1799 (99% degree of alcoholysis) was used, and the rest are the same.

[0056] Comparative Example 5

[0057] Compared with Example 1, the difference is that the high-temperature welding temperature in step S5 is 150°C, and the rest are the same.

[0058] The recyclable PVA / biomass thermoplastic foam materials prepared in all examples and comparative examples were tested for performance according to the following methods: 1. Density measurement Refer to GB / T6343-2009 "Determination of Apparent Density of Foamed Plastics and Rubber". The sample size is 50mm × 50mm × 25mm. After conditioning at 23℃±2℃ and 50%±5% relative humidity for 24 hours, the mass is weighed, and the dimensions are measured to calculate the volume. Five samples are tested for each sample, and the arithmetic mean is taken.

[0059] 2. Compression strength determination

[0060] Refer to GB / T8813-2020 "Determination of Compression Properties of Rigid Foamed Plastics". The sample size is 50mm × 50mm × 25mm. The compression direction is perpendicular to the direction of cell elongation during foaming. The compression rate is 5mm / min. Record the compressive force when the compressive strain reaches 10%. Five samples are tested for each sample, and the arithmetic mean is taken.

[0061] 3. Water absorption rate determination

[0062] Refer to GB / T8810-2005 "Determination of Water Absorption Rate of Rigid Foamed Plastics". Weigh the dried sample (W0), immerse it in deionized water at 23℃±2℃ for 24 hours, remove it, wipe off the surface moisture, and weigh it (W1). The water absorption rate is calculated using the formula: WA=(W1-W0) / W0×100%. Five samples are tested for each sample, and the arithmetic mean is taken.

[0063] 4. Determination of latent heat of phase change

[0064] Differential scanning calorimetry (DSC) was used for determination. Sample amounts were 5-10 mg, under a nitrogen atmosphere, and the temperature was increased from -20℃ to 20℃ at a rate of 5℃ / min. The phase transition enthalpy was calculated by integration. Three samples were tested for each sample, and the arithmetic mean was taken.

[0065] 5. Cyclic performance test

[0066] The samples were subjected to 10 cycles of "water absorption-freezing-cooling-drying": 24 hours of water absorption until saturation, 12 hours of freezing at -20°C, 4 hours of thawing at room temperature, and 24 hours of drying at 60°C. After each cycle, the water retention rate (weighed after centrifugation at 1000 rpm for 5 minutes after saturation) and compressive strength were measured. The water retention rate retention rate (10th cycle water retention rate / 1st cycle water retention rate × 100%) and compressive strength retention rate (10th cycle compressive strength / 1st cycle compressive strength × 100%) after the 10th cycle were calculated. Five samples were tested for each sample, and the arithmetic mean was taken.

[0067] The results are shown in Table 1: Table 1

[0068] As can be seen from Table 1, all performance indicators of all embodiments (1-8) meet or exceed the requirements of the guidelines, proving that the technical solution of the present invention has good universality and adjustability.

[0069] The comparison of Examples 1-3 shows that the ratio of straw powder to shell powder can be adjusted for performance. Example 2 (30 parts straw powder) has the highest compressive strength (0.92 MPa), indicating that the straw powder has a significant effect on enhancing rigidity; Example 3 (20 parts shell powder) has the highest water absorption rate (720%) and the highest latent heat of phase change (301 J / g), indicating that the porous structure of the shell powder improves water absorption and heat capacity; Example 1 (20:10) has the best overall performance.

[0070] The comparison of Examples 1, 5, and 6 shows that magnesium chloride dosages of 0.5-3.0 parts are all acceptable, but 1.5 parts (Example 1) exhibits the best overall performance. Too low a dosage (0.5 parts) results in decreased compressive strength and cycle performance; a higher dosage (3.0 parts) still maintains excellent performance, demonstrating the wide adaptability of the coordinate bonds.

[0071] Example 1 (88%) and Example 4 (92%) showed comparable and excellent performance. Comparative Example 4 (99%) showed a comprehensive decline in performance, with a density of 0.26 g / cm³, compressive strength of 0.62 MPa, and water absorption of 480%, demonstrating that excessive crystallinity leads to processing difficulties and decreased hydrophilicity, thus verifying the necessity of the 88%-92% range.

[0072] Comparative Example 1 (without coordinating metal salt) showed a compressive strength lower than the guideline requirements, and its water absorption and water retention rate after 10 cycles were significantly worse than Example 1. This is because the lack of coordination bond reinforcement results in weak interfacial bonding between PVA and biomass filler, low stress transfer efficiency, and insufficient melt strength leading to cell merging and coarsening during foaming, increasing structural defects. Comparative Example 2 (without modified shell powder) showed a water absorption rate lower than the guideline requirements, and its compressive strength was also close to the lower limit, demonstrating the key contribution of shell powder to water absorption and mechanical reinforcement. The porous structure formed by calcination and activation of modified shell powder not only improves water absorption through capillary action, but its heterogeneous nucleation also significantly refines the cells, indirectly enhancing mechanical properties. Comparative Example 3 (without dual network) showed a water retention rate after 10 cycles that was far lower than Example 1, demonstrating the decisive role of the polyacrylic acid second network in maintaining the cyclic water absorption capacity. In the dual network structure, the strong water absorption of PAAS compensates for the decrease in water absorption caused by the change in crystallinity after repeated water absorption and drying of PVA, ensuring the recyclability of the material. As previously mentioned, Comparative Example 4 (excessive PVA hydrolysis) showed a comprehensive decline in performance. Comparative Example 5 (insufficient welding temperature) had a compressive strength of 0.58 MPa, slightly lower than the guideline requirement, demonstrating the necessity of high-temperature welding above 170°C. Only at this temperature can the coordination bonds be fully dissociated and the PVA molecular chains be fully diffused and entangled, achieving both chemical and physical interface healing.

[0073] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A recyclable PVA / biomass thermoplastic foam material, characterized in that, By weight, it includes the following components: 100 parts of polyvinyl alcohol (PVA); 10-30 parts modified straw powder; 5-20 parts of modified shell powder; Plasticizer 20-40 parts; 2-5 parts of thermal decomposition inhibitor; 0.5-3 parts of coordination metal salt; The modified straw powder is straw powder that has been treated with alkali and surface-modified with a silane coupling agent, with a particle size of 500-800 mesh; The modified seashell powder is seashell powder that has been calcined and ball-milled, with a particle size of 800-1000 mesh; The coordination metal salt is selected from Mg²⁺. + Ca² + or Zn² + Salts.

2. The recyclable PVA / biomass thermoplastic foam material according to claim 1, characterized in that, The degree of alcoholysis of the polyvinyl alcohol is 88%-92%, and the degree of polymerization is 1500-2000.

3. The recyclable PVA / biomass thermoplastic foam material according to claim 1, characterized in that, The modified straw powder is prepared by the following steps: Soak straw powder in a 3-8 wt% alkaline solution for 1-3 hours, wash until neutral, dry, then place in an ethanol solution, add 1-3% by mass of silane coupling agent KH550 or KH560, react at 50-70℃ for 0.5-2 hours, filter and dry to obtain the final product.

4. The recyclable PVA / biomass thermoplastic foam material according to claim 1, characterized in that, The modified seashell powder is prepared by the following steps: After cleaning and crushing the waste seashells, they are calcined at 700-900℃ for 1-3 hours, then wet-milled in a ball mill for 6-10 hours, and dried to obtain micro-nano-scale modified seashell powder.

5. The recyclable PVA / biomass thermoplastic foam material according to claim 1, characterized in that, The plasticizer is at least one of glycerol, sorbitol, and polyethylene glycol; the thermal decomposition inhibitor is at least one of calcium stearate, zinc stearate, or pentaerythritol; and the coordinating metal salt is at least one of magnesium chloride, calcium chloride, or zinc acetate.

6. The recyclable PVA / biomass thermoplastic foam material according to claim 1, characterized in that, The preparation method of the recyclable PVA / biomass thermoplastic foam material includes the following steps: S1. PVA, modified straw powder, modified shell powder, plasticizer, thermal decomposition inhibitor and coordination metal salt are mixed evenly according to the ratio, and then melt-blended and extruded to granulate to obtain PVA / biomass composite material particles. S2. Place the composite material particles obtained in step S1 in a high-pressure reactor, introduce CO2, heat to 120-150℃, pressurize to 12-20MPa, so that CO2 reaches a supercritical state, maintain pressure for 2-5 hours for infiltration, then quickly release pressure to foam, and cool to obtain PVA / biomass composite foamed beads. S3. Immerse the foamed beads obtained in step S2 into a solution containing acrylic monomers, crosslinking agents and initiators. After vacuum-assisted impregnation, heat up to carry out in-situ polymerization reaction to form a second network in the foam skeleton. After washing and drying, composite foam beads are obtained. S4. Fill the composite foam beads obtained in step S3 into the mold, heat to 170-230℃ to fuse the bead interface, hold pressure, cool and demold to obtain recyclable PVA / biomass thermoplastic foam material.

7. The recyclable PVA / biomass thermoplastic foam material according to claim 6, characterized in that, The foaming temperature in step S2 is 125-135℃, the foaming pressure is 14-18MPa, and the pressure holding and permeation time is 2.5-3.5h.

8. A recyclable PVA / biomass thermoplastic foam material according to claim 6, characterized in that, The acrylic monomer in step S3 is at least one of acrylic acid, methacrylic acid, or acrylamide; the crosslinking agent is N,N'-methylenebisacrylamide; the initiator is potassium persulfate or ammonium persulfate; the in-situ polymerization reaction temperature is 50-70℃, and the reaction time is 1-3h.

9. A recyclable PVA / biomass thermoplastic foam material according to claim 6, characterized in that, The heating method described in step S4 is high-frequency electromagnetic heating or hot air heating, the heating time is 10-30 minutes, and the welding pressure is 0.2-0.5 MPa.

10. A recyclable PVA / biomass thermoplastic foam material according to claim 1, characterized in that, The recyclable PVA / biomass thermoplastic foam material is used in cold chain transportation packaging.

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