Marine environment self-degradable plastic material, packaging bag and preparation method of packaging bag

Abstract

The present invention discloses a marine environment self-degradable plastic material, and relates to the technical field of degradable plastics, and the marine environment self-degradable plastic material comprises the following components by weight: 55-65 parts of polyvinyl alcohol; 8 to 12 parts of sodium alginate; 8 to 16 parts of shell powder; 3-10 parts of a plasticizer; 0.5 to 3 parts of compatilizer; the shell powder and the sodium alginate are applied to design of degradable plastics, high-concentration magnesium ions and sodium ions in seawater are used as natural triggering agents, response degradation of the material is achieved, the material is stable and can be normally used and stored in low-ion-strength environments such as fresh water or air, and the material can be normally used and stored in high-ion-strength environments such as seawater. Degradation is triggered, marine plastic pollution caused by long-term accumulation is avoided, and the problem that existing degradable plastic lacks environmental selectivity is solved.

Description

Technical Field

[0001] This invention relates to the technical field, specifically to a self-degradable plastic material for marine environments, packaging bags, and their preparation methods. Background Technology

[0002] With the continued growth in global consumption of plastic products, plastic pollution, especially marine plastic pollution, has become a major environmental problem that urgently needs to be addressed. Currently, biodegradable plastics on the market mainly include bio-based or biodegradable materials such as polylactic acid (PLA), polyhydroxyalkanoates (PHA), and polybutylene adipate / terephthalate (PBAT). However, these materials have many limitations: Although polylactic acid (PLA) is derived from renewable resources, its degradation requires specific composting conditions and is extremely slow in natural environments, especially marine environments. Polyhydroxyalkanoates (PHA), while possessing good biodegradability, are expensive to produce, costing 5-8 times more than traditional plastics, making large-scale adoption difficult. While PBAT can degrade in soil, its degradation rate in seawater is equally slow and requires stringent conditions.

[0003] In recent years, researchers have attempted to develop environmentally responsive degradable materials. For example, photodegradable plastics degrade under ultraviolet light by adding photosensitizers. However, these materials cannot degrade in the dark environment of the seabed, and the ultraviolet light triggering lacks selectivity, easily leading to misdegradation during daily use. Another example is pH-responsive degradable materials, which utilize the pH difference between seawater and freshwater to trigger degradation. However, seawater has a pH of only 8–8.3, not significantly different from freshwater's 7–7.5, resulting in insufficient trigger sensitivity, and misjudgment is common in transitional environments such as estuaries and saline lakes. Yet another approach is enzymatically degradable materials, which degrade through the activation of halophilic enzymes in high-salt environments. However, these enzymes have poor stability, short shelf life, and high cost.

[0004] Therefore, there is a need to develop a cost-effective plastic material that can stably trigger self-degradation in marine environments and is not prone to accidental degradation during normal use. Summary of the Invention

[0005] The purpose of this invention is to provide a cost-effective plastic material that can stably trigger self-degradation in a marine environment and is not prone to accidental degradation during normal use.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: On one hand, the present invention provides a marine environment self-degradable plastic material, comprising the following components in parts by weight: 55-65 parts of polyvinyl alcohol, 8-12 parts of sodium alginate, 8-16 parts of shell powder, 3-10 parts of plasticizer, 0.5-3 parts of compatibilizer, and 0.1-1 parts of lubricant.

[0007] Preferably, the polyvinyl alcohol has a degree of hydrolysis of 87-89% and a degree of polymerization of 1700-2000. Controlling the degree of hydrolysis to 87-89% ensures that a small number of acetate groups remain on the PVA molecular chain. The presence of these hydrophobic groups reduces the overall hydrophilicity of the material, improves its stability in freshwater environments, and does not significantly affect the material's biodegradability. The degree of polymerization of 1700-2000 ensures that the material has sufficient molecular weight, resulting in good mechanical strength after film formation, with a tensile strength reaching 15-20 MPa, meeting the requirements for packaging bags.

[0008] Preferably, the sodium alginate has a molecular weight of 100,000 to 500,000, and the molar ratio of mannuronic acid to guluronic acid is 1.2 to 2.5. The M / G ratio is a key structural parameter of sodium alginate. When the G unit content is high, the resulting cross-linked network is rigid but brittle; when the M unit content is high, the material has good flexibility but insufficient strength. Controlling the M / G ratio at 1.5 to 2.0 allows the material to maintain sufficient strength while possessing good flexibility and elongation at break, with an elongation at break reaching 150% to 250%. A molecular weight of 100,000 to 500,000 ensures that the sodium alginate has an appropriate chain length, which can form an effective interpenetrating network with PVA without causing excessively high solution viscosity due to excessively high molecular weight, making it difficult to process.

[0009] Preferably, the shell powder has a particle size of 1–10 μm and a calcium carbonate content of not less than 90%. The shell powder is derived from aquatic processing waste such as oyster shells, scallop shells, or clam shells, obtained through washing, drying, crushing, and sieving. The main component of the shell powder is calcium carbonate (CaCO3), with a purity greater than 90% and a particle size distribution of 1–10 μm. This particle size range is chosen based on the following considerations: excessively large particle sizes lead to uneven dispersion in the polymer matrix, affecting the material's uniformity and mechanical properties; excessively small particle sizes, such as less than 0.5 μm, while exhibiting good dispersibility, result in a large specific surface area that easily leads to agglomeration and may also cause slow dissolution in fresh water, affecting material stability. A particle size of 1–10 μm ensures both good dispersibility and stability in fresh water, while also allowing for rapid dissolution in seawater in response to magnesium ions.

[0010] Preferably, the shell powder has a bimodal particle size distribution, comprising fine shell powder of 1–3 μm and coarse shell powder of 8–10 μm. The coarse shell powder comprises 8–10 parts, and the fine shell powder comprises 2–5 parts. The coarse particles provide the primary physical cross-linking, resulting in low cost. A small amount of fine particles acts as a trigger, rapidly initiating degradation. The degradation process involves the initial dissolution of fine shell powder → localized breakdown of physical cross-linking → seawater infiltration → further dissolution of coarse particles.

[0011] Preferably, the mixture also includes 5-8 parts of sodium carboxymethyl cellulose and 3-5 parts of pH-sensitive polymer microspheres. The pH-sensitive microspheres are prepared using polymethacrylic acid or polyacrylic acid. These microspheres are stable at pH < 7.5 and undergo carboxyl deprotonation at pH > 8.0, causing the microspheres to swell and dissolve. The microspheres are encapsulated with sodium bicarbonate or citric acid as a degradation promoter.

[0012] Preferably, the plasticizer comprises 3-5 parts glycerol and 2-3 parts sorbitol; the additives comprise 1-2 parts propylene glycol alginate and 0.5-1 parts calcium stearate. Glycerol and sorbitol, as plasticizers, lower the glass transition temperature of the material, improving its flexibility and processing performance. Propylene glycol alginate, as a compatibilizer, contains both hydrophilic alginate segments and hydrophobic propylene glycol ester segments in its molecular structure, which can improve the interfacial compatibility between PVA and sodium alginate, increasing the interpenetration between the two phases and thus enhancing the overall performance of the material. Calcium stearate, as a lubricant, can reduce melt viscosity, improve processing fluidity, and prevent sticking during processing. Simultaneously, calcium stearate acts as an internal lubricant in the material, improving the dispersion uniformity of the shell powder particles.

[0013] The biodegradable plastic material for marine environments provided by this invention has the following degradation principle: In a normal environment (ionic strength below 0.01 M), calcium carbonate in seashell powder exists stably in the polymer network as a physical cross-linking node. Simultaneously, PVA and sodium alginate form a cross-linked network through hydrogen bonds, maintaining a tensile strength of 15–20 MPa. In a seawater environment (ionic strength approximately 0.7 M), the high ionic strength significantly reduces the activity coefficient of calcium carbonate through the salt effect, increasing its apparent solubility by 2–3 times. Simultaneously, magnesium ions (Mg ions) in seawater… 2+ (Approximately 1300 mg / L) adsorbs onto the surface of calcium carbonate particles, forming a loose hydrated magnesium layer, disrupting the original crystal structure and increasing solubility. This process can be simplified as follows: While the hydrated magnesium layer temporarily inhibits dissolution, it also makes the particle surface porous and increases its sensitivity to subsequent dissolution. Within 48–120 hours, the shell powder particles gradually dissolve due to the salt effect and surface porosification, and the physical cross-linked network begins to collapse; simultaneously, the high concentration of sodium ions (Na+) in seawater... + (Approximately 10500 mg / L) The electrostatic shielding effect weakens the hydrogen bonding between the hydroxyl groups of PVA and the carboxyl groups of sodium alginate, and competitively binds to the carboxyl groups of sodium alginate, further disrupting the chemical cross-linking network. Under the synergistic effect of these two mechanisms, the material completely fragments into fragments smaller than 5 cm within 7–10 days. The fragmented material is further degraded in the marine environment through microbial activity.

[0014] Adding sodium carboxymethyl cellulose can enhance pH responsiveness. It contains a large number of carboxyl groups, which can form a denser negatively charged network with sodium alginate and also form hydrogen bonds with the hydroxyl groups of PVA, enhancing interfacial compatibility. These carboxyl groups are very sensitive to pH; at pH below 6.5–7.5, the carboxyl groups are partially protonated, and the molecular chain relatively contracts; at pH above 8.0–8.3, the carboxyl groups are deprotonated, electrostatic repulsion increases, and the molecular chain extends. In high pH environments, it will react with Na+ in seawater. + Ca 2+ Mg 2+ The strong electrostatic interactions between plasmas disrupt the hydrogen bonds between CMC and Na molecular chains, leading to the rapid disintegration of the overall cross-linked network of the material and accelerating the initiation of the degradation process. This pH-responsive behavior can be combined with pH-sensitive microspheres, as the microspheres dissolve in seawater, releasing sodium bicarbonate and increasing the local pH and ionic strength. The combination of these two components enables more reliable and faster selective degradation in seawater. Furthermore, sodium carboxymethyl cellulose (CMC) possesses excellent film-forming and thickening properties, improving the rheological properties of the slurry and making it easier to process. As a natural polysaccharide derivative, it exhibits excellent biodegradability, being degraded by various cellulases to produce glucose and carboxymethyl glucose, which are completely non-toxic. Other components of the material, such as PVA, are degraded by alcohol dehydrogenases and esterases to acetaldehyde and acetic acid, ultimately mineralizing into carbon dioxide and water. Sodium alginate is degraded by alginate lyase to guluronic acid and mannulate, ultimately mineralizing. The dissolved calcium ions can be utilized by marine organisms such as corals, demonstrating sufficient environmental friendliness.

[0015] On the other hand, the present invention provides a marine-environmentally degradable packaging bag made of marine-environmentally degradable plastic material.

[0016] Furthermore, the present invention also provides a method for manufacturing a marine-environmentally biodegradable packaging bag, comprising the following steps: S1. Prepare a 12-15 wt% polyvinyl alcohol (PVA) solution. The specific steps are as follows: Heat deionized water to 85-90°C. While stirring, add modified PVA powder in batches, with each addition not exceeding 10% of the total amount. Allow 10-15 minutes between additions to avoid clumping from adding too much at once. Stir and dissolve for 2-3 hours until completely dissolved. Maintain the temperature at 85-90°C during dissolution. Excessive temperature will cause PVA molecular chain degradation, while excessively low temperatures will slow the dissolution process. After complete dissolution, lower the temperature to 70°C and maintain this temperature. At this point, the PVA solution concentration is 12-15 wt%. Lowering the temperature to 70°C is to prevent excessive temperature from degrading sodium alginate when mixing with the sodium alginate solution later, while maintaining sufficient fluidity for easy mixing.

[0017] S2. To prepare a sodium alginate solution with a concentration of 3–5 wt%, sodium alginate powder is slowly added to deionized water at 20–25°C. The solution is then dispersed using a high-speed disperser at 2000–3000 r / min for 30 minutes until completely dissolved. Excessive dissolution temperature of sodium alginate can lead to molecular chain breakage, a decrease in molecular weight, and negatively impact the final material properties. High-speed dispersion breaks up agglomerates of sodium alginate powder, promotes water molecule penetration and swelling, and accelerates the dissolution rate. The final product is a sodium alginate solution with a concentration of 3–5 wt%.

[0018] S3. Preparation of Shell Powder Suspension: The first step is the pretreatment of the shell powder. Shellfish waste is rinsed three times with fresh water to remove surface salts and impurities. Then, it is soaked in a 0.5% sodium hydroxide solution for 2 hours to remove residual organic matter. Next, it is rapidly rinsed with 0.1 mol / L dilute hydrochloric acid to remove surface soluble salts. Finally, it is rinsed with pure water until neutral and dried at 80–100℃ until the moisture content is less than 1%. The dried shells are then pulverized using a ceramic ball mill for 24 hours, followed by wet sieving to retain a particle size range of 1–10 μm. The sieving medium is deionized water. The key to this pretreatment process is removing residual salts and organic matter from the shell surface to prevent these impurities from affecting material properties. Simultaneously, the rapid treatment with dilute acid removes the loose surface layer, resulting in a more uniform particle morphology and more controllable dissolution behavior in the final shell powder. Pretreated seashell powder was dispersed in deionized water according to the formula ratio, with a seashell powder to water weight ratio of 1:3 to 1:5. 0.1–0.3 parts by weight of a dispersant such as sodium polyacrylate or sodium hexametaphosphate were added. The mixture was dispersed using a high-speed disperser at 3000–4000 r / min for 15–20 minutes, followed by ultrasonic treatment at 400 W power and 40 kHz frequency for 10 minutes to obtain a uniform and stable seashell powder suspension. The dispersant's role is to adsorb onto the surface of the seashell powder particles, providing steric hindrance or electrostatic repulsion to prevent particle agglomeration. Ultrasonic treatment further disperses agglomerates through cavitation, improving dispersion uniformity.

[0019] S4. Slowly add the PVA solution prepared in S1 to the sodium alginate solution prepared in S2 while stirring. Stir for 10 minutes to allow the two polymers to fully interpenetrate. During the mixing process, allow the temperature to naturally drop to 60-65℃. Then, add glycerol, sorbitol, propylene glycol alginate, and calcium stearate sequentially, stirring for 5 minutes after each addition to ensure thorough dispersion. Finally, slowly add the shell powder suspension prepared in step 3 and disperse it for 20 minutes using a high-shear disperser at 3000 rpm to ensure uniform dispersion of the shell powder in the polymer matrix. During dispersion, the temperature needs to be controlled at 60-65℃. Too low a temperature will result in excessively high viscosity, making dispersion difficult, while too high a temperature may cause local cross-linking. After dispersion, transfer the composite slurry to an ultrasonic device and degas it for 10 minutes using a power of 600W and a frequency of 40kHz to remove air bubbles. Degassing is a critical step; residual air bubbles will form defects after film formation, severely affecting the mechanical properties and appearance quality of the material. The final product is a composite slurry with a solid content of 15–18 wt% and a pH of 6.5–7.5. pH control is crucial; excessive acidity can lead to sodium alginate gelation, while excessive alkalinity can affect the stability of PVA.

[0020] S5. Place the composite slurry in a vacuum degassing tank and degas for 15-20 minutes under a vacuum of -0.08 MPa and a temperature of 50-55°C. Vacuum degassing can further remove microbubbles that ultrasonic degassing could not completely remove, especially bubbles inside the slurry. The degassing temperature of 50-55°C is to reduce the viscosity of the slurry, which is conducive to the escape of bubbles, while avoiding cross-linking reactions caused by excessively high temperatures.

[0021] S6. The slurry obtained in S5 is dried and then fed into a blown film machine for blown film forming.

[0022] S7 is a series of post-processing steps for bag making, followed by quality inspection. The inspection items include indicators such as thickness uniformity, tensile strength, elongation at break, transparency, and haze. Qualified products are packaged and put into storage.

[0023] In particular, S6 can also be film-formed using a casting method. The cast wet film enters a segmented drying system and then a hot-pressing process. Heating and pressurizing promote the formation of more hydrogen bonds between the PVA and sodium alginate molecular chains, enhancing the interpenetration between the two phases and improving the material's mechanical properties. This allows the shell powder particles to be more tightly embedded in the polymer network, forming a stable physical cross-linked structure, further reducing the moisture content and improving the material's dimensional stability. The hot-pressing temperature needs to be higher than the glass transition temperature of PVA to ensure sufficient molecular chain mobility, and lower than the melting point of PVA to prevent melting. Simultaneously, the thermal stability of sodium alginate must be fully considered. 110–130℃ is the optimal hot-pressing temperature range. At this temperature, PVA is in a highly elastic state, allowing molecular chain rearrangement and cross-linking without melting or degradation, while also preventing a decrease in the molecular weight of sodium alginate at higher temperatures. A pressure of 2–4 MPa and a time of 20–40 seconds ensure the compactness of the material's interior, eliminate residual pores, and at the same time ensure that the shell powder particles are in close contact with the matrix, avoiding the deterioration of material performance caused by excessively long-term high-temperature treatment.

[0024] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention applies shell powder and sodium alginate to the design of biodegradable plastics, utilizing the high concentration of magnesium and sodium ions in seawater as natural triggers to achieve responsive degradation of the material. The material is stable in freshwater or air environments with an ion strength of less than 1000 ppm, allowing for normal use and storage. However, in seawater environments with an ion strength of approximately 35000 ppm, degradation is triggered, avoiding long-term accumulation and marine plastic pollution, and solving the problem of the lack of environmental selectivity in existing biodegradable plastics.

[0025] 2. This invention constructs a dual cross-linked network structure consisting of physical cross-linking of shell powder and chemical cross-linking of PVA and sodium alginate. Shell powder, as a physical cross-linking node, exists stably in the normal environment, maintaining the mechanical strength of the material. After dissolving in seawater, the physical cross-linking network collapses. At the same time, sodium ions in seawater disrupt the hydrogen bond cross-linking between PVA and sodium alginate. The synergistic effect of the dual mechanism increases the degradation rate and the degradation effect is significantly better than that of materials with a single mechanism.

[0026] 3. This invention realizes the high-value utilization of aquatic waste. China generates about 2 million tons of shell waste every year. The traditional treatment methods are landfill or use as low-value filler. This invention uses shell powder as a functional component in biodegradable plastic materials, giving waste shells new application value, which is in line with the concept of circular economy and sustainable development.

[0027] 4. The degradation products of the material of this invention are environmentally friendly. The calcium ions dissolved from the calcium carbonate can be utilized by marine organisms such as corals. PVA and sodium alginate are completely mineralized into carbon dioxide and water through microbial action. The entire degradation process does not produce microplastic particles and leaves no toxic or harmful substances, thus achieving an ecological closed loop for the material.

[0028] 5. The production cost of the material of this invention is controllable. The main raw materials, PVA and sodium alginate, are mature products produced in industrialized manner. The cost of shell powder is extremely low and can even be obtained through recycling. The amount of plasticizer and additives used is small. The overall cost is about RMB 15,000 to 18,000 per ton, which is higher than traditional plastics but lower than high-end biodegradable plastics, and has commercial application prospects.

[0029] The technical solution of the present invention will be further described in detail below through embodiments. Detailed Implementation

[0030] To enable those skilled in the art to better understand the present application, the technical solutions in specific embodiments of the present application will be clearly and completely described below. Unless otherwise defined, the technical or scientific terms used in this invention should have the ordinary meaning understood by those skilled in the art.

[0031] In all the following examples and comparative examples, the material properties were tested under the same conditions: Freshwater environment: Ionic strength ≤500ppm, 25℃.

[0032] Simulated seawater environment: artificial seawater, ionic strength approximately 35,000 ppm; Mg 2+ Content approximately 1300 mg / L, Na + The content is approximately 10,500 mg / L.

[0033] The specifications of the raw materials used are as follows: Polyvinyl alcohol, degree of alcoholysis 88%, degree of polymerization 1700~2000.

[0034] Polyvinyl alcohol, degree of alcoholysis 99%, degree of polymerization 1700.

[0035] Sodium alginate has a molecular weight of 100,000 to 500,000 and an M / G ratio of 1.5.

[0036] Hydroxypropyl starch, a nonionic modified starch, with a degree of substitution of 0.1 to 0.2.

[0037] The raw material for the shell powder is waste oyster shells. After cleaning, removing the meat, soaking in a 0.5% NaOH solution to remove organic matter, rinsing with dilute hydrochloric acid, and calcining at 350℃, two grades of shell powder are obtained through ball milling and air classification. Laser particle size analyzer testing revealed: fine-particle shell powder with a particle size distribution range of 1–3 µm, median diameter D50 = 2.1 µm, and D90 < 3.2 µm; and coarse-particle shell powder with a particle size distribution range of 8–10 µm, median diameter D50 = 9.2 µm, and D90 < 10.5 µm. The shell powder used in this invention has a CaCO3 content ≥ 96%.

[0038] For industrial calcium carbonate, commercially available heavy calcium carbonate with an average particle size of 5µm and a D50 of 5.1µm is selected.

[0039] Example 1 The materials were prepared according to the following weight ratio: 60 parts polyvinyl alcohol (PVA~88), 10 parts sodium alginate, 4 parts fine granulated shell powder (1~3µm), 10 parts coarse granulated shell powder (8~10µm), 5 parts glycerin, 3 parts sorbitol, 2 parts propylene glycol alginate, 0.5 parts calcium stearate, and approximately 2.5 parts compatibilizer and lubricant.

[0040] The plastic product was prepared as follows: Deionized water was heated to 87°C, and modified PVA powder was added in batches, stirring for 2.5 hours until completely dissolved to obtain a 13.5% PVA solution. The solution was then cooled to 70°C for later use. Sodium alginate powder was added to deionized water at room temperature and dispersed at 2500 rpm for 30 minutes to obtain a 4% sodium alginate solution. Pretreated shell powder was dispersed in deionized water at a weight ratio of 1:4, and 0.2 parts of sodium polyacrylate dispersant were added. The mixture was dispersed at 3500 rpm for 18 minutes, followed by ultrasonic treatment at 400W and 40kHz for 10 minutes. The 70°C PVA solution was slowly added to the sodium alginate solution, and stirred for 10 minutes. Glycerin, sorbitol, propylene glycol alginate, and calcium stearate were added sequentially, stirring for 5 minutes each time. Finally, the shell powder suspension was added and dispersed at 3000 r / min for 20 minutes under high shear, followed by ultrasonic degassing at 600 W and 40 kHz for 10 minutes to obtain the composite slurry. Vacuum degassing was then performed for 18 minutes at a vacuum of ~0.08 MPa and a temperature of 52℃. The slurry was then cast into a film. Segmented drying was carried out: stage 1 at 45℃ / 65%RH / 12 minutes, stage 2 at 65℃ / 45%RH / 25 minutes, and stage 3 at 85℃ / 25%RH / 18 minutes, with a final moisture content of 10%. Hot-press crosslinking was then performed at 130℃ and 3.5 MPa, reducing the moisture content to 6.5%. The slurry was then cooled, peeled, electrostatically dusted, and slit. Quality inspection was subsequently conducted.

[0041] Example 2 This embodiment is a high-strength directional formulation, and the materials are prepared according to the following weight ratio: 65 parts polyvinyl alcohol (PVA~88), 5 parts sodium alginate, 2 parts fine granular shell powder (1~3µm), 14 parts coarse granular shell powder (8~10µm), the plasticizer and additives are the same as in Example 1, and the preparation method is the same as in Example 1.

[0042] Example 3 This embodiment is a rapid degradation-oriented formulation, and the materials are prepared according to the following weight ratio: 55 parts polyvinyl alcohol (PVA~88), 15 parts sodium alginate, 6 parts fine granular shell powder (1~3µm), 8 parts coarse granular shell powder (8~10µm), the plasticizer and additives are the same as in Example 1, and the preparation method is the same as in Example 1.

[0043] Example 4 This embodiment presents an enhanced rapid degradation formulation incorporating pH-sensitive microspheres. The materials were prepared according to the following weight ratios: 60 parts polyvinyl alcohol (PVA~88), 8 parts sodium alginate, 3 parts fine-particle shell powder (1~3 μm), 8 parts coarse-particle shell powder (8~10 μm), 5 parts sodium carboxymethyl cellulose (degree of substitution 0.8~1.0), and 3 parts plasticizer and additives as in Example 1. Methacrylic acid was used as the monomer, ethylene glycol dimethacrylate as the crosslinking agent, and potassium persulfate as the initiator. The mass ratio of MAA to EGDMA was 9:1. The mixture was reacted at 60°C for 5 hours. After washing and drying, pH-sensitive microspheres were obtained, with sodium bicarbonate encapsulated within the microspheres as a degradation promoter.

[0044] The preparation method further includes slowly adding sodium carboxymethyl cellulose powder to deionized water and stirring to dissolve it at room temperature for 1 hour to obtain a sodium carboxymethyl cellulose solution of a certain concentration. Then, sodium alginate solution is added to PVA solution and stirred for 10 minutes, followed by the addition of the prepared sodium carboxymethyl cellulose solution. After the shell powder is fully dispersed, the pre-prepared pH-sensitive microsphere suspension (microsphere to water mass ratio 1:3) is slowly added to the composite slurry and stirred thoroughly. The remaining steps are the same as in Example 1.

[0045] Comparative Example 1 This comparative example is a single coarse particle control. The materials were prepared according to the following weight ratios: 60 parts polyvinyl alcohol (PVA~88), 10 parts sodium alginate, 14 parts fine granular shell powder (1~3µm), and 0 parts coarse granular shell powder (8~10µm). The plasticizer and additives were the same as in Example 1, and the preparation method was the same as in Example 1.

[0046] Comparative Example 2 This comparative example is a single fine particle control. The materials were prepared according to the following weight ratio: 60 parts polyvinyl alcohol (PVA~88), 10 parts sodium alginate, and 14 parts coarse-particle shell powder (8~10µm). No fine-particle shell powder was added. The plasticizer and additives were the same as in Example 1, and the preparation method was the same as in Example 1.

[0047] Comparative Example 3 This comparative example is a control of ordinary ore calcium carbonate. The materials were prepared according to the following weight ratio: 60 parts polyvinyl alcohol (PVA~88), 10 parts sodium alginate, 14 parts industrial grade heavy calcium carbonate (5µm), plasticizer and additives as in Example 1, and preparation method as in Example 1.

[0048] Comparative Example 4 This comparative example is a nonionic polymer substitute control. The materials were prepared according to the following weight ratios: 60 parts polyvinyl alcohol (PVA~88), 10 parts hydroxypropyl starch, 4 parts fine granular shell powder (1~3µm), 10 parts coarse granular shell powder (8~10µm), plasticizers and additives were the same as in Example 1, and the preparation method was the same as in Example 1.

[0049] Comparative Example 5 This comparative example is a control lacking sodium alginate. The materials were prepared according to the following weight ratio: 70 parts polyvinyl alcohol (PVA~88), 0 parts sodium alginate, 4 parts fine granular shell powder (1~3µm), 10 parts coarse granular shell powder (8~10µm). The plasticizer and additives were the same as in Example 1, and the preparation method was the same as in Example 1.

[0050] Comparative Example 6 This comparative example is a control lacking shell powder. The materials were prepared according to the following weight ratios: 72 parts polyvinyl alcohol (PVA~88), 12 parts sodium alginate, 0 parts fine-particle shell powder (1~3µm), and 0 parts coarse-particle shell powder (8~10µm). The plasticizer and additives were the same as in Example 1, and the preparation method was the same as in Example 1.

[0051] Comparative Example 7 This comparative example is a high-crystallinity matrix control. The materials were prepared according to the following weight ratio: 60 parts of polyvinyl alcohol (PVA~99, degree of alcoholysis>95%), the rest were the same as in Example 1, and the preparation method was the same as in Example 1.

[0052] Table 1: Formulations of Examples and Comparative Examples (parts by weight)

[0053] Table 2: Comparison of Performance Test Results

[0054] Note: Freshwater stability is defined as the mass loss rate over 48 hours; seawater degradation is triggered by the appearance of visible cracks; and complete seawater fragmentation is defined as fragments smaller than 5 cm.

[0055] By comparing the above data, the following conclusions can be drawn: 1. Comparison of Example 1 with Comparative Examples 1 and 2 revealed that while Comparative Example 1 exhibited acceptable mechanical strength, its small specific surface area of ​​the shell powder limited its contact area with seawater, resulting in slow salt effects and magnesium ion adsorption, with cracks appearing within a 92-hour window, failing to meet the requirements for rapid degradation. Comparative Example 2, with its large specific surface area of ​​fine-particle shell powder, achieved rapid degradation in just 22 hours, but was prone to agglomeration, leading to increased internal defects and a tensile strength reduction to 14.8 MPa. Furthermore, it exhibited a high water absorption and swelling rate in freshwater, with a mass loss exceeding 3.5%. Example 1, through a combination of fine and coarse particles, utilized fine particles as a degradation trigger to rapidly respond to magnesium ions and form pores, while coarse particles served as a framework to maintain mechanical strength. The results showed that Example 1 achieved a tensile strength of 18.8 MPa and a degradation time of 4.5 days, balancing high strength with a suitable degradation rate, and demonstrating good freshwater stability, proving the effective role of the bimodal particle size distribution of the shell powder.

[0056] 2. A comparison between Example 1 and Comparative Example 3 revealed that the comparative example, using ordinary industrial calcium carbonate, although its particle size was between that of the coarse and fine powders of this invention, took more than 30 days to disintegrate in seawater. This indicates that the bio-derived shell powder possesses a unique porous layered structure and biomineralized crystal form, exhibiting higher ion exchange activity with magnesium ions compared to calcite derived from minerals. This is a key material selection factor for achieving rapid degradation in this invention.

[0057] 3. Comparison of Example 1 and Comparative Example 4 revealed that Comparative Example 4 used nonionic hydroxypropyl starch instead of sodium alginate. Although both have similar hydrophilicity, starch lacks carboxyl groups (~COO). - ), cannot react with Na in seawater + Strong ionic interactions occur. Experimental results show that the degradation time increases dramatically to over 45 days, strongly demonstrating that the technical concept of using sodium alginate's polyelectrolyte properties as an ion switch is real and effective.

[0058] 4. Comparison of Example 1 and Comparative Example 5 revealed that Comparative Example 5 removed sodium alginate, relying solely on PVA and shell powder. Although the PVA matrix has high strength, it is deficient in sodium in seawater. + The mechanism of destruction of the chemical cross-linked network is that there is no PVA-sodium alginate hydrogen bond dissociation process. The high-strength PVA matrix cannot be rapidly disintegrated by dissolving shell powder alone, and the fragmentation time is extended to more than 20 days.

[0059] 5. After comparing Example 1 with Comparative Example 6, it was found that Comparative Example 6, which removed shell powder, exhibited two fatal defects. On the one hand, it lost its environmental selectivity. Due to the lack of the locking effect of the hydrophobic calcium carbonate skeleton, the material underwent severe water absorption and swelling in fresh water, resulting in extreme instability of the product during normal use, such as becoming sticky when exposed to rain. On the other hand, the degradation mode changed. In seawater, due to the absence of the pore-forming effect formed by magnesium ions attacking shell powder, the material could not undergo rapid physical disintegration and fragmentation. Instead, it exhibited a slow overall swelling and dissolution into a soft mud-like state, and the complete degradation time was actually longer than that of Example 1.

[0060] Comparative Examples 5 and 6 together demonstrate the dual cross-linked network structure of physical cross-linking of shell powder and chemical cross-linking of PVA sodium alginate. The synergistic effect of the dual mechanisms increases the degradation rate and the degradation effect is significantly better than that of materials with a single mechanism.

[0061] 6. Comparison of Example 1 and Comparative Example 7 revealed that Comparative Example 7, using PVA with a high degree of hydrolysis (99%), exhibited extremely strong hydrogen bonds between its molecular chains and high crystallinity, resulting in almost no swelling in seawater at room temperature. Seawater could not penetrate the material, preventing the shell powder inside from fully contacting the seawater environment. The salt effect and magnesium ion action were physically blocked, and the material remained undegraded even after being immersed in seawater for six months. This conversely demonstrates the necessity of limiting the degree of PVA hydrolysis to 87-89% in this invention; that is, it is essential to ensure adequate swelling of the matrix to establish ion transport channels, allowing seawater to penetrate the material and fully contact the shell powder.

[0062] 7. A comparison of Example 4 and Example 1 revealed that the introduction of pH-sensitive microspheres and CMC-Na led to a more rapid initiation of material degradation, while maintaining high freshwater stability. This indicates that the pH-sensitive microsphere system can more accurately identify the seawater environment, achieving more controllable and rapid degradation while ensuring freshwater stability. Furthermore, a comparison of Example 4 and Example 3 showed that while Example 3 also achieved rapid degradation by increasing the amount of sodium alginate, its tensile strength was lower, and its stability in freshwater was slightly worse. Example 4, by introducing the pH-sensitive microsphere system, achieved a degradation rate comparable to Example 3 while maintaining high tensile strength and excellent freshwater stability, resulting in a more balanced overall performance.

[0063] Finally, it should be noted that the described embodiments are merely some, not all, of the embodiments of the present invention. Those skilled in the art will understand that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the claims and their equivalents; that is, all other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A self-degradable plastic material for marine environments, characterized in that, It includes the following components in parts by weight: 55-65 parts polyvinyl alcohol, 8-12 parts sodium alginate, 8-16 parts shell powder, 3-10 parts plasticizer, 0.5-3 parts compatibilizer, and 0.1-1 parts lubricant.

2. The marine environment self-degradable plastic material according to claim 1, characterized in that, The degree of alcoholysis of the polyvinyl alcohol is 87-89%, and the degree of polymerization is 1700-2000.

3. The marine environment self-degradable plastic material according to claim 1, characterized in that, The sodium alginate has a molecular weight of 100,000 to 500,000, and the molar ratio of mannuronic acid to guluronic acid is 1.2 to 2.

5.

4. The marine environment self-degradable plastic material according to claim 1, characterized in that, The shell powder has a particle size of 1-10 μm and a calcium carbonate content of not less than 90%.

5. The marine environment self-degradable plastic material according to claim 4, characterized in that, The shell powder has a bimodal particle size distribution, including fine shell powder of 1-3 μm and coarse shell powder of 8-10 μm.

6. The marine environment self-degradable plastic material according to claim 1, characterized in that, It also includes 5-8 parts of sodium carboxymethyl cellulose and 3-5 parts of pH-sensitive polymer microspheres.

7. The marine environment self-degradable plastic material according to claim 1, characterized in that, The plasticizer comprises 3-5 parts glycerol and 2-3 parts sorbitol; the auxiliary agent comprises 1-2 parts propylene glycol alginate and 0.5-1 parts calcium stearate.

8. A marine-environmentally biodegradable packaging bag, characterized in that, Made using the marine environment self-degradable plastic material as described in any one of claims 1 to 7.

9. A method for manufacturing a marine-environmentally biodegradable packaging bag, characterized in that, Includes the following steps: S1. Prepare a polyvinyl alcohol solution with a concentration of 12-15 wt%; S2. Prepare a sodium alginate solution with a concentration of 3-5 wt%; S3. Prepare a seashell powder suspension; S4. The polyvinyl alcohol solution is added to the sodium alginate solution and mixed. Plasticizer, additives and shell powder suspension are added in sequence. After high shear dispersion, ultrasonic degassing is performed to obtain a composite slurry. S5. Vacuum degas the composite slurry; S6, blown film forming; S7, Post-processing.

10. A method for manufacturing a marine-environmentally biodegradable packaging bag according to claim 9, characterized in that, S3 specifically involves: mixing shell powder and deionized water at a weight ratio of 1:3 to 1:5, adding a dispersant, dispersing at high speed, and then ultrasonically treating to prepare a shell powder suspension.

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