Degradable breathable film and method for making the same

Abstract

This invention discloses a biodegradable breathable membrane and its preparation method, belonging to the field of thin film materials. The breathable membrane comprises, by weight, 60 parts modified polylactic acid, 12 parts modified natural cellulose, 1.5 parts plasticizer, 3.5 parts compatibilizer, 1 part lubricant, 0.1-2 parts calcium stearate, and 0.5-5 parts microgel particles. The microgel particles can be pre-crosslinked calcium alginate microgel particles, calcium-aluminum ion gradient crosslinked microgel particles, or pore-controlled calcium-aluminum ion gradient crosslinked microgel particles. Through the synergistic effect of calcium stearate and specific microgel particles, this membrane significantly improves its thermal stability, resistance to damp heat aging, and durability under harsh environments while ensuring the material's biodegradability, breathability, and antibacterial and oxygen-barrier functions. The preparation method includes steps such as raw material mixing, melt blending and granulation, and blow molding, which is simple and suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of thin film materials, and more particularly to a biodegradable breathable membrane and its preparation method. Background Technology

[0002] Polylactic acid (PLA) is considered a potential environmentally friendly alternative to traditional plastic packaging due to its biodegradability. However, PLA itself suffers from drawbacks such as high brittleness, poor thermal stability, and easy hydrolysis, limiting its application in demanding packaging scenarios. To overcome these shortcomings, existing technologies focus on blending and modifying PLA. For example, one disclosed technique effectively improves the mechanical strength, antibacterial properties, and oxygen barrier properties of a composite film by introducing PLA modified with cerium complexes and natural cellulose modified with isophorone diisocyanate and polydopamine. This technology represents progress in functionalizing PLA-based materials.

[0003] However, existing technologies primarily focus on the introduction and preliminary compounding of functional components, failing to provide a systematic solution to the fundamental challenge that PLA materials face in processing and practical use environments—namely, the synergistic inhibition of thermo-oxidative degradation and hydrolytic degradation. Specifically, under harsh conditions such as high-temperature processing or high humidity and acidity, PLA segments are prone to breakage, leading to rapid degradation of material properties. Simply introducing conventional stabilizers may interfere with the effectiveness of existing functional centers in the system (such as antibacterial and oxygen-barrier cerium ions). Furthermore, when such films are used to package fresh foods with high water activity, a high-humidity microenvironment easily forms inside the packaging, accelerating the hydrolysis of the PLA matrix and potentially causing excessive swelling of hydrophilic fillers, triggering a vicious cycle of declining mechanical and barrier properties. Therefore, how to significantly improve the processing stability, environmental durability, and adaptability to complex application scenarios while maintaining the material's excellent basic properties remains a pressing technical problem to be solved in this field. Summary of the Invention

[0004] This application provides a biodegradable breathable membrane and its preparation method, which solves the technical problems of insufficient processing thermal stability, easy degradation and aging in humid or acidic environments, and mutual interference between the components added to improve stability and the functional centers, making it difficult to maintain the long-term mechanical properties, barrier properties and antibacterial properties of the material in a coordinated manner.

[0005] This application provides a biodegradable breathable membrane, comprising the following raw materials by weight:

[0006] 60 parts of modified polylactic acid;

[0007] 12 parts of modified natural cellulose;

[0008] 1.5 parts plasticizer;

[0009] Compatibilizer 3.5 parts;

[0010] 1 part lubricant;

[0011] 0.1-2 parts of calcium stearate, wherein the calcium stearate has a particle size D50 ≤ 5 μm;

[0012] 0.5-5 parts of microgel particles, wherein the microgel particles are pre-crosslinked calcium alginate microgel particles with a particle size of 5-10 μm.

[0013] Furthermore, the preparation method of the pre-crosslinked calcium alginate microgel particles includes the following steps: adding a 3.5% sodium alginate aqueous solution dropwise to an 8% calcium chloride aqueous solution for crosslinking and solidification, collecting the gel particles and washing them; then dispersing the gel particles in a 1% glutaraldehyde aqueous solution, adjusting the pH of the system to 4.5, and reacting to carry out pre-crosslinking enhancement; washing, freeze-drying, and sieving to obtain pre-crosslinked calcium alginate microgel particles with a particle size of 5-10 μm.

[0014] Furthermore, the microgel particles are calcium-aluminum ion gradient cross-linked microgel particles, wherein the molar ratio of calcium ions to aluminum ions is 2:1.

[0015] Furthermore, the preparation method of the calcium-aluminum ion gradient crosslinked microgel particles includes the following steps: adding a 3.5% sodium alginate aqueous solution dropwise to a mixed crosslinking solution containing calcium ions and aluminum ions for initial crosslinking, wherein the calcium ion concentration is 1 mol / L and the aluminum ion concentration is 0.5 mol / L; then adding ammonia water dropwise to the system to uniformly adjust the pH from 5.8 to 8.0 within 90 minutes; collecting the gel particles, washing, freeze-drying, and sieving to obtain calcium-aluminum ion gradient crosslinked microgel particles with a particle size of 5-10 μm.

[0016] Furthermore, the calcium-aluminum ion gradient crosslinked microgel particles are calcium-aluminum ion gradient crosslinked microgel particles that have undergone pore regulation treatment. The pore regulation treatment includes preparing slow-frozen macroporous microgel particles and fast-frozen dense microgel particles, and mixing the two at a dry basis mass ratio of 1:2.

[0017] Furthermore, the slow-frozen macroporous microgel particles have an average particle size of 8-15 μm, a porosity greater than 80%, and an average pore size of 100-300 nm.

[0018] The fast-frozen dense microgel particles have an average particle size of 5-10 μm, a porosity of 60%-70%, and an average pore size of less than 50 nm.

[0019] Furthermore, the plasticizer is epoxidized soybean oil; the compatibilizer is styrene-maleic anhydride-acrylonitrile copolymer; the lubricant is ethylene bis-stearamide; and the calcium stearate is food grade with a particle size D50 of no more than 5 μm.

[0020] A method for preparing a biodegradable breathable membrane includes the following steps:

[0021] S1. Weigh the modified polylactic acid, modified natural cellulose, plasticizer, compatibilizer, lubricant, microgel particles and the remaining calcium stearate according to the weight ratio;

[0022] S2. Put all the raw materials weighed in step S1 into a high-speed mixer and mix at 1500 r / min for 25 minutes at room temperature to obtain a premix;

[0023] S3. The premix obtained in step S2 is fed into a twin-screw extruder for melt blending and extrusion granulation. The extruder temperature is set sequentially from the feed port to the die head as follows: Zone 1 170℃, Zone 2 175℃, Zone 3 180℃, Zone 4 185℃, and Die head 190℃. The screw speed is 250 r / min. After water cooling and pelletizing, blown film masterbatch is obtained.

[0024] S4. The blown film masterbatch obtained in step S3 is fed into a blown film machine to blow-form a film. The screw temperature of the blown film machine is controlled at 165°C, the die head temperature is controlled at 175°C, the blow-up ratio is adjusted to 2.2:1, the traction speed is 15m / min, and the film thickness is controlled at 35μm to obtain the biodegradable breathable film.

[0025] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0026] By introducing a synergistic system of calcium stearate and specific microgel particles, the processing thermal stability and environmental durability of polylactic acid-based composite materials are enhanced through multi-dimensional synergistic effects such as chemical neutralization, ion management and physical barrier, thus solving the problem of balancing the mechanical properties, long-term functionality and durability of biodegradable materials.

[0027] Calcium stearate, as a fast acid acceptor, can immediately neutralize carboxyl groups generated in the early stages of processing or hydrolysis, thus inhibiting autocatalytic degradation. Meanwhile, microgel particles, as long-lasting ion reservoirs and moisture adsorbents, can slowly release calcium ions and reduce the water activity around the matrix, providing continuous protection. The two complement each other in terms of time and space.

[0028] By using pre-crosslinked calcium alginate or calcium-aluminum ion gradient crosslinked microgels, calcium ions can be effectively locked, reducing the concentration of free calcium ions and decreasing the risk of competitive coordination with cerium functional centers. This ensures the efficient operation of the antibacterial and oxygen coordination barrier mechanism centered on cerium complexes.

[0029] Further, calcium-aluminum ion gradient crosslinking microgels were adopted, especially by forming a dense and stable coordination network around the microgel with aluminum ions, which improved the long-term stability and functional durability of the material under harsh chemical environments such as acidic or high-salt conditions, and expanded the application range of the material.

[0030] By controlling the pore size of microgel particles, particles with different pore sizes and porosity characteristics were prepared and compounded in proportion, achieving dynamic and intelligent management of the humidity of the internal microenvironment of the packaging. This effectively reduced the generation of condensate, delayed the performance degradation of the film in high humidity environments, and created a more stable preservation environment for packaged food.

[0031] Microgel particles, as soft organic-inorganic hybrid fillers, can improve interfacial compatibility and absorb and disperse stress through elastic deformation, thereby improving the toughness of materials while avoiding stress concentration and achieving a balanced improvement in mechanical properties. Detailed Implementation

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention; the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0033] Example 1: A biodegradable breathable membrane, comprising the following raw materials by weight:

[0034] 60 parts of modified polylactic acid;

[0035] 12 parts of modified natural cellulose;

[0036] 1.5 parts plasticizer;

[0037] Compatibilizer 3.5 parts;

[0038] 1 part lubricant;

[0039] 0.1-2 parts calcium stearate;

[0040] 0.5-5 parts of microgel particles, wherein the microgel particles are pre-crosslinked calcium alginate microgel particles;

[0041] The preparation of the modified PLA includes the following steps:

[0042] A1. Dissolve 1 part by weight of cerium nitrate in 8.75 parts by weight of deionized water in a reaction vessel, then add 4 parts by weight of sodium citrate, react at 35°C for 4 hours, add 25% ammonia water to adjust the pH to 7, let stand for 10 hours, centrifuge, filter, and dry to obtain the cerium complex.

[0043] A2. Add 2 parts by weight of cerium complex, 16 parts by weight of lactic acid and 10 parts by weight of toluene obtained in step A1 to a reaction vessel, and simultaneously add the first part of calcium stearate, which accounts for 45% of the total amount of calcium stearate added. React at 140°C for 2 hours, then evacuate under vacuum for 4 hours at a vacuum degree of 0.05 MPa. After drying, the modified polylactic acid is obtained.

[0044] The preparation of the modified natural cellulose includes the following steps:

[0045] B1. Add 1 part by weight of cellulose nanocrystals, 2.5 parts by weight of isophorone diisocyanate and 0.001 parts by weight of tetraisopropyl titanate to a reaction vessel and react at 75°C for 4 hours to obtain activated natural cellulose.

[0046] B2. Add 3 parts by weight of activated natural cellulose and 0.15 parts by weight of dopamine obtained in step B1 to a reaction vessel, react at 35°C for 1.5 h, add 5 parts by weight of tris(hydroxymethyl)aminomethane buffer solution with pH 8, stir for 22 h, wash the obtained product with deionized water until pH 7, filter and dry to obtain the modified natural cellulose.

[0047] The preparation of the pre-crosslinked calcium alginate microgel particles includes the following steps:

[0048] C1. Dissolve sodium alginate in deionized water to prepare a 3.5% (w / w) solution. Stir at 35°C for 3 hours until completely dissolved, and let stand for 2 hours to remove bubbles, as the aqueous phase.

[0049] C2. The aqueous phase obtained in step C1 is added dropwise to an 8% calcium chloride aqueous solution at a rate of 2 drops per second, and crosslinked and cured at 25°C for 1 hour. The resulting gel particles are collected and washed three times with deionized water.

[0050] C3. Disperse the gel particles obtained in step C2 in a 1% glutaraldehyde aqueous solution, adjust the pH of the system to 4.5 with hydrochloric acid, and react at 45°C for 3 hours to carry out pre-crosslinking reinforcement;

[0051] C4. The product after step C3 is washed three times with deionized water, freeze-dried under vacuum for 36 hours, and then passed through 400-mesh and 800-mesh sieves to obtain the pre-crosslinked calcium alginate microgel particles with a particle size of 5-10 μm.

[0052] The plasticizer is epoxidized soybean oil;

[0053] The compatibilizer is a styrene-maleic anhydride-acrylonitrile copolymer;

[0054] The lubricant is ethylene bis-stearamide;

[0055] The calcium stearate is commercially available food grade with a particle size D50 ≤ 5 μm.

[0056] The method for preparing the biodegradable breathable membrane includes the following steps:

[0057] S1. Weigh the modified polylactic acid, modified natural cellulose, plasticizer, compatibilizer, lubricant, pre-crosslinked calcium alginate microgel particles and the remaining calcium stearate (i.e., the second part of calcium stearate) according to the weight ratio.

[0058] S2. Put all the raw materials weighed in step S1 into a high-speed mixer and mix them at 1500 r / min for 25 min at room temperature to obtain a premix.

[0059] S3. The premix obtained in step S2 is fed into a twin-screw extruder for melt blending and extrusion granulation. The extruder temperature is set sequentially from the feed port to the die head as follows: Zone 1 170℃, Zone 2 175℃, Zone 3 180℃, Zone 4 185℃, and Die head 190℃. The screw speed is 250 r / min. After water cooling and pelletizing, blown film masterbatch is obtained.

[0060] S4. The blown film masterbatch obtained in step S3 is fed into a blown film machine to blow-form a film. The screw temperature of the blown film machine is controlled at 165°C, the die temperature is controlled at 175°C, the blow-up ratio is adjusted to 2.2:1, the traction speed is 15m / min, and the film thickness is controlled at 35μm to obtain the biodegradable breathable film.

[0061] Experiments were conducted on the technical solution of this embodiment to verify the effects of the individual and synergistic addition of calcium stearate and pre-crosslinked calcium alginate microgel particles on the film performance.

[0062] The experimental groups are shown in Table 1 below:

[0063] Table 1 Experimental Grouping in Example 1

[0064]

[0065] The basic formula remains unchanged, using the following raw materials in parts by weight: modified PLA: 60 parts, modified natural cellulose: 12 parts, plasticizer (epoxidized soybean oil): 1.5 parts, compatibilizer (styrene-maleic anhydride-acrylonitrile copolymer): 3.5 parts, lubricant (ethylene bis-stearamide): 1.0 part.

[0066] Test the following items:

[0067] 1. Mechanical Properties: The tensile strength of the film was measured using a universal testing machine according to GB / T 1040.2-2006. The film was cut into standard type 1B dumbbell-shaped specimens and stretched at a constant rate of 50 mm / min until the specimen broke. The maximum load was recorded and the tensile strength was calculated. Each group of samples was tested five times and the average value was taken.

[0068] 2. Tear resistance: Refer to JIS K7128-3:1998 to test the right-angle tear strength of the film. Cut the film into standard right-angle tear specimens and tear them at a rate of 200 mm / min on a material testing machine. Record the maximum force value during the tearing process as the tear strength. Test five specimens in each direction (MD and TD) and calculate the average value.

[0069] 3. Antibacterial Properties: The antibacterial activity of the membrane against *Escherichia coli* was evaluated using the plate count method. The membrane was uniformly cut into 10mm diameter discs and placed at the bottom of a 24-well plate. 100 μL of a solution (approximately 1×10⁻⁶) was added to each well. 5 A bacterial suspension of CFU / mL was prepared to evenly cover the sample surface. The well plates were incubated at 37°C for 6 hours. Subsequently, each film sample was transferred to a tube containing 10 mL of phosphate-buffered saline (PBS) and shaken at 150 rpm for 10 minutes to elute the bacteria. 60 μL of the eluent was evenly spread onto nutrient agar plates and incubated at 37°C for 24 hours. The number of viable colonies was counted and compared with the blank control to calculate the inhibition rate. The experiment was repeated three times.

[0070] 4. Oxygen Barrier Performance: The oxygen permeability of the membrane was determined using a VAC-V1 differential pressure gas permeability tester. The membrane was cut to the specified size and installed in the test chamber. Under the conditions of a test temperature of 23℃ and a relative humidity of 0%, the volume of oxygen permeating per unit area of ​​the membrane per unit time was measured. The results were expressed as cubic centimeters per square meter·24h·0.1 MPa [cm²]. 3 / (m 2 [24h·0.1MPa] indicates.

[0071] 5. Thermal stability: The thermal decomposition behavior of the thin film was evaluated using a thermogravimetric analyzer (TGA). Approximately 5 mg of the thin film sample was weighed and placed in an alumina crucible. Under a high-purity nitrogen atmosphere with a flow rate of 50 mL / min, the temperature was increased from 30 °C to 600 °C at a rate of 10 °C / min. The change in sample weight with temperature was recorded, and the temperature at which 5% weight loss occurred (Td5%) was used as the evaluation index for thermal stability.

[0072] 6. Resistance to damp heat aging: The film sample was placed in a constant temperature and humidity test chamber and subjected to accelerated aging for 7 consecutive days at 70℃ and 80% relative humidity. After aging, the sample was equilibrated in a standard environment (23℃, 50%RH) for 24 hours, and then its tensile strength and oxygen permeability were tested again. The performance retention rate was calculated to characterize its durability.

[0073] The test results are shown in Table 2 below:

[0074] Table 2 Experimental Detection Results of Example 1

[0075]

[0076] The technical solutions described in the embodiments of this application have at least the following technical effects or advantages:

[0077] This embodiment introduces calcium stearate and pre-crosslinked calcium alginate microgel particles to construct a synergistic stabilizing system. This system aims to simultaneously address the thermo-oxidative / hydrolytic degradation problems that easily occur in polylactic acid (PLA)-based composites during processing and use, as well as the problem of inorganic stabilizers competing with cerium (Ce)-based antibacterial and oxygen-barrier functional centers, thus weakening their core functions.

[0078] Experimental results show that the sample with the optimal ratio (1.0 part calcium stearate and 3.0 parts calcium alginate microgel) achieved synergistic improvement in key properties. The initial thermal decomposition temperature (Td5%) of the material was significantly higher than that of the unadded system, indicating enhanced thermal stability. After accelerated aging at 70℃ and 80% relative humidity, its tensile strength retention rate was greatly improved, proving that its resistance to damp heat aging was fundamentally improved. At the same time, the oxygen permeability of the film was further reduced, the antibacterial rate against Escherichia coli remained at a high level, and the tensile and tear strengths were improved in a balanced manner.

[0079] The technical principle behind these effects stems from the unique multi-level synergistic effect between calcium stearate and calcium alginate microgels. First, at the functional isolation and ion management level, the pre-crosslinked calcium alginate three-dimensional network can effectively lock a portion of calcium ions through coordination, significantly reducing the concentration and mobility of free calcium ions in the system. This greatly alleviates the risk of calcium ions competing for coordination with functional cerium ions, thus ensuring the complete and efficient operation of the antibacterial and oxygen coordination barrier mechanism centered on cerium complexes. Second, at the stabilization mechanism level, the two complement each other temporally and spatially. Calcium stearate, as a highly efficient acid acceptor, can rapidly neutralize the carboxyl termini generated during high-temperature processing or initial hydrolysis of PLA, immediately inhibiting the autocatalytic degradation chain reaction and providing rapid protection. Meanwhile, the calcium alginate microgel acts as a long-term reservoir and physical barrier. Its network structure not only slowly releases calcium ions for continuous stability but also adsorbs moisture from the environment, reducing the water activity around the PLA matrix and physically slowing down the hydrolysis process. Finally, at the mechanical enhancement level, microgel particles, as a soft organic-inorganic hybrid filler, can be uniformly dispersed between the PLA matrix and modified cellulose. Their surface active groups help improve interfacial compatibility, while their own elastic deformation capacity can absorb and disperse stress, effectively passivating crack tips. This improves the material's toughness while avoiding stress concentration problems caused by calcium stearate particles.

[0080] Example 2: The above examples, through the compounding of calcium stearate and calcium alginate microgels, synergistically enhance the processing stability and environmental durability of polylactic acid-based composite materials from multiple dimensions, including chemical neutralization, ion management, interface modification, and physical barriers, without sacrificing the core functions of the material. This provides an effective approach to solving the problem of balancing the mechanical properties, long-term functionality, and durability of biodegradable packaging materials. To further improve the overall performance of the material, further improvements were made based on Example 1.

[0081] The microgel particles are calcium-aluminum ion gradient cross-linked microgel particles, and the preparation of the calcium-aluminum ion gradient cross-linked microgel particles includes the following steps:

[0082] G1. Dissolve sodium alginate in deionized water to prepare a solution with a mass concentration of 3.5%. Stir at 35°C for 3 hours until completely dissolved, and let stand for 2 hours to remove bubbles, as the aqueous phase.

[0083] G2. Weigh out calcium chloride hexahydrate and aluminum chloride hexahydrate separately, dissolve them together in deionized water, and prepare a mixed crosslinked solution with a total metal ion concentration of 1.5 mol / L, wherein the calcium ion concentration is 1.0 mol / L and the aluminum ion concentration is 0.5 mol / L (i.e., Ca2+). 2+ And Al 3+ (Molar ratio is 2:1).

[0084] G3. The aqueous phase obtained in step G1 is added dropwise at a constant rate of 2 drops per second to the mixed crosslinking solution prepared in step G2, and the mixture is stirred continuously at 25°C to carry out preliminary crosslinking for 1 hour.

[0085] G4. Under continuous stirring, slowly add 10% ammonia solution to the reaction system of step G3, adjusting the pH of the system uniformly from the initial 5.8 to 8.0 within 90 minutes. This process promotes the gradient distribution and coordination structure transformation of aluminum ions inside and on the surface of the gel particles;

[0086] G5. Stop stirring, collect the gel particles generated by the reaction, wash with deionized water 4 times, and then wash with 50% ethanol aqueous solution 2 times to thoroughly remove residual ions and impurities.

[0087] G6. The wet gel washed in step G5 is subjected to vacuum freeze-drying for 36 hours. The dried particles are then passed through 400-mesh and 800-mesh sieves to obtain calcium-aluminum ion gradient cross-linked microgel particles with a particle size of 5-10 μm.

[0088] This embodiment is based on the experiment group 6 in embodiment 1. Experiment group 7 is the experimental group. The difference between experimental group 7 and experimental group 6 is that the microgel particles in experimental group 7 are calcium-aluminum ion gradient cross-linked microgel particles.

[0089] Based on the performance testing in Example 1, acid resistance environment testing and aluminum ion migration testing were added;

[0090] Acid resistance test: Each group of film samples was immersed in a citrate-disodium hydrogen phosphate buffer solution at pH 3.5 and placed in a constant temperature shaker (100 rpm) at 40℃. The samples were removed on days 1, 3, and 7, rinsed with deionized water, dried, and equilibrated for 24 hours under standard conditions. Their tensile strength and oxygen permeability were then tested, and the performance retention rate was calculated.

[0091] Aluminum ion migration detection: Following the national food safety standard GB 31604.1-2015, the film sample was immersed in a 4% (volume fraction) acetic acid solution at 40℃ for 24 hours. The aluminum content in the immersion solution was determined using inductively coupled plasma optical emission spectrometry (ICP-OES).

[0092] The performance test results are shown in Tables 3 and 4 below:

[0093] Table 3 Performance test results of Example 2

[0094]

[0095] Table 4 Performance test results of Example 2

[0096]

[0097] The technical solutions described in the embodiments of this application have at least the following technical effects or advantages:

[0098] The core purpose of introducing calcium-aluminum ion gradient crosslinked microgels in this scheme is to further improve the long-term stability and functional durability of composite film materials under harsh chemical environments such as acidic or high-salt conditions. This addresses the problems of the original calcium-based microgel system experiencing rapid swelling and disintegration of the network structure under low pH conditions, as well as the premature release of loaded calcium ions, which leads to insufficient long-term protection of the polylactic acid matrix and exacerbates potential interference with cerium functional centers.

[0099] Experimental results show that the introduction of calcium-aluminum ion-gradient crosslinked microgels significantly improved two key durability indicators of the film while maintaining and even optimizing its basic mechanical properties, oxygen barrier properties, and antibacterial rate. First, the performance degradation rate of the film under acidic corrosion was significantly slowed down, and its retention rate of mechanical and barrier properties after immersion in pH 3.5 buffer for 7 days was significantly improved compared to the control group using the original calcium-based microgel. Second, the thermal stability index (Td5%) of the material was also further improved. These data indicate that the chemical stability and environmental adaptability of the material have been substantially enhanced.

[0100] The reason for these effects lies in the fundamental improvement in chemical stability brought about by the introduction of aluminum ions and their gradient distribution structure. Aluminum ions have a higher charge density and coordination ability than calcium ions. Under controlled pH conditions, gradient cross-linking makes aluminum ions more inclined to form a stable and dense coordination network in the peripheral region of the microgel particles. This gradient structure, with increasing aluminum ion concentration from the inside out, constructs a transition zone of chemical stability from strong to weak at the microscopic level. In an acidic environment, the outer aluminum-rich dense network can preferentially and effectively block the penetration and attack of hydrogen ions, thereby protecting the relatively fragile calcium cross-linked regions and the encapsulated calcium ions, delaying the disintegration process of the entire microgel network. Simultaneously, aluminum ions and their hydrolysis products can also coordinate with the carboxyl groups at the ends of polylactic acid chains, providing additional anchoring sites for the polymer chains and inhibiting their hydrolytic chain breakage. By constructing a microgel-reinforced phase with a chemical stability gradient, and in conjunction with the original calcium stearate rapid neutralization mechanism, a more durable and robust protection is provided for the polylactic acid matrix from both physical barrier and chemical bonding dimensions. This further expands the effective protection of the material from ordinary environments to more demanding acidic or high ionic strength environments, significantly improving the reliability and applicability of biodegradable packaging materials in applications.

[0101] This improved solution addresses the shortcomings of the original synergistic system in providing insufficient long-term protection against extreme chemical environments. While the calcium-based microgel in the original system can lock in calcium and provide buffering, its calcium ion cross-linking network is sensitive to strong acids. Under prolonged or high-intensity acid erosion, the gel structure prematurely disintegrates, leading to a decline in its long-term sustained-release and physical protective functions. The calcium-aluminum gradient microgel, through its robust aluminum-rich shell, transforms the microgel from a consumable buffer into a protective structure. It not only protects the trapped calcium ions for a longer period, achieving more sustained ion replenishment, but also maintains its structure in harsh environments, continuously enhancing interface and moisture management. This embodiment, by strengthening the bulk durability of the microgel as a defensive structure, fundamentally extends the functional lifespan of the material in harsh applications such as acidic or high-salt environments, thus expanding its applicability.

[0102] Example 3: In practical application, it was found that when this biodegradable breathable film was used to package watermelons, a large amount of condensation formed inside the packaging. The inner surface of the film became sticky and fogged due to continuous high humidity, reducing the mechanical stiffness of the packaging. Research showed that when this product was used to package foods with high water activity (such as fresh-cut fruits and vegetables), the internal microenvironment became saturated with humidity, triggering a vicious cycle of excessive swelling of microgels and PLA hydrolysis. To solve this problem, further improvements were made to the scheme in Example 3.

[0103] The calcium-aluminum ion gradient crosslinked microgel particles were subjected to pore regulation treatment to prepare two types of microgel particles with different pore characteristics.

[0104] The calcium-aluminum ion gradient crosslinked microgel particles include slow-frozen macroporous microgel particles and fast-frozen dense microgel particles, with a mass ratio of 1:2.

[0105] The preparation of the two types of microgel particles with different pore characteristics includes the following steps:

[0106] P1. Following steps G1 to G5 in the improved scheme A, washed calcium-aluminum ion gradient crosslinked wet gel particles were prepared.

[0107] P2. Preparation of slow-frozen macroporous microgel particles, specifically:

[0108] a. Spread the wet gel particles evenly in a shallow dish, with a layer thickness of 5 mm.

[0109] b. Place it in a programmed cooling freezer and slowly cool it from room temperature to -20°C at a rate of 0.5°C per minute, and maintain it at that temperature for 4 hours to complete the slow freezing process.

[0110] c. The slow-frozen samples were then subjected to vacuum freeze-drying for 36 hours.

[0111] d. After drying, the particles are sieved to obtain slow-frozen macroporous microgel particles with an average particle size of 8-15 μm, a porosity greater than 80%, and an average pore size in the range of 100-300 nm.

[0112] P3. Preparation of rapidly frozen dense microgel particles, specifically as follows:

[0113] a. Slowly drip wet gel particles into n-hexane pre-cooled to -196°C with liquid nitrogen, causing them to freeze instantly into solid beads.

[0114] b. Quickly transfer the frozen beads to the freeze dryer cold trap, which has been pre-cooled to -40°C.

[0115] c. Perform vacuum freeze drying for 36 hours.

[0116] d. After drying, the particles are sieved to obtain rapidly frozen dense microgel particles with an average particle size of 5-10 μm, a porosity of 60%-70%, and an average pore size of less than 50 nm.

[0117] P4. The slow-frozen macroporous microgel particles obtained in step P2 and the fast-frozen dense microgel particles obtained in step P3 are physically mixed at a dry basis mass ratio of 1:2 to obtain the final calcium-aluminum ion gradient crosslinked microgel particles with pore regulation.

[0118] Based on Experiment 7 of Example 2, the technical solution of this embodiment was tested. Experiment 8 was used as Experiment 8. The difference between Experiment 8 and Experiment 7 is that Experiment 8 uses pore-controlled calcium-aluminum ion gradient crosslinked microgel particles (i.e., a mixture of slow-frozen macroporous and fast-frozen dense types at a dry basis mass ratio of 1:2). The rest of the formulation and process parameters are exactly the same.

[0119] Application scenario testing: Two sets of films were made into packaging bags of the same specifications for packaging a fixed quantity of freshly cut lettuce (e.g., 100g per bag), and sealed. The bags were placed in a 4°C refrigerator to simulate cold chain sales and storage, and were observed and tested regularly.

[0120] 1. Monitoring of the internal microenvironment of packaging:

[0121] (1) Observation and weighing of condensate inside the bag: On the 1st, 3rd, 5th and 7th day of storage, open the packaging bag, gently absorb all visible condensate on the inner wall of the bag with pre-dried and weighed filter paper, weigh immediately, and calculate the mass of condensate. At the same time, record the time of condensate appearance and distribution.

[0122] (2) Measurement of film moisture absorption rate: After 7 days of storage, the film was taken out, the surface was quickly wiped dry, and the percentage increase in its weight compared to before packaging was measured.

[0123] 2. Monitoring of thin film performance changes:

[0124] Performance retention rate after storage: After 7 days of storage, the film was removed from the packaging and equilibrated under standard conditions for 48 hours. Its oxygen permeability and right-angle tear strength were then tested and compared with the initial values ​​before packaging to calculate the retention rate.

[0125] Testing revealed that in a comparative test of packaged fresh-cut lettuce, experimental group 8, employing pore-controlled microgels, exhibited superior microenvironment management and film performance retention. Regarding microenvironment monitoring within the packaging, by day 5 of storage, only minimal or no significant condensation was observed inside the packaging of experimental group 8, while experimental group 7 showed more and more widely distributed condensation. After 7 days of storage, the moisture absorption rate of the film in experimental group 8 was approximately 3%, significantly lower than the 5% of experimental group 7. In terms of monitoring changes in film performance, after 7 days of storage, the oxygen permeability retention rate of experimental group 8 was approximately 92%, and the tear strength retention rate was approximately 88%, both significantly higher than the 85% and 80% of experimental group 7, respectively.

[0126] The technical solutions described in the embodiments of this application have at least the following technical effects or advantages:

[0127] This solution aims to address the degradation of material properties and accelerated food spoilage caused by oversaturated humidity within the packaging microenvironment of biodegradable and breathable membranes when used to package foods with high water activity. This solution significantly improves humidity management within the packaging, specifically by drastically reducing condensation and lowering the film's own moisture absorption rate. This effective humidity control directly slows down the performance degradation of the film in humid environments, ensuring that the film maintains high retention rates of oxygen barrier properties and mechanical strength during later stages of storage.

[0128] The technical principle behind these effects lies in the artificial construction of two complementary pore structures within a chemically homogeneous microgel community through differentiated freeze-drying processes. The macroporous particles formed by slow freezing possess high porosity and an open pore structure, enabling them to rapidly adsorb large amounts of gaseous and liquid moisture. They act as an immediate humidity buffer, quickly controlling humidity levels below the condensation point when humidity inside the packaging rises sharply. Conversely, the dense particles formed by rapid freezing have smaller pore sizes and a more robust network structure. Their hygroscopic kinetics are slower, and their primary function is to maintain the overall structural rigidity of the gel and serve as a long-term reservoir of calcium and aluminum ions, providing continuous neutralization and stabilization during slight hydrolysis of the PLA matrix. When these two types of particles are mixed in a specific ratio, a dynamic humidity management mechanism is formed in the composite material, working synergistically on both rapid moisture absorption and long-term moisture stabilization timescales.

[0129] This embodiment of the solution actively intervenes in the evolution path of the packaging microenvironment at the level of physical adsorption by precisely controlling the physical structure of the microgel. The result is the avoidance of a vicious cycle where high humidity causes microgel swelling, which in turn accelerates PLA hydrolysis, ultimately leading to even higher environmental humidity. This improves the durability of the film material in high-humidity applications and creates a more stable and suitable preservation environment for the packaged food, thus achieving a synergistic solution to both material performance maintenance and food shelf-life extension.

[0130] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. For those skilled in the art, the present invention can have various modifications and variations. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A biodegradable breathable membrane, characterized in that, By weight, it includes the following ingredients: 60 parts of polylactic acid modified with cerium complex; 12 parts of natural cellulose modified with isophorone diisocyanate and polydopamine; 1.5 parts plasticizer; Compatibilizer 3.5 parts; 1 part lubricant; 0.1-2 parts of calcium stearate, wherein the calcium stearate has a particle size D50 ≤ 5 μm; 0.5-5 parts of microgel particles, wherein the microgel particles are pre-crosslinked calcium alginate microgel particles with a particle size of 5-10 μm; Alternatively, the microgel particles are calcium-aluminum ion gradient cross-linked microgel particles, wherein the molar ratio of calcium ions to aluminum ions is 2:

1.

2. The biodegradable breathable membrane as described in claim 1, characterized in that, The method for preparing the pre-crosslinked calcium alginate microgel particles, The process includes the following steps: adding a 3.5% sodium alginate aqueous solution dropwise to an 8% calcium chloride aqueous solution for cross-linking and curing, collecting the gel particles and washing them; then dispersing the gel particles in a 1% glutaraldehyde aqueous solution, adjusting the pH of the system to 4.5, and reacting to carry out pre-cross-linking reinforcement. After washing, freeze-drying, and sieving, pre-crosslinked calcium alginate microgel particles with a particle size of 5-10 μm were obtained.

3. The biodegradable breathable membrane as described in claim 1, characterized in that, The method for preparing the calcium-aluminum ion gradient crosslinked microgel particles, The process includes the following steps: a 3.5% sodium alginate aqueous solution is added dropwise to a mixed crosslinking solution containing calcium and aluminum ions for initial crosslinking, wherein the calcium ion concentration is 1 mol / L and the aluminum ion concentration is 0.5 mol / L; then ammonia is added dropwise to the system to uniformly adjust the pH from 5.8 to 8.0 within 90 minutes; the gel particles are collected, washed, freeze-dried, and sieved to obtain calcium-aluminum ion gradient crosslinked microgel particles with a particle size of 5-10 μm.

4. The biodegradable breathable membrane as described in claim 1, characterized in that, The calcium-aluminum ion gradient crosslinked microgel particles are calcium-aluminum ion gradient crosslinked microgel particles that have undergone pore regulation treatment. The pore regulation treatment includes preparing slow-frozen macroporous microgel particles and fast-frozen dense microgel particles, and mixing the two at a dry basis mass ratio of 1:

2.

5. The biodegradable breathable membrane as described in claim 4, characterized in that, The slow-frozen macroporous microgel particles have an average particle size of 8-15 μm, a porosity greater than 80%, and an average pore size of 100-300 nm. The fast-frozen dense microgel particles have an average particle size of 5-10 μm, a porosity of 60%-70%, and an average pore size of less than 50 nm.

6. The biodegradable breathable membrane as described in claim 1, characterized in that, The plasticizer is epoxidized soybean oil; the compatibilizer is styrene-maleic anhydride-acrylonitrile copolymer; the lubricant is ethylene bis-stearamide; and the calcium stearate is food grade.

7. A method for preparing a biodegradable breathable membrane as described in any one of claims 1-6, characterized in that, Includes the following steps: S1. Weigh the modified polylactic acid, modified natural cellulose, plasticizer, compatibilizer, lubricant, microgel particles and calcium stearate according to the weight ratio; S2. Put all the raw materials weighed in step S1 into a high-speed mixer and mix at 1500 r / min for 25 minutes at room temperature to obtain a premix; S3. The premix obtained in step S2 is fed into a twin-screw extruder for melt blending and extrusion granulation. The extruder temperature is set sequentially from the feed port to the die head as follows: Zone 1 170℃, Zone 2 175℃, Zone 3 180℃, Zone 4 185℃, and Die head 190℃. The screw speed is 250 r / min. After water cooling and pelletizing, blown film masterbatch is obtained. S4. The blown film masterbatch obtained in step S3 is fed into a blown film machine to blow-form a film. The screw temperature of the blown film machine is controlled at 165°C, the die head temperature is controlled at 175°C, the blow-up ratio is adjusted to 2.2:1, the traction speed is 15m / min, and the film thickness is controlled at 35μm to obtain the biodegradable breathable film.

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