Fully degradable wrapping film and its preparation method and application

Abstract

The application provides a fully degradable wrapping film and a preparation method and application thereof, and belongs to the technical field of wrapping films. The fully degradable wrapping film comprises, from inside to outside, an inner layer, a middle layer and an outer layer arranged in sequence; the outer layer comprises, with the mass percentage of the outer layer being 100%, 30%-70% of a first biodegradable polyester with rigid chains, 15%-50% of biomass fillers and 5-25% of toughening fibers; the middle layer comprises, with the mass percentage of the middle layer being 100%, 40%-90% of a second biodegradable polyester with flexible chains and 5%-30% of hydrophobically modified starch; and the inner layer comprises, with the mass percentage of the inner layer being 100%, 25%-65% of a water-soluble polymer, 5%-25% of a biomass degradation promoter and 20%-60% of a natural resin tackifier. The wrapping film realizes the unity of full degradability, good mechanical properties, environmental adaptability and cost economy through the synergy of the three layers, and can effectively adapt to the packaging requirements of multiple scenes such as sea transportation and high-value logistics.

Description

Technical Field

[0001] This application relates to the field of stretch film technology, specifically to a fully degradable stretch film, its preparation method, and its application. Background Technology

[0002] Logistics packaging stretch film, with its strong wrapping force, self-adhesion and shrinkage properties, is used in conjunction with pallets to tightly and securely bundle products into a unit. It is widely used for the whole-unit packaging of products in various fields such as hardware, mining, chemical, pharmaceutical, food and machinery.

[0003] Currently, traditional stretch films on the market mainly use petroleum-based polymers as the base material. Although cost-effective, they have significant problems: long degradation cycles, easy environmental pollution, and microplastics produced after aging can harm the ecology and health. While alternatives such as wooden crates and kraft paper exist, wooden crates have drawbacks such as high cost, low operational efficiency, and inability to fit irregularly shaped goods. Kraft paper suffers from low tensile strength, weak puncture resistance, and poor waterproof and moisture-proof properties. Pure bio-based polymers and bio-based stretch films formed by blends of them also have issues with tensile strength, puncture strength, degradation speed, and applicable scenarios. Therefore, there is an urgent need for a stretch film that balances full degradability, good mechanical properties, and cost-effectiveness to meet the needs of various scenarios, including maritime transport. Summary of the Invention

[0004] In view of this, in order to at least partially solve at least one of the aforementioned technical problems, this application provides a fully degradable wrapping film and its preparation method and application.

[0005] To achieve the above objectives, the technical solution of this application is as follows:

[0006] According to one embodiment of this application, a fully degradable wrapping film is provided, comprising an inner layer, a middle layer, and an outer layer arranged sequentially from the inside out.

[0007] The outer layer comprises 100% by weight and includes: 30% to 70% of a first biodegradable polyester with rigid chains, 15% to 50% of biomass filler, and 5% to 25% of toughening fiber.

[0008] The middle layer comprises 100% by weight, and includes: 40% to 90% of a second biodegradable polyester with a flexible chain and 5% to 30% of hydrophobically modified starch.

[0009] The inner layer comprises 100% by weight and includes: 25% to 65% water-soluble polymer, 5% to 25% biomass degradation promoter, and 20% to 60% natural resin tackifier.

[0010] In some embodiments, the first biodegradable polyester includes at least one of polylactic acid, polyhydroxyalkanoate, copolymer of polylactic acid and polyglycolic acid, and copolymer of polylactic acid and polycaprolactone.

[0011] Biomass fillers include at least one of rice husk ash, bamboo powder, wheat straw ash, lignin, microcrystalline cellulose, and chitosan.

[0012] Toughening fibers include at least one of cellulose fibers, protein fibers, and starch-based fibers. Cellulose fibers are selected from at least one of regenerated cellulose fibers and microfibrillated cellulose. Protein fibers include silk fibroin fibers. Starch-based fibers include modified starch fibers.

[0013] The second biodegradable polyester includes at least one of polybutylene terephthalate, polybutylene succinate, and polycaprolactone.

[0014] Hydrophobically modified starch includes at least one of acetylated starch, adipate-esterified starch, polylactic acid-grafted starch, and lignocellulose powder.

[0015] Water-soluble polymers include at least one of polyvinyl alcohol, polyvinyl acetate, modified starch, polyglutamic acid, and pullulan.

[0016] Biomass degradation promoters include at least one of cassava starch, corn starch, potato starch, and amylopectin.

[0017] Natural resin tackifiers include at least one of rosin glycerol esters, hydrogenated rosin esters, terpene resins, and shellac.

[0018] In some embodiments, the thickness of the inner layer is 15% to 25% of the thickness of the fully degradable stretch film; the thickness of the middle layer is 45% to 55% of the thickness of the fully degradable stretch film; and the thickness of the outer layer is 25% to 35% of the thickness of the fully degradable stretch film.

[0019] In some embodiments, the second biodegradable polyester is 100% by mass, and the second biodegradable polyester includes 5% to 50% recycled biodegradable polyester; the recycled biodegradable polyester includes at least one of recycled polybutylene terephthalate, recycled polybutylene succinate, and recycled polylactic acid.

[0020] In some embodiments, the outer layer further includes: bio-based plasticizers and / or biodegradable plasticizers.

[0021] In some embodiments, the bio-based plasticizer includes at least one of acetylated tributyl citrate, triethyl citrate, and epoxidized soybean oil, and the biodegradable plasticizer is selected from polyethylene glycol.

[0022] In some embodiments, the outer layer is 100% by mass, and the amount of bio-based plasticizer or biodegradable plasticizer is 1% to 8%.

[0023] In some embodiments, the middle layer further includes at least one of: a nano-reinforcing material and a compatibilizer;

[0024] In some embodiments, the nano-reinforcing material includes at least one of layered silicates, nano-silica, cellulose nanocrystals, and bio-based carbon dots.

[0025] In some embodiments, the compatibilizer includes at least one of titanate coupling agents, aluminate coupling agents, and hydrolyzable silane coupling agents.

[0026] In some embodiments, the mass percentage of the middle layer is 100%, the amount of nano-reinforcing material is 0.05% to 1.5%, and the amount of compatibilizer is 0.2% to 3%.

[0027] In some embodiments, the inner layer further includes at least one of: a mildew inhibitor and a moisture absorber;

[0028] In some embodiments, the antifungal agent includes at least one of sodium benzoate, potassium sorbate, natamycin, and chitosan.

[0029] In some embodiments, the moisture-proofing agent includes nano-sized moisture-proofing fillers selected from at least one of nano-silica, nano-calcium carbonate, and modified kaolin.

[0030] In some implementations, the amount of antifungal agent is 0.1% to 2% and the amount of moisture-proof agent is 0.5% to 5%, with the inner layer accounting for 100% by mass.

[0031] In some embodiments, the fully degradable stretch film has the following properties: tensile strength of 350% to 400%; puncture strength of 250 to 280 g / μm; tensile strength of 35 to 40 MPa; and light transmittance of 85% to 88%.

[0032] According to another embodiment of this application, a method for preparing the above-mentioned fully degradable wrapping film is provided, comprising:

[0033] The raw materials for the outer and middle layers are added to different extruders for extrusion casting to obtain a cast film with a composite middle and outer layer.

[0034] The raw materials for the inner layer are mixed and then sprayed onto the middle layer surface of the cast film to obtain a composite film.

[0035] The composite film is first stretched longitudinally, then stretched transversely, and then heat-set, corona-treated, and cured to obtain a fully degradable wrapping film.

[0036] In some embodiments, the extrusion temperature of the outer layer is 160~170°C; the extrusion temperature of the middle layer is 140~150°C; and the temperature of the cast film during inner layer spraying is 130~140°C.

[0037] In some implementations, the longitudinal stretching ratio is 2.8 to 3.5:1, and the transverse stretching ratio is 2.3 to 3.0:1.

[0038] In some embodiments, the heat setting temperature is 30~40°C;

[0039] In some implementations, the conditions for corona treatment are: power 3~5kW, unwinding speed 10~20m / min;

[0040] In some embodiments, the curing conditions are: temperature 20℃~30℃, humidity 50%~60%, and curing time 12~48h.

[0041] According to another embodiment of this application, an application of the above-described fully degradable stretch film in product packaging is provided.

[0042] Compared with existing technologies, the technical solution provided in this application has at least the following beneficial effects: The fully degradable stretch film of this application achieves functional complementarity and performance improvement through a three-layer synergistic design. The outer layer uses a first biodegradable polyester with rigid chains as the matrix, combined with biomass fillers and toughening fibers to improve hardness, puncture strength, UV resistance, and abrasion resistance, forming a reliable outer layer protection. The middle layer uses a second biodegradable polyester with flexible chains combined with hydrophobically modified starch to optimize tensile strength and creep resistance, ensuring the stability of the stretch film's shape. The inner layer, through the combination of water-soluble polymers, natural resin tackifiers, and biomass degradation promoters, achieves stable and durable self-adhesion while possessing good moisture-proof performance and efficient degradation characteristics. Moreover, all three layers are made of degradable materials with wide availability. The synergistic effect of the three layers enables the stretch film to possess full degradability, good mechanical properties, environmental adaptability, and cost-effectiveness. This fully degradable stretch film can be widely used in the packaging of various products such as electronics, electrical appliances, and cold chain food, and is especially suitable for harsh scenarios such as sea freight and high-humidity storage, taking into account both cargo safety protection and environmental protection requirements. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments.

[0044] In the following detailed description, numerous specific details are set forth for ease of explanation to provide a thorough understanding of the embodiments of this application. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0045] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The term "comprising" as used herein indicates the presence of features, steps, or operations, but does not exclude the presence or addition of one or more other features.

[0046] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0047] Due to the non-degradability, microplastic pollution, and low recycling rate of traditional petroleum-based stretch films, various alternatives have emerged in the industry, but all have significant drawbacks: wooden crates are costly and have poor adaptability; kraft paper wrapping films lack tensile strength and puncture resistance, and have weak environmental adaptability; among bio-based stretch films, pure polylactic acid (PLA) films have insufficient mechanical properties; blends of polybutylene terephthalate (PBAT) and polylactic acid (PLA) films degrade slowly, have weak resistance to environmental erosion, and insufficient performance, making it difficult to meet the long-term protection requirements of scenarios such as maritime packaging; starch-based stretch films are susceptible to humidity and prone to mold growth; and polyhydroxyalkanoate (PHA)-based films are too expensive and have low elongation. None of the existing solutions can simultaneously achieve full degradability, good mechanical properties, and cost-effectiveness.

[0048] In the process of realizing the concept of this application, it was found that if a multi-layer composite structure can be designed, and the core functions of wear resistance and anti-aging, high tensile strength and creep resistance, self-adhesive moisture-proof and mildew-proof can be given to the film layer through precise selection and synergistic ratio of materials in each layer, while adopting a biodegradable material system, the environmental pain points of traditional stretch film and the performance and cost shortcomings of existing alternatives can be solved, thus achieving a unity of environmental protection, practicality and economy.

[0049] Based on this, this application proposes a fully degradable stretch film, its preparation method, and its application. The outer layer uses biodegradable polyester as the base material, combined with biomass fillers and toughening fibers to ensure mechanical strength, transparency, anti-aging properties, and surface abrasion resistance. The middle layer uses flexible biodegradable polyester as the main body, combined with hydrophobically modified starch and recycled materials to provide high elongation at break, creep resistance, and impact cushioning capacity. The inner layer uses water-soluble polymer as the core, combined with natural tackifiers and degradation promoters to achieve self-adhesive, moisture-proof, and mildew-proof functions. Simultaneously, by combining three-layer co-extrusion casting, stretching and orientation, etc., a stretch film that balances full degradability, good mechanical properties, environmental adaptability, and cost-effectiveness is finally obtained, effectively adapting to packaging needs in various scenarios such as maritime transport and high-value logistics.

[0050] Specifically, according to one aspect of this application, a fully degradable wrapping film is provided, comprising an inner layer, a middle layer, and an outer layer arranged sequentially from the inside out.

[0051] The outer layer comprises 100% by weight and includes: 30% to 70% of a first biodegradable polyester with rigid chains, 15% to 50% of biomass filler, and 5% to 25% of toughening fiber.

[0052] The middle layer comprises 100% by weight, and includes: 40% to 90% of a second biodegradable polyester with a flexible chain and 5% to 30% of hydrophobically modified starch.

[0053] The inner layer comprises 100% by weight and includes: 25% to 65% water-soluble polymer, 5% to 25% biomass degradation promoter, and 20% to 60% natural resin tackifier.

[0054] In some specific embodiments, based on the total mass of the outer layer, the mass percentage of the first biodegradable polyester can be, for example, 30%, 40%, 50%, 60%, 70%, etc., to provide stable structural support and basic mechanical strength as the outer layer matrix. If the percentage is too low, the wrapping film is prone to brittleness and difficult to form; if it is too high, it will lead to uneven dispersion of biomass filler and toughening fiber. The mass percentage of biomass filler can be, for example, 15%, 20%, 30%, 40%, 50%, etc. An appropriate amount of biomass filler can form a nano-filling network in the first biodegradable polyester, improving the hardness, wear resistance and anti-aging properties of the wrapping film. An inappropriate percentage will result in insufficient protective effect or decreased mechanical properties. The mass percentage of toughening fiber can be, for example, 5%, 10%, 15%, 20%, 25%, etc. An appropriate amount of toughening fiber can reduce its crystallinity and enhance toughness and puncture resistance by forming hydrogen bonds with the first biodegradable polyester.

[0055] In some specific embodiments, based on the total mass of the middle layer, the mass percentage of the second biodegradable polyester can be, for example, 40%, 50%, 60%, 70%, 80%, 90%, etc., serving as the matrix material of the middle layer and providing the stretchability and creep resistance of the wrapping film; the mass percentage of the hydrophobically modified starch can be, for example, 5%, 10%, 15%, 20%, 25%, 30%, etc. An appropriate amount of hydrophobically modified starch can interact with the second biodegradable polyester through its hydrophobic groups, optimizing the interfacial compatibility and moisture resistance between the second biodegradable polyester and the hydrophobically modified starch, preventing the film material from bulging in high humidity environments, and providing degradation sites. An imbalance in the proportion will affect the protective effect and mechanical stability.

[0056] In some specific embodiments, based on the total mass of the inner layer, the mass percentage of the water-soluble polymer can be, for example, 25%, 30%, 40%, 50%, 60%, 65%, etc., serving as the basis for the adhesive film formation of the inner layer. If the percentage is too low, the self-adhesion will be insufficient; if it is too high, the hygroscopicity will be too strong. The mass percentage of the biomass degradation promoter can be, for example, 5%, 10%, 15%, 20%, 25%, etc. Appropriate addition can provide sufficient initial attack points for microorganisms and accelerate the degradation process of the membrane material. An improper percentage will affect the degradation efficiency or cause the stretch film to become brittle. The mass percentage of the natural resin tackifier can be, for example, 20%, 30%, 40%, 50%, 60%, etc. The active groups contained in the natural resin can form a stable bond with the water-soluble polymer, enhancing the adhesion and durability. If the percentage is too high, the tackifier is prone to migration and precipitation, causing adhesion on the surface of the stretch film, and may also affect the light transmittance and degradation performance of the stretch film.

[0057] According to embodiments of this application, the outer layer is composed of a first biodegradable polyester with rigid chains, biomass filler, and toughening fibers working synergistically. The polyester provides a stable structure and basic strength, while the biomass filler forms a nano-network, improving properties such as hardness, abrasion resistance, and UV shielding. The toughening fibers form hydrogen bonds with the polyester, preventing embrittlement and enhancing puncture resistance. The middle layer is primarily composed of a second biodegradable polyester with flexible chains, ensuring tensile strength and creep resistance. Hydrophobically modified starch optimizes its compatibility and moisture resistance with the second biodegradable polyester, while also providing degradation sites. The inner layer is composed of a water-soluble polymer, a natural resin tackifier, and a biomass degradation promoter. The water-soluble polymer constructs an adhesive coating, the tackifier enhances adhesive durability, and the degradation promoter accelerates the disintegration of the stretch film, ensuring its degradation performance. The outer protective layer, the middle support layer, and the inner functional adaptation layer work synergistically to achieve a balance between the stretch film's full degradability, good mechanical properties, environmental adaptability, and cost-effectiveness, adapting to various packaging needs.

[0058] According to embodiments of this application, the first biodegradable polyester includes at least one of polylactic acid (PLA), polyhydroxyalkanoate (PHA), a copolymer of polylactic acid and polyglycolic acid (PLA-PGA), and a copolymer of polylactic acid and polycaprolactone (PLA-PCL). The first biodegradable polyester is selected from biodegradable rigid or semi-rigid polyesters. These polyesters have regular molecular chain structures and excellent mechanical strength, providing a stable structural framework for the outer layer, ensuring the basic rigidity and morphological stability of the membrane material, while also possessing good biodegradability, avoiding the environmental burden caused by the non-degradability of the matrix material.

[0059] Biomass fillers include at least one of rice husk ash, bamboo powder, wheat straw ash, lignin, microcrystalline cellulose, and chitosan. Biomass fillers are selected from biodegradable natural inorganic / organic reinforcing fillers. These fillers are not only widely available and inexpensive, but also possess natural functional properties. For example, rice husk ash contains over 90% amorphous silica (SiO2), which can form a uniformly dispersed nanoscale filling network in the first biodegradable polyester matrix. On the one hand, this improves the hardness of the polyester matrix, enhances surface wear resistance, and resists frictional loss during sea transport and logistics. On the other hand, amorphous SiO2 has excellent ultraviolet (UV) shielding capabilities (shielding rate up to 85%), increasing the UV shielding rate of the stretch film, effectively inhibiting degradation of the outer layer of the stretch film caused by photoaging, and extending the service life of the stretch film.

[0060] Toughening fibers include at least one of cellulose fibers, protein fibers, and starch-based fibers. Cellulose fibers are selected from at least one of regenerated cellulose fibers and microfibrillated cellulose; protein fibers include silk fibroin fibers; and starch-based fibers include modified starch fibers. The hydroxyl groups on the toughening fiber molecular chains can form stable hydrogen bonds with the first biodegradable polyester (such as PLA), which can reduce the crystallinity of the polyester, preventing membrane embrittlement caused by excessive crystallinity, and inhibit crack propagation under external impact, thereby significantly enhancing the toughness and puncture strength of the membrane, solving the problems of rigid polyester's easy breakage and insufficient puncture resistance. Different types of toughening fibers can be flexibly combined according to actual performance requirements to further optimize the mechanical balance of the outer layer.

[0061] According to the embodiments of this application, the outer layer of the stretch film is designed to provide basic strength through a matrix polyester, enhance functional properties through biomass fillers, and optimize mechanical toughness through toughening fibers, thereby ensuring the mechanical properties, structural stability, and environmental adaptability of the stretch film.

[0062] According to embodiments of this application, the second biodegradable polyester includes at least one of polybutylene terephthalate (PBAT), polybutylene succinate (PBS), and polycaprolactone (PCL). The second biodegradable polyester is a biodegradable flexible polyester with good chain segment flexibility and high elongation at break, providing high elasticity and tensile properties for the middle layer of the stretch film. Simultaneously, it possesses good biodegradability, maintaining consistency with the biodegradable material system of the outer and inner layers, ensuring the overall complete degradability of the stretch film.

[0063] Hydrophobically modified starch includes at least one of acetylated starch, adipate-esterified starch, polylactic acid-grafted starch, and lignocellulose powder. The hydrophobic groups of the modified starch can form good interactions with the ester bonds on the molecular chain of a second biodegradable polyester (such as PBAT), significantly improving the interfacial bonding between starch and the polyester matrix and avoiding internal defects caused by poor compatibility of traditional unmodified starch. The modified starch exhibits significantly enhanced hydrophobicity, effectively resisting high-humidity environments in maritime transport and warehousing scenarios, preventing starch from absorbing moisture and expanding, which can lead to bulging and cracking of the film. This significantly improves the retention rate of the intermediate layer's tensile strength, solving the problem of performance degradation under high humidity in existing starch-based films. Furthermore, starch materials are widely available and inexpensive; starch is a natural biodegradable polysaccharide that can provide initial contact sites for microorganisms, accelerating the degradation process of the intermediate layer and even the entire winding film.

[0064] According to the embodiments of this application, the middle layer of the stretch film provides high elasticity and tensile strength through flexible polyester. The synergistic design of hydrophobically modified starch optimizes the interfacial compatibility and water resistance between starch and flexible polyester and reduces costs, ensuring the mechanical property stability of the middle layer of the stretch film, while taking into account degradation efficiency and cost economy, thus providing support for the stretch film to adapt to harsh logistics scenarios such as long-term sea freight and high stacking.

[0065] According to embodiments of this application, the water-soluble polymer includes at least one of polyvinyl alcohol, polyvinyl acetate, modified starch, polyglutamic acid, and pullulan. The molecular chains of this type of polymer contain polar groups such as hydroxyl and carboxyl groups, which can form hydrogen bonds with water molecules to provide basic adhesion and ensure the initial adhesion between film layers. Simultaneously, it is biodegradable and can be flexibly adjusted according to adhesion requirements to adapt to the adhesion strength requirements of different packaging scenarios.

[0066] Biomass degradation accelerators include at least one of cassava starch, corn starch, potato starch, and amylopectin. These biomass degradation accelerators are widely available, inexpensive, and their molecular structure is easily decomposed and utilized by microorganisms: as an inner layer component, they provide initial sites of action for microorganisms in the environment, accelerating the erosion of water-soluble polymers, thickeners, and other components by microorganisms, thus promoting the disintegration of the inner layer.

[0067] Natural resin tackifiers include at least one of rosin glycerol esters, hydrogenated rosin esters, terpene resins, and shellac. These natural resins contain active groups such as carboxyl and hydroxyl groups, which can form stable bonds with water-soluble polymers (such as PVA) through esterification reactions. Their cyclic molecular structure enhances the durability of adhesion, avoiding the defects of traditional migratory tackifiers such as easy volatility and rapid adhesion decay. Furthermore, their initial tack and peel strength can be flexibly adjusted through the formulation ratio to meet the immediate bonding requirements after pre-stretching of the stretch film.

[0068] According to the embodiments of this application, the inner layer is designed with a synergistic approach: a water-soluble polymer forms an adhesive film base, a natural resin tackifier enhances the long-lasting self-adhesion, and a biomass degradation promoter accelerates the disintegration of the inner layer. This approach not only ensures the adhesive stability of the stretch film during storage and transportation but also enables it to respond quickly to the degradation environment. It works in conjunction with the outer and middle layer materials to achieve overall complete degradation while taking into account both processing adaptability and cost-effectiveness.

[0069] According to embodiments of this application, the thickness of the inner layer is 15% to 25% of the thickness of the fully degradable wrapping film, for example, it can be 15%, 17%, 20%, 22%, 25% of the thickness of the fully degradable wrapping film, etc.

[0070] The thickness of the middle layer is 45% to 55% of the thickness of the fully degradable stretch film, for example, it can be 45%, 47%, 50%, 52%, 55% of the thickness of the fully degradable stretch film.

[0071] The thickness of the outer layer is 25% to 35% of the thickness of the fully degradable wrapping film, for example, it can be 25%, 27%, 30%, 32%, 35% of the thickness of the fully degradable wrapping film.

[0072] According to the embodiments of this application, the middle layer of the stretch film dominates mechanical properties, the outer layer enhances environmental adaptability, and the inner layer ensures adhesion and degradation. The thickness ratio of the three layers matches the material characteristics and functions of each layer, which not only avoids the performance defects caused by a single layer being too thick or too thin, but also achieves a balance between full degradability, mechanical properties, environmental adaptability, and cost economy.

[0073] According to embodiments of this application, the second biodegradable polyester comprises 5% to 50% recycled biodegradable polyester, with a mass percentage of 100%. For example, it may include recycled biodegradable polyester comprising 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, and 50%. The recycled biodegradable polyester is obtained by processing recovered biodegradable polyester through a depolymerization-repolymerization process. The loss of mechanical properties can generally be controlled within 10%, thus maintaining the tensile and creep-resistant mechanical properties of the middle layer while achieving the recycling of biodegradable materials. Simultaneously, it effectively reduces raw material costs and further optimizes the product's cost-effectiveness.

[0074] Recycled biodegradable polyesters include at least one of recycled polybutylene terephthalate (recycled PBAT), recycled polybutylene succinate (recycled PBS), and recycled polylactic acid (recycled PLA).

[0075] According to embodiments of this application, the outer layer further includes: bio-based plasticizers and / or biodegradable plasticizers, which further improve the processing fluidity and low-temperature toughness of the inner layer material, and are suitable for large-scale production processes such as co-extrusion casting and low-temperature logistics scenarios.

[0076] Bio-based plasticizers include at least one of acetylated tributyl citrate (ATBC), triethyl citrate, and epoxidized soybean oil, while biodegradable plasticizers are selected from polyethylene glycol. Taking ATBC as an example, its molecules can insert into the spaces between the first biodegradable polyester molecular chains, disrupting intermolecular forces to lower the glass transition temperature (Tg), improving melt flow during material processing, and preventing problems such as clumping and unevenness during inner layer extrusion. Simultaneously, it enhances the toughness of the outer layer at low temperatures, preventing brittleness. Furthermore, these plasticizers themselves possess biodegradability or environmental degradability and do not affect the overall fully degradable characteristics of the stretch film.

[0077] The outer layer is 100% by mass, and the amount of bio-based plasticizer or biodegradable plasticizer is 1% to 8%, such as 1%, 2%, 4%, 5%, 6%, 8%, etc. It can be flexibly adjusted according to the type of biodegradable polyester in the outer layer, processing parameters and target performance requirements, to ensure that the mechanical properties and degradation efficiency of the outer layer are not damaged while optimizing processing performance and low temperature adaptability.

[0078] According to embodiments of this application, the middle layer further includes at least one of: a nano-reinforcing material and a compatibilizer.

[0079] Nanomaterials include at least one of layered silicates, nano-silica, cellulose nanocrystals, and bio-based carbon dots. These materials, due to their nanoscale size effect, can form a uniformly dispersed three-dimensional network structure within the middle layer, effectively dispersing stress generated during the use of the wound membrane and inhibiting molecular chain slippage, thereby significantly improving the membrane's creep resistance and overall strength.

[0080] Compatibilizers include at least one of titanate coupling agents, aluminate coupling agents, and hydrolyzable silane coupling agents to improve the interfacial compatibility between the second biodegradable polyester and components such as hydrophobically modified starch. Taking titanate coupling agents as an example, their alkoxy groups can react with the hydroxyl groups of starch molecules to form stable chemical bonds, reducing interfacial defects between different components, significantly improving interfacial bonding strength, and enhancing the tensile strength of the middle layer.

[0081] With the middle layer accounting for 100% by mass, the amount of nano-reinforcing material is 0.05% to 1.5%, for example, 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, etc.; the amount of compatibilizer is 0.2% to 3%, for example, 0.2%, 0.5%, 0.8%, 1.0%, 2.0%, 3.0%, etc.

[0082] According to embodiments of this application, the inner layer further includes at least one of an antifungal agent and a moisture-proof agent.

[0083] The antifungal agents include at least one of sodium benzoate, potassium sorbate, natamycin, chitosan, and their derivatives. All are environmentally friendly antifungal agents, leaving no residue and compatible with degradation systems. These agents effectively prevent mold growth in stretch film during humid storage and long-term transportation by disrupting microbial cell membranes and inhibiting metabolic reproduction. This avoids film adhesion degradation and structural damage, ensuring packaging integrity and cargo safety.

[0084] The moisture-proofing agent includes nano-level moisture-proofing fillers, which are selected from at least one of nano-silica, nano-calcium carbonate, and modified kaolin. These nano-level moisture-proofing fillers can form a dense physical barrier network in the inner layer, significantly reducing the permeability of external moisture and preventing viscosity fluctuations and membrane bulging caused by moisture absorption by water-soluble polymers, starch, and other components in the inner layer, thereby further improving the performance stability of the stretch film in humid environments.

[0085] The amount of antifungal agent used, based on the mass percentage of the inner layer, is 0.1% to 2%, for example, 0.1%, 0.3%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, etc.; the amount of moisture-proof agent used is 0.5% to 5%, for example, 0.5%, 1.0%, 1.5%, 2.0%, 3.0%, 4.0%, 5.0%, etc.

[0086] According to embodiments of this application, the fully degradable stretch film meets at least one of the following conditions: tensile strength of 350%~400%, for example, 350%, 360%, 370%, 380%, 390%, 400%, etc.; puncture strength of 250~280 g / μm, for example, 250 g / μm, 260 g / μm, 270 g / μm, 280 g / μm, etc.; tensile strength of 35~40 MPa, for example, 35 MPa, 36 MPa, 37 MPa, 38 MPa, 39 MPa, 40 MPa, etc.; and light transmittance of 85%~88%, for example, 85%, 86%, 87%, 88%, etc.

[0087] In terms of mechanics, the stretch film of this application has good tensile strength, ensuring the retraction and binding force after pre-stretching. Its high puncture strength and tensile strength can withstand friction, compression and impact from sharp objects during logistics. In terms of practicality, its suitable light transmittance facilitates cargo sorting and verification. Combined with its UV resistance, moisture-proof, and mildew-proof properties, it is suitable for various scenarios such as ocean shipping and high-value logistics. In terms of environmental protection, the stretch film adopts a biodegradable material system, achieving a synergistic unity of environmental protection, mechanical performance and practical adaptability.

[0088] According to another embodiment of this application, a method for preparing the above-described fully degradable wrapping film is provided, comprising the following steps.

[0089] The raw materials for the outer and middle layers are added to different extruders for extrusion casting to obtain a cast film with a composite middle and outer layer.

[0090] The raw materials for the inner layer are mixed and then sprayed onto the middle layer surface of the cast film to obtain a composite film.

[0091] The composite film is first stretched longitudinally, then stretched transversely, and then heat-set, corona-treated, and cured to obtain a fully degradable wrapping film.

[0092] According to embodiments of this application, the preparation method of fully degradable stretch film adopts a step-by-step composite process: first, a composite cast film of middle and outer layers is obtained by co-extrusion casting, and then a composite film containing the inner layer is obtained by spray coating. This process can control the thickness ratio and interfacial bonding force of each layer, effectively avoiding problems such as phase separation caused by differences in processing temperature and melt viscosity of different layer materials, and ensuring the stable function of each layer. On this basis, biaxial stretching combined with heat setting treatment promotes the formation of an ordered orientation structure of molecular chains in the film, significantly improving the mechanical properties of the stretch film such as tensile strength, tensile strength, and puncture strength, while reducing creep deformation rate; subsequent corona treatment can enhance the surface tension of the film material and the interlayer bonding strength, while curing treatment can promote the interfacial fusion of each component and the migration balance of additives, ensuring uniform and stable product performance. The overall preparation process is simple and efficient, highly compatible with existing stretch film production lines, and can be adapted to the needs of different application scenarios by flexibly adjusting process parameters.

[0093] According to embodiments of this application, the extrusion temperature of the outer layer is 160~170℃, for example, 160℃, 162℃, 165℃, 168℃, 170℃, etc. This temperature allows the first biodegradable polyester to fully melt, ensuring that the biomass filler and toughening fiber are uniformly dispersed in the matrix to form a stable functional network; it also avoids excessively high temperatures that could lead to thermal degradation of the polyester, preventing a decrease in the mechanical properties of the wound film. Simultaneously, it is matched with a co-extrusion casting process to ensure the UV resistance, abrasion resistance, and other functional properties of the outer layer.

[0094] The extrusion temperature of the middle layer is 140~150℃, for example, 140℃, 142℃, 145℃, 147℃, 150℃, etc. This temperature can meet the melt processing requirements of the second biodegradable polyester, and is also suitable for the dispersion stability of hydrophobic modified starch, etc. If the temperature is too high, it will cause the polyester molecular chain to be excessively relaxed, affecting the creep resistance of the wound film, and may also cause thermal decomposition of hydrophobic modified starch, destroying the mechanical load-bearing function of the middle layer.

[0095] During inner layer spraying, the temperature of the cast film is 130~140℃, for example, 130℃, 132℃, 135℃, 137℃, 140℃, etc. During the spraying operation, the substrate supporting the inner layer (i.e., the outer / middle layer composite cast film) needs to be kept within an appropriate temperature range. This temperature range allows the inner layer material to soften moderately, ensuring that a continuous and uniform viscous coating can be formed on the surface of the cast film after spraying; at the same time, it avoids excessively high temperatures that cause the water-soluble polymer to decompose and the natural resin tackifier to volatilize and become ineffective, or excessively low temperatures that cause the coating to cure too quickly and have insufficient adhesion, thereby ensuring the stability of the self-adhesive and moisture-proof functions of the inner layer.

[0096] In some specific embodiments, the inner layer can be sprayed using an electrostatic spraying system, with a spraying voltage of, for example, 50kV and a spraying rate of, for example, 2kg / min.

[0097] According to embodiments of this application, the stretch ratio for longitudinal stretching (MDO) is 2.8 to 3.5:1, such as 2.8:1, 3.0:1, 3.1:1, 3.3:1, 3.5:1, etc.; the stretch ratio for transverse stretching (TDO) is 2.3 to 3.0:1, such as 2.3:1, 2.5:1, 2.8:1, 3.0:1, etc. An appropriate stretch ratio can promote the orderly arrangement of molecular chains within the film, significantly improving the tensile strength and breaking strength of the wrapping film, thus meeting the requirements for bidirectional mechanical properties during pre-stretch packaging.

[0098] In some specific embodiments, in order to ensure that the membrane material is subjected to uniform stress during biaxial stretching and to avoid interlayer peeling or stretching cracks, the composite membrane needs to be preheated before stretching. The preheating roller temperature is set as follows: 60°C for the outer layer and 50°C for the middle layer.

[0099] According to embodiments of this application, the heat setting temperature is 30~40℃, for example, it can be 30℃, 32℃, 35℃, 38℃, 40℃, etc. This temperature range can not only prevent the stretch film from melting and failing due to excessive temperature, but also fix the orientation structure of the stretched molecular chains through heat setting, thereby further improving the tensile strength and puncture strength of the stretch film.

[0100] According to embodiments of this application, the corona treatment conditions are: power 3~5kW, for example, 3 kW, 3.5 kW, 4 kW, 4.5 kW, 5 kW, etc.; unwinding speed 10~20 m / min, for example, 10 m / min, 13 m / min, 15 m / min, 18 m / min, 20 m / min, etc. Modifying the surface of the stretch film using plasma generated by high-frequency discharge can increase the number of hydroxyl groups (-OH) on the surface of the water-soluble polymer in the inner layer of the stretch film, thereby improving surface activity and adhesion performance.

[0101] According to embodiments of this application, the curing conditions are: temperature 20℃~30℃, humidity 50%~60%, and curing time 12~48h. Utilizing a lower curing temperature can prevent starch retrogradation in the inner and middle layers, preventing the stretch film from experiencing decreased toughness and viscosity fluctuations due to starch crystallization. A suitable humidity environment promotes the uniform diffusion of natural resin tackifiers onto the surface of the inner layer of the stretch film, further increasing the initial adhesion of the inner layer and ensuring adhesive stability during the packaging process.

[0102] In some specific embodiments, the curing temperature can be 20°C, 22°C, 25°C, 28°C, 30°C, etc., the humidity can be 50%, 52%, 55%, 58%, 60%, etc., and the curing time can be 12h, 18h, 20h, 24h, 30h, 36h, 48h, etc.

[0103] In some specific embodiments, the preparation method of the fully degradable stretch film also includes a slitting and winding process: the slitting accuracy is controlled to a width tolerance of ±0.2mm, the winding tension is controlled to ≤5N, and the winding method adopts center winding. This ensures that the width of the stretch film is uniform, meeting the specification requirements of different packaging scenarios; the low tension control combined with the center winding method can effectively avoid appearance defects such as indentations and wrinkles on the surface of the stretch film caused by extrusion, while preventing adhesion or stretching deformation between film layers, ensuring the stability of the mechanical properties and appearance quality of the finished film.

[0104] According to another embodiment of this application, an application of the above-described fully degradable stretch film in product packaging is provided.

[0105] According to embodiments of this application, the fully degradable stretch film relies on a fully biodegradable material system, which can degrade quickly in the natural environment without plastic residue pollution, thus meeting the needs of environmentally friendly packaging. The high tensile strength, puncture strength, and tensile strength of the fully degradable stretch film can be adapted to the bundling and packaging needs of various products such as electronics, cold chain food, building materials, and furniture, effectively resisting friction, compression, and puncture by sharp objects during transportation, reducing the damage rate of goods, and its good light transmittance facilitates the sorting and verification of goods in the warehousing process. The fully degradable stretch film also has moisture-proof, mildew-proof, and UV-resistant properties, making it suitable for harsh application scenarios such as sea transportation and warehousing in rainy areas. At the same time, its stable self-adhesion and good compatibility with existing production lines not only improve packaging operation efficiency but also control application costs, possessing outstanding value for large-scale promotion.

[0106] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with specific embodiments. Unless otherwise specified, all methods described in the embodiments are conventional and can be performed according to the techniques or conditions described in the literature or the product manual.

[0107] In the specific embodiments of this application, the sources of some of the main raw materials are briefly described below.

[0108] Polylactic acid (PLA): Commercially available, optical purity ≥96%.

[0109] Rice husk ash: Ash from agricultural waste rice husks after controlled-temperature combustion, with an amorphous silica (SiO2) content of ≥90%.

[0110] Regenerated cellulose fiber: Microfibrillated cellulose (MFC) obtained from recycled pulp, cotton pulp or wood pulp by mechanical or chemo-mechanical methods, with a fiber length ranging from 50 to 100 μm.

[0111] Polyvinyl alcohol (PVA): Commercially available, degree of alcoholysis 88%; particle size ≤50μm.

[0112] Acetylated starch: Obtained by esterification of corn starch with acetic anhydride.

[0113] Recycled polybutylene terephthalate (recycled PBAT): The recycled material of PBAT products (films) is obtained through washing, drying, depolymerization, and repolymerization processes.

[0114] In the following examples, all other chemicals and raw materials were commercially available or prepared using recognized processing methods. It should be noted that the specific examples described below are merely illustrative, and the scope of protection of this application is not limited thereto.

[0115] Example 1

[0116] Embodiment 1 of this application provides a fully degradable wrapping film, which includes an inner layer, a middle layer and an outer layer arranged sequentially from the inside out.

[0117] The outer layer is 100% by mass and consists of: 50% polylactic acid (PLA), 30% rice husk ash, and 20% regenerated cellulose fiber (50~100μm in length).

[0118] The middle layer comprises 100% by weight and consists of: 50% polybutylene terephthalate (PBAT), 30% acetylated starch, and 20% recycled PBAT.

[0119] The inner layer comprises 100% by mass and consists of: 40% polyvinyl alcohol (PVA) (88% degree of hydrolysis, powder particle size ≤50μm), 50% pre-dried cassava starch (moisture content ≤8%), and 10% rosin glycerol ester.

[0120] The preparation method of the fully degradable wrapping film includes the following steps (1) to (5).

[0121] (1) The PLA particles and rice husk ash were pre-dried to a moisture content of ≤0.5%, passed through an 800-mesh sieve, and mixed with regenerated cellulose fibers. The mixture was placed in a high-speed mixer and pre-dispersed for 10 minutes at 60°C and 300 rpm to obtain pre-dispersed material. The pre-dispersed material was added to a co-rotating twin-screw extruder for granulation. The process parameters were: rotation speed 200 rpm, temperature gradient 160°C (feeding section), 170°C (melting section), 165°C (die head section). Vacuum degassing was performed during granulation to remove moisture and volatiles, and the outer layer raw material was obtained.

[0122] (2) Add PBAT resin, acetylated starch and recycled PBAT to a mixer and shear and mix for 15 minutes at 60 rpm and 120 ℃ to obtain a premix; add the premix to a single screw extruder for extrusion granulation at a temperature range of 140~160 ℃ and filter through an 80 mesh screen to obtain the middle layer raw material.

[0123] (3) The outer layer material and the middle layer material are added to different extruders for extrusion casting, and the extrusion temperatures are 160~170 and 140~150 respectively, to obtain a cast film with a composite middle layer and outer layer;

[0124] (4) PVA powder, acetylated cassava starch and rosin glycerol are dry mixed for 5 minutes at room temperature and 200 rpm using a dry powder mixer to obtain a dry powder mixture; the dry powder mixture is uniformly sprayed onto the middle layer surface of the cast film using an electrostatic spraying system (voltage 50kV, spraying rate 2kg / min) to obtain a three-layer composite film.

[0125] (5) The three-layer composite film is first stretched longitudinally with a stretch ratio of 3:1, and then stretched transversely with a stretch ratio of 2.5:1. It is then heat-set at 40℃, followed by corona treatment at a power of 5kW and a processing speed of 20m / min. Finally, it is cured at 30℃ and 60% humidity for 48 hours to obtain a fully degradable wrapping film. The outer layer thickness is 30% of the total thickness of the fully degradable wrapping film; the middle layer thickness is 50% of the total thickness of the fully degradable wrapping film; and the inner layer thickness is 20% of the total thickness of the fully degradable wrapping film.

[0126] Comparative Example 1

[0127] Comparative Example 1 of this application provides a linear low-density polyethylene (LLDPE) stretch film.

[0128] Comparative Example 2

[0129] Comparative Example 2 of this application provides a 100% polylactic acid (PLA) stretch film.

[0130] Comparative Example 3

[0131] Comparative Example 3 of this application provides a polybutylene terephthalate-adipate and polylactic acid (PBAT / PLA) blended winding film, which comprises 60% PBAT, 30% PLA, and 10% starch / calcium carbonate filler.

[0132] Comparative Example 4

[0133] Comparative Example 4 of this application provides a starch-based stretch film comprising 65% modified starch, 20% PBS, and 15% PBAT.

[0134] Comparative Example 5

[0135] Comparative Example 4 of this application provides a polyhydroxyalkanoate (PHA) based winding film.

[0136] The mechanical properties, light transmittance, durability, and degradation performance of the fully degradable stretch film in Example 1 of this application and the stretch films in Comparative Examples 1 to 5 were tested. The specific test methods are as follows.

[0137] (1) Tensile strength and fracture strength test

[0138] Test standard: Refer to GB / T 1040.3-2006 (Plastics - Determination of tensile properties - Part 3: Test conditions for films and sheets).

[0139] Sample: Take a standard dumbbell-shaped or strip-shaped sample.

[0140] Conditions: Typically conducted under standard conditions of room temperature (23±2℃) and relative humidity of 50%±10%, with a tensile speed set to (e.g., 500±50) mm / min until the specimen breaks. Record the maximum load (used to calculate the breaking strength) and the elongation at fracture (tensile strength).

[0141] (2) Puncture strength

[0142] Test standard: Refer to the test method for puncture strength in GB / T 10004-2008 (dry lamination and extrusion lamination of plastic composite films and bags for packaging).

[0143] Method summary: Using a puncture probe of specified size and shape, the fixed membrane sample is punctured vertically at a constant speed. The maximum force required to puncture the membrane sample is recorded and converted into force per unit thickness (g / μm).

[0144] (3) Light transmittance

[0145] Test standard: Refer to GB / T 2410-2008 (Determination of light transmittance and haze of transparent plastics).

[0146] (4) Adhesion (initial adhesion)

[0147] Test standard: Refer to GB / T 4852-2002 (Test method for initial tack of pressure-sensitive adhesive tape - rolling ball method).

[0148] Method summary: The membrane sample is attached to a standard test plate (or another identical membrane sample) under a certain pressure, and then peeled off at a constant angle and speed. The average force value (N / cm) during the peeling process is recorded.

[0149] (5) Salt spray tolerance

[0150] Test standard: Refer to GB / T 1771-2007 (Determination of resistance to neutral salt spray of paints and varnishes).

[0151] Conditions: Prepare a (5±1)% sodium chloride solution and spray it continuously in a salt spray chamber at (35±2)℃ for 720 hours. After removing the sample, allow it to recover under standard conditions for 24 hours, and then measure its viscosity retention rate (viscosity after treatment / initial viscosity × 100%) according to the viscosity test method described above.

[0152] (6) Resistance to compressive creep

[0153] Test standard: Designed with reference to the ASTM D2990-17 compression creep test principle.

[0154] Method summary: The membrane sample of specified size is placed in a constant temperature and humidity environment, and a constant pressure (0.8 MPa) is applied for a certain period of time (30 days). The thickness change of the sample is measured periodically, and the final deformation rate is calculated.

[0155] (7) Composting degradation

[0156] Standard reference: Refer to GB / T 19277.1-2011 (Determination of final aerobic biodegradability of materials under controlled composting conditions - using the method of measuring released carbon dioxide).

[0157] Brief description of conditions: A membrane sample of known dry weight is buried in mature, active compost and placed in a controlled composting device at (58±2)℃ with moderate aeration. The amount of carbon dioxide released is measured periodically, or the sample is periodically removed, cleaned, dried, and weighed to calculate the mass loss rate. "6-month degradation" refers to the membrane sample reaching a high biodegradation rate (e.g., ≥90%) or physically fragmenting to unrecognizable levels within 6 months under these conditions.

[0158] (8) Seawater degradation

[0159] Standard reference: Refer to GB / T 40612-2021.

[0160] Brief description of conditions: Immerse the membrane sample in real marine environment or artificial seawater prepared according to standard, and conduct aerobic culture at room temperature or simulated ocean temperature (e.g., 30°C). Monitor carbon dioxide release or observe and weigh periodically to assess the degree of degradation.

[0161] (9) Soil degradation

[0162] Standard reference: Refer to GB / T 22047-2008.

[0163] Brief description of conditions: Bury the membrane sample in biologically active natural soil or standard soil, maintain soil temperature and humidity (25℃, 40-60% of field capacity), and periodically sample to assess mass loss, morphological changes or loss of mechanical properties.

[0164] Unless otherwise specified, performance tests shall be conducted in accordance with the above standard methods or equivalent methods.

[0165] Table 1 shows a comparison of the performance indicators of the stretch film in Examples 1 to 5.

[0166] Table 1

[0167]

[0168] As shown in Table 1, the LLDPE stretch film in Comparative Example 1 has a wide temperature range and good salt spray resistance, but it is non-degradable and easily causes environmental pollution, thus failing to meet environmental protection requirements. The PLA stretch film in Comparative Example 2 has low elongation and puncture strength, is easily punctured, and is only degradable for industrial composting, making it suitable for short-distance transportation and low-strength packaging. The PBAT / PLA blend stretch film in Comparative Example 3 has an elongation close to that of LLDPE stretch film, but insufficient puncture strength and slow degradation rate. The starch-based stretch film in Comparative Example 4 has low puncture strength, its performance deteriorates significantly in high humidity environments, and it is prone to damage and mold growth. The PHA-based stretch film in Comparative Example 5 has good puncture strength, but low elongation; however, PHA material is expensive. The fully degradable stretch film of this application outperforms traditional LLDPE stretch film in terms of mechanical properties: its elongation reaches 350%~400%, puncture strength reaches 250~280g / μm, tensile strength reaches 35~40MPa, and its compressive creep deformation rate is only 1.2%, exhibiting excellent tensile, puncture, and deformation resistance, which can meet the mechanical requirements of cargo bundling and packaging and long-distance transportation. Meanwhile, the initial tack of this fully degradable stretch film is 1.2~1.5N / cm, ensuring ease of packaging operations; its light transmittance is 85%-88%, and its temperature resistance range is -30℃~70℃, meeting the requirements of conventional warehousing, sorting, and transportation environments; its salt spray resistance reaches 90% after 720 hours, demonstrating good weather resistance and suitability for humid and high-salt-spray environments such as sea transport. In addition, the fully degradable stretch film can degrade in about 6 months (composting / seawater environment), which solves the problem of the non-degradability of traditional LLDPE stretch film. It takes into account the full degradability, good mechanical properties, environmental adaptability and cost economy, and has outstanding application value.

[0169] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this application. It should be understood that the above descriptions are merely specific embodiments of this application and are not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A fully degradable wrapping film, characterized in that, The wrapping film comprises, from the inside out, an inner layer, a middle layer, and an outer layer arranged sequentially. The outer layer comprises, by weight percentage, 100%, 30% to 70% of a first biodegradable polyester with rigid chains, 15% to 50% of biomass filler, and 5% to 25% of toughening fibers; The middle layer comprises 100% by mass and consists of: 40% to 90% of a second biodegradable polyester with flexible chains and 5% to 30% of hydrophobically modified starch. The inner layer comprises 100% by mass and includes: 25% to 65% water-soluble polymer, 5% to 25% biomass degradation promoter, and 20% to 60% natural resin tackifier.

2. The fully degradable wrapping film according to claim 1, characterized in that, The first biodegradable polyester includes at least one of polylactic acid, polyhydroxyalkanoate, copolymer of polylactic acid and polyglycolic acid, and copolymer of polylactic acid and polycaprolactone. The biomass filler includes at least one of rice husk ash, bamboo powder, wheat straw ash, lignin, microcrystalline cellulose, and chitosan; The toughening fiber includes at least one of cellulose fibers, protein fibers, and starch-based fibers; the cellulose fibers are selected from at least one of regenerated cellulose fibers and microfibrillated cellulose; the protein fibers include silk fibroin fibers; and the starch-based fibers include modified starch fibers. The second biodegradable polyester includes at least one of polybutylene terephthalate, polybutylene succinate, and poly(ε-caprolactone). The hydrophobically modified starch includes at least one of acetylated starch, adipate-esterified starch, polylactic acid-grafted starch, and lignocellulose powder; The water-soluble polymer includes at least one of polyvinyl alcohol, polyvinyl acetate, modified starch, polyglutamic acid, and pullulan. The biomass degradation promoter includes at least one of cassava starch, corn starch, potato starch, and amylopectin. The natural resin tackifier includes at least one of rosin glycerol ester, hydrogenated rosin ester, terpene resin, and shellac.

3. The stretch film according to claim 1 or 2, characterized in that, The thickness of the inner layer is 15% to 25% of the thickness of the fully degradable wrapping film; The thickness of the middle layer is 45% to 55% of the thickness of the fully degradable wrapping film; The thickness of the outer layer is 25% to 35% of the thickness of the fully degradable wrapping film.

4. The fully degradable wrapping film according to claim 1, characterized in that, The second biodegradable polyester comprises 5% to 50% recycled biodegradable polyester, with the second biodegradable polyester comprising 100% by mass. The recycled biodegradable polyester includes at least one of recycled polybutylene terephthalate, recycled polybutylene succinate, and recycled polylactic acid.

5. The fully degradable wrapping film according to claim 1, characterized in that, The outer layer also includes: bio-based plasticizers and / or biodegradable plasticizers; The bio-based plasticizer includes at least one of acetylated tributyl citrate, triethyl citrate, and epoxidized soybean oil, and the biodegradable plasticizer is selected from polyethylene glycol; With the outer layer comprising 100% by mass, the amount of the bio-based plasticizer or biodegradable plasticizer is 1% to 8%; The middle layer also includes at least one of the following: nano-reinforcing materials and compatibilizers; The nano-reinforcing material includes at least one of layered silicates, nano-silica, cellulose nanocrystals, and bio-based carbon dots. The compatibilizer includes at least one of titanate coupling agents, aluminate coupling agents, and hydrolyzable silane coupling agents. With the middle layer comprising 100% by mass, the amount of the nano-reinforcing material being 0.05% to 1.5%, and the amount of the compatibilizer being 0.2% to 3%; The inner layer also includes at least one of: antifungal agent and moisture-proof agent; The antifungal agent includes at least one of sodium benzoate, potassium sorbate, natamycin, and chitosan; The moisture-proof agent includes nano-level moisture-proof filler, which is selected from at least one of nano-silica, nano-calcium carbonate, and modified kaolin. With the inner layer accounting for 100% by mass, the amount of the antifungal agent is 0.1% to 2%; and the amount of the moisture-proof agent is 0.5% to 5%.

6. The fully degradable wrapping film according to claim 1, characterized in that, The fully degradable wrapping film meets at least one of the following conditions: The elongation rate is 350%~400%; The puncture strength is 250~280 g / μm; The fracture strength is 35~40 MPa; Light transmittance is 85%~88%.

7. A method for preparing a fully degradable wrapping film according to any one of claims 1 to 6, characterized in that, The preparation method includes: The raw materials for the outer and middle layers are respectively added to different extruders for extrusion casting to obtain a cast film composed of the middle and outer layers; The raw materials of the inner layer are mixed and then sprayed onto the middle layer surface of the cast film to obtain a composite film; The composite film is first stretched longitudinally, then stretched laterally, and then subjected to heat setting, corona treatment, and curing treatment to obtain a fully degradable wrapping film.

8. The preparation method according to claim 7, characterized in that, The extrusion temperature of the outer layer is 160~170℃; The extrusion temperature of the middle layer is 140~150℃; During the inner layer spraying, the temperature of the cast film is 130~140℃; The longitudinal stretching ratio is 2.8~3.5:1, and the transverse stretching ratio is 2.3~3.0:

1.

9. The preparation method according to claim 7, characterized in that, The heat setting temperature is 30~40℃; The conditions for the corona treatment are: power 3~5kW, unwinding speed 10~20m / min; The curing conditions are: temperature 20℃~30℃, humidity 50%~60%, and curing time 12~48h.

10. The application of a fully degradable stretch film as described in any one of claims 1 to 6 in product packaging.

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