A fully bio-based degradable food packaging composite film and a preparation method thereof

Abstract

A fully bio-based biodegradable food packaging composite film and its preparation method are disclosed, belonging to the field of food packaging materials technology. The composite film adopts a three-layer adhesive-free composite structure, consisting of a high-barrier outer layer, a toughening middle layer, and a heat-sealing inner layer from the outside to the inside. The high-barrier outer layer includes PLA, modified bamboo pulp nanocellulose, and chitosan. The toughening middle layer consists of PHBHHx from PHA and a bio-based compatibilizer. The heat-sealing inner layer includes a reactive blend of PLA and PHBHHx. This invention achieves chemical bonding of the three-layer interface through reactive extrusion and utilizes the synergistic combination effect of nanocellulose and chitosan, as well as the gradient crystallization synergistic effect of PLA and PHBHHx, to significantly improve the overall performance of the composite film.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, and more particularly to food packaging, specifically a fully bio-based biodegradable food packaging composite film and its preparation method. Background Technology

[0002] With the depletion of fossil fuels, the traditional petroleum-based plastic packaging materials industry is seeking new solutions. Simultaneously, the environmental problems caused by these petroleum-based plastic packaging materials are prompting the search for biodegradable materials. Polylactic acid (PLA) and polybutylene terephthalate (PBAT) are currently the two most widely used biodegradable materials. PLA has good biocompatibility, high tensile strength, and transparency, but suffers from poor toughness, inadequate heat-sealing performance, and insufficient barrier properties. PBAT, on the other hand, possesses excellent flexibility and processability, but has lower tensile strength and poorer barrier properties.

[0003] However, with current technology, PBAT cannot be fully bio-based. PBAT is polymerized from terephthalic acid (PTA), adipic acid (AA), and 1,4-butanediol (BDO). In current mainstream processes, all three monomers are petroleum-based. Although bio-based BDO has been commercially produced and bio-based AA has achieved small-scale pilot production, there is still no mature large-scale commercial technology for bio-based PTA. Currently, even the most advanced bio-based PBAT products on the market have a bio-based content of only 36%-65%. In addition, the production process of PBAT still relies on fossil fuels, resulting in a high carbon footprint, which falls short of the true concept of green and sustainable development.

[0004] In addition, polyhydroxyalkanoates (PHAs) are a class of natural polyesters produced by microbial fermentation. They are currently the only biodegradable materials that are 100% bio-based and can be completely degraded in soil, seawater, and compost. PHA family members include short-chain PHAs such as PHB and PHBV, medium- and long-chain PHAs such as PHBHHx, and hetero-chain PHAs such as P34HB and P4HB. Among them, poly(3-carboxybutyrate-co-3-hydroxyhexanoate) PHBHHx, by introducing 3-hydroxyacetic acid copolymer units, significantly improves the brittleness of PHB, exhibiting good flexibility, processing performance, and heat-sealing properties, with an elongation at break exceeding 200%, approaching that of traditional polyethylene (PE). Furthermore, PHA has better barrier properties against oxygen and water vapor than PLA, and also possesses natural biocompatibility and certain antibacterial properties, making it ideal for food packaging.

[0005] However, PHBHHx materials alone suffer from low tensile strength and high cost, while PLA has high tensile strength and transparency, but poor toughness and heat-sealing performance. Therefore, researchers have begun to explore combining these different materials through various methods, using structural design and synergistic modification to select products that possess the advantages of both materials. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention proposes a fully bio-based biodegradable food packaging composite film and its preparation method.

[0007] The technical problem to be solved by the present invention is achieved through the following technical solution: A fully bio-based biodegradable food packaging composite film adopts a three-layer adhesive-free composite structure, consisting of a high-barrier outer layer, a toughening middle layer, and a heat-sealing inner layer from the inside out. The polymer matrix of the composite film is derived from renewable materials and does not contain petroleum-based polyester polymers. The high-barrier outer layer uses bio-based polylactic acid (PLA) as the matrix and is compounded with natural bio-based barrier reinforcing components. The toughening middle layer uses bio-based polyhydroxyalkanoate (PHBHHx) as the matrix. The heat-sealing inner layer uses a blend of bio-based PLA and bio-based PHBHHx as the matrix.

[0008] The raw materials for each layer, by weight, are as follows: The high-barrier outer layer comprises: 84-91 parts of bio-based polylactic acid (PLA), 3-8 parts of modified bamboo pulp nanocellulose, 2-5 parts of chitosan, 0.5-2 parts of bio-based epoxy chain extender, and 0.1-0.5 parts of food-grade antioxidant. The toughening middle layer includes: 87-94 parts of bio-based polyhydroxyalkanoate PHBHHx, 3-8 parts of bio-based compatibilizer, 0.5-2 parts of food-grade slip agent, and 0.1-0.5 parts of food-grade antioxidant; The heat-sealed inner layer includes: 42-58 parts of bio-based PLA, 28-53 parts of bio-based PHBHHx, 1-3 parts of bio-based epoxy chain extender, 0.5-2 parts of food-grade opening agent, and 0.1-0.5 parts of food-grade antioxidant.

[0009] In this invention, the modified bamboo pulp nanocellulose is prepared by dispersing bamboo pulp nanocellulose in deionized water to prepare a suspension with a mass fraction of 5-8%, adding silane coupling agent KH550, stirring and reacting at 60-70℃ for 2-3 h, and then filtering, washing and drying to obtain the product; the amount of silane coupling agent KH550 is 3-5% of the mass of nanocellulose.

[0010] In this invention, the chitosan has a degree of deacetylation ≥90% and a molecular weight of 5×10⁻⁶. 4 -2×10 5 Da originates from shrimp and crab shells or edible fungi mycelium.

[0011] In this invention, the bio-based epoxy chain extender is a bio-based multi-epi-epoxide compound with an epoxy value ≥ 0.45 mol / 100g, which is obtained from soybean oil or castor oil through an epoxidation reaction.

[0012] In this invention, the bio-based compatibilizer is maleic acid-grafted PLA with a grafting rate of 0.8-1.5%, prepared by melt grafting.

[0013] In this invention, the bio-based PHBHHx is poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), wherein the content of 3-hydroxyhexanoate Hx units is 8-12 mol%, obtained by microbial fermentation, and the bio-based content is not less than 99.9%.

[0014] In this invention, the food-grade antioxidant is one or more of antioxidant 1010, antioxidant 168, and natural tocopherol, and all components comply with the GB 4806.10-2016 standard for the use of additives in plastic materials and products for food contact.

[0015] In this invention, the food-grade slip agent is one or more of the following: erucamide, oleamide, and glyceryl stearate.

[0016] In this invention, the food-grade opening agent is one or more of fumed silica, precipitated silica, and food-grade talc, and the average particle size of the powder is controlled at 2-5 μm to avoid problems such as crystal points or excessive surface roughness after film formation.

[0017] In this invention, the high-barrier outer layer has a thickness of 15-25 μm, the toughening middle layer has a thickness of 20-35 μm, the heat-sealing inner layer has a thickness of 10-20 μm, and the total thickness of the composite film is 45-80 μm.

[0018] Based on the above raw materials, a method for preparing a fully bio-based biodegradable food packaging composite film includes the following steps: S1 outer layer masterbatch preparation: Bio-based PLA, modified bamboo pulp nanocellulose, chitosan, bio-based epoxy chain extender and antioxidant of the outer layer raw materials are mixed in proportion and added to a twin-screw extruder for reactive extrusion granulation. The extrusion temperature is 160-185℃, the screw speed is 200-300 r / min, and the material residence time is 2-3 min to obtain the outer layer masterbatch. S2 intermediate layer masterbatch preparation: Bio-based PHBHHx, bio-based compatibilizer, slip agent and antioxidant of intermediate layer raw material are mixed in proportion, added to twin screw extruder for melt blending and granulation, the extrusion temperature is 135-160℃, the screw speed is 250-350 r / min, and intermediate layer masterbatch is obtained. S3 Inner Layer Masterbatch Preparation: Bio-based PLA, bio-based PHBHHx, bio-based epoxy chain extender, opening agent and antioxidant of the inner layer raw materials are mixed in proportion and added to a twin-screw extruder for reactive extrusion granulation. The extrusion temperature is 145-170℃, the screw speed is 220-320 r / min, and the material residence time is 1.5-2.5 min to obtain the inner layer masterbatch. S4 Three-Layer Co-Extrusion Blown Film: The outer layer masterbatch, middle layer masterbatch, and inner layer masterbatch are respectively added to the three extruders of the three-layer co-extrusion blown film machine. The outer layer extrusion temperature is 165-190℃, the middle layer extrusion temperature is 140-165℃, the inner layer extrusion temperature is 150-175℃, the die temperature is 168-182℃, the blow-up ratio is (2.5-3.5):1, and the traction speed is 15-25 m / min to obtain a three-layer composite film. S5 Online Plasma Treatment: The three-layer composite membrane obtained in step S4 is treated with a low-temperature plasma treatment device to treat the outer surface of the composite membrane. The treatment power is 300-500 W and the treatment time is 5-10 s. A mixed gas of argon and oxygen with a volume ratio of 9:1 is used to obtain the final product.

[0019] Compared with the prior art, the present invention has the following advantages: (1) This application completely abandons the petroleum-based PBAT component commonly used in traditional biodegradable membranes. The polymer matrix is ​​entirely made from renewable biomass: bio-based PLA is obtained by fermentation of corn starch, and PHBHHx is synthesized by microbial fermentation. The bio-based content of both is over 99%. At the additive level, plant-derived biomass additives such as erucamide are preferred. The overall bio-based content is ≥99%, which completely eliminates the dependence on fossil resources from the raw material end. At the same time, the biodegradability is high, there is no residue, and the carbon footprint is significantly reduced. It can meet the dual environmental protection requirements of composting degradation and marine degradation at the same time. (2) This application adopts a three-layer supply structure consisting of a high-barrier outer layer, a toughened middle layer, and a heat-sealing inner layer. The functions of each layer are clearly defined: the outer layer provides barrier, rigidity, and printability; the middle layer provides overall flexibility and puncture resistance; and the inner layer ensures low-temperature heat-sealing performance and the safety of the contents. The three layers complement each other, solving the inherent contradiction of a single bio-based material being either "strong and brittle" or "tough and weak". The three layers achieve interfacial chemical bonding through reactive extrusion during the co-extrusion process, eliminating the need for additional adhesive coating. This avoids the safety risk of small adhesive molecules migrating into food and solves the problem of the adhesive layer hindering biodegradation. The interlayer peel strength is high, and there is no risk of delamination during use. (3) The synergistic effect of this application is obvious: the modified nanocellulose builds a one-dimensional nano network in the PLA matrix, which greatly extends the diffusion path of gas and water vapor, and provides a mechanical reinforcement skeleton. Chitosan fills the gaps in the network and forms a dense interface with nanocellulose and PLA molecular chains through hydrogen bonds, further blocking the diffusion channels. At the same time, it gives the film a certain natural antibacterial property. The two work together to reduce oxygen permeability and increase tensile strength. By controlling the processing temperature and cooling rate of each layer, the outer layer PLA forms a high degree of crystallinity, ensuring barrier properties and surface modification. The inner layer PLA / PHBHHx blend maintains a low degree of crystallinity, ensuring low-temperature heat sealing and flexibility. The structural solution solves the coordination between barrier properties and heat sealing properties. The epoxy chain extender reacts with the terminal carboxyl and terminal hydroxyl groups of PLA and PHBHHx in the molten state, which not only improves the compatibility of the blend phase, but also forms covalent bonds between the layers, thus improving the mechanical stability and interlayer bonding of the blend system. (4) The composite film obtained in this application has excellent comprehensive performance. Its oxygen permeability, water vapor permeability, tensile strength, elongation at break and heat seal strength are all better than conventional PLA / PBAT blend film. It can meet most food packaging requirements. In particular, it can maintain good flexibility in low temperature environment (-18℃) and is suitable for frozen food packaging. (5) This application adopts twin-screw reactive extrusion and three-layer co-extrusion blown film process. The equipment is fully compatible with the existing biodegradable film production line. No additional investment is required. The production line is highly efficient and suitable for large-scale industrial production. By optimizing the formula design, the amount of PHBHHx is controlled at 40-50%, which effectively reduces the material cost and makes the product competitive in the market. Attached Figure Description

[0020] Figure 1 This is a comparison chart of the barrier properties of different formulations of the present invention; Figure 2 This is a comparison chart of the mechanical and heat-sealing properties of different formulations of the present invention; Figure 3 Radar chart showing the comprehensive performance of different PHA materials in this invention; Figure 4 This is a graph showing the change in biodegradability rate of the present invention over time. Detailed Implementation

[0021] The present invention will be further described below with reference to the accompanying drawings and specific preferred embodiments, but this does not limit the scope of protection of the present invention.

[0022] Some of the raw materials and instruments used in the embodiments of this invention are shown below. Unless otherwise specified, the other raw materials, equipment and testing standards are all commercially available products, conventional industrial equipment and national / industry general standards in this field.

[0023] Materials and Instruments Bio-based polylactic acid (PLA): NatureWorks 4032D, NatureWorks Inc., USA, with a number-average molecular weight of approximately 100,000 and a bio-based content of ≥99%; All-bio-based polyhydroxy fatty acid ester PHBHHx: P34HB, Blue Crystal Microorganism, 3-hydroxyhexanoate Hx unit molar content 10%, bio-based content ≥99.9%; Bamboo pulp nanocellulose: Xinsen Biotechnology, diameter 10-30 nm, length 1-5 μm, solid content 92%; Chitosan: Jinke Biotechnology, degree of deacetylation 92%, molecular weight 1.2×10 5 Da; Bio-based epoxy chain extender: epoxy value 0.52 mol / 100g, obtained by oxidation of soybean oil; Bio-based compatibilizer: maleic acid grafted PLA, grafting rate 1.2%, matrix is ​​PLA 4032D; Food-grade slip agent: plant-derived erucamide, AkzoNobel; Food-grade opening agent: Fumed silica, HDK N20, Wacker Chemie, average particle size approximately 2 μm; Food-grade antioxidant: Antioxidant 1010 (pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]), BASF; Silane coupling agent KH550: Shuguang Chemical, industrial grade.

[0024] All polyester raw materials were dried in a vacuum drying oven at 60℃ for 10 hours before use, with the moisture content controlled to be ≤200ppm, to avoid hydrolysis and degradation during processing.

[0025] Preparation of modified bamboo pulp nanocellulose Weigh a certain amount of bamboo pulp nanocellulose and disperse it in deionized water to prepare a 6% (w / w) suspension. First, shear the suspension at high speed for 30 min, then sonicate it for 20 min until it is uniformly dispersed. Add 4% (w / w) of silane coupling agent KH550 (dry basis of nanocellulose), heat to 65℃, and stir for 2.5 h. After the reaction is complete, filter the mixture, wash the filter cake three times with deionized water, dry it in a vacuum drying oven at 60℃ for 12 h, and then pulverize it to obtain modified bamboo pulp nanocellulose.

[0026] Example 1

[0027] This embodiment presents a preferred embodiment of a fully bio-based biodegradable food packaging composite film and its preparation method.

[0028] 1.1 Raw material selection The high-barrier outer layer comprises, by weight: 87 parts PLA, 5 parts modified bamboo pulp nanocellulose, 3 parts chitosan, 1.2 parts bio-based epoxy chain extender, and 0.3 parts antioxidant 1010; The toughening intermediate layer comprises, by weight: 91 parts PHBHHx, 5 parts bio-based compatibilizer, 1 part erucamide, and 0.2 parts antioxidant 1010; The heat-sealed inner layer comprises, by weight: 50 parts PLA, 44 parts PHBHHx, 2 parts bio-based epoxy chain extender, 1 part fumed silica, and 0.3 parts antioxidant 1010.

[0029] 1.2 Preparation Scheme (1) Preparation of outer layer masterbatch: Weigh out each raw material of the outer layer according to the proportion, add them to a high-speed mixer and mix for 8 min at a mixing speed of 300 r / min; add the mixture to a parallel twin-screw extruder for reactive extrusion granulation. The temperatures of each section of the extruder from the feed port to the die head are 160℃, 165℃, 170℃, 175℃, 180℃, and 185℃ respectively. The screw speed is 250 r / min and the material residence time is about 2.5 min. After water cooling and pelletizing, the outer layer masterbatch is obtained. (2) Preparation of intermediate layer masterbatch: Weigh each raw material of the intermediate layer according to the proportion, mix at high speed for 5 min and then add to a twin-screw extruder for melt blending and granulation. The temperatures of each section are 135℃, 140℃, 145℃, 150℃, 155℃ and 160℃ respectively. The screw speed is 300 r / min. Water cooling and pelletizing are used to obtain intermediate layer masterbatch. (3) Preparation of inner layer masterbatch: Weigh each raw material of the inner layer according to the proportion, mix at high speed for 8 min and then add to a twin-screw extruder for reactive extrusion granulation. The temperatures of each section are 145℃, 150℃, 155℃, 160℃, 165℃ and 170℃ respectively. The screw speed is 280 r / min and the material residence time is about 2 min. Water cooling and pelletizing are used to obtain inner layer masterbatch. (4) Three-layer co-extrusion blown film: Three kinds of masterbatch are added to the three extruders of the three-layer co-extrusion blown film machine. The outer layer extrusion temperature is 165-190℃, the middle layer extrusion temperature is 140-165℃, the inner layer extrusion temperature is 150-175℃, the die temperature is 175℃, the blow ratio is 3:1, the traction speed is 20 m / min, and the thickness ratio of each layer is controlled by adjusting the speed of each extruder to obtain the three-layer co-extrusion base film. (5) Online plasma treatment: The base film is introduced into a low-temperature plasma treatment unit and the outer surface is treated with argon-oxygen mixed plasma. The gas volume ratio Ar:O2=9:1, the treatment power is 400 W, the treatment line speed is synchronized with the blown film traction speed, the treatment time is about 8 s, and the final composite film product is obtained by winding. The total thickness is 60 μm, and the thicknesses of each layer from the inside to the outside are 20 μm, 25 μm and 15 μm respectively.

[0030] Example 2

[0031] This embodiment is based on Embodiment 1, and a new embodiment is formed by adjusting the amount of some raw materials.

[0032] 2.1 Raw material selection The high-barrier outer layer comprises, by weight: 84 parts PLA, 8 parts modified bamboo pulp nanocellulose, 4 parts chitosan, 1.8 parts bio-based epoxy chain extender, and 0.4 parts antioxidant 1010; The toughening intermediate layer, by weight, comprises: 87 parts PHBHHx, 8 parts bio-based compatibilizer, 1.5 parts erucamide, and 0.3 parts antioxidant 1010; The heat-sealed inner layer comprises, by weight: 42 parts PLA, 53 parts PHBHHx, 3 parts bio-based epoxy chain extender, 1.5 parts fumed silica, and 0.4 parts antioxidant 1010.

[0033] 2.2 Preparation Scheme The preparation method and process parameters are the same as in Example 1, and the final composite membrane product is obtained with a total thickness of 80 μm and the thicknesses of each layer from the inside to the outside are 25 μm, 35 μm and 20 μm respectively.

[0034] Example 3

[0035] This embodiment is based on Embodiment 1, and a new embodiment is formed by adjusting the amount of some raw materials.

[0036] 3.1 Raw material selection The high-barrier outer layer comprises, by weight: 91 parts PLA, 3 parts modified bamboo pulp nanocellulose, 2 parts chitosan, 0.6 parts bio-based epoxy chain extender, and 0.2 parts antioxidant 1010; The toughening intermediate layer comprises, by weight: 94 parts PHBHHx, 3 parts bio-based compatibilizer, 0.5 parts erucamide, and 0.1 parts antioxidant 1010; The heat-sealed inner layer comprises, by weight: 58 parts PLA, 38 parts PHBHHx, 1 part bio-based epoxy chain extender, 0.5 parts fumed silica, and 0.2 parts antioxidant 1010.

[0037] 3.2 Preparation Scheme The preparation method and process parameters are the same as in Example 1, and the final composite membrane product is obtained with a total thickness of 45 μm and the thicknesses of each layer from the inside to the outside are 15 μm, 20 μm and 10 μm, respectively.

[0038] Comparative test To verify the technical effect and material selection of the present invention, a parallel comparative experiment was set up. The total thickness of all comparative samples was controlled at 60 μm, and the preparation process was completely consistent with that of Example 1 except for the variables.

[0039] 4.1 Tests to verify the technical effects of the present invention 4.1.1 Experimental Objective The performance and effectiveness of the three-layer functional partition structure and the high-barrier outer layer components of this invention are verified, and compared with existing mainstream biodegradable packaging films.

[0040] 4.1.2 Sample Setup Experimental group: Example 1 (this invention); Comparative Example 1: Existing PLA / PBAT monolayer blend membrane (commercially available mainstream formulation, PBAT bio-based content approximately 36%). Comparative Example 2: Three-layer co-extruded film without barrier fillers (the high-barrier outer layer does not contain nanocellulose and chitosan). Comparative Example 3: Three-layer co-extruded film with only modified nanocellulose added (high-barrier outer layer without chitosan); Comparative Example 4: Three-layer co-extruded film with only chitosan added (high-barrier outer layer without modified nanocellulose).

[0041] 4.1.3 Test Results The core performance was tested in accordance with national standards, and the results are shown in Table 1.

[0042] Table 1 compares the solution of the present invention with existing technologies and solutions involving adjusting the barrier packing.

[0043] 4.1.4 Results Analysis ( Figure 1 , 2 4) (1) Compared with the mainstream PLA / PBAT system of Comparative Example 1, the oxygen permeability of Example 1 decreased by 56.5%, the water vapor permeability decreased by 50.6%, the tensile strength increased by 39.1%, the heat seal strength increased by 52.0%, and the bio-based content increased from 61.2% to 100%, truly achieving a dual breakthrough in environmental protection attributes and performance.

[0044] (2) Comparing Comparative Examples 2, 3, and 4, it can be seen that adding nanocellulose or chitosan alone can only achieve limited performance improvement. After the two are combined, the oxygen permeability is reduced by 48.4% compared with the unfilled group, which is much higher than the combined effect of adding nanocellulose and chitosan alone. This proves that the nanonetwork built by the modified nanocellulose and the interface filling of chitosan form a significant synergistic effect, achieving a synergistic modification effect.

[0045] 4.2 Experiments on the selection of PHA family materials 4.2.1 Experimental Objective The PHA family includes three major structural categories: short-chain PHA, medium- and long-chain PHA, and hetero-chain PHA. Different family members exhibit different comprehensive performances in the composite membrane system of this invention based on their own characteristics. The technical rationality and economic feasibility of selecting PHBHHx are demonstrated by weighing four dimensions: mechanical properties, barrier properties, processing feasibility, and material cost.

[0046] 4.2.2 Sample Setup All samples maintained the same three-layer structure, filler formulation, and preparation process, except that the PHBHHx in the toughening middle layer and heat-sealing inner layer was replaced with the corresponding PHA material: ① Short-chain PHA (SCL-PHA): Comparative Example 5 uses PHBV (poly-3-hydroxybutyrate-co-3-hydroxyvalerate), which represents the chain-severed copolymer PHA with monomer carbon number C4-C5; ②Medium- and long-chain PHA (MCL-PHA): Comparative Example 6 uses PHHxO (poly-3-hydroxyhexanoate-co-3-hydroxyoctanoate), which represents pure medium- and long-chain copolymer PHA with monomer carbon number C6-C8; ③ Heterochain PHA: Comparative Example 7 uses PHB-b-PEG block copolymer, the main chain of which contains both ester and ether heteroatoms, representing modified heterochain PHA with heteroatoms introduced into the main chain; ④ Selection of the present invention: Example 1, using PHBHHx (short chain-medium long chain copolymer PHA).

[0047] 4.2.3 Test Results The tests were conducted from three dimensions: performance, mass production capability, and overall cost. The results are shown in Table 2.

[0048] Table 2 compares the material selection schemes of this invention with those of other PHA family materials.

[0049] 4.2.4 Results Analysis ( Figure 3 ) (1) Short-chain PHA (PHBV, comparative example 5): Its advantages are high crystallinity, good barrier performance and slightly lower raw material cost; however, its shortcomings are prominent: the elongation at break is less than 90%, the low-temperature impact resistance is poor, and it cannot meet the basic requirements of food packaging for rubbing, dropping and low-temperature storage and transportation; at the same time, the processing window is narrow, crystal points are easy to appear on the film surface, the yield is low, and it is only suitable for rigid packaging scenarios and cannot be used for flexible food packaging films.

[0050] (2) Medium- and long-chain PHA (PHHxO, comparative example 6): The advantages are excellent flexibility, elongation at break of over 500%, and excellent low-temperature toughness and heat-sealing performance; however, the long molecular side chains result in low crystallinity, tensile strength of less than 30 MPa, and a significant decrease in barrier performance, which cannot meet the barrier requirements for food preservation; in addition, the raw material price is high, the film bubble is relatively soft, the blown film stability is poor, and the overall cost performance is low.

[0051] (3) Heterochain PHA (PHB-b-PEG, Comparative Example 7): The compatibility and toughness were improved by introducing ether blocks, and the low temperature performance was good; however, the strong hydrophilicity of PEG segments led to a significant decrease in water vapor barrier performance, which was not conducive to food preservation in high humidity environments; at the same time, the block copolymer preparation process was complicated, the raw material cost was the highest, the industrial mass production maturity was low, and it did not yet have the conditions for large-scale application.

[0052] (4) The selected material PHBHHx in this invention: As a copolymer of short-chain and medium-to-long-chain monomers, it perfectly balances various properties: it retains the high strength and high barrier properties of short-chain PHA, while also possessing the flexibility and heat-sealing properties of medium-to-long-chain PHA; it has a wide processing window, good compatibility with existing general blown film production lines, and a yield rate as high as 94%; the material cost is moderate, and the overall cost is competitive in the market after controlling the amount used through three-layer functional zoning. Considering the three dimensions of comprehensive performance, processability, and cost, PHBHHx is the optimal choice for the scenario of fully bio-based flexible food packaging film.

[0053] Application Preservation Test The composite film prepared in Example 1, the blended film of Comparative Example 1, and ordinary PE food packaging film were selected to conduct packaging tests on fresh Red Beauty strawberries. Each group packaged 500 g of strawberries and stored them in a refrigerated environment at 4°C and 85% relative humidity for 15 days. The weight loss rate and rot rate of the strawberries were measured.

[0054] The results showed that after 15 days of storage, the strawberries packaged in Example 1 were bright red and plump, with a weight loss rate of 4.5% and a rot rate of 7.2%; the strawberries packaged in Comparative Example 1 showed slight loss of gloss, with some fruits developing soft and rotten spots, a weight loss rate of 6.8%, and a rot rate of 15.6%; the strawberries packaged in ordinary PE film were noticeably wilted, with a high proportion of moldy and rotten fruits, a weight loss rate of 8.6%, and a rot rate of 23.5%. This indicates that the composite film of the present invention has a superior barrier preservation effect and can effectively extend the shelf life of fresh food.

Claims

1. A fully bio-based biodegradable food packaging composite film, characterized in that, The composite film employs a three-layer adhesive-free composite structure, consisting of a high-barrier outer layer, a toughening middle layer, and a heat-sealing inner layer from the inside out. The polymer matrix of the composite film is derived from renewable materials and does not contain petroleum-based polyester polymers. Specifically, the high-barrier outer layer uses bio-based polylactic acid (PLA) as the matrix, compounded with natural bio-based barrier reinforcing components; the toughening middle layer uses bio-based polyhydroxyalkanoate (PHBHHx) as the matrix; and the heat-sealing inner layer uses a blend of bio-based PLA and bio-based PHBHHx as the matrix.

2. The composite membrane according to claim 1, characterized in that, The raw materials for each layer are, by weight, as follows: The high-barrier outer layer comprises: 84-91 parts of bio-based polylactic acid (PLA), 3-8 parts of modified bamboo pulp nanocellulose, 2-5 parts of chitosan, 0.5-2 parts of bio-based epoxy chain extender, and 0.1-0.5 parts of food-grade antioxidant. The toughening middle layer includes: 87-94 parts of bio-based polyhydroxyalkanoate PHBHHx, 3-8 parts of bio-based compatibilizer, 0.5-2 parts of food-grade slip agent, and 0.1-0.5 parts of food-grade antioxidant; The heat-sealed inner layer includes: 42-58 parts of bio-based PLA, 28-53 parts of bio-based PHBHHx, 1-3 parts of bio-based epoxy chain extender, 0.5-2 parts of food-grade opening agent, and 0.1-0.5 parts of food-grade antioxidant.

3. The composite membrane according to claim 2, characterized in that, The modified bamboo pulp nanocellulose is prepared by dispersing bamboo pulp nanocellulose in deionized water to prepare a suspension with a mass fraction of 5-8%, adding silane coupling agent KH550, stirring and reacting at 60-70℃ for 2-3 h, and then filtering, washing and drying to obtain the product; the amount of silane coupling agent KH550 is 3-5% of the mass of nanocellulose.

4. The composite membrane according to claim 2, characterized in that, The chitosan has a degree of deacetylation ≥90% and a molecular weight of 5×10⁻⁶. 4 -2×10 5 Da.

5. The composite membrane according to claim 2, characterized in that: The bio-based PHBHHx is poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), wherein the content of 3-hydroxyhexanoate Hx units is 8-12 mol%, obtained by microbial fermentation, and the bio-based content is not less than 99.9%.

6. The composite membrane according to claim 2, characterized in that, The bio-based epoxy chain extender is a bio-based multi-epi-epoxide compound with an epoxy value ≥ 0.45 mol / 100g; the bio-based compatibilizer is maleic acid-grafted PLA with a grafting rate of 0.8-1.5%, prepared by melt grafting; the food-grade antioxidant is one or more of antioxidant 1010, antioxidant 168, and natural tocopherol; the food-grade slip agent is one or more of plant-derived erucamide, oleamide, and glyceryl stearate; the food-grade opening agent is one or more of fumed silica, precipitated silica, and food-grade talc, with the average particle size of the powder controlled at 2-5 μm.

7. The composite membrane according to claim 2, characterized in that, The high-barrier outer layer has a thickness of 15-25 μm, the toughening middle layer has a thickness of 20-35 μm, the heat-sealing inner layer has a thickness of 10-20 μm, and the total thickness of the composite film is 45-80 μm.

8. A method for preparing a fully bio-based biodegradable food packaging composite film as described in any one of claims 1-7, characterized in that, Includes the following steps: S1 outer layer masterbatch preparation: Bio-based PLA, modified bamboo pulp nanocellulose, chitosan, bio-based epoxy chain extender and antioxidant of the outer layer raw materials are mixed in proportion and added to a twin-screw extruder for reactive extrusion granulation. The extrusion temperature is 160-185℃, the screw speed is 200-300 r / min, and the material residence time is 2-3 min to obtain the outer layer masterbatch. S2 intermediate layer masterbatch preparation: Bio-based PHBHHx, bio-based compatibilizer, slip agent and antioxidant of intermediate layer raw material are mixed in proportion, added to twin screw extruder for melt blending and granulation, the extrusion temperature is 135-160℃, the screw speed is 250-350r / min, and intermediate layer masterbatch is obtained. S3 Inner Layer Masterbatch Preparation: Bio-based PLA, bio-based PHBHHx, bio-based epoxy chain extender, opening agent and antioxidant of the inner layer raw materials are mixed in proportion and added to a twin-screw extruder for reactive extrusion granulation. The extrusion temperature is 145-170℃, the screw speed is 220-320 r / min, and the material residence time is 1.5-2.5 min to obtain the inner layer masterbatch. S4 Three-Layer Co-Extrusion Blown Film: The outer layer masterbatch, middle layer masterbatch, and inner layer masterbatch are respectively added to the three extruders of the three-layer co-extrusion blown film machine. The outer layer extrusion temperature is 165-190℃, the middle layer extrusion temperature is 140-165℃, the inner layer extrusion temperature is 150-175℃, the die temperature is 168-182℃, the blow-up ratio is (2.5-3.5):1, and the traction speed is 15-25 m / min to obtain a three-layer composite film. S5 Online Plasma Treatment: The three-layer composite membrane obtained in step S4 is treated with a low-temperature plasma treatment device to treat the outer surface of the composite membrane. The treatment power is 300-500 W and the treatment time is 5-10 s. A mixed gas of argon and oxygen with a volume ratio of 9:1 is used to obtain the final product.

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