A polypropylene carbonate biodegradable composite film and a preparation method thereof

Abstract

The mass fraction composition of the polypropylene carbonate (PPC) / nano chitosan (NCS) / nano cellulose (NFC) biodegradable antibacterial composite film disclosed by the application is: 100 parts of PPC, 1-10 parts of NCS, 1-14 parts of NFC, 1-10 parts of essential oil, 0.5-1 part of lubricant, 0.5-1 part of stabilizer and 0.5-1 part of surfactant; the application realizes one-pot preparation of NCS and NFC, and the process is simple, convenient and efficient. Compared with pure PPC material, NFC in the composite material acts as a reinforcing phase, and NCS acts as an antibacterial phase, so that the mechanical properties of PPC can be improved, and good antibacterial properties can be endowed. In addition, by adding essential oil, the synergistic effect of NCS and essential oil is triggered to further improve the antibacterial properties of the film. The preparation method of the application is simple, efficient and low in cost, the equipment requirement is low, the amount of additives is small, the industrial production can be easily realized, and the application has wide market application prospect and commercial value.

Description

Technical Field

[0001] This invention belongs to the field of biodegradable plastic composite film technology, specifically a biodegradable antibacterial composite film of polypropylene carbonate / nano-chitosan / nano-cellulose and its preparation method. Background Technology

[0002] Polypropylene carbonate (PPC) is a high-toughness, biodegradable polyester polymerized from propylene oxide and carbon dioxide (CO2). PPC possesses biodegradability and biocompatibility, and can effectively mitigate the greenhouse effect and reduce dependence on fossil resources, thus attracting increasing attention. This material shows significant application potential in packaging films, textile fibers, and electronic packaging. However, due to its relatively low strength, insufficient dimensional stability, and lack of antibacterial properties, it is not advantageous for applications such as food packaging. Therefore, there is an urgent need to improve the mechanical properties, dimensional stability, and antibacterial properties of PPC composites without compromising their toughness and biodegradability.

[0003] Nanofibrillated cellulose (NFC) is a novel cellulose derivative with a unique structure, large surface area, and wide availability, making it suitable as a low-cost nanofiller to reinforce and toughen polymer composites. NFC-reinforced composites exhibit advantages such as biocompatibility, excellent mechanical properties, wear resistance, and biodegradability. For example, Hu et al. (Compos. Sci. Technol. 2013, 78: 63-68) prepared PPC composites using 0.1 wt% NFC as a reinforcing material and found that their tensile strength and Young's modulus increased to 19 MPa and 1414 MPa, respectively, with tensile strength increasing 10-fold and Young's modulus increasing 7-fold. Guo et al. (Polym. Adv. Technol. 2020, 31(4): 853-863) found that the tensile strength of PPC composites increased with increasing NFC content. When the NFC content increased to 1 wt%, the tensile strength of the composite increased from 22.1 MPa to 29.8 MPa. Introducing NFC into PPC can not only achieve mechanical strengthening but also significantly improve the shape and dimensional stability of PPC. However, Tian et al. (Polym.Adv.Technol.2020,31(11):2408-2421) pointed out that due to the influence of various factors such as microorganisms and oxygen, there are many limitations in the storage and preservation of food. The antimicrobial properties of PPC reinforced films need to be strengthened, and currently they still cannot meet the requirements for improving food safety and shelf life.

[0004] Adding chitosan (CS) is an effective method to impart antibacterial properties to PPC / NFC composites without affecting their mechanical properties and dimensional stability. CS is a product of partially removing acetyl groups from the natural polysaccharide chitin, possessing multiple physiological functions such as biodegradability, biocompatibility, non-toxicity, and antibacterial activity. It exhibits high antibacterial activity against a variety of pathogenic and putrefactive microorganisms (including fungi, Gram-positive bacteria, and Gram-negative bacteria). Quan et al. (RSCAdv.2021,11:10121-10129) reported that PPC / CS composites showed high antibacterial activity against Escherichia coli and Staphylococcus aureus. Furthermore, reducing the size of chitosan particles at the nanoscale, i.e., nano-chitosan (NCS), can increase the specific surface area and improve antibacterial activity. The molecular structure of NCS can promote the interaction between NFC and PPC molecules. These interactions may involve hydrogen bonds, van der Waals forces, and mechanical interlocking, which contribute to enhanced plasticity, flowability, and mechanical properties. Murugesan et al. (Processes. 2020, 8(9): 1173) have shown that NCS and its derivatives are among the best nanomaterials for providing antibacterial activity. However, due to the hydrophilic nature of CS, its water vapor permeability (WVP) is high, which is not conducive to the preparation of thin films. To solve this problem, hydrophobic materials such as lipids, or essential oils, are usually added to CS. Essential oils can improve the antibacterial activity of NCS, and they have a synergistic effect in antibacterial properties. When essential oils are loaded into NCS nanostructures, the antibacterial activity is enhanced. Therefore, this invention proposes a convenient preparation method of NFC and NCS and develops a biodegradable composite film based on PPC. NFC is used to improve the mechanical properties and dimensional stability of the PPC composite film, and the antibacterial properties of the film are further improved by inducing the synergistic effect of NCS and essential oils. Summary of the Invention

[0005] The problem this invention aims to solve is to address the issues of low strength, poor dimensional stability, and lack of antibacterial properties in existing PPC materials, and to propose a high-performance biodegradable PPC / NCS / NFC composite film and its preparation method.

[0006] The technical solution adopted by the present invention to solve the above-mentioned technical problems is as follows: a PPC / NCS / NFC composite film, wherein the mass fraction is: 100 parts of PPC, 1-10 parts of NCS, 1-14 parts of NFC, 1-10 parts of essential oil, 0.5-1 parts of lubricant, 0.5-1 parts of stabilizer, and 0.5-1 parts of surfactant.

[0007] Preferably, the NFC source is one or a combination of two or more of the following biomass powders: wood powder, bamboo powder, corn stalk powder, rice stalk powder, and cotton stalk powder.

[0008] Preferably, the essential oil is one or a combination of two or more of the following: orange peel essential oil, mugwort essential oil, cinnamon essential oil, and hyssop essential oil.

[0009] Preferably, the lubricant is one or a combination of two or more of stearic acid, ethylene bis-stearamide, oleamide, and erucamide. Preferably, the stabilizer is one or a combination of two or more of antioxidant 1010, antioxidant 168, antioxidant 1076, and antioxidant 1790, and the surfactant is one or a combination of two or more of Tween-80, Tween-85, mannose erythritol ester-a, and mannose erythritol ester-b.

[0010] The above-mentioned method for preparing a PPC / NCS / NFC composite film includes the following steps:

[0011] 1. One-pot preparation of NFC and NCS

[0012] 1) Mix 10-30 parts of biomass powder with 70-90 parts of sodium hydroxide solution, stir in a water bath at 75°C for 6 hours, filter after the reaction is complete, wash the solid sample obtained by filtration until neutral, add a mixed solution of 80-90 parts of sodium chlorite solution and 4-5 parts of glacial acetic acid, heat and stir at 70-80°C for 1-2 hours, then wash with pure water until neutral, and dry to dryness;

[0013] 2) Place 1-14 portions of the dried sample obtained in 1) and 1-10 portions of CS in a container, add pure water to adjust the solid-liquid ratio to 1:10-1:20, and then place the mixture in a homogenizer to perform high-speed shearing at 10000-18000 rpm for 0.5-2 hours. Finally, centrifuge at 10000-15000 rpm for 3-5 minutes, and freeze-dry to obtain a mixed powder of NFC and NCS.

[0014] Preferably, in step 1), the biomass powder has a mesh size of 80-150, the sodium hydroxide solution concentration is 10-20 wt%, and the sodium chlorite solution concentration is 10-15 wt%.

[0015] Reducing the size of CS at the nanoscale can improve antibacterial activity. In step 2) of the method described in this invention, the solute sample obtained in step 1) and CS are processed into nanoscale mixed particles by high-speed shearing, which increases the specific surface area of ​​both. Furthermore, the molecular structure of CS is very similar to that of cellulose, differing only in the functional groups attached to the second carbon in the repeating unit. This method allows for uniform pre-dispersion of the two phases, resulting in good compatibility between NFC and NCS, which is beneficial for uniform blending them in PPC. Step 2) of the method described in this invention simultaneously achieves a one-pot preparation of NFC and NCS, making the process simple and efficient.

[0016] 2. Preparation of PPC / NCS / NFC composite films

[0017] a: First, add 1 to 10 parts of essential oil to the NFC and NCS mixed powder obtained in step 2) above, then add 0.5 to 1 part of surfactant, and mix in a high-speed mixer for 20 to 40 minutes. Then add 100 parts of PPC, 0.5 to 1 part of lubricant, and 0.5 to 1 part of stabilizer and continue mixing for 30 to 60 minutes.

[0018] b: The mixture obtained in step a is melt-extruded and granulated in a twin-screw extruder at a temperature of 110-180℃ and a screw speed of 60-200rpm to obtain PPC / NCS / NFC composite particles;

[0019] c: The composite particles obtained in step b are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine, blown film machine, or cast film machine.

[0020] Preferably, in step c, the pre-compression pressure of the flat vulcanizing machine is 0.5–0.7 MPa, the pre-compression time is 150–180 s, the pressurization pressure is 2.5–3.5 MPa, the pressurization time is 150–180 s, and the hot-compression temperature is 140–150 °C.

[0021] Preferably, the processing temperature of the blown film machine in step c is 120–160°C.

[0022] Preferably, the processing temperature of the casting film machine in step c is 120–160°C.

[0023] Preferably, the thickness of the PPC / NCS / NFC composite film in step c is 0.05 mm to 0.8 mm.

[0024] In the method of this invention, NFC and NCS are fully mixed with essential oil in step a, which enhances the synergistic antibacterial effect of NCS and essential oil. At the same time, step a can also improve the dispersibility of NFC and NCS with PPC in advance, strengthen the uniform combination of the two nanophases of NFC and NCS with PPC during melt processing, and reduce agglomeration.

[0025] The method of this invention achieves interfacial entanglement between NFC and PPC molecular chains through step a. The high strength and stiffness of NFC are the main reasons for its ability to enhance the PPC composite film. Furthermore, the amino and hydroxyl functional groups on NCS exhibit a certain plasticizing effect, which can improve the rheological properties of PPC and enhance the interfacial interaction between NFC and PPC. This enhanced interfacial interaction reduces crack initiation and propagation, thereby improving the overall strength and toughness of the composite material.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] 1. This invention employs a one-step preparation method, simultaneously preparing NFC and NCS, resulting in a simple and efficient process. Furthermore, compared to pure PPC materials, the composite material, with NFC as a reinforcing phase and NCS as an antibacterial phase, improves the mechanical properties of PPC and imparts excellent antibacterial properties. Compared to simply adding NFC and NCS to PPC, the method of this invention allows NFC and NCS to be more easily dispersed within PPC, thereby enhancing its mechanical properties.

[0028] 2. This invention enhances the antibacterial activity of NCS through the synergistic effect of NCS and essential oils, resulting in superior antibacterial properties in the PPC / NCS / NFC composite film. NCS, being positively charged, can electrostatically interact with negatively charged essential oils during high mixing, leading to a synergistic antibacterial effect. Furthermore, the hydrophobic nature of the essential oils effectively reduces the water vapor permeability of NCS when added, which is beneficial for film preparation.

[0029] 3. Compared with traditional PPC composite film preparation methods, the method of this invention is convenient to process, requires less equipment, and uses less additives. In this invention, NFC forms a mechanical strengthening effect with the PPC matrix, and the unique network structure of NFC locks the molecular chains of PPC, thereby preventing their slippage and effectively improving the dimensional stability of the PPC matrix. Attached Figure Description

[0030] Figure 1 Tensile properties of PPC / NCS / NFC composite films with different NFC addition amounts;

[0031] Figure 2 SEM images of PPC / NCS / NFC composite films with different NFC addition amounts (a: Example 1; b: Example 2; c: Example 3; d: Example 4; e: Example 5);

[0032] Figure 3 Dimensional stability of PPC / NCS / NFC composite films with different NFC addition amounts;

[0033] Figure 4 Antibacterial activity of PPC / NCS / NFC composite films with different NCS addition amounts against Escherichia coli;

[0034] Figure 5 Antibacterial properties of PPC / NCS / NFC composite films with different NCS addition amounts against Staphylococcus aureus;

[0035] Figure 6 Antibacterial activity of PPC / NCS / NFC composite films with different essential oil additions against Escherichia coli;

[0036] Figure 7 Antibacterial properties of PPC / NCS / NFC composite films with different essential oil additions against Staphylococcus aureus;

[0037] Figure 8 Tensile properties of PPC / NCS / NFC composite films with different shear treatment times;

[0038] Figure 9 SEM images of PPC / NCS / NFC composite films with different shearing times (a: Example 16; b: Example 17);

[0039] Figure 10 Tensile properties of PPC / NCS / NFC composite films at different shearing speeds. Detailed Implementation

[0040] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0041] The mass fraction composition of the PPC / NCS / NFC composite films in Examples 1 to 5 is shown in Table 1. The PPC / NCS / NFC composite films in Examples 1 to 5 are designated as C1, C2, C3, C4, and C5, respectively, for single-factor experiments.

[0042] In Examples 1 to 5: the NFC source is corn stalk powder, the essential oil is orange peel essential oil, the lubricant is a 1:1 mixture of stearic acid and ethylene bis-stearamide, the stabilizer is antioxidant 1010, and the surfactant is Tween-80.

[0043] Table 1

[0044] Serial Number serial number PPC NCS NFC essential oils lubricant stabilizer surfactants Shearing time / h Example 1 C1 100 8 0 5 0.5 1 0.5 2 Example 2 C2 100 8 2 5 0.5 1 0.5 2 Example 3 C3 100 8 6 5 0.5 1 0.5 2 Example 4 C4 100 8 10 5 0.5 1 0.5 2 Example 5 C5 100 8 14 5 0.5 1 0.5 2

[0045] The preparation method of the PPC / NCS / NFC composite film in Example 1 includes the following steps:

[0046] 1. One-pot preparation of NFC and NCS

[0047] 1) Mix 14 parts of corn stalk powder with 90 parts of 20wt% sodium hydroxide solution, stir in a water bath at 75℃ for 6h, filter after the reaction, wash the solid sample obtained by filtration until neutral, add 90 parts of 15wt% sodium chlorite solution and 4 parts of glacial acetic acid mixed solution, heat and stir at 80℃ for 2h, then wash with pure water until neutral, and dry to dryness;

[0048] 2) Place the 0 and 8 portions of dried CS obtained in 1) into a container, add pure water to adjust the solid-liquid ratio to 1:15, and then place the mixture in a homogenizer to perform high-speed shearing at 16000 rpm for 2 hours. Finally, centrifuge at 12000 rpm for 5 minutes, and freeze-dry to obtain a mixed powder of NFC and NCS.

[0049] 2. Preparation of PPC / NCS / NFC composite films

[0050] a: First, add 5 parts of orange peel essential oil to the NFC and NCS mixed powder obtained in step 2) above, then add 0.5 parts of surfactant, mix in a high-speed mixer for 40 minutes, then add 100 parts of PPC, 0.5 parts of lubricant, and 1 part of stabilizer and continue mixing for 60 minutes.

[0051] b: The mixture obtained in step a is melt-extruded and granulated in a twin-screw extruder at a temperature of 180°C and a screw speed of 120 rpm to obtain PPC / NCS / NFC composite particles;

[0052] c: The composite particles obtained in step b are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine. The pre-compression pressure of the flat vulcanizing machine is 0.5MPa, the pre-compression time is 180s, the pressurization pressure is 2.5MPa, the pressurization time is 150s, and the hot-pressing temperature is 150℃.

[0053] The thickness of the composite film is approximately 0.31 mm.

[0054] The preparation methods of the PPC / NCS / NFC composite films in Examples 2 to 5 are basically the same as those in Example 1. The difference is that in step 2) of the preparation of Examples 2 to 5, the mass fractions of the dried sample obtained in step 1) are 2 parts, 6 parts, 10 parts and 14 parts, respectively, to prepare the PPC / NCS / NFC composite films of Examples 2 to 5.

[0055] For the PPC / NCS / NFC composite films of Examples 1 to 5, tensile test samples were prepared using a cutting tool. The mechanical properties of the PPC / NCS / NFC composite films of Examples 1 to 5 were tested in groups according to the method of national standard GB / T1040.1-2018. The tensile test samples were dumbbell-shaped specimens with a length of 50 mm, a gauge length of 30 mm, a center width of 4 mm, and a thickness of approximately 0.31 mm. The tensile speed was 20 mm / min, and at least 5 specimens were tested in each group. The average values ​​of the tensile strength, tensile modulus, and tensile elongation at break were taken. The results are as follows: Figure 1 As shown.

[0056] from Figure 1It can be seen that the tensile strength and elastic modulus of the PPC / NCS / NFC composite films increased after the addition of NFC, and the tensile strength and elastic modulus of all composite films were higher than those of the sample with 0 parts of NFC (C1). Compared with C1, the tensile strength of C4 increased from 9.80 MPa to 15.03 MPa, an increase of 53%; the elastic modulus increased from 2.82 MPa to 4.25 MPa, an increase of 51%. This improvement is mainly attributed to the interfacial entanglement between NFC and PPC molecular chains. NFC has a nanofiber structure and can form mechanical chains with PPC molecular chains. Generally, NFC plays a role in restricting the movement of PPC molecular chains and effectively forming a network structure in composite materials, thereby significantly enhancing the mechanical properties of the composite materials. In addition, all PPC composite films (C1-C5) exhibited excellent ductility, with tensile fracture strain exceeding 400%, which is mainly attributed to the flexibility of PPC.

[0057] The PPC / NCS / NFC composite films of Examples 1 to 5 were observed at 10,000x magnification at 3kV after being quenched with liquid nitrogen. Figure 2 Scanning electron microscopy (SEM) images of the fracture surfaces of the PPC / NCS / NFC composite films are shown. The fracture surface (C1) of the film without NFC exhibits a certain wrinkled structure, indicating that the composite material possesses ductile fracture characteristics. With increasing NFC content, white stripes appear in the SEM images; these white stripes represent NFC at the fracture cross-section, which is well embedded in the matrix, enhancing the mechanical properties of the composite. A network structure formed by NFC and the PPC matrix is ​​observed in C5, which facilitates mechanical interlocking with polymer molecules. No micron-scale aggregates of NFC were observed in any of the SEM observations, further demonstrating that good dispersion of NFC and the formation of the network structure are crucial for enhancing the mechanical properties of the composite. This is consistent with the improved mechanical properties observed in tensile tests.

[0058] Examples 1-5 of the PPC / NCS / NFC composite films were subjected to external force at 25°C. The composites with different NFC contents were completely folded to 0° and held in this folded state for 1 minute. After the external force was removed, the angles formed by the composites increased until they remained constant. The angles formed by the composites at different times were recorded to analyze the recovery rate of different groups. The results are as follows: Figure 3As shown in the figure, observations revealed that, within the same recovery time, the recovery angle of the composite material containing NFC was significantly better than that of C1. Specifically, at a recovery time of 3 seconds, the former had a recovery angle of 105°, while the latter's recovery angle was greater than 130°. Furthermore, with the gradual increase of the NFC proportion, the recovery angle and dimensional stability of the PPC composite material were enhanced. Among them, C5 exhibited the best dimensional stability, with a recovery angle of 138° at 3 seconds, 168° at 30 seconds, 170° at 1 minute, and reaching 173° at 5 minutes. This improvement is attributed to the nanofiber structure of NFC, which encapsulates the PPC molecular chains and supports their extension.

[0059] The mass fraction composition of the PPC / NCS / NFC composite films of Examples 6 to 10 is shown in Table 2. The PPC / NCS / NFC composite films of Examples 6 to 10 are respectively designated as S1, S2, S3, S4 and S5 for single-factor experiments.

[0060] In Examples 6 to 10: the NFC source is bamboo powder, the lubricant is a 1:1 mixture of oleamide and erucamide, the stabilizer is antioxidant 1076, and the surfactant is Tween-85.

[0061] Table 2

[0062] Serial Number serial number PPC NCS NFC essential oils lubricant stabilizer surfactants Shearing time / h Example 6 S1 100 0 14 0 0.5 0.5 0.5 2 Example 7 S2 100 2 14 0 0.5 0.5 0.5 2 Example 8 S3 100 5 14 0 0.5 0.5 0.5 2 Example 9 S4 100 7 14 0 0.5 0.5 0.5 2 Example 10 S5 100 10 14 0 0.5 0.5 0.5 2

[0063] The preparation method of the PPC / NCS / NFC composite film in Example 6 includes the following steps:

[0064] 1. One-pot preparation of NFC and NCS

[0065] 1) Mix 20 parts of bamboo powder with 85 parts of 20wt% sodium hydroxide solution, stir in a water bath at 75℃ for 6 hours, filter after the reaction is complete, wash the solid sample obtained by filtration until neutral, add 85 parts of 15wt% sodium chlorite solution and 4 parts of glacial acetic acid mixed solution, heat and stir at 70℃ for 2 hours, then wash with pure water until neutral, and dry to dryness;

[0066] 2) Place 14 portions of the dried sample obtained in 1) and 0 portions of CS in a container, add pure water to adjust the solid-liquid ratio to 1:10, and then place the mixture in a homogenizer to perform high-speed shearing at 10,000 rpm for 2 hours. Finally, centrifuge at 10,000 rpm for 3 minutes, and freeze-dry to obtain a mixed powder of NFC and NCS.

[0067] 2. Preparation of PPC / NCS / NFC composite films

[0068] a: First, add 0 parts of essential oil to the mixed powder of NFC and NCS obtained in step 2) above, then add 0.5 parts of surfactant, mix in a high-speed mixer for 20 minutes, then add 100 parts of PPC, 0.5 parts of lubricant and 0.5 parts of stabilizer and continue mixing for 30 minutes.

[0069] b: The mixture obtained in step a is melt-extruded and granulated in a twin-screw extruder at a temperature of 150°C and a screw speed of 100 rpm to obtain PPC / NCS / NFC composite particles;

[0070] c: The composite particles obtained in step b are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine. The pre-compression pressure of the flat vulcanizing machine is 0.5 MPa, the pre-compression time is 150 s, the pressurization pressure is 2.5 MPa, the pressurization time is 150 s, and the hot-pressing temperature is 140℃.

[0071] The composite film thickness is approximately 0.30 mm.

[0072] The preparation methods of the PPC / NCS / NFC composite films in Examples 7 to 10 are basically the same as those in Example 6, except that in step 2) of the preparation of Examples 7 to 10, the mass fraction of CS is 2 parts, 5 parts, 7 parts and 10 parts respectively, and the PPC / NCS / NFC composite films of Examples 7 to 10 are prepared respectively.

[0073] For the PPC / NCS / NFC composite films of Examples 6-10, antibacterial test samples were prepared using a cutter. The antibacterial properties of the PPC / NCS / NFC composite films of Examples 6-10 were tested in groups according to the method of national standard GB / T 31402-2015. The tensile test samples were circular specimens with a diameter of 50 mm and a thickness of 3 mm. They were incubated with suspensions of *Escherichia coli* and *Staphylococcus aureus* in a water bath shaker at 160 rpm and 36 ± 1 °C for 60 min. A group of samples without the composite material was used as a control group. 100 μL of the solution was diluted to 10⁵ CFU / mL. Then, 500 μL of the diluted bacterial solution was applied to a culture dish containing LB broth (pH = 7) agar using an L-shaped rod. The culture dishes were incubated at 37 °C for 48 h. After 48 h, the bacterial growth status was observed, and the colony count was recorded.

[0074] The antibacterial properties of PPC / NCS / NFC composite films against Escherichia coli are as follows: Figure 4 As shown, the antibacterial properties against Staphylococcus aureus are as follows: Figure 5As shown in the figure. Observation of samples cultured for 48 hours revealed that the composite film (S1) with only PPC and NFC added had almost no inhibitory effect on *Escherichia coli* and *Staphylococcus aureus*. Composite films with 7 and 10 parts NCS added showed similar antibacterial activity, both significantly stronger than the other groups. Furthermore, the composite films with added NCS (S2–S5) showed greater inhibitory effect on *Staphylococcus aureus* than on *Escherichia coli*. This is because NCS has a greater inhibitory effect on Gram-positive bacteria (*Staphylococcus aureus*) than on Gram-negative bacteria (*Escherichia coli*), and the NCS surface contains a large number of amino groups, including positively charged –NH4+. 3+ The groups on NCS (non-cationic phosphodiesterate) bind more readily to the negatively charged membranes of Gram-positive pathogens, and electrostatic interactions exist between NCS and bacterial cell walls. In this sense, because the cell walls of Gram-positive microorganisms consist of a thick layer of peptidoglycan and a polymer called teichoic acid, whose backbone is highly charged by negatively charged phosphate groups, it can establish electrostatic interactions with cationic antimicrobial compounds (such as NCS). Therefore, Gram-positive microorganisms are more susceptible to NCS than Gram-negative microorganisms, thus exhibiting superior antimicrobial properties.

[0075] The mass fraction composition of the PPC / NCS / NFC composite films of Examples 11 to 15 is shown in Table 3. The PPC / NCS / NFC composite films of Examples 11 to 15 are respectively represented by numbers K1, K2, K3, K4 and K5 for single-factor experiments.

[0076] In Examples 11 to 15: the NFC source is rice straw powder, the essential oil is Artemisia argyi essential oil, the lubricant is a mixture of ethylene bis-stearamide, oleamide and erucamide in a 1:2:1 ratio, the stabilizer is antioxidant 1076, and the surfactant is mannoerythritol ester-a.

[0077] Table 3

[0078] Serial Number serial number PPC NCS NFC essential oils lubricant stabilizer surfactants Shearing time / h Example 11 K1 100 7 14 0 0.5 0.5 0.5 2 Example 12 K2 100 7 14 2 0.5 0.5 0.5 2 Example 13 K3 100 7 14 5 0.5 0.5 0.5 2 Example 14 K4 100 7 14 7 0.5 0.5 0.5 2 Example 15 K5 100 7 14 10 0.5 0.5 0.5 2

[0079] The preparation method of the PPC / NCS / NFC composite film in Example 11 includes the following steps:

[0080] 1. One-pot preparation of NFC and NCS

[0081] 1) Mix 25 parts of rice straw powder with 90 parts of 20wt% sodium hydroxide solution, stir in a water bath at 75℃ for 6h, filter after the reaction, wash the solid sample obtained by filtration until neutral, add 85 parts of 15wt% sodium chlorite solution and 5 parts of glacial acetic acid mixed solution, heat and stir at 80℃ for 2h, then wash with pure water until neutral, and dry to dryness;

[0082] 2) Place 14 portions of the dried sample and 7 portions of CS obtained in 1) into a container, add pure water to adjust the solid-liquid ratio to 1:20, and then place the mixture in a homogenizer to perform high-speed shearing at 13000 rpm for 2 hours. Finally, centrifuge at 15000 rpm for 3 minutes, and freeze-dry to obtain a mixed powder of NFC and NCS.

[0083] 2. Preparation of PPC / NCS / NFC composite films

[0084] a: First, add 0 parts of Artemisia argyi essential oil to the mixed powder of NFC and NCS obtained in step 2) above, then add 0.5 parts of surfactant, mix in a high-speed mixer for 30 minutes, then add 100 parts of PPC, 0.5 parts of lubricant and 0.5 parts of stabilizer and continue mixing for 50 minutes.

[0085] b: The mixture obtained in step a is melt-extruded and granulated in a twin-screw extruder at a temperature of 160°C and a screw speed of 120 rpm to obtain PPC / NCS / NFC composite particles;

[0086] c: The composite particles obtained in step b are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine. The pre-compression pressure of the flat vulcanizing machine is 0.7 MPa, the pre-compression time is 180 s, the pressurization pressure is 3.5 MPa, the pressurization time is 180 s, and the hot-pressing temperature is 150 ℃.

[0087] The composite film thickness is approximately 0.33 mm.

[0088] The preparation methods of the PPC / NCS / NFC composite films in Examples 12 to 15 are basically the same as those in Example 11, except that in preparation step a of Examples 12 to 15, the mass fractions of Artemisia argyi essential oil are 2 parts, 5 parts, 7 parts, and 10 parts, respectively, to prepare the PPC / NCS / NFC composite films of Examples 12 to 15.

[0089] For the PPC / NCS / NFC composite films of Examples 11-15, antibacterial test samples were prepared using a cutter. The antibacterial properties of the PPC / NCS / NFC composite films of Examples 11-15 were tested in groups according to the method of national standard GB / T 31402-2015. The tensile test samples were circular specimens with a diameter of 50 mm and a thickness of 3 mm. They were incubated with suspensions of *Escherichia coli* and *Staphylococcus aureus* in a water bath shaker at 160 rpm and 36 ± 1 °C for 60 min. A group of samples without the composite material was used as a control group. 100 μL of the solution was diluted to 10⁵ CFU / mL. Then, 500 μL of the diluted bacterial solution was applied to a culture dish containing LB broth (pH = 7) agar using an L-shaped rod. The culture dishes were incubated at 37 °C for 48 h. After 48 h, the bacterial growth status was observed, and the colony count was recorded.

[0090] The antibacterial properties of PPC / NCS / NFC composite films against Escherichia coli are as follows: Figure 6 As shown, the antibacterial properties against Staphylococcus aureus are as follows: Figure 7 As shown in the figure, observation of samples cultured for 48 hours revealed that the composite films with added essential oils (K2-K5) exhibited superior antibacterial properties compared to those without essential oils (K1 and S1-S5), demonstrating a synergistic effect in antibacterial activity. This is because NCS carries a positive charge and can electrostatically interact with the negatively charged essential oils, and the hydrophobic essential oils can improve the hydrophilic properties of chitosan, enhancing its reactivity. The composite film with 5 parts essential oil (K3) showed the strongest inhibitory effect against *Escherichia coli* and *Staphylococcus aureus*. This is because excessive essential oils may result in insufficient electrostatic binding with CS, thus reducing their synergistic effect. The composite films (K2-K5) with added essential oils have a greater inhibitory effect on Staphylococcus aureus than on Escherichia coli. This is because Gram-positive bacteria (Staphylococcus aureus) are more easily inhibited by essential oils than Gram-negative bacteria (Escherichia coli). This is because the outer membrane of Gram-negative bacteria is protected by hydrophilic lipopolysaccharides, which reduces their sensitivity to essential oils, thereby preventing essential oils from entering and destroying bacterial cells.

[0091] The mass fraction composition of the PPC / NCS / NFC composite films of Examples 16 and 17 is shown in Table 4. The PPC / NCS / NFC composite films of Examples 16 and 17 are respectively designated as F1 and F2 for single-factor experiments.

[0092] In Examples 16 and 17: the NFC source is wood flour, the essential oil is cinnamon essential oil, the lubricant is a mixture of ethylene bis-stearamide, oleamide, and erucamide in a 2:1:1 ratio, the stabilizer is antioxidant 1010, and the surfactant is mannoerythritol ester-b.

[0093] Table 4

[0094] Serial Number serial number PPC NCS NFC essential oils lubricant stabilizer surfactants Shearing time / h Example 16 F1 100 7 2 5 0.5 0.5 1 0.5 Example 17 F2 100 7 2 5 0.5 0.5 1 2

[0095] The preparation method of the PPC / NCS / NFC composite film in Example 16 includes the following steps:

[0096] 1. One-pot preparation of NFC and NCS

[0097] 1) Mix 20 parts of wood flour with 70 parts of 10wt% sodium hydroxide solution, stir in a water bath at 75°C for 6 hours, filter after the reaction is complete, wash the solid sample obtained by filtration until neutral, add a mixed solution of 80 parts of 10wt% sodium chlorite solution and 4 parts of glacial acetic acid, heat and stir at 70°C for 1 hour, then wash with pure water until neutral, and dry to dryness;

[0098] 2) Place two and seven portions of the dried sample obtained in 1) into a container, add pure water to adjust the solid-liquid ratio to 1:15, and then place the mixture in a homogenizer to perform high-speed shearing at 11,000 rpm for 0.5 h. Finally, centrifuge at 12,000 rpm for 3 min, and freeze-dry to obtain a mixed powder of NFC and NCS.

[0099] 2. Preparation of PPC / NCS / NFC composite films

[0100] a: First, add 5 parts of cinnamon essential oil to the mixed powder of NFC and NCS obtained in step 2) above, then add 1 part of surfactant, and mix in a high-speed mixer for 30 minutes. Then add 100 parts of PPC, 0.5 parts of lubricant, and 0.5 parts of stabilizer and continue mixing for 40 minutes.

[0101] b: The mixture obtained in step a is melt-extruded and granulated in a twin-screw extruder at a temperature of 180°C and a screw speed of 160 rpm to obtain PPC / NCS / NFC composite particles;

[0102] c: The composite particles obtained in step b are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine. The pre-compression pressure of the flat vulcanizing machine is 0.7 MPa, the pre-compression time is 180 s, the pressurization pressure is 3.5 MPa, the pressurization time is 180 s, and the hot-pressing temperature is 150 ℃.

[0103] The composite film thickness is approximately 0.30 mm.

[0104] The preparation method of the PPC / NCS / NFC composite film in Example 17 is basically the same as that in Example 16, except that the shearing time of the homogenizer in step 2) of the preparation method in Example 17 is 2 hours.

[0105] For the PPC / NCS / NFC composite films of Examples 16 and 17, tensile test samples were prepared using a cutter. The mechanical properties of the PPC / NCS / NFC composite films of Examples 16 and 17 were tested in groups according to the method of national standard GB / T1040.1-2018. The tensile test samples were dumbbell-shaped specimens with a length of 50 mm, a gauge length of 30 mm, a center width of 4 mm, and a thickness of approximately 0.30 mm. The tensile speed was 20 mm / min, and at least 5 specimens were tested in each group. The average values ​​of the tensile strength, tensile modulus, and tensile elongation at break were taken. The results are as follows: Figure 8 As shown.

[0106] Compared to sample F1, which underwent short-time shearing, F2 exhibited better mechanical properties after 2 hours of shearing. Specifically, the tensile strength increased from 8.01 MPa to 11.25 MPa, a 40.4% increase, and the elastic modulus increased from 2.32 MPa to 3.38 MPa, a 45.3% increase. This improvement can be attributed to the shearing force transforming both cellulose and CS into nanoscale particles. Since NFC and NCS have very similar molecular structures, they exhibit good compatibility and stronger performance. Simultaneously, under high-speed shearing, NFC and NCS are uniformly mixed, and bridging hydrogen bonds form between the highly dispersed NFC and NCS, thereby improving film performance. In contrast, F1, due to its excessively short shearing time, did not undergo sufficient shearing of cellulose and CS, explaining its lower tensile strength compared to F2.

[0107] For the PPC / NCS / NFC composite films of Examples 16 and 17, after being quenched with liquid nitrogen, they were observed at 10,000x magnification at 3kV. Figure 9 A scanning electron microscope image of the fractured surface of the PPC / NCS / NFC composite film is shown. Figure 9 The PPC / NCS / NFC composite film exhibited an inhomogeneous phase structure and showed large-particle fillers, indicating that NFC and NCS were unevenly distributed in the matrix in the insufficiently sheared sample. This reduced the compatibility of PPC, NFC, and NCS, and the larger particle size also led to larger porosity, which is the main reason for the low mechanical properties. After sufficient shearing, as... Figure 9 b. The particle size and porosity of both NFC and NCS are reduced, which leads to an improvement in the mechanical properties of the samples.

[0108] The mass fraction composition of the PPC / NCS / NFC composite films of Examples 18 to 22 is shown in Table 5. The PPC / NCS / NFC composite films of Examples 18 to 22 are respectively represented by numbers H1, H2, H3, H4, and H5 for single-factor experiments.

[0109] In Examples 18 to 22: the NFC source was cotton stalk powder, the essential oil was hyssop essential oil, the lubricant was a mixture of ethylene bis-stearamide, oleamide, and erucamide in a 1:1:2 ratio, the stabilizer was antioxidant 1790, and the surfactant was Tween-80.

[0110] Table 5

[0111] Serial Number serial number PPC NCS NFC essential oils lubricant stabilizer surfactants Shear speed / rpm Example 18 H1 100 7 14 5 1 0.5 0.5 10000 Example 19 H2 100 7 14 5 1 0.5 0.5 12000 Example 20 H3 100 7 14 5 1 0.5 0.5 14000 Example 21 H4 100 7 14 5 1 0.5 0.5 16000 Example 22 H5 100 7 14 5 1 0.5 0.5 18000

[0112] The preparation method of the PPC / NCS / NFC composite film in Example 18 includes the following steps:

[0113] 1. One-pot preparation of NFC and NCS

[0114] 1) Mix 25 parts of cotton stalk powder with 80 parts of 20wt% sodium hydroxide solution, stir in a water bath at 75℃ for 6h, filter after the reaction is complete, wash the solid sample obtained by filtration until neutral, add 90 parts of 15wt% sodium chlorite solution and 5 parts of glacial acetic acid mixed solution, heat and stir at 80℃ for 2h, then wash with pure water until neutral, and dry to dryness;

[0115] 2) Place 14 portions of the dried sample and 7 portions of CS obtained in 1) into a container, add pure water to adjust the solid-liquid ratio to 1:20, and then place the mixture in a homogenizer to perform high-speed shearing at 10,000 rpm for 2 hours. Finally, centrifuge at 15,000 rpm for 3 minutes, and freeze-dry to obtain a mixed powder of NFC and NCS.

[0116] 2. Preparation of PPC / NCS / NFC composite films

[0117] a: First, add 5 parts of hyssop essential oil to the mixed powder of NFC and NCS obtained in step 2) above, then add 0.5 parts of surfactant, mix in a high-speed mixer for 30 minutes, then add 100 parts of PPC, 1 part of lubricant and 0.5 parts of stabilizer and continue to mix for 30 minutes.

[0118] b: The mixture obtained in step a is melt-extruded and granulated in a twin-screw extruder at a temperature of 180°C and a screw speed of 160 rpm.

[0119] rpm, to obtain PPC / NCS / NFC composite particles;

[0120] c: The composite particles obtained in step b are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine. The pre-compression pressure of the flat vulcanizing machine is 0.7 MPa, the pre-compression time is 180 s, the pressurization pressure is 3.5 MPa, the pressurization time is 180 s, and the hot-pressing temperature is 140℃.

[0121] The composite film thickness is approximately 0.29 mm.

[0122] The preparation methods of the PPC / NCS / NFC composite films in Examples 19 to 22 are basically the same as those in Example 18, except that the homogenization speeds in step 2) of the preparation methods in Examples 19 to 22 are 12000 rpm, 14000 rpm, 16000 rpm, and 18000 rpm, respectively.

[0123] For the PPC / NCS / NFC composite films of Examples 19 to 22, tensile test samples were prepared using a cutting tool. The mechanical properties of the PPC / NCS / NFC composite films of Examples 19 to 22 were tested in groups according to the method of national standard GB / T1040.1-2018. The tensile test samples were dumbbell-shaped specimens with a length of 50 mm, a gauge length of 30 mm, a center width of 4 mm, and a thickness of approximately 0.29 mm. The tensile speed was 20 mm / min, and at least 5 specimens were tested in each group. The average values ​​of the tensile strength, tensile modulus, and tensile elongation at break were taken. The results are as follows: Figure 10 As shown.

[0124] from Figure 10 It can be seen that shear rate improves the tensile strength and elastic modulus of the PPC / NCS / NFC composite film, reaching its maximum at 16000 rpm (H3). Specifically, the tensile strength increased from 13.12 MPa to 16.53 MPa, an increase of 26.1%, and the elastic modulus increased from 3.73 MPa to 5.74 MPa, an increase of 53.9%. These results confirm that shear rate has a significant effect on improving the mechanical properties of the PPC / NCS / NFC composite film. This may be because as the shear rate increases, the cellulose and CS particles are subjected to greater mechanical and frictional forces, causing the particles to gradually become smaller. Smaller particles have a larger surface area, which increases the contact area between the material and other components, improves the interfacial bonding strength, and reduces porosity and internal defects. This improves the dispersion uniformity of nanoparticles in the composite material. Uniform dispersion of nanoparticles is beneficial to improving the mechanical properties of the material, and increases the density and compactness of the material. However, when the shearing speed reaches 18,000 rpm, the mechanical properties of the PPC / NCS / NFC composite film decrease. This may be because high-speed shearing causes intense collisions and friction between particles, resulting in excessive wear and even peeling of the particle surface, which in turn causes the particles to deform or even break.

Claims

1. A polypropylene carbonate (PPC) / nano-chitosan (NCS) / nano-cellulose (NFC) composite film, characterized in that, The mass fraction of the PPC / NCS / NFC composite film comprises: 100 parts PPC, 1-10 parts NCS, 1-14 parts NFC, 1-10 parts essential oil, 0.5-1 part lubricant, 0.5-1 part stabilizer, and 0.5-1 part surfactant; the lubricant is one or a combination of two or more of stearic acid, ethylene bis-stearamide, oleamide, and erucamide; the stabilizer is one or a combination of two or more of antioxidant 1010, antioxidant 168, antioxidant 1076, and antioxidant 1790; the surfactant is one or a combination of two or more of Tween-80, Tween-85, mannose erythritol ester-a, and mannose erythritol ester-b; the preparation method of the PPC / NCS / NFC composite film includes the following steps: 1) Mix 10-30 parts of biomass powder with 70-90 parts of sodium hydroxide solution, stir in a water bath at 75°C for 6 hours, filter after the reaction is complete, wash the solid sample obtained by filtration until neutral, add a mixed solution of 80-90 parts of sodium chlorite solution and 4-5 parts of glacial acetic acid, heat and stir at 70-80°C for 1-2 hours, then wash with pure water until neutral, and dry to dryness; 2) Place 1-14 parts of the dried sample and 1-10 parts of chitosan (CS) obtained in step 1) into a container, add pure water to adjust the solid-liquid ratio to 1:10-1:20, and then put it into a homogenizer to perform high-speed shearing of the solid-liquid mixture at 10000-18000 rpm for 0.5-2 hours; finally, centrifuge at 10000-15000 rpm for 3-5 minutes, and freeze-dry to obtain a mixed powder of NFC and NCS; 3) First, add 1 to 10 parts of essential oil to the NFC and NCS mixed powder obtained in step 2), then add 0.5 to 1 part of surfactant, and mix in a high-speed mixer for 20 to 40 minutes. Then add 100 parts of PPC, 0.5 to 1 part of lubricant, and 0.5 to 1 part of stabilizer and continue mixing for 30 to 60 minutes. 4) The mixture obtained in step 3) is melt-extruded and granulated in a twin-screw extruder at a temperature of 110-180℃ and a screw speed of 60-200 rpm to obtain PPC / NCS / NFC composite particles. 5) The composite particles obtained in step 4) are processed into PPC / NCS / NFC composite films using a flat vulcanizing machine, blown film machine, or cast film machine.

2. The PPC / NCS / NFC composite film according to claim 1, characterized in that, The nanocellulose is derived from one or more of the following biomass powders: wood powder, bamboo powder, corn stalk powder, rice stalk powder, and cotton stalk powder.

3. The PPC / NCS / NFC composite film according to claim 1, characterized in that, The biomass powder has a mesh size of 80-150.

4. The PPC / NCS / NFC composite film according to claim 1, characterized in that, In step 5), the pre-compression pressure of the flat vulcanizing machine is 0.5-0.7 MPa, the pre-compression time is 150-180 s, the pressurization pressure is 2.5-3.5 MPa, the pressurization time is 150-180 s, and the hot-pressing temperature is 140-150℃; the processing temperature of the blown film machine is 120-160℃; and the processing temperature of the cast film machine is 120-160℃.

5. The PPC / NCS / NFC composite film according to claim 1, characterized in that, In step 5), the thickness of the PPC / NCS / NFC composite film is 0.05 mm to 0.8 mm.

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