Method for green preparation of polylactic acid composite film by synergistic effect of fluorescent carbon quantum dots and cellulose film and polylactic acid thin film

Abstract

This invention provides a green method for preparing polylactic acid (PLA) composite films using fluorescent carbon quantum dots and cellulose membranes in synergistic processes. The prepared PLA composite film exhibits good hydrophilicity, excellent transparency, and high haze; it is opaque to UV and high-energy blue light; its tensile strength is not less than 35 MPa; its hemolysis rate is close to 0; and it possesses antibacterial activity, making it suitable for food packaging. The method includes the following steps: a) preparing a cellulose suspension and forming a film from the cellulose suspension; b) immersing the film obtained in step a) in a carbon quantum dot solution and then drying it to obtain a cellulose membrane containing carbon quantum dots; c) subjecting the pure PLA membrane to oxygen plasma treatment or alkali treatment followed by washing to obtain a PLA membrane; d) coating the PLA membrane with an adhesive and hot-pressing it with the carbon quantum dot-containing cellulose membrane to obtain the PLA composite film.

Description

Technical Field

[0001] This invention relates to the field of nanomaterial preparation technology, specifically a method for the green preparation of carbon quantum dot / cellulose / polylactic acid composite membranes using fluorescent carbon quantum dots and cellulose membranes in synergistic polylactic acid films. Background Technology

[0002] With the continued growth of global energy demand, the depletion of fossil fuels, price volatility, and accelerating global warming are becoming increasingly severe problems, making the development of renewable energy and efficient energy conversion pathways an urgent need. Biomass energy, as an important renewable energy source, currently contributes approximately 45±10 EJ to global energy supply. Rice (Oryza sativa L.), a major global food crop, generates a large amount of inedible biomass residues during its production. These residues mainly consist of cellulose, hemicellulose, lignin, as well as proteins, starch, extracts, and inorganic substances. However, these resources have not been fully utilized from the source of production, resulting not only in serious resource waste but also hindering environmental and economic development.

[0003] Carbon quantum dots (CQDs), as an emerging quasi-spherical nanomaterial, typically have a size of less than 10 nm and possess a three-dimensional multilayer graphite structure. Their lateral and longitudinal dimensions are similar, and their surfaces exhibit clear lattice fringes and abundant chemical groups, displaying unique intrinsic-state luminescence and quantum confinement effects. Compared to traditional quantum dots and organic dyes containing heavy metals such as cadmium and lead, CQDs not only possess superior optical properties such as high photostability, tunable fluorescence emission, and high quantum yield, but also offer advantages such as excellent biocompatibility, low toxicity, ease of functionalization, and low cost, showing broad application prospects in fields such as bioimaging, sensing and detection, and functional packaging.

[0004] Cellulose is a linear polymer composed of D-glucose units linked by β-1,4-glycosidic bonds, with its basic structural unit being a long poly(β-1,4-glucanopyranoside) chain. These molecular chains self-assemble through dense intramolecular and intermolecular hydrogen bonding to form highly crystalline nanofibers with widths of only 1.5–3.5 nm, and further aggregate into basic fibrous structures with sizes of approximately 1–10 nm. Due to its high specific strength, low coefficient of thermal expansion, and good biodegradability, cellulose is often considered an ideal bio-based reinforcing material.

[0005] Polylactic acid (PLA) is a biodegradable thermoplastic polymer formed by the dehydration condensation of lactic acid monomers. It is typically produced from starch derived from renewable resources such as corn and potatoes through processes including saccharification, fermentation, and ring-opening polymerization. PLA possesses good biocompatibility, high mechanical strength, strong hydrophobicity, and excellent processing performance and transparency. However, its low crystallization rate, insufficient mechanical toughness, and poor thermal stability limit its practical applications. To improve the physical and functional properties of PLA films, researchers often introduce nanofillers, such as nanocellulose, metal nanoparticles (Ag, ZnO, SiO2, etc.), and natural bioactive substances (such as essential oils), through physical blending or chemical polymerization. In this system, cellulose, as a reinforcing phase, can significantly improve the mechanical and thermal properties of PLA; while carbon quantum dots, as highly tunable nanomaterials, are expected to improve the thermal stability of the matrix and impart antibacterial activity and fluorescence properties to the material when introduced into PLA composite film systems, thus expanding its applications in high-end fields such as smart packaging and biosensing.

[0006] In summary, this study aims to explore strategies for the high-value comprehensive utilization of rice straw. Targeting the approximately 30% residue remaining after extracting raw materials for pulp molding products from rice straw, carbon quantum dots were prepared using a hydrothermal method, while cellulose was extracted through alkali treatment and bleaching. The resulting cellulose and carbon quantum dots were then synergistically applied to the preparation of carbon quantum dot / cellulose / polylactic acid composite films. This research has significant scientific and practical value for developing next-generation intelligent packaging materials and achieving the efficient conversion and high-value utilization of agricultural waste. Summary of the Invention

[0007] The technical problem to be solved by this invention is to address the shortcomings of existing methods for preparing carbon quantum dot / cellulose / polylactic acid composite films, such as high equipment requirements, limited film-forming methods, high reagent toxicity, and significant potential harm to human health. This invention provides a green method for preparing polylactic acid composite films using fluorescent carbon quantum dots and cellulose films in synergistic polylactic acid film production. This offers a novel approach to preparing carbon quantum dot / cellulose / polylactic acid composite films. The resulting carbon quantum dot / cellulose / polylactic acid composite films exhibit good hydrophilicity, excellent transparency, and high haze; they are opaque to UV light and high-energy blue light; have a tensile strength of not less than 35 MPa; and exhibit hemolysis rates close to zero. They also possess antibacterial activity and can be used for food packaging.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for the green preparation of carbon quantum dot / cellulose / polylactic acid composite membranes using fluorescent carbon quantum dots and cellulose membranes in synergistic polylactic acid film preparation includes the following steps:

[0010] a. Prepare cellulose into a cellulose suspension, and then fabricate the cellulose suspension into a thin film;

[0011] b. Immerse the film obtained in step a into a carbon quantum dot solution, then remove and dry it to obtain a cellulose membrane containing carbon quantum dots.

[0012] c. Treat the pure polylactic acid film with oxygen plasma or treat it with alkali and wash it to obtain a polylactic acid film.

[0013] d. Coating a polylactic acid film with an adhesive and stacking it with a cellulose film containing carbon quantum dots, followed by hot pressing, yields a carbon quantum dot / cellulose / polylactic acid composite film.

[0014] As an improvement, in step a, cellulose is prepared into a cellulose suspension through chemical purification and mechanical treatment;

[0015] As an improvement, the cellulose purification process in step a is as follows:

[0016] a1: Prepare a 0.4 wt% KH-550 anhydrous ethanol solution with a cellulose to anhydrous ethanol volume ratio of 1 g: 10 mL, adjust the pH of the solution to 4.5–5.5 with glacial acetic acid, and then place it in a constant temperature water bath at 20–30 ℃ and stir to allow it to fully hydrolyze, thus obtaining a silanol solution.

[0017] a2: Add cellulose to the above silanol solution, stir the reaction in a water bath at 40-60 °C, wash the product multiple times with anhydrous ethanol to remove residual silanol, and finally dry the product to obtain precursor material d.

[0018] The mechanical processing is as follows: the obtained precursor material d is prepared into a mixed solution with a concentration of 1-3 wt%, and the mixed solution is stirred to obtain a cellulose suspension, denoted as K-CNF suspension.

[0019] As an improvement, the process of preparing a thin film from K-CNF suspension is as follows: Wet filter paper and lay it flat on a glass plate, then place a square silica gel plate frame on the filter paper; take 6-9g of K-CNF suspension and add pure water to it until the total weight is 11g, mix evenly at room temperature to obtain mixed solution a; use a dropper to draw mixed solution a and spread it evenly in the square groove of the silica gel plate, then place it in a 30-50 ℃ forced-air drying oven for 3-5h to prepare a KH-550 modified cellulose membrane, denoted as K-CNF membrane.

[0020] As an improvement, the specific process of step b is as follows: Take 1 mL of carbon quantum dot solution and 4 mL of pure water and mix them evenly at room temperature to obtain mixed solution b; place the K-CNF membrane in a petri dish, add mixed solution b until it covers the K-CNF membrane, and soak for 1 min; take out the soaked membrane and lay it flat on a glass plate, and then place it in a 30-50 ℃ forced-air drying oven for 3-5 h to prepare a cellulose membrane containing carbon quantum dots, denoted as KR membrane.

[0021] As an improvement, in step c, the process of treating the pure polylactic acid film with alkali and washing it to obtain a polylactic acid film is as follows: prepare a sodium hydroxide solution with a concentration of 8-12wt%, immerse the polylactic acid film in it for 10-50 minutes, take it out and wash it with running water until the film surface is neutral, and obtain the alkali-treated PLA film.

[0022] As an improvement, the preparation method of the adhesive in step d is as follows: prepare an aqueous solution of polyvinyl alcohol with a concentration of 8-14wt%, place it in a hydrothermal stirring environment at 80-100℃, add citric acid according to the solid-liquid ratio of polyvinyl alcohol aqueous solution to citric acid of 100g: 1-3g, and mix evenly to obtain the adhesive.

[0023] As an improvement, during hot pressing in step d, the polylactic acid film coated with adhesive and the cellulose film containing carbon quantum dots are stacked one on top of the other and laminated at 70-90℃ and 4-6MPa for 10-20 minutes; then laminated at room temperature until the composite film cools to room temperature.

[0024] As an improvement, the carbon quantum dot solution preparation method in step b is as follows: the residue after extracting pulp from rice straw as raw material for molded products is prepared into a carbon quantum dot solution by hydrothermal synthesis. The carbon quantum dot solution is then filtered, centrifuged, and dialyzed to obtain the carbon quantum dot solution used for film preparation, which is the carbon quantum dot solution in step b.

[0025] As an improvement, the carbon quantum dot solution preparation method in step b is as follows: 30% of the residue after extracting pulp from rice straw for molding products is crushed and passed through a 90-mesh sieve as a precursor material. 4-6 g of the residue is weighed and placed in a beaker, 50-70 mL of purified water is added, and the mixture is stirred and mixed evenly. The mixture is then transferred to a reaction vessel and heated at 180-220 ℃ for 5-7 hours. After the reaction is completed, the mixture is allowed to cool naturally to room temperature. The solution is then filtered using a microporous membrane, centrifuged, and dialyzed. The resulting solution is the rice straw-based carbon quantum dot solution (R-CQDs).

[0026] The beneficial effects of this invention are as follows:

[0027] This invention utilizes approximately 30% of the residue remaining after extracting pulp from rice straw for molding products to achieve high-value conversion. On one hand, it uses low-cost and readily available biomass waste to prepare widely applicable carbon quantum dots. On the other hand, it extracts cellulose from biomass waste through alkali treatment and bleaching. By combining a layer-by-layer self-packing method and a hot-pressing method, cellulose, carbon quantum dots, and polylactic acid are hot-pressed to achieve the goal of economically, efficiently, and safely preparing polylactic acid composite membranes. The composite membrane prepared by this invention has good fluorescence, tensile, and ultraviolet blocking properties. Attached Figure Description

[0028] Figure 1 , 2 These are photographs of rice straw-based carbon quantum dot solutions under 365nm ultraviolet light and fluorescent light, respectively.

[0029] Figure 3 , 4 5, 6, 7, 8, and 9 are the 3D fluorescence spectrum, fluorescence lifetime spectrum, UV-Vis transmittance spectrum, transmission electron microscope image, Zeta potential, X-ray diffraction pattern, and Fourier transform infrared spectrum of carbon quantum dot (R-CQDs) powder, respectively.

[0030] Figure 10 These are antibacterial test images of carbon quantum dots, where image A shows the antibacterial test for Escherichia coli and image B shows the antibacterial test for Staphylococcus aureus.

[0031] Figure 11 These are photographs of PLA film (a), D-PLA film (b), KR film (c), KRD composite film (d), and KRDK composite film under fluorescent light (top) and 365 nm ultraviolet light (bottom).

[0032] Figure 12 These are photographs of KD-10(a), KD-20(b), KD-30(c), KD-40(d), and KD-50(e) under fluorescent light (top) and 365nm ultraviolet light (bottom).

[0033] Figure 13 This is a UV-Vis transmittance diagram of K-CNF film, KRD composite film, and KRDK composite film;

[0034] Figure 14 This is a graph showing the UV-Vis transmittance of KD-10, KD-20, KD-30, KD-40, and KD-50 composite films;

[0035] Figure 15 These are Fourier transform infrared spectra of K-CNF membrane, KRD composite membrane, and D-PLA membrane;

[0036] Figure 16These are Fourier transform infrared spectra of KD-10, KD-20, KD-30, KD-40, and KD-50 composite films;

[0037] Figure 17 These are Fourier transform infrared spectra of P-10, P-20, P-30, P-40, and P-50, etc.

[0038] Figure 18 These are the tensile strength and elastic modulus of the composite membrane.

[0039] Figure 19 , 20 Figures 21, 22, 23, 24, and 25 are tensile cross-sectional views of KRD, KRDK, KD-10, KD-20, KD-30, KD-40, and KD-50 composite membranes, respectively.

[0040] Figure 26 This is the result of a hemolysis rate test on composite membranes, etc.

[0041] Figure 27 These are the hemolysis rate test results for P-10, P-20, P-30, P-40, and P-50, etc.

[0042] Figure 28 These are antibacterial test diagrams of blank sample (①), PLA membrane (②), D-PLA membrane (③), KR membrane (④), and KRD membrane (⑤). Diagram A shows the antibacterial test for Escherichia coli, and diagram B shows the antibacterial test for Staphylococcus aureus. Detailed Implementation

[0043] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings:

[0044] Example 1: Preparation of rice straw-based carbon quantum dot solution

[0045] Includes the following steps:

[0046] S1. Using the residue of about 30% after extracting pulp from rice straw as raw material for molded products as precursor material, the precursor material is pretreated as follows: the precursor material is stirred and crushed, and then screened with a 90-mesh molecular sieve to obtain a 90-mesh precursor material. It is then placed in an oven at (105±2)℃ to dry until the mass deviation between two adjacent weighings is less than 0.2% (time difference not less than 1h), and stored in a desiccator in a sealed container for later use.

[0047] S2. The pretreated precursor material is prepared into a carbon quantum dot solution by hydrothermal synthesis, including the following steps:

[0048] S21. Take 5g of 90-mesh oven-dried precursor material and place it in 60mL of deionized water, then stir and mix thoroughly.

[0049] S22. Pour the mixture into a 100mL high-temperature and high-pressure reactor lined with polytetrafluoroethylene, and place the reactor in a 200℃ oven for 6 hours to obtain a carbon quantum dot solution.

[0050] S3. The carbon quantum dot solution is filtered, centrifuged, dialyzed, and cooled to dry, including the following steps:

[0051] S31. Remove the reactor and allow it to cool naturally to room temperature. Pour out the carbon quantum dot solution and filter it using a microporous membrane with a pore size of 0.22 μm.

[0052] S32. Place the filtered carbon quantum dot solution in a centrifuge and centrifuge at 1000 rpm for 10 min to separate the reaction residue and the yellow reaction liquid.

[0053] S33. Pre-treat the dialysis bag with a pore size of 1000 Da, then pour the reaction supernatant into the pre-treated dialysis bag and dialyze for 24 h to remove inorganic salts from the carbon quantum dots and obtain a purified carbon quantum dot solution.

[0054] S34. The purified carbon quantum dot solution was placed in a freeze dryer and freeze-dried under vacuum for 48 hours to obtain carbon quantum dot (R-CQDs) solid powder.

[0055] S35. The surface morphology of carbon quantum dot (R-CQDs) solid powder was observed by transmission electron microscopy, the crystal structure of carbon dots was determined by XRD, the fluorescence performance was determined by 3D fluorescence spectrum and fluorescence lifetime spectrum, the surface group composition was determined by Zeta potential and Fourier transform infrared spectrum, the shielding performance was determined by ultraviolet-visible light transmittance, and the antibacterial performance was determined by antibacterial test.

[0056] Regarding optical properties, the fluorescence spectrum and fluorescence quantum yield of rice straw-based carbon quantum dots were tested. The carbon quantum dots exhibited strong blue fluorescence emission under 365 nm excitation, with the optimal emission peak located at 450 nm. The fluorescence quantum yield of the carbon quantum dots was 1.53%.

[0057] from Figure 1 , 2 As can be seen, R-CQDs appear bright blue under 365nm ultraviolet light and pale yellow under fluorescent light.

[0058] The fluorescence spectrometer used was FluoroMaxPlus (HORIBA, France). Figure 3As shown, the fluorescence spectra of R-CQDs change significantly with the excitation wavelength, exhibiting a marked excitation dependence. This is attributed to surface defects and size distribution differences caused by heteroatom functional groups on their surface. Under 365 nm excitation, R-CQDs show strong blue fluorescence emission, with the optimal emission peak located at 450 nm.

[0059] For fluorescence lifetime (FluoroMaxPlus, HORIBA, France), see [link / reference]. Figure 4 The fluorescence lifetime decay curves of R-CQDs conform to the double exponential decay model, with an average fluorescence lifetime τ of 5.64 ns (where τ1 = 6.595 ns and τ2 = 3.151 ns). The average time for excited-state molecules of R-CQDs to return to the ground state is short, indicating that their fluorescence lifetime is in the nanosecond range, and no phosphorescence or afterglow was observed. The quantum yield of R-CQDs is 1.53%.

[0060] Table 1:

[0061]

[0062] For UV-Vis transmittance (Lambda 950, PerkinElmer, USA), please refer to [reference needed]. Figure 5 Carbon quantum dots exhibit the highest transmittance (96%) in the 200-800 nm range, while the highest transmittances for high-energy blue light, UVA, UVB, and UVC are 66.8%, 43.1%, 4.5%, and 4.2%, respectively.

[0063] The morphological characteristics of carbon quantum dots were tested using transmission electron microscopy, such as... Figure 6 As shown, the TEM (Regulus 8100, Hitachi, Japan) images of R-CQDs show that: R-CQDs exhibit partial aggregation, but no macromolecular impurities are present. The average particle size of R-CQDs is 3.16 ± 0.54 nm, with a particle size range of 2.00 to 4.50 nm.

[0064] The elemental composition was analyzed, and its Zeta potential and X-ray diffraction were measured, as shown in the figures below. Figure 7 , 8 Zeta potential: R-CQDs remain negatively charged in the pH range of 6-8, due to the abundance of carboxyl functional groups on their surface. XRD (XRD Ultima IV, Rigaku, Japan): R-CQDs exhibit a broad peak at 22.6°, indicating the presence of a disordered carbon structure with low crystallinity, which may be related to the presence of graphitic carbon (002) planes.

[0065] from Figure 9As shown in the Fourier transform infrared (FTIR) spectra, carbon quantum dots exhibit surface functional groups such as hydroxyl groups, carbon-carbon double bonds, and carbonyl groups. FTIR (Nicolet iS10, Thermo Fisher Scientific, USA): The raw materials (precursor materials for preparing carbon quantum dots), R-CQDs, and R-CQDs residues all show stretching vibration peaks of OH / NH, ≡CH, and out-of-plane bending vibrations of CH near 3330-3350 cm⁻¹, 2890-3000 cm⁻¹, and 1050 cm⁻¹. Furthermore, R-CQDs show stretching vibration absorption peaks of CH, CH₄, C=C / CN, and carbonyl groups (COC) at 2932 cm⁻¹, 2876 cm⁻¹, 1395 cm⁻¹, and 1020 cm⁻¹, respectively, while the peak at 1585 cm⁻¹ corresponds to the superimposed vibrations of the aromatic ring skeleton and C=O. Both the raw material and the residue of R-CQDs exhibit C=C stretching vibrations near 1620 cm⁻¹ and 1590 cm⁻¹. The specific composition of the raw material (the precursor material for preparing carbon quantum dots) is shown in Table 2.

[0066] Table 2:

[0067] Component Name Content / wt% Component Name Content / wt% Moisture 18 manganese sulfate 0.1 crude protein 4.3 Titanium sulfate 0.05 crude fiber 68 Alumina 0.05 Crude fat 1.8 silicates 5 potassium sulfate 1.0 disodium hydrogen phosphate 0.5 superphosphate 0.5 amino acids 0.6 Magnesium sulfate 0.1 / /

[0068] like Figure 10 The antibacterial properties of carbon quantum dots (R-CQDs) were tested. Plate counting was used to evaluate the antibacterial characteristics of R-CQDs against Gram-negative *Escherichia coli* and Gram-positive *Staphylococcus aureus*. The untreated control group ① showed dense bacterial colonies for both strains, indicating normal proliferation without intervention. The culture dish ② containing R-CQDs showed a decrease in *E. coli* colonies and a further reduction in the survival rate of *Staphylococcus aureus*, with inhibition rates of 99% for both *E. coli* and *Staphylococcus aureus*.

[0069] Example 2: Preparation of cellulose membrane containing carbon quantum dots, i.e., KR membrane

[0070] S4: Preparation of cellulose fiber suspension (K-CNF)

[0071] Cellulose was purchased from Guilin Sailuna Technology Co., Ltd. K-CNF suspension was prepared through chemical purification and mechanical treatment.

[0072] The purification process is as follows:

[0073] S41: A 0.4 wt% KH-550 anhydrous ethanol solution was prepared by mixing cellulose (purchased from Guilin Sailuna Technology Co., Ltd.) and anhydrous ethanol at a volume ratio of 1 g: 10 mL. The pH of the solution was adjusted to 4.5–5.5 with glacial acetic acid. The solution was then placed in a 25 ℃ constant temperature water bath and stirred for 1 h to allow for complete hydrolysis, yielding a silanol solution. Cellulose was added to the above silanol solution and reacted in a 50 ℃ water bath with stirring for 1 h. After the reaction was completed, the product was washed multiple times with anhydrous ethanol to remove residual silanol. Finally, the product was dried in a 70 ℃ oven until completely dry to obtain precursor material d.

[0074] Mechanical processing: The obtained precursor material d was added to pure water to prepare a mixed solution with a concentration of 2wt%. The mixed solution was stirred for 40 times to obtain a cellulose suspension, denoted as K-CNF suspension.

[0075] S5: Preparation of KR membrane (cellulose membrane containing carbon quantum dots)

[0076] S51: Wet the double-loop qualitative filter paper with purified water and lay it flat on a glass plate. Cut out a square frame (5cm×5cm) of appropriate size in the silica gel plate and place it on the surface of the filter paper. Take 7.6g of K-CNF suspension and add purified water to it until the total weight is 11g. Mix it evenly at room temperature to obtain mixed solution a. Use a dropper to draw mixed solution a and spread it evenly in the square groove (5cm×5cm) of the silica gel plate. Then place the entire device in a 40 ℃ forced-air drying oven for 4h to prepare the KH-550 modified cellulose membrane, which is denoted as K-CNF membrane.

[0077] S52: Take 1 mL of the purified carbon quantum dot solution R-CQDs obtained in step S33 and mix it with 4 mL of pure water at room temperature to obtain mixed solution b; place the K-CNF membrane in a petri dish, add mixed solution b until it covers the K-CNF membrane, and soak for 1 min; take out the soaked membrane and lay it flat on a glass plate, and then place the entire device in a 40℃ forced-air drying oven for 4 h to prepare a cellulose membrane containing carbon quantum dots, denoted as KR membrane.

[0078] Example 3: Preparation of Adhesive

[0079] S6. Preparation of adhesive (PNMS), including the following steps:

[0080] S61. Prepare an aqueous solution of polyvinyl alcohol (PVA) with a concentration of 11.7 wt%, and stir it in a water bath at 90°C for 1 h.

[0081] S62. Add citric acid according to the solid-liquid ratio of PVA aqueous solution to citric acid of 50g:1g, and stir in a water bath at 90℃ until the mixture is uniform to obtain adhesive (PNMS); put PNMS into a sealed container, defoam and then use.

[0082] Example 4: Preparation of polylactic acid films P-10, P-20, P-30, P-40 and P-50 after alkali treatment

[0083] S7. The polylactic acid film (thickness 0.0456±0.0008mm, size 5×5cm) is subjected to alkali treatment and washed, including the following steps:

[0084] S71. Prepare a 10wt% NaOH solution and immerse the polylactic acid film for 10 min, 20 min, 30 min, 40 min and 50 min respectively.

[0085] S72. Rinse the polylactic acid film with running water until it is neutral, and let it air dry until the mass deviation between two consecutive weighings is less than 0.2% (time difference not less than 1 hour). The resulting polylactic acid film after alkali treatment is called P-10, P-20, P-30, P-40 and P-50 respectively.

[0086] Example 5: Oxygen plasma treatment of polylactic acid film (preparation of D-PLA film)

[0087] S8. Perform oxygen plasma treatment on the polylactic acid film (thickness 0.0456±0.0008mm, size 5×5cm), including the following steps:

[0088] S81. Fix the polylactic acid film inside the machine cavity and set the cavity parameters as follows: 70W power and 16.0Kpa vacuum.

[0089] S82. Oxygen, as a reaction gas, is discharged into the chamber at a flow rate of 600 sccm. The surface of the polylactic acid film is subjected to oxygen plasma bombardment for 100 s to obtain a plasma-treated polylactic acid film (D-PLA), which is then stored in the dark.

[0090] Example 6: Preparation of polylactic acid composite films KD-10, KD-20, KD-30, KD-40 and KD-50

[0091] S9. Preparation of polylactic acid composite membrane, comprising the following steps:

[0092] S91. Immerse P-10, P-20, P-30, P-40 and P-50 obtained in step s72 in PNMS obtained in step s62 for 2 min respectively;

[0093] S92. Using tweezers, gently hold P-10, P-20, P-30, P-40 and P-50 in the air respectively, so that the PNMS not attached to the film drips down;

[0094] S93. P-10, P-20, P-30, P-40 and P-50 are laid flat on top of a KR membrane obtained in step S52. Then, another KR membrane is laid flat on top of P-10, P-20, P-30, P-40 and P-50 respectively. The entire device is then placed at 80℃ and 5MPa for lamination for 15min to prepare polylactic acid composite membranes, which are denoted as KD-10, KD-20, KD-30, KD-40 and KD-50 respectively.

[0095] KD-10, KD-20, KD-30, KD-40 and KD-50 all consist of two layers of KR membrane and one layer of alkali-treated PLA membrane.

[0096] Example 7: Preparation of polylactic acid composite membrane KRD

[0097] After coating a D-PLA membrane obtained in Example 5 with an adhesive (PNMS) according to steps s91 and 92, it was laid flat on top of a KR membrane obtained in step S52 and laminated at 80°C and 5MPa for 15 minutes to prepare a polylactic acid composite membrane, denoted as KRD.

[0098] Example 8: Preparation of polylactic acid composite membrane KRDK

[0099] After coating a D-PLA film obtained in Example 5 with an adhesive (PNMS) according to steps s91 and 92, it is laid flat on top of a KR film obtained in step s52. Then, another KR film is laid flat on top of the D-PLA film coated with adhesive. The film is then laminated at 80°C and 5MPa for 15 minutes to prepare a polylactic acid composite film, denoted as KRDK.

[0100] The composite films prepared in each embodiment were subjected to UV fluorescence evaluation, water contact angle, transmittance, haze, thickness testing, UV-Vis transmittance, FTIR, tensile testing, and SEM (tensile cross-section) tests. The surface functional group composition of the composite film was determined by Fourier transform infrared spectroscopy of the polylactic acid film, and the hydrophilicity, haze, and transmittance of the composite film were determined by the water contact angle. Mechanical tensile analysis was performed to determine the tensile strength and elastic modulus of the composite film, and thermogravimetric analysis was conducted to analyze its thermal stability.

[0101] like Figure 11PLA film, D-PLA film, KR film, KRD composite film and KRDK composite film all have high transparency under fluorescent light; PLA film and D-PLA film have no fluorescence under ultraviolet light, while KR film, KRD composite film and KRDK composite film exhibit a bright blue color under ultraviolet light, which is due to the fluorescence properties of carbon quantum dots.

[0102] Both types of composite films exhibit high transparency under fluorescent light and display a bright blue color under 365nm ultraviolet light. For example... Figure 12 The KD-10, KD-20, KD-30, KD-40 and KD-50 composite films all exhibit high transparency under fluorescent light and display a bright blue color under ultraviolet light, which is due to the fluorescence properties of carbon quantum dots.

[0103] Water contact tests were conducted on the plasma-treated D-PLA film. After plasma treatment, the PLA film changed from hydrophobic to hydrophilic, and its water contact angle did not change significantly after two months of treatment. The water contact angle was tested using an FCA2000A2 instrument (manufactured by Shanghai Aifeisi Precision Instruments Co., Ltd.), and the data are shown in Table 3.

[0104] The water contact angle of the PLA film changed from hydrophobic to hydrophilic after plasma treatment. This is because oxygen plasma treatment introduces oxygen-containing polar groups such as hydroxyl and carboxyl groups onto the PLA surface, increasing the film's hydrophilicity. After 0, 30, and 60 days of plasma treatment, the water contact angle of the PLA film at 0s, 30s, and 60s showed little change, indicating that these polar groups did not migrate extensively into the bulk material during storage, nor were they covered by airborne contaminants (such as hydrocarbons). The hydrophilic modification effect of oxygen plasma on the PLA film exhibits good aging stability, maintaining no significant degradation for 60 days, and the treated surface state demonstrates good environmental tolerance.

[0105] Table 3:

[0106]

[0107] Water contact test, light transmittance and haze test were performed on cellulose and composite membranes. The data are shown in Tables 4 and 5. The films all have good hydrophilicity, good transparency and high haze.

[0108] Table 4:

[0109]

[0110] The films all exhibit good hydrophilicity, excellent transparency, and high haze.

[0111] Table 5:

[0112]

[0113] Thickness tests were conducted on the cellulose membrane and the composite membrane using a digital micrometer. The data are shown in Tables 6 and 7. The thickness of the composite membrane was approximately 0.1 mm.

[0114] Table 6:

[0115] serial number D-PLA PLA K-CNF KR KRD 1 0.047 0.048 0.011 0.018 0.08 2 0.046 0.043 0.012 0.018 0.089 3 0.045 0.044 0.009 0.019 0.088 4 0.047 0.042 0.011 0.020 0.090 5 0.045 0.044 0.017 0.021 0.088 6 0.046 0.051 0.015 0.017 0.082 7 0.047 0.046 0.015 0.020 0.092 8 0.046 0.045 0.017 0.019 0.081 9 0.044 0.047 0.012 0.018 0.078 10 0.047 0.046 0.012 0.021 0.08 average value 0.046 0.0456 0.0131 0.0191 0.0848 Standard deviation 0.001 0.002497999 0.002586503 0.0013 0.004812484 Standard error 0.000316228 0.000789937 0.000817924 0.000411096 0.001521841 Thickness (example) 0.046±0.0003 0.0456±0.0008 0.0131±0.0008 0.0191±0.0004 0.0848±0.0015 unit mm mm mm mm mm

[0116] Table 7:

[0117] serial number KRDK KD-10 KD-20 KD-30 KD-40 KD-50 1 0.096 0.107 0.110 0.111 0.097 0.116 2 0.104 0.111 0.107 0.112 0.100 0.113 3 0.098 0.109 0.111 0.110 0.102 0.113 4 0.101 0.108 0.108 0.112 0.099 0.113 5 0.101 0.109 0.113 0.112 0.098 0.110 6 0.102 0.114 0.117 0.109 0.106 0.112 7 0.092 0.110 0.113 0.110 0.104 0.111 8 0.093 0.105 0.111 0.112 0.098 0.107 9 0.1 0.112 0.108 0.115 0.103 0.112 10 0.098 0.115 0.106 0.111 0.104 0.113 average value 0.0985 0.11 0.1104 0.1114 0.1011 0.112 Standard deviation 0.003694591 0.002932576 0.003168596 0.00156205 0.002947881 0.002236068 Standard error 0.001168332 0.000927362 0.001001998 0.000493964 0.000932202 0.000707107 Thickness (example) 0.0985±0.0011 0.11±0.0009 0.1104±0.0010 0.1114±0.0005 0.1011±0.0009 0.112±0.0007 unit mm mm mm mm mm mm

[0118] The UV-Vis transmittance of cellulose membrane and composite membrane was tested and compared with that of carbon quantum dots. Both cellulose membrane and composite membrane showed that they were almost transparent to UV light and high-energy blue light.

[0119] The UV-Vis transmittance test was conducted using a Lambda 950 (PerkinElmer, USA), see [link / reference]. Figure 13 , 14 The modified cellulose membrane and composite membrane are almost opaque to UV light and HEBL; the composite membrane containing only one layer of treated PLA membrane has the highest transmittance of 8.3% in the 200-800nm ​​range, while the composite membrane containing two layers of treated PLA membrane has a transmittance of 2.5%; as the number of composite membrane layers increases, its transmittance in the 250-800nm ​​range decreases.

[0120] The composite films are almost completely opaque to UV light and HEBL. The highest transmittance of KD-10, KD-20, KD-30, KD-40, and KD-50 composite films in the 200-800 nm range is 3.0%, 5.1%, 3.4%, 4.0%, and 4.6%, respectively. The highest transmittance of HEBL is 2.1%, 3.6%, 2.1%, 2.8%, and 2.7%. The highest transmittance of UV-A is 1.9%, 3.2%, 1.8%, 2.5%, and 2.2%, respectively. The highest transmittance of UV-B is 1.1%, 1.9%, 0.9%, 1.5%, and 1.1%, respectively. The highest transmittance of UV-C is 0.8%, 1.5%, 0.6%, 1.1%, and 0.7%, respectively.

[0121] Fourier transform infrared spectroscopy (FTIR) was performed on cellulose membranes and composite membranes (Nicolet iS10, Thermo Fisher Scientific, USA).

[0122] like Figure 15The KR, D-PLA, and KRD composite membranes all exhibit ≡CH stretching vibration peaks, C=O stretching vibration peaks, CC skeleton stretching vibration peaks, benzene ring skeleton vibration peaks, and CH out-of-plane bending vibrations in the vicinity of 3307 cm⁻¹, 1750 cm⁻¹~1710 cm⁻¹, 1170 cm⁻¹, 1450 cm⁻¹, and 1225 cm⁻¹~680 cm⁻¹. Furthermore, the CO stretching vibration and the OH in-plane bending vibration are coupled, producing two absorption bands at 1420 cm⁻¹ and 1300~1200 cm⁻¹.

[0123] like Figure 16 The peaks for stretching vibrations of OH / NH are 3340 cm⁻¹ to 3200 cm⁻¹; the peaks for stretching vibrations of CH are 2950 cm⁻¹ to 2890 cm⁻¹; the stretching vibrations of CO and the in-plane bending vibrations of OH are coupled, producing two absorption bands at 1420 cm⁻¹ and 1300 to 1200 cm⁻¹ (wood spectroscopy); the peaks for stretching vibrations of the CC skeleton are 1170 cm⁻¹ to 1110 cm⁻¹; and the peaks for out-of-plane bending vibrations of CH are 1030 cm⁻¹ to 890 cm⁻¹.

[0124] like Figure 17 The peaks around 1755 cm⁻¹ and 1720 cm⁻¹ are C=O stretching vibration peaks. After plasma treatment, the intensity at 1755 cm⁻¹ is higher than that at 1720 cm⁻¹, which is attributed to the introduction of -COOH by plasma treatment. As the alkali treatment time increases, the peak at 1750 cm⁻¹ broadens, and the peak intensity near 1270 cm⁻¹ increases. This is attributed to the hydrolysis of the PLA surface caused by alkali treatment, which produces some medium molecular weight oligomers. The peak at 1450 cm⁻¹ is the benzene ring skeleton vibration peak. Due to the coupling of the two in-plane bending vibrations (CH₂), the 1370 peak splits into two peaks of basically equal intensity, which appear around 1380 and 1360 cm⁻¹, respectively. The peaks around 1177 cm⁻¹ and 1125 cm⁻¹ are CC skeleton stretching vibration peaks. The peaks from 1078 cm⁻¹ to 865 cm⁻¹ are CH out-of-plane bending vibrations.

[0125] Tensile tests were performed on the composite membrane (using a CMT4204 universal testing machine). The tensile strength and elastic modulus of the composite membrane both increased with the increase of PLA membrane alkali treatment time.

[0126] like Figure 18The tensile strengths of KD-10, KD-20, KD-30, KD-40, and KD-50 were 38.37±0.59 MPa, 41.77±0.93 MPa, 50.58±0.33 MPa, 59.44±2.27 MPa, 64.29±0.68 MPa, 65.74±1.69 MPa, and 70.89±1.76 MPa, respectively. The tensile strength of the composite membrane increased with the duration of alkali treatment of the PLA membrane. This is because, with prolonged alkali treatment, the surface etching effect of the 15wt% NaOH solution on the pure PLA film became increasingly significant. This, along with physical anchoring and chemical cross-linking, jointly drove the continuous improvement of interfacial adhesion strength. At the physical level, prolonged etching continuously deepens the erosion of the PLA surface, significantly increasing its micro-roughness and porosity. The resulting microcracks provide space for component penetration. Under high-temperature and high-pressure hot pressing conditions of 80℃ and 5MPa, these microporous structures cause the PVA / citric acid crosslinking agent and cellulose layer to be deeply compressed and embedded into the PLA matrix, thereby constructing a strong mechanical anchoring (interlocking) effect.

[0127] Meanwhile, chemical-dimensional interfacial modification is achieved through base-induced ester bond hydrolysis, exposing a large number of terminal hydroxyl (-OH) and carboxyl (-COOH) groups on the PLA surface. With increasing treatment time, the density of these polar groups continuously increases, forming a dense hydrogen bond network with the hydroxyl groups in PVA and undergoing efficient esterification with the polycarboxyl crosslinking agent citric acid. Simultaneously, they also exhibit strong chemical interactions with amino or silanol groups on the surface of the KH550-treated cellulose membrane, forming dense translayer chemical bonds.

[0128] The synergistic effect of this physical interlocking and chemical cross-linking causes the interfacial bonding force between the PLA layer and the cellulose layer to steadily increase with treatment time. In tensile tests, this strengthened interface effectively inhibits debonding or delamination, ensuring that stress can be smoothly transferred from the PLA matrix to the high-strength cellulose backbone. Although the PLA matrix may suffer slight damage due to hydrolysis, the gain effect brought about by the improved interfacial properties significantly dominates the overall mechanical performance of the composite film, ultimately driving its tensile strength to show a clear monotonic increasing trend with increasing alkali treatment time.

[0129] The elastic moduli of KD-10, KD-20, KD-30, KD-40, and KD-50 were 1278.72±156.43, 1206.21±117.12, 2869.87±132.08, 2360.33±167.63, 2381.36±25.06, 2897.75±65.44, and 4056.01±129.83, respectively. The elastic modulus of the composite membrane initially decreased and then increased with increasing alkali treatment time of the PLA membrane. This is because in the initial stage of treatment with 15wt% NaOH solution, the strong alkali induced severe hydrolysis of the ester bonds on the PLA surface, leading to molecular chain breakage and a significant decrease in molecular weight. These low-molecular-weight degradation products remaining in the system produced a significant "internal plasticizing" effect, enhancing the mobility of chain segments and making the originally dense surface structure more porous with numerous micro-defects. At this stage, although the interlayer interface is gradually being established, the severe stiffness loss of the PLA matrix surface layer dominates the overall mechanical performance of the material, forming a physical "low-modulus intermediate transition zone." Because the crosslinking agent has not yet built a sufficiently strong and tough continuous network at this stage, this weak interfacial layer is prone to elastic deformation in the early stages of stress. This local "softening" effect masks the reinforcing effect of the interface, ultimately leading to a decrease in the overall initial stiffness and elastic modulus of the composite film.

[0130] However, with further extension of the alkali treatment time, the reaction mechanism underwent a significant transformation, and the interface strengthening effect began to outweigh the negative impact of degradation. Because the alkali solution selectively degrades the polymer, it preferentially attacks the loose, large-volume amorphous regions in the PLA structure, allowing the tightly packed, highly rigid crystalline regions to be relatively preserved. This significant increase in surface local crystallinity effectively compensated for the initial stiffness loss structurally, laying the foundation for modulus recovery. Simultaneously, prolonged deep etching created a deep and interconnected porous structure on the PLA surface. Under high-temperature and high-pressure hot pressing conditions of 80℃ and 5MPa, PVA, flexible cellulose segments, and citric acid molecules were deeply penetrated and compressed into the pores, transforming the interface from a traditional "two-dimensional contact" into a thick and interwoven "three-dimensional anchoring layer."

[0131] From a chemical perspective, deep hydrolysis exposed a vast number of carboxyl and hydroxyl reaction sites. Citric acid, a multi-carboxyl crosslinking agent, induced extremely dense three-dimensional crosslinking of PVA, aminated cellulose, and PLA surface end groups at high temperatures. This high-density crosslinking network constructed a high-strength "hard shell" in situ at the interface, significantly restricting molecular chain slippage. This reinforcing effect, driven by both physical anchoring and high-density chemical crosslinking, not only gradually "repaired" and strengthened the originally porous PLA surface layer but also formed an interface reinforcement layer with a modulus higher than the original matrix. Ultimately, the increased rigidity of the interface region dominated the material's mechanical behavior, leading to a significant recovery in the overall elastic modulus of the composite film in the later stages.

[0132] The tensile cross-section of the composite membrane is analyzed, based on the mechanical tensile cross-section of the composite membrane. Figure 19-25 It can be seen that after stretching, the KR layer in all composite films is always tightly bonded to the adhesive. This is because cellulose itself is rich in hydroxyl groups (-OH). After modification with KH-550, a siloxane network is grafted onto the surface of cellulose, and a large number of amino groups (-NH2) are also introduced. During hot pressing, the carboxyl groups (-COOH) in the adhesive will undergo an amidation reaction with the -NH2 on the KR surface to form extremely strong covalent bonds (amide bonds). At the same time, the -COOH in citric acid will also undergo an esterification reaction with the -OH contained in PVA and cellulose itself.

[0133] In contrast, KRD ( Figure 19 ) and KRDK ( Figure 20 Severe delamination occurred between the adhesive and D-PLA after stretching. This is because although plasma treatment introduced oxygen-containing groups into the PLA surface, improving its hydrophilicity, the interaction between the PLA and the adhesive was mainly physical adsorption and hydrogen bonding, making it difficult to form large-scale covalent cross-links at 80°C. Therefore, after stretching, the adhesive adhered tightly to KR, and no adhesive residue remained on the D-PLA surface. Furthermore, the adhesive itself exhibits high hardness and brittleness after curing, making it unable to alleviate tensile stress through plastic deformation and highly susceptible to brittle fracture. Consequently, obvious blocky delamination was observed on the adhesive fracture surface after stretching.

[0134] Immersion in high-concentration NaOH causes chemical corrosion and degradation of the PLA surface, resulting in a loose, porous, and even brittle polymer network. This generates a large amount of -COOH and -OH (saponification reaction). Under hot pressing at 80℃, the -COOH / -OH on the PLA surface undergoes esterification and cross-linking with components in the adhesive, thus increasing the chemical bonding force at the interface. Therefore, when the tensile force is applied to P-10, large-area clean peeling does not occur; instead, obvious oblique cracks appear due to stress concentration. As the immersion time increases (30 min), the degree of hydrolysis and saponification on the PLA surface deepens. The density of the hydrogen bond network formed between the polar groups on the PLA surface and the adhesive gradually increases, and the amorphous areas are severely corroded and dissolved. The PLA surface is no longer a single unit but a blocky, brittle surface layer separated by countless microcracks. Therefore, when the tensile force is applied to P-30, obvious blocky peeling occurs.

[0135] When the alkaline etching time was further extended to 40 min and 50 min, the fracture morphology underwent a significant qualitative change. With the formation of deep alkali etching channels, numerous micro- and nano-scale pores and honeycomb-like channels were etched deep into the matrix on the PLA surface and its subsurface. This allowed the adhesive to penetrate deeply under capillary effects, forming a microscopic "tree root"-like interpenetrating network (IPN) and strong mechanical interlocking after hot-pressing curing. Under this deep cross-linking and macroscopic anchoring, tensile loads were efficiently transferred to the deep interior of the PLA. Due to the higher degree of freedom of the surface PLA molecular chains resulting from the deep hydrolysis at 40 min and 50 min, under the applied uniaxial tensile shear stress, the strongly anchored PLA surface polymer and the infiltrated adhesive molecules were forced to undergo intense localized plastic deformation at the edges of the micropores. As the tensile displacement increases, these polymer chains undergo high-ratio stretching, orientation, and rearrangement along the force direction, becoming thinner and leaving a large number of dense, continuous, and highly oriented micro / nano-scale filaments on the tensile fracture surface, exhibiting a characteristic "filamentous drawing" structure. Since 50 min (P-50) has a deeper alkaline etching depth and a thicker physical interpenetrating layer compared to 40 min (P-40), it experiences greater plastic tensile work and deformation displacement before fracture. Consequently, the filamentous drawing structure on the P-50 fracture surface is thicker, denser, and longer. This indicates that deep alkaline etching combined with micro-anchoring can promote the transformation of interfacial failure from brittle interfacial peeling to a high-toughness plastic deformation fracture mode.

[0136] The adhesive thicknesses in KD-10, KD-30, KD-40, and KD-50 are 7.023±0.145um, 5.098±0.181um, 5.890±0.051um, and 8.292±0.058um, respectively.

[0137] Furthermore, the tensile cross-sectional images of the composite films show that KR and the adhesive remained tightly bonded after stretching. This is because cellulose itself is rich in hydroxyl groups (-OH). After modification with KH-550, a siloxane network was grafted onto the cellulose surface, and a large number of amino groups (-NH2) were also introduced. During hot pressing, the carboxyl groups (-COOH) in the adhesive react with the -NH2 on the KR surface to form extremely strong covalent bonds (amide bonds). At the same time, the -COOH in citric acid also reacts with the -OH groups contained in PVA and cellulose itself to undergo esterification. In contrast, KRD showed severe delamination between the adhesive and D-PLA after stretching. This is because although plasma treatment introduced oxygen-containing groups to the PLA surface, improving its hydrophilicity, the interaction between it and the adhesive was mainly physical adsorption and hydrogen bonding, making it difficult to form large-scale covalent cross-links with the adhesive at 80°C. Therefore, after stretching, the adhesive and KR remained tightly bonded, and no adhesive residue remained on the D-PLA surface. The adhesive itself has high hardness and brittleness after curing, and cannot relieve tensile stress through plastic deformation. It is very easy to cause brittle fracture. Therefore, obvious blocky peeling appears on the cross-section of the adhesive after stretching.

[0138] Immersion in high-concentration NaOH causes chemical corrosion and degradation of the PLA surface, resulting in a loose, porous, and even embrittled polymer network. This also generates a large amount of -COOH and -OH (saponification reaction), which corresponds to the peak intensity changes near 1750 cm⁻¹ and 1270 cm⁻¹ in the Fourier transform infrared spectrum. Under hot pressing at 80℃, the -COOH / -OH on the PLA surface undergoes esterification crosslinking with components in the adhesive, thus increasing the chemical bonding force at the interface. Therefore, when the tensile force was transmitted to P-10, no large-area clean peeling occurred. Due to stress concentration, obvious oblique cracks appeared. As the soaking time increased (30 min), the degree of hydrolysis and saponification of the PLA surface deepened. The density of the hydrogen bond network formed between the polar groups on the PLA surface and the adhesive gradually increased. The amorphous area was severely corroded and dissolved. The PLA surface was no longer a whole, but a blocky brittle surface layer separated by countless microcracks. Therefore, when the tensile force was transmitted to P-30, obvious blocky peeling occurred.

[0139] When the alkaline etching time was further extended to 40 min and 50 min, the fracture morphology underwent a significant qualitative change. With the formation of deep alkali etching channels, numerous micro- and nano-scale pores and honeycomb-like channels were etched deep into the matrix on the PLA surface and its subsurface. This allowed the adhesive to penetrate deeply under capillary effects, forming a microscopic "tree root"-like interpenetrating network (IPN) and strong mechanical interlocking after hot-pressing curing. Under this deep cross-linking and macroscopic anchoring, tensile loads were efficiently transferred to the deep interior of the PLA. Due to the higher degree of freedom of the surface PLA molecular chains resulting from the deep hydrolysis at 40 min and 50 min, under the applied uniaxial tensile shear stress, the strongly anchored PLA surface polymer and the infiltrated adhesive molecules were forced to undergo intense localized plastic deformation at the edges of the micropores. As the tensile displacement increases, these polymer chains undergo high-ratio stretching, orientation, and rearrangement along the force direction, becoming thinner and leaving a large number of dense, continuous, and highly oriented micro / nano-scale filaments on the tensile fracture surface, exhibiting a characteristic "filamentous drawing" structure. Since 50 min (P-50) has a deeper alkaline etching depth and a thicker physical interpenetrating layer compared to 40 min (P-40), it experiences greater plastic tensile work and deformation displacement before fracture. Consequently, the filamentous drawing structure on the P-50 fracture surface is thicker, denser, and longer. This indicates that deep alkaline etching combined with micro-anchoring can promote the transformation of interfacial failure from brittle interfacial peeling to a high-toughness plastic deformation fracture mode.

[0140] Partial safety evaluations were conducted on the composite films, and all results met the current standards for food packaging films.

[0141] Given the safety assessment of film materials used in food packaging, such as Figure 26 As shown, the hemolysis rate of pure water is approximately 99%, while the hemolysis rates of PBS, KR, KRD, KRDK, KD-10, KD-20, KD-30, KD-40, and KD-50 are all below 5%. Among them, the hemolysis rate of KRDK is approximately 4%, while the hemolysis rates of KR, KRD, KD-10, KD-20, KD-30, KD-40, and KD-50 are all close to 0, which meets the requirements of the current standard (European Commission Regulation, 2011) for films used in food packaging.

[0142] See Figure 27The hemolysis rates of PBS, PLA, D-PLA, P-10, P-20, P-30, P-40, and P-50 are all below 5%, with P-30 and P-50 having hemolysis rates of approximately 4%, P-40 having a hemolysis rate of approximately 1%, and PLA, D-PLA, P-10, and P-20 having hemolysis rates close to 0, which meets the requirements of the current standard (European Commission Regulation, 2011) for films used in food packaging.

[0143] See Figure 28 The antibacterial properties of PLA membranes, plasma-treated PLA membranes, carbon quantum dot-containing cellulose membranes, and KRD composite membranes against Gram-negative *Escherichia coli* and Gram-positive *Staphylococcus aureus* were evaluated using plate counting. The untreated control group showed dense bacterial colonies for both strains, indicating normal proliferation without intervention. The carbon quantum dot-containing KR membranes achieved a 99.9% inhibition rate against *Staphylococcus aureus*, while pure polylactic acid membranes showed no antibacterial activity against either *Escherichia coli* or *Staphylococcus aureus* before or after plasma treatment.

[0144] The culture dishes corresponding to KR and KRD membranes showed a decrease in the number of Escherichia coli colonies, while the survival rate of Staphylococcus aureus further decreased. The inhibition rates of Escherichia coli against KR membrane and KRD membrane were approximately 36.89% and 27.18%, respectively, while the inhibition rate of Staphylococcus aureus against both reached 99.9%. These results indicate that KR and KRD membranes possess antibacterial activity compared to PLA membranes before and after plasma treatment.

[0145] To explore the application potential of composite films, preliminary preservation tests were conducted on white mushrooms using KRD film and KRDK film. The data are shown in Table 8. The test results show that the weight loss rate of KRD film and KRDK film is lower than that of PVC preservation film.

[0146] Table 8:

[0147] Days Blank sample (g) PVC food wrap (g) K / R / D(g) K / R / D / K (g) Day 1 8.281 12.546 12.772 12.767 Day 2 7.210 12.472 12.751 12.745 weightlessness 1.071 0.074 0.021 0.022 Day 3 6.403 12.410 12.735 12.729 weightlessness 0.807 0.062 0.016 0.016 Day 4 5.457 12.341 12.718 12.713 weightlessness 0.946 0.069 0.017 0.016 Day 5 4.846 12.278 12.702 12.701 weightlessness 0.611 0.063 0.016 0.012 Day 6 4.317 12.212 12.686 12.684 weightlessness 0.529 0.066 0.016 0.017 Day 7 3.809 12.145 12.665 12.665 weightlessness 0.508 0.067 0.021 0.019 Day 8 3.183 12.081 12.650 12.652 weightlessness 0.626 0.064 0.015 0.013 Day 9 2.630 12.012 12.632 12.632 weightlessness 0.553 0.069 0.018 0.018 Day 10 2.031 11.941 12.614 12.617 weightlessness 0.599 0.071 0.018 0.015 Day 11 1.684 11.881 12.605 12.606 weightlessness 0.347 0.060 0.009 0.008 Day 12 1.343 11.805 12.576 12.575 weightlessness 0.341 0.076 0.029 0.031 Day 13 1.045 11.738 12.559 12.558 weightlessness 0.298 0.067 0.017 0.017 Day 14 0.829 11.672 12.545 12.545 weightlessness 0.216 0.066 0.014 0.013

[0148] Example 9: Another method for preparing purified carbon quantum dot solution

[0149] Nitrogen-doped carbon quantum dot solution was prepared using rice straw as raw material, according to the method in CN108409984B (application number 2018102510530).

[0150] A dialysis bag with a pore size of 1000 Da was pretreated, and then the resulting nitrogen-doped carbon quantum dot solution was poured into the pretreated dialysis bag and dialyzed for 24 hours to remove inorganic salts from the carbon quantum dots, resulting in a purified carbon quantum dot solution.

[0151] Example 10: Another method for preparing purified carbon quantum dot solution

[0152] A carbon quantum dot solution was prepared according to the method in CN110157423B (application number 2019105704418);

[0153] A dialysis bag with a pore size of 1000 Da was pretreated, and then the resulting carbon quantum dot solution was poured into the pretreated dialysis bag and dialyzed for 24 hours to remove inorganic salts from the carbon quantum dots, resulting in a purified carbon quantum dot solution.

[0154] Example 11

[0155] The purified carbon quantum dot solution R-CQDs obtained in step S33 of Example 2 was replaced with the purified carbon quantum dot solution obtained in Example 9 or 10, while keeping other conditions unchanged in Example 2, and a KR membrane was prepared.

[0156] Example 12

[0157] The KR membranes in Examples 6, 7, and 8 were replaced with the KR membrane prepared in Example 11, while other conditions remained unchanged, to obtain polylactic acid composite membranes KD-10, KD-20, KD-30, KD-40, KD-50, KRD, and KRDK, respectively.

[0158] The polylactic acid composite film obtained in Example 12 was subjected to UV fluorescence evaluation, water contact angle, transmittance, haze, thickness test, UV-Vis transmittance, FTIR, tensile test, SEM (tensile cross section), hemolysis rate, antibacterial test and other tests. The results were basically the same as the polylactic acid composite films obtained in Examples 6, 7 and 8.

[0159] This invention discloses a green method for preparing polylactic acid composite membranes using fluorescent carbon quantum dots and cellulose membranes in synergistic polylactic acid membranes, comprising: (1) using approximately 30% of the residue after extracting pulp from rice straw as raw material for molded products as a precursor material, pre-treating it, and preparing a carbon quantum dot solution from the pre-treated precursor material using a hydrothermal synthesis method, filtering, centrifuging, and dialysis the carbon quantum dot solution to obtain the carbon quantum dot solution used for membrane preparation; (2) preparing cellulose into a cellulose suspension; (3) using pure polylactic acid membrane as a matrix, treating it with alkali and drying it to absolute dryness to obtain the polylactic acid membrane used for membrane preparation; (4) after preparing the cellulose suspension into a membrane, adding the carbon quantum dot solution, drying it to absolute dryness, then adding an adhesive and hot-pressing it with the pre-treated polylactic acid membrane to obtain the polylactic acid composite membrane. This invention utilizes common biomass raw materials to prepare carbon quantum dots and cellulose, which has the advantages of being green and environmentally friendly. Moreover, no toxic solutions are involved in the membrane treatment process and the preparation of the composite membrane. This invention overcomes the shortcomings of previous polylactic acid composite membrane preparation processes, such as high equipment and method costs, high reagent toxicity, and potential harm to human health. It opens up a completely new approach for the preparation of polylactic acid films.

[0160] The scope of protection of this invention includes, but is not limited to, the above embodiments. The scope of protection of this invention is defined by the claims. Any substitutions, modifications, or improvements to this technology that are easily conceived by those skilled in the art fall within the scope of protection of this invention.

Claims

1. A method for the green preparation of polylactic acid composite membranes using fluorescent carbon quantum dots and cellulose membranes in synergistic polylactic acid film preparation, characterized by: Includes the following steps: a. Prepare cellulose into a cellulose suspension, and then fabricate the cellulose suspension into a thin film; b. Immerse the film obtained in step a into a carbon quantum dot solution, then remove and dry it to obtain a cellulose membrane containing carbon quantum dots. c. Treat the pure polylactic acid film with oxygen plasma or treat it with alkali and wash it to obtain a polylactic acid film. d. Coating a polylactic acid film with an adhesive and stacking it with a cellulose film containing carbon quantum dots, followed by hot pressing, yields a carbon quantum dot / cellulose / polylactic acid composite film.

2. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 1, characterized in that: In step a, cellulose is prepared into a cellulose suspension through chemical purification and mechanical treatment.

3. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 2, characterized in that: In step a, the cellulose purification process is as follows: a1: Prepare a 0.4 wt% KH-550 anhydrous ethanol solution with a cellulose to anhydrous ethanol volume ratio of 1 g: 10 mL, adjust the pH of the solution to 4.5–5.5 with glacial acetic acid, and then place it in a 20-30°C constant temperature water bath and stir to allow it to fully hydrolyze, thus obtaining a silanol solution. a2: Add cellulose to the above silanol solution, stir the reaction in a water bath at 40-60 °C, wash the product multiple times with anhydrous ethanol to remove residual silanol, and finally dry the product to obtain precursor material d. The mechanical processing is as follows: the obtained precursor material d is prepared into a mixed solution with a concentration of 1-3 wt%, and the mixed solution is stirred to obtain a cellulose suspension, denoted as K-CNF suspension.

4. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 3, characterized in that: The process of preparing a thin film from K-CNF suspension is as follows: Wet filter paper and lay it flat on a glass plate, then place a square silica gel plate frame on the filter paper; take 6-9g of K-CNF suspension and add purified water to it until the total weight is 11g, mix evenly at room temperature to obtain mixed solution a; use a dropper to draw mixed solution a and spread it evenly in the square groove of the silica gel plate, then place it in a 30-50℃ forced-air drying oven for 3-5h to prepare a KH-550 modified cellulose membrane, denoted as K-CNF membrane.

5. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 4, characterized in that: The specific process of step b: Take 1 mL of carbon quantum dot solution and 4 mL of pure water and mix them evenly at room temperature to obtain mixed solution b; place the K-CNF membrane in a petri dish, add mixed solution b until it covers the K-CNF membrane, and soak for 1 min; take out the soaked membrane and lay it flat on a glass plate, and then place it in a 30-50 ℃ forced-air drying oven for 3-5 h to prepare a cellulose membrane containing carbon quantum dots, denoted as KR membrane.

6. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 1, characterized in that: In step c, the process of treating the pure polylactic acid film with alkali and washing it to obtain a polylactic acid film is as follows: prepare a sodium hydroxide solution with a concentration of 8-12wt%, immerse the polylactic acid film in it for 10-50 minutes, take it out and wash it with running water until the film surface is neutral, and obtain the alkali-treated PLA film.

7. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 1, characterized in that: The adhesive preparation method in step d is as follows: Prepare a polyvinyl alcohol aqueous solution with a concentration of 8-14wt%, place it in a hydrothermal stirring environment at 80-100℃, add citric acid according to the solid-liquid ratio of polyvinyl alcohol aqueous solution to citric acid of 100g: 1-3g, and mix evenly to obtain the adhesive.

8. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 1, characterized in that: During hot pressing in step d, the polylactic acid film coated with adhesive and the cellulose film containing carbon quantum dots are stacked one on top of the other and laminated at 70-90℃ and 4-6MPa for 10-20 minutes; then laminated at room temperature until the composite film cools to room temperature.

9. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 1, characterized in that: The carbon quantum dot solution preparation method in step b: The residue after extracting pulp from rice straw as raw material for molded products is used to prepare a carbon quantum dot solution by hydrothermal synthesis. The carbon quantum dot solution is filtered, centrifuged, and dialyzed to obtain the carbon quantum dot solution used for film making, which is the carbon quantum dot solution in step b.

10. The method for preparing carbon quantum dot / cellulose / polylactic acid composite membrane as described in claim 9, characterized in that: The preparation method of the carbon quantum dot solution in step b: After extracting raw materials for paper pulp molding products from rice straw, 30% of the residue is crushed and passed through a 90-mesh sieve as a precursor material. Weigh 4-6 g of the residue and place it in a beaker. Add 50-70 mL of purified water, stir and mix evenly, and transfer it to a reaction vessel. The reaction vessel is heated at 180-220 ℃ for 5-7 hours. After the reaction is completed, allow it to cool naturally to room temperature. Filter the solution using a microporous membrane, and then centrifuge and dialyze it. The resulting solution is the rice straw-based carbon quantum dot solution.

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