Preparation method of lignin nanoparticle / polyester composite packaging film

Abstract

The invention relates to a preparation method of a lignin nanoparticle / polyester composite packaging film, and belongs to the field of lignin high-valued application. The industrial sulfate lignin is subjected to molecular weight grading through a green organic solvent, LNPs is prepared by combining an anti-solvent precipitation technology, and the LNPs is doped into PBAT to prepare the LNPs / PBAT composite packaging film. The obtained composite film is remarkably superior to a sample doped with original lignin in the aspects of mechanical property, ultraviolet barrier property, oxidation resistance and antibacterial property, and the method is green in process, high in controllability and suitable for large-scale application and has good lignin high-value utilization and packaging material development prospects.

Description

TECHNICAL FIELD

[0001] The present application relates to a preparation method of a lignin nanoparticle / polyester composite packaging film, and belongs to the field of high-value application of lignin. BACKGROUND

[0002] Sulfate lignin is a by-product of the Kraft process in the paper industry, and accounts for a large proportion in industrial lignin. At present, most of the industrial sulfate lignin is mainly used as fuel, but this utilization method is low in efficiency and fails to fully exert its value. In fact, sulfate lignin is rich in functional groups such as phenolic hydroxyl, alcoholic hydroxyl, carboxyl and methoxyl, and has great application potential in composite materials. However, due to its complex structure, low reactivity, poor solubility and other problems, it still faces great challenges to convert it into high-value-added products.

[0003] In order to solve the problem of the non-uniformity of sulfate lignin in structure and morphology, different chemical reactions are often used to modify lignin to produce high-value-added materials. Compared with modified lignin, lignin nanofication is a more green treatment method. After nanofication, the inherent heterogeneity, poor dispersibility and large particle size of lignin can be largely solved. Lignin nanoparticles (LNPs) can greatly reduce the heterogeneity of lignin compared with original lignin, but as a kind of nanomaterial, it is still subject to the complexity of lignin raw materials, and the size and morphology of the particles are often not stable enough. Therefore, there is an urgent need to explore a simple, green and environmentally friendly method for preparing LNPs with uniform particles.

[0004] Polybutylene adipate terephthalate (PBAT) is a polyester with good biodegradability and excellent physical properties, so it is widely used in packaging films and other fields. Compared with common degradable polyesters such as polylactic acid, PBAT has better flexibility, so it has more advantages in flexible packaging. However, PBAT also has some disadvantages, such as poor light stability and low mechanical strength, which will affect its application range. Lignin is widely available and low in price, but due to its complex structure and uneven properties, it still has some challenges to be efficiently utilized. However, research has found that lignin and other degradable materials can be combined to give the material antibacterial, antioxidant, and hydrophobic properties, making it have application value in many fields. Especially in packaging materials, lignin-based composites have great development potential. Combining lignin PBAT can not only reduce production costs, but also improve material performance, such as improving water vapor and oxygen barrier properties, UV shielding ability, thermal stability and mechanical strength, making PBAT more widely used. However, the compatibility of lignin and PBAT is not very good, and usually requires additional modification treatment to make them better combined, but this will also increase the complexity and cost of production. In contrast, LNPs as a high-quality filler can significantly improve the compatibility of lignin PBAT and reduce the difficulty of preparing composite materials. However, even so, how to ensure the uniform dispersion of LNPs and make them stably added to the PBAT matrix is still an important technical problem to be solved. SUMMARY

[0005] The purpose of the present application is to provide a preparation method of lignin nanoparticle / polyester composite packaging film, which uses a green organic solvent system to fractionate industrial sulfate lignin, and uses the LNPs prepared after fractionation to prepare a lignin nanoparticle / polyester composite packaging film, solving the above problems.

[0006] In order to achieve the above purpose, the technical scheme of the present application is as follows:

[0007] A preparation method of lignin nanoparticle / polyester composite packaging film, the preparation method comprising the following steps:

[0008] (1) Dissolving and separating industrial sulfate lignin by solvent to obtain lignin fractions with different molecular weights;

[0009] (2) Dissolve the lignin fractions obtained in step (1) in a γ-valerolactone aqueous solution, ultrasonic, centrifugal to remove the insoluble part, to obtain a lignin solution;

[0010] (3) Dilute the lignin solution obtained in step (2) with deionized water for anti-solvent precipitation;

[0011] (4) centrifuging the diluted solution, washing the precipitate, and freeze-drying to obtain the lignin nanoparticles;

[0012] (5) adding polybutylene adipate terephthalate into a solvent, stirring and dissolving, adding the lignin nanoparticles obtained in step (4), and ultrasonic dissolving to obtain a film solution;

[0013] (6) pouring the film solution obtained in step (5) into a mold, drying to obtain a lignin nanoparticle / polyester composite packaging film.

[0014] In the above technical solution, further, in step (1), the preparation method of the lignin fractions with different molecular weights comprises the following steps:

[0015] 1) adding industrial kraft lignin into a solvent S1, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, vacuum rotary evaporation of the supernatant to recover ethanol, and obtaining a first fraction K1, wherein the solvent S1 is a mixed solution of ethanol and water with a volume ratio of 2:8;

[0016] 2) adding the precipitate obtained in step 1) into a solvent S2, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, vacuum rotary evaporation of the supernatant to recover ethanol, and obtaining a second fraction K2, wherein the solvent S2 is a mixed solution of ethanol and water with a volume ratio of 4:6;

[0017] 3) adding the precipitate obtained in step 2) into a solvent S3, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, vacuum rotary evaporation of the supernatant to recover ethanol, and obtaining a third fraction K3, wherein the solvent S3 is a mixed solution of ethanol and water with a volume ratio of 6:4;

[0018] 4) adding the precipitate obtained in step 3) into a solvent S4, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, wherein the precipitate is a fifth fraction K5, vacuum rotary evaporation of the supernatant to recover ethanol, adding water to precipitate the lignin, centrifuging to obtain a fourth fraction K4, and wherein the solvent S4 is a mixed solution of ethanol and γ-valerolactone with a volume ratio of 9:1.

[0019] In the above technical solution, further, in step (2), the K0, K2-K3 use a γ-valerolactone aqueous solution with a volume fraction of 50%, and the K4-K5 use a γ-valerolactone aqueous solution with a volume fraction of 60%.

[0020] In the above technical solution, further, in step (3), the volume fraction of γ-valerolactone in the diluted solution is 5%.

[0021] In the above technical solution, further, in step (5), the stirring temperature is 50-70°C.

[0022] In the technical solution, further, in step (5), the solvent is tetrahydrofuran (THF) or N,N-dimethylformamide (DMF).

[0023] In the technical solution, further, for the thin film with THF as the solvent, the film liquid is poured into a mold and naturally air-dried for 12 hours to form a film; for the thin film with DMF as the solvent, the film liquid is first dried at 50 DEG C for 6 hours, and then vacuum dried at 70 DEG C for 2 hours to remove DMF.

[0024] The present application has the following beneficial effects:

[0025] The method of the present application obtains fractions with different molecular weights and similar structures by green solvent fractionation, further prepares LNPs, and blends the LNPs with PBAT as nano-filler, which significantly improves the mechanical properties, ultraviolet blocking property, antioxidant property and antibacterial property of the obtained thin film, and provides a new path for the development of degradable bio-based packaging materials. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 For GPC chromatograms and molecular weights and yields of the original lignin K0 and the fractionated lignins K1-K5 in Example 1, (a) is the GPC chromatogram, (b) is the molecular weight and yield;

[0027] Figure 2 For infrared spectra of the original lignin K0 and the fractionated lignins K1-K5 in Example 1, (a) is the infrared spectrum, (b) is a local enlarged view;

[0028] Figure 3 For functional group content diagrams of the original lignin K0 and the fractionated lignins K1-K5 in Example 1, (a) is the quantitative 31 P nuclear magnetic resonance spectrum, (b) is the -OH and -carboxyl content of the original lignin and the fractionated lignin;

[0029] Figure 4 For SEM images of the LNPs prepared from the fractionated lignin and the original lignin in Example 2 and their corresponding particle size distribution diagrams;

[0030] Figure 5 For the tensile effect diagram of the thin film in Test Example 1, (a) is the tensile strain curve of the PBAT thin film added with different mass fractions of K0 and L0, (b) is the tensile strain curve of the THF-PBAT-LNPs thin film, (c) is the tensile strain curve of the DMF-PBAT-LNPs thin film, (d) is the strain and tensile strength of the PBAT thin film added with different mass fractions of K0 and L0 at the time of breaking, (e) is the strain and tensile strength of the THF-PBAT-LNPs thin film at the time of breaking, and (f) is the strain and tensile strength of the DMF-PBAT-LNPs thin film at the time of breaking;

[0031] Figure 6 For the UV-Vis transmittance curve and appearance of the film in Test Example 2, (a) is the UV-Vis transmittance curve of THF-PBAT-LNPs film, (b) is the UV-Vis transmittance curve of DMF-PBAT-LNPs film, (c) is the actual appearance picture of the film;

[0032] Figure 7 For the infrared spectrum test graph of the film in Test Example 3, (a) is THF-PBAT-LNPs film, (b) is DMF-PBAT-LNPs film;

[0033] Figure 8 For the surface morphology comparison graph of the film in Test Example 4, (a) is the film surface scanning electron microscope image; (b) is the film cross-section scanning electron microscope image;

[0034] Figure 9 For the DPPH clearance rate comparison graph of PBAT-LNPs film in Test Example 5, (a) is THF-PBAT-LNPs film, (b) is DMF-PBAT-LNPs film;

[0035] Figure 10 For the barrier effect graph of the film in Test Example 6, (a) is the WVPR of PBAT-LNPs film, (b) is the OTR of PBAT-LNPs film;

[0036] Figure 11 For the antibacterial performance graph of PBAT-LNPs film in Test Example 7;

[0037] Figure 12 For the fresh-keeping effect graph of PBAT-LNPs film on strawberries in Test Example 7;

[0038] Figure 13 For the appearance and strength graph of the recycled film in Test Example 8, (a) is the appearance of the PBAT-LNPs film prepared by recycling, (b) is the tensile curve of the PBAT-LNPs film prepared by recycling. DETAILED DESCRIPTION

[0039] The following non-limiting examples can enable those of ordinary skill in the art to more fully understand the present application, but in no way limit the present application.

[0040] The following industrial kraft lignin (KL) was purchased from a company in Shandong, and the specific chemical composition is shown in the following table.

[0041] Table 1 Chemical composition of industrial kraft lignin

[0042] Acid insoluble lignin Acid soluble lignin Carbohydrate Ash Other 82.19 2.05 8.27 3.39 4.10

[0043] Example 1

[0044] 1) The industrial kraft lignin (K0) was added to solvent S1, stirred at room temperature, centrifuged, and the supernatant was recovered by vacuum rotary evaporation to recover the ethanol, obtaining the first fraction K1, the solvent S1 was a mixture of ethanol and water in a volume ratio of 2:8;

[0045] 2) The precipitate obtained in step 1) was added to solvent S2, stirred at room temperature, centrifuged, and the supernatant was recovered by vacuum rotary evaporation to recover the ethanol, obtaining the second fraction K2, the solvent S2 was a mixture of ethanol and water in a volume ratio of 4:6;

[0046] 3) The precipitate obtained in step 2) was added to solvent S3, stirred at room temperature, centrifuged, and the supernatant was recovered by vacuum rotary evaporation to recover the ethanol, obtaining the third fraction K3, the solvent S3 was a mixture of ethanol and water in a volume ratio of 6:4;

[0047] 4) The precipitate obtained in step 3) was added to solvent S4, stirred at room temperature, centrifuged, and the supernatant was recovered by vacuum rotary evaporation to recover the ethanol, obtaining the fourth fraction K4, the solvent S4 was a mixture of ethanol and γ-valerolactone in a volume ratio of 9:1.

[0048] The original lignin (K0) and the fractions of lignin (K1-K5) were subjected to molecular weight detection after acetylation. The acetylation step of the lignin was to add 100 mg of lignin sample to 2 ml of acetic anhydride / pyridine (1 / 1, v / v) solution under nitrogen atmosphere in the dark for 72 h, then the solution was added to 100 ml of ice water to precipitate the lignin by centrifugation, and the centrifugal washing was repeated three times to completely remove the residual pyridine. The acetylated lignin sample was completely dissolved in THF (2.0 mg mL -1 ) and tested for molecular weight after filtering the suspended matter with a 0.22 μm nylon filter.

[0049] Table 2 Molecular weight and yield of each lignin fraction

[0050] K1 K2 K3 K4 K5 K0 Mn / g·mol -1 ]]> 875 1401 2683 3935 9066 2622 Mw / g·mol -1 ]] 2016 2995 4673 7176 20190 6737 Dispersion 2.30 2.14 1.74 1.82 2.23 2.57 Yield / % 10.1 16.4 29.7 18.2 12.6 —

[0051] By solvent fractionation, five fractions with gradually increasing molecular weight were obtained, from the lowest molecular weight K1 (Mw 2016 g mol -1 ) to the highest molecular weight K5 (Mw 20190 g mol -1The principle of solvent fractionation lies in the difference in solubility of lignin with different molecular weights in organic solvents of different proportions. Low molecular weight lignin has higher solubility, while high molecular weight lignin has lower solubility, thus obtaining fractions of lignin with different molecular weights.

[0052] Infrared spectroscopy analysis was performed on the original lignin (K0) and the lignin fractions (K1-K5):

[0053] FTIR spectra at 4000-500 cm⁻¹ -1 The chemical structure of lignin components was analyzed within the specified range using the potassium bromide tableting method.

[0054] according to Figure 2 Analysis shows it is located at 3420cm. -1 The absorption peak can be attributed to the absorption of aliphatic and phenolic hydroxyl groups (-OH groups), while the 2938 cm⁻¹ peak is attributable to the absorption of these groups. -1 The absorption peak at 1601 cm⁻¹ corresponds to the vibrations of the methyl and methylene groups. -1 1514cm -1 1425cm -1 The absorption band at this point is typical of lignin aromatic skeleton vibrations, while the vibrational band of CH deformation associated with aromatic ring vibrations appears at 1460 cm⁻¹. -1 It was observed at 1270cm. -1 The absorption peak at 1032 cm⁻¹ is due to the CO stretching vibration in the G unit. -1 The observed signal is related to CO-related deformation vibrations in primary alcohols. Infrared spectral characterization of lignin before and after fractionation treatment showed no significant structural changes.

[0055] Functional group analysis of primary lignin (K0) and lignin fractions (K1-K5):

[0056] Lignin 31 For the determination of P, cyclohexanol solution and chromium acetylacetone solution were dissolved in pyridine / deuterium chloroform solvent (1.6:1, v / v) to prepare an internal standard / relaxant (cyclohexanol solution 4.0 mg / mL, chromium acetylacetone solution 3.6 mg / mL). 25 mg of dried lignin was dissolved in 400 μL of pyridine / deuterium chloroform solvent (pyridine / CDCl3 (1.6:1, v / v), followed by the addition of 150 μL of internal standard / relaxant and thorough dissolution. Finally, 100 μL of 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) was added for phosphorylation, and quantification was performed. 31P NMR analysis. Due to the different carbohydrate contents in each lignin fraction that are difficult to dissolve in pyridine / deuterated chloroform solvent, the actual dissolved lignin mass needs to be used when performing quantitative calculation, and the functional group content calculation formula is as follows:

[0057]

[0058] Wherein, A: functional group content (mmol / g);

[0059] ρ: cyclohexanol concentration (mg / ml);

[0060] A1: integral area of hydroxyl in cyclohexanol;

[0061] A2: integral area of hydroxyl peak in lignin structure;

[0062] m: mass of the actual dissolved lignin sample (g);

[0063] According to Figure 3 (a) analysis, 31 The P spectrum is divided into five regions, aliphatic OH (149.5-145.2 ppm), C5 substituted OH (144.5-142.3 ppm), guaiacyl OH (141.6-138.8 ppm), p-hydroxyphenyl OH (138.2-137.0 ppm), carboxyl (136.3-133.8 ppm). As shown in Figure 3 (b), the industrial kraft lignin sample is rich in hydroxyl and carboxyl functional groups. In the lignin fractionated, with the breakage of β-O-4 bond, the lignin fraction with low molecular weight shows higher content of phenolic hydroxyl. With the gradual increase of molecular weight, the number of phenolic hydroxyl and carboxyl in each fraction shows a significant decreasing trend, while the number of aliphatic hydroxyl changes relatively small.

[0064] Example 2

[0065] Preparation of LNPs from lignin with different molecular weights:

[0066] (1) Dissolve the raw lignin K0 of Example 1 and lignin fractions with different molecular weights K2-K5 in γ-valerolactone aqueous solution (K0, K2-K3 use γ-valerolactone aqueous solution with a volume fraction of 50%, K4-K5 use γ-valerolactone aqueous solution with a volume fraction of 60%), ultrasonic, centrifugation to remove the undissolved part, to obtain a lignin solution;

[0067] (2) Dilute the lignin solution obtained in step (1) with deionized water, and the volume fraction of γ-valerolactone in the diluted solution is 5%, and perform anti-solvent precipitation;

[0068] (3) Centrifuge the diluted solution and wash the precipitate with deionized water at least 3 times to remove residual γ-valerol. After freeze-drying for 3 days, lignin nanoparticles of different molecular weights are obtained and are denoted as L0, L2, L3, L4 and L5, respectively.

[0069] like Figure 4 As shown, lignin L0 prepared by anti-solvent precipitation is transformed into nanoparticles with an average size of 159.1 nm, but there is a problem of particle size inhomogeneity, with the largest size reaching 378 nm. Compared with the L2 fraction, the size and inhomogeneity of the L3 fraction are reduced, with an average particle size of 180.7 nm. The average particle size of the L4 fraction is further reduced to 103.8 nm, showing a significant decreasing trend in size and inhomogeneity compared with the L0 fraction. The weight-average molecular weight of K5 reaches 20190 g·mol⁻¹, which is significantly larger than that of other fractions. The average particle size of L5 prepared from K5 is reduced to 56.7 nm, and the size distribution becomes very uniform.

[0070] Example 3

[0071] Preparation of lignin nanoparticle / polyester composite packaging film:

[0072] (1) Polybutylene terephthalate (PBAT) was added to tetrahydrofuran (THF) / N,N-dimethylformamide (DMF) (0.04 g / mL) and stirred at 60 °C to dissolve. After dissolution, the film solution was added to LNPs in Example 2 and ultrasonically treated to achieve uniform dispersion.

[0073] (2) Pour the well-dispersed membrane solution into a glass culture dish and dry it to obtain a thin film.

[0074] The THF-PBAT film prepared with THF as solvent is named T0, the film with added virgin lignin is named TK0, and the THF-PBAT films with added LNPs of different molecular weights are named TL0, TL2, TL3, and TL4. Similarly, the films prepared with DMF as solvent are named D0, DK0, DL0, DL2, DL3, DL4, and DL5, respectively.

[0075] Test Example 1

[0076] Effect of different mass fractions of LNPs on the tensile strength of the film:

[0077] like Figure 5As shown, the optimal addition amount of LNPs in PBAT is 8wt% (tensile strength 10.7MPa) when THF is used as the solvent, which is better than 6wt% (8.5MPa) of the original lignin K0, and both are higher than the pure PBAT film T0 (6.6MPa). With the increase of the addition amount of LNPs, the tensile strength shows a trend of first rising and then falling, indicating that the dispersibility has a significant influence on the mechanical properties. Further investigation of the effect of LNPs with different particle sizes shows that the film prepared by small particle size LNPs (TL4) has the highest tensile strength (11.1MPa), which is better than that of large particle size samples (TL2, TL3), but the stress decreases slightly, indicating that small size LNPs have better dispersibility, which helps to enhance the mechanical properties. Although the high molecular weight LNPs (L5) have smaller particle size, they have poor dispersibility in THF, resulting in insufficient compatibility of the film.

[0078] In order to improve the dispersibility of high molecular weight LNPs, DMF with better dispersibility was used to prepare films for comparison. Figure 5 (c) and 5(f) show that the tensile strength of the film is further improved under the DMF system, and the DL5 sample reaches 14.2MPa, indicating that DMF helps to improve the interface compatibility and distribution uniformity of LNPs and PBAT, and enhances the mechanical properties of the film.

[0079] Test Example 2

[0080] Effect of adding LNPs on the light transmittance of the film:

[0081] The film was cut into a rectangle of 50mm x 15mm, and the ultraviolet light blocking performance of the PBAT-LNPs film was tested by ultraviolet spectrophotometer in the scanning range of 200-800nm, and the results are shown in Figure 6 .

[0082] According to Figure 6 analysis, the ultraviolet shielding ability of the film is obviously improved after adding LNPs in the PBAT matrix, especially the blocking effect in the UVA band becomes better. This shows that LNPs can effectively absorb ultraviolet light and enhance the anti-UV ability of the film. However, due to the color of lignin itself, the transmittance of PBAT-LNPs film in the visible light region decreases. Especially the lignin with high molecular weight has deeper color, so the transmittance of PBAT film prepared by high molecular weight LNPs in the visible light region will further decrease. In contrast, when DMF is used as the solvent, the dispersibility of LNPs is better, so the transmittance of PBAT-LNPs film in the visible light band prepared by DMF system is better than that of PBAT-LNPs film prepared by THF system.

[0083] Test Example 3

[0084] Infrared spectrum of the film after adding LNPs:

[0085] Characteristic peaks of PBAT-LNPs thin films were measured using an ATR-FTIR spectrometer. The scanning range was 450–4000 cm⁻¹. -1 The spectrometer was set to perform 64 scans with a resolution of 4 cm⁻¹. -1 The results are shown Figure 7 .

[0086] according to Figure 7 Analysis showed that the characteristic absorption peaks of the sample film did not change significantly after the addition of LNPs filler. At 2961 cm⁻¹... -1 An absorption peak for the tensile vibration of CH was observed at 1710 cm⁻¹. -1 The absorption peak at 1266 cm⁻¹ is due to the C=O stretching vibration. -1 and 1103cm -1 An absorption peak belonging to CO was observed at 726 cm⁻¹. -1 The absorption peaks present belong to -CH2. The hydroxyl absorption peak of the PBAT-LNPs film is similar to that of the pure PBAT film at 3939 cm⁻¹. -1 The absorption peak broadened and red-shifted compared to the absorption region, indicating that hydrogen bonds formed between the hydroxyl groups of LNPs and PBAT. The strong intermolecular hydrogen bonding between the hydroxyl groups of LNPs and PBAT significantly enhances the mechanical properties of the film, providing greater potential for the application of LNPs in composite materials.

[0087] Test Example 4

[0088] Effect of adding LNPs on the surface morphology of the thin film:

[0089] The surface morphology of PBAT and PBAT-LNPs films was observed using scanning electron microscopy. For the cross-section of the films, the films were first subjected to brittle fracture in liquid nitrogen to maintain their structural integrity. The prepared film samples were then sputtered with gold for further morphological observation. The results are shown in [Figure number missing]. Figure 8 .

[0090] according to Figure 8 Analysis showed that, compared with the sample with directly added sulfate lignin, the surface roughness of the film sample with added LNPs was significantly reduced, indicating that LNPs have better dispersibility and uniform distribution in the PBAT matrix.

[0091] Test Example 5

[0092] The effect of adding LNPs on the antioxidant properties of the film:

[0093] Cut 0.3g of the film sample into pieces and add them to 6mL of methanol solution to completely immerse them. Soak at room temperature for 24 hours. Then, mix 2mL of the supernatant with 2mL of DPPH methanol solution (60mg / L) and store in the dark for 1 hour. Record the absorbance values ​​of different film sample solutions at 517nm. The free radical scavenging rate (RSA) is calculated as follows:

[0094] RSA = [1 - A1 / A0] * 100%

[0095] Where A0 is the UV absorbance value of the DPPH methanol aqueous solution, and A1 is the UV absorbance value of the mixture of the film and the DPPH methanol solution. All experiments were repeated three times to obtain the average value. The results are shown in [Figure number missing]. Figure 9 .

[0096] according to Figure 9 Analysis showed that all films with added LNPs exhibited higher DPPH scavenging rates than pure PBAT films, mainly due to the higher solubility, higher specific surface area, and smaller size of LNPs. These characteristics significantly enhanced the proton-donating capacity of lignin phenyl groups, thereby improving antioxidant performance.

[0097] Test Example 6

[0098] The effect of adding LNPs on the barrier properties of the film:

[0099] Water vapor transport rate (WVTR) of the membrane was measured using a permeameter. Oxygen transport rate (OTR) of the membrane was measured using a gas permeameter (25±2℃ and 50%RH). Results are shown in [Table missing]. Figure 10 .

[0100] according to Figure 10 Analysis showed that with the introduction of LNPs, the WVTR and OTR of the PBAT-LNPs film gradually decreased. This performance improvement is mainly attributed to the physical barrier formed by the uniform dispersion of LNPs in the PBAT matrix, which causes the gas to encounter a longer and more tortuous path when passing through the film, thereby significantly improving the water vapor and oxygen barrier performance.

[0101] Test Example 7

[0102] The effect of adding LNPs on the antibacterial and preservation properties of the film:

[0103] Antibacterial property: PBAT and PBAT-LNPs composite films were cut into discs with a diameter of 2 cm. The discs were irradiated under ultraviolet light to kill bacteria, then immersed in a bacterial culture suspension containing 105 CFU / mL of bacteria; then cultured on a shaker at 37°C for 12 h; subsequently, the discs washed with distilled water were placed in a PBS solution and ultrasonicated for 5 min to obtain a bacterial solution; the bacterial solution was diluted 100 times with a PBS solution and then coated on a broth solid culture medium, and the bacterial growth was observed after incubation at 37°C for 12 h. The results are shown in Figure 11 .

[0104] Fresh-keeping property: The film prepared with DMF as the solvent and having good performance was used to test the fresh-keeping property of strawberries. The results are shown in Figure 12 .

[0105] According to the analysis of Figure 11 and Figure 12 , the composite film showed significant antibacterial properties after the addition of LNPs, and had good inhibition effect on the growth of E. coli and S. aureus. The benzene ring structure and phenol unit of LNPs endow them with excellent antibacterial ability. Compared with the original lignin, LNPs have a larger specific surface area, and more functional groups are distributed on the surface of LNPs, so that they show better antibacterial performance. Small-sized LNPs can make the film have better antibacterial and fresh-keeping properties.

[0106] Test Example 8

[0107] Effect of the addition of LNPs on the renewable property of the film:

[0108] The prepared PBAT-LNPs film samples were redissolved in DMF, and after drying again, the film (D0', DL0', DL5') was prepared. The appearance and tensile properties of the film prepared by recycling were tested, and the results are shown in Figure 13 .

[0109] According to the analysis of Figure 13 , the PBAT film prepared by recycling not only maintained high stability in appearance, but also was not significantly affected in performance.

[0110] The above examples are only preferred embodiments of the present application, and are not limited to the embodiments. The protection scope of the present application should be limited by the scope defined by the claims. Other different forms of changes or variations can be made on the basis of the above description. The obvious changes or variations derived therefrom are still within the protection scope of the present application.

Claims

1. A method for preparing a lignin nanoparticle / polyester composite packaging film, characterized by: The preparation method comprises the following steps: (1) separating industrial kraft lignin by solvent dissolution to obtain lignin fractions with different molecular weights; (2) dissolving the lignin fraction obtained in step (1) in a gamma-valerolactone aqueous solution, ultrasonicating, and centrifuging to obtain a lignin solution; (3) diluting the lignin solution obtained in step (2) with deionized water and performing anti-solvent precipitation; (4) centrifuging the diluted solution, washing the precipitate, and freeze-drying to obtain lignin nanoparticles; (5) adding polybutylene adipate terephthalate to a solvent, stirring and dissolving, adding the lignin nanoparticles obtained in step (4), ultrasonicating and dissolving to obtain a film solution; (6) pouring the film solution obtained in step (5) into a mold, drying to obtain a lignin nanoparticle / polyester composite packaging film.

2. The method of claim 1, wherein: In step (1), the preparation method of lignin fractions with different molecular weights comprises the following steps: 1) adding industrial kraft lignin to a solvent S1, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, vacuum rotary evaporation of the supernatant to recover ethanol to obtain a first fraction K1, and the solvent S1 is a mixed solution of ethanol and water with a volume ratio of 2:8; 2) adding the precipitate obtained in step 1) to a solvent S2, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, vacuum rotary evaporation of the supernatant to recover ethanol to obtain a second fraction K2, and the solvent S2 is a mixed solution of ethanol and water with a volume ratio of 4:6; 3) adding the precipitate obtained in step 2) to a solvent S3, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, vacuum rotary evaporation of the supernatant to recover ethanol to obtain a third fraction K3, and the solvent S3 is a mixed solution of ethanol and water with a volume ratio of 6:4; 4) adding the precipitate obtained in step 3) to a solvent S4, stirring at room temperature, centrifuging to obtain a precipitate and a supernatant, the precipitate being a fifth fraction K5, vacuum rotary evaporation of the supernatant to recover ethanol, adding water to precipitate the lignin, centrifuging to obtain a fourth fraction K4, and the solvent S4 is a mixed solution of ethanol and gamma-valerolactone with a volume ratio of 9:

1.

3. The method of claim 2, wherein: In step (2), K1-K3 uses a gamma-valerolactone aqueous solution with a volume fraction of 50%, and K4-K5 uses a gamma-valerolactone aqueous solution with a volume fraction of 60%.

4. The method of claim 1, wherein: In step (3), the volume fraction of gamma-valerolactone in the diluted solution is 5%.

5. The method of claim 1, wherein: In step (5), the stirring temperature is 50-70°C.

6. The method of claim 1, wherein: In step (5), the solvent is tetrahydrofuran or N,N-dimethylformamide.

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