A cellulose / lignin-based high-strength UV-resistant composite film and its preparation method

Abstract

This invention discloses a cellulose / lignin-based high-strength UV-resistant composite film and its preparation method. The preparation method includes the following steps: raw wood veneer is sequentially subjected to delignification and hemicellulose removal treatment to obtain raw wood cellulose; lignin nanoparticles are prepared; polyvinyl alcohol and a plasticizer are dissolved in water to obtain a polyvinyl alcohol aqueous solution; the lignin nanoparticles are dispersed in the polyvinyl alcohol aqueous solution to obtain a polyvinyl alcohol suspension containing lignin nanoparticles; the raw wood cellulose is immersed in the polyvinyl alcohol suspension containing lignin nanoparticles and repeatedly subjected to microwave treatment; finally, the film is obtained after extrusion and drying. This invention utilizes the interaction between lignin nanoparticles, PVA, and cellulose to prepare a composite film with high mechanical strength, flexibility, strong UV resistance, and is safe and biodegradable, showing great application potential.

Description

Technical Field

[0001] This invention belongs to the field of green biomass film technology, specifically relating to a cellulose / lignin-based high-strength UV-resistant composite film and its preparation method. Background Technology

[0002] Currently, wood-based biomaterials are a viable alternative to fossil fuels for producing materials and chemicals, while also playing a crucial role in maintaining environmental sustainability. The polymerization of lignocellulose biopolymers has greater economic potential compared to the production of platform chemicals from wood, as it consumes less energy and is generally less expensive to produce. Cellulose and lignin are the two main structural components of wood. Cellulose is optically transparent, while lignin strongly absorbs light in the visible light region. Both of these wood components possess excellent ductility and strength, UV shielding, water resistance, structural controllability, biocompatibility, and low cytotoxicity. Therefore, the nanostructuring and recombination of cellulose and lignin can be used for biodegradable packaging of food and beverages and as thin-film materials for biomedicine.

[0003] Polyvinyl alcohol (PVA) is a highly crystalline, water-soluble, non-toxic, and biodegradable synthetic polymer with excellent film-forming properties. The preparation of cellulose fiber and lignin-based PVA composite films has been reported. For example, the addition of cellulose microfibers (CMF), cellulose nanocrystals (CNC), and carbon nanofibers (CNF) to PVA films has resulted in nanocomposite films with high tensile strength. Furthermore, UV-resistant PVA composite films have been prepared by impregnating PVA with lignin nanoparticles. However, cellulose-based PVA films prepared via bottom-up methods lack sufficient mechanical strength and stability due to the absence of an ordered skeletal structure, and are expensive to produce. In contrast, lignin impregnation of PVA reduces the mechanical properties of PVA composite films. Therefore, it is necessary to further identify biomass materials that can be mixed with a PVA matrix to provide composite films with robust mechanical properties.

[0004] Wood is composed of honeycomb-like cells, whose cell walls consist of a nanocomposite layer of oriented cellulose microfibrils. Cellulose microfibrils provide wood with toughness and strength. Lignin, also a major component of the cell wall, helps to hold cells together. Therefore, lignin imparts rigidity and resistance to decay to wood. Previous studies have shown that the recombination of cellulose and lignin results in new materials with low density, low thermal conductivity, attractive mechanical properties, and good optical properties. For example, Bian et al. demonstrated that films prepared from lignocellulose nanofiber suspensions exhibit excellent hydrophobicity and thermal stability. Sadeghifar et al. found that lignin and cellulose form uniform films through a Click chemical reaction, overcoming the inhomogeneity of cellulose / lignin mixtures, resulting in films with high optical transparency and strong flexibility. In similar studies, the incorporation of gum lignin particles into cellulose nanofibers (CNFs) has been investigated as a strategy to obtain robust, waterproof, and UV-shielding films.

[0005] To date, some studies on the preparation of nanocellulose-based PVA films have been reported, but existing biomass films suffer from drawbacks such as poor UV resistance and poor mechanical properties. Furthermore, there are few reports on the preparation of high-strength, multifunctional films using bio-derived materials. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a cellulose / lignin-based high-strength UV-resistant composite film and its preparation method.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing a cellulose / lignin-based high-strength UV-resistant composite film includes the following steps:

[0009] (1) Wood veneer is subjected to delignification and hemicellulose removal treatment in sequence to obtain wood cellulose (WCS); further, the wood veneer is balsa wood veneer, poplar veneer, pine veneer or fir veneer; the thickness of the wood veneer is 1-5 mm. The delignification treatment method is as follows: the wood veneer is immersed in a NaClO2 aqueous solution with a pH of 4-5 and kept at 90-100℃ for 6-8 hours to complete the delignification; the hemicellulose removal treatment method is as follows: the wood veneer that has undergone delignification treatment is immersed in a 6-10% NaOH solution and kept at 70-90℃ for 3-5 hours to remove hemicellulose, and after washing, wood cellulose is obtained.

[0010] (2) Preparation of lignin nanoparticles: The method is as follows: dissolve lignin raw material in an organic solvent to obtain lignin solution; under stirring conditions, add lignin solution dropwise to water to obtain suspension; put the suspension into a dialysis bag and put the dialysis bag into water for dialysis treatment for 12-48h to remove organic solvent; finally, after drying, lignin nanoparticles (LNPs) are obtained.

[0011] (3) Polyvinyl alcohol (PVA) and plasticizer are dissolved in water to obtain a polyvinyl alcohol aqueous solution; lignin nanoparticles are dispersed in the polyvinyl alcohol aqueous solution to obtain a polyvinyl alcohol suspension containing lignin nanoparticles; further, the plasticizer is decaglycerol ester; the content of lignin nanoparticles in the polyvinyl alcohol suspension containing lignin nanoparticles is 1-8 wt.%.

[0012] (4) The cellulose from the raw wood is immersed in a polyvinyl alcohol suspension containing lignin nanoparticles and subjected to repeated microwave treatments. Finally, the product is obtained by extrusion and drying, namely, a cellulose / lignin-based high-strength UV-resistant composite film. Further, the microwave treatment frequency is 2450MHz, the power is 800W; the duration of each microwave treatment is 0.5-2min; the number of microwave treatments is repeated 10-20 times; the extrusion pressure is 0.4-0.6kg, and the time is 0.3-0.5min.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] (1) The raw materials of the high-strength UV-resistant composite film of the present invention are wood veneer and lignin, which are in line with the concept of green and sustainable development. The preparation method of wood cellulose and lignin nanoparticles is simple and the raw materials are green and pollution-free. The added polyvinyl alcohol and decaglycerol are also safe and biodegradable materials. The prepared composite film has high strength, good light transmittance, strong UV resistance, and is safe and biodegradable.

[0015] (2) The present invention adds polyvinyl alcohol and decaglycerol to the film to improve the toughness of the film; the addition of an appropriate amount of lignin nanoparticles retains the good light transmittance of the film while improving the film’s UV resistance and thermal stability; the original wood cellulose retains the original cellulose skeleton structure of wood, providing high-strength mechanical properties for the composite film, similar to a steel frame, which significantly improves the mechanical properties of the film.

[0016] (3) Based on the strong hydrogen bond interaction between lignin nanoparticles, PVA and cellulose, the present invention has good interfacial compatibility and binding between the components. The composite film prepared has high mechanical strength, flexibility and UV protection properties, and is expected to become a very useful biodegradable biological film in food packaging and biomedical applications. Attached Figure Description

[0017] Figure 1 Structural characterization diagrams of WV, WCS, EHL, and LNPs;

[0018] Figure 2 Top view and cross-sectional view of the composite membrane with 4 wt.% LNPs prepared for the example;

[0019] Figure 3 Infrared spectra of composite films with different LNP contents;

[0020] Figure 4 The UV-Vis spectra of composite films with different LNP contents;

[0021] Figure 5 Thermogravimetric spectra of composite films with different LNP contents;

[0022] Figure 6 Tensile mechanical test results for composite films with different LNPs contents;

[0023] Figure 7 Figure 1 shows the bending test results of a composite membrane with 4 wt.% LNPs content in different directions. Detailed Implementation

[0024] The present invention will be further described below with reference to embodiments, so that those skilled in the art can better understand and implement the present invention, but the embodiments are not intended to limit the present invention.

[0025] In addition, unless otherwise specified, the preparation processes in the following embodiments are all conventional methods in the prior art, and therefore will not be described in detail.

[0026] Example

[0027] A method for preparing a cellulose / lignin-based high-strength UV-resistant composite film includes the following steps:

[0028] (1) Preparation of wood cellulose (WCS):

[0029] Balsa wood veneer was selected. Balsa wood veneer with dimensions of 50×50×2mm (length×width×height) was soaked in a 2wt.% NaClO2 aqueous solution, and the pH was adjusted to 4.6 with acetic acid. Then, it was heated to 100℃ and reacted for 7 hours. After cooling, the sample was taken out and placed in an 8% NaOH solution and heated at 80℃ for 4 hours. The resulting sample was taken out of the solution and rinsed repeatedly three times with an ethanol aqueous solution at room temperature to remove residual chemicals and obtain the product WCS. Then, it was stored in deionized water at 5℃.

[0030] (2) Preparation of lignin nanoparticles (LNPs):

[0031] 10 mg of enzymatically hydrolyzed lignin (EHL) was dissolved in 10 mL of THF, and then magnetically stirred at room temperature to obtain a lignin solution. The lignin solution was added to 40 mL of deionized water using a peristaltic pump at a flow rate of 2 mL / min, and stirred at 600 rpm for 6 h to obtain a suspension. The suspension was placed in a dialysis bag, and the dialysis bag was immersed in excess deionized water for 24 h, during which time it was replaced twice with fresh deionized water. The resulting suspension was freeze-dried to obtain the product LNPs.

[0032] (3) Preparation of a polyvinyl alcohol suspension containing lignin nanoparticles:

[0033] Polyvinyl alcohol (PVA) was dissolved in water at 95°C to obtain a 5 w / v PVA aqueous solution. After cooling, decaglycerol was added to the PVA aqueous solution at a ratio of PVA:decaglycerol = 15:100. Then, different amounts of LNPs were added to the PVA aqueous solution to obtain polyvinyl alcohol suspensions with LNP concentrations of 0, 1 wt.%, 2 wt.%, 4 wt.%, and 8 wt.%.

[0034] (4) WCS was immersed in 20 mL of polyvinyl alcohol suspensions containing different LNPs concentrations. The mixture was repeatedly heated in a microwave oven (2450 MHz, 800 W) for 1 min, followed by cyclic cooling for 5 min, until the solution volume decreased to 10 mL. Finally, the obtained samples were treated under 0.5 kg pressure for 0.5 min and then air-dried at a constant temperature to obtain the composite membrane. The composite membranes were named according to the LNPs concentration in the polyvinyl alcohol suspension, as 0 LNPs / WCS / PVA, 1 LNPs / WCS / PVA, 2 LNPs / WCS / PVA, 4 LNPs / WCS / PVA, and 8 LNPs / WCS / PVA, respectively.

[0035] Comparative Example

[0036] Preparation of pure PVA film: Polyvinyl alcohol (PVA) was dissolved in water at 95°C to prepare a 5 w / v% PVA aqueous solution; 20 mL of the PVA solution prepared above was poured into a mold (length × width × height: 80 × 80 × 10 mm) and air-dried at a constant temperature to obtain a pure PVA film.

[0037] Structural and performance characterization

[0038] (1) Structural characterization of WCS and LNPs

[0039] Figure 1Figure a shows the structural characterization of wood veneer (WV). It can be seen that wood veneer has an anisotropic porous microstructure, with a honeycomb structure in cross-section and vertically arranged fibrous tracheids in longitudinal section. Figure 1 Figure b shows the structural characterization of WCS. It can be seen that after delignification and hemicellulose treatment, WCS retains the neatly arranged cell wall structure of the original wood. In addition, the cell wall exhibits a higher porosity than before chemical treatment. Figure 1 c). Regularly arranged cellulose is an excellent structural material that can improve the mechanical properties of biofilm materials. Figure 1 Figure d shows a TEM image of enzymatically hydrolyzed lignin, which can be seen to be an irregular blocky structure. Figure e shows the LNPs prepared in step (2). Figure f shows the particle size distribution of the LNPs, which can be seen to be regular spherical particles with a particle size of about 270 nm. The unique chemical composition and structure of nano-lignin give it good optical properties.

[0040] (2) Structural characterization of the composite membrane prepared in the examples

[0041] The microstructure of the composite films prepared in the examples was studied using scanning electron microscopy (SEM). The results are as follows: Figure 2 As shown. Figure 2 Images b and c are SEM images observed at low magnification. They show that the composite membrane surface is smooth and flat, the fiber orientation is obvious, and no phase separation was observed. Figure 2 The composite membranes in Figures e and f show a more compact layered structure. All pores and channels within the WCS are completely filled with nano-LNPs, enabling the cellulose fibers to adhere neatly together and improving the mechanical properties of the nanocomposite membrane. Figure 2 The h and i figures show that LNPs and PVA fillers are uniformly dispersed in WCS, indicating that PVA, LNPs and WCS have good compatibility, adhesion, intermolecular binding and affinity.

[0042] (3) FT-IR analysis of the composite membrane prepared in the example

[0043] The chemical structure of the composite film and the interactions between PVA, WCS and LNPs were determined by FT-IR analysis. Figure 3 Nanocomposite membrane at 3280 cm⁻¹ -1 Different hydroxyl vibration peaks appeared nearby, which is related to the hydrogen bonds between the hydroxyl groups of glucose monomers in WCS and the oxygen-containing functional groups on the surfaces of PVA and LNPs; the characteristic absorption peak of pure PVA film is: 2930 cm⁻¹. -1 (Asymmetric stretching vibration of the CH2 group), 2904 cm⁻¹ -1 (CH stretching), 1665cm -1(Modification of the HOH group), 1563cm -1 and 1423cm -1 (C=C stretching), 1367cm -1 and 1086cm -1 (Stretching of CH groups), 1231cm -1 (CO stretch) and 1138cm -1 (COC stretching). The FT-IR spectra of all composite films showed characteristic peaks of PVA, indicating that no chemical reaction occurred during the film preparation process.

[0044] (4) Study on the UV resistance of the composite film

[0045] To study the UV resistance of biomass composite membranes, the UV-Vis absorption spectra of pure PVA membranes and biomass composite membranes of 0LNPs / WCS / PVA, 1LNPs / WCS / PVA, 2LNPs / WCS / PVA, 4LNPs / WCS / PVA, and 8LNPs / WCS / PVA were tested in the wavelength range of 230-800 nm. Figure 4 The transmittance of pure PVA film and 0LNPs / WCS / PVA composite film at 500 nm was 72.6% and 37.5%, respectively. In contrast, the transmittance of the lignin-containing nanocomposite film was close to zero across the entire ultraviolet wavelength range. Furthermore, the LNPs / WCS / PVA film maintained strong transmittance in the visible light region, especially when the LNPs concentration was less than 4 wt.%, the visible light transmittance of the prepared nanocomposite film exceeded 20%. Due to the chromogenic groups on the lignin, the color of the nanocomposite film darkened with increasing LNPs concentration, and the optical transmittance of the film decreased. Figure 4 As shown.

[0046] (5) Thermal stability study of composite membranes

[0047] Thermal stability is an important performance parameter of biomass composite films. Figure 5 As shown, the initial thermal degradation temperature of pure PVA film is 220.1℃, with a 5% mass loss after degradation. However, the initial thermal degradation temperature of the 0LNPs / WCS / PVA film (228.4℃) is basically consistent with that of pure PVA; compared with pure PVA film, all nanocomposite films containing LNPs exhibit higher initial thermal degradation temperatures and lower mass losses before 300℃. These results are due to the lower initial degradation temperature of lignin (133℃). The maximum decomposition temperature (Tmax) of the film is as follows... Figure 5As shown in Figure b, the Tmax of the pure PVA film is 256.5℃, while the Tmax of the 1LNPs / WCS / PVA composite film increases to 263.6℃. The results indicate that the Tmax of the LNPs / WCS / PVA film gradually increases with increasing LNPs concentration. The superior thermal properties of the LNPs / WCS / PVA composite film are beneficial for expanding its application areas.

[0048] (6) Mechanical properties study of composite membranes

[0049] The mechanical properties of the PVA and composite films were evaluated by tensile strength testing, such as... Figure 6 As shown, the tensile strength of the LNPs / WCS / PVA film increased by more than 300.0% compared to the pure PVA film, attributed to the high mechanical strength of WCS and the strong interaction between its nanofibers and LNPs / PVA. However, the stiffness of WCS also led to a significant decrease in the film's strain properties. Figure 6 As shown in Figure b, with the increase of LNP concentration, the Young's modulus of the composite film first increases slightly and then gradually decreases, while the elongation at break gradually decreases. This is attributed to the rigid structure of LNPs, which can restrict the molecular movement of polymers such as WCS and PVA, leading to a certain degree of microphase separation in the composite film. The composite film exhibits good flexibility in all directions, and after the bending stress is released, it completely maintains its original shape. Figure 7 ).

[0050] Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing a cellulose / lignin-based high-strength UV-resistant composite film, characterized in that: Includes the following steps: (1) Log veneer is successively treated with lignin removal and hemifiber removal to obtain log cellulose; (2) Preparation of lignin nanoparticles; (3) Polyvinyl alcohol and plasticizer are dissolved in water to obtain a polyvinyl alcohol aqueous solution; lignin nanoparticles are dispersed in the polyvinyl alcohol aqueous solution to obtain a polyvinyl alcohol suspension containing lignin nanoparticles; (4) The raw wood cellulose is immersed in a polyvinyl alcohol suspension containing lignin nanoparticles and subjected to repeated microwave treatment. Finally, the target product, namely cellulose / lignin-based high-strength UV-resistant composite film, is obtained after extrusion and drying. In step (1), the method of delignification treatment is as follows: the log veneer is immersed in a NaClO2 aqueous solution with a pH of 4-5 and kept at 90-100℃ for 6-8 hours to complete the delignification; the method of de-fiber treatment is as follows: the log veneer that has undergone delignification treatment is immersed in a 6-10% NaOH solution and kept at 70-90℃ for 3-5 hours, and after washing, the log cellulose is obtained. In step (3), the plasticizer is decaglycerol; the content of lignin nanoparticles in the polyvinyl alcohol suspension containing lignin nanoparticles is 1-8 wt.%.

2. The method for preparing the cellulose / lignin-based high-strength UV-resistant composite film according to claim 1, characterized in that: In step (1), the log veneer is balsa wood veneer, poplar veneer, pine veneer or fir veneer; the thickness of the log veneer is 1 to 5 mm.

3. The method for preparing the cellulose / lignin-based high-strength UV-resistant composite film according to claim 1, characterized in that: In step (2), the method for preparing lignin nanoparticles is as follows: dissolve lignin raw material in an organic solvent to obtain a lignin solution; under stirring conditions, add the lignin solution dropwise to water to obtain a suspension; put the suspension into a dialysis bag, and put the dialysis bag into water for dialysis treatment to remove the organic solvent; finally, after drying, lignin nanoparticles are obtained.

4. The method for preparing the cellulose / lignin-based high-strength UV-resistant composite film according to claim 3, characterized in that: The dialysis treatment time is 12 to 48 hours.

5. The method for preparing the cellulose / lignin-based high-strength UV-resistant composite film according to claim 1, characterized in that: In step (4), the frequency of the microwave treatment is 2450MHz and the power is 800W; the duration of each microwave treatment is 0.5-2min; and the microwave treatment is repeated 10-20 times.

6. The method for preparing the cellulose / lignin-based high-strength UV-resistant composite film according to claim 1, characterized in that: In step (4), the pressure of the extrusion is 0.4-0.6 kg and the time is 0.3-0.5 min.

7. A cellulose / lignin-based high-strength UV-resistant composite film, characterized in that: It is prepared by the preparation method described in any one of claims 1 to 6.

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