Bamboo-based conversion film capable of replacing plastic and preparation method of bamboo-based conversion film

Abstract

The invention discloses a bamboo-based conversion film capable of replacing plastic and a preparation method of the bamboo-based conversion film, and belongs to the technical field of food packaging. The preparation method comprises the following steps: carrying out pretreatment, crosslinking and delignification treatment on bamboo wood, soaking the bamboo wood in deionized water, carrying out TEMPO oxidation treatment on the obtained delignified bamboo wood, washing the delignified bamboo wood with deionized water, carrying out resin impregnation, soaking the delignified bamboo wood for a certain time, taking out the delignified bamboo wood, and airing the delignified bamboo wood to obtain the bamboo-based conversion film capable of replacing plastic. According to the preparation method, the integrity of bamboo wood is maintained through pretreatment and crosslinking, delignification and TEMPO oxidation enhance transparency (visible light transmittance is 55-62%), PVA-tea polyphenol filling forms a compact structure, so that the film has high tensile strength (greater than or equal to 7.33 MPa), proper elongation at break (40.58-61.32%), oxidation resistance and degradability, and the problems of integrity and transparency during film forming of a bamboo-based material are solved; the prepared film can replace plastic and is suitable for the field of food packaging, and environmental pollution is reduced.

Description

Technical Field

[0001] This invention belongs to the field of food packaging technology; specifically, it relates to a bamboo-based conversion film that can replace plastic and its preparation method. Background Technology

[0002] Bamboo, as a renewable, low-carbon, and environmentally friendly biomass resource, has attracted widespread attention due to its rapid growth, abundant resources, and excellent mechanical properties. Bamboo possesses superior mechanical properties; the average tensile strength and modulus of a single bamboo fiber are 1.6 GPa and 33 GPa, respectively, exceeding those of other known natural fibers such as cotton and hemp. Therefore, bamboo has been widely used in the preparation of various structural composite materials. However, traditional bamboo substrates have poor mechanical properties and limited functionality, failing to meet the ever-changing needs of people's lives. Therefore, researchers are dedicated to exploring new strategies for modifying bamboo units, including delignification, microwave treatment, and densification, to transform bamboo into sustainable, environmentally friendly, and cost-effective high-performance composite materials. Furthermore, chemical or physical treatments further enhance the pore structure of bamboo, promoting the development of various bamboo composite materials with added or reinforcing properties to better meet diverse applications. Among these, transparent bamboo (TB), as a functional product developed through structural adjustment strategies, has attracted widespread attention due to its unique performance characteristics.

[0003] Natural bamboo appears opaque within the visible light spectrum. Its transparency is intricately linked to its chemical composition and structure. Chemically, bamboo is primarily composed of cellulose, hemicellulose, and lignin. Cellulose and hemicellulose are colorless, while lignin imparts color. These colored components cause significant light scattering and absorption in the visible light spectrum, resulting in opacity. Structurally, bamboo, as a natural organic composite material, is mainly composed of vascular bundles (composed of thick-walled tissues, vessels, and sieve tubes), with numerous internal pores. When visible light shines on the bamboo surface, some is reflected, and some penetrates the interior. The substances inside the bamboo, such as water and air, have a different refractive index than the cell wall components, scattering, refracting, reflecting, and absorbing some of the incoming visible light, making the bamboo material appear opaque.

[0004] In addition, the high density and low porosity of bamboo can hinder the penetration of treatment agents during chemical processing, and the lack of lateral structure makes it difficult to maintain its integrity during processing. These factors pose challenges to the preparation of bamboo-based transparent materials (such as transparent bamboo films).

[0005] Therefore, developing a technology that can convert bamboo-based materials into films while maintaining their integrity, enhancing transparency, and possessing biodegradable and antioxidant properties is of great significance for replacing plastics and reducing white pollution. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problems of difficulty in maintaining integrity and insufficient transparency during the conversion of bamboo-based materials into films, and to provide a bamboo-based conversion film that can replace plastic and its preparation method. The film has high mechanical strength, good translucency, biodegradability and antioxidant properties, and can be applied in the food packaging field.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for preparing a bamboo-based conversion film that can replace plastic includes the following steps: S1. Pretreatment: After the bamboo is cut in diameter, it is soaked in an alkaline solution for pretreatment. S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water for crosslinking treatment; S3, Delignification: After treating cross-linked bamboo with delignification, soak it in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo is subjected to TEMPO oxidation treatment, and then washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: Immerse TEMPO oxidized bamboo in a PVA (polyvinyl alcohol resin)-tea polyphenol mixed solution for 24-30 hours to obtain a bamboo-based conversion film.

[0008] As a further description of the above technical solution, the bamboo material mentioned in step S1 is a diameter section of any one of single bamboo, nan bamboo, moso bamboo, square bamboo, or black bamboo, with a thickness of 0.5 to 1.5 mm.

[0009] As a further description of the above technical solution, the alkaline solution in step S1 is a 1% NaOH solution; the pretreatment temperature is 80-85℃, and the time is 1-3h.

[0010] As a further description of the above technical solution, the crosslinking treatment in step S2 is carried out at a temperature of 60-65°C for 1-3 hours.

[0011] As a further description of the above technical solution, the solution used for delignification treatment in step S3 is a 1-1.5 wt% sodium chlorite solution, and the pH value is adjusted to 4.6 by glacial acetic acid; the delignification treatment temperature is 80-85℃, and the time is 6-12h.

[0012] As a further description of the above technical solution, the solution for the TEMPO oxidation treatment in step S4 is prepared as follows: 0.032g TEMPO, 0.2g sodium bromide and 5mL sodium hypochlorite solution are dissolved in 200mL water and the pH is adjusted to 10; the temperature for the TEMPO oxidation treatment is 20-25℃ and the time is 24-60h.

[0013] As a further description of the above technical solution, in step S5, the mass ratio of PVA to tea polyphenols in the mixed solution of PVA and tea polyphenols is (5-10):(0-5), and the soaking temperature is 20-25℃.

[0014] The present invention also provides a bamboo-based conversion film that can replace plastic, which is prepared by the above preparation method, is semi-transparent, has degradability and antioxidant properties, and has a tensile strength of 7.33 to 9.14 MPa.

[0015] Compared with the prior art, the beneficial effects of the present invention are: This invention uses cellulose as the matrix and PVA as the filler. The cellulose framework derived from bamboo, prepared by pretreatment, cross-linking treatment, delignification treatment and TEMPO oxidation treatment, is used as the matrix. It is then immersed in a mixed solution of PVA and tea polyphenols. PVA and tea polyphenols are filled into the cellulose framework by solution displacement method to obtain a translucent, high mechanical strength and water-resistant bamboo-based transparent film. Due to the high density and low porosity of bamboo, and the crucial role of lignin in its composition, direct delignification treatment of bamboo is detrimental to maintaining its integrity. This invention addresses the issue of bamboo's susceptibility to damage during delignification and other subsequent treatments through pretreatment and cross-linking processes, effectively preserving the integrity of the film structure. Delignification effectively removes lignin without damaging cellulose, reducing light scattering and making it translucent. TEMPO oxidation introduces carboxyl groups to enhance hydrophilicity and interfacial properties, facilitating the uniform filling of PVA and tea polyphenols into the cellulose framework, reducing interfacial light reflection, and achieving good translucency. The uniform deposition of PVA and tea polyphenols within the cellulose framework forms a dense structure, giving the film high tensile strength (7.33–9.14 MPa), suitable elongation at break (40.58–61.32%), and good translucency (visible light transmittance 55%–62%), meeting packaging requirements. Tea polyphenols also impart antioxidant properties to the film, extending food shelf life. PVA enhances the film's water resistance and flexibility.

[0016] The raw material used in this invention is natural bamboo. The bamboo-based conversion film prepared is biodegradable, avoiding plastic pollution and meeting the needs of green development. Detailed Implementation

[0017] The following examples further illustrate the specific technical content of the present invention, but it should not be construed as limiting the scope of the subject matter of the present invention to the following examples. All technologies implemented based on the above content of the present invention fall within the scope of the present invention.

[0018] Example 1 A method for preparing a bamboo-based conversion film that can replace plastic includes the following steps: S1. Pretreatment: Select a single bamboo diameter section with a thickness of 1 mm, soak it in 1% NaOH solution, and pretreat it at 80℃ for 3 hours to obtain pretreated bamboo material. S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water and crosslinked at 60℃ for 3 hours to obtain crosslinked bamboo. S3, Delignification: At 80℃, the cross-linked bamboo was treated with 1wt% sodium chlorite solution (pH adjusted to 4.6 by glacial acetic acid) for 6 hours to remove delignification, and then soaked in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo was immersed in TEMPO solution (0.032g TEMPO, 0.2g sodium bromide, 5mL sodium hypochlorite dissolved in 200mL water, pH adjusted to 10), and TEMPO was oxidized at 20℃ for 48 h. Then it was washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: TEMPO oxidized bamboo is immersed in a PVA-tea polyphenol mixed melt with a mass ratio of 8:2 and soaked at 20°C for 30 hours to obtain a translucent bamboo-based conversion film.

[0019] Example 2 A method for preparing a bamboo-based conversion film that can replace plastic includes the following steps: S1. Pretreatment: Select bamboo sections with a thickness of 0.5 mm, soak them in 1% NaOH solution, and treat them at 80℃ for 3 hours to obtain pretreated bamboo material; S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water and crosslinked at 60°C for 3 hours to obtain crosslinked bamboo. S3, Delignification: At 80℃, the cross-linked bamboo was treated with 1.5wt% sodium chlorite solution (pH adjusted to 4.6 by glacial acetic acid) for 6 hours to remove delignification, and then soaked in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo was immersed in TEMPO solution (same as in Example 1) and subjected to TEMPO oxidation treatment at 20°C for 48 h. Then it was washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: TEMPO oxidized bamboo is immersed in a PVA-tea polyphenol mixed melt with a mass ratio of 7:3 and soaked at 20°C for 28 hours to obtain a translucent bamboo-based conversion film.

[0020] Example 3 A method for preparing a bamboo-based conversion film that can replace plastic includes the following steps: S1. Pretreatment: Select bamboo sections with a diameter of 1.2 mm and soak them in 1% NaOH solution. Treat them at 85℃ for 1 hour to obtain pretreated bamboo. S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water and crosslinked at 65°C for 1 hour to obtain crosslinked bamboo. S3, Delignification: At 85℃, the cross-linked bamboo was treated with 1wt% sodium chlorite solution (pH adjusted to 4.6 by glacial acetic acid) for 8 hours to remove delignification, and then soaked in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo was immersed in TEMPO solution (same as in Example 1) and subjected to TEMPO oxidation treatment at 25°C for 60 h. Then it was washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: TEMPO oxidized bamboo is immersed in a PVA-tea polyphenol mixed melt with a mass ratio of 6:4 and soaked at 25°C for 24 hours to obtain a translucent bamboo-based conversion film.

[0021] Comparative Example 1 A method for preparing a bamboo-based conversion thin film includes the following steps: S1, Delignification: Select a single bamboo diameter section with a thickness of 1mm, perform delignification treatment, and then soak it in deionized water to obtain delignified bamboo material. S2, TEMPO oxidation: Delignified bamboo is subjected to TEMPO oxidation treatment, and then washed with deionized water to obtain TEMPO oxidized bamboo. S3. Resin Impregnation: TEMPO oxidized bamboo is immersed in a PVA-tea polyphenol mixed melt with a mass ratio of 8:2 and soaked at 20°C for 30 hours to obtain a bamboo-based conversion film.

[0022] The difference between Comparative Example 1 and Example 1 is that no pretreatment and cross-linking treatment were performed.

[0023] Comparative Example 2 A method for preparing a bamboo-based conversion thin film includes the following steps: S1. Pretreatment: Select a single bamboo diameter section with a thickness of 1mm and soak it in an alkaline solution for pretreatment to obtain pretreated bamboo material. S2, Delignification: After pre-treated bamboo is treated with delignification, it is soaked in deionized water to obtain delignified bamboo. S3, TEMPO oxidation: Delignified bamboo is subjected to TEMPO oxidation treatment, and then washed with deionized water to obtain TEMPO oxidized bamboo. S4. Resin Impregnation: TEMPO oxidized bamboo is immersed in a PVA-tea polyphenol mixed melt with a mass ratio of 8:2 and soaked at 20°C for 30 hours to obtain a bamboo-based conversion film.

[0024] The difference between Comparative Example 2 and Example 1 is that no cross-linking treatment was performed.

[0025] Comparative Example 3 A method for preparing a bamboo-based conversion thin film includes the following steps: S1. Pretreatment: Select a single bamboo diameter section with a thickness of 1mm and soak it in an alkaline solution for pretreatment to obtain pretreated bamboo material. S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water and crosslinked at 60℃ for 3 hours to obtain crosslinked bamboo. S3, Delignification: After cross-linked bamboo is treated with delignification, it is soaked in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo is subjected to TEMPO oxidation treatment, and then washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: Immerse TEMPO oxidized bamboo in PVA melt at 20°C for 30 hours to obtain bamboo-based conversion film.

[0026] The difference between Comparative Example 3 and Example 1 is that tea polyphenols were not added in the resin impregnation step.

[0027] Comparative Example 4 A method for preparing a bamboo-based conversion thin film includes the following steps: S1. Pretreatment: Select a single bamboo diameter section with a thickness of 1mm and soak it in an alkaline solution for pretreatment to obtain pretreated bamboo material. S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water and crosslinked at 60℃ for 3 hours to obtain crosslinked bamboo. S3, Delignification: After cross-linked bamboo is treated with delignification, it is soaked in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo is subjected to TEMPO oxidation treatment, and then washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: Immerse TEMPO oxidized bamboo in an epoxy resin solution at 20°C for 30 hours to obtain a bamboo-based conversion film.

[0028] The difference between Comparative Example 4 and Example 1 lies in the resin raw materials used in the resin impregnation step.

[0029] Comparative Example 5 A method for preparing a bamboo-based conversion thin film includes the following steps: S1, Delignification: Select a single bamboo diameter section with a thickness of 1mm, perform delignification treatment, and then soak it in deionized water to obtain delignified bamboo material. S2. Oxidation: The delignified bamboo is oxidized and then washed with deionized water to obtain oxidized bamboo. S3. Resin Impregnation: Immerse oxidized bamboo in a PVA-tea polyphenol mixed melt with a mass ratio of 8:2 and soak at 20°C for 30 hours to obtain a bamboo-based conversion film.

[0030] The difference between Comparative Example 5 and Example 1 is that no cross-linking treatment was performed, and only ordinary oxidation treatment was performed.

[0031] Performance testing: The bamboo-based conversion films prepared in Examples 1-3 and Comparative Examples 1-5 were tested for tensile strength, elongation at break, and transparency (visible light transmittance), and the results are shown in Table 1.

[0032] Table 1. Test results of bamboo-based conversion films prepared in Examples 1-3 and Comparative Examples 1-5

[0033] The tensile strength test results in Table 1 show that the bamboo-based conversion films prepared in Examples 1-3 have high tensile strength (7.33-9.14 MPa), exhibiting strong tensile resistance. This is attributed to the dense cellulose framework formed by pretreatment, crosslinking, and TEMPO oxidation, as well as the uniform filling of PVA-tea polyphenols, which makes the film structure stable and able to withstand greater tensile forces. The tensile strength of the films in Comparative Examples 1-5 is significantly lower than that in Example 1. Among them, Comparative Example 1 (4.85 MPa) and Comparative Example 4 (5.40 MPa) performed the worst, indicating that the lack of key process steps or mismatch of raw materials can lead to a loose structure of bamboo-based materials and a significant decrease in mechanical strength.

[0034] The results of the elongation at break test in Table 1 show that the elongation at break of the bamboo-based conversion films in Examples 1-3 are all at a moderate level (40.58-61.32%), which ensures that the film has a certain degree of flexibility to adapt to packaging operations, while avoiding deformation caused by excessive stretching. This meets the "rigid-flexibility balance" requirement of food packaging and satisfies the strength requirements of packaging films in the food industry. In contrast, the films in Comparative Examples 1-5 have a relatively high elongation at break, which makes them prone to stretching and deformation due to excessive flexibility.

[0035] The transparency test results in Table 1 show that the bamboo-based conversion films of Examples 1-3 have a visible light transmittance range of 55%–62%, exhibiting good translucency. This is because the delignification treatment removes colored lignin, and TEMPO oxidation enhances the compatibility of bamboo with PVA and tea polyphenols, reducing light scattering and allowing the film to transmit more visible light, thus meeting the basic requirement of food packaging for "visible contents." In contrast, the visible light transmittance of Comparative Examples 1-5 ranges only from 28% to 50%, with significantly lower transparency, failing to achieve good translucency and limiting their application in transparent packaging scenarios.

[0036] This invention uses cellulose as the matrix and PVA as the filler. A bamboo-derived cellulose framework, prepared by pretreatment, cross-linking, delignification, and TEMPO oxidation of bamboo, is used as the matrix. This framework is then immersed in a PVA-tea polyphenol mixed solution. PVA and tea polyphenols are then filled into the cellulose framework through a solution displacement method, resulting in a semi-transparent, high-mechanical-strength, water-resistant, all-wood-based bamboo-based conversion film. This film is also biodegradable, which can improve the white pollution problem caused by traditional plastic packaging materials and provide a new approach for converting bamboo-based materials into transparent composite materials.

[0037] The above embodiments describe the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are only illustrative of the principles of the present invention. Various changes and modifications can be made to the present invention without departing from the scope of the principles of the present invention, and all such changes and modifications fall within the protection scope of the present invention.

Claims

1. A method for preparing a bamboo-based conversion film that can replace plastic, characterized in that, Includes the following steps: S1. Pretreatment: The bamboo is pretreated by soaking it in an alkaline solution; S2, Crosslinking: The pretreated bamboo is immersed in a mixture of PEG, NaOH, ethanol and deionized water for crosslinking treatment; S3, Delignification: After treating cross-linked bamboo with delignification, soak it in deionized water to obtain delignified bamboo. S4, TEMPO oxidation: Delignified bamboo is subjected to TEMPO oxidation treatment, and then washed with deionized water to obtain TEMPO oxidized bamboo. S5. Resin Impregnation: Immerse TEMPO oxidized bamboo in a PVA-tea polyphenol mixed solution for 24-30 h to obtain a bamboo-based conversion film.

2. The method for preparing a bamboo-based conversion film that can replace plastic according to claim 1, characterized in that: The bamboo material mentioned in step S1 is a diameter section of any one of single bamboo, moso bamboo, fuso bamboo, square bamboo, or black bamboo, with a thickness of 0.5 to 1.5 mm.

3. The method for preparing a bamboo-based conversion film that can replace plastic according to claim 2, characterized in that: The alkaline solution mentioned in step S1 is a 1% NaOH solution; the pretreatment temperature is 80-85℃ and the time is 1-3 h.

4. The method for preparing a bamboo-based conversion film that can replace plastic according to claim 1, characterized in that: The crosslinking treatment in step S2 is performed at a temperature of 60–65°C for 1–3 hours.

5. The method for preparing a bamboo-based conversion film that can replace plastic according to claim 1, characterized in that: The delignification treatment in step S3 uses a 1-1.5 wt% sodium chlorite solution, and the pH is adjusted to 4.6 by glacial acetic acid; the delignification treatment temperature is 80-85℃, and the time is 6-12 h.

6. The method for preparing a bamboo-based conversion film that can replace plastic according to claim 1, characterized in that: The solution for the TEMPO oxidation treatment in step S4 is prepared as follows: 0.032 g TEMPO, 0.2 g sodium bromide and 5 mL sodium hypochlorite solution are dissolved in 200 mL water and the pH is adjusted to 10; the temperature for the TEMPO oxidation treatment is 20-25℃ and the time is 24-60 h.

7. The method for preparing a bamboo-based conversion film that can replace plastic according to claim 1, characterized in that: In step S5, the mass ratio of PVA to tea polyphenols in the PVA-tea polyphenol mixed solution is (5-10):(0-5), and the soaking temperature is 20-25℃.

8. A bamboo-based conversion film prepared by the preparation method according to any one of claims 1-7, characterized in that: The bamboo-based conversion film is semi-transparent, biodegradable, and antioxidant, with a tensile strength ≥7.33 MPa.

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