Method for preparing polyvinyl alcohol flame-retardant film from coffee shell carbon dots and application of polyvinyl alcohol flame-retardant film

Abstract

The invention provides a method for preparing a polyvinyl alcohol flame-retardant film by using coffee shell carbon dots and application, and belongs to the technical field of composites.The method comprises the following steps that a coffee shell carbon dot solution and polyvinyl alcohol are mixed, heated in an oil bath and then cooled to the room temperature, the cooled coffee shell carbon dot-polyvinyl alcohol solution is subjected to tape casting to form a film, and the polyvinyl alcohol flame-retardant film is obtained. And drying to obtain the coffee shell carbon dot-based polyvinyl alcohol flame-retardant film. The coffee shells are used as the raw material to prepare the carbon dots, caffeic acid and chlorogenic acid in the coffee shells are mainly used, resource utilization of waste is achieved, the production cost of the flame-retardant film is reduced, and the prepared flame-retardant film is light, has certain mechanical strength and is suitable for various fields needing light flame-retardant materials. The flame retardant property is excellent, flame spreading can be effectively delayed or prevented, the safety of the material is improved, and the preparation method is simple and is more suitable for popularization and application.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite materials, and particularly relates to a method for preparing a polyvinyl alcohol flame-retardant film using coffee shell carbon dots and its application. Background Art

[0002] Flame-retardant films are a type of functional film material that can effectively inhibit or delay the spread of flames. They have the characteristics of being lightweight, flexible, and easy to process, and are widely used in fields such as electronics and electrical appliances, building decoration, and transportation. For example, in the field of electronics and electrical appliances, flame-retardant films can be used for the insulating layer of printed circuit boards to prevent fires caused by short circuits; in the field of building decoration, flame-retardant films can be used for the surface coatings of materials such as wallpapers and floors to improve the fire safety level of buildings; in the field of transportation, flame-retardant films can be used for interior materials of vehicles such as airplanes and high-speed trains to reduce the fire risk. With the continuous improvement of people's awareness of safety and environmental protection, the demand for flame-retardant films is increasing day by day, and higher requirements are also put forward for their performance.

[0003] However, traditional flame-retardant films still have many defects in practical applications. First of all, traditional flame retardants such as aluminum hydroxide and magnesium hydroxide often need to be added in large amounts to achieve the desired flame-retardant effect, which will significantly reduce the mechanical properties and transparency of the film, restricting its application range. Secondly, some flame retardants such as halogen-based flame retardants will produce toxic and harmful gases during combustion, which will cause harm to the environment and human health, and this is contrary to the current development concept of green environmental protection. In addition, although some new flame retardants such as nano flame retardants and organophosphorus flame retardants have high flame-retardant efficiency, their preparation processes are complex and the costs are high, making it difficult to achieve large-scale industrial production. These problems seriously restrict the further development and application of flame-retardant films.

[0004] Therefore, the present invention proposes a method for preparing a PVA flame-retardant film using coffee shell carbon dots. Summary of the Invention

[0005] The mechanical properties and flame-retardant properties of composite materials are usually improved by adding expensive nanoparticles (such as MXene, graphene oxide, boron nitride, etc.) or complex chemical modifications. However, these methods are not only costly but also have complex preparation processes, restricting their large-scale applications. To solve the above technical problems, the present invention proposes a method for preparing a PVA flame-retardant film using coffee shell carbon dots and its application. The polyvinyl alcohol flame-retardant film obtained by the present invention can achieve similar effects to the above-mentioned expensive nanoparticle or complex chemical modification films, while avoiding the use of expensive materials and simplifying the preparation process. The present invention utilizes the interfacial interaction between coffee shell carbon dots (CS-CDs) and the polyvinyl alcohol (PVA) matrix to not only improve the mechanical strength of the film but also significantly enhance its flame-retardant performance, providing a new idea for the development of high-performance and low-cost flame-retardant materials.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention:

[0008] A method for preparing a polyvinyl alcohol flame-retardant film using coffee shell carbon dots, comprising the following steps:

[0009] Mix the coffee shell carbon dots (CS-CDs) solution with polyvinyl alcohol (PVA), heat it in an oil bath and then cool it to room temperature. Cast the cooled coffee shell carbon dot-polyvinyl alcohol solution into a film, and obtain a coffee shell carbon dot-based polyvinyl alcohol flame-retardant film after drying.

[0010] The coffee shell carbon dots (CS-CDs) of the present invention are different from other carbon dots in that their excellent flame-retardant properties mainly depend on the caffeic acid and chlorogenic acid rich in coffee shells:

[0011] (1) Specificity of functional groups

[0012] Caffeic acid (4-hydroxycinnamic acid) and chlorogenic acid (ester of caffeic acid and quinic acid) are both polyphenolic compounds, rich in catechol hydroxyl groups (-OH), carboxylic acid groups (-COOH) and unsaturated double bonds (C=C). These groups are retained and anchored on the surface of the carbon dots during the carbon dot preparation process (hydrothermal carbonization) to form a unique surface functional group array. Among them, the formed catechol structure can provide Lewis basic sites, which can form hydrogen bonds with the polar groups (-NH-, -OH) of the polymer, enhance the interfacial compatibility, and avoid carbon dot aggregation. The formed carboxylic acid groups undergo decarboxylation reactions (-COOH→CO2↑) at high temperatures (>200 °C), absorb heat (about 150 kJ / mol) and release non-combustible gases, diluting the concentration of combustible gases. The formed π-π conjugate system can improve the thermal stability of the carbon dots (decomposition temperature >500 °C), and at the same time promote the graphitization degree of the carbon layer. In contrast, the surface of traditional carbon dots (such as glucose-based carbon dots) is mainly composed of alcohol hydroxyl groups (-OH), lacking carboxylic acids and conjugated double bonds, resulting in less gas release during thermal decomposition (CO2 release amount <40% of CS-CDs) and low char formation efficiency.

[0013] (2) Thermal decomposition behavior - multiple flame-retardant synergistic effect

[0014] The flame retardancy of CS-CDs mainly relies on the thermal decomposition process. First is the dehydration stage (100 - 200 °C), where surface hydroxyl groups condense, releasing H2O and reducing the surface temperature of the material. In the decarboxylation / ester hydrolysis stage (200 - 350 °C), caffeic acid decarboxylates to form styrene, which further cyclizes to form aromatic carbon precursors. The ester bond of chlorogenic acid breaks, releasing caffeic acid and quinic acid. Quinic acid dehydrates to form a cyclic ether structure, enhancing the crosslinking of the carbon layer. CO2 and CO are released during this stage, forming a physical barrier. Subsequently, in the carbonization stage (>350 °C), the remaining polyphenol backbone carbonizes to form a nitrogen / oxygen-containing graphitic carbon layer, which rapidly conducts heat away.

[0015] (3) Acid-base synergistic carbonization mechanism

[0016] The carboxylic acid groups (acidic) on the surface of CS-CDs react with polyvinyl alcohol (PVA) to form a crosslinked network, improving the mechanical strength of the carbon layer.

[0017] (4) Chemical flame retardancy by free radical capture - phenolic structure

[0018] The phenolic hydroxyl groups (-OH) of caffeic acid and chlorogenic acid release H + during the combustion process, capturing free radicals in the flame to terminate the chain combustion reaction.

[0019] Furthermore, the mass ratio of the coffee shell carbon dot solution to polyvinyl alcohol is 6:1.

[0020] Furthermore, the preparation method of the coffee shell carbon dot solution includes the following steps:

[0021] Add coffee shells to deionized water for hydrothermal reaction. After the hydrothermal reaction is completed, cool to room temperature, centrifuge to obtain the supernatant, and after drying treatment, a coffee shell carbon dot solution with a mass concentration of 1% is obtained.

[0022] In the preparation method of the coffee shell carbon dot solution, the dosage ratio of the coffee shells to deionized water is 1 g:100 mL.

[0023] In the preparation method of the coffee shell carbon dot solution, the temperature of the hydrothermal reaction is 180 °C and the time is 10 h.

[0024] In the preparation method of the coffee shell carbon dot solution, the drying temperature is 80 °C and the time is 12 h.

[0025] Furthermore, the temperature of the oil bath heating is 95 °C and the time is 5 h.

[0026] Furthermore, the drying temperature is 30 °C and the time is 6 h.

[0027] In the present invention, a strong interfacial bonding formed between the coffee shell carbon dots and the PVA matrix through non-covalent bonds such as π-π interactions, hydrogen bonds, and van der Waals forces effectively optimizes the energy dissipation mechanism. The unique caffeic acid and chlorogenic acid in the coffee shell for firing carbon dots are the greatest features that distinguish these carbon dots from other carbon dots, significantly enhancing the fracture resistance and overall ductility of the thin film material. In addition, the uniform dispersion of the coffee shell carbon dots and their unique micro-carbonized structure further improve the toughness and strength of the thin film, endowing it with excellent ductility and mechanical properties while maintaining high strength. Moreover, the unique graphitized conjugated structure and micro-carbonized characteristics of the coffee shell carbon dots can effectively promote the formation of a carbon layer during the combustion process, inhibiting the transfer of heat and oxygen, thereby significantly enhancing the flame retardancy of the material.

[0028] The second technical solution of the present invention:

[0029] A coffee shell carbon dot-based polyvinyl alcohol flame-retardant thin film, prepared according to the described method, and the thickness of the coffee shell carbon dot-based polyvinyl alcohol flame-retardant thin film is 0.25 mm.

[0030] The third technical solution of the present invention:

[0031] The application of the described coffee shell carbon dot-based polyvinyl alcohol flame-retardant thin film in the preparation of flame-retardant film materials.

[0032] Compared with the prior art, the present invention has the following advantages and technical effects:

[0033] (1) As a novel carbon-based nanomaterial, carbon dots (CDs) have attracted extensive attention due to their unique structure and properties. Carbon dots have characteristics such as small size, large specific surface area, and rich surface functional groups, showing great application potential in fields such as bioimaging, photocatalysis, and sensors. In the present invention, CS-CDs are used as raw materials to be mixed with PVA to prepare a flame-retardant thin film because the coffee shell carbon dots have good flame retardancy. Their carbon core structure can form a dense carbon layer at high temperatures, effectively isolating the transfer of heat and oxygen, thereby inhibiting combustion. Compared with traditional flame retardants, the coffee shell carbon dots obtained in the present invention not only have high flame retardancy efficiency but also are environmentally friendly and non-toxic, and do not release harmful gases during combustion. Therefore, introducing coffee shell carbon dots into the preparation of flame-retardant thin films in the present invention is expected to overcome the defects of traditional flame-retardant thin films and promote the development of flame-retardant thin films towards high efficiency, environmental protection, and multi-functionality.

[0034] (2) The carbon dots used in the present invention are prepared using coffee shells as raw materials, realizing the resource utilization of waste and reducing environmental pollution, which is in line with the concept of sustainable development. As agricultural waste, coffee shells are widely sourced and low-cost, reducing the production cost of the flame-retardant thin film.

[0035] (3) The flame-retardant film prepared by the present invention is lightweight and has a certain mechanical strength, is suitable for various fields requiring lightweight flame-retardant materials, and has excellent flame-retardant performance, can effectively delay or prevent the spread of fire, and enhance the safety of materials.

[0036] (4) The preparation method of the flame-retardant film of the present invention is simple and more suitable for popularization and application. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention. In the drawings:

[0038] Figure 1 are the stress-strain curves of the films of Example 1 and Comparative Examples 1-2;

[0039] Figure 2 are the test results of the stress (breaking strength) of the films of Example 1 and Comparative Examples 1-2;

[0040] Figure 3 are the test results of the breaking strain of the films of Example 1 and Comparative Examples 1-2;

[0041] Figure 4 are the test results of the toughness of the films of Example 1 and Comparative Examples 1-2;

[0042] Figure 5 are the test results of the limiting oxygen index (LOI) of the films of Example 1 and Comparative Examples 1-3. DETAILED DESCRIPTION OF THE INVENTION

[0043] Now, various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation to the present invention, but should be understood as a more detailed description of certain aspects, characteristics and implementation schemes of the present invention.

[0044] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. In addition, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range and each smaller range between any other stated value or intermediate value within the stated range are also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0045] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although this invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of this invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to those documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0046] Without departing from the scope or spirit of this invention, various improvements and changes can be made to the specific embodiments of the description of this invention, which are obvious to those skilled in the art. Other embodiments obtained from the description of this invention are obvious to those skilled in the art. The description and examples of this invention are merely exemplary.

[0047] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0048] An embodiment of this invention provides a method for preparing a polyvinyl alcohol flame-retardant film using coffee shell carbon dots, comprising the following steps:

[0049] Mix a coffee shell carbon dots (CS-CDs) solution with polyvinyl alcohol (PVA), heat it in an oil bath and then cool it to room temperature. Cast the cooled coffee shell carbon dots-polyvinyl alcohol solution into a film, and after drying, a coffee shell carbon dots-based polyvinyl alcohol flame-retardant film is obtained.

[0050] In a preferred embodiment of this invention, the mass ratio of the coffee shell carbon dots solution to polyvinyl alcohol is 6:1.

[0051] In a preferred embodiment of this invention, the method for preparing the coffee shell carbon dots solution comprises the following steps:

[0052] Add coffee shells to deionized water for hydrothermal reaction. After the hydrothermal reaction is completed, cool it to room temperature, centrifuge to obtain the supernatant, and after drying, a coffee shell carbon dots solution is obtained;

[0053] In the method for preparing the coffee shell carbon dots solution, the dosage ratio of coffee shells to deionized water is 1 g:100 mL, the temperature of the hydrothermal reaction is 180 °C, the time is 10 h, the drying temperature is 80 °C, and the time is 12 h.

[0054] In a preferred embodiment of this invention, the temperature of the oil bath heating is 95 °C and the time is 5 h.

[0055] In a preferred embodiment of this invention, the drying temperature is 30 °C and the time is 6 h.

[0056] An embodiment of the present invention also provides a coffee shell carbon dot-based polyvinyl alcohol flame retardant film, which is prepared according to the above method, and the thickness of the coffee shell carbon dot-based polyvinyl alcohol flame retardant film is 0.25 mm.

[0057] In a preferred embodiment of the present invention, room temperature refers to 20 - 30 °C.

[0058] All raw materials used in the present invention are obtained by purchasing in the market, and polyvinyl alcohol is purchased from Chengdu Jiacai Technology Co., Ltd.

[0059] The technical solution of the present invention will be further described below through examples.

[0060] Example 1

[0061] A method for preparing a polyvinyl alcohol flame retardant film using coffee shell carbon dots includes the following steps:

[0062] (1) Grind 1 g of coffee shells to 100 μm using a grinder, add 100 mL of deionized water, place it in a reaction kettle, heat it at 180 °C for 10 h, and wait for the reaction kettle to cool naturally to room temperature to obtain an initial coffee shell carbon dot solution;

[0063] (2) Centrifuge the prepared initial coffee shell carbon dot solution using a centrifuge and extract the supernatant. Put the extracted supernatant into an oven and dry it at 80 °C for 12 h to remove excess water to obtain a coffee shell carbon dot solution with a mass concentration of 1%;

[0064] (3) Take 12 g of the coffee shell carbon dot solution cooled to room temperature, add 2 g of polyvinyl alcohol, pour it into a round-bottom flask, and heat it using an oil bath at 95 °C for 5 h to obtain a coffee shell carbon dot - polyvinyl alcohol solution;

[0065] (4) Pour the coffee shell carbon dot - polyvinyl alcohol solution cooled to room temperature into a mold for casting into a film, and place it in an oven to dry at 30 °C for 6 h to obtain a coffee shell carbon dot-based polyvinyl alcohol flame retardant film.

[0066] Comparative Example 1

[0067] A method for preparing a polyvinyl alcohol film includes the following steps:

[0068] Add 1 g of polyvinyl alcohol to 100 mL of deionized water, pour it into a round-bottom flask, and heat it using an oil bath at 95 °C for 4 h to obtain a polyvinyl alcohol solution. After cooling, pour it into a mold for casting into a film and place it in an oven to dry at 30 °C for 6 h to obtain a polyvinyl alcohol film.

[0069] Comparative Example 2

[0070] A method for preparing a polyvinyl alcohol flame retardant film using coffee shells includes the following steps:

[0071] Crush 1 g of coffee husks to 100 μm using a grinder, add them to 100 mL of deionized water, then add 1.96 g of polyvinyl alcohol. Pour the mixture into a round-bottom flask and heat it in an oil bath at 95 °C for 4 h to obtain a coffee husk-polyvinyl alcohol solution. After cooling, place it in an oven and dry it at 30 °C for 6 h to obtain a coffee husk-polyvinyl alcohol film.

[0072] Comparative Example 3

[0073] A method for preparing a polyvinyl alcohol flame-retardant film using coffee husk carbon dots includes the following steps:

[0074] (1) Crush 1 g of coffee husks to 500 μm using a grinder, add them to 100 mL of deionized water, place the mixture in a reaction kettle, and heat it at 160 °C for 8 h. Wait for the reaction kettle to cool naturally to room temperature to obtain an initial coffee husk carbon dot solution;

[0075] (2) Centrifuge the prepared initial coffee husk carbon dot solution using a centrifuge and extract the supernatant. Place the extracted supernatant in an oven and dry it at 90 °C for 12 h to remove excess moisture to obtain a 1% coffee husk carbon dot solution by mass concentration;

[0076] (3) Take 6 g of the coffee husk carbon dot solution cooled to room temperature, add 2 g of polyvinyl alcohol, pour the mixture into a round-bottom flask, and heat it in an oil bath at 95 °C for 5 h to obtain a coffee husk carbon dot-polyvinyl alcohol solution;

[0077] (4) Pour the coffee husk carbon dot-polyvinyl alcohol solution cooled to room temperature into a mold to cast a film, place it in an oven and dry it at 30 °C for 6 h to obtain a coffee husk carbon dot-based polyvinyl alcohol flame-retardant film.

[0078] Performance testing

[0079] Cut the films prepared in Example 1 and Comparative Examples 1-2 into rectangular specimens (5 cm × 1 cm × 0.25 mm), and conduct tensile property tests at room temperature. The results are shown in Table 1.

[0080] Uniaxial tensile measurements were carried out using an electronic universal testing machine (Shenzhen Sun Technology Co., Ltd., China), and the extension speed was 5 mm / min.

[0081] Toughness is calculated according to the area under the stress-strain curve using the following formula:

[0082] ΔU = ∫σds

[0083] In the formula, σ and ε are the stress and strain of the film respectively, and △U is the toughness.

[0084] Table 1 Results of tensile strength and toughness determination of the films in Example 1 and Comparative Examples 1-2

[0085] Specimen Tensile strength (MPa) <![CDATA[Toughness (MJ / m 3 )]]> Example 1 185 227 Comparative Example 1 32 5.38 Comparative Example 2 23 2.89

[0086] As can be seen from the data in Table 1, the tensile strength and toughness of the film prepared in Example 1 are the largest, reaching 185 MPa and 227 MJ / m 3 . Moreover, by comparing the data changes of Comparative Examples 1-2 with those of Example 1, it can be seen that the mechanical properties of the film doped with coffee shell carbon dots are much higher than those of the film without doped coffee shell carbon dots, indicating that the film with excellent mechanical properties can be prepared in the examples of the present invention.

[0087] Figure 1 Figure 1 is the stress-strain curve of the films of Example 1 and Comparative Examples 1-2. From Figure 1 it can be seen that both the pure polyvinyl alcohol (PVA) film (Comparative Example 1) and the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) show relatively low mechanical properties, and their elongation at break and tensile strength are both limited, unable to meet the requirements of high-performance materials. Specifically, the stress-strain curves of the pure polyvinyl alcohol (PVA) film (Comparative Example 1) and the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) show that their elongation at break is relatively low, showing poor ductility and not reaching the ideal level. In contrast, the coffee shell carbon dot-based polyvinyl alcohol film prepared in Example 1 shows a significant improvement in mechanical properties. Its stress-strain curve shows that the elongation at break reaches 224%, and at the same time, the tensile strength is also significantly improved. This excellent mechanical property is attributed to the strong binding interface formed between the coffee shell carbon dots and the PVA matrix through non-covalent bonds such as π-π interactions, hydrogen bonds, and van der Waals forces, effectively improving the stress transfer efficiency and the overall ductility of the material. In addition, the uniform dispersion and micro-carbonized structure of the coffee shell carbon dots further enhance the toughness and strength of the film, enabling it to have excellent mechanical properties while maintaining high ductility. This result indicates that the coffee shell carbon dot-based polyvinyl alcohol film has broad application potential in the fields of high-performance flame retardant materials and flexible films.

[0088] Figure 2 Figure 2 is the test result of the stress (fracture strength) of the films of Example 1 and Comparative Examples 1-2. From Figure 2 it can be seen that the pure polyvinyl alcohol (PVA) film (Comparative Example 1) and the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) show relatively low stress. In contrast, the coffee shell carbon dot-based polyvinyl alcohol film prepared in Example 1 has more excellent stress, showing a significant improvement compared to Comparative Examples 1 and 2.

[0089] Figure 3 Figure 3 is the test result of the fracture strain of the films of Example 1 and Comparative Examples 1-2. From Figure 3It can be seen that the pure polyvinyl alcohol (PVA) film (Comparative Example 1) and the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) exhibit lower strain. In contrast, the coffee shell carbon dot-based polyvinyl alcohol film prepared in Example 1 has more excellent strain, showing a significant improvement compared to Comparative Examples 1 and 2.

[0090] Figure 4 Figure shows the toughness test results of the films in Example 1 and Comparative Examples 1-2. It can be seen from the toughness test results that the pure polyvinyl alcohol (PVA) film (Comparative Example 1) and the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) have low toughness and are difficult to meet the toughness requirements of high-performance materials. Specifically, the pure polyvinyl alcohol (PVA) film (Comparative Example 1) and the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) have low toughness values and show poor fracture resistance. In contrast, the coffee shell carbon dot-based polyvinyl alcohol film prepared in Example 1 exhibits excellent toughness performance, with a significantly increased toughness value reaching 227 MJ / m 3 , far exceeding the films of Comparative Examples 1 and 2. This significant performance improvement is mainly attributed to the strong interfacial bonding formed between the coffee shell carbon dots and the PVA matrix through non-covalent bonds such as π-π interactions, hydrogen bonds, and van der Waals forces, effectively optimizing the energy dissipation mechanism and significantly enhancing the fracture resistance of the material. In addition, the uniform dispersion of the coffee shell carbon dots and their unique micro-carbonized structure further improve the toughness of the film, enabling it to have both excellent ductility while maintaining high strength. This result indicates that the coffee shell carbon dot-based polyvinyl alcohol film of the present invention has important application potential in the fields of high-performance flame retardant materials and toughness-enhanced films.

[0091] Figure 5The limiting oxygen index (LOI) test results of the films in Example 1 and Comparative Examples 1-3 are shown. It can be seen from the LOI test results that the LOI value of the pure polyvinyl alcohol (PVA) film (Comparative Example 1) is 19.6%, showing poor flame retardant performance and being difficult to meet the requirements of high-performance flame retardant materials. The LOI value of the ordinary coffee shell polyvinyl alcohol film (Comparative Example 2) is slightly increased to 21.6%, indicating that the addition of coffee shells has a certain improvement on the flame retardant performance, but the improvement range is limited and still cannot meet the higher standard flame retardant requirements. In contrast, the coffee shell carbon dot-based polyvinyl alcohol film prepared in Example 1 shows a significant improvement in flame retardant performance, with its LOI value reaching as high as 28.9%, far exceeding the films in Comparative Examples 1 and 2. This significant performance improvement is mainly attributed to the introduction of coffee shell carbon dots. Its unique graphitized conjugated structure and micro-carbonization characteristics can effectively promote the formation of a carbon layer during combustion, inhibit the transfer of heat and oxygen, thus significantly improving the flame retardant performance of the material. In addition, the strong interfacial bonding formed between the coffee shell carbon dots and the PVA matrix through non-covalent bonds such as π-π interaction, hydrogen bond and van der Waals force further enhances the stability and flame retardant efficiency of the material. Compared with Example 1, the LOI value in Comparative Example 3 also decreases due to the reduction in the addition amount of the carbon dot solution. This result shows that the coffee shell carbon dot-based polyvinyl alcohol film of the present invention has important application potential in the field of high-performance flame retardant materials, providing a new idea for the development of efficient and environmentally friendly flame retardant materials.

[0092] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims

1. A method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots, characterized in that: The following steps are involved: The coffee shell carbon dot solution is mixed with polyvinyl alcohol, heated in an oil bath and then cooled to room temperature, the cooled coffee shell carbon dot-polyvinyl alcohol solution is cast into a film, and the coffee shell carbon dot-based polyvinyl alcohol flame retardant film is obtained after drying.

2. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 1, characterized in that: The mass ratio of the coffee shell carbon dot solution to polyvinyl alcohol is 6:

1.

3. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 1, characterized in that: The method for preparing the coffee shell carbon dot solution comprises the following steps: The coffee shells were added into water for hydrothermal reaction, cooled to room temperature after the hydrothermal reaction, centrifuged to obtain the supernatant, and dried to obtain a coffee shell carbon dot solution with a mass concentration of 1%.

4. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 3, characterized in that: The usage ratio of the coffee shell to water is 1g:100mL.

5. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 3, characterized in that: The temperature of the hydrothermal reaction is 180° C. and the time is 10 h.

6. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 3, characterized in that: The drying temperature is 80° C. and the drying time is 12 h.

7. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 1, characterized in that: The oil bath was heated at 95°C for 5 hours.

8. The method for preparing polyvinyl alcohol flame retardant film using coffee shell carbon dots according to claim 1, characterized in that: The drying temperature is 30° C. and the drying time is 6 hours.

9. A coffee shell carbon dot-based polyvinyl alcohol flame retardant film, characterized in that: Prepared according to the method according to any one of claims 1 to 8, the coffee shell carbon dot-based polyvinyl alcohol flame retardant film has a thickness of 0.25 mm.

10. Use of the coffee husk carbon dot-based polyvinyl alcohol flame retardant film as claimed in claim 9 in the preparation of flame retardant film materials.

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