Preparation of colorless high-transparency flame-retardant phosphonated nanocellulose reinforced pva film based on secondary swelling

Abstract

The application provides a kind of colorless high-transparency flame-retardant phosphonated nanocellulose reinforced PVA film based on secondary swelling, which is prepared by the following steps: firstly, cellulose is preliminarily swelled by NaOH solution; then, a certain amount of urea and NH4H2PO4 are added and effectively mixed with cellulose; after drying the moisture, solid-phase reaction is carried out; while the secondary swelling is carried out, the subsequent phosphonation degree is increased; then, homogeneous treatment is carried out; finally, high-phosphorus-content phosphonated nanocellulose is prepared; as a filler, flame-retardant and colorless transparent reinforced PVA film can be prepared by solution molding. By the secondary swelling treatment, the problem of yellowing and blackening of cellulose during the phosphonation reaction process is prevented; under the premise of improving the mechanical properties and not damaging the transparency, the prepared reinforced PVA film can achieve better flame-retardant effect by adding a smaller amount of flame-retardant component.

Description

Technical Field

[0001] The invention belongs to the technical field of nanocellulose reinforced PVA film technology and relates to a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film prepared based on secondary swelling. Background Art

[0002] In recent years, environmentally friendly polymer materials have attracted increasing research attention due to their potential significant environmental benefits. Polyvinyl alcohol (PVA), as an environmentally friendly polymer, possesses excellent mechanical properties, excellent oil and solvent resistance, and is non-toxic. It can also be produced industrially through non-petroleum-based methods, offering broad application prospects in packaging, soft electronics, electronic information, and other fields. However, PVA has a low limiting oxygen index of only 19-20%, making it flammable and prone to molten droplets, making it a fire hazard. To facilitate its application, the prior art has disclosed a number of flame retardants that can be applied to PVA, including halogen flame retardants, phosphorus-based flame retardants, and inorganic salt flame retardants. However, among these flame retardants, halogen flame retardants have been gradually banned due to their potential toxicity. While some classic halogen-free flame retardants, such as ammonium polyphosphate and magnesium hydroxide, are effective for PVA, they require large additions (>25 wt%). Furthermore, due to their poor compatibility with the PVA matrix, these flame retardants are prone to agglomeration and precipitation, leading to problems such as decreased mechanical properties and low flame retardancy. Developing flame retardants that are highly efficient, environmentally friendly, and have no side effects (especially on mechanical properties and transparency) has always been an ideal flame retardant solution.

[0003] In existing research, the excellent performance and environmental friendliness of bio-based flame retardants have been proven. Bio-based materials are a renewable resource that can be regenerated by photosynthesis. Cellulose, as the most abundant biopolymer in nature, produces 1.1 billion tons of biomass per year. 10 ~1.10 11 Tons, present in various plants, bacteria, and fungi. Cellulose contains a large number of hydroxyl groups and has the characteristics of easy functionalization: there are three active hydroxyl functional groups at the C-2, C-3 and C-6 positions. The phosphorylation of cellulose is the most widely studied modification method. The phosphorylation reaction of cellulose can be achieved by hydroxyl functionalization with phosphoric acid or its salts, phosphoric acid, polyphosphorus compounds, phosphorus pentoxide or phosphorus pentasulfide, halogenated derivatives, phosphoric acid anhydride, or by grafting phosphorus-containing functional groups, which can improve the carbon-forming ability of cellulose and thus improve its flame retardant properties. For example: Khakalo et al. [1] Enzyme-assisted pulp decomposition is carried out under mild mechanical treatment, and the fiberized material is effectively mixed with (NH4)2HPO4 in the presence of urea, followed by cellulose phosphorylation reaction. The phosphorylated cellulose prepared is used to prepare flame-retardant fiber paper.

[0004] However, in the above methods and existing literature, the prepared phosphorylated cellulose usually has serious yellowing and blackening phenomena, and the transparency of the product is also seriously affected. In order to solve the above problems, Xie et al. [2] A transparent phosphorylated cellulose film was prepared by a similar method. To ensure high transparency of the film (above 80%), the phosphorus content of the sample could only reach a maximum of 6.17%, making it difficult to use as a flame retardant.

[0005] Based on the above research, it was found that in the existing technology, the phosphorus content of phosphorylated cellulose and its easy degradation and discoloration have become a "seesaw" problem. If a colorless and transparent phosphorylated nanocellulose reinforced PVA film can be prepared without reducing the phosphorus content of phosphorylated cellulose, it will provide new ideas and solutions for flame-retardant reinforced PVA film.

[0006] [1] Khakalo, A.; Jaiswal, AK; Kumar, V.; Gestranius, M.; Kangas, H.; Tammelin, T., Production of High-Solid-Content Fire-Retardant Phosphorylated Cellulose Microfibrils.ACS Sustainable Chemistry&Engineering 2021,9(36),12365-12375.

[0007] [2] Xie Hong. Preparation of phosphorylated nanocellulose and research on the properties of its nanopaper. Master, 2021. Summary of the Invention

[0008] In order to solve the problems in the above-mentioned prior art, the present invention provides a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film prepared based on secondary swelling. The preparation method uses NaOH solution to initially swell the cellulose, then adds a certain amount of urea and NH4H2PO4 to effectively mix with the cellulose, and after drying the moisture, undergoes a solid-phase reaction. During the secondary swelling, the subsequent phosphorylation degree is increased (the P element content is 10-12.2%), and then undergoes homogenization to prepare phosphorylated nanocellulose with a high phosphorus content. As a filler, a flame-retardant, colorless and transparent reinforced PVA film can be prepared. While improving mechanical properties and not destroying transparency, a better flame retardant effect can be achieved by adding a smaller amount of flame retardant components.

[0009] To achieve the above objectives, the present invention is implemented by adopting a technical solution consisting of the following technical measures.

[0010] The present invention provides a method for preparing a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film based on secondary swelling, which mainly comprises the following steps:

[0011] (1) immersing cellulose in a NaOH solution with a mass concentration of 2 to 15 wt% and stirring at a temperature of 30 to 80° C. for 10 to 60 minutes to obtain a primary swollen cellulose suspension;

[0012] (2) the once swollen cellulose suspension obtained in step (1) is fully mixed with the phosphorylation modifier, after which water is removed and the mixture is reacted in a closed environment at a temperature of 140 to 160° C. for 0.5 to 2 hours. After the reaction time is up, the mixture is cooled, the solid is separated, and the solid is washed to obtain phosphorylated cellulose;

[0013] The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the primary swollen cellulose suspension is 1:(1.8-3.6):(4.8-9.8);

[0014] (3) preparing a phosphorylated cellulose suspension having a mass concentration of 3 to 5 wt% by preparing the phosphorylated cellulose obtained in step (2), and homogenizing the phosphorylated cellulose suspension to obtain a phosphorylated nanocellulose suspension;

[0015] (4) completely dissolving PVA in deionized water to prepare a PVA aqueous solution with a mass concentration of 5 to 15 wt%, mixing the phosphorylated nanocellulose suspension obtained in step (3) with the PVA aqueous solution as a film-forming slurry, and performing film-forming based on a conventional solution method to obtain a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film;

[0016] The mass ratio of phosphorylated nanocellulose to PVA in the membrane-making slurry is 1:(5-20).

[0017] In this article, the cellulose in step (1) can be obtained homemade or directly from the market. Those skilled in the art can select a suitable source of cellulose based on cost and production process requirements.

[0018] In one technical solution, the cellulose in step (1) is prepared by extracting biomass materials. Generally speaking, the biomass materials are crop materials collected during agricultural production, such as waste parts (such as straw) of crops such as wheat, corn, rice, barley, cotton, soybeans, and sugarcane.

[0019] It should be noted that the aforementioned cellulose is extracted from biomass materials. The specific process / method for extracting cellulose from biomass materials is currently documented in relevant technical literature. Those skilled in the art can directly refer to the content of such relevant technical literature or directly purchase cellulose extracted from biomass materials. It should be emphasized that in the embodiments herein, the cellulose utilized is extracted from sugarcane and bamboo pulp, but this does not constitute a sole designation or limitation regarding the source of the cellulose.

[0020] The inventive point of the present invention is that, during the process of conducting experiments to analyze the "seesaw" problem between the phosphorus content of phosphorylated cellulose and its easy degradation and discoloration in the prior art, the inventor accidentally discovered that the NaOH solution commonly used in the process of removing lignin from the biomass material used (sugarcane bamboo pulp used in the experiment) can also swell the cellulose powder well, and the cellulose swollen with the NaOH solution can significantly increase the phosphorus content of the phosphorylated cellulose in the subsequent phosphorylation reaction. At the same time, the high-phosphorus content phosphorylated cellulose prepared does not have the problem of yellowing or blackening, and is a pure white product. The reinforced PVA film prepared using it as a filler retains the colorless and highly transparent properties to the greatest extent.

[0021] Through further analysis, it is speculated that this is because the cellulose swollen with NaOH solution can effectively improve the reaction efficiency of the subsequent phosphorylation reaction, retain the crystal form and crystallinity of cellulose to the greatest extent, and can prepare phosphorylated cellulose with a high phosphorus content at a lower temperature and in a shorter reaction time, thus avoiding the yellowing and blackening of cellulose caused by degradation.

[0022] Based on the above invention, it is important to note that in step (1), the stirring at 30-80°C for 10-60 minutes is required. If the stirring time exceeds 60 minutes, the NaOH solution will excessively swell the cellulose, causing the cellulose crystals to swell and dissolve, reducing the crystallinity of the cellulose and affecting the subsequent preparation of phosphorylated cellulose. More preferably, the stirring is carried out at 50-80°C for 20-45 minutes.

[0023] Based on the above invention, it is important to note that the reaction in step (2) is carried out in a closed environment at a temperature of 140-160° C. for 0.5-2 h. If the reaction time exceeds 2 h or the temperature is higher than 160° C., excessive phosphorylation reaction will occur, and the cellulose will turn yellow and black due to degradation.

[0024] It should be noted that when the primary swelled cellulose suspension obtained in step (1) is subjected to step (2), the primary swelled cellulose suspension needs to be fully mixed with the phosphorylated modifier. Therefore, if the viscosity of the primary swelled cellulose suspension is too high, it will be detrimental to the dispersion effect of the phosphorylated modifier. Those skilled in the art should know that the viscosity of the primary swelled cellulose suspension can be reduced by diluting it. In order to better illustrate the present invention and provide a technical solution for reference, in step (2), the primary swelled cellulose suspension obtained in step (1) is adjusted to a mass concentration of 3 to 10 wt%, and then fully mixed with the phosphorylated modifier. Under laboratory conditions, the sufficient mixing in step (2) can be achieved by mechanical stirring and / or ultrasonic treatment.

[0025] Furthermore, in step (2), the primary swelled cellulose suspension and the phosphorylated modifier are thoroughly mixed and then dehydrated, followed by a reaction in a closed environment at 140-160° C. for 0.5-2 hours. This is essentially a solid-phase reaction in a molten urea state, with the molten urea simultaneously causing a secondary swelling of the cellulose. Typically, the dehydration step involves drying at 60-110° C. under laboratory conditions.

[0026] In this context, the homogenization treatment in step (3) is to further micronize and homogenize the phosphorylated cellulose using a homogenizer. To ensure that the resulting phosphorylated nanocellulose has higher transparency when used to reinforce the PVA film, in one preferred technical solution, the phosphorylated nanocellulose obtained after the homogenization treatment in step (3) has an average length greater than 1000 nm and an average diameter ≤ 5 nm.

[0027] It should be noted that those skilled in the art should be aware that during the homogenization process, the specific concentration of the phosphorylated cellulose suspension and the homogenization process parameters depend on the homogenizer used. Those skilled in the art can directly refer to the specific guidance on how to homogenize cellulose into nanocellulose in the prior art.

[0028] In this article, the Chinese name of the PVA in step (4) is polyvinyl alcohol, and conventional industrial PVA raw materials in this technical field can be directly selected. Those skilled in the art can select appropriate PVA brands according to specific needs and process requirements, or refer to conventional PVA brands in the application field of the final product, especially the selection of PVA raw materials suitable as raw materials for film products in the prior art.

[0029] In one technical solution, the PVA in step (4) preferably has a polymerization degree of 1700 to 2600 and an alcoholysis degree of 88 to 99%.

[0030] In one of the technical solutions, the PVA aqueous solution in step (4) may also be added with additives conventionally used in PVA processing and molding to achieve further functional expansion / process assistance of the product. For the specific selection of additives, those skilled in the art may refer to the existing technology or existing literature, such as defoamers, antioxidants, heat stabilizers, plasticizers, antibacterial agents and other processing aids / functional additives. It is noted that the PVA aqueous solution in step (1) may include or not include additives conventionally used in PVA processing and molding; in the following preferred technical solution and specific implementation, in order to minimize the influencing factors in the comparative experiment, no additives are added to the PVA aqueous solution, but this does not mean that appropriate additives cannot be added to the PVA aqueous solution. However, it should be noted that the addition of the above-mentioned additives is based on not affecting the transparency of the prepared film. When selecting additives, additives that will not cause dyeing / color difference to the product should be selected, and the total amount of additives added should not exceed 10wt%.

[0031] Herein, the film is formed based on a conventional solution method in step (4). Those skilled in the art may refer to conventional process modes / methods for forming films based on a solution method in the prior art.

[0032] In order to better illustrate the present invention and provide a technical solution for reference, the film-making method described in step (4) is based on the conventional solution method, which is to defoam the film-making slurry at a temperature of 25 to 80°C, pour it into a mold and dry it at a temperature of 25 to 80°C. After demolding, a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film is prepared.

[0033] The present invention has the following beneficial effects:

[0034] 1. The present invention provides a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film prepared based on secondary swelling, which has the flame-retardant functionality of a high phosphorus content while retaining the colorless and transparent properties of the PVA film to the greatest extent.

[0035] 2. The present invention provides a method for preparing a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film based on secondary swelling. The secondary swelling treatment prevents the yellowing and blackening of cellulose during the phosphorylation reaction.

[0036] 3. The preparation method of the present invention is simple, and by preparing phosphorylated nanocellulose with a high phosphorus content, a flame-retardant, colorless and transparent reinforced PVA film can be prepared as a filler. On the premise of improving mechanical properties and not destroying transparency, a smaller amount of flame-retardant components can be added to obtain a better flame-retardant effect, and the film is suitable for use as a packaging material. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1Comparative photographs of the cellulose suspension, the once-swelled cellulose suspension, the phosphorylated cellulose suspension, and the phosphorylated nanocellulose suspension in Example 1 of the present invention. From left to right, the cellulose suspension, the once-swelled cellulose suspension, the phosphorylated cellulose suspension, and the phosphorylated nanocellulose suspension were all left to stand for 12 hours.

[0038] Figure 2 The XPS comparison spectra of the phosphorylated cellulose and the cellulose raw materials used in Example 1 and Comparative Example 2 are shown. The phosphorylated cellulose without swelling corresponds to the phosphorylated cellulose prepared in Comparative Example 2, and the phosphorylated cellulose with swelling corresponds to the phosphorylated cellulose prepared in Example 1.

[0039] Figure 3 Comparative photos of phosphorylated cellulose in Example 1 (left) of the present invention and Comparative Example 3 (right).

[0040] Figure 4 The following are UV spectra comparison results and transparency verification images of the thin film samples prepared in Examples 1 to 4 and Comparative Example 1 of the present invention. Figure a shows the UV spectra comparison results of the thin film samples prepared in Examples 1 to 4 and Comparative Example 1, with 0wt%, 5wt%, 10wt%, 12.5wt%, and 15wt% corresponding to the thin film samples prepared in Comparative Example 1, Example 5, Example 4, Example 3, Example 2, and Example 1, respectively. Figure b1 shows the transparency verification image of the thin film sample prepared in Comparative Example 1, and Figure b2 shows the transparency verification image of the thin film sample prepared in Example 2.

[0041] Figure 5 This is a comparison diagram of the XRD results of the raw cellulose, the intermediate phosphorylated cellulose and the phosphorylated nanocellulose in Example 1 of the present invention.

[0042] Figure 6 Graph comparing the results of vertical combustion experiments on the film samples prepared in Example 2 (bottom) of the present invention and Comparative Example 1 (top). DETAILED DESCRIPTION

[0043] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples, but it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than for limiting the claims of the invention. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters for implementation. It is particularly important to point out that all similar replacements and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention. Although it is believed that those of ordinary skill in the art fully understand the following terms, the following definitions are still stated to help illustrate the subject matter disclosed by the present invention.

[0044] The present invention provides a method for preparing a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film based on secondary swelling, which mainly comprises the following steps:

[0045] (1) immersing cellulose in a NaOH solution with a mass concentration of 2 to 15 wt% and stirring at a temperature of 30 to 80° C. for 10 to 60 minutes to obtain a primary swollen cellulose suspension;

[0046] (2) the once swollen cellulose suspension obtained in step (1) is fully mixed with the phosphorylation modifier, after which water is removed and the mixture is reacted in a closed environment at a temperature of 140 to 160° C. for 0.5 to 2 hours. After the reaction time is up, the mixture is cooled, the solid is separated, and the solid is washed to obtain phosphorylated cellulose;

[0047] The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the primary swollen cellulose suspension is 1:(1.8-3.6):(4.8-9.8);

[0048] (3) preparing a phosphorylated cellulose suspension having a mass concentration of 3 to 5 wt% by preparing the phosphorylated cellulose obtained in step (2), and homogenizing the phosphorylated cellulose suspension to obtain a phosphorylated nanocellulose suspension;

[0049] (4) completely dissolving PVA in deionized water to prepare a PVA aqueous solution with a mass concentration of 5 to 15 wt%, mixing the phosphorylated nanocellulose suspension obtained in step (3) with the PVA aqueous solution as a film-forming slurry, and performing film-forming based on a conventional solution method to obtain a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film;

[0050] The mass ratio of phosphorylated nanocellulose to PVA in the membrane-making slurry is 1:(5-20).

[0051] In the present application, the cellulose in step (1) can be obtained by self-preparation or directly purchased from the market. The skilled in the art can choose the appropriate source of cellulose according to the cost and the requirement of production process.

[0052] In one embodiment, the cellulose in step (1) is prepared based on biomass materials. Generally, the biomass materials are the agricultural crop materials collected during the agricultural production process, such as the waste parts (e.g. straw) of crops such as wheat, corn, rice, barley, cotton, soybean, and sugarcane.

[0053] It should be noted that the cellulose prepared based on biomass materials is prepared by a specific process / method for extracting cellulose from biomass materials, which is disclosed in the related technical documents. The skilled in the art can directly refer to the content of the related technical documents or directly purchase the cellulose prepared based on biomass materials. It should be emphasized that the cellulose used in the present application is prepared based on sugarcane bamboo pulp, but this is not the only specified or limited source of cellulose.

[0054] The application point of the present application is that during the analysis of the "seesaw" problem of the phosphorus content and the easy degradation and discoloration of the phosphated cellulose in the prior art, the inventors accidentally found that the NaOH solution commonly used in the process of removing lignin from the biomass material (sugarcane bamboo pulp used in the experiment) can also swell the cellulose powder well. The cellulose swelled by the NaOH solution can significantly improve the phosphorus content of the phosphated cellulose in the subsequent phosphating reaction, and the high-phosphorus-content phosphated cellulose prepared does not have the problem of yellowing and blackening, and is a pure white product. The enhanced PVA film prepared by using the cellulose as a filler retains the properties of colorless and high transparency to the greatest extent.

[0055] Through further analysis, it is speculated that this is because the cellulose swelled by the NaOH solution can effectively improve the reaction efficiency of the subsequent phosphating reaction, maximally retain the crystal form and crystallinity of the cellulose, and can prepare high-phosphorus-content phosphated cellulose at a lower temperature and in a shorter reaction time, thereby avoiding the yellowing and blackening phenomenon caused by the degradation of the cellulose.

[0056] Based on the above invention, it is important to note that in step (1), stirring at 30-80°C for 10-60 minutes is required. If the stirring time exceeds 60 minutes, the NaOH solution will overswell the cellulose, causing the cellulose crystals to swell and dissolve, reducing the crystallinity of the cellulose and affecting the subsequent preparation of phosphorylated cellulose. In one preferred embodiment, stirring at 50-80°C for 20-45 minutes is further preferred.

[0057] Based on the above invention, it is important to note that the reaction in step (2) is carried out in a closed environment at a temperature of 140-160° C. for 0.5-2 h. If the reaction time exceeds 2 h or the temperature is higher than 160° C., excessive phosphorylation reaction will occur, and the cellulose will turn yellow and black due to degradation.

[0058] It should be noted that when the primary swelled cellulose suspension obtained in step (1) is subjected to step (2), the primary swelled cellulose suspension needs to be fully mixed with the phosphorylated modifier. Therefore, if the viscosity of the primary swelled cellulose suspension is too high, it will be detrimental to the dispersion effect of the phosphorylated modifier. Those skilled in the art should know that the viscosity of the primary swelled cellulose suspension can be reduced by diluting it. In order to better illustrate the present invention and provide a technical solution for reference, in step (2), the primary swelled cellulose suspension obtained in step (1) is adjusted to a mass concentration of 3 to 10 wt%, and then fully mixed with the phosphorylated modifier. Under laboratory conditions, the sufficient mixing in step (2) can be achieved by mechanical stirring and / or ultrasonic treatment.

[0059] Furthermore, in step (2), the primary swelled cellulose suspension and the phosphorylated modifier are thoroughly mixed and then dehydrated, followed by a reaction in a closed environment at 140-160° C. for 0.5-2 hours. This is essentially a solid-phase reaction in a molten urea state, with the molten urea simultaneously causing a secondary swelling of the cellulose. Typically, the dehydration step involves drying at 60-110° C. under laboratory conditions.

[0060] In one embodiment, the mass ratio of the solid mass of the primary swelled cellulose suspension to the NH4H2PO4 in step (2) is 1:(1.8-3.6), for example, 1:1.8, 1:1.9, 1:2.0, 1:2.1, 1:2.2, 1:2.3, 1:2.4, 1:2.5, 1:2.6, 1:2.7, 1:2.8, 1:2.9, 1:3.0, 1:3.1, 1:3.2, 1:3.3, 1:3.4, 1:3.5, 1:3.6 or any range or point value therebetween; the mass ratio of the solid mass of the primary swelled cellulose suspension to the urea in step (2) is 1:(4.8-9.8), for example, 1:4.8, 1:4.9, 1:5.0, 1:5.1, 1:5.2, 1:5.3, 1:5. 4, 1:5.5, 1:5.6, 1:5.7, 1:5.8, 1:5.9, 1:6.0, 1:6.1, 1:6.2, 1:6.3, 1:6.4, 1:6.5, 1:6.6, 1:6.7, 1:6.8, 1:6.9, 1:7, 1:7.1, 1:7.2, 1:7.3, 1:7.4, 1:7.5, 1:7.6, 1:7.7, 1:7.8, 1:7.9, 1:8.0, 1:8.1, 1:8.2, 1:8.3, 1:8.4, 1:8.5, 1:8.6, 1:8.7, 1:8.8, 1:8.9, 1:9.0, 1:9.1, 1:9.2, 1:9.3, 1:9.4, 1:9.5, 1:9.6, 1:9.7, 1:9.8 or any range or point value therebetween.

[0061] In this context, the homogenization treatment in step (3) is to further micronize and homogenize the phosphorylated cellulose using a homogenizer. To ensure that the resulting phosphorylated nanocellulose has higher transparency when used to reinforce the PVA film, in one preferred embodiment, the phosphorylated nanocellulose obtained after the homogenization treatment in step (3) has an average length greater than 1000 nm and an average diameter ≤ 5 nm.

[0062] It should be noted that those skilled in the art should be aware that during the homogenization process, the specific concentration of the phosphorylated cellulose suspension and the homogenization process parameters depend on the homogenizer used. Those skilled in the art can directly refer to the specific guidance on how to homogenize cellulose into nanocellulose in the prior art.

[0063] In this article, the Chinese name of the PVA in step (4) is polyvinyl alcohol, and conventional industrial PVA raw materials in this technical field can be directly selected. Those skilled in the art can select appropriate PVA brands according to specific needs and process requirements, or refer to conventional PVA brands in the application field of the final product, especially the selection of PVA raw materials suitable as raw materials for film products in the prior art.

[0064] In one embodiment, the PVA in step (4) preferably has a degree of polymerization of 1700 to 2600 and a degree of alcoholysis of 88 to 99%.

[0065] In one embodiment, the PVA aqueous solution in step (4) may also be added with additives conventionally used in PVA processing and molding to achieve further functional expansion / process assistance of the product. For the specific selection of additives, those skilled in the art may refer to the existing technology or existing literature, such as defoaming agents, antioxidants, heat stabilizers, plasticizers, antibacterial agents and other processing aids / functional additives. It is noted that the PVA aqueous solution in step (1) may include or not include additives conventionally used in PVA processing and molding; in the following preferred technical solutions and specific embodiments, in order to minimize the influencing factors in the comparative experiment, no additives are added to the PVA aqueous solution, but this does not mean that appropriate additives cannot be added to the PVA aqueous solution. However, it should be noted that the addition of the above-mentioned additives is based on not affecting the transparency of the prepared film. When selecting additives, additives that will not cause dyeing / color difference to the product should be selected, and the total amount of additives added should not exceed 10wt%.

[0066] Herein, the film is formed based on a conventional solution method in step (4). Those skilled in the art may refer to conventional process modes / methods for forming films based on a solution method in the prior art.

[0067] In order to better illustrate the present invention and provide an embodiment for reference, the film-forming process described in step (4) is based on a conventional solution method, wherein the film-forming slurry is defoamed at a temperature of 25 to 80°C, poured into a mold, and then dried at a temperature of 25 to 80°C. After demolding, a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film is prepared.

[0068] The present invention will be further explained in detail below with reference to the examples. However, it will be appreciated by those skilled in the art that these examples are provided for illustrative purposes only and are not intended to limit the present invention.

[0069] Example

[0070] The embodiments of the present application will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are merely illustrative of the present application and should not be considered as limiting the scope of the present application. In the examples, if no specific conditions are indicated, the conditions are carried out according to normal conditions or manufacturer recommendations. The reagents used or the instruments that are not indicated by the manufacturer are conventional products that can be obtained commercially. The application should not be construed as being limited to the specific examples described.

[0071] 1. Raw materials

[0072] Polyvinyl alcohol (PVA) (degree of polymerization 1700, degree of alcoholysis 99%), supplied by Sichuan Weilun Co., Ltd.

[0073] The cellulose was obtained from commercially available sugarcane pulp as raw material;

[0074] NH4H2PO4, urea, and NaOH were provided by Aladdin;

[0075] Distilled water was used throughout the experiment.

[0076] 2. Test Method

[0077] 1) Tensile properties test

[0078] The film samples were cut into dumbbell-shaped specimens and subjected to tensile testing at a speed of 20 mm / min using an electronic universal material testing machine.

[0079] 2) Scanning electron microscope test (SEM)

[0080] The micromorphology of cellulose, film samples, and carbon residue was analyzed using an Apreo S HiVo SEM at 10 kV. All samples were plated with platinum before testing. Before cross-sectional analysis, the samples were subjected to brittle fracture by immersion in liquid nitrogen. The surface element content of the film cross-section and carbon residue was determined using EDS (Octane Elect Super).

[0081] 3) X-ray photoelectron spectroscopy (XPS)

[0082] The composition of the surface chemical bonds of phosphorylated cellulose and residual carbon samples was analyzed. The original spectra were corrected and calibrated with the carbon-carbon single bond peak at 284.6 eV, and quantitative analysis was performed using CasaXPS software.

[0083] 4) Homogenization

[0084] Homogenization was performed using a microfluidizer (NOOZLE Nano) at a pressure of 6000 psi.

[0085] 5) Limiting Oxygen Index (LOI)

[0086] According to the standard of ASTM D 2863-77, the LOI value was measured using the LOI tester (Nanjing Jiangning Analytical Instrument Co., Ltd.) with a size of 100 × 50 × 0.1 mm. 3 .

[0087] 6) Vertical combustion test

[0088] The flame retardant grade is tested by UL-94 (Dongguan Jinte Instrument Co., Ltd.), in accordance with GB / T 2408-2008 standard, and the dimensions are 125.0×50.0×0.1mm 3 .

[0089] 7) Conical Combustion Calorimetry

[0090] According to ISO 5660 standard, the flame retardant properties of the film samples were characterized using a British FTT-0476 cone calorimeter (size 100×100×4mm3).

[0091] 8) Transmission electron microscopy (TEM)

[0092] Transmission electron microscopy (TEM) images of phosphorylated nanocellulose were obtained using a Tecnai G2 F20 S-TWIN (FEI, USA) at an accelerating voltage of 60 kV. Samples were prepared by dropping aqueous sample solutions onto copper grids and stained with 2% phosphotungstic acid negative stain for 3 minutes before testing.

[0093] 9) Thermogravimetric analysis-infrared analysis

[0094] The gaseous volatiles at 50–900 °C were analyzed using TG-FTIR (TGA8000-FTIR spectrometer, Perkin Elmer) at a heating rate of 30 °C / min in an air atmosphere.

[0095] 10) Ultraviolet-visible spectrophotometer (UV)

[0096] The UV spectra of the film samples were obtained using a Shimadzu UV-1750 UV-visible spectrophotometer with a slit width of 2 mm.

[0097] 11) Thermogravimetric analysis (TGA)

[0098] The thermal stability of the samples was measured using TAQ50 (TA Instrument Co. Ltd, New Castle, DE, USA) in a nitrogen atmosphere.

[0099] 3. Preparation Method

[0100] (1) immersing cellulose in a NaOH solution with a mass concentration of 2 wt% and stirring at 60°C for 30 min. After the time is up, a primary swollen cellulose suspension with a cellulose mass concentration of 10 wt% is obtained;

[0101] (2) The once swollen cellulose suspension obtained in step (1) and the phosphorylation modifier are thoroughly mixed and then dried at 105° C. to remove moisture. The mixture is reacted in a closed environment at 145° C. for 1.5 hours. After the reaction time is up, the mixture is cooled, vacuum filtered, and the solid is separated and washed to obtain phosphorylated cellulose.

[0102] The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the primary swollen cellulose suspension is 1:1.8:7.35;

[0103] (3) preparing a 4 wt% phosphorylated cellulose suspension by mixing the phosphorylated cellulose obtained in step (2) with water, and homogenizing the mixture to obtain a phosphorylated nanocellulose suspension;

[0104] (4) PVA is completely dissolved in deionized water to prepare a PVA aqueous solution with a mass concentration of 10 wt%, the phosphorylated nanocellulose suspension obtained in step (3) is mixed with the PVA aqueous solution as a film-forming slurry, and a film is formed based on a conventional solution method to obtain a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film;

[0105] Wherein, the mass ratio of phosphorylated nanocellulose to PVA in the film-making slurry is 1:(5-20);

[0106] The film-making method based on the conventional solution method is to vacuum defoam the film-making slurry at a temperature of 80°C, pour it into a mold and dry it at a temperature of 60°C. After demolding, a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film is prepared as a film sample. By controlling the mass of the solid matter therein, the thickness of the prepared film sample is controlled at 80 to 100 μm.

[0107] Example 1

[0108] Example 1 is a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film prepared as a film sample according to the above-mentioned "3. Preparation method", wherein the mass ratio of phosphorylated nanocellulose to PVA in the film-making slurry in step (4) is 1:5.67, that is, the mass concentration of phosphorylated nanocellulose in the solid matter of the film-making slurry is 15wt%.

[0109] Example 2

[0110] Example 2 is a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film prepared as a film sample according to the above-mentioned "3. Preparation method", wherein the mass ratio of phosphorylated nanocellulose to PVA in the film-making slurry in step (4) is 1:7, that is, the mass concentration of phosphorylated nanocellulose in the solid matter of the film-making slurry is 12.5wt%.

[0111] Example 3

[0112] Example 3 is a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film prepared as a film sample according to the above "3. Preparation method", wherein the mass ratio of phosphorylated nanocellulose to PVA in the film-making slurry in step (4) is 1:9, that is, the mass concentration of phosphorylated nanocellulose in the solid matter of the film-making slurry is 10wt%.

[0113] Example 4

[0114] Example 4 is a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film prepared as a film sample according to the above "3. Preparation method", wherein the mass ratio of phosphorylated nanocellulose to PVA in the film-making slurry in step (4) is 1:19, that is, the mass concentration of phosphorylated nanocellulose in the solid matter of the film-making slurry is 5wt%.

[0115] Verification Example 1

[0116] In order to verify the swelling effect of the secondary swelling in the present invention, cellulose and water were mixed to prepare a cellulose suspension with a cellulose concentration of 1 wt% as verification sample 1;

[0117] The cellulose was immersed in a NaOH solution with a mass concentration of 2 wt% and stirred at 60°C for 30 minutes. After the time was up, a primary swelled cellulose suspension with a cellulose mass concentration of 1 wt% was obtained as the second verification sample.

[0118] A primary swelled cellulose suspension having a cellulose mass concentration of 1 wt% was prepared according to step (2) of the above “3. Preparation Method” to obtain phosphorylated cellulose, and the cellulose suspension was mixed with water to prepare a phosphorylated cellulose suspension having a phosphorylated cellulose mass concentration of 1 wt% as verification sample three;

[0119] The phosphorylated nanocellulose suspension obtained by homogenizing a 1 wt% phosphorylated cellulose suspension was used as verification sample four.

[0120] Comparative Example 1

[0121] In Comparative Example 1, without adding phosphorylated nanocellulose, PVA was directly prepared based on a conventional solution method to obtain a PVA film with the same thickness of 80 to 100 μm as a comparative sample.

[0122] Comparative Example 2

[0123] In Comparative Example 2, without undergoing a single swelling treatment with a NaOH solution, cellulose was mixed with water to prepare a cellulose suspension having a cellulose mass concentration of 10 wt%. The cellulose suspension and the phosphorylation modifier were thoroughly mixed, then dried at 105° C. to remove moisture. The mixture was then reacted in a closed environment at 145° C. for 1.5 h. After the reaction time expired, the mixture was cooled, separated by vacuum filtration, and washed to obtain phosphorylated cellulose.

[0124] The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the cellulose suspension is 1:1.8:7.35.

[0125] Comparative Example 3

[0126] In Comparative Example 3, the cellulose was not swollen once with the NaOH solution. In order to prepare phosphorylated cellulose having the same phosphorus content as in Examples 1 to 4, the following steps were specifically performed:

[0127] The cellulose suspension is prepared by mixing cellulose and water to form a cellulose mass concentration of 10 wt %, and the cellulose suspension and the phosphorylation modifier are thoroughly mixed and then dried at 105° C. to remove moisture. The mixture is reacted in a closed environment at 170° C. for 1.5 hours. After the reaction time is up, the mixture is cooled, vacuum filtered, and the solid is separated and washed to obtain phosphorylated cellulose.

[0128] The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the cellulose suspension is 1:1.8:7.35.

[0129] The phosphorus content of the prepared phosphorylated cellulose is about 10%.

[0130] Comparative Example 4

[0131] In Comparative Example 4, the cellulose was not swollen once with the NaOH solution. In order to prepare phosphorylated cellulose having the same phosphorus content as in Examples 1 to 4, the following steps were specifically performed:

[0132] The cellulose suspension is prepared by mixing cellulose and water to form a cellulose mass concentration of 10 wt %, and the cellulose suspension and the phosphorylation modifier are thoroughly mixed and then dried at 105° C. to remove moisture. The mixture is reacted in a closed environment at 145° C. for 3 hours. After the reaction time is up, the mixture is cooled, separated by vacuum filtration, and washed to obtain phosphorylated cellulose.

[0133] The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the cellulose suspension is 1:1.8:7.35.

[0134] The phosphorus content of the prepared phosphorylated cellulose is about 10%.

[0135] 4. Test Results

[0136] like Figure 1 As shown, in Verification Example 1, the pretreatment with NaOH solution causes the cellulose to swell, which can further promote the phosphorylation modification of the cellulose.

[0137] like Figure 2 As shown, the same phosphorylation reaction was performed on cellulose with and without a NaOH solution swelling by XPS. Quantitative analysis showed that the phosphorus content of the phosphorylated cellulose without a NaOH solution swelling was only about 4-6%, while after a single swelling, the phosphorus content increased greatly to about 10-12.2%.

[0138] like Figure 3 As shown, in Comparative Examples 3 and 4, the phosphorylated cellulose with the same phosphorus content as in Examples 1-4 could only be obtained by increasing the reaction temperature or extending the reaction time. However, excessive phosphorylation often causes cellulose decomposition, resulting in yellowing and blackening. However, after a single swelling treatment with a NaOH solution, the phosphorylated cellulose produced retained its original color.

[0139] Adding flame retardants to traditional flame retardant methods will cause the transparency of film products to decrease significantly. Figure 4 As shown, the UV spectra and images show that the flame-retardant PVA film samples prepared in Examples 1-4 of the present invention are colorless and have excellent transparency. Within the wavelength range of 500-800 nm, the film sample containing 12.5 wt% phosphorylated nanocellulose has a transparency of 82.5%, nearly comparable to pure PVA film (86.0%). The film sample containing 12.5 wt% phosphorylated nanocellulose can pass the UL-94 V-0 rating and achieve an LOI value of 28%.

[0140] like Figure 5 As shown in the figure, it can be seen that the cellulose, phosphorylated cellulose and phosphorylated nanocellulose after homogenization that have undergone a single swelling treatment with NaON all retain the characteristic peaks of the original cellulose, and their crystal structures remain consistent. However, due to mechanical and physical effects, the diffraction peak of phosphorylated nanocellulose at 34.6° disappears, indicating that the mechanical effect has destroyed its crystallinity.

[0141] like Figure 6 As shown, the flame retardancy of the film samples of Example 2 and Comparative Example 1 was compared using the UL-94 test. Due to the flammability of PVA and the thinness of the film, it did not receive a UL-94 rating. However, the addition of phosphorylated nanocellulose improved the flame retardancy of the reinforced PVA film sample, achieving a VTM-0 rating for self-extinguishing upon leaving a flame, ultimately passing the UL-94 test.

[0142] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film based on secondary swelling, characterized in that The main steps include: (1) Immerse the cellulose in a NaOH solution with a mass concentration of 2-15 wt% and stir at a temperature of 30-80 °C for 10-60 min. After the time is up, a primary swollen cellulose suspension is obtained; (2) The once swollen cellulose suspension obtained in step (1) is fully mixed with the phosphorylation modifier, and then the water is removed. The mixture is reacted in a closed environment at a temperature of 140-160° C. for 0.5-2 h. After the reaction time is up, the solid is cooled and separated, and then washed to obtain phosphorylated cellulose; The phosphorylation modifier is prepared by mixing NH4H2PO4 and urea, and the ratio of the solid mass to the mass of NH4H2PO4 and urea in the primary swollen cellulose suspension is 1:(1.8-3.6):(4.8-9.8); (3) preparing a phosphorylated cellulose suspension having a mass concentration of 3 to 5 wt% by weight by preparing the phosphorylated cellulose obtained in step (2), and homogenizing the phosphorylated cellulose suspension to obtain a phosphorylated nanocellulose suspension; (4) PVA is completely dissolved in deionized water to prepare a PVA aqueous solution with a mass concentration of 5 to 15 wt%, the phosphorylated nanocellulose suspension obtained in step (3) is mixed with the PVA aqueous solution as a film-making slurry, and a film is formed based on a conventional solution method to obtain a colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film; The mass ratio of phosphorylated nanocellulose to PVA in the membrane slurry is 1:(5-20).

2. The method according to claim 1, wherein: The cellulose in step (1) is prepared by extracting biomass materials.

3. The method according to claim 1, wherein: In step (1), the cellulose is immersed in a NaOH solution with a mass concentration of 2-15 wt% and stirred at a temperature of 50-80° C. for 20-45 minutes.

4. The method according to claim 1, wherein: The phosphorylated nanocellulose obtained after homogenization in step (3) has an average length of >1000 nm and an average diameter of ≤5 nm.

5. The method according to claim 1, wherein: The PVA in step (4) is a PVA with a degree of polymerization of 1700-2600 and a degree of alcoholysis of 88-99%.

6. The method according to claim 1, wherein: The PVA aqueous solution in step (4) also includes auxiliary agents used in PVA processing and molding, and the total amount of the auxiliary agents added is not higher than 10 wt%.

7. A colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film prepared by the method for preparing a colorless, highly transparent, flame-retardant, phosphorylated nanocellulose reinforced PVA film based on secondary swelling as described in claim 1.

8. Use of the colorless, highly transparent, flame-retardant phosphorylated nanocellulose reinforced PVA film as claimed in claim 7 as a packaging material.

Images