Environment-friendly flame-retardant polyvinyl alcohol film and preparation method thereof

Abstract

The invention discloses an environment-friendly flame-retardant polyvinyl alcohol film and a preparation method thereof, and relates to the technical field of polyvinyl alcohol films. The polyvinyl alcohol film is prepared from the following components in parts by weight: 90 to 110 parts of polyvinyl alcohol, 0.4 to 1.2 parts of polydopamine modified cellulose nanocrystals, 8 to 15 parts of chitosan / phytic acid / boron nitride composite flame retardant and 0.6 to 0.9 part of plasticizer. The preparation method comprises the following steps: adding polyvinyl alcohol into deionized water, dissolving and mixing to obtain a polyvinyl alcohol aqueous solution, adding glycerol, continuously stirring for 1-2 hours, sequentially adding a chitosan / phytic acid / boron nitride composite flame retardant and polydopamine modified cellulose nanocrystals, and uniformly stirring and mixing at 50-70 DEG C to obtain a blended solution; and carrying out film casting on the blended solution. The limit oxygen index of the film is greater than or equal to 30%, the vertical combustion grade is V-0, the tensile strength is greater than or equal to 40%, the elongation at break is greater than or equal to 140%, and the light transmittance is greater than or equal to 85%.

Description

Technical Field

[0001] The present invention relates to the technical field of polyvinyl alcohol films, and in particular to an environmentally friendly flame-retardant polyvinyl alcohol film and a preparation method thereof. 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, exhibits excellent mechanical properties, excellent oil and solvent resistance, and is non-toxic. Furthermore, it can be produced industrially through non-petroleum-based methods, offering broad application prospects in packaging, soft electronics, and electronic information technology. However, PVA has several drawbacks that limit its application. First, PVA is flammable—due to its composition and chemical structure, its limiting oxygen index is relatively low, only 19-20%. Furthermore, combustion produces large amounts of black smoke and dripping molten droplets, significantly limiting its practical application in areas such as construction. Second, PVA has limited toughness—it is prone to cracking during processing, and its relatively low elongation poses challenges for film production. Third, PVA's high hydroxyl content makes it water-sensitive—it swells and dissolves upon contact with water, thus limiting its application in humid environments.

[0003] For better application, many flame retardants that can be applied to PVA have been disclosed in the prior art, including halogen flame retardants, phosphorus flame retardants, and inorganic salt flame retardants. However, among these flame retardants, halogen flame retardants have gradually been banned due to their potential toxicity, and some classic halogen-free flame retardants such as ammonium polyphosphate and magnesium hydroxide are effective for PVA, but need to be added in large quantities (>25wt%), and due to the poor compatibility of flame retardants with the PVA matrix, they are easy to agglomerate and precipitate, resulting in problems such as decreased mechanical properties of the product and low flame retardant durability. The development of flame retardants that are efficient, environmentally friendly, and have no side effects (especially on mechanical properties and transparency) has always been an ideal flame retardant solution. Based on this, there is an urgent need for a high-performance flame retardant material to further improve the flame retardant ability of polyvinyl alcohol film while reducing its potential pollution. Summary of the Invention

[0004] The purpose of the present invention is to provide an environmentally friendly flame-retardant polyvinyl alcohol film and a preparation method thereof, so as to solve the problems of insufficient flame retardancy and poor environmental performance of the polyvinyl alcohol film.

[0005] The purpose of the present invention can be achieved through the following technical solutions:

[0006] In a first aspect, an environmentally friendly flame-retardant polyvinyl alcohol film comprises the following raw materials in parts by weight:

[0007] 90-110 parts of polyvinyl alcohol, 0.4-1.2 parts of polydopamine-modified cellulose nanocrystals, 8-15 parts of chitosan / phytic acid / boron nitride composite flame retardant, and 0.6-0.9 parts of plasticizer.

[0008] As a further embodiment of the present invention, the preparation method of the chitosan / phytic acid / boron nitride composite flame retardant comprises the following steps:

[0009] S1.Quaternized chitosan:

[0010] Dissolve chitosan in acetic acid solution, mix well, add epoxypropyltrimethylammonium chloride (ETA), stir and react at 55-65°C for 12-24 hours, precipitate with ethanol, centrifuge, wash, and freeze-dry to obtain quaternized chitosan powder;

[0011] The amino group (-NH2) in the chitosan molecule undergoes a ring-opening reaction with the epoxy group of glycidyltrimethylammonium chloride to generate quaternized chitosan. By introducing quaternary ammonium cationic groups, the water solubility of chitosan and its electrostatic interaction ability with phytic acid are enhanced; the cationic properties of quaternized chitosan can subsequently combine with phytic acid (polyanion) through electrostatic interaction to form a stable complex.

[0012] S2. Boron Nitride Functionalization:

[0013] Add boron nitride to deionized water, disperse it evenly by ultrasonication, then add 3-aminopropyltriethoxysilane (APTES), and stir and react at 55-65°C for 2-3 hours to obtain a functionalized boron nitride dispersion;

[0014] The boron nitride (BN) surface is silanized with a silane coupling agent to graft amino groups (-NH2). The hydrolyzed silanol groups (-Si-OH) of APTES condense with hydroxyl groups (-OH) on the BN surface to form Si-OB bonds, simultaneously introducing terminal amino groups. This functionalized BN exhibits improved dispersibility, and the amino groups further interact with phytic acid or quaternized chitosan, enhancing the interfacial bonding of the composite.

[0015] S3. Add quaternized chitosan to deionized water, add phytic acid, stir and mix thoroughly, then add functionalized boron nitride dispersion, ultrasonicate for 2-3 hours, collect the product by centrifugation, and freeze-dry to obtain a chitosan / phytic acid / boron nitride composite flame retardant.

[0016] Quaternized chitosan (positively charged) and phytic acid (negatively charged) form a polyelectrolyte complex through electrostatic attraction. Functionalized BN is embedded in the chitosan-phytic acid network through hydrogen bonding or electrostatic interactions, forming a three-dimensional composite structure. Phytic acid acts as an acid and gas source, quaternized chitosan as a carbon source, and BN as a thermal barrier, all three synergistically exerting an expansion flame retardant and thermal conductivity-inhibiting mechanism.

[0017] Furthermore, in S1, the mass percentage of the acetic acid solution is 1-3%.

[0018] Furthermore, in S1, the ratio of chitosan, acetic acid solution and epoxypropyltrimethylammonium chloride is 1 g:50 mL:1.5-2.0 g.

[0019] Furthermore, in S2, the usage ratio of the boron nitride, deionized water, and 3-aminopropyltriethoxysilane is 0.1 g:50 mL:0.05-0.1 g.

[0020] Furthermore, in S3, the usage ratio of the quaternized chitosan, deionized water, phytic acid, and functionalized boron nitride dispersion is 1 g:50 mL:1 g:50 mL.

[0021] As a further embodiment of the present invention, the method for preparing polydopamine-modified cellulose nanocrystals comprises the following steps:

[0022] Microcrystalline cellulose is dispersed in deionized water to obtain a uniform suspension; dopamine hydrochloride is added to the suspension, and the pH value is adjusted to 8-9 with Tris-HCl buffer, and the suspension is stirred at room temperature (25-30° C.) for 20-28 hours, and the suspension is centrifuged, purified, and freeze-dried to obtain polydopamine-modified cellulose nanocrystals.

[0023] Furthermore, the solid content of the suspension is 1-2 wt %.

[0024] Furthermore, the amount of dopamine hydrochloride used is 10-16 wt % of the amount of microcrystalline cellulose used.

[0025] As a further embodiment of the present invention, the plasticizer is glycerol.

[0026] As a further solution of the present invention, the alcoholysis degree of the polyvinyl alcohol is 99 mol%, including but not limited to any one of polyvinyl alcohols with models 0599, 1799, and 2499.

[0027] In a second aspect, a method for preparing an environmentally friendly flame-retardant polyvinyl alcohol film is provided, which is used to prepare the environmentally friendly flame-retardant polyvinyl alcohol film described in the first aspect, comprising the following steps:

[0028] Polyvinyl alcohol is added to deionized water, dissolved and mixed to obtain a polyvinyl alcohol aqueous solution, and glycerol is added and stirred for 1-2 hours, and then a chitosan / phytic acid / boron nitride composite flame retardant and polydopamine-modified cellulose nanocrystals are added in sequence, and stirred and mixed evenly at 50-70°C to obtain a blended solution; the blended solution is cast into a film to obtain an environmentally friendly flame-retardant polyvinyl alcohol film.

[0029] As a further solution of the present invention, the mass fraction of the polyvinyl alcohol aqueous solution is 15-25%.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. The present invention provides an environmentally friendly flame-retardant polyvinyl alcohol film. A chitosan / phytic acid / boron nitride composite flame retardant is introduced into the polyvinyl alcohol film system. Phytic acid is pyrolyzed to generate phosphoric acid to promote the dehydration of polyvinyl alcohol into charcoal. Quaternized chitosan is used as a carbon source to form an expanded charcoal layer. Boron nitride is dispersed in the charcoal layer through silane modification. Boron nitride utilizes its high thermal conductivity to quickly disperse local heat, lowering the film's combustion temperature, enhancing its thermal stability, and isolating it from heat and oxygen. The nano-enhancement effect of boron nitride and the interfacial interaction of the chitosan-phytic acid network can improve the tensile strength and toughness of the polyvinyl alcohol film, avoiding the degradation of mechanical properties caused by traditional flame retardants. Chitosan and phytic acid are bio-based materials with good compatibility with biodegradable polyvinyl alcohol. The composite flame retardant is non-toxic and easy to disperse evenly, avoiding phase separation. Furthermore, polydopamine-modified cellulose nanocrystals serve as a compatibilizer to further enhance the interfacial compatibility between the composite flame retardant and polyvinyl alcohol, preventing phase separation.

[0032] 2. The environmentally friendly flame-retardant polyvinyl alcohol film prepared by the present invention adopts bio-based raw materials to prepare flame-retardant materials, forming a physical cross-linking network between the polyvinyl alcohol molecular chains. While improving the flame retardant properties, it has good mechanical properties and light transmittance. The film has good biodegradability and is environmentally friendly. DETAILED DESCRIPTION

[0033] The specific embodiments of the present invention are described in detail below, but it should be understood that the protection scope of the present invention is not limited by the specific embodiments.

[0034] The terms used in the examples of this application are for the purpose of describing specific implementation rules only and are not intended to limit this application. The singular forms "a", "an", "the" and "the" used in the implementation rules of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0035] It should be understood that in the various embodiments of the present application, the size of the serial number of each process does not mean the order of execution. Some or all steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the implementation regulations of this application.

[0036] The weights of the relevant components mentioned in the examples of this application may not only refer to the specific content of each component, but also represent the weight ratio between the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the examples of this application, it is within the scope disclosed in the examples of this application. Specifically, the mass described in the examples of this application may be a mass unit known in the chemical industry, such as μg, mg, g, kg, etc.

[0037] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0038] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this application can be purchased from the market or prepared by existing methods; specifically, in the following examples and comparative examples, polyvinyl alcohol is PVA1799; the deacetylation degree of chitosan is ≥90%, and the boron nitride is hexagonal nanosheets.

[0039] Preparation Example 1

[0040] This preparation example provides a method for preparing a chitosan / phytic acid / boron nitride composite flame retardant, comprising the following steps:

[0041] S1.Quaternized chitosan:

[0042] 1 g of chitosan was dissolved in 50 mL of 2% acetic acid solution and mixed well. 1.8 g of epoxypropyltrimethylammonium chloride was added and stirred at 60°C for 20 h. The mixture was ethanol precipitated, centrifuged, washed, and freeze-dried to obtain quaternized chitosan powder.

[0043] S2. Boron Nitride Functionalization:

[0044] 0.1 g of boron nitride was added to 50 mL of deionized water and ultrasonically dispersed uniformly. Then 0.08 g of APTES was added and stirred at 60 ° C for 3 h to obtain a functionalized boron nitride dispersion.

[0045] S3. Add 1 g of quaternized chitosan to 50 mL of deionized water, add 1 g of phytic acid, stir and mix well, then add 50 mL of functionalized boron nitride dispersion, ultrasonicate for 3 h, collect the product by centrifugation, and freeze-dry to obtain a chitosan / phytic acid / boron nitride composite flame retardant.

[0046] Preparation Example 2

[0047] This preparation example provides a method for preparing polydopamine-modified cellulose nanocrystals, comprising the following steps:

[0048] Microcrystalline cellulose was dispersed in deionized water to obtain a uniform suspension with a solid content of 1.5 wt%; dopamine hydrochloride was added to the suspension in an amount of 14 wt% of the mass of the microcrystalline cellulose, and the pH was adjusted to 8.5 with Tris-HCl buffer. The mixture was stirred at room temperature for 24 hours, and then centrifuged, purified, and freeze-dried to obtain polydopamine-modified cellulose nanocrystals.

[0049] Example 1

[0050] An environmentally friendly flame-retardant polyvinyl alcohol film comprises the following raw materials in parts by weight:

[0051] 100 parts of polyvinyl alcohol, 0.8 parts of polydopamine-modified cellulose nanocrystals prepared in Preparation Example 2, 12 parts of the chitosan / phytic acid / boron nitride composite flame retardant prepared in Preparation Example 1, and 0.8 parts of glycerol;

[0052] The method for preparing the environmentally friendly flame-retardant polyvinyl alcohol film comprises the following steps:

[0053] Polyvinyl alcohol was added to deionized water, heated and stirred at 90°C until completely dissolved to obtain a 20% mass fraction of polyvinyl alcohol aqueous solution. After adding glycerol and continuing to stir for 2 hours, chitosan / phytic acid / boron nitride composite flame retardant and polydopamine-modified cellulose nanocrystals were added in sequence, and stirred and mixed evenly at 60°C to obtain a blended solution. The blended solution was ultrasonically degassed for 25 minutes, poured into a casting mold, dried at 50°C for 20 hours, and peeled off to obtain an environmentally friendly flame-retardant polyvinyl alcohol film.

[0054] Example 2

[0055] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that only the weight portion of the polydopamine-modified cellulose nanocrystals prepared in Preparation Example 2 is replaced by 0.4 parts;

[0056] The preparation method is consistent with that in Example 1.

[0057] Example 3

[0058] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that only the weight portion of the polydopamine-modified cellulose nanocrystals prepared in Preparation Example 2 is replaced by 1.2 parts;

[0059] Example 4

[0060] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that only the weight parts of the chitosan / phytic acid / boron nitride composite flame retardant prepared in Preparation Example 1 are replaced with 8 parts;

[0061] The preparation method is consistent with that in Example 1.

[0062] Example 5

[0063] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that only the weight parts of the chitosan / phytic acid / boron nitride composite flame retardant prepared in Preparation Example 1 are replaced with 15 parts;

[0064] The preparation method is consistent with that in Example 1.

[0065] Comparative Preparation Example 1

[0066] This comparative preparation example provides a preparation method for a chitosan / phytic acid / boron nitride composite flame retardant. The method differs from Preparation Example 1 in that the chitosan is not quaternized. The method specifically includes the following steps:

[0067] S1. Boron Nitride Functionalization:

[0068] 0.1 g of boron nitride was added to 50 mL of deionized water and ultrasonically dispersed uniformly. Then 0.08 g of APTES was added and stirred at 60 ° C for 3 h to obtain a functionalized boron nitride dispersion.

[0069] S2. Add 1 g of quaternized chitosan to 50 mL of 2% acetic acid solution, add 1 g of phytic acid, stir and mix thoroughly, then add 50 mL of functionalized boron nitride dispersion. Ultrasonicate for 3 h, collect the product by centrifugation, and freeze-dry to obtain a chitosan / phytic acid / boron nitride composite flame retardant.

[0070] Comparative Preparation Example 2

[0071] This comparative preparation example provides a preparation method for a chitosan / phytic acid / boron nitride composite flame retardant. The method differs from Preparation Example 1 in that the boron nitride is not functionalized. The method specifically includes the following steps:

[0072] S1. Same as Preparation Example 1;

[0073] S2. 0.1 g of boron nitride was added to 50 mL of deionized water and ultrasonically dispersed to obtain a boron nitride dispersion;

[0074] S3. Add 1 g of quaternized chitosan to 50 mL of deionized water, add 1 g of phytic acid, stir and mix well, then add 50 mL of boron nitride dispersion, ultrasonicate for 3 h, collect the product by centrifugation, and freeze-dry to obtain a chitosan / phytic acid / boron nitride composite flame retardant.

[0075] Comparative Preparation Example 3

[0076] This comparative preparation example provides a preparation method for a chitosan / phytic acid composite flame retardant. The difference from Preparation Example 1 is that no functionalized boron nitride is added. The method specifically includes the following steps:

[0077] S1.Quaternized chitosan:

[0078] 1 g of chitosan was dissolved in 50 mL of 2% acetic acid solution and mixed well. 1.8 g of epoxypropyltrimethylammonium chloride was added and stirred at 60°C for 20 h. The mixture was ethanol precipitated, centrifuged, washed, and freeze-dried to obtain quaternized chitosan powder.

[0079] S2. Add 1 g of quaternized chitosan to 50 mL of deionized water, add 1 g of phytic acid, and ultrasonicate for 3 h. Collect the product by centrifugation and freeze-dry to obtain a chitosan / phytic acid composite flame retardant.

[0080] Comparative Example 1

[0081] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that the chitosan / phytic acid / boron nitride composite flame retardant prepared in Preparation Example 1 is replaced by the one prepared in Comparative Preparation Example 1;

[0082] The preparation method is consistent with that in Example 1.

[0083] Comparative Example 2

[0084] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that the chitosan / phytic acid / boron nitride composite flame retardant prepared in Preparation Example 1 is replaced by the one prepared in Comparative Preparation Example 2;

[0085] The preparation method is consistent with that in Example 1.

[0086] Comparative Example 3

[0087] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that the chitosan / phytic acid / boron nitride composite flame retardant prepared in Preparation Example 1 is replaced by the one prepared in Comparative Preparation Example 3;

[0088] The preparation method is consistent with that in Example 1.

[0089] Comparative Example 4

[0090] An environmentally friendly flame-retardant polyvinyl alcohol film, which differs from Example 1 in that the polydopamine-modified cellulose nanocrystals prepared in Preparation Example 2 are replaced by microcrystalline cellulose;

[0091] The preparation method is consistent with that in Example 1.

[0092] The performance tests were conducted on the polyvinyl alcohol films prepared in Examples 1-5 and Comparative Examples 1-4:

[0093] (1) Oxygen index test (LOI): Use an oxygen index meter to test the limiting oxygen index of the sample. The sample size is 120×50×0.5mm 3 , the testing standard is GB / T 2406.2-2009.

[0094] (2) Vertical combustion test (UL-94): Use a horizontal vertical combustion tester to conduct a vertical combustion test on the sample. The sample size is 130×13×0.5mm 3 , the test standard is ASTM D3801.

[0095] (3) Mechanical properties test: GB 13022-1991 plastic film tensile properties test method was used.

[0096] (4) Light transmittance test: refer to ASTM D1003 standard test method for light transmittance and haze of plastics.

[0097] The above test results are shown in Table 1.

[0098] Table 1

[0099] project LOI UL-94 tensile strength Elongation at break Light transmittance unit (％) (grade) (MPa) (％) (％) Example 1 34 V-0 52 151 87 Example 2 33 V-0 46 144 88 Example 3 34 V-0 53 154 85 Example 4 31 V-0 49 148 88 Example 5 34 V-0 50 149 86 Comparative Example 1 25 V-1 39 128 80 Comparative Example 2 28 V-0 42 139 75 Comparative Example 3 26 V-1 40 130 84 Comparative Example 4 26 V-1 35 126 70

[0100] It can be seen from the test results in Table 1 that the mechanical properties, flame retardant properties and light transmittance of the environmentally friendly flame retardant polyvinyl alcohol film prepared by the present invention all reach a good level; compared with Example 1 and Comparative Example 1, since the quaternized chitosan in Example 1 forms a stronger electrostatic interaction with the phytic acid containing a phosphoric acid group through the cationic properties, the phosphorus-nitrogen flame retardant system is more stable and the flame retardant properties are relatively better; the quaternized chitosan can improve the compatibility of the flame retardant with the polyvinyl alcohol matrix, avoid stress concentration, and its mechanical properties are relatively good; the unquaternized chitosan and phytic acid complex is prone to phase separation, forming a microscopic uneven structure, which leads to light scattering. Comparing Example 1 with Comparative Example 2, the silane pretreatment of boron nitride can improve the dispersibility of boron nitride in the polyvinyl alcohol matrix, avoid agglomeration, enhance interfacial compatibility, and reduce light scattering, thereby more effectively exerting the flame retardant effect and improving the mechanical properties and light transmittance; Comparing Example 1 with Comparative Example 3, since the boron nitride with a lamellar structure plays a physical barrier role in the system, when the flame retardant is not added, the flame retardant is only based on the chemical flame retardant mechanism of chitosan / phytic acid, and the flame retardant efficiency is reduced. At the same time, the polyvinyl alcohol matrix lacks the reinforcing effect of nanofillers, and the mechanical properties are reduced; Comparing Example 1 with In comparative example 4, the thermal decomposition products of polydopamine in polydopamine-modified cellulose nanocrystals can promote the formation of a dense carbon layer in the polyvinyl alcohol matrix to isolate heat and oxygen. At the same time, the phenolic hydroxyl groups and amino groups can neutralize free radicals in the combustion process. The unmodified cellulose nanocrystals only serve as inert fillers and lack active flame retardant function. They may even cause local combustion acceleration due to uneven dispersion. In addition, the unmodified cellulose nanocrystals are easily aggregated due to hydrogen bonding to form stress defects. The polydopamine-modified cellulose nanocrystals reduce the refractive index difference with polyvinyl alcohol, thereby reducing light scattering.

[0101] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus.

[0102] The above disclosures are only a few specific embodiments of the present invention. However, the embodiments of the present invention are not limited thereto. Any changes that can be conceived by those skilled in the art should fall within the scope of protection of the present invention.

Claims

1. An environmentally friendly flame-retardant polyvinyl alcohol film, characterized in that: According to parts by weight, it includes the following raw materials: 90-110 parts of polyvinyl alcohol, 0.4-1.2 parts of polydopamine-modified cellulose nanocrystals, 8-15 parts of chitosan / phytic acid / boron nitride composite flame retardant, and 0.6-0.9 parts of plasticizer.

2. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 1, characterized in that: The preparation method of the chitosan / phytic acid / boron nitride composite flame retardant comprises the following steps: S1. Chitosan was dissolved in acetic acid solution and mixed uniformly. Then glycidyltrimethylammonium chloride was added and stirred at 55-65°C for 12-24h. The mixture was precipitated with ethanol, centrifuged, washed, and freeze-dried to obtain quaternized chitosan powder. S2. Boron nitride was added to deionized water and ultrasonically dispersed uniformly. APTES was then added and stirred at 55-65°C for 2-3 hours to obtain a functionalized boron nitride dispersion. S3. Add quaternized chitosan to deionized water, add phytic acid, stir and mix thoroughly, then add functionalized boron nitride dispersion, ultrasonicate for 2-3 hours, collect the product by centrifugation, and freeze-dry to obtain a chitosan / phytic acid / boron nitride composite flame retardant.

3. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 2, characterized in that: In S1, the usage ratio of the chitosan, acetic acid solution, and epoxypropyltrimethylammonium chloride is 1 g:50 mL:1.5-2.0 g.

4. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 2, characterized in that: In S2, the usage ratio of the boron nitride, deionized water, and APTES is 0.1 g:50 mL:0.05-0.1 g.

5. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 2, characterized in that: In S3, the usage ratio of the quaternized chitosan, deionized water, phytic acid, and functionalized boron nitride dispersion is 1 g:50 mL:1 g:50 mL.

6. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 1, characterized in that: The method for preparing polydopamine-modified cellulose nanocrystals comprises the following steps: Microcrystalline cellulose is dispersed in deionized water to obtain a uniform suspension; dopamine hydrochloride is added to the suspension, and the pH value is adjusted to 8-9 with Tris-HCl buffer, and the suspension is stirred at room temperature for 20-28 hours. The suspension is then centrifuged, purified, and freeze-dried to obtain polydopamine-modified cellulose nanocrystals.

7. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 6, characterized in that: The solid content of the suspension is 1-2 wt %; the amount of dopamine hydrochloride used is 10-16 wt % of the amount of microcrystalline cellulose used.

8. The environmentally friendly flame-retardant polyvinyl alcohol film according to claim 1, characterized in that: The alcoholysis degree of the polyvinyl alcohol is 99 mol%.

9. A method for preparing an environmentally friendly flame-retardant polyvinyl alcohol film, for preparing the environmentally friendly flame-retardant polyvinyl alcohol film according to claim 1, characterized in that: The following steps are involved: Add polyvinyl alcohol to deionized water, dissolve and mix to obtain a polyvinyl alcohol aqueous solution, then add glycerol and continue stirring for 1-2 hours, then add chitosan / phytic acid / boron nitride composite flame retardant and polydopamine-modified cellulose nanocrystals in sequence, and stir and mix evenly at 50-70°C to obtain a blend solution; The blended solution is cast into a film to obtain an environmentally friendly flame-retardant polyvinyl alcohol film.

10. The method for preparing an environmentally friendly flame-retardant polyvinyl alcohol film according to claim 9, characterized in that: The mass fraction of the polyvinyl alcohol aqueous solution is 15-25%.