Polyvinyl alcohol composite film, and preparation method and application thereof

Abstract

The application discloses a polyvinyl alcohol composite material film and a preparation method and application thereof. By adding a reinforcing agent cellulose nanocrystal and a crosslinking agent citric acid in the polyvinyl alcohol, the water resistance and ultraviolet shielding performance of the polyvinyl alcohol film are significantly improved, and the tensile strength is also improved. The polyvinyl alcohol composite material film prepared by the application has high transparency under visible light, and the transmittance at 300 nm is only 17.3%, and has high ultraviolet shielding performance. The application prepares the high-performance polyvinyl alcohol composite material film through green materials and a simple process, and has wide application prospects.

Description

Technical Field

[0001] This invention belongs to the field of polyvinyl alcohol composite films, specifically relating to a polyvinyl alcohol composite film, its preparation method, and its application. Background Technology

[0002] Polyvinyl alcohol (PVA), a synthetic polymer with excellent physicochemical stability and biodegradability, has been widely used in textiles, chemicals, construction, agriculture, and medicine due to its good flexibility, ductility, and processability. It is an ideal matrix material with both practical value and environmental potential. However, some problems still exist in the practical application of PVA.

[0003] Polyvinyl alcohol (PVA) has poor water resistance and is easily hygroscopic and soluble. The high hydroxyl content in its molecular chain gives it strong hydrophilicity. Upon contact with moisture, water molecules easily penetrate the molecular chains, disrupting intermolecular forces and altering the material's state. Secondly, PVA has limited functionality, lacking UV shielding. Its molecular chain is a saturated carbon chain, lacking π-π conjugated systems and aromatic rings, thus lacking chromophores that absorb ultraviolet light. These defects make PVA prone to performance degradation and structural failure under humid and light-exposed environments, severely limiting its applications in packaging, protection, and long-term service. Summary of the Invention

[0004] To address the problem that existing polyvinyl alcohol (PVA) composite films cannot simultaneously achieve water resistance and UV resistance, this invention proposes a PVA composite film and its preparation method. This invention introduces cellulose nanocrystals as a reinforcing agent and citric acid as a crosslinking agent into PVA. Citric acid can covalently link PVA and cellulose nanocrystals through esterification, simultaneously introducing a conjugated structure capable of absorbing UV rays. During the reaction, PVA consumes hydroxyl groups, and crosslinking makes the material denser, thereby improving the water resistance and UV shielding performance of PVA. The PVA composite film prepared by this invention exhibits high transparency under visible light, while its transmittance at 300 nm is only 17.3%, demonstrating excellent UV shielding performance. This invention uses green materials and a simple process to prepare a high-performance PVA composite film, which has broad application prospects.

[0005] One of the technical solutions of the present invention is to provide a method for preparing a polyvinyl alcohol composite film, which includes the following steps: (1) Dissolve citric acid in an aqueous dispersion of cellulose nanocrystals, with a mass ratio of citric acid to cellulose nanocrystals of 7:1-1:1; (2) Add the mixed solution obtained in step (1) to the polyvinyl alcohol aqueous solution and mix evenly to obtain a mixed liquid. Then remove the air from the mixed liquid. The mass ratio of polyvinyl alcohol to citric acid is 13-14.2:1. (3) Pour the mixed liquid obtained in step (2) into a mold, heat it in an oven at 60°C for 6 hours, and then heat it in an oven at 150°C for 15 minutes to undergo an esterification reaction to obtain a polyvinyl alcohol composite film.

[0006] Furthermore, the concentration of the cellulose nanocrystal aqueous dispersion is 0.2wt%-1.7wt%.

[0007] Furthermore, the concentration of the polyvinyl alcohol aqueous solution is 6.2-6.6 wt%.

[0008] Furthermore, in step (2), the mixing method is ultrasonic treatment for 30 minutes.

[0009] Furthermore, the method for removing air from the mixed liquid is to evacuate for 1 hour.

[0010] Furthermore, the method for preparing the polyvinyl alcohol aqueous solution is as follows: after dispersing polyvinyl alcohol in deionized water, stir it in an oil bath at 90°C for 1 hour.

[0011] During heat treatment, the hydroxyl groups of polyvinyl alcohol / cellulose nanocrystals undergo esterification with the carboxyl groups of citric acid, forming ester bonds and constructing a dense three-dimensional cross-linked network structure within the composite film. Simultaneously, the cellulose nanocrystals not only function as a reinforcing filler but also participate in the construction of the cross-linked network through their surface hydroxyl groups, thereby enhancing the interfacial interaction between the cellulose nanocrystals and the polyvinyl alcohol matrix. This interconnected network effectively restricts the movement of polyvinyl alcohol molecular chains and strengthens intermolecular interactions, significantly improving the tensile strength and structural stability of the composite film. Furthermore, the consumption of hydrophilic hydroxyl groups during esterification, coupled with the tight cross-linked structure, reduces moisture penetration and molecular diffusion within the film, thus improving waterproof performance and reducing water vapor transmission. In addition, the addition of cellulose nanocrystals and the denser network structure enhance the film's UV protection capability by increasing light scattering and reducing UV transmission. Due to these enhanced mechanical, barrier, and UV protection properties, the cross-linked polyvinyl alcohol / cellulose nanocrystal / citric acid composite film exhibits better preservation performance.

[0012] The second technical solution of the present invention is to provide a polyvinyl alcohol composite film prepared by the above method.

[0013] The third technical solution of the present invention is to provide the above-mentioned polyvinyl alcohol composite film applications, including applications in the field of food packaging and applications in the field of agricultural film.

[0014] The advantages of this invention are: This invention designs a synergistic system of polyvinyl alcohol, cellulose nanocrystals, and citric acid to prepare a polyvinyl alcohol composite film that simultaneously possesses water resistance, UV shielding, and biodegradability. The tensile strength of this material exceeds 30 MPa, the UV blocking rate is as high as 82.7%, and it can be immersed in water for 24 hours without complete decomposition, which is significantly better than pure polyvinyl alcohol films. Attached Figure Description

[0015] Figure 1 This is the stress-strain curve of pure polyvinyl alcohol.

[0016] Figure 2 This is the stress-strain curve of the polyvinyl alcohol composite film in this invention.

[0017] Figure 3 This is the light transmittance curve of a pure polyvinyl alcohol film.

[0018] Figure 4 This is the light transmittance curve of the polyvinyl alcohol composite film in this invention.

[0019] Figure 5 This refers to the expansion degree of the polyvinyl alcohol composite film in this invention. Detailed Implementation

[0020] The following examples are provided to further illustrate the present invention and are intended to explain the invention, not to limit its scope. Unless otherwise specified, all figures are expressed in parts by weight and weight percentages.

[0021] Unless otherwise specified, the raw materials used in this invention are all conventional commercially available products; unless otherwise specified, the methods used in this invention are all conventional methods in the field.

[0022] The embodiments of the present invention will be further described below with reference to several examples.

[0023] It should be understood that the described embodiments are merely some, not all, of the embodiments in this application. All other embodiments obtained by those skilled in the art based on the embodiments in this application without inventive effort are within the scope of protection of this application.

[0024] The terminology used in the embodiments of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application. The singular forms “a,” “the,” and “the” used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0025] Example 1 (1) Dissolve 4.95g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.6wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.05g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 0.2wt% cellulose nanocrystal suspension; (3) Add 0.35g of citric acid to the cellulose nanocrystal suspension and stir for 20min; (4) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min to dissolve the citric acid in the cellulose nanocrystal suspension; (5) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (6) Pour the defoamed liquid into a mold, place it in an oven at 60°C for 6 hours, and then heat it in an oven at 150°C for 15 minutes to obtain a polyvinyl alcohol composite film.

[0026] Example 2 (1) Dissolve 4.85g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.5wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.15g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 0.7wt% cellulose nanocrystal suspension; (3) Add 0.35g of citric acid to the cellulose nanocrystal suspension and stir for 20min to dissolve the citric acid in the cellulose nanocrystal suspension; (4) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (5) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (6) Pour the defoamed liquid into a mold, place it in an oven at 60°C for 6 hours, and then heat it in an oven at 150°C for 15 minutes to obtain a polyvinyl alcohol composite film.

[0027] Example 3 (1) Dissolve 4.75g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.4wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.25g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 1.2wt% cellulose nanocrystal suspension; (3) Add 0.35g of citric acid to the cellulose nanocrystal suspension and stir for 20min to dissolve the citric acid in the cellulose nanocrystal suspension; (4) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (5) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (6) Pour the defoamed liquid into a mold, place it in an oven at 60°C for 6 hours, and then heat it in an oven at 150°C for 15 minutes to obtain a polyvinyl alcohol composite film. Figure 2 The tensile stress and strain of the polyvinyl alcohol composite film in this embodiment are shown. The tensile strength of the polyvinyl alcohol composite film exceeds 30 MPa, which is higher than that of the pure polyvinyl alcohol film, and the elastic modulus is also improved. This performance improvement comes from two aspects: firstly, the reinforcing agent cellulose nanocrystals can be uniformly dispersed in polyvinyl alcohol; secondly, citric acid forms a tight three-dimensional structure with polyvinyl alcohol and cellulose nanocrystals through a cross-linking reaction, which can restrict the slippage and relative movement of the chains and effectively transfer stress from the matrix to the cellulose nanocrystals.

[0028] Example 4 (1) Dissolve 4.65g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.2wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.35g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 1.7wt% cellulose nanocrystal suspension; (3) Add 0.35g of citric acid to the cellulose nanocrystal suspension and stir for 20min to dissolve the citric acid in the cellulose nanocrystal suspension; (4) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (5) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (6) Pour the defoamed liquid into a mold, place it in an oven at 60°C for 6 hours, and then heat it in an oven at 150°C for 15 minutes to obtain a polyvinyl alcohol composite film.

[0029] Comparative Example 1 (1) Dissolve 5g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.7wt% polyvinyl alcohol aqueous solution. (2) The polyvinyl alcohol solution was ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes. (3) Pour the defoamed solution into a mold and place it in an oven at 60°C for 6 hours to obtain a pure polyvinyl alcohol film; Figure 1 The tensile stress-strain diagram is shown for the pure polyvinyl alcohol film in Comparative Example 1.

[0030] The prepared pure polyvinyl alcohol film has a transmittance of up to 67.9% at 300 nm. Figure 3 ), while the transmittance of the polyvinyl alcohol composite film in Example 3 was as low as 17.3% ( Figure 4 This is because citric acid produces derivatives with conjugated structures during heating at 150°C, and the cross-linking effect causes the cellulose nanocrystals to be uniformly dispersed and the film to be dense, thus improving the UV shielding performance of the polyvinyl alcohol composite film.

[0031] Comparative Example 2 (1) Dissolve 4.95g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.6wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.05g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 0.2wt% cellulose nanocrystal suspension; (3) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (4) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuum defoamed for 60 minutes; (5) Pour the defoamed liquid into a mold and place it in an oven at 60°C for 6 hours to obtain a polyvinyl alcohol / cellulose nanocrystal film.

[0032] Comparative Example 3 (1) Dissolve 4.85g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.5wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.15g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 0.7wt% cellulose nanocrystal suspension; (3) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (4) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (5) Pour the defoamed liquid into a mold and place it in an oven at 60°C for 6 hours to obtain a polyvinyl alcohol / cellulose nanocrystal film.

[0033] Comparative Example 4 (1) Dissolve 4.75g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.4wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.25g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 1.2wt% cellulose nanocrystal suspension; (3) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (4) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (5) Pour the defoamed liquid into a mold and place it in an oven at 60°C for 6 hours to obtain a polyvinyl alcohol / cellulose nanocrystal film.

[0034] Comparative Example 5 (1) Dissolve 4.65g of polyvinyl alcohol in 70ml of deionized water and stir in an oil bath at 90℃ for 1h to obtain a 6.2wt% polyvinyl alcohol aqueous solution. (2) Disperse 0.35g of cellulose nanocrystals in 20ml of deionized water and stir at room temperature for 30min to obtain a uniformly dispersed 1.7wt% cellulose nanocrystal suspension; (3) Add the cellulose nanocrystal suspension to the polyvinyl alcohol solution and stir for 20 min; (4) The mixed liquid is ultrasonically vibrated in an ultrasonic cleaner for 30 minutes, and then vacuumed to remove bubbles for 60 minutes; (5) Pour the defoamed liquid into a mold and place it in an oven at 60°C for 6 hours to obtain a polyvinyl alcohol / cellulose nanocrystal film.

[0035] like Figure 5 As shown, Comparative Examples 1 and 2 completely decomposed after soaking in water for 24 hours, while Examples 2, 3, and 4 remained in water and swelled. During the crosslinking reaction, polyvinyl alcohol and cellulose nanocrystals consumed hydroxyl groups and formed tight covalent bonds with citric acid, thus preventing complete decomposition in water. Furthermore, the more compact the three-dimensional structure, the smaller the swelling of the polyvinyl alcohol composite film.

[0036] Comparative Example 6 When citric acid is replaced with an equimolar amount of acetic anhydride, the resulting PVA composite membrane completely dissolves in water, exhibiting a tensile strength of 27 MPa. This is because acetic anhydride contains only single reactive functional groups, making it difficult to promote the crosslinking of polyvinyl alcohol and cellulose to construct a dense three-dimensional network structure. Consequently, the resulting composite membrane is inferior to the citric acid-crosslinked modified PVA composite membrane in terms of tensile strength and water resistance.

[0037] Comparative Example 7 The difference from Example 1 is that 0.35 g of choline chloride was added in step (3). It was found that the PVA composite film with added choline chloride dissolved in water, and the tensile strength and elastic modulus decreased, while the elongation at break increased.

[0038] The above embodiments detail the structure, features, and effects of the present invention. The above descriptions are merely preferred embodiments of the present invention. Any changes made in accordance with the concept of the present invention, or equivalent embodiments modified to have equivalent variations, shall still fall within the scope of protection of the present invention if they do not exceed the scope covered by the specification.

Claims

1. A method for preparing a polyvinyl alcohol composite film, characterized in that, It includes the following steps: (1) Dissolve citric acid in an aqueous dispersion of cellulose nanocrystals, with a mass ratio of citric acid to cellulose nanocrystals of 7:1-1:1; (2) Add the mixed solution obtained in step (1) to the polyvinyl alcohol aqueous solution and mix evenly to obtain a mixed liquid. Then remove the air from the mixed liquid. The mass ratio of polyvinyl alcohol to citric acid is 13-14.2:

1. (3) Pour the mixed liquid obtained in step (2) into a mold, heat it in an oven at 60°C for 6 hours, and then heat it in an oven at 150°C for 15 minutes to undergo an esterification reaction to obtain a polyvinyl alcohol composite film.

2. The method according to claim 1, characterized in that, The concentration of the cellulose nanocrystal aqueous dispersion is 0.2wt%-1.7wt%.

3. The method according to claim 1, characterized in that, The concentration of the polyvinyl alcohol aqueous solution is 6.2-6.6 wt%.

4. The method according to claim 1, characterized in that, The mixing method in step (2) is ultrasonic treatment for 30 minutes.

5. The method according to claim 1, characterized in that, The method for removing air from the mixed liquid is to evacuate for 1 hour.

6. The method according to claim 1, characterized in that, The method for preparing the polyvinyl alcohol aqueous solution is as follows: after dispersing polyvinyl alcohol in deionized water, stir it in an oil bath at 90°C for 1 hour.

7. A polyvinyl alcohol composite film prepared by the method according to any one of claims 1-6.

8. The application of a polyvinyl alcohol composite film as described in claim 7.

9. The application according to claim 8, characterized in that, Applications in the food packaging industry.

10. The application according to claim 8, characterized in that, Applications in the field of agricultural films.

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