Polyvinyl alcohol / nylon composite film with high barrier property and water boiling resistance and preparation method

Abstract

The invention discloses a polyvinyl alcohol / nylon composite film with high barrier property and water boiling resistance and a preparation method, and belongs to the field of film materials. The technical problem that an existing gas barrier material composed of polyvinyl alcohol and a nylon film cannot have good water boiling resistance and gas barrier property at the same time is solved. The polyvinyl alcohol / nylon composite film with excellent water boiling resistance and gas barrier property is prepared by taking a proper amount of micromolecular aldehydes as a cross-linking agent and dilute acid as a catalyst and controlling the molecular weight of the polyvinyl alcohol and the cross-linking ratio of the aldehydes to hydroxyl groups to reduce the influence of the cross-linking effect on the crystallinity of the PVA. The composite film can still keep excellent helium and oxygen barrier properties after being cooked in water at 100 DEG C for 1 hour, and in addition, the prepared composite film has excellent compatibility and can be compounded with other materials to prepare a multi-layer composite material with excellent gas barrier properties and water cooking resistance.

Description

Technical Field

[0001] The invention belongs to the field of membrane materials, and relates to a polyvinyl alcohol / nylon composite membrane with high barrier properties and water boiling resistance and a preparation method thereof. Background Art

[0002] Inflatable objects such as airships, moored balls, and lifeboats are often used at high altitudes or outdoors. The preparation of gas barrier materials with stable performance and resistance to water boiling is of great significance to improving the reliability, safety, and service life of inflatable objects. Nylon membrane has good mechanical properties and gas barrier properties, and its molecular chain structure is compact and has good chemical corrosion resistance.

[0003] Polyvinyl alcohol (PVA) has good solvent resistance, wear resistance and excellent film-forming properties. It is a green, safe, environmentally friendly and low-cost material. It is widely used in air filtration materials, oil-water separation materials, packaging materials and other fields. Polyvinyl alcohol is a semi-crystalline polymer with good gas barrier properties. It is a commonly used gas barrier material and has important application value in inflatable objects such as airships and moored balls. However, polyvinyl alcohol has poor water resistance and contains a large number of hydroxyl groups in its molecular chain. In a humid environment, the hydroxyl groups in polyvinyl alcohol can react with water in the air to form hydrogen bonds, destroying the aggregated structure of the material and causing the gas barrier performance of the material to decrease.

[0004] Composite biaxially oriented nylon film (BPOA) and polyvinyl alcohol can produce materials with good mechanical properties and gas barrier properties. At present, the main methods for improving the water resistance of polyvinyl alcohol / nylon film gas barrier materials include waterproof coating, physical blending and chemical bonding. Waterproof coating method and physical modification often require the addition of hydrophobic polymer fillers. For multi-layer composite gas barrier material structures, these two methods will affect the adhesion between material interfaces, reduce the creep properties and mechanical properties of the materials, and affect the performance stability of gas barrier materials; the polyvinyl alcohol / nylon film gas barrier material prepared by chemical modification (chemical bonding) has stable and adjustable performance. The use of cross-linking agents can reduce the proportion of hydrophilic groups in the PVA matrix to significantly improve the water resistance of the material. However, to achieve good water resistance, it is necessary to use a cross-linking agent that is too large or the proportion of the cross-linking agent is too high, which will destroy the aggregated structure of the material and reduce the gas barrier performance of the material. Summary of the invention

[0005] In order to solve the technical problem that the gas barrier material composed of polyvinyl alcohol and nylon film cannot have good water boiling resistance and gas barrier property at the same time, the present invention provides a polyvinyl alcohol / nylon composite film with both high barrier property and water boiling resistance and a preparation method thereof.

[0006] In order to achieve the above purpose, the following technical solutions are adopted:

[0007] One of the purposes of the present invention is to provide a polyvinyl alcohol / nylon composite film having both high barrier properties and water boiling resistance. The raw materials of the polyvinyl alcohol / nylon composite film include polyvinyl alcohol, a cross-linking agent, an acid catalyst, deionized water and a nylon film.

[0008] It is further defined that the cross-linking agent is a small molecule aldehyde compound.

[0009] It is further defined that the small molecule aldehyde compound is one or more of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, glyoxal, glutaraldehyde, and adipaldehyde.

[0010] It is further defined that the molar ratio of the hydroxyl group in the polyvinyl alcohol to the aldehyde group in the small molecule aldehyde compound is 100:(0.5-2.5).

[0011] It is further defined that the number average molecular weight of the polyvinyl alcohol is 8.9w to 9.8w.

[0012] It is further defined that the acid catalyst is one or more of hydrochloric acid, sulfuric acid, acetic acid, and phosphoric acid.

[0013] The second object of the present invention is to provide a method for preparing the polyvinyl alcohol / nylon composite film having both high barrier properties and water boiling resistance, comprising:

[0014] (1) Weighing polyvinyl alcohol, adding deionized water, and fully dissolving the polyvinyl alcohol under heating conditions to prepare a polyvinyl alcohol solution of a certain concentration;

[0015] (2) diluting the small molecule aldehyde compound into a dilute aldehyde solution of a certain concentration;

[0016] (3) diluting the acid catalyst into a dilute acid solution of a certain concentration;

[0017] (4) mixing a polyvinyl alcohol solution with a dilute aldehyde solution according to a molar ratio of hydroxyl groups to aldehyde groups to obtain a mixed solution, adding a dilute acid solution to the mixed solution to adjust the pH, and then mixing the mixed solution with the adjusted pH at room temperature to obtain a polyvinyl alcohol coating;

[0018] (5) The polyvinyl alcohol coating is coated on the nylon film, and then placed in an oven for heat treatment to form a polyvinyl alcohol coating, thereby obtaining a polyvinyl alcohol / nylon composite film with both high barrier properties and water boiling resistance.

[0019] It is further defined that the volume ratio of the polyvinyl alcohol solution in (1) to the aldehyde dilute solution in (2) is 100:(1-6).

[0020] It is further defined that the mass concentration of the polyvinyl alcohol solution in (1) is 0.5-10%.

[0021] It is further defined that the mass concentration of the aldehyde dilute solution in (2) is 0.1 to 20%.

[0022] It is further defined that the molar concentration of the dilute acid solution in (3) is 0.01 to 10 mol / L.

[0023] It is further defined that the heat treatment temperature in (5) is 50 to 120° C. and the heat treatment time is 10 to 360 min.

[0024] It is further defined that the thickness of the polyvinyl alcohol coating in (5) is 1 to 200 μm.

[0025] The beneficial effects achieved by the present invention are as follows:

[0026] (1) The molecular weight of polyvinyl alcohol determines its solubility. The larger the molecular weight, the worse its solubility and the more difficult its subsequent processing. Polyvinyl alcohol with too small a molecular weight has poor water resistance and is very easy to fall off from the surface of the material even after cross-linking. The present invention uses polyvinyl alcohol with a molecular weight of 8.9w to 9.8w as a raw material, which not only ensures that it has good processability and can be well coated on the nylon film, but also improves the water boiling resistance of the polyvinyl alcohol / nylon composite film. In addition, due to its good processability and not easy to fall off, the composite film also has good gas barrier properties.

[0027] (2) The small molecule aldehydes used in the present invention are small in size, have mild reaction conditions with hydroxyl groups, and are low in cost. By controlling the molar ratio of hydroxyl groups to aldehyde groups and thus the degree of crosslinking, the effect of crosslinking on the crystallinity of PVA can be reduced. If the crosslinking degree is too high, the viscosity of the polyvinyl alcohol coating will be too high and the coating will be uneven. If the crosslinking degree is too low, the polyvinyl alcohol coating will be too thin and will not be able to play a role in water boiling resistance and gas barrier when coated on the nylon film. The present invention improves the water boiling resistance of the composite film while taking into account the gas barrier performance. In addition, the aldehyde groups in the small molecule aldehydes react with the hydroxyl groups in the polyvinyl alcohol to form ether bonds through an aldol condensation reaction. The ether bonds are relatively stable under neutral, alkaline, and weakly acidic conditions. The prepared material has good acid and alkali resistance.

[0028] (3) The present invention prepares the acid catalyst into a dilute acid solution to catalyze the aldol condensation, which can both accelerate the crosslinking reaction speed and prevent the reaction from being too rapid. Dilute acid will catalyze the crosslinking reaction, because too slow a reaction speed will lead to insufficient crosslinking, affecting the water boiling resistance and gas barrier properties of the composite film. However, the concentration of dilute acid is lower than that of traditional concentrated acid, and it can prevent the crosslinking reaction speed from being too fast, resulting in an increase in the viscosity of the system and thus causing uneven coating on the composite film. In addition, dilute acid can reduce the acid's corrosive effect on the machine.

[0029] (4) The polyvinyl alcohol / nylon composite film provided by the present invention has excellent water boiling resistance and gas barrier properties. After being boiled in room temperature water and 100°C water for 1 hour, it can still maintain excellent helium and oxygen gas barrier properties. The gas permeability of the material changes little or even remains unchanged. In addition, the polyvinyl alcohol / nylon composite film of the present invention has good compatibility and can be composited with other materials to prepare a multilayer composite material with excellent gas barrier properties and water boiling resistance. Moreover, the preparation method of the polyvinyl alcohol / nylon composite film of the present invention is simple, the raw materials are simple, and the cost is saved, and it has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is the FT-IR spectrum of the polyvinyl alcohol / nylon composite membrane prepared in Comparative Example 1;

[0031] Figure 2 is the FT-IR spectrum of the polyvinyl alcohol / nylon composite membrane prepared in Example 3;

[0032] Figure 3 This is the FT-IR image of the polyvinyl alcohol / nylon composite membrane prepared in Comparative Example 1 after being boiled in water;

[0033] Figure 4 This is the FT-IR image of the polyvinyl alcohol / nylon composite membrane prepared in Example 3 after being boiled in water. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific implementation methods of the present invention are described in detail below in combination with the embodiments of the specification. In the following description, many specific details are elaborated to facilitate a full understanding of the present invention, but the present invention can also be implemented in other ways different from those described herein. Those skilled in the art can make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0035] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor does it refer to a separate or selective embodiment that is mutually exclusive with other embodiments.

[0036] The experimental methods used in the following examples are conventional methods unless otherwise specified. The materials, reagents, methods and instruments used are conventional materials, reagents, methods and instruments in the art unless otherwise specified, and can be obtained through commercial channels by those skilled in the art.

[0037] Example 1

[0038] 2.0 g of PVA with a molecular weight of 9.0w was added to deionized water and fully dissolved under heating at 80°C to prepare 100 mL of a PVA solution with a mass concentration of 2%. 1.0 mL of commercially available glutaraldehyde (i.e., a cross-linking agent) with a mass concentration of 50% was prepared into 10.0 mL of a glutaraldehyde dilute solution with a mass concentration of 5%. 1.0 mL of commercially available 12 mol / L concentrated hydrochloric acid was prepared into 120.0 mL of a 0.1 mol / L dilute hydrochloric acid solution. 11 mL of a PVA solution with a mass concentration of 2% and 25 μL of a glutaraldehyde solution with a mass concentration of 5% were mixed to form a hydroxyl-containing crosslinking agent. A mixed solution with an aldehyde molar ratio of 100:0.5, 1 mL of a 0.1 mol / L dilute hydrochloric acid solution is added to the mixed solution to adjust the pH to 1-2, and then the mixed solution with adjusted pH is mixed evenly at room temperature to obtain a polyvinyl alcohol coating, and the polyvinyl alcohol coating is coated on both sides of a nylon membrane (biaxially oriented nylon membrane, model BPOA), the thickness of the polyvinyl alcohol coating is 2 μm, and then the nylon membrane is placed in an oven at 80°C for 2 hours to obtain a polyvinyl alcohol / nylon composite membrane, the crosslinking degree of this embodiment is 1%, and the polyvinyl alcohol / nylon composite membrane prepared in this embodiment is tested for helium permeability and contact angle.

[0039] Example 2

[0040] The difference between this embodiment and embodiment 1 is that the amounts of PVA solution and glutaraldehyde solution used are 11 mL and 50 μL respectively, the molar ratio of hydroxyl group to aldehyde group in the mixed solution formed by the two is 100:1.0, the crosslinking degree of this embodiment is 2%, and the remaining process steps and parameter settings are the same as those in embodiment 1.

[0041] The helium permeability and contact angle tests were performed on the polyvinyl alcohol / nylon composite membrane prepared in this example.

[0042] Example 3

[0043] The difference between this embodiment and embodiment 1 is that the amounts of PVA solution and glutaraldehyde solution used are 11 mL and 75 μL respectively, the molar ratio of hydroxyl group to aldehyde group in the mixed solution formed by the two is 100:1.5, the crosslinking degree of this embodiment is 3%, and the remaining process steps and parameter settings are the same as those in embodiment 1.

[0044] The polyvinyl alcohol / nylon composite membrane prepared in this example was characterized by Fourier transform infrared spectroscopy (FT-IR) and tested for helium permeability and contact angle.

[0045] Example 4

[0046] The difference between this embodiment and embodiment 1 is that the amounts of PVA solution and glutaraldehyde solution used are 11 mL and 100 μL respectively, the molar ratio of hydroxyl group to aldehyde group in the mixed solution formed by the two is 100:2.0, the mixed solution with adjusted pH is coated on one side of the nylon membrane, and the other side is not coated, the crosslinking degree of this embodiment is 4%, and the remaining process steps and parameter settings are the same as those in embodiment 1.

[0047] The helium permeability and contact angle tests were performed on the polyvinyl alcohol / nylon composite membrane prepared in this example.

[0048] Example 5

[0049] The difference between this embodiment and embodiment 1 is that the amounts of PVA solution and glutaraldehyde solution used are 11 mL and 125 μL respectively, the molar ratio of hydroxyl group to aldehyde group in the mixed solution formed by the two is 100:2.5, the crosslinking degree of this embodiment is 5%, and the remaining process steps and parameter settings are the same as those in embodiment 1.

[0050] The helium permeability, oxygen permeability and contact angle tests were performed on the polyvinyl alcohol / nylon composite membrane prepared in this example.

[0051] Comparative Example 1

[0052] PVA was added to deionized water and fully dissolved under heating conditions to prepare a PVA solution with a mass fraction of 2%. The PVA solution was coated on both sides of the nylon membrane with a polyvinyl alcohol coating thickness of 2 μm. The membrane was then placed in an oven at 80°C for reaction for 2 hours to obtain a polyvinyl alcohol / nylon composite membrane. The crosslinking degree of this comparative example was 0.

[0053] The polyvinyl alcohol / nylon composite membrane prepared in this comparative example was characterized by Fourier transform infrared spectroscopy (FT-IR) and tested for helium permeability and contact angle.

[0054] Comparative Example 2

[0055] The gas barrier material of Comparative Example 2 is a commercially available high-barrier film HG film.

[0056] The oxygen permeation rate of the commercially available high barrier film HG film was tested.

[0057] The polyvinyl alcohol / nylon composite films of the above-mentioned embodiments and comparative examples were characterized and tested for performance:

[0058] 1. Characterization of composite membrane by Fourier transform infrared spectroscopy (FT-IR) before water cooking

[0059] Nicolet iS 5 Fourier transform infrared spectroscopy produced by Thermo Fisher Scientific was used to characterize the coating of polyvinyl alcohol on the surface of nylon membrane. The test conditions were: resolution 4 cm -1; Scanning rate 0.47cm / s; Background and sample scanned 32 times, scanning range 400~4000cm -1 , Figure 1 and Figure 2 FT-IR spectra of the polyvinyl alcohol / nylon composite membranes prepared in Comparative Example 1 and Example 3 before being boiled in 100°C boiling water.

[0060] PVA and nylon membrane as raw materials, 3270cm -1 and 1080cm -1 The characteristic peak near is the absorption peak of secondary hydroxyl group, which is the characteristic peak of PVA. This peak does not exist in pure nylon membrane; 1640cm -1 The characteristic peak at is the stretching vibration peak of C=O, which is the absorption band I of amide, 1560cm -1 The characteristic peaks at are the angular vibration of NH and the absorption band II of amide. These two peaks are characteristic peaks of nylon membrane and do not exist in pure PVA. Figure 1 and Figure 2 It can be found that in the spectra of Comparative Example 1 and Example 3, both have -1 Typical secondary hydroxyl absorption peaks appeared at 1640 cm -1 and 1540cm -1 The characteristic absorption peak of the nylon membrane appeared at , and the result showed that the nylon membrane in the polyvinyl alcohol / nylon composite membranes prepared in Comparative Example 1 and Example 3 was successfully wrapped by polyvinyl alcohol.

[0061] 2. Helium permeability

[0062] The gas barrier properties of the polyvinyl alcohol / nylon composite film before and after water boiling were measured using the PERME VAC-V2 differential pressure gas permeometer produced by Labthink / Languang Company in China. The boiling conditions were to boil the polyvinyl alcohol / nylon composite film in 100℃ boiling water for 1 hour, take it out and place it in an oven at 80℃ for 24 hours, and then test it at room temperature for 24 hours. In order to reduce the experimental error, three pieces of material were randomly selected for measurement, and the final value was the average of the three measurement data. The results are shown in Table 1.

[0063] Q in Table 1 初始 is the original helium permeation of the polyvinyl alcohol / nylon composite membrane, Q 处理 It is the helium permeability measured after the polyvinyl alcohol / nylon composite membrane is boiled in 100℃ water for 1h.

[0064] Table 1 Helium permeation results of polyvinyl alcohol / nylon membrane before and after water boiling

[0065] sample <![CDATA[Q 初始 (L / m 2 ×24h×atm)]]> <![CDATA[Q 处理 (L / m 2 ×24h×atm)]]> Comparative Example 1 1066 2685 Example 1 1191 1403 Example 2 1117 911 Example 3 1021 1012 Example 4 1335 1350 Example 5 887 913

[0066] From the data in Table 1, it can be seen that after the polyvinyl alcohol / nylon composite membrane prepared in Comparative Example 1 was boiled in water, its gas permeability increased rapidly to 2685 L / m 2 ×24h×atm, compared with the untreated sample, the helium permeation of the material increased by 151.7%; the polyvinyl alcohol / nylon composite membranes prepared in Examples 1 to 5 had relatively small increases in helium permeation before and after water boiling, especially when the crosslinking degree of polyvinyl alcohol was ≥2%, that is, Examples 2 to 5, compared with before boiling, the helium permeation of the composite membrane after water boiling was basically unchanged. In summary, the polyvinyl alcohol / nylon composite membrane prepared by the method of the present invention has a stronger bonding strength between the coating formed by the modified polyvinyl alcohol and the nylon membrane, and the modified polyvinyl alcohol coating has good water boiling resistance, and after boiling in water, it still maintains good gas barrier properties.

[0067] 3. Oxygen permeability

[0068] The oxygen barrier properties of the composite membranes before and after water treatment were measured using the GTR-7006 differential pressure gas permeometer produced by China Jinan Sike Testing Technology Co., Ltd. The cooking conditions were to boil the polyvinyl alcohol / nylon composite membrane in 100°C boiling water for 1 hour, take it out and place it in an oven at 80°C for 24 hours, and then test it at room temperature for 24 hours. In order to reduce the experimental error, three pieces of material were randomly selected for measurement, and the final value was the average of the three measurement data. The results are shown in Table 2.

[0069] Q in Table 2 初始 is the original oxygen permeability of the material, Q 处理 It is the oxygen permeability measured after the material is cooked in 100℃ water for 1h.

[0070] Table 2 Results of oxygen permeation of polyvinyl alcohol / nylon membrane before and after water boiling

[0071] sample <![CDATA[Q 初始 (L / m 2 ×24h×atm)]]> <![CDATA[Q 处理 (L / m 2 ×24h×atm)]]> Comparative Example 2 1.492 1.150 Example 5 0.752 0.714

[0072] It can be seen from the data in Table 2 that after the polyvinyl alcohol / nylon composite membrane prepared in Example 5 was boiled in water, its oxygen permeability increased rapidly to almost no significant change, and the oxygen permeability before and after water boiling treatment was significantly lower than that of the commercially available high barrier membrane HG membrane.

[0073] 4. Characterization of composite membrane by Fourier transform infrared spectroscopy (FT-IR) after water boiling

[0074] Nicolet iS 5 Fourier transform infrared spectroscopy produced by Thermo Fisher Scientific was used to characterize the distribution of polyvinyl alcohol on the surface of nylon membrane after water boiling. The test conditions were: resolution 4 cm -1 ; Scanning rate 0.47cm / s; Background and sample scanned 16 times, scanning range 400~4000cm-1 , the results are as follows Figure 3 and Figure 4 shown.

[0075] Depend on Figure 3 and Figure 4 It can be found that after the polyvinyl alcohol / nylon composite membrane prepared in Comparative Example 1 was boiled in water, the material 1080cm -1 The secondary hydroxyl absorption peak at 1080 cm-1 basically disappeared, indicating that the modified polyvinyl alcohol coating on the surface of the material was basically washed away; in comparison, after the polyvinyl alcohol / nylon composite membrane prepared in Example 3 was boiled in water, the material -1 The secondary hydroxyl absorption peak intensity at the location changes little, indicating that the modified PVA coating on the surface of Example 3 falls off less or does not fall off, further indicating that the bonding strength between the modified polyvinyl alcohol and the nylon membrane is stronger, and the modified polyvinyl alcohol coating has better water boiling resistance.

[0076] 5. Contact angle test.

[0077] The effect of modification on the wetting behavior of the material surface was measured using the AC100D contact angle tester produced by China Yingnuo Company. The results are shown in Table 2. The test liquid volume was 5.0 μL. In order to reduce the experimental error, three pieces of material were randomly selected for measurement. The final value was the average of the three measurement data. The results are shown in Table 2.

[0078] Table 3 Contact angle test results of polyvinyl alcohol / nylon composite membrane.

[0079] sample 0s / ° 10s / ° 20s / ° 30s / ° 40s / ° 50s / ° 60s / ° Comparative Example 1 61.26 60.45 60.17 59.74 58.78 58.11 56.99 Example 1 73.59 73.57 72.54 71.89 70.73 70.25 69.89 Example 2 79.56 77.68 76.49 76.37 76.09 75.23 74.75 Example 3 75.99 75.55 75.37 74.45 74.12 73.84 73.5 Example 5 69.30 68.88 68.71 68.47 68.29 68.29 68.11

[0080] From the data in Table 3, it can be seen that the contact angle of the polyvinyl alcohol / nylon composite film prepared in Comparative Example 1 is 61.26°; the contact angle of the polyvinyl alcohol / nylon composite film prepared in Example 1 is increased to 73.59° after modification, the contact angle of the composite film of Example 2 is increased to 79.56°, the contact angle of Example 3 is 75.99°, the contact angle of the composite film decreases, and the contact angle of the composite film of Example 5 further decreases to 69.3°. In summary, the surface contact angles of the polyvinyl alcohol / nylon composite films prepared in all embodiments are greater than those in Comparative Example 1. When the crosslinking degree of PVA is ≤2%, that is, Examples 1 to 2, the contact angle of the composite film increases with the increase of the crosslinking degree. When the crosslinking degree of PVA is ≥3%, that is, Examples 3 to 5, the contact angle of the composite film decreases with the increase of the crosslinking degree. In addition, it was found that after the droplet stayed on the surface of the material for 60s, the surface contact angles of the composite films of the embodiments did not decrease significantly, indicating that the material has good stability in the air state.

[0081] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A polyvinyl alcohol / nylon composite film having both high barrier properties and water boiling resistance, characterized in that: The raw materials for preparing the polyvinyl alcohol / nylon composite membrane include polyvinyl alcohol, a cross-linking agent, an acid catalyst, deionized water and a nylon membrane; The cross-linking agent is a small molecule aldehyde compound; The molar ratio of the hydroxyl group in the polyvinyl alcohol to the aldehyde group in the small molecular aldehyde compound is 100:(0.5-2.5).

2. The polyvinyl alcohol / nylon composite film according to claim 1, characterized in that: The number average molecular weight of the polyvinyl alcohol is 8.9w-9.8w.

3. The polyvinyl alcohol / nylon composite film according to claim 1, characterized in that: The acid catalyst is one or more of hydrochloric acid, sulfuric acid, acetic acid and phosphoric acid.

4. The polyvinyl alcohol / nylon composite film according to claim 1, characterized in that: The small molecule aldehyde compound is one or more of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, glyoxal, glutaraldehyde, and adipaldehyde.

5. A method for preparing a polyvinyl alcohol / nylon composite film having both high barrier properties and water boiling resistance as claimed in any one of claims 1 to 4, characterized in that: include: (1) Weighing polyvinyl alcohol, adding deionized water, and fully dissolving the polyvinyl alcohol under heating conditions to prepare a polyvinyl alcohol solution of a certain mass concentration; (2) diluting the small molecule aldehyde compound into a dilute aldehyde solution of a certain mass concentration; (3) diluting the acid catalyst into a dilute acid solution of a certain concentration; (4) mixing a polyvinyl alcohol solution with a dilute aldehyde solution according to a molar ratio of hydroxyl groups to aldehyde groups to obtain a mixed solution, adding a dilute acid solution to the mixed solution to adjust the pH, and then mixing the mixed solution with the adjusted pH at room temperature to obtain a polyvinyl alcohol coating; (5) The polyvinyl alcohol coating is coated on the nylon film, and then placed in an oven for heat treatment to form a polyvinyl alcohol coating, thereby obtaining a high-barrier and water-resistant polyvinyl alcohol / nylon composite film.

6. The preparation method according to claim 5, characterized in that: The volume ratio of the polyvinyl alcohol solution in (1) to the aldehyde dilute solution in (2) is 100:(1-6).

7. The preparation method according to claim 5, characterized in that: The mass concentration of the polyvinyl alcohol solution in (1) is 0.5-10%.

8. The preparation method according to claim 5, characterized in that: The mass concentration of the aldehyde dilute solution in (2) is 0.1-20%.

9. The preparation method according to claim 5, characterized in that: The molar concentration of the dilute acid solution in (3) is 0.01 to 10 mol / L.

10. The preparation method according to claim 5, characterized in that: The heat treatment temperature in (5) is 50-120° C., the heat treatment time is 10-360 min, and the thickness of the polyvinyl alcohol coating is 1-200 μm.

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