A lignin-based chemically cross-linked polyvinyl alcohol composite film and a preparation method thereof

Abstract

The application discloses a lignin-based chemical cross-linking polyvinyl alcohol composite film and a preparation method thereof, and belongs to the technical field of high polymer materials. The preparation method provided by the application comprises the following steps: lignin and succinic anhydride are dissolved in a solvent, and a succinic anhydride esterified lignin solution is obtained by reacting in the presence of a catalyst; polyvinyl alcohol is added into the succinic anhydride esterified lignin solution, and a film is obtained by drying after reaction. The preparation method is simple, low in cost and suitable for large-scale preparation. The lignin-based chemical cross-linking polyvinyl alcohol composite film obtained by the method has an elongation at break of more than 200% in a high-humidity environment of 90% RH, good resilience after soaking, a resilience rate of more than 90%, and even up to 100%, and is suitable for working environments with high humidity and high strain requirements for materials, and has good degradation performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, and particularly relates to a lignin-based chemically cross-linked polyvinyl alcohol composite film and a preparation method thereof. BACKGROUND

[0002] The information disclosed in the background of the present application is merely intended to increase the understanding of the overall background of the present application and should not be necessarily regarded as acknowledging or implicitly suggesting that this information constitutes the prior art known to those skilled in the art.

[0003] Industrial lignin is mainly derived from the pulp and papermaking process in the form of by-products, which is considered as one of the potential sources of biomass energy and can be converted into fuel and other chemicals. However, the main utilization of lignin in the current pulp and paper industry system is still in the low-value stage. Lignin is either directly discharged into nature for natural degradation or simply collected and burned for industrial production to provide heat value, which causes environmental pollution and waste of resources.

[0004] Lignin / polyvinyl alcohol composite film is one of the effective ways to utilize lignin, and both lignin and polyvinyl alcohol are naturally degradable substances, so they have a broad application prospect in film materials. The current lignin / polyvinyl alcohol composite film is mainly physical blending film, but the compatibility between lignin and polyvinyl alcohol is poor, and the mechanical properties of the obtained blending film are poor. Cross-linking lignin and polyvinyl alcohol with cross-linking agents to prepare film materials is one of the important ways to improve the mechanical properties of the film. Epoxy chloropropane, formaldehyde and glutaraldehyde are the most widely used cross-linking agents at present. However, due to the presence of epoxy groups, epoxy chloropropane degrades slowly, formaldehyde itself is highly toxic, and if released in large quantities into the environment, it may have a negative impact on the ecological system. Glutaraldehyde is greatly affected by the pH of the environment, and extreme pH can even cause the hydrolysis of glutaraldehyde, affecting the cross-linking performance. In addition, the lignin / polyvinyl alcohol composite film prepared by using the above cross-linking agents may be damaged in a high humidity environment, has poor resilience or even no resilience, and it is difficult to simultaneously satisfy good degradability and good resilience in a high humidity environment, so the practical value is poor.

[0005] Therefore, it is an urgent problem to provide a lignin / polyvinyl alcohol composite film with good degradability and good resilience in a high humidity environment. SUMMARY

[0006] Therefore, the present application provides a lignin-based chemical cross-linking polyvinyl alcohol composite film and a preparation method thereof.

[0007] In a first aspect, the present application provides a preparation method of a lignin-based chemical cross-linking polyvinyl alcohol composite film, comprising the following steps:

[0008] dissolving lignin and succinic anhydride in a solvent to obtain a succinic anhydride esterified lignin solution in the presence of a catalyst;

[0009] adding polyvinyl alcohol to the succinic anhydride esterified lignin solution, and drying to form a film after reaction.

[0010] Preferably, the mass fraction of the lignin is 30-50% of the total mass of the lignin, succinic anhydride and polyvinyl alcohol.

[0011] Further, the mass ratio of the succinic anhydride to the polyvinyl alcohol is 1:(0.8-1.2).

[0012] Preferably, the total mass of the lignin, succinic anhydride and polyvinyl alcohol to the amount of the solvent is 1g:(2.5-5)mL.

[0013] Preferably, the solvent is selected from one or more of dimethyl sulfoxide, N,N-dimethylformamide or N,N-dimethylacetamide.

[0014] Preferably, the catalyst is selected from N,N-dimethylbenzylamine 、 acetone or 1-methylimidazole.

[0015] Preferably, the mass fraction of the catalyst is 15-25% of the mass of the succinic anhydride.

[0016] Preferably, in the step of obtaining the succinic anhydride esterified lignin solution in the presence of the catalyst, the reaction temperature is 55-70°C, and the reaction time is 5-10h.

[0017] Preferably, in the step of drying to form a film after reaction, the reaction temperature is 55-70°C, and the reaction time is 5-10h.

[0018] Preferably, in the step of drying to form a film after reaction, the drying is vacuum drying, the drying temperature is 55-70°C, and the drying time is 8-24h.

[0019] In a second aspect, the present application provides a lignin-based chemical cross-linking polyvinyl alcohol composite film prepared by the above preparation method.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] (1) The present application esterifies and modifies lignin with succinic anhydride, then esterifies and crosslinks with polyvinyl alcohol through carboxyl, forming a crosslinked network with three-dimensional spatial structure, and finally a lignin-based chemical crosslinking polyvinyl alcohol composite film is prepared by solution casting method. The preparation method of the present application is simple, low in cost and suitable for large-scale preparation.

[0022] (2) The lignin-based chemical crosslinking polyvinyl alcohol composite film prepared by the present application forms a polyvinyl alcohol-succinic anhydride-lignin crosslinked network, and the moisture resistance of the material is significantly improved. The elongation at break can still be maintained at more than 200% in a high humidity environment of 90% RH. Moreover, due to the formation of a good crosslinked skeleton network, the resilience after soaking is good, with a resilience rate of more than 90%, even up to 100%, thus expanding the use environment of polyvinyl alcohol film and being suitable for working environments that are long-term in high humidity and have high strain requirements for materials.

[0023] (3) The lignin-based chemical crosslinking polyvinyl alcohol composite film prepared by the present application has good degradability and becomes brittle and wrinkled after landfill treatment, with good environmental performance. BRIEF DESCRIPTION OF DRAWINGS

[0024] The drawings accompanying the specification of the present application serve to provide a further understanding of the present application, and the illustrative embodiments of the present application and their descriptions serve to explain the present application, and do not constitute an improper limitation of the present application. Obviously, for those skilled in the art, other drawings can also be obtained from these drawings without creative labor.

[0025] Figure 1 is a picture of the resilience performance test process of the composite film sample of the present application examples 1-3 and comparative example 1 in a dry state;

[0026] Figure 2 is a picture of the resilience performance test process of the composite film sample of the present application examples 1-3 after soaking.

[0027] Figure 3 is a picture of the degradation performance test process of the composite film sample of the present application example 3 during natural landfill, wherein a is the initial sample; b is the sample after the first cycle, c is the sample after the second cycle, and d is the sample after the third cycle. DETAILED DESCRIPTION

[0028] It should be noted that the following detailed description is exemplary in nature and is intended to provide further description of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0029] The application provides a preparation method of a lignin-based chemical cross-linked polyvinyl alcohol composite film, comprising the following steps:

[0030] The lignin and succinic anhydride are dissolved in a solvent, and the succinic anhydride esterified lignin solution is obtained by reaction in the presence of a catalyst;

[0031] The polyvinyl alcohol is added to the succinic anhydride esterified lignin solution, and the film is obtained by drying after reaction.

[0032] In the application, the three-dimensional network structure of lignin provides a large number of reaction sites for esterification, and the lignin and succinic anhydride occur esterification under the action of a catalyst, and then the esterification occurs with the hydroxyl groups of polyvinyl alcohol, forming a cross-linked network with a three-dimensional space structure. Depending on the large steric hindrance and rich functional groups of lignin, water molecules are difficult to diffuse in the material, and the hydrophilic functional groups of lignin compete with the hydroxyl groups of polyvinyl alcohol to form hydrogen bonds with water, and the esterification consumes part of the hydroxyl groups of polyvinyl alcohol, so that the combination of water molecules and the hydroxyl groups of polyvinyl alcohol is effectively prevented, and the influence of the hydroxyl groups on polyvinyl alcohol is reduced. Therefore, the lignin-based chemical cross-linked polyvinyl alcohol composite film has excellent moisture resistance compared with the polyvinyl alcohol film, and the prepared lignin-based chemical cross-linked polyvinyl alcohol composite film has high elongation at break and high resilience in a high-humidity environment.

[0033] In the application, the mass fraction of the lignin is 30-50% of the total mass of the lignin, succinic anhydride and polyvinyl alcohol. The content of lignin will affect the mechanical properties, when the content of lignin is too low, the cross-linked network is not dense enough, and thus the moisture resistance and resilience are poor, and when the content of lignin is too high, too many nucleation sites are formed, and due to the three-dimensional structure of lignin, the distance between polyvinyl alcohol molecules is too large, and thus the breaking stress is seriously reduced.

[0034] In the application, the mass ratio of the succinic anhydride and polyvinyl alcohol is 1:(0.8-1.2), and the succinic anhydride is used as a cross-linking agent, and the amount of the succinic anhydride is large (about 30-40 wt% of the total mass of the lignin, succinic anhydride and polyvinyl alcohol), which is beneficial to the reaction with the hydroxyl groups, and in addition, the succinic anhydride has low cost, wide sources and good degradability.

[0035] In the present application, the total mass of lignin, succinic anhydride and polyvinyl alcohol is 1g, and the amount of solvent is 2.5-5mL. The amount of solvent can ensure that the solute is fully dissolved, so that the modification is more uniform, thereby facilitating the preparation of a film with uniform performance; and can also avoid waste caused by excessive solvent.

[0036] In the present application, the solvent is selected from one or more of dimethyl sulfoxide, N,N-dimethylformamide or N,N-dimethylacetamide, and the solvent should have good solubility for lignin, polyvinyl alcohol, succinic anhydride and catalyst to ensure sufficient crosslinking.

[0037] In the present application, the catalyst is selected from N,N-dimethylbenzylamine, acetone or 1-methylimidazole, and is preferably N,N-dimethylbenzylamine. The mass fraction of the catalyst is 15-25% of the mass of succinic anhydride. When the catalyst is N,N-dimethylbenzylamine, it has good Lewis base properties, provides a large number of electron clouds for the reaction, interacts with the carbonyl oxygen in the carboxylic acid, and this interaction can enhance the electropositivity of the carbonyl carbon, making it more susceptible to attack by alcohol, thereby accelerating the esterification reaction.

[0038] In the step of obtaining a succinic anhydride esterified lignin solution in the presence of a catalyst in the present application, the reaction temperature is 55-70℃, and more preferably 58-65℃, and the reaction time is 5-10h.

[0039] In the step of drying into a film after the reaction in the present application, the reaction temperature is 55-70℃, and more preferably 58-65℃, and the reaction time is 5-10h.

[0040] In the step of drying into a film after the reaction in the present application, the drying is vacuum drying, the drying temperature is 55-70℃, and the drying time is 8-24h.

[0041] The reaction process of the present application is carried out under stirring conditions, and the present application does not make special limitations on the stirring conditions, and the commonly used stirring conditions in the art can be used.

[0042] The present application does not make special limitations on the specific drying and film forming step, and the solution casting method commonly used in the art can be used.

[0043] The present application also provides a lignin-based chemically crosslinked polyvinyl alcohol composite film prepared by the above preparation method. The lignin-based chemically crosslinked polyvinyl alcohol composite film of the present application has good mechanical properties, and the elongation at break can be as high as 400% or more under dry conditions, and can still be maintained at 200% or more in a high humidity environment, and also has good resilience in a high humidity environment, thereby expanding the application field of the lignin-based chemically crosslinked polyvinyl alcohol composite film.

[0044] The technical solutions of the present application are further described below in combination with specific examples.

[0045] Example 1

[0046] The present example provides a lignin-based chemically cross-linked polyvinyl alcohol film, wherein the mass fraction of lignin is 30% of the total mass of lignin, succinic anhydride and polyvinyl alcohol. The specific preparation method is as follows:

[0047] Take 4.29 g of lignin powder, take 5 g of succinic anhydride powder, add 50 mL of dimethyl sulfoxide and 1 g of N,N-dimethylbenzylamine, and magnetically stir at 60°C for 6 h to obtain a succinic anhydride esterified lignin solution. Take 5 g of polyvinyl alcohol powder and add it to the succinic anhydride esterified lignin solution, and magnetically stir at 60°C for 6 h to obtain a cross-linked reaction solution. Transfer the cross-linked reaction solution to a polystyrene culture dish and dry it in a vacuum drying oven at 60°C for 12 h. After cooling to room temperature, a lignin-based chemically cross-linked polyvinyl alcohol composite film is obtained.

[0048] Example 2

[0049] The present example provides a lignin-based chemically cross-linked polyvinyl alcohol composite film, wherein the mass fraction of lignin is 40% of the total mass of lignin, succinic anhydride and polyvinyl alcohol. The specific preparation method is as follows:

[0050] Take 6.67 g of lignin powder, take 5 g of succinic anhydride powder, add 50 mL of dimethyl sulfoxide and 1 g of N,N-dimethylbenzylamine, and magnetically stir at 60°C for 6 h to obtain a succinic anhydride esterified lignin solution. Take 5 g of polyvinyl alcohol powder and add it to the succinic anhydride esterified lignin solution, and magnetically stir at 60°C for 6 h to obtain a cross-linked reaction solution. Transfer the cross-linked reaction solution to a polystyrene culture dish and dry it in a vacuum drying oven at 60°C for 12 h. After cooling to room temperature, a lignin-based chemically cross-linked polyvinyl alcohol composite film is obtained.

[0051] Example 3

[0052] The present example provides a lignin-based chemically cross-linked polyvinyl alcohol composite film, wherein the mass fraction of lignin is 50% of the total mass of lignin, succinic anhydride and polyvinyl alcohol. The specific preparation method is as follows:

[0053] Take 10 g of lignin powder, take 5 g of succinic anhydride powder, add 50 mL of dimethyl sulfoxide and 1 g of N,N-dimethyl benzylamine, and stir magnetically at 60°C for 6 h to obtain a succinic anhydride esterified lignin solution. Take 5 g of polyvinyl alcohol powder and add it to the succinic anhydride esterified lignin solution, stir magnetically at 60°C for 6 h to obtain a solution after crosslinking reaction. Transfer the solution after crosslinking reaction to a polystyrene culture dish, dry in a vacuum drying oven at 60°C for 12 h, and cool to room temperature to obtain a lignin-based chemically crosslinked polyvinyl alcohol composite film.

[0054] Comparative Example 1

[0055] The present comparative example is compared with Example 1, the difference being that the mass fraction of lignin in the present comparative example is 10% of the total mass of lignin, succinic anhydride and polyvinyl alcohol. The specific preparation method is as follows:

[0056] Take 10 g of lignin powder, take 5 g of succinic anhydride powder, add 50 mL of dimethyl sulfoxide and 1 g of N,N-dimethyl benzylamine, and stir magnetically at 60°C for 6 h to obtain a succinic anhydride esterified lignin solution. Take 5 g of polyvinyl alcohol powder and add it to the succinic anhydride esterified lignin solution, stir magnetically at 60°C for 6 h to obtain a solution after crosslinking reaction. Transfer the solution after crosslinking reaction to a polystyrene culture dish, dry in a vacuum drying oven at 60°C for 12 h, and cool to room temperature to obtain a lignin-based chemically crosslinked polyvinyl alcohol composite film.

[0057] Comparative Example 2

[0058] The present comparative example is compared with Example 1, the difference being that the mass fraction of lignin in the present comparative example is 20% of the total mass of lignin, succinic anhydride and polyvinyl alcohol. The specific preparation method is as follows:

[0059] Take 10 g of lignin powder, take 5 g of succinic anhydride powder, add 50 mL of dimethyl sulfoxide and 1 g of N,N-dimethyl benzylamine, and stir magnetically at 60°C for 6 h to obtain a succinic anhydride esterified lignin solution. Take 5 g of polyvinyl alcohol powder and add it to the succinic anhydride esterified lignin solution, stir magnetically at 60°C for 6 h to obtain a solution after crosslinking reaction. Transfer the solution after crosslinking reaction to a polystyrene culture dish, dry in a vacuum drying oven at 60°C for 12 h, and cool to room temperature to obtain a lignin-based chemically crosslinked polyvinyl alcohol composite film.

[0060] Comparative Example 3

[0061] The present comparative example provides a preparation method of a polyvinyl alcohol film, the specific steps being as follows:

[0062] The polyvinyl alcohol powder 5 g was weighed and added to 50 mL dimethyl sulfoxide, and then stirred magnetically at 60 °C for 12 h. The obtained polyvinyl alcohol solution was transferred to a polystyrene culture dish and dried in a vacuum drying oven at 60 °C for 12 h. After cooling to room temperature, the polyvinyl alcohol film was obtained.

[0063] Test Example

[0064] 1. Mechanical property test:

[0065] The tensile test of the lignin-based chemically cross-linked polyvinyl alcohol film of Examples 1-3 and Comparative Examples 1-2 and the polyvinyl alcohol film of Comparative Example 3 was performed in a dry state and after exposure to 90% air humidity (90% RH) for 72 h using a texture analyzer at ambient temperature. The composite film was made into a dumbbell-shaped sample (length = 20 mm, width = 4 mm, thickness = 0.1 mm) and used for the tensile test at a speed of 60 mm / min. The test results are shown in Table 1.

[0066] Table 1. Film tensile test data in dry state and high humidity state

[0067]

[0068] N / A in Table 1 for Comparative Example 1 and Comparative Example 3 means that the material is too curled after being treated under 90% RH humidity for 72 hours and cut into a dumbbell-shaped sample, and is no longer suitable for film tensile test. As can be seen from Table 1, with the increase of the amount of lignin, the breaking strain of the lignin-based polyvinyl alcohol film gradually increases, and when the amount of lignin is 50%, the breaking strain is as high as 1216%. The breaking stress increases first and then decreases with the addition of lignin content. After being treated under 90% RH humidity for 72 hours, the breaking stress of all the sample of the examples is significantly improved, among which the sample of Example 1 (lignin content 30%) has the smallest decrease in breaking strain and the largest increase in breaking stress; when the lignin content is more than 20%, the composite film can still maintain good film morphology (satisfy the tensile test conditions) under high humidity environment treatment; when the lignin content is more than 30%, the breaking elongation under high humidity environment can be maintained above 200%, and the mechanical property is good.

[0069] 2. Resilience test:

[0070] Rebound property test in dry state: The lignin-based chemically crosslinked polyvinyl alcohol films of Examples 1-3, Comparative Examples 1-2, and the polyvinyl alcohol film of Comparative Example 3 were each made into a rectangular sample (length = 50 mm, width = 10 mm). The dry film rectangular sample was stretched to 3 times the initial length, held for 1 minute, and then released, and the length of the released film was measured (Note: the polyvinyl alcohol film of Comparative Example 3 could not be stretched to 3 times the initial length, so only a stretch from 40 cm to 70 cm was performed for the rebound property test).

[0071] Rebound property test after immersion: The lignin-based chemically crosslinked polyvinyl alcohol films of Examples 1-3, Comparative Examples 1-2, and the polyvinyl alcohol film of Comparative Example 3 were each cut into a rectangle (length = 40 or 50 mm, width = 10 mm), immersed in a glass bottle filled with tap water for 24 hours, and then the length of the film was measured after immersion. The wet film rectangular sample was then stretched to 3 times the initial length, held for 1 minute, and then released, and the length of the released film was measured.

[0072] The results of the rebound property test in dry state and after immersion are shown in Table 2, and the pictures of the test process are shown in Figure 1 and Figure 2

[0073] Table 2: Results of the rebound property test of the films in dry state and after immersion

[0074]

[0075]

[0076] Note: The formula for calculating the rebound rate in Table 2 is: Rebound rate (%) = (Total elongation - Non-recoverable elongation) / Total elongation x 100%; "N / A" in Table 2 indicates that the material swelled severely after immersion, such as Comparative Example 1 and Comparative Example 2, or the material was completely dissolved after immersion, such as Comparative Example 3, so the rebound property test could not be performed or was not suitable.

[0077] As can be seen from Table 2, the addition of lignin to the material provided the material with a rebound property, and the samples with a high lignin content, such as Examples 1-3, maintained the rebound property after immersion and showed a trend of improvement.

[0078] 3. Degradation performance test:

[0079] The lignin-based chemically crosslinked polyvinyl alcohol film of Example 3 (lignin content of 50 wt%) was used for the degradation performance test. The sample was cut into a semicircle for easy observation, and was buried in the natural soil at a depth of about 2 cm. The sample was observed and photographed every 4 days.

[0080] As can be seen from Table 2, the addition of lignin to the material provided the material with a rebound property, and the samples with a high lignin content, such as Examples 1-3, maintained the rebound property after immersion and showed a trend of improvement. Figure 3 ​As shown, it can be seen that after the end of the first cycle, the sample surface has obvious wrinkles; at the end of the second cycle, the sample has a tendency to shrink; at the end of the third cycle, the sample of Example 3 has a serious shrinkage. It can be seen that the lignin-based chemically cross-linked polyvinyl alcohol film of Example 3 becomes brittle and wrinkled after landfill treatment, which is consistent with the characteristics of the degradation process, and it can be seen that even if the lignin content reaches 50%, the lignin-based chemically cross-linked polyvinyl alcohol film still has good degradation performance.

[0081] The preferred embodiments of the present application have been described above with the aid of drawing figures, and are not intended to limit the present application, and for those skilled in the art, the present application can have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for preparing a lignin-based chemically crosslinked polyvinyl alcohol composite film, characterized by, The method comprises the following steps: dissolving lignin and succinic anhydride in a solvent, and reacting in the presence of a catalyst to obtain a succinic anhydride esterified lignin solution; adding polyvinyl alcohol to the succinic anhydride esterified lignin solution, and drying after reaction to form a film, thereby obtaining the lignin-based chemical crosslinking polyvinyl alcohol composite film; the catalyst is N,N-dimethylbenzylamine; the mass fraction of the lignin is 30-50% of the total mass of the lignin, succinic anhydride and polyvinyl alcohol; the mass ratio of the succinic anhydride to the polyvinyl alcohol is 1:(0.8-1.2).

2. The production method according to claim 1, wherein the total mass of the lignin, succinic anhydride and polyvinyl alcohol to the amount of the solvent is 1g:(2.5-5)mL.

3. The production method according to claim 1, wherein the solvent is selected from one or more of dimethyl sulfoxide, N,N-dimethylformamide or N,N-dimethylacetamide.

4. The production method according to claim 1, wherein the mass fraction of the catalyst is 15-25% of the mass of the succinic anhydride.

5. The production method according to claim 1, wherein in the step of reacting in the presence of the catalyst to obtain the succinic anhydride esterified lignin solution, the reaction temperature is 55-70℃, and the reaction time is 5-10h.

6. The production method according to claim 1, wherein in the step of drying after reaction to form a film, the reaction temperature is 55-70℃, and the reaction time is 5-10h.

7. The production method according to claim 1, wherein in the step of drying after reaction to form a film, the drying is vacuum drying, the drying temperature is 55-70℃, and the drying time is 8-24h.

8. The lignin-based chemical crosslinking polyvinyl alcohol composite film prepared by the preparation method in any one of claims 1-7.

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