Chitosan wet strength agent and preparation method thereof

Abstract

The invention relates to the technical field of papermaking, and particularly provides a chitosan wet strength agent and a preparation method thereof. The chitosan wet strength agent is prepared from the following components in percentage by mass: 1 to 5 percent of maleic anhydride, 3 to 15 percent of nano cellulose, 1 to 5 percent of chitosan, 0.1 to 1 percent of dispersing agent, 0.1 to 0.5 percent of pH (Potential of Hydrogen) regulator, 0.01 to 0.1 percent of initiator and the balance of deionized water. Chitosan and nano cellulose molecules are covalently bridged with the surface of the cellulose fiber at the same time through the in-situ grafting effect of maleic anhydride; the high specific surface area of the nanocellulose forms a physical enhanced network, and the cationic characteristic of the chitosan improves the electrostatic adsorption and retention rate of the auxiliary agent on the fiber; the three components synergistically construct a fiber-assistant composite cross-linked structure, so that the paper humidity is improved, and the defects that the traditional wet strength agent is poor in environmental protection property and the single modification way is limited in performance are overcome.

Description

Technical Field

[0001] The present invention relates to the technical field of papermaking. Specifically, the present invention relates to a chitosan wet strength agent and a preparation method thereof. Background Art

[0002] In the papermaking industry, the strength of paper drops sharply when it comes into contact with water. Usually, a wet strength agent needs to be added to maintain its tensile and tear resistance. Currently, mainstream wet strength agents, such as polyamide epichlorohydrin resin, can effectively improve wet strength, but toxic substances such as formaldehyde and epichlorohydrin are likely to remain during production or use. Moreover, such resins need to react under acidic conditions and are difficult to adapt to modern neutral / alkaline papermaking processes.

[0003] As a natural cationic polymer, chitosan can bind to fibers through hydrogen bonds and form a surface coating, and has been tried to replace synthetic wet strength agents. However, single chitosan still has the defects of limited wet strength enhancement, low retention rate, and poor hydrolysis resistance.

[0004] Therefore, how to improve the problems of poor environmental protection of existing wet strength agents and limited wet strength effects of single modification methods has become a technical problem that the industry urgently needs to break through. Summary of the Invention

[0005] The present invention provides a chitosan wet strength agent and a preparation method thereof. Through the in-situ grafting action of maleic anhydride, chitosan and nanocellulose molecules are covalently bridged to the surface of cellulose fibers at the same time; the high specific surface area of nanocellulose forms a physical reinforcement network, and the cationic characteristics of chitosan improve the electrostatic adsorption and retention rate of the additive on the fibers; the three cooperate to construct a "fiber - additive" composite cross-linked structure, thereby improving the humidity of paper and overcoming the defects of poor environmental protection of traditional wet strength agents and limited performance of single modification methods.

[0006] The present invention provides a preparation method of a chitosan wet strength agent. The chitosan wet strength agent contains the following components in mass percentages: maleic anhydride: 1 - 5%, nanocellulose: 3 - 15%, chitosan: 1 - 5%, dispersant: 0.1 - 1%, pH regulator: 0.1 - 0.5%, initiator: 0.01 - 0.1%, and the balance is deionized water;

[0007] Among them, the preparation method includes the following steps: S100. Add chitosan to deionized water, dissolve it under stirring conditions, and add a pH regulator for a first adjustment treatment to obtain a chitosan solution; S200. Disperse nanocellulose in deionized water containing a dispersant, perform ultrasonic treatment, and add a pH regulator for a second adjustment treatment to obtain a suspension; S300. After mixing the chitosan solution and the suspension, add maleic anhydride and an initiator to obtain a reaction solution; S400. Filter the reaction solution and cool it to room temperature to obtain a chitosan wet strength agent.

[0008] In any of the above technical solutions, step S300 specifically includes: S310, mixing the chitosan solution and the suspension to obtain a first mixed solution; S320, dropping maleic anhydride into the first mixed solution under a nitrogen atmosphere, and performing stirring treatment at 60 - 80 °C for 2 - 4 h to obtain a second mixed solution; S330, adding a polyhydroxy initiator to the second mixed solution, and reacting at 60 - 80 °C for 2 - 4 h to obtain a third mixed solution; S340, adding an amino / aldehyde group initiator to the third mixed solution to obtain a reaction solution.

[0009] In any of the above technical solutions, in step S330, the polyhydroxy initiator includes at least one of ammonium persulfate and potassium persulfate; and / or in step S340, the amino / aldehyde group initiator includes at least one of azodiisobutyramidine hydrochloride and glutaraldehyde.

[0010] In any of the above technical solutions, in step S310, the pH value of the first mixed solution is 4 - 5; and / or in step S320, the dropping rate of maleic anhydride is 1 - 2 mL / min; and / or in step S320, the stirring rate is 300 - 600 rpm.

[0011] In any of the above technical solutions, the dispersant includes at least one of polyvinyl alcohol and polyacrylamide; and / or the pH regulator includes at least one of acetic acid and sodium hydroxide.

[0012] In any of the above technical solutions, in S100, the pH value of the primary adjustment treatment is 4 - 6; in S200, the pH value of the secondary adjustment treatment is 7 - 8.

[0013] In any of the above technical solutions, the mass ratio of cellulose to chitosan is (1 - 3):1.

[0014] In any of the above technical solutions, in step S200, the power of the ultrasonic treatment is 200 - 500 W, the frequency is 20 - 40 kHz, the time is 30 - 60 min, and there is an intermittent period of 1 - 2 min every 3 - 6 min of ultrasonic treatment; in step S400, the filtration treatment uses a 0.2 - 0.5 μm microporous filter membrane, and the cooling rate is 2 - 5 °C / min.

[0015] In any of the above technical solutions, the degree of deacetylation of chitosan is 80 - 95%, and the diameter of the nanocellulose is 20 - 100 nm.

[0016] The present invention also provides a chitosan wet strength agent prepared by any of the above methods.

[0017] After adopting the technical solution of the present invention, the achievable technical effects are as follows:

[0018] 1. Introduce the cross-linking reaction of maleic anhydride and chitosan to generate water-resistant chemical bonds, effectively enhancing the mechanical strength of paper in the wet state. At the same time, when chitosan is deposited on the surface of the pulp under alkaline conditions, it can significantly improve the wet strength.

[0019] 2. Adopt the combination of nanocellulose and dispersant to enhance the dispersibility and stability of the wet strength agent, avoid precipitation, and ensure the long-term use effect. Nanocellulose, as a green biomass resource, has good dispersibility and stability, and can effectively enhance the wet strength performance of paper.

[0020] 3. The introduction of chitosan improves the film-forming property of the wet strength agent and its adhesion to fibers, enhancing the overall strength and durability of the paper. Chitosan, as a natural polymer, has good film-forming property and adhesiveness, and can form a uniform film on the fiber surface to improve the strength and durability of the paper. Specific embodiments

[0021] In order to more clearly understand the above-mentioned objects, features and advantages of the present invention, the present invention will be further described in detail below in conjunction with specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other. Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below. To make the above-mentioned objects, features and advantages of this aspect more obvious and understandable, the specific embodiments of this aspect will be described in detail below.

[0022] In the paper-making industry, the strength of paper drops sharply when it encounters water. Usually, a wet strength agent needs to be added to maintain its tensile and tear resistance. Currently, the mainstream wet strength agents, such as polyamide epichlorohydrin resin, can effectively improve the wet strength, but toxic substances such as formaldehyde and epichlorohydrin are likely to remain during production or use, and such resins need to react under acidic conditions and are difficult to adapt to modern neutral / alkaline papermaking processes.

[0023] Chitosan, as a natural cationic polymer, can bind to fibers through hydrogen bonds and form a surface coating, and has been tried to replace synthetic wet strength agents. However, single chitosan still has the defects of limited wet strength enhancement, low retention rate, and poor hydrolysis resistance.

[0024] Therefore, this embodiment provides a chitosan wet strength agent and a preparation method thereof. The chitosan wet strength agent contains the following components in mass percentage: maleic anhydride: 1 - 5%, nano - cellulose: 3 - 15%, chitosan: 1 - 5%, dispersant: 0.1 - 1%, pH regulator: 0.1 - 0.5%, initiator: 0.01 - 0.1%, and the balance is deionized water; through the in - situ grafting action of maleic anhydride, chitosan and nano - cellulose molecules are covalently bridged to the surface of cellulose fibers at the same time; the high specific surface area of nano - cellulose forms a physical reinforcement network, and the cationic property of chitosan improves the electrostatic adsorption and retention rate of the auxiliary agent on the fibers; the three cooperate to construct a "fiber - auxiliary agent" composite cross - linked structure, thereby improving the paper humidity and overcoming the defects of poor environmental protection of traditional wet strength agents and limited performance of single modification methods.

[0025] Preferably,

[0026] The preparation method includes the following steps:

[0027] S100. Add chitosan to deionized water, dissolve it under stirring conditions, and add a pH regulator for the first adjustment treatment to obtain a chitosan solution;

[0028] S200. Disperse nano - cellulose in deionized water containing a dispersant, perform ultrasonic treatment, and add a pH regulator for the second adjustment treatment to obtain a suspension;

[0029] S300. After mixing the chitosan solution and the suspension, add maleic anhydride and an initiator to obtain a reaction solution;

[0030] S400. Filter the reaction solution and cool it to room temperature to obtain the chitosan wet strength agent.

[0031] Preferably, in step S100, the solution is adjusted to weakly acidic by the pH regulator, and the pH value is in the range of 4 - 6 to activate the amino group of chitosan and provide active sites for subsequent reactions with maleic anhydride. The pH regulator can be at least one of acetic acid and citric acid; if the pH is too low, chitosan will undergo acidolysis, resulting in molecular chain breakage and reducing the mechanical strength of the final product; if the pH is too high, chitosan cannot be completely dissolved, forming flocculent suspensions, resulting in uneven solutions, and the undissolved particles will hinder the dispersion of nano - cellulose, and local brittle regions may appear in the final product.

[0032] Furthermore, the degree of deacetylation of chitosan is 80 - 95% to ensure that sufficient free amino groups participate in subsequent reactions; if the degree of deacetylation is too low, the number of amino groups is insufficient, resulting in a decrease in the grafting rate of maleic anhydride.

[0033] Preferably, in step S200, nanocellulose is dispersed in deionized water containing a dispersant, ultrasonic treatment is carried out, and a pH regulator is added to adjust the pH value to 7-8 to obtain a uniform suspension; nanocellulose has a high surface area and strong hydrogen bond interactions, and is prone to agglomeration. Ultrasonic treatment and a dispersant are used together to disperse nanocellulose; among them, ultrasonic treatment can effectively destroy its agglomerated structure. The power of ultrasonic treatment is 200-500 W, the frequency is 20-40 kHz, and the time is 30-60 min, and there is an intermittent period of 1-2 min every 3-6 min of ultrasonic treatment. Excessive ultrasonic power may damage the nanocellulose structure, and it needs to be treated in batches to avoid gelation; the dispersant helps to stabilize the dispersion system and ensure the stability of nanocellulose in the aqueous phase. The dispersant can be at least one of polyvinyl alcohol and polyacrylamide, and the steric hindrance effect is used to maintain the stability of nanocellulose in the aqueous phase. If there is agglomeration in the suspension, a locally high-concentration region will be formed when mixed with the chitosan solution, resulting in uneven grafting reaction and reducing the maleic anhydride grafting efficiency.

[0034] Furthermore, to adjust the pH of the nanocellulose suspension to 7-8, a sodium hydroxide solution can be used to keep the pH away from the isoelectric point of nanocellulose, and electrostatic repulsion is generated by enhancing the surface negative charge to effectively prevent agglomeration; the diameter of nanocellulose is 20-100 nm, and the smaller size helps to increase the specific surface area and enhance the interfacial binding with chitosan.

[0035] Furthermore, the mass ratio of nanocellulose to the chitosan is (1-3):1. Nanocellulose can self-assemble into a continuous three-dimensional scaffold network in the chitosan matrix, providing a strong mechanical skeleton for the composite system; it can not only effectively disperse the applied stress, but also build more contact points between the chitosan molecular chains, optimize the load transfer, thereby forming a similar nano-reinforced structure in the paper and greatly improving the wet tensile strength. In addition, the abundant hydroxyl groups on the surface of nanocellulose and the amino groups of chitosan can interact tightly through hydrogen bonds; chitosan itself is positively charged under weak acid to neutral conditions, and can further enhance the interfacial binding force through electrostatic adsorption with the surface of nanocellulose and cellulose fibers. Within this ratio range, chitosan molecules can uniformly coat nanocellulose, inhibit fiber agglomeration, and maintain good nano-scale dispersibility; when the ratio of nanocellulose exceeds 3:1, excessive nanocellulose is prone to entanglement and phase separation, resulting in an uneven network and a decrease in the strengthening effect. In the wet state, the closely arranged nanocellulose network can slow down the rapid penetration of water, and cooperate with the covalent cross-linking and electrostatic adsorption of chitosan to jointly delay the destruction of hydrogen bonds between fibers, significantly improving the durability of wet strength.

[0036] Preferably, in step S300, the chitosan solution is mixed with the nanocellulose suspension, maleic anhydride and an initiator are added, and the reaction is carried out at 60 - 80 °C for 2 - 4 hours under nitrogen protection to complete the crosslinking reaction. Maleic anhydride, as a crosslinking agent, can form covalent bonds between chitosan and nanocellulose, enhancing the wet strength performance. The initiator initiates a free radical reaction under mild conditions to ensure the high efficiency and controllability of the reaction.

[0037] Further, step S300 specifically includes:

[0038] S310. After mixing the chitosan solution with the suspension, a first mixed solution is obtained;

[0039] S320. Under a nitrogen atmosphere, maleic anhydride is dropped into the first mixed solution, and stirring treatment is carried out at 60 - 80 °C for 2 - 4 h to obtain a second mixed solution;

[0040] S330. A polyhydroxy initiator is added to the second mixed solution, and the reaction is carried out at 60 - 80 °C for 2 - 4 h to obtain a third mixed solution;

[0041] S340. An amino / aldehyde group-containing initiator is added to the third mixed solution to obtain a reaction solution.

[0042] Preferably, in step S310, the chitosan solution is mixed with the suspension to obtain a first mixed solution. The pH value of the first mixed solution is 4 - 5, which helps to uniformly disperse chitosan and nanocellulose and ensure the uniformity of subsequent reactions; and within this pH range, the amino group of chitosan is in a partially protonated state and carries a positive charge, which is conducive to its combination with negatively charged nanocellulose through electrostatic interaction, promoting the crosslinking reaction and thus improving the performance of the final wet strength agent; at lower or higher pH values, chitosan may precipitate or its solubility may decrease, affecting the efficiency of the crosslinking reaction and the wet strength performance.

[0043] For example, in step S320, under a nitrogen atmosphere, maleic anhydride is dropped into the first mixed solution and stirred at 60 - 80 °C for 2 - 4 hours to obtain a second mixed solution; maleic anhydride, as a crosslinking agent, reacts with the amino group in chitosan to form an amide bond, enhancing the crosslinking degree of chitosan and improving the wet strength performance; carrying out the reaction under a nitrogen atmosphere helps to prevent the occurrence of oxidation reactions and ensure the smooth progress of the crosslinking reaction. Among them, the dropping rate of maleic anhydride is 1 - 2 mL / min, and the stirring treatment rate is 300 - 600 rpm. Slowly dropping can ensure the uniform distribution of maleic anhydride in the reaction system and promote a uniform crosslinking reaction; an appropriate stirring rate helps to evenly distribute the heat in the reaction system, avoid local overheating or cooling, and ensure the stable progress of the reaction.

[0044] Specifically, in step S330, a polyhydroxy initiator is added to the second mixed solution, and the reaction is carried out at 60-80°C for 2-4 hours to obtain a third mixed solution. The polyhydroxy initiator includes at least one of ammonium persulfate and potassium persulfate. Ammonium persulfate or potassium persulfate decomposes under heating conditions to generate free radicals, which can initiate a crosslinking reaction between chitosan and nanocellulose. Such initiators have high reactivity and can effectively promote the crosslinking reaction at low temperatures, thereby increasing the degree of crosslinking. In step S340, an amino / aldehyde-containing initiator is added to the third mixed solution to obtain a reaction solution. The amino / aldehyde-containing initiator includes at least one of azobisisobutylamidine hydrochloride and glutaraldehyde. Azobisisobutylamidine hydrochloride or glutaraldehyde provides amino or aldehyde groups during the reaction, participates in the crosslinking reaction, and enhances the stability of the crosslinked network. This further promotes the crosslinking reaction, forming a stronger network structure and improving wet strength. In general, the polyhydroxy initiator is added in step S330 for free radical grafting, and the amino / aldehyde-containing initiator is added in step S340 for crosslinking.

[0045] Preferably, in step S400, the reaction solution is filtered to remove unreacted impurities and cooled to room temperature to obtain the final chitosan wet strength agent. The filtration treatment can remove unreacted substances and improve the purity of the product. After cooling to room temperature, the product is stable and easy to store and use. The filtration treatment uses a 0.2-0.5μm microporous filter membrane with a cooling rate of 2-5℃ / min. Gradient cooling can inhibit the excessive crystallization of chitosan molecular chains and avoid increased brittleness of the wet strength agent after film formation.

[0046] Example 1

[0047] This embodiment provides a chitosan wet strength agent and a preparation method thereof. The chitosan wet strength agent comprises the following components in percentage by mass:

[0048] Maleic anhydride: 2%, nanocellulose: 5%, chitosan: 5%, polyvinyl alcohol: 0.5%, acetic acid: 0.3%, sodium hydroxide: 0.2%, ammonium persulfate: 0.03%, azobisisobutylamidine hydrochloride: 0.03%, and the balance is deionized water;

[0049] Wherein, the preparation method comprises the following steps:

[0050] S100, adding chitosan with a deacetylation degree of 90% into deionized water, dissolving it under stirring, and adding acetic acid to adjust the pH value to 5, performing a first adjustment treatment to obtain a chitosan solution;

[0051] S200. Disperse nanocellulose with a diameter of 30 nm in deionized water containing polyvinyl alcohol. Perform ultrasonic treatment at a power of 300 W, a frequency of 30 kHz, and a time of 40 min, with an intermittent period of 1 min every 5 min of ultrasonic treatment. Add sodium hydroxide to adjust the pH value to 7 and perform a secondary adjustment treatment to obtain a suspension;

[0052] S310. Mix the chitosan solution and the suspension, and adjust the pH value to 5 to obtain a first mixed solution;

[0053] S320. Under a nitrogen atmosphere, dropwise add maleic anhydride to the first mixed solution at a dropping rate of 1 mL / min, and perform stirring treatment at 70 °C and a stirring rate of 450 rpm for 3 h to obtain a second mixed solution;

[0054] S330. Add ammonium persulfate to the second mixed solution and react at 70 °C for 3 h to obtain a third mixed solution;

[0055] S340. Add azobisisobutyramidine hydrochloride to the third mixed solution and react at 50 °C for 3 h to obtain a reaction solution;

[0056] S400. Filter the reaction solution through a 0.3-μm microporous membrane at a cooling rate of 3 °C / min, and cool it to room temperature to obtain the chitosan wet strength agent.

[0057] Example 2

[0058] This example provides a chitosan wet strength agent and its preparation method. The chitosan wet strength agent contains the following components in mass percentages:

[0059] Maleic anhydride: 5%, nanocellulose: 15%, chitosan: 5%, polyacrylamide: 1%, acetic acid: 0.5%, sodium hydroxide: 0.5%, potassium persulfate: 0.05%, glutaraldehyde: 0.05%, and the balance is deionized water;

[0060] Among them, the preparation method includes the following steps:

[0061] S100. Add chitosan with a deacetylation degree of 95% to deionized water, dissolve it under stirring conditions, and add acetic acid to adjust the pH value to 6 for a primary adjustment treatment to obtain a chitosan solution;

[0062] S200. Disperse nanocellulose with a diameter of 100 nm in deionized water containing polyacrylamide. Perform ultrasonic treatment at a power of 500 W, a frequency of 40 kHz, and a time of 60 min, with an intermittent period of 2 min every 6 min of ultrasonic treatment. Add sodium hydroxide to adjust the pH value to 8 and perform a secondary adjustment treatment to obtain a suspension;

[0063] S310. After mixing the chitosan solution and the suspension, adjust the pH value to 5 to obtain a first mixed solution;

[0064] S320. Under a nitrogen atmosphere, add maleic anhydride dropwise to the first mixed solution at a dropping rate of 2 mL / min, and carry out stirring treatment at 80 °C with a stirring rate of 600 rpm for 4 h to obtain a second mixed solution;

[0065] S330. Add potassium persulfate to the second mixed solution and react at 80 °C for 4 h to obtain a third mixed solution;

[0066] S340. Add glutaraldehyde to the third mixed solution and react at 60 °C for 4 h to obtain a reaction solution;

[0067] S400. Filter the reaction solution through a 0.5-μm microporous filter membrane at a cooling rate of 2 °C / min, and cool it to room temperature to obtain the chitosan wet strength agent.

[0068] Example 3

[0069] This example provides a chitosan wet strength agent and a preparation method thereof. The chitosan wet strength agent contains the following components in mass percentage:

[0070] Maleic anhydride: 1%, nanocellulose: 4%, chitosan: 2%, polyvinyl alcohol: 1%, citric acid: 0.1%, sodium hydroxide: 0.1%, ammonium persulfate: 0.01%, azobisisobutyramidine hydrochloride: 0.01%, and the balance is deionized water;

[0071] Among them, the preparation method includes the following steps:

[0072] S100. Add chitosan with a deacetylation degree of 80% to deionized water, dissolve it under stirring conditions, and add citric acid to adjust the pH value to 4 for a primary adjustment treatment to obtain a chitosan solution;

[0073] S200. Disperse nanocellulose with a diameter of 20 nm in deionized water containing polyvinyl alcohol, carry out ultrasonic treatment at a power of 200 W, a frequency of 20 kHz, and a time of 30 min, with an intermittent period of 1 min every 3 min of ultrasonic treatment, and add sodium hydroxide to adjust the pH value to 7 for a secondary adjustment treatment to obtain a suspension;

[0074] S310. After mixing the chitosan solution and the suspension, adjust the pH value to 5 to obtain a first mixed solution;

[0075] S320. Under a nitrogen atmosphere, add maleic anhydride dropwise to the first mixed solution at a dropping rate of 2 mL / min, and carry out stirring treatment at 60 °C with a stirring rate of 300 rpm for 2 h to obtain a second mixed solution;

[0076] S330. Add potassium persulfate to the second mixed solution, and react at 60 °C for 2 h to obtain a third mixed solution;

[0077] S340. Add glutaraldehyde to the third mixed solution, and react at 50 °C for 2 h to obtain a reaction solution;

[0078] S400. Filter the reaction solution through a 0.2-μm microporous membrane with a cooling rate of 5 °C / min, and cool to room temperature to obtain the chitosan wet strength agent.

[0079] Comparative Example 1

[0080] This comparative example provides a chitosan wet strength agent, which is obtained by purchasing from outside.

[0081] Comparative Example 2

[0082] This comparative example provides a wet strength agent, which is obtained by purchasing PAE resin from outside.

[0083] Test data

[0084] Put Examples 1-5 and Comparative Examples 1-2 into a gas chromatograph to calculate the formaldehyde concentration, and the results are shown in Table 1; calculated by mass percentage, add Examples 1-3 and Comparative Examples 1-2 to the pulp at 0.5%, and perform conventional forming, pressing, drying and other steps to obtain paper; take the paper samples of Examples 1-3 and Comparative Examples 1-2, with a size of 50 mm * 200 mm, soak them completely wet in deionized water, stretch them to break through a tensile testing machine, record the maximum tensile force value, and calculate the wet tensile strength and retention rate; apply a vertical pressure through a spherical bursting strength tester or a bursting strength tester to measure the maximum force value at the time of rupture, and the results are shown in Table 1:

[0085] Table 1

[0086]

[0087] The wet tensile strength of Example 1 reaches 2.3 kN / m, and the retention rate is 18.5%, which is significantly higher than that of Comparative Examples 1-2, proving the synergistic enhancement of the three-dimensional network of nanocellulose and the covalent cross-linking of chitosan-maleic anhydride; the strength of Example 2 is increased to 2.8 kN / m due to the stronger physical support provided by the high content of nanocellulose; the performance of Example 3 is the lowest but still better than that of Comparative Examples 1-2; Comparative Example 1 is obtained by purchasing from outside, without introducing nanocellulose and maleic anhydride cross-linking, and the wet strength retention rate is only 6.5%, indicating that the modification effect of traditional chitosan is limited.

[0088] The bursting strength of Examples 1-3 is 2.1-2.9 times that of Comparative Example 1. Due to the high modulus network of the nanocellulose in this application, the crack propagation is retarded, and the chitosan-maleic anhydride covalent bond remains stable in a wet environment. The bursting strength of Comparative Example 1 is low because the single chitosan relies on hydrogen bonds and is easily dissociated in water.

[0089] No formaldehyde residue was detected in Examples 1-3, meeting the food-grade / medical material standards. However, in Comparative Examples 1-2, the residual formaldehyde was 72 ppm and 78 ppm, far exceeding the national standard limit.

[0090] Generally speaking, through the nanocellulose-chitosan-maleic anhydride synergistic system, the wet tensile strength and bursting strength of Examples 1-3 are significantly better than those of Comparative Examples 1-2, and there is no formaldehyde residue, meeting the environmental protection and high-performance requirements of modern papermaking processes.

[0091] In the description of this specification, the descriptions of terms such as "one embodiment", "some embodiments", "specific embodiments", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or instance. Moreover, the specific features, structures, materials, or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention.

Claims

1. A preparation method of a chitosan wet strength agent, characterized in that, The chitosan wet strength agent contains the following components in mass percentages: Maleic anhydride: 1 - 5%, nano - cellulose: 3 - 15%, chitosan: 1 - 5%, dispersant: 0.1 - 1%, pH regulator: 0.1 - 1%, initiator: 0.01 - 0.1%, and the balance is deionized water; Among them, the preparation method includes the following steps: S100. Add the chitosan to deionized water, dissolve it under stirring conditions, and add the pH regulator for the first adjustment treatment to obtain a chitosan solution; S200. Disperse the nano - cellulose in deionized water containing the dispersant, perform ultrasonic treatment, and add the pH regulator for the second adjustment treatment to obtain a suspension; S300. After mixing the chitosan solution and the suspension, add maleic anhydride and the initiator to obtain a reaction solution; S400. Filter the reaction solution and cool it to room temperature to obtain the chitosan wet strength agent.

2. The preparation method according to claim 1, characterized in that, The step S300 specifically includes: S310. After mixing the chitosan solution and the suspension, obtain a first mixed solution; S320. Under a nitrogen atmosphere, dropwise add maleic anhydride to the first mixed solution, and perform stirring treatment at 60 - 80 °C for 2 - 4 h to obtain a second mixed solution; S330. Add a polyhydroxy - type initiator to the second mixed solution, and react at 60 - 80 °C for 2 - 4 h to obtain a third mixed solution; S340. Add an amino / aldehyde - group - containing initiator to the third mixed solution, and react at 50 - 60 °C for 2 - 4 h to obtain the reaction solution.

3. According to the preparation method described in claim 2, characterized in that in step S330, the polyhydroxy - type initiator includes at least one of ammonium persulfate and potassium persulfate; and / or in step S340, the amino / aldehyde - group - containing initiator includes at least one of azodiisobutyramidine hydrochloride and glutaraldehyde.

4. According to the preparation method described in claim 2, characterized in that in step S310, the pH value of the first mixed solution is 5 - 6; and / or in step S320, the dropping rate of maleic anhydride is 1 - 2 mL / min; and / or in step S320, the stirring rate is 300 - 600 rpm.

5. According to the preparation method described in claim 1, characterized in that the dispersant includes at least one of polyvinyl alcohol and polyacrylamide; and / or the pH regulator includes at least one of acetic acid, citric acid, and sodium hydroxide.

6. According to the preparation method described in claim 1, characterized in that in S100, the pH value of the first adjustment treatment is 4 - 6; in S200, the pH value of the second adjustment treatment is 7 - 8.

7. The preparation method according to claim 1, characterized in that, The mass ratio of the nano - cellulose to the chitosan is (1 - 3):

1.

8. According to the preparation method described in claim 1, characterized in that in step S200, the power of the ultrasonic treatment is 200 - 500 W, the frequency is 20 - 40 kHz, the time is 30 - 60 min, and there is an intermittent period of 1 - 2 min every 3 - 6 min of ultrasonic treatment; In step S400, the filtration treatment uses a 0.2 - 0.5 μm microporous filter membrane, and the cooling rate is 2 - 5 °C / min.

9. The preparation method according to claim 1, characterized in that, The degree of deacetylation of the chitosan is 80 - 95%, and the diameter of the nanocellulose is 20 - 100 nm.

10. A chitosan wet strength agent, characterized in that, The chitosan wet strength agent is prepared by any one of claims 1 - 9.

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