Cationic polyurethane wet strength agent for papermaking and preparation method thereof

Abstract

The invention discloses a cationic polyurethane wet strength agent for papermaking and a preparation method thereof, belonging to the technical field of papermaking additive processing, and is used to solve the technical problem that the effect of the cationic polyurethane wet strength agent in the prior art on the wet tensile strength of a papermaking sample needs to be further improved; the invention comprises the following components by weight: 60-70 parts of a polyurethane emulsion, 4-6 parts of a modified polyethyleneimine and 2.8-3.2 parts of a chitosan mixture, and the cationic polyurethane emulsion containing a quaternary ammonium salt block is prepared, and the modified polyethyleneimine and the chitosan mixture are coordinated with each other, so as to effectively improve the wet tensile strength and the tensile strength retention rate of the papermaking sample.

Description

Technical Field

[0001] The invention relates to the technical field of papermaking additive processing, and in particular to a cationic polyurethane wet strength agent for papermaking and a preparation method thereof. Background Art

[0002] In the papermaking industry, wet strength is an important indicator to measure the mechanical properties of paper in a wet environment. Especially for special-purpose papers such as packaging paper, medical paper, label paper, etc., high wet strength is an indispensable performance requirement. Wet strength agent is a crucial auxiliary agent that can significantly improve the strength of paper in a wet state, so that the paper can still maintain good physical properties in a humid environment.

[0003] In the prior art, a Chinese invention patent with publication number CN105085906A discloses a method for preparing a papermaking wet strength agent, comprising the following steps: (1) subjecting a polyamine, adipic acid ester, a dimer acid and a catalyst to a polymerization reaction under heating conditions to obtain a polyamide polyamine prepolymer aqueous solution; (2) adding water, a cationic etherifying agent and epichlorohydrin to the prepolymer solution, reacting at room temperature and then heating the solution for further reaction, and finally adding an acid to adjust the pH value to obtain a papermaking wet strength agent with a solid content of 15-35%. The advantages of the method are that the polymerization process is mild, stable and controllable, the product is not prone to over-reaction and gelation, the solid content is high, the monomer reaction is complete, the product has a high degree of polymerization and good bonding performance, the product has a low curing temperature, is easy to mature and has good water resistance.

[0004] However, the polyurethane wet strength agent in the prior art is not cationic enough and has a weak binding force with negatively charged cellulose fibers. In addition, the polyurethane wet strength agent is affected by its dispersibility in aqueous solution, which will lead to the uniformity of its distribution in the papermaking pulp, so that the wet tensile strength of the papermaking sample needs to be further improved.

[0005] In view of the technical defects in this aspect, a solution is now proposed. Summary of the invention

[0006] The object of the present invention is to provide a cationic polyurethane wet strength agent for papermaking and a preparation method thereof, so as to solve the technical problem that the effect of the cationic polyurethane wet strength agent on the wet tensile strength of papermaking samples in the prior art needs to be further improved.

[0007] The object of the present invention can be achieved by the following technical scheme: A cationic polyurethane wet strength agent for papermaking, comprising the following components in parts by weight: 60-70 parts of polyurethane emulsion, 4-6 parts of modified polyethyleneimine and 2.8-3.2 parts of chitosan mixture;

[0008] The preparation method of the polyurethane emulsion comprises the following steps: under the protection of inert gas, polytetrahydrofuran ether diol, dihydroxy quaternary ammonium salt, catalyst and tetrahydrofuran are mixed, the temperature of the reaction system is raised to 60-65° C., a diisocyanate solution is added dropwise to the reaction system, the reaction is carried out by heat preservation for 2-3 hours, trimethylolethane is added dropwise to the reaction system, the reaction is carried out by heat preservation for 50-60 minutes, 2-(2-aminoethyl) 1,3-propylene glycol is added to the reaction system, the reaction is carried out by heat preservation for 40-60 minutes, and post-treatment is performed to obtain the polyurethane emulsion.

[0009] The synthetic reaction formula of polyurethane emulsion is:

[0010]

[0011] Where:

[0012] ;

[0013] .

[0014] The synthetic reaction mechanism of polyurethane emulsion is:

[0015] During the reaction, the hydroxyl groups on the polytetramethylene ether diol or dihydroxy quaternary ammonium salt molecules react with the isocyanate groups on the isophorone diisocyanate molecules to generate urethane bonds, thereby forming a polyurethane prepolymer co-embedded with the polytetramethylene ether diol and the dihydroxy quaternary ammonium salt. The three hydroxyl groups in the trimethylolethane react with the isocyanate groups in the prepolymer to generate a polyurethane having a three-dimensional network structure. The amino groups on the 2-(2-aminoethyl)1,3-propanediol molecules have high activity and will preferentially react with the terminal isocyanate groups on the polyurethane molecules to form dihydroxyl end capping on the polyurethane molecules. After high-speed emulsification and dispersion of solvent water, the amount of water added is controlled to prepare polyurethane emulsions with different solid contents.

[0016] Furthermore, the chitosan mixture consists of chitosan quaternary ammonium salt and carboxymethyl cellulose in a weight ratio of 3:1.

[0017] Furthermore, the amount ratio of the polytetrahydrofuran ether diol, dihydroxy quaternary ammonium salt, catalyst, tetrahydrofuran, diisocyanate solution, trimethylolethane and 2-(2-aminoethyl) 1,3-propylene glycol is 5g:2g:0.1g:8mL:8g:2g:1g, the catalyst is dibutyltin dilaurate, the diisocyanate solution is composed of isophorone diisocyanate and tetrahydrofuran in a ratio of 1g:1g, and the post-treatment includes: after the reaction is completed, the temperature of the reaction system is reduced to room temperature, purified water is added to the reaction system, and emulsification and dispersion are performed for 30-50 minutes to obtain a polyurethane emulsion with a solid content of 15-20%.

[0018] Furthermore, the preparation method of the dihydroxy quaternary ammonium salt is: 1,1'-(butylimino)dimethylol and 1-chlorobutane are stirred and mixed in a closed environment, the reaction system is placed in a heat exchange medium at a temperature of 140-150°C, the reaction is kept warm for 35-40 hours, and post-processed to obtain the dihydroxy quaternary ammonium salt.

[0019] The synthetic reaction formula of dihydroxy quaternary ammonium salt is:

[0020]

[0021] The synthetic reaction mechanism of dihydroxy quaternary ammonium salt is:

[0022] In the reaction process, the nitrogen atom with a lone pair of electrons in 1,1'-(butylimino)dimethylol is used as a nucleophilic active site to attack the carbon atom in 1-chlorobutane, and the nitrogen atom forms a partial bond with the carbon atom. At the same time, the chlorine atom acts as a leaving group, and the bond between the chlorine atom and the carbon atom is gradually broken and separated, forming a five-coordinated transition state. The nitrogen atom forms a stable CN bond with the carbon atom to generate a quaternary ammonium salt, and a dihydroxy quaternary ammonium salt is prepared;

[0023] The mass spectrometry data of dihydroxy quaternary ammonium salt are:

[0024] m / z: 225.1496 (100.0%), 227.1466 (32.0%), 226.1529 (10.8%), 228.1500 (3.5%); molecular weight is 225.76.

[0025] Furthermore, the usage ratio of 1,1'-(butylimino)dimethylol and 1-chlorobutane is 1 mol:1.3 mol, and the post-treatment includes: after the reaction is completed, the temperature of the reaction system is reduced to 60-70°C, and low-boiling substances are evaporated under reduced pressure to obtain a dihydroxy quaternary ammonium salt.

[0026] Furthermore, the preparation method of the modified polyethyleneimine solution is: mix polyethyleneimine, ethanol solution, catalyst and sodium carbonate, stir until the system is dissolved, increase the temperature of the reaction system to 50-60°C, add epichlorohydrin to the reaction system, keep the reaction warm for 2-3h, and post-treat to obtain modified polyethyleneimine.

[0027] The synthetic reaction formula of modified polyethyleneimine solution is:

[0028]

[0029] The synthetic reaction mechanism of modified polyethyleneimine is:

[0030] During the reaction, ethanol is used as an organic solvent to reduce the polarity of water and the possibility of epoxy ring opening. Tetrabutylammonium bromide is used as a phase transfer catalyst, and sodium carbonate provides weak alkaline conditions to promote the nucleophilic substitution reaction between the halogen on the epichlorohydrin molecule and the amino group in the polyethyleneimine, while avoiding the epoxy ring opening condensation, thereby preparing epoxy-modified modified polyethyleneimine.

[0031] Furthermore, the dosage ratio of the polyethyleneimine, the ethanol solution, the catalyst, the sodium carbonate and the epichlorohydrin is 5g:10mL:0.1g:1.5g:1.3g, the ethanol solution is a 60-70vol% ethanol aqueous solution, the catalyst is tetrabutylammonium bromide, and the post-treatment includes: after the reaction is completed, adjusting the pH of the reaction system to 7, raising the temperature of the reaction system to 70°C, and removing low-boiling substances under reduced pressure to obtain modified polyethyleneimine.

[0032] The invention also provides a method for preparing a cationic polyurethane wet strength agent for papermaking, which comprises uniformly mixing a polyurethane emulsion, a modified polyethyleneimine solution and a chitosan mixture to obtain the cationic wet strength agent.

[0033] The present invention has the following beneficial effects:

[0034] 1. The cationic polyurethane wet strength agent for papermaking of the present invention is prepared by using isophorone diisocyanate as a crosslinking agent, trimethylolethane as a crosslinking agent, and 2-(2-aminoethyl) 1,3-propylene glycol as a capping agent to obtain a polyurethane emulsion of block copolymerization and crosslinking of polytetrahydrofuran ether diol and dihydroxy quaternary ammonium salt. The quaternary ammonium salt introduced into the polyurethane molecular chain can be combined with negatively charged cellulose fibers in papermaking pulp through electrostatic adsorption to enhance the hydrogen bond network between pulp fibers. The polytetrahydrofuran ether diol in the polyurethane molecule is a flexible hydrophobic segment, which undergoes dynamic rearrangement in a wet environment, partially compensates for the destroyed bonding through hydrogen bond reorganization, and resists water penetration with the crosslinked network of the polyurethane molecules, slows down the destruction of hydrogen bonds between fibers, and maintains wet strength.

[0035] 2. The cationic polyurethane wet strength agent for papermaking of the present invention is modified by modifying polyethyleneimine, and a large number of non-hydrophilic epoxy groups are modified on the hydrophilic polyethyleneimine molecular chain, so that it is easier to disperse in the polyurethane emulsion, and the hydrophobic segment of the polyurethane and the hydrophilic segment of the modified polyethyleneimine work together to improve the stability of the paper in a wet environment. The unreacted amine groups on the modified polyethyleneimine molecular chain can form hydrogen bonds with water molecules in water, and accept protons in water to carry positive charges, which can be combined with negatively charged cellulose fibers in papermaking pulp through electrostatic adsorption. In addition, the epoxy group will open the ring under appropriate conditions and undergo a condensation reaction with active groups such as hydroxyl or amino groups in the papermaking pulp molecules to generate covalent bonds such as imine and ether bonds. The formation of these covalent bonds will not be broken due to the wetting of the paper, thereby further improving the tensile strength of the papermaking sample under wet conditions.

[0036] 3. The cationic polyurethane wet strength agent for papermaking of the present invention is prepared by mixing chitosan quaternary ammonium salt and carboxymethyl cellulose to form a chitosan mixture. The chitosan quaternary ammonium salt molecules contain cationic groups, which can be combined with negatively charged cellulose fibers through electrostatic adsorption. The chitosan quaternary ammonium salt and the modified polyethyleneimine in the polyurethane emulsion cooperate with each other to further enhance the hydrogen bond network between papermaking fibers. The hydrophilic properties of the chitosan quaternary ammonium salt and the hydroxymethyl cellulose regulate the permeability of the wet strength agent, so that the wet strength agent can be more evenly dispersed in the papermaking slurry, promote more uniform crosslinking, and further improve the wet tensile strength of the papermaking sample. DETAILED DESCRIPTION

[0037] The technical solution of the present invention will be clearly and completely described below in conjunction with the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] In the present application, the molecular weight of polytetramethylene ether glycol is 650±25, and the hydroxyl number is 166.2-179.5 mgKOH / g;

[0039] In this application, the chitosan quaternary ammonium salt is selected from Green Union (Jining) Chemical Technology Co., Ltd., the chemical name is hydroxypropyltrimethylammonium chloride chitosan, and the content is 90%;

[0040] In the present application, polyethyleneimine was selected from Hubei Yamade Biopharmaceutical Co., Ltd., with a molecular weight of 5000, a density of 1.030 g / mL at 25°C, a boiling point of 250°C (lit), and a melting point of 59-60°C. Example 1

[0041] This embodiment provides a method for preparing a cationic polyurethane wet strength agent for papermaking, comprising the following steps:

[0042] S1. Preparation of dihydroxy quaternary ammonium salt

[0043] Weigh: 133.2 g of 1,1'-(butylimino)dimethylol and 120.3 g of 1-chlorobutane, add into a sealed three-necked flask, fix the three-necked flask in an oil bath with mechanical stirring and stir, set the oil bath temperature to 140°C, keep warm for 35 hours, lower the temperature of the three-necked flask to 60°C, evaporate low boiling points under reduced pressure to obtain a dihydroxy quaternary ammonium salt.

[0044] S2. Preparation of polyurethane emulsion

[0045] isophorone diisocyanate and tetrahydrofuran are uniformly mixed in a weight ratio of 1:1 to obtain a diisocyanate solution;

[0046] Weigh: 50 g of polytetramethylene glycol, 20 g of dihydroxy quaternary ammonium salt, 1 g of dibutyltin dilaurate and 80 mL of tetrahydrofuran, add into a three-necked flask protected by nitrogen and stir, raise the temperature of the three-necked flask to 60°C, add 80 g of diisocyanate solution dropwise into the three-necked flask, keep warm and react for 2 h, add 20 g of trimethylolethane dropwise into the three-necked flask, keep warm and react for 50 min, add 10 g of 2-(2-aminoethyl)1,3-propylene glycol into the three-necked flask, keep warm and react for 40 min, lower the temperature of the three-necked flask to room temperature, add purified water into the three-necked flask, emulsify and disperse for 30 min, and obtain a polyurethane emulsion with a solid content of 15%.

[0047] S3. Preparation of modified polyvinyl amide solution

[0048] Weigh: 50 g of polyethyleneimine, 100 mL of 60 vol% ethanol aqueous solution, 1 g of tetrabutylammonium bromide and 15 g of sodium carbonate, add into a three-necked flask, raise the temperature of the three-necked flask to 50°C, stir until the system is dissolved, add 13 g of epichlorohydrin into the three-necked flask, keep warm for 2 h, adjust the pH of the reaction system to 7, raise the temperature of the reaction system to 70°C, and remove low-boiling substances under reduced pressure to obtain modified polyethyleneimine.

[0049] S4. Preparation of chitosan mixture

[0050] Chitosan quaternary ammonium salt and carboxymethyl cellulose are mixed in a weight ratio of 3:1 to obtain a chitosan mixture.

[0051] S5. Preparation of cationic wet strength agent

[0052] Weigh 60 parts of polyurethane emulsion, 15 parts of modified polyethyleneimine solution and 2.8 parts of chitosan mixture by weight and mix them evenly to obtain a cationic wet strength agent. Example 2

[0053] This embodiment provides a method for preparing a cationic polyurethane wet strength agent for papermaking, comprising the following steps:

[0054] S1. Preparation of dihydroxy quaternary ammonium salt

[0055] Weigh: 133.2 g of 1,1'-(butylimino)dimethylol and 120.3 g of 1-chlorobutane, add into a sealed three-necked flask, fix the three-necked flask in an oil bath with mechanical stirring and stir, set the oil bath temperature to 145°C, keep warm for 37 hours, reduce the temperature of the three-necked flask to 65°C, evaporate low boiling points under reduced pressure to obtain a dihydroxy quaternary ammonium salt.

[0056] S2. Preparation of polyurethane emulsion

[0057] isophorone diisocyanate and tetrahydrofuran are uniformly mixed in a weight ratio of 1:1 to obtain a diisocyanate solution;

[0058] Weigh: 50 g of polytetramethylene glycol, 20 g of dihydroxy quaternary ammonium salt, 1 g of dibutyltin dilaurate and 80 mL of tetrahydrofuran, add into a three-necked flask protected by nitrogen and stir, the temperature of the three-necked flask is raised to 63°C, 80 g of diisocyanate solution is added dropwise into the three-necked flask, and the reaction is kept warm for 2.5 h, 20 g of trimethylolethane is added dropwise into the three-necked flask, and the reaction is kept warm for 55 min, 10 g of 2-(2-aminoethyl)1,3-propylene glycol is added into the three-necked flask, and the reaction is kept warm for 50 min, the temperature of the three-necked flask is lowered to room temperature, purified water is added into the three-necked flask, and emulsification and dispersion are carried out for 40 min to obtain a polyurethane emulsion with a solid content of 17%.

[0059] S3. Preparation of modified polyvinyl amide solution

[0060] Weigh: 50 g of polyethyleneimine, 100 mL of 65 vol% ethanol aqueous solution, 1 g of tetrabutylammonium bromide and 15 g of sodium carbonate, add into a three-necked flask, raise the temperature of the three-necked flask to 55°C, stir until the system is dissolved, add 13 g of epichlorohydrin into the three-necked flask, keep warm for 2.5 h, adjust the pH of the reaction system to 7, raise the temperature of the reaction system to 70°C, and remove low-boiling substances under reduced pressure to obtain modified polyethyleneimine.

[0061] S4. Preparation of chitosan mixture

[0062] Chitosan quaternary ammonium salt and carboxymethyl cellulose are mixed in a weight ratio of 3:1 to obtain a chitosan mixture.

[0063] S5. Preparation of cationic wet strength agent

[0064] Weigh 65 parts of polyurethane emulsion, 17 parts of modified polyethyleneimine solution and 3 parts of chitosan mixture by weight and mix them evenly to obtain a cationic wet strength agent. Example 3

[0065] This embodiment provides a method for preparing a cationic polyurethane wet strength agent for papermaking, comprising the following steps:

[0066] S1. Preparation of dihydroxy quaternary ammonium salt

[0067] Weigh: 133.2 g of 1,1'-(butylimino)dimethylol and 120.3 g of 1-chlorobutane, add into a sealed three-necked flask, fix the three-necked flask in an oil bath with mechanical stirring and stir, set the oil bath temperature to 150°C, keep warm for 40 hours, lower the temperature of the three-necked flask to 70°C, evaporate low boiling points under reduced pressure to obtain a dihydroxy quaternary ammonium salt.

[0068] S2. Preparation of polyurethane emulsion

[0069] isophorone diisocyanate and tetrahydrofuran are uniformly mixed in a weight ratio of 1:1 to obtain a diisocyanate solution;

[0070] Weigh: 50 g of polytetramethylene glycol, 20 g of dihydroxy quaternary ammonium salt, 1 g of dibutyltin dilaurate and 80 mL of tetrahydrofuran, add into a three-necked flask protected by nitrogen and stir, raise the temperature of the three-necked flask to 65°C, add 80 g of diisocyanate solution dropwise into the three-necked flask, keep warm and react for 3 h, add 20 g of trimethylolethane dropwise into the three-necked flask, keep warm and react for 60 min, add 10 g of 2-(2-aminoethyl)1,3-propylene glycol into the three-necked flask, keep warm and react for 60 min, lower the temperature of the three-necked flask to room temperature, add purified water into the three-necked flask, emulsify and disperse for 50 min, and obtain a polyurethane emulsion with a solid content of 20%.

[0071] S3. Preparation of modified polyvinyl amide solution

[0072] Weigh: 50 g of polyethyleneimine, 100 mL of 70 vol% ethanol aqueous solution, 1 g of tetrabutylammonium bromide and 15 g of sodium carbonate, add into a three-necked flask, raise the temperature of the three-necked flask to 60°C, stir until the system is dissolved, add 13 g of epichlorohydrin into the three-necked flask, keep warm for 3 h, adjust the pH of the reaction system to 7, raise the temperature of the reaction system to 70°C, and remove low-boiling substances under reduced pressure to obtain modified polyethyleneimine.

[0073] S4. Preparation of chitosan mixture

[0074] Chitosan quaternary ammonium salt and carboxymethyl cellulose are mixed in a weight ratio of 3:1 to obtain a chitosan mixture.

[0075] S5. Preparation of cationic wet strength agent

[0076] Weigh 70 parts of polyurethane emulsion, 4-6 parts of modified polyethyleneimine and 3.2 parts of chitosan mixture by weight and mix them evenly to obtain a cationic wet strength agent.

[0077] Comparative Example 1

[0078] The difference between this comparative example and Example 3 is that the amount of dihydroxy quaternary ammonium salt added in step S2 is 10 g.

[0079] Comparative Example 2

[0080] The difference between this comparative example and Example 3 is that step S3 is eliminated, and the modified polyethyleneimine in step S5 is replaced by the polyethyleneimine in step S3.

[0081] Comparative Example 3

[0082] The difference between this comparative example and Example 3 is that step S4 is omitted and no chitosan mixture is added in step S5.

[0083] Performance Test:

[0084] The cationic wet strength agent prepared in Examples 1-3 and Comparative Examples 1-3 was added to the papermaking pulp at an addition amount of 20 kg / t to prepare paper with a basis weight of 100 g / m 2 The papermaking samples prepared without adding cationic wet strength agent were used as blank group;

[0085] The tensile strength of the papermaking samples prepared in Examples 1-3 and Comparative Examples 1-3 was measured with reference to the standard GB / T12914-2018 "Determination of tensile strength of paper and paperboard - constant rate tensile method (20 mm / min)", i.e., dry tensile strength;

[0086] The tensile strength of the paper samples prepared in Examples 1-3 and Comparative Examples 1-3 after immersion in water, i.e., wet tensile strength, was measured with reference to the standard GB / T 465.2-2008 "Determination of tensile strength of paper and paperboard after immersion in water";

[0087] According to the formula , the retention rate of the tensile strength of the papermaking samples prepared in Examples 1-3 and Comparative Examples 1-3 after soaking was determined, where P 湿 is the wet tensile strength of the sample, P 干 is the wet tensile strength of the sample. The specific test results are shown in Table 1 below.

[0088]

[0089] Data Analysis:

[0090] Comparative analysis of the data in Table 1 above shows that the cationic wet strength agent for papermaking prepared by the present invention is added to the papermaking pulp at an addition amount of 20 kg / t, and the wet tensile strength of the prepared papermaking sample reaches 12.15 kN / m, and the tensile strength retention rate reaches 22.41%. All test data are better than those of the comparative example, indicating that the present invention effectively improves the wet tensile strength and tensile strength retention rate of the papermaking sample by preparing a cationic polyurethane emulsion containing quaternary ammonium salt blocks and cooperating with a modified polyethyleneimine and chitosan mixture.

[0091] The preferred embodiments of the present invention disclosed above are only used to help explain the present invention. The preferred embodiments do not describe all the details in detail, nor do they limit the invention to only specific implementation methods. Obviously, many modifications and changes can be made according to the content of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well. The present invention is limited only by the claims and their full scope and equivalents.

Claims

1. A cationic polyurethane wet strength agent for papermaking, comprising a polyurethane emulsion, a modified polyethyleneimine and a chitosan mixture obtained by fully mixing, wherein the polytetrahydrofuran ether diol, a dihydroxy quaternary ammonium salt, a catalyst and tetrahydrofuran are mixed under the protection of an inert gas, the temperature of the reaction system is raised to 60-65° C., a diisocyanate solution is added dropwise to the reaction system, and the reaction is carried out by heat preservation for 2-3 hours, trimethylolethane is added dropwise to the reaction system, and the reaction is carried out by heat preservation for 50-60 minutes, 2-(2-aminoethyl) 1,3-propylene glycol is added to the reaction system, and the reaction is carried out by heat preservation for 40-60 minutes, and post-treatment is carried out to obtain a polyurethane emulsion; The chitosan mixture consists of chitosan quaternary ammonium salt and carboxymethyl cellulose in a weight ratio of 3:1; Mix polyethyleneimine, ethanol solution, catalyst and sodium carbonate, stir until the system is dissolved, raise the temperature of the reaction system to 50-60°C, add epichlorohydrin to the reaction system, keep the temperature for reaction for 2-3h, and post-treat to obtain modified polyethyleneimine; By weight, 60-70 parts of polyurethane emulsion, 4-6 parts of modified polyethyleneimine and 2.8-3.2 parts of chitosan mixture are fully mixed to obtain the cationic polyurethane wet strength agent.

2. A cationic polyurethane wet strength agent for papermaking according to claim 1, characterized in that: The dosage ratio of the polytetrahydrofuran ether diol, dihydroxy quaternary ammonium salt, catalyst, tetrahydrofuran, diisocyanate solution, trimethylolethane and 2-(2-aminoethyl) 1,3-propylene glycol is 5g:2g:0.1g:8mL:8g:2g:1g, the catalyst is dibutyltin dilaurate, the diisocyanate solution is composed of isophorone diisocyanate and tetrahydrofuran in a ratio of 1g:1g, and the post-treatment comprises: after the reaction is completed, the temperature of the reaction system is lowered to room temperature, purified water is added to the reaction system, and emulsification and dispersion are performed for 30-50 minutes to obtain a polyurethane emulsion with a solid content of 15-20%.

3. A cationic polyurethane wet strength agent for papermaking according to claim 1, characterized in that: The dihydroxy quaternary ammonium salt is obtained by stirring and mixing 1,1'-(butylimino)dimethylol and 1-chlorobutane in a closed environment, placing the reaction system in a heat exchange medium at a temperature of 140-150° C., keeping the temperature for reaction for 35-40 hours, and post-processing.

4. A cationic polyurethane wet strength agent for papermaking according to claim 3, characterized in that: The usage ratio of 1,1'-(butylimino)dimethylol and 1-chlorobutane is 1 mol:1.3 mol. The post-treatment comprises: after the reaction is completed, the temperature of the reaction system is reduced to 60-70° C., and low boiling substances are distilled off under reduced pressure to obtain a dihydroxy quaternary ammonium salt.

5. A cationic polyurethane wet strength agent for papermaking according to claim 1, characterized in that: The dosage ratio of the polyethyleneimine, ethanol solution, catalyst, sodium carbonate and epichlorohydrin is 5g:10mL:0.1g:1.5g:1.3g, the ethanol solution is a 60-70vol% ethanol aqueous solution, the catalyst is tetrabutylammonium bromide, and the post-treatment comprises: after the reaction is completed, adjusting the pH of the reaction system to 7, raising the temperature of the reaction system to 70°C, and removing low-boiling substances under reduced pressure to obtain modified polyethyleneimine.

6. The method for preparing a cationic polyurethane wet strength agent for papermaking according to any one of claims 1 to 5, characterized in that: The polyurethane emulsion, the modified polyethyleneimine solution and the chitosan mixture are uniformly mixed to obtain a cationic wet strength agent.

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