A guanidine-containing chitosan-based wet strength agent, its preparation method and application

Abstract

This invention discloses a method for preparing a chitosan-based wet strength agent containing guanidine groups. By grafting dendritic polyamide-amines and amino compounds, the number of amino groups on the grafted chitosan powder is increased, thereby increasing the guanidine content in the chitosan-based wet strength agent after reaction with cyanamide compounds. The preparation method used in this invention employs mild reaction conditions and is halogen-free throughout the process, avoiding the release of harmful substances during the reaction. This invention also discloses the prepared chitosan-based wet strength agent containing guanidine groups, which exhibits good water solubility and a positive charge content >3 mmol / g. The positive charge in the wet strength agent can strongly electrostatically adsorb the negative charge of hydroxyl groups and other groups on plant fibers. This invention further discloses the application of the chitosan-based wet strength agent containing guanidine groups in papermaking. Paper made using this wet strength agent has superior mechanical properties and antibacterial effects.

Description

Technical Field

[0001] This invention relates to the field of wet strength agents, specifically to a chitosan-based wet strength agent containing guanidine groups, its preparation method, and its application. Background Technology

[0002] Wet strength agents are papermaking additives that can significantly improve the wet strength of paper. These additive molecules can undergo physical or chemical cross-linking with plant fibers, weakening the interaction between plant fibers and water, thus giving the paper high mechanical strength even after it is wet or soaked. Currently, commonly used wet strength agents in industrial and commercial applications include polyamide-aminopolyamine epichlorohydrin (PAE), urea-formaldehyde (UF), and melamine-formaldehyde (MF), which have mature application technologies and significant wet strength effects. However, due to limitations in the chemical structure and synthesis routes of these products, the above-mentioned wet strength agents inevitably have the problem of toxic and harmful substance leakage during production and application, such as aldehydes and organochlorines, which limits their application in the development of paper products and functionalization. For example, the National Food Safety Standard for Food Contact Paper and Paperboard Materials and Products (GB4806.8-2022) has put forward specific regulations on the safety of papermaking raw materials and the physicochemical indicators of paper (especially organochlorine content).

[0003] CN116854867A discloses a hydrophobic associative hyperbranched papermaking wet strength agent and its preparation method. This method utilizes a copolymerization reaction initiated by cationic monomers, amide monomers, and functional monomers with branched monomers under nitrogen protection, followed by purification to obtain the papermaking wet strength agent. The preparation process involves numerous chemical raw materials and a complex system; the preparation process is difficult to control in terms of temperature, feeding sequence, and proportioning, and requires continuous monitoring of temperature and viscosity in the reaction system to avoid over-reaction.

[0004] Solenis, an American company, has refined its molecular design and synthetic routes to reduce side reactions and lower the content of free organochlorine. It has also developed a third-generation PAE wet strength agent, Kymene 5720 (US7932349, US8101710, EP2046265, KR101387870, etc.), using membrane separation technology. This agent still contains 0.5% organochlorine residue.

[0005] CN118422520A discloses a method for preparing an aliphatic PAE-type high paper wet strength agent. This method involves alicyclic polyamines, polyaliphatic amines, acrylates, and epichlorohydrin through a multi-step controlled process to prepare the wet strength agent resin. This method uses large amounts of organic reagents and concentrated acids (such as sulfuric acid, nitric acid, and hydrochloric acid) as raw materials, and the reaction process is complex and cumbersome. Even with a refined preparation process, the system may still contain a significant amount of residual organic chlorine components.

[0006] Reference 1 (Zhou Yuxi, et al. Enhancing Cross-Linking Network for Superior Wet Strength of Paper by Sustainable Hyperbranched Polyimines. ACS Appl Mater Interfaces., 16, 15383-15393, 2024, doi:10.1021 / acsami.4c01403.) discloses a hyperbranched wet strength agent crosslinked with oxidized amylopectin and polyamines. This agent provides wet strength through multiple interactions between nitrogen-containing functional structures and plant fibers. However, the key process of oxidizing starch requires the use of strong oxidizing agents. Furthermore, the large number of aldehyde groups generated reacts with polyamines, which easily leads to crosslinking and reduces the efficiency of functional interactions.

[0007] In summary, the current research and development of wet strength agents still suffers from shortcomings such as highly toxic chemical raw materials, complex preparation processes, low energy efficiency, and low wet strength. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for preparing a chitosan-based wet strength agent containing guanidine groups. This method involves mild reaction conditions, is halogen-free throughout the process, avoids the leakage of harmful substances during the reaction, and produces a chitosan-based wet strength agent with high guanidine content, rich in positive charge, and good water solubility.

[0009] A method for preparing a chitosan-based wet strength agent containing a guanidine group includes the following steps:

[0010] (1) Chitosan was subjected to a solid-liquid grafting reaction with dendritic polyamide-amine. After solid-liquid separation, alcohol washing and drying, chitosan grafted with dendritic polyamide-amine was obtained. Then, it was subjected to a solid-liquid grafting reaction with an amino compound solution. After the reaction was completed, the mixture was subjected to solid-liquid separation, alcohol washing and drying to obtain chitosan grafted with amino.

[0011] (2) The chitosan grafted with amino groups obtained in step (1) is reacted with cyanamide compounds in an acidic aqueous solution, and after precipitation, washing and drying, a chitosan-based wet strength agent containing guanidine groups is obtained.

[0012] This invention uses chitosan, dendritic polyamide-amine, and amino compounds as raw materials. First, a solid-liquid reaction is carried out to obtain chitosan rich in amino functional groups. Then, a guanidine reaction is carried out to obtain a chitosan-based wet strength agent containing guanidine groups. The material does not require pretreatment, the preparation process is simple and easy to operate, and the whole process is halogen-free and aldehyde-free, avoiding the leakage of harmful substances during the reaction.

[0013] Preferably, in step (1), the chitosan powder has a particle size of 200-600 mesh and a molecular weight of 20000-80000 g / mol.

[0014] Preferably, in step (1), the dendritic polyamide-amine is a tetra-armed or octa-armed polyamide-amine, and the end is a methoxy group.

[0015] In this invention, the dendritic polyamide is tetra- or octa-armed with terminal methoxy groups, and has a molecular weight of 404 or 1204.6 g / mol. The structure of the tetra- or octa-armed polyamide-amine with terminal methoxy groups is shown below:

[0016]

[0017] Because the solid-liquid grafting reaction of powder has few sites and large steric hindrance, the degree of cross-linking is low. In this invention, chitosan is subjected to a solid-liquid grafting reaction with dendritic polyamide-amine. The amino groups on the surface of chitosan undergo an amidation reaction with the methoxy groups at the ends of the dendritic polyamide-amine. Then, chitosan powder grafted with dendritic polyamide-amine (terminal methoxy groups) is obtained by simple solid-liquid separation (such as centrifugation and filtration).

[0018] Preferably, in step (1), the mass ratio of chitosan to dendritic polyamide-amine is 5 to 10:1.

[0019] Preferably, in step (1), the amino compound is ethylenediamine, 1,2-diaminopropane, or 1,3-diaminopropane.

[0020] These amines have low molecular weight, good solubility, and are easy to remove, so they are unlikely to remain in the final chitosan-based wet strength agent.

[0021] In this invention, chitosan grafted with dendritic polyamide-amine can undergo a similar solid-liquid reaction with amino compounds, converting surface methoxy groups into amino terminals via Michael addition. The introduction of dendritic polyamide-amine side chains, combined with the reaction with amino compounds, increases the number of amino groups on the chitosan surface. Subsequently, amino-grafted chitosan is obtained through simple solid-liquid separation (e.g., centrifugation, filtration).

[0022] Preferably, in step (1), the mass ratio of chitosan to amino compound in the grafted dendritic polyamide-amine is 5 to 10:1.

[0023] Preferably, in step (1), the temperature of the solid-liquid grafting reaction is 20-40°C and the reaction time is 10-48h.

[0024] Preferably, in step (2), the cyanamide compound is cyanamide, dicyandiamide, or N,N-diethylcyanamide.

[0025] In this invention, the cyanamide compound is protonated in an acidic system and can undergo an addition reaction with ammonia to generate guanidine.

[0026] Preferably, in step (2), the mass ratio of the cyanamide compound to the chitosan grafted with amino groups is 2 to 6:1.

[0027] In this invention, chitosan with surface-grafted amino groups is more easily protonated in acidic aqueous solution. The chitosan molecules after powder dissolution exhibit a branched morphology, which allows both the main chain amino group and the dendritic amino group to undergo guanidine reaction with cyanamide compounds, ultimately preparing a chitosan-based wet strength agent containing guanidine groups. This wet strength agent has good water solubility and multiple branching and active sites.

[0028] Preferably, in step (2), the acidic aqueous solution is a hydrochloric acid aqueous solution with a concentration of 0.2 to 0.6 M.

[0029] Preferably, in step (2), the reaction temperature is 60-100°C and the reaction time is 4-14 hours.

[0030] The present invention also provides a chitosan-based wet strength agent containing guanidine groups prepared by the above preparation method. This chitosan-based wet strength agent contains abundant guanidine functional groups and positive charges, and has high water solubility. It can better provide branched network structure and adsorption sites for binding with plant fibers, which is beneficial to reducing the direct binding of water with plant fibers, thereby improving dry strength and wet strength performance.

[0031] Preferably, the nitrogen atom content in the guanidine-containing chitosan-based wet strength agent is >15 at.%.

[0032] In this invention, chitosan powder undergoes a two-step solid-liquid grafting reaction followed by a reaction with a cyanamide compound to prepare a chitosan-based wet strength agent containing guanidine groups. This method yields a chitosan-based wet strength agent with a higher guanidine group content compared to directly reacting chitosan with a cyanamide compound. Since guanidine groups are rich in nitrogen atoms, the guanidine group content can be characterized by testing the amount of nitrogen atoms present.

[0033] Preferably, the positive charge content of the guanidine-containing chitosan-based wet strength agent is >3 mmol / g.

[0034] The guanidine-containing chitosan-based wet strength agent prepared by this invention has a high positive charge content, which can disrupt the hydrogen bonding of chitosan molecules, making the wet strength agent more loosely structured and more water-soluble. At the same time, the positive charge in the wet strength agent can have a strong electrostatic adsorption effect with the negative charge of hydroxyl groups and other groups on plant fibers, resulting in paper with better mechanical properties.

[0035] This invention also provides the application of the above-mentioned guanidine-containing chitosan-based wet strength agent in papermaking. Because the wet strength agent of this invention has high water solubility, a branched structure, and abundant cations, it can bind to plant fibers rich in hydroxyl groups, exhibiting superior binding force to anionic plant fibers. In a specific application example of this invention, at the same addition amount, the wet strength agent of this invention has higher dry and wet strength than PAE resin. When the wet strength agent addition amount is 1 wt%, the resulting paper has a dry strength gain of over 50% and a tensile strength >50 MPa, while simultaneously having a wet strength gain of over 300% and a tensile strength >3 MPa. Furthermore, the paper prepared using the wet strength agent obtained by this invention also exhibits good antibacterial effects.

[0036] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0037] (1) In this invention, the guanidine-containing chitosan-based wet strength agent increases the number of amino groups on the grafted chitosan by grafting dendritic polyamide-amine and amino compounds, thereby increasing the guanidine content in the chitosan-based wet strength agent after reaction with cyanamide compounds. The preparation method used has mild reaction conditions and is halogen-free throughout the process, avoiding the leakage of harmful substances during the reaction.

[0038] (2) The guanidine-containing chitosan-based wet strength agent prepared by the present invention has good water solubility and positive charge content >3 mmol / g, which can better bind with plant fibers.

[0039] (3) Paper prepared using the guanidine-containing chitosan-based wet strength agent obtained by the present invention has good dry strength and wet strength properties, and also has antibacterial effect. Attached Figure Description

[0040] Figure 1 Images of the synthesized dendritic polyamide-amine (g 1.5), purchased chitosan powder, the guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Examples 1 and 2, and the guanidine-only chitosan powder.

[0041] Figure 2 The infrared spectra are those of the guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Example 1, and the purchased chitosan powder.

[0042] Figure 3 The images show the water solubility test results of the guanidine-containing chitosan-based wet strength agent prepared in Example 1 and the guanidine-only chitosan powder prepared in Comparative Example 2.

[0043] Figure 4 The X-ray photoelectron spectroscopy (XPS) analysis statistics are shown for the guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Examples 1 and 2, and the guanidine-only chitosan powder.

[0044] Figure 5 The contact angle test diagrams are for the paper prepared using Examples 1, 2, and 4.

[0045] Figure 6 Microscopic images of the surface morphology of the paper prepared using Examples 1, 2 and 4.

[0046] Figure 7 The graphs show the dry strength test results of the paper prepared using Examples 1 to 4.

[0047] Figure 8 The wet strength test diagrams are for the paper prepared using Examples 1 to 4.

[0048] Figure 9 The graph shows the antibacterial effect of the paper prepared using Examples 1, 2, and 4 against Escherichia coli and Staphylococcus aureus. Detailed Implementation

[0049] The present invention will be further described in detail below with reference to the embodiments, but the implementation of the present invention is not limited to the following embodiments.

[0050] All raw materials used in this invention are commercially available.

[0051] Preparation of an eight-arm polyamide-amine with terminal methoxy groups (g 1.5):

[0052] (1) Ethylenediamine (16 g, 0.266 mol) was dissolved in 75 mL of methanol. The solution was slowly added dropwise to a methanol (100 mL) solution of methyl acrylate (172 g, 2 mol). The reaction was stirred and kept at 20°C for 24 hours. After vacuum distillation, the resulting solution yielded a pale yellow liquid (0.5 g, yield 72%, 83 g, 0.194 mol).

[0053] (2) Dissolve g0.5 (18 g, 0.042 mol) obtained in step (1) in 210 mL of methanol, and add it dropwise to a methanol (30 mL) solution of ethylenediamine (130.6 g, 2.18 mol). Keep the reaction stirred and keep the temperature at 20 degrees Celsius for 24 hours. After vacuum distillation, the resulting solution yields a pale yellow liquid g1 (yield 90.5%, 21 g, 0.038 mol).

[0054] (3) Dissolve g1 (18g, 0.033mol) obtained in step (2) in 165mL of methanol. Slowly add the solution dropwise to a methanol (34mL) solution of methyl acrylate (35g, 0.407mol). Keep the reaction stirred and maintain the temperature at 20°C for 24 hours. After vacuum distillation, the resulting solution yields an eight-arm polyamide-amine with terminal methoxy groups (g1.5), which is a pale yellow liquid (yield 99.5%, 39.8g, 0.033mol).

[0055] Example 1

[0056] (1) 6g of pale yellow chitosan powder (purchased from Anhui Zesheng Technology Co., Ltd., product number 9DDUAEWX, sieved to 400 mesh particles) was dispersed in 40ml of methanol, and 1g of terminal methoxy-terminated eight-arm polyamide-amine was added. The mixture was reacted at 25℃ for 24h. After filtration and washing with methanol, the solid product was separated to obtain 6.08g of chitosan powder grafted with dendritic polyamide-amine. The chitosan powder grafted with dendritic polyamide-amine was dispersed in 20ml of methanol, and 1g of ethylenediamine was added. The mixture was reacted at 25℃ for 24h. After filtration and washing with methanol, the solid product was separated to obtain 6.1g of chitosan powder grafted with amino groups.

[0057] (2) Weigh 1.2g of the chitosan powder with grafted amino groups obtained in step (1), dissolve it in 40 ml of 0.5M hydrochloric acid, add 3.6g of dicyandiamide, react at 90℃ for 12h, and separate 1.01g of chitosan-based wet strength agent containing guanidine groups by methanol precipitation and washing.

[0058] Example 2

[0059] (1) 6g of pale yellow chitosan powder (400 mesh) was dispersed in 40ml of methanol, and 1g of terminal methoxy-terminated eight-arm polyamide-amine was added. The mixture was reacted at 40℃ for 12h. After filtration and washing with methanol, the solid product was separated to obtain 6.12g of chitosan powder grafted with dendritic polyamide-amine. The chitosan powder grafted with dendritic polyamide-amine was dispersed in 20ml of methanol, and 1g of ethylenediamine was added. The mixture was reacted at 40℃ for 12h. After filtration and washing with methanol, the solid product was separated to obtain 6.15g of chitosan powder grafted with amino groups.

[0060] (2) Weigh 1.2g of the chitosan powder with grafted amino groups obtained in step (1), dissolve it in 40 ml of 0.2M hydrochloric acid, add 7.2g of dicyandiamide, react at 80℃ for 4h, and separate 1.22g of chitosan-based wet strength agent containing guanidine groups by methanol precipitation and washing.

[0061] Example 3

[0062] (1) 6g of pale yellow chitosan powder (400 mesh) was dispersed in 40ml of methanol, and 1g of terminal methoxy-terminated eight-arm polyamide-amine was added. The mixture was reacted at 25℃ for 24h. After filtration and washing with methanol, the solid product was separated to obtain 6.12g of chitosan powder grafted with dendritic polyamide-amine. The chitosan powder grafted with dendritic polyamide-amine was dispersed in 20ml of methanol, and 1g of 1,3-diaminopropane was added. The mixture was reacted at 40℃ for 24h. After filtration and washing with methanol, the solid product was separated to obtain 6.13g of chitosan powder grafted with amino.

[0063] (2) Weigh 1.2g of the chitosan powder with grafted amino groups obtained in step (1), dissolve it in 40 ml of 0.5M hydrochloric acid, add 3.6g of cyanamide, react at 70℃ for 8h, and separate 1.10g of chitosan-based wet strength agent containing guanidine groups by methanol precipitation and washing.

[0064] Comparative Example 1

[0065] 6g of pale yellow chitosan powder (400 mesh) was dispersed in 40ml of methanol, and 1g of terminal methoxylated eight-arm polyamide-amine was added. The mixture was reacted at 40℃ for 12h. After filtration and washing with methanol, the solid product was separated, yielding 6.12g of chitosan powder grafted with dendritic polyamide-amine. The chitosan powder grafted with dendritic polyamide-amine was dispersed in 20ml of methanol, and 1g of ethylenediamine was added. The mixture was reacted at 40℃ for 12h. After filtration and washing with methanol, the solid product was separated, yielding 6.15g of chitosan powder grafted with amino groups.

[0066] Comparative Example 2

[0067] 1.2 g of chitosan powder (400 mesh) was dissolved in 40 mL of 0.5 M hydrochloric acid, and 3.6 g of dicyandiamide was added. The mixture was reacted at 90 °C for 12 h. After precipitation with methanol and washing, 1.15 g of guanidine-treated chitosan powder was obtained.

[0068] Application Example 1

[0069] The hardwood pulp was beaten (40 beaters), and 3g of the absolutely dry pulp was weighed out and filtered using a solvent separation system. The wet paper cake was then pressed into sheets and dried at 70°C to obtain paper of uniform thickness.

[0070] Application Example 2

[0071] The preparation method is the same as in Application Example 1, except that 1 wt% of the chitosan-based wet strength agent prepared in Example 1 is also added.

[0072] Application Example 3

[0073] The preparation method is the same as that of Application Example 2, except that the added wet strength agent is the chitosan-based wet strength agent prepared in Example 2 at 2 wt%.

[0074] Application Example 4

[0075] The preparation method is the same as in Application Example 2, except that the wet strength agent added is 1 wt% PAE (purchased from Mingxiang Chemical Technology (Shandong) Group Co., Ltd., product number MX-1201).

[0076] Sample Analysis

[0077] I. Sample Appearance

[0078] The morphology of the prepared octyl-amino-amine with terminal methoxy groups (g1.5), the chitosan-based wet strength agent containing guanidine groups prepared in Example 1, the chitosan powder with grafted amino groups prepared in Comparative Examples 1 and 2, and the chitosan powder with only guanidine groups were compared.

[0079] Figure 1 Images are shown of the synthesized dendritic polyamide-amine (g 1.5), purchased chitosan powder, the guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Examples 1 and 2, and the guanidine-only chitosan powder. In Comparative Example 1, the appearance of the chitosan powder did not change significantly after the grafting reaction because small molecules were grafted onto the macroscopic particles, resulting in a lower loading. In Comparative Example 2, the chitosan reacted with dicyandiamide in the dissolved state to generate abundant guanidine functional groups, and the C=N---O interaction between the molecular chains resulted in a yellow color. In Example 1, due to the introduction of more external amino functional groups through grafting, more guanidine groups were generated during the reaction with dicyandiamide in the dissolved state, resulting in a stronger color effect and a deeper color than in Comparative Example 2.

[0080] II. Infrared Spectral Characterization

[0081] The guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Example 1, and the purchased chitosan powder were characterized by infrared spectroscopy.

[0082] Figure 2 The infrared spectra of the guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Example 1, and the purchased chitosan powder are shown. In Example 1, the chitosan powder containing OH (3350 cm⁻¹) has... -1 ), C=O(1735cm) -1 ), -NH(1575cm -1 ), C=NH + (1340cm -1 ), C = N (1685cm) -1The functional structure is different from that in Comparative Example 1, indicating that the reaction successfully grafted dendritic polyamide-amine and carried out guanidine reaction.

[0083] III. Water solubility test

[0084] Equal amounts of the guanidine-containing chitosan-based wet strength agent prepared in Example 1 and the guanidine-only chitosan powder prepared in Comparative Example 2 were weighed and placed in water to observe their water solubility.

[0085] Figure 3 The figures show the water solubility test results of the guanidine-containing chitosan-based wet strength agent prepared in Example 1 and the guanidine-only chitosan powder prepared in Comparative Example 2. Compared with Comparative Example 2, the chitosan grafted with dendritic polyamide-amine in Example 1 has more amino groups, which is conducive to the guanidine reaction and can obtain a molecular structure with more cationic groups, thus having better water solubility. That is, it can also be well dissolved in water without adjusting the pH, which is convenient for compounding with plant fibers in the papermaking process.

[0086] IV. Charge Measurement

[0087] Weigh 50 mg of the guanidine-containing chitosan-based wet strength agent prepared in Example 1 and the guanidine-only chitosan powder prepared in Comparative Example 2, respectively, and dissolve them in 10 mL of pure water. Sonicate the samples to ensure complete dissolution. Then, take 1 mL of the solution and dilute it with 9 mL of pure water, and place it in a test container. Titrate with a 1 mmol / L anionic standard solution. The instrument automatically records the volume of standard solution consumed during the titration potential change. The number of cations in the samples of Example 1 and Comparative Example 2 is then calculated. Each sample is tested twice.

[0088] Cation content (mmol / g) = V × C / M

[0089] Where V is the required volume of the standard anion (usually tested in milliliters, mL), C is the concentration of the anion titrant (mol / L), and M is the mass of the resin being tested (g).

[0090] Anion demand tests showed that the cation content in Example 1 (~3.5 mmol / g) was higher than that in Example 2 (~2.6 mmol / g).

[0091] V. Nitrogen Content Test

[0092] The powder sample was spread on a tape and placed in the X-ray energy dispersive spectrometer chamber for degassing. After degassing, scanning tests were performed.

[0093] Figure 4X-ray photoelectron spectroscopy (XPS) analysis statistics of the guanidine-containing chitosan-based wet strength agent prepared in Example 1, the amino-grafted chitosan powder prepared in Comparative Examples 1 and 2, and the guanidine-only chitosan powder are shown. The nitrogen content of Example 1 is >15 at.%, significantly higher than that of Comparative Examples 1 and 2. Since the nitrogen atom content of guanidine is higher than that of amino groups, this indicates that the chitosan wet strength agent prepared after grafting dendritic polyamide-amine and amino compounds has a higher guanidine content. The higher nitrogen content of Comparative Example 2 compared to Comparative Example 1 is because the grafting amount in the solution state of the guanidineization reaction may be higher than that in the solid-liquid grafting of dendritic polyamide-amine.

[0094] VI. Surface morphology and hydrophilicity / hydrophobicity tests

[0095] The prepared paper was placed flat on the stage of a contact angle meter (Krüss DSA-30 tester). The entire process of a 5 μL water droplet from contact to residence / penetration into the paper was observed in video mode. The contact angle values ​​at different time points were obtained by normalizing the video photo nodes. The hydrophilicity and hydrophobicity of the paper prepared in Examples 1, 2 and 4 were determined by the contact angle test.

[0096] Figure 5 The contact angle test diagrams for the paper prepared using Examples 1, 2, and 4 are shown. Figure 6 Microscopic images of the surface morphology of the papers obtained in Application Examples 1, 2, and 4 are shown. The paper obtained in Application Example 1 exhibits extremely strong hydrophilicity; water droplets are attracted into the pores of the paper upon contact, and the paper swells. The fiber structure of Application Example 1 shows relatively independent and distinct fibers. The papers obtained in Application Examples 2 and 4 show relatively weaker hydrophilicity because the wet-strength molecules encapsulate the plant fibers (as can be seen in the fiber structure), and the carbon chains present in the wet-strength molecules have a certain degree of hydrophobicity. Notably, Application Example 2 exhibits superior hydrophobicity, possibly due to its branched structure and abundant cationic matrix, which allows for better binding with the plant fibers and a high coating rate.

[0097] VII. Dry and Wet Strength Performance Tests

[0098] The dry and wet strength properties of the paper prepared in Examples 1-4 were tested according to the following methods. The dry strength test strips (approximately 15mm × 10mm × 0.3mm) were tested using a WDW-100 universal electronic tensile testing machine: the actual width and thickness were measured during testing, a 1kN sensor was used, and the machine head speed was 10mm·min. -1 The wet strength test specimen was tested using dynamic thermomechanical methods (TA, Q800). The maximum force was 18 N. Therefore, a paper strip approximately 10 mm × 2 mm × 0.3 mm was used. The actual width and thickness were measured during the test, and the paper strip was completely submerged in water for 25 minutes before testing. The tensile displacement was controlled at 2 mm / min. -1 .

[0099] Figure 7 and Figure 8 The figures show the dry strength and wet strength test results of the paper prepared in Application Examples 1 to 4, respectively. The paper in Application Example 1 has a dry strength of <20 MPa and a wet strength of <0.5 MPa. When the wet strength agent is added at 1 wt%, the paper prepared has a dry strength gain of more than 50% and a tensile strength gain of >50 MPa, and a wet strength gain of more than 300% and a tensile strength gain of >3 MPa.

[0100] 8. Antibacterial test

[0101] Take the prepared bacterial solution and dilute it to prepare a 1×10⁻⁶ solution. 6 CFU / mL bacterial suspension; take a 1×1cm sample. 2 In Application Examples 1, 2, and 4, paper samples were immersed in 3 mL of *E. coli* and *Staphylococcus aureus* solutions, respectively, and incubated at 37°C in a shaking incubator for 24 h to allow bacterial adhesion. Subsequently, the samples were removed from the culture medium and rinsed three times with 50 mL of deionized water to remove any non-adhering bacteria. The samples were then sonicated for 2 min in test tubes containing 2 mL of deionized water, and the bacteria adhering to the samples were collected. The isolated bacterial solutions were diluted 10-fold and 100-fold sequentially and plated onto agar plates. After incubation at 37°C for 18 h, the colony count was multiplied by the dilution factor, and the result was the average of three measurements.

[0102] Figure 9 The figures show the antibacterial effects of the paper prepared in Examples 1, 2, and 4 against Escherichia coli and Staphylococcus aureus. Since the chitosan-based wet strength agent of the present invention contains a large number of guanidine groups, the paper prepared using the chitosan-based wet strength agent of the present invention has a significantly better antibacterial effect against Escherichia coli and Staphylococcus aureus than the paper prepared using commercially available PAE.

[0103] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A process for the preparation of a guanidinium-containing chitosan-based wet strength agent, characterized in that, The method comprises the following steps: (1) performing a solid-liquid grafting reaction of chitosan and dendritic polyamide-amine, wherein the dendritic polyamide-amine has a methoxy group as a terminal group, and an amido reaction occurs between the amino group on the surface of the chitosan and the methoxy group terminal of the dendritic polyamide-amine, and then solid-liquid separation, alcohol washing and drying are performed to obtain chitosan grafted with dendritic polyamide-amine, and then a solid-liquid grafting reaction of the chitosan grafted with dendritic polyamide-amine and an amino compound solution is performed, and after the reaction, solid-liquid separation, alcohol washing and drying are performed on the mixture to obtain chitosan grafted with amino groups; (2) reacting the chitosan grafted with amino groups obtained in step (1) with a cyanamide compound in an acidic aqueous solution, and then performing precipitation, washing and drying to obtain a guanidyl-containing chitosan-based wet strength agent.

2. The process for preparing a guanidyl group-containing chitosan-based wet strength agent according to claim 1, characterized by, In step (1), the dendritic polyamide-amine is a four-arm or eight-arm polyamide-amine.

3. The method for preparing the chitosan-based wet strength agent containing guanidine groups according to claim 1, characterized in that, In step (1), the mass ratio of the chitosan to the dendritic polyamide-amine is 5-10:

1.

4. The method for preparing the chitosan-based wet strength agent containing guanidine groups according to claim 1, characterized in that, In step (1), the amino compound is one of ethylenediamine, 1,2-diaminopropane and 1,3-diaminopropane.

5. The method for preparing the guanidine-containing chitosan-based wet strength agent according to claim 1, characterized in that, In step (1), the mass ratio of the chitosan grafted with dendritic polyamide-amine to the amino compound is 5-10:

1.

6. The method for preparing the chitosan-based wet strength agent containing guanidine groups according to claim 1, characterized in that, In step (2), the cyanamide compound is cyanamide, dicyanamide or N,N-diethyl cyanamide.

7. The method for preparing the chitosan-based wet strength agent containing guanidine groups according to claim 1, characterized in that, In step (2), the mass ratio of the cyanamide compound to the chitosan grafted with amino groups is 2-6:

1.

8. The guanidyl-containing chitosan-based wet strength agent prepared by the preparation method according to any one of claims 1-6.

9. The guanidinium-containing chitosan-based wet strength agent according to claim 8, characterized in that, The guanidyl-containing chitosan-based wet strength agent has an N atom content of >15 at.%, and a positive charge content of >3 mmol / g.

10. Application of the guanidyl-containing chitosan-based wet strength agent according to claim 8 or 9 in papermaking.

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