A crosslinked water-soluble polymer paper strengthening agent and a method for preparing the same

Abstract

The application discloses a crosslinked network water-soluble polymer paper reinforcing agent and a preparation method thereof, and belongs to the field of paper reinforcing. At 60-120 DEG C, a mixed solution containing polyvinyl alcohol and sodium carboxymethyl cellulose is stirred, a catalyst and a crosslinking agent containing amino are added, the catalyst is zinc chloride or aluminum chloride, and then heat preservation treatment is carried out; the obtained reaction system is cooled, a silane coupling agent is added, and then heat preservation treatment is carried out for 0.5-2 hours; the pH of the obtained reaction system is adjusted to neutral, and the crosslinked network water-soluble polymer paper reinforcing agent is obtained, which has a wide application prospect in the production of various papers (such as packaging paper, writing paper, printing paper and the like).

Description

Technical Field

[0001] This invention belongs to the field of paper reinforcement, specifically relating to a cross-linked network water-soluble polymeric paper reinforcing agent and its preparation method. Background Technology

[0002] Paper reinforcement refers to improving the physical properties and durability of paper through various methods and technologies. Generally, it mainly involves selecting high-strength, high-quality fiber raw materials, such as chemical wood pulp fibers, to improve the basic strength of the paper, or introducing special fibers or artificial polymers to further enhance the strength and durability of the paper. The use of paper reinforcing agents (such as amphoteric polymers) can effectively improve properties such as paper stiffness, breaking length, and folding endurance. Paper reinforcing agents enhance the bonding force between fibers by forming hydrogen bonds and cross-linking bonds.

[0003] Currently, commonly used paper strengthening agents include modified starch, polyacrylamide and its derivatives, and urea-formaldehyde resin and its modified products. Modified starch is one of the most widely used paper strengthening agents, but it suffers from low filtration rates, failing to meet high dry strength and folding endurance requirements. Polyacrylamide and its derivatives, when used as dry strength agents, show little or no improvement in tear strength, and may even decrease it. Urea-formaldehyde resin and its modified products exhibit uneven interaction with fibers, difficulty in controlling dilution and local concentration, and also pose environmental concerns regarding formaldehyde release.

[0004] CMC possesses excellent water solubility, thickening, water retention, and adhesion properties, which can improve the dry and wet strength of paper, enhance paper uniformity, and can be used in combination with other dry and wet strength agents to further improve paper performance. PVA has excellent film-forming and adhesion properties, forming a protective film on the paper surface, improving paper strength and water resistance. After crosslinking with CMC, it forms a more stable three-dimensional network structure, significantly improving paper mechanical properties, reducing the hydrophilicity of CMC and PVA, minimizing the impact of moisture on paper strength, and adjusting the system viscosity and flowability, resulting in more uniform coating of the reinforcing agent, improving paper surface quality and printability. Therefore, theoretically, CMC and PVA can be prepared as paper reinforcing agents. However, there are some problems with the crosslinking of PVA and CMC: First, it requires harsh reaction conditions, such as high temperature, high pressure, or special catalysts, which not only increases production costs but also places high demands on equipment; second, it is impossible to simultaneously achieve a good crosslinking effect and a balance of material properties. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a cross-linked network water-soluble polymeric paper reinforcing agent and its preparation method, which solves the problems of the harsh reaction conditions required for PVA and CMC cross-linking and the inability to simultaneously achieve a good cross-linking effect and a good balance of material properties. Based on the polymer polymerization reaction, polyvinyl alcohol and sodium carboxymethyl cellulose system are cross-linked to form a cross-linked network structure. With the help of organosilicon modification, the solid content of the reinforcing agent is increased, the adhesion is enhanced, and a paper reinforcing agent with better performance is prepared.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing a cross-linked network water-soluble polymeric paper reinforcing agent includes the following steps;

[0008] S1, At 60-120℃, a catalyst and an amino-containing crosslinking agent are added to a mixture of polyvinyl alcohol and sodium carboxymethyl cellulose while stirring. The catalyst is zinc chloride or aluminum chloride. The mass ratio of polyvinyl alcohol to sodium carboxymethyl cellulose is (1-5):(1-3). After heat treatment, the amino group of the crosslinking agent reacts with the hydroxyl group of polyvinyl alcohol and the carboxyl group of sodium carboxymethyl cellulose to gradually form an intermediate product with a crosslinked structure, thus obtaining reaction system a.

[0009] S2, cool reaction system a, add silane coupling agent, and then keep it warm for 0.5-2 hours to obtain reaction system b. Adjust the pH of reaction system b to neutral to obtain cross-linked network water-soluble polymer paper strengthening agent.

[0010] Preferably, S1 obtains the mixture according to the following process:

[0011] Polyvinyl alcohol and sodium carboxymethyl cellulose were dissolved in deionized water at 60-120℃ and 50-80℃, respectively, to obtain a polyvinyl alcohol solution with a mass percentage of 5%-20% and a sodium carboxymethyl cellulose solution with a mass percentage of 2%-10%.

[0012] The polyvinyl alcohol solution and sodium carboxymethyl cellulose solution are mixed and stirred at 50-60°C for 30-60 minutes at a stirring speed of 300-500 r / min to obtain the mixture.

[0013] Preferably, the crosslinking agent in S1 is urea.

[0014] Preferably, the urea is 3%-15% of the total mass of polyvinyl alcohol and sodium carboxymethyl cellulose, and the catalyst is 3% of the mass of urea.

[0015] Preferably, S1 is added to the mixed aqueous solution of the crosslinking agent and the catalyst by means of stirring and dripping, and the dripping time is controlled within 30-150 min.

[0016] Preferably, the heat preservation treatment described in S1 is carried out at 60-120℃ for 0.5-2 hours.

[0017] Preferably, in step S2, the reaction system a is cooled to 65-75°C before the silane coupling agent is added.

[0018] Preferably, the silane coupling agent is (3-glycidylpropoxy)trimethoxysilane, and (3-glycidylpropoxy)trimethoxysilane is 1%-10% of the total mass of polyvinyl alcohol and sodium carboxymethyl cellulose.

[0019] Preferably, S2 is added dropwise to (3-glycidylpropoxy)trimethoxysilane, with the addition time controlled between 30 and 120 minutes.

[0020] A cross-linked network water-soluble polymeric paper reinforcing agent obtained by the preparation method of any one of the above-described cross-linked network water-soluble polymeric paper reinforcing agents.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] This invention discloses a method for preparing a cross-linked network water-soluble polymeric paper strengthening agent. Under the catalysis of zinc chloride or aluminum chloride, and under controlled temperature conditions, the amino groups of the cross-linking agent react with the hydroxyl groups of PVA and the carboxyl groups of CMC to gradually form an intermediate product with a cross-linked structure. Subsequent cooling promotes a condensation reaction between the silane coupling agent and the terminal hydroxyl groups of the intermediate product, achieving end-capping and yielding the cross-linked network water-soluble polymeric paper strengthening agent. This cross-linking system can be achieved under relatively mild conditions, without requiring extreme temperature or pressure reaction conditions, thus improving the strength and moisture resistance of paper. This strengthening agent exhibits good compatibility, good flowability, uniform film formation, strong adhesion, good compatibility with various synthetic latexes, good resistance to mildew, stable performance, fast drying speed, ease of use, and low cost. Zinc chloride or aluminum chloride is common, relatively inexpensive, and widely available, contributing to reduced overall production costs. Zinc chloride or aluminum chloride has relatively low toxicity, posing less harm to operators and the environment during production compared to some heavy metal catalysts or highly corrosive catalysts, making this invention more advantageous in terms of environmental protection and safety. Compared to some crosslinking processes that require high temperature and pressure, this invention achieves good crosslinking results at lower temperatures and normal pressures, thereby reducing reliance on specialized equipment and further lowering production costs. The raw materials used in this invention are non-toxic and pollution-free, making them more suitable for industrial production. When used in conjunction with other additives in the papermaking process (such as sizing agents and fillers), they do not produce adverse reactions and do not negatively affect the whiteness, printability, or other important properties of the paper. The paper strengthening agent of this invention has broad application prospects in the production of various types of paper (such as packaging paper, writing paper, and printing paper), meeting the paper industry's demand for improved paper strength while maintaining good water solubility, thus enabling its application potential in more fields.

[0023] Furthermore, this invention uses urea as a crosslinking agent. The amino groups in the urea molecule can more easily react with the hydroxyl groups on the PVA molecular chain and the carboxyl groups on the CMC molecular chain. Under the catalysis of zinc chloride or aluminum chloride, the reaction can proceed more quickly and fully, and the reaction conditions are milder. It does not require complex equipment and special catalysts, which greatly reduces the production cost. This forms a crosslinked network structure between molecules, enhancing the mechanical properties, integrity, and stability of the system.

[0024] Furthermore, this invention employs KH560 end-capping. The (3-glycidylpropoxy)trimethoxysilane moiety in the KH560 molecular structure possesses unique chemical activity, enabling it to specifically react with the active sites after PVA / CMC crosslinking, achieving highly efficient end-capping. This high efficiency allows for more precise control over the material's surface properties, such as improved hydrophobicity, enhanced compatibility with starch-based or polyacrylamide-based materials, and improved interaction with paper fibers, allowing it to perform better in applications requiring moisture and water resistance. Compared to traditional end-capping technologies, KH560 end-capping brings more functionalities to the material. Attached Figure Description

[0025] Figure 1 The infrared spectrum of G3-PVA / CMC obtained in Example 3 of this invention is shown.

[0026] Figure 2 The thermal stability analysis curve of G3-PVA / CMC obtained in Example 3 of this invention is shown.

[0027] Figure 3 This is a graph showing the effect of urea dosage on the dry and wet tensile strength of paper according to the present invention.

[0028] Figure 4 This diagram illustrates the effect of urea dosage on paper folding endurance and tear strength according to the present invention.

[0029] Figure 5 This is a graph showing the effect of urea dosage on the ring crush index and surface water absorption of paper according to the present invention. Detailed Implementation

[0030] The following examples further illustrate the above-described content of the present invention, but it should not be construed as limiting the scope of the subject matter of the present invention to the following examples. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention.

[0031] This invention discloses a cross-linked network water-soluble polymeric paper reinforcing agent. The raw materials include component I, component II, component III, a silane coupling agent, and a catalyst. The mass ratio of component I (polyvinyl alcohol) to component II (sodium carboxymethyl cellulose) is (1-5):(1-3). The mass of component III (urea, cross-linked) is 3-15% of the total mass of components I and II. The mass of the silane coupling agent ((3-glycidylpropoxy)trimethoxysilane) is 1-10% of the total mass of components I and II. The catalyst (zinc chloride or aluminum chloride) is 3% of the mass of urea.

[0032] This invention discloses a method for preparing a cross-linked network water-soluble polymeric paper reinforcing agent, comprising the following steps:

[0033] PVA (polyvinyl alcohol) and CMC-Na (sodium carboxymethyl cellulose, hereinafter referred to as CMC for simplicity) are dissolved separately in deionized water. The dissolution temperature of PVA is 60-120℃, and that of CMC is 50-80℃. Then, the PVA solution (5-20% by mass) and the CMC solution (2-10% by mass, a pale yellow transparent solution) are mixed and stirred at 50-60℃ for 30-60 minutes at a stirring speed of 300-500 r / min to obtain a homogeneous PVA / CMC mixed solution.

[0034] Polyvinyl alcohol has a degree of polymerization between 300 and 3000, a degree of alcoholysis between 88% and 98%, and an aqueous solution mass concentration between 5% and 20%.

[0035] The PVA / CMC mixed solution was heated to 60-120℃, and then a mixed aqueous solution of urea and zinc chloride or aluminum chloride (dissolved at room temperature, with a urea to water mass ratio of 1:30) was slowly added dropwise while stirring at a speed of 300 r / min. Zinc chloride or aluminum chloride was used as a catalyst, and the addition time was controlled at 30-150 min. After the addition was completed, the solution was kept at this temperature for 0.5-2 hours. During this process, the amino groups of urea reacted with the hydroxyl groups of PVA and the carboxyl groups of CMC, gradually forming a cross-linked structure. After the reaction was completed, a cross-linked intermediate product was obtained.

[0036] The cross-linked product was cooled to 65-75℃, and KH560 (3-glycidylpropoxy)trimethoxysilane) was slowly added dropwise over a time of 30-120 min. After the addition was completed, the product was kept at this temperature for 0.5-2 hours with a stirring speed of 200-400 r / min. The silanol produced by the hydrolysis of KH560 underwent a condensation reaction with the terminal hydroxyl groups of the cross-linked intermediate product, thus achieving end-capping. The pH was then adjusted to neutral with 26% ammonia water to obtain a cross-linked network water-soluble polymer paper strengthening agent in the form of an emulsion.

[0037] The reaction equation for the cross-linking of CMC (carboxymethyl cellulose) and PVA (polyvinyl alcohol) with urea (catalyzed by zinc chloride) is as follows:

[0038] The reaction of the carboxyl group (-COOH) of CMC with urea (H2N-CONH2):

[0039] Under the catalysis of zinc chloride or aluminum chloride, the reaction formula is -COOH+H2N-CONH2→CO-NH-CONH2+H2O.

[0040] The reaction of the hydroxyl group (-OH) of PVA with urea: -OH + H₂N-CONH₂ → -O-CO-NH-NH₂ + H₂O

[0041] The end-capping reaction equation for KH560 (γ-(2,3-epoxypropoxy)propyltrimethoxysilane) is as follows:

[0042] The terminal hydroxyl group (-OH) of CMC or PVA reacts with the epoxy group (CH2-CH-CH2O) of KH560:

[0043] -OH+CH2-CH-CH2O→HO-CH2-CH(OH)-CH2-O-polymer chain

[0044] Example 1

[0045] PVA with a degree of polymerization of 1799 and a degree of alcoholysis of 98% was selected. The PVA was slowly added to deionized water and stirred at 90°C for 2 hours until completely dissolved, yielding a 10% (w / w) PVA solution. CMC with a degree of substitution of 0.7 was selected. The CMC was slowly added to deionized water at room temperature and stirred at 60°C for 3 hours, yielding a 5% (w / w) CMC solution.

[0046] PVA solution and CMC solution were mixed at a solid content mass ratio of 5:2 and stirred at 60℃ for 20 minutes at a stirring speed of 200 r / min to obtain a homogeneous PVA / CMC mixed solution.

[0047] A PVA / CMC mixed solution was heated to 80°C, and then a mixed aqueous solution of urea and zinc chloride was added dropwise. The amount of urea added was 5% of the total mass of PVA and CMC. Zinc chloride was added as a catalyst, at a rate of 3% of the urea mass, over a dropping time of 60 min. After the addition was complete, the mixture was kept at this temperature for 2 h, with continuous stirring at a speed of 300 r / min. Under the catalysis of zinc chloride, the amino groups in the urea molecules crosslinked with the hydroxyl groups on the PVA and CMC molecular chains. After the reaction was complete, the temperature was lowered to 70°C, and KH560 was slowly added dropwise to the reaction system at a rate of 3% of the total mass of PVA and CMC, over a dropping time of 30 min. After the addition was complete, the mixture was stirred at 70°C for 2 h at a stirring speed of 200 r / min. The silanol produced by the hydrolysis of KH560 underwent a condensation reaction with the hydroxyl groups of the crosslinking intermediates, thus achieving end-capping. The pH was adjusted to neutral using 26% ammonia water by volume to obtain a cross-linked network water-soluble polymer paper strengthening agent, namely sample G1-PVA / CMC.

[0048] Example 2

[0049] PVA with a degree of polymerization of 1799 and a degree of alcoholysis of 98% was selected. The PVA was slowly added to deionized water and stirred at 90°C for 2 hours until completely dissolved, yielding a 10% (w / w) PVA solution. CMC with a degree of substitution of 0.7 was selected. The CMC was slowly added to deionized water at room temperature and stirred at 60°C for 3 hours, yielding a 5% (w / w) CMC solution.

[0050] PVA solution and CMC solution were mixed at a solid content mass ratio of 5:2 and stirred at 60℃ for 30 minutes at a stirring speed of 200 r / min to obtain a homogeneous PVA / CMC mixed solution.

[0051] A PVA / CMC mixed solution was heated to 80°C, and then a mixed aqueous solution of urea and zinc chloride was added dropwise. The amount of urea added was 8% of the total mass of PVA and CMC. Zinc chloride was added as a catalyst, with the amount of zinc chloride added being 3% of the mass of urea. The addition time was controlled at 60 min. After the addition was completed, the mixture was kept at the same temperature for 2 h, with continuous stirring at a speed of 300 r / min during the reaction. Under the catalysis of zinc chloride, the amino groups in the urea molecules crosslinked with the hydroxyl groups on the PVA and CMC molecular chains. After the reaction was completed, the temperature was lowered to 70°C, and KH560 was slowly added dropwise to the reaction system. The amount of KH560 added was 3% of the total mass of PVA and CMC, with the addition time controlled at 30 min. After the addition was completed, the mixture was stirred at 70°C for 2 h at a stirring speed of 200 r / min. The silanol produced by the hydrolysis of KH560 underwent a condensation reaction with the hydroxyl groups of the crosslinking intermediate, thereby achieving end-capping. The pH was adjusted to neutral using ammonia solution with a volume fraction of 26% to obtain a cross-linked network water-soluble polymer paper reinforcing agent, namely sample G2-PVA / CMC.

[0052] Example 3

[0053] PVA with a degree of polymerization of 1799 and a degree of alcoholysis of 98% was selected. The PVA was slowly added to deionized water and stirred at 90°C for 2 hours until completely dissolved, yielding a 10% (w / w) PVA solution. CMC with a degree of substitution of 0.7 was selected. The CMC was slowly added to deionized water at room temperature and stirred at 60°C for 3 hours, yielding a 5% (w / w) CMC solution.

[0054] PVA solution and CMC solution were mixed at a solid content mass ratio of 5:2 and stirred at 70℃ for 30 minutes at a stirring speed of 200 r / min to obtain a homogeneous PVA / CMC mixed solution.

[0055] A PVA / CMC mixed solution was heated to 80°C, and then a mixed aqueous solution of urea and zinc chloride was added dropwise. The amount of urea added was 10% of the total mass of PVA and CMC. Zinc chloride was added as a catalyst, at a rate of 3% of the urea mass, over a dropping time of 60 min. After the addition was complete, the mixture was kept at this temperature for 2 h, with continuous stirring at a speed of 300 r / min. Under the catalysis of zinc chloride, the amino groups in the urea molecules crosslinked with the hydroxyl groups on the PVA and CMC molecular chains. After the reaction was complete, the temperature was lowered to 70°C, and KH560 was slowly added dropwise to the reaction system at a rate of 3% of the total mass of PVA and CMC, over a dropping time of 30 min. After the addition was complete, the mixture was stirred at 70°C for 2 h at a stirring speed of 200 r / min. The silanol produced by the hydrolysis of KH560 underwent a condensation reaction with the hydroxyl groups of the crosslinking intermediate, thus achieving end-capping. The pH was adjusted to neutral using ammonia solution with a volume fraction of 26% to obtain a cross-linked network water-soluble polymer paper reinforcing agent, namely sample G3-PVA / CMC.

[0056] Example 4

[0057] PVA with a degree of polymerization of 1799 and a degree of alcoholysis of 98% was selected. The PVA was slowly added to deionized water and stirred at 90°C for 2 hours until completely dissolved, yielding a 10% (w / w) PVA solution. CMC with a degree of substitution of 0.7 was selected. The CMC was slowly added to deionized water at room temperature and stirred at 60°C for 3 hours, yielding a 5% (w / w) CMC solution.

[0058] PVA solution and CMC solution were mixed at a solid content mass ratio of 5:2 and stirred at 70℃ for 30 minutes at a stirring speed of 200 r / min to obtain a homogeneous PVA / CMC mixed solution.

[0059] A PVA / CMC mixed solution was heated to 80°C, and then a mixed aqueous solution of urea and zinc chloride was added dropwise. The amount of urea added was 15% of the total mass of PVA and CMC. Zinc chloride was added as a catalyst, at a rate of 3% of the urea mass, over a dropping time of 60 min. After the addition was complete, the mixture was kept at this temperature for 2 h, with continuous stirring at a speed of 300 r / min. Under the catalysis of zinc chloride, the amino groups in the urea molecules crosslinked with the hydroxyl groups on the PVA and CMC molecular chains. After the reaction was complete, the temperature was lowered to 70°C, and KH560 was slowly added dropwise to the reaction system at a rate of 3% of the total mass of PVA and CMC, over a dropping time of 30 min. After the addition was complete, the mixture was stirred at 70°C for 2 h at a stirring speed of 200 r / min. The silanol produced by the hydrolysis of KH560 underwent a condensation reaction with the hydroxyl groups of the crosslinking intermediates, thus achieving end-capping. The pH was adjusted to neutral using 26% ammonia water by volume to obtain a cross-linked network water-soluble polymer paper strengthening agent, namely sample G4-PVA / CMC.

[0060] Structural and performance testing

[0061] FT-IR Analysis of Cross-linked Water-soluble Polymer Paper Strengthening Agent (G-PVA / CMC)

[0062] Figure 1 This is the infrared spectrum of G-PVA / CMC at 3565 cm⁻¹. -1 The broad, strong absorption peak is due to the stretching vibration of the hydroxyl group (-OH). This peak is broad and strong due to the presence of water; it is located at 2937 cm⁻¹. -1 The appearance of a stretching vibration absorption peak at saturated CH bonds is characteristic of alkyl compounds and consistent with the carbon chain structure in KH560; at 2063 cm⁻¹... -1 The characteristic absorption peak of -C≡N- is located at 1635 cm⁻¹. -1 The presence of a strong C=O stretching vibration absorption peak at the point confirms the existence of the cross-linked structure of PVA and CMC, which is consistent with the structural characteristics of G-PVA / CMC.

[0063] Thermogravimetric analysis of cross-linked network water-soluble polymeric paper strengthening agent (G-PVA / CMC)

[0064] The mass loss and integral curves of the emulsion after heating after drying were tested and analyzed to investigate the effect of temperature on polymer stability. The results are as follows: Figure 2 As shown. From Figure 2It can be seen that as the temperature increases, the mass loss of the reinforcing agent exhibits three distinct stages, eventually reaching an equilibrium state; the mass loss rate shows a trend of initially slow, then accelerating, and finally gradually stabilizing. In the first stage, at a temperature not exceeding 200 ℃, the mass of the reinforcing agent changed by 7.88%, which is due to the evaporation of moisture. In the stage of 200-300 ℃, a small amount of pyrolysis of side chain groups on the molecular chain occurred. In the third stage, when the temperature does not exceed 300-400 ℃, the mass loss rate accelerated significantly, with a mass change as high as 44.25%, which is mainly characterized by the breakage of the main chain segments of the paper reinforcing agent.

[0065] The cross-linked network water-soluble polymer paper reinforcing agent prepared above was used to sizing the surface of corrugated base paper waste paper: the base paper was cut into 15cm×30cm pieces, impregnated with an appropriate amount of G-PVA / CMC sizing agent for 3 minutes, and then placed on a roller coater. The sizing agent was evenly coated on the paper using the sizing roller, so that the cross-linked network water-soluble polymer paper reinforcing agent was adsorbed onto the fibers through hydrogen bonding and electrostatic interaction. Finally, it was vacuum dried at 105℃ for 10 minutes, and the physical properties of the finished paper were tested.

[0066] The physical properties of the paper were tested and calculated according to national standards. The tensile index was determined according to GB / T 12914-2008, "Determination of Tensile Strength of Paper and Paperboard"; the tear index was determined according to GB / T 455-2002, "Determination of Tear Strength of Paper and Paperboard"; the folding endurance was determined according to GB / T 457-2008, "Determination of Folding Endurance of Paper and Paperboard"; the ring crush index was determined according to GB / T 2679.8-2016, "Determination of Ring Crush Strength of Paper and Paperboard"; and the water absorption test (60 Scobb value) was determined according to GB / T 1540-2002, "Determination of Water Absorption of Paper and Paperboard".

[0067] Effect of urea dosage on paper strength properties

[0068] To determine the optimal urea content, the effect of urea addition as a cross-linked network water-soluble polymer paper strengthening agent on the paper strength properties was investigated. The results are as follows: Figure 3 , Figure 4 and Figure 5 As shown, the dry and wet tensile indices of sized paper increase with increasing urea content. This is because the cross-linked network water-soluble polymer paper strengthening agent is electrostatically attracted to the paper fibers and adsorbed onto the fiber surface. Furthermore, the hydroxyl functional groups on the polymer chains also form hydrogen bonds with the hydroxyl groups on the fibers, thereby increasing the toughness and strength between the fibers. When the urea content is 8.0%, the paper's dry tensile index is 78.3 Nm / g, wet tensile index is 4.63 Nm / g, folding endurance is 11 times, and tear index is 6.6 Nm / g. 2 / g, ring pressure index 4.7 Nmg -1Surface water absorption rate: 84.3 g / m 2 Compared with paper without internal sizing, the strengths increased by 318.7%, 69.8%, 100%, 522.6%, 50.4%, and 18.9%, respectively. However, as the urea content increased, both the dry and wet strength indices of the paper decreased. This is because excessive urea competes with PVA and CMC for reaction sites, interfering with the normal cross-linking process, resulting in incomplete or unstable cross-linked structures, and thus reducing the strength of the paper.

Claims

1. A method for preparing a cross-linked network water-soluble polymeric paper reinforcing agent, characterized in that, Includes the following steps; S1, polyvinyl alcohol and sodium carboxymethyl cellulose are dissolved in deionized water at 60-120℃ and 50-80℃ respectively to obtain a polyvinyl alcohol solution with a mass percentage of 5%-20% and a sodium carboxymethyl cellulose solution with a mass percentage of 2%-10%. The polyvinyl alcohol solution and sodium carboxymethyl cellulose solution are mixed and stirred at 50-60℃ for 30-60 min at a stirring speed of 300-500 r / min to obtain a mixture containing polyvinyl alcohol and sodium carboxymethyl cellulose. At 60-120℃, a catalyst and an amino-containing crosslinking agent are added to the mixture containing polyvinyl alcohol and sodium carboxymethyl cellulose while stirring. The crosslinking agent is urea, and the catalyst is zinc chloride or aluminum chloride. The mass ratio of polyvinyl alcohol to sodium carboxymethyl cellulose is (1-5):(1-3). The urea is 3%-15% of the total mass of polyvinyl alcohol and sodium carboxymethyl cellulose, and the catalyst is 3% of the mass of urea. After heat treatment, reaction system a is obtained. S2, cool reaction system a, add a silane coupling agent, which is (3-glycidylpropoxy)trimethoxysilane, and the (3-glycidylpropoxy)trimethoxysilane is 1%-10% of the total mass of polyvinyl alcohol and sodium carboxymethyl cellulose. Then keep it warm for 0.5-2 hours to obtain reaction system b. Adjust the pH of reaction system b to neutral to obtain a cross-linked network water-soluble polymer paper strengthening agent.

2. The method for preparing the cross-linked network water-soluble polymeric paper reinforcing agent according to claim 1, characterized in that, S1 is added dropwise while stirring, with the addition time controlled between 30-150 min.

3. The method for preparing the cross-linked network water-soluble polymeric paper reinforcing agent according to claim 1, characterized in that, The heat preservation treatment described in S1 is carried out at 60-120℃ for 0.5-2 hours.

4. The method for preparing the cross-linked network water-soluble polymeric paper reinforcing agent according to claim 1, characterized in that, S2 cools reaction system a to 65-75℃ and then adds a silane coupling agent.

5. The method for preparing the cross-linked network water-soluble polymeric paper reinforcing agent according to claim 1, characterized in that, S2 is added dropwise with (3-glycidylpropoxy)trimethoxysilane, and the addition time is controlled between 30-120 min.

6. A cross-linked network water-soluble polymeric paper reinforcing agent obtained by the preparation method of the cross-linked network water-soluble polymeric paper reinforcing agent according to any one of claims 1-5.

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