Amphoteric polyacrylamide dry strength agent aqueous solution and preparation method thereof

Abstract

The invention provides an amphoteric polyacrylamide dry strength agent aqueous solution and a preparation method thereof.The preparation method comprises the following steps that acrylamide, an active monomer, an anionic monomer, a chelating agent, a chain transfer agent and water are stirred and mixed to be uniform, then a redox initiator is added to initiate a polymerization reaction, and a first reaction mixed solution is obtained; dropwise adding a grafting monomer into the first reaction mixed solution, and reacting to obtain a second reaction mixed solution; wherein the grafting monomer is an unsaturated ester compound containing hydroxyl; uniformly mixing a mixed solution formed by acrylamide, a cationic monomer, a cross-linking agent and water with the second reaction mixed solution, and initiating a polymerization reaction through an azo initiator to obtain an aqueous solution of the amphoteric polyacrylamide dry strength agent. The dry strength agent aqueous solution prepared by the method is moderate in solid content and excellent in solubility, the reaction heat in the polymerization process is easy to control, and the production stability is high; compared with traditional cationic or common amphoteric polyacrylamide, the dry strength agent can more effectively improve the tensile strength and bursting strength of cultural paper, packaging paper and specialty paper.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to an aqueous solution of amphoteric polyacrylamide dry strength agent and its preparation method. Background Technology

[0002] Under the development trend of greening, high-value-added and low-cost papermaking industry, amphoteric polyacrylamide dry strength agent has gradually become the core additive in the production of packaging paper, cultural paper and specialty paper because it has both the fiber adsorption capacity of cationic groups and the environmental tolerance of anionic groups.

[0003] However, existing amphoteric polyacrylamide dry strength agents still face significant technical bottlenecks in terms of preparation processes and molecular mechanisms. Regarding molecular mechanisms, traditional amphoteric polyacrylamide dry strength agents primarily rely on electrostatic adsorption. The hydrogen bonds provided by the acrylamide units are weak and easily disrupted, resulting in limited improvement in paper tensile and burst strength, making it difficult to meet the requirements of high-strength corrugated base paper, food packaging paper, and other high-end paper types. In terms of preparation processes, existing amphoteric polyacrylamide dry strength agent aqueous solutions are mostly prepared using a one-step mixed copolymerization method. This method is prone to polymerization fluctuations and localized overheating due to differences in monomer reactivity, leading to excessive cross-linking and gelation of molecular chains and a wide molecular weight distribution. Furthermore, the polymer easily forms aggregates through hydrogen bond entanglement, resulting in uneven dispersion in the pulp, causing localized high concentrations and bright spots, while simultaneously reducing agent utilization and increasing costs.

[0004] Therefore, there is an urgent need to provide an aqueous solution of amphoteric polyacrylamide dry strength agent and its preparation method. Summary of the Invention

[0005] This invention provides an aqueous solution of amphoteric polyacrylamide dry strength agent and its preparation method, which can solve the problems of existing aqueous solutions of amphoteric polyacrylamide dry strength agent being prone to agglomeration in pulp and having insufficient bonding strength with paper.

[0006] In a first aspect, the present invention provides a method for preparing an aqueous solution of amphoteric polyacrylamide dry strength agent, the preparation method comprising the following steps: (1) Mix acrylamide, active monomer, anionic monomer, chelating agent, chain transfer agent and water, then add redox initiator to initiate polymerization reaction to obtain the first reaction mixture; (2) The grafted monomer is added dropwise to the first reaction mixture, and a second reaction mixture is obtained after the reaction; wherein the grafted monomer is an unsaturated ester compound containing hydroxyl groups; (3) Mix the mixed solution of acrylamide, cationic monomer, crosslinking agent and water with the second reaction mixture, and initiate the polymerization reaction with an azo initiator to obtain the aqueous solution of the amphoteric polyacrylamide dry strength agent.

[0007] Preferably, in step (1), the active monomer is N-hydroxymethylacrylamide; the anionic monomer is at least one of fumaric acid, maleic acid, citric acid or itaconic acid; the chelating agent is at least one of disodium ethylenediaminetetraacetate, triethanolamine or trisodium aminotriacetate; and the chain transfer agent is at least one of isopropanol, methanol, thiol or sodium formate.

[0008] Preferably, the redox initiator is a group consisting of ammonium persulfate-sodium bisulfite, tert-butyl hydroperoxide-pyrosulfite, or potassium persulfate-thiosulfate.

[0009] Preferably, in steps (1) and (2), based on the mass of acrylamide, the amounts of each reaction raw material are as follows: 0.5-2% active monomer, 5-10% anionic monomer, 0.1-0.3% chelating agent, 0.2-0.8% chain transfer agent, 0.1-0.3% redox initiator, 2-10% grafting monomer, and 7-9% water.

[0010] Preferably, in step (1), the initiation temperature is 63~65℃, and the reaction is kept at 87~93℃ for 20~30 minutes; Preferably, in step (2), the grafting monomer is hydroxyethyl methacrylate.

[0011] Preferably, the dropping rate of the grafted monomer is 0.3~0.5 mL / min, and the reaction time is 10~15 min.

[0012] Preferably, in step (3), the cationic monomer is at least one of methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, dimethylaminoethyl acrylate, or dimethylaminoethyl methacrylate; and the crosslinking agent is at least one of ethylene glycol diacrylate, N,N-dimethylacrylamide, or methylenebisacrylamide.

[0013] Preferably, the azo initiator is at least one of azobisisobutyronitrile hydrochloride, azobisisobutyronitrile, azobisisoheptanenitrile, or azobisisobutylamidine hydrochloride.

[0014] Preferably, the mass ratio of acrylamide in step (3) to acrylamide in step (1) is 100:(15~20).

[0015] Preferably, in step (3), based on the mass of acrylamide, the amounts of each reaction raw material are: 10-30% cationic monomer, 0.01-0.08% crosslinking agent, 0.1-0.3% azo initiator, and 6-6.5%.

[0016] Preferably, in step (3), the initiation temperature is 83~85℃, and the reaction is carried out at 90~95℃ for 60~90 minutes.

[0017] Preferably, in step (1), before adding the initiator to initiate the polymerization reaction, the step further includes purging the reaction system with nitrogen to remove oxygen and adjusting the pH of the reaction system to 3.0~3.5.

[0018] Preferably, in step (3), before adding the initiator to initiate the polymerization reaction, the step of adjusting the pH of the reaction system to 3.0~3.5 is also included.

[0019] Preferably, in step (3), after the reaction is complete, a step of adding a terminator to the reaction system is also included.

[0020] More preferably, the terminating agent is at least one of hydroquinone, hydroquinone methyl ether, or tert-butylcatechol.

[0021] More preferably, in step (3), the terminating agent is 0.3-0.5% based on the mass of acrylamide.

[0022] Preferably, the effective content of the amphoteric polyacrylamide dry strength agent aqueous solution is 13-17%, and the apparent viscosity is 1000-3000 MPa·s.

[0023] Secondly, embodiments of the present invention also provide an aqueous solution of amphoteric polyacrylamide dry strength agent, which is prepared by any of the preparation methods described in the first aspect above.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects: (1) In this invention, by introducing hydroxyl-containing unsaturated ester compounds during the polymerization reaction of polyacrylamide, the bonding ability between the dry strength agent aqueous solution and pulp fiber can be significantly enhanced. During use, the hydroxyl groups introduced in the dry strength agent can form strong hydrogen bonds with the hydroxyl groups on the fiber surface. The bonding strength is much higher than that of the weak hydrogen bonds in traditional polyacrylamide, thereby effectively compensating for the lack of bonding force between fibers and achieving synergistic enhancement of electrostatic adsorption and hydrogen bonding. Furthermore, the dry strength agent aqueous solution product retains the cationic groups of amphoteric polyacrylamide, which can ensure that the dry strength agent is quickly and stably adsorbed on the negatively charged fiber surface and prevented from being lost with water. At the same time, the strong hydrogen bonds provided by the hydroxyl-containing unsaturated ester compounds further strengthen the bonding between fibers, forming a stable three-dimensional network of fiber-dry strength agent-fiber. The synergistic effect of these two mechanisms fundamentally compensates for the defects of insufficient hydrogen bond binding sites and weak bonding force in traditional dry strength agent products, and can significantly improve the key physical properties of paper such as tensile strength and bursting strength.

[0025] (2) In this invention, a three-step synthesis process is used. In the first step, acrylamide, active monomer, anionic monomer, chelating agent, chain transfer agent and water are polymerized under the action of a redox initiator to form an acrylamide-anionic prepolymer, ensuring a uniform main chain structure. In the second step, after the main chain of the prepolymer is formed, hydroxyl-containing unsaturated ester compounds are introduced dropwise. This not only allows them to better match the reactivity of the anionic units in the prepolymer, but also avoids intermolecular self-polymerization at high concentrations, thereby ensuring that hydroxyl functional groups can be efficiently and directionally grafted onto the polymer main chain. In the third step, a mixed solution containing acrylamide, cationic monomer and crosslinking agent is copolymerized with the product of the second step. By strictly controlling the order of addition and temperature, disordered copolymerization caused by excessively high local concentrations of cationic monomer is effectively avoided, and the problem of "too fast in the early stage and insufficient in the later stage" caused by the difference in reactivity of different monomers is alleviated. This inhibits local overheating and excessive crosslinking of molecular chains from the source, and significantly reduces the probability of gel particle formation. Thus, by precisely controlling the reaction sequence and conditions through a three-step process, the polymer molecular chains are fully extended and uniformly dispersed in the aqueous solution, fundamentally preventing agglomeration and stratification caused by hydrogen bond entanglement. The resulting dry strength agent aqueous solution exhibits excellent dispersion stability and can be uniformly distributed in the pulp during application, completely solving the appearance defects such as "bright spots" and "spots" on the paper caused by agglomeration. This significantly improves paper uniformity, surface quality, and additive utilization, while ensuring batch-to-batch stability of product quality. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0027] Based on one or more technical problems existing with amphoteric polyacrylamide as a dry strength agent in papermaking, embodiments of the present invention provide a method for preparing an aqueous solution of amphoteric polyacrylamide dry strength agent, the preparation method comprising the following steps: (1) Mix acrylamide, active monomer, anionic monomer, chelating agent, chain transfer agent and water, then add redox initiator to initiate polymerization reaction to obtain the first reaction mixture; (2) The grafted monomer is added dropwise to the first reaction mixture, and a second reaction mixture is obtained after the reaction; wherein the grafted monomer is an unsaturated ester compound containing hydroxyl groups; (3) Mix the mixed solution of acrylamide, cationic monomer, crosslinking agent and water with the second reaction mixture, and initiate the polymerization reaction with an azo initiator to obtain the aqueous solution of the amphoteric polyacrylamide dry strength agent.

[0028] In this embodiment of the invention, by introducing hydroxyl-containing unsaturated ester compounds during the polymerization reaction of polyacrylamide, the bonding ability between the dry strength agent aqueous solution and pulp fibers can be significantly enhanced. During use, the hydroxyl groups introduced in the dry strength agent can form strong hydrogen bonds with the hydroxyl groups on the fiber surface. The bonding strength is much higher than that of the weak hydrogen bonds in traditional polyacrylamide, thereby effectively compensating for the insufficient bonding force between fibers and achieving synergistic enhancement of electrostatic adsorption and hydrogen bonding. Furthermore, the resulting dry strength agent aqueous solution product retains the cationic groups of amphoteric polyacrylamide, which can ensure that the dry strength agent is quickly and stably adsorbed on the negatively charged fiber surface and prevented from being lost with water. At the same time, the strong hydrogen bonds provided by the hydroxyl-containing unsaturated ester compounds further strengthen the bonding between fibers, forming a stable three-dimensional network of fiber-dry strength agent-fiber. The synergistic effect of these two mechanisms fundamentally compensates for the defects of insufficient hydrogen bonding sites and weak bonding force in traditional dry strength agent products, and can significantly improve the key physical properties of paper such as tensile strength and bursting strength.

[0029] Considering that when hydroxyl-containing unsaturated ester compounds are copolymerized with other monomers to form polyacrylamide copolymers, they tend to preferentially react with cationic monomers to form random copolymers rather than being directionally grafted onto the polyacrylamide backbone, resulting in insufficient grafting efficiency, difficulty in fully utilizing strong hydrogen bonding, and the tendency for free monomers to undergo self-polymerization, affecting paper uniformity, this invention innovatively employs a three-step synthesis process. The first step involves initiating a polymerization reaction between acrylamide, reactive monomers, anionic monomers, chelating agents, chain transfer agents, and water under the action of a redox initiator to form an acrylamide-anionic prepolymer, ensuring a uniform backbone structure. The second step, after the prepolymer backbone is formed, introduces hydroxyl-containing unsaturated ester compounds dropwise. This not only better matches the reactivity of the anionic units in the prepolymer but also prevents intermolecular self-polymerization at high concentrations, thus ensuring that hydroxyl functional groups can be efficiently and directionally grafted onto the polymer backbone. In the third step, a mixed solution containing acrylamide, cationic monomers, and crosslinking agents is copolymerized with the product from the second step. By strictly controlling the order of addition and temperature, disordered copolymerization caused by excessively high local concentrations of cationic monomers is effectively avoided, and the problem of "excessive initial reaction and insufficient later reaction" caused by differences in the reactivity of different monomers is alleviated. This inhibits local overheating and excessive crosslinking of molecular chains from the source, significantly reducing the probability of gel particle formation. Thus, through the precise control of the reaction sequence and conditions in the three-step process, the polymer molecular chains are fully extended and uniformly dispersed in the aqueous solution, fundamentally preventing agglomeration and stratification caused by hydrogen bond entanglement. The resulting dry strength agent aqueous solution has excellent dispersion stability and can be uniformly distributed in pulp during application, completely solving the appearance defects such as "bright spots" and "spots" on the paper caused by agglomeration, significantly improving paper uniformity, surface quality, and additive utilization, while ensuring batch-to-batch stability of product quality.

[0030] According to some preferred embodiments, in step (1), the active monomer is N-hydroxymethylacrylamide; the anionic monomer is at least one of fumaric acid, maleic acid, citric acid or itaconic acid; the chelating agent is at least one of disodium ethylenediaminetetraacetate, triethanolamine or trisodium aminotriacetate; the chain transfer agent is at least one of isopropanol, methanol, thiol or sodium formate; the redox initiator is a group consisting of ammonium persulfate-sodium bisulfite, tert-butyl hydroperoxide-pyrosulfite or potassium persulfate-thiosulfate; preferably, the mass ratio of oxidant to reductant in the redox initiator is 1:1.

[0031] According to some preferred embodiments, in steps (1) and (2), based on the mass of acrylamide, the amounts of each reactant are as follows: active monomer 0.5-2% (e.g., 0.5%, 1.0%, 1.5%, or 2%), anionic monomer 5-10% (e.g., 5%, 6%, 7%, 8%, 9%, or 10%), chelating agent 0.1-0.3% (e.g., 0.1%, 0.2%, 0.25%, or 0.3%), chain transfer... The agent is 0.2-0.8% (e.g., 0.2%, 0.3%, 0.5%, 0.6% or 0.8%), the redox initiator is 0.1-0.3% (e.g., 0.1%, 0.2%, 0.25% or 0.3%), the grafting monomer is 2-10% (e.g., 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%), and the water is 7-9% (e.g., 7%, 7.5%, 8%, 8.5% or 9%).

[0032] According to some preferred embodiments, in step (1), the initiation temperature is 63~65℃ (for example, it can be 63℃, 64℃ or 65℃), and after reacting to 87~93℃ (for example, it can be 87℃, 88℃, 89℃, 90℃, 91℃, 92℃ or 93℃), it is kept at this temperature for 20~30min (for example, it can be 20min, 22min, 25min, 28min or 30min). In this embodiment of the invention, acrylamide is first polymerized with the aforementioned active monomers, anionic monomers, chelating agents, chain transfer agents, and water under the action of a redox agent to form a structurally controllable acrylamide-anionic prepolymer. Furthermore, precise control of the content of each component during the reaction facilitates the construction of a polymer backbone with a uniform structure, well-defined reaction sites, and a concentrated molecular weight distribution, thus laying the foundation for subsequent reactions. The addition of active monomers introduces reactive sites into the polymer chain and adjusts the polymer molecular weight. If the content is too low, it is not conducive to introducing sufficient active sites on the main chain, leading to a decrease in the grafting efficiency of hydroxyl-containing unsaturated ester compounds in the second step. Conversely, if the content is too high, it easily leads to excessive cross-linking or the formation of localized gels during polymerization, resulting in a decrease in the solubility of the prepolymer. Meanwhile, if the content of anionic monomers is too low, the negative charge content in the prepolymer will be too low, which will not be conducive to its combination with the subsequent cationic monomers; while if the content is too high, the prepolymer as a whole will have too strong anionicity, which may cause it to react strongly with the cationic monomers in the subsequent steps to produce precipitation or microgels, and affect the adsorption performance of the final dry strength agent on the surface of the negatively charged fiber.

[0033] According to some preferred embodiments, in step (2), the grafting monomer is hydroxyethyl methacrylate.

[0034] In this embodiment of the invention, the grafting monomer is preferably hydroxyethyl methacrylate. The hydroxyl groups in its molecular chain can form strong hydrogen bonds with the hydroxyl groups on the surface of pulp fibers. This bonding force is much stronger and more stable than the weak hydrogen bonds provided by the acrylamide units in ordinary polyacrylamide. It effectively strengthens the connection between fibers like a molecular bridge. At the same time, its ester structure can also improve the compatibility between the polymer and the fiber, promoting a more uniform distribution of the molecular chain on the fiber surface and forming a comprehensive and tight bond with the fiber. In addition, this monomer can also improve the flexibility of the polymer molecular chain, avoiding the increase in paper brittleness caused by the excessive rigidity of the molecular chain in traditional dry strength agents. Thus, while improving the strength of the paper, it also significantly improves its toughness.

[0035] According to some preferred embodiments, the dropping rate of the grafted monomer is 0.3 to 0.5 mL / min (e.g., 0.3 mL / min, 0.4 mL / min or 0.5 mL / min), and the reaction time is 10 to 15 min (e.g., 10 min, 11 min, 12 min, 13 min, 14 min or 15 min).

[0036] Considering the high reactivity of hydroxyethyl methacrylate (HME) and anionic monomers, and their similar free radical reactivity, they can form stable copolymer segments. Therefore, in this embodiment of the invention, acrylamide is first polymerized with anionic monomers to form a well-defined acrylamide-anionic prepolymer. The anionic units (containing active hydrogen sites) uniformly distributed on the main chain of this prepolymer can weakly interact with the ester groups in the subsequently introduced hydroxyethyl methacrylate molecules, thereby guiding the free radicals of hydroxyethyl methacrylate to preferentially attack the prepolymer main chain, effectively avoiding intermolecular self-polymerization or disordered copolymerization with subsequent cationic monomers. After the prepolymer main chain is formed, hydroxyethyl methacrylate is introduced dropwise, and the dropwise addition rate of the grafting monomer is precisely controlled, which is beneficial to achieve efficient grafting of hydroxyethyl methacrylate while ensuring system stability.

[0037] Experiments of this invention have confirmed that if the dropping rate is too fast, the probability of collisions between hydroxyethyl methacrylate (HME) molecules increases significantly. Its free radicals are more likely to react with other HME molecules for self-polymerization, rather than attacking the active sites of the main chain. This leads to a decrease in grafting efficiency, and a large amount of ungrafted HME will self-polymerize to form low-molecular-weight homopolymers. These homopolymers not only fail to contribute effective hydrogen bonding but may also adsorb onto the fiber surface, forming an inert layer and thus reducing paper uniformity. Conversely, if the dropping rate is too slow, HME will remain in the reaction system for too long. Especially under acidic conditions, its ester groups are prone to hydrolysis, generating glycolic acid, resulting in a reduction in the number of effective hydroxyl groups. Simultaneously, the pH fluctuations caused by the hydrolysis products may further inhibit the polymerization activity of subsequent cationic monomers.

[0038] According to some preferred embodiments, in step (3), the cationic monomer is at least one of methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, dimethylaminoethyl acrylate, or dimethylaminoethyl methacrylate; the crosslinking agent is at least one of ethylene glycol diacrylate, N,N-dimethylacrylamide, or methylenebisacrylamide; and the azo initiator is at least one of azobisisobutyronitrile hydrochloride, azobisisobutyronitrile, azobisisoheptanenitrile, or azobisisobutylamidine hydrochloride.

[0039] According to some preferred embodiments, the mass ratio of acrylamide in step (3) to that in step (1) is 100:(15~20) (e.g., 100:15, 100:16, 100:17, 100:18, 100:19 or 100:20); in step (3), based on the mass of acrylamide, the amounts of each reactant are: 10~30% cationic monomer (e.g., 10%, 15%, 20%, 25% or 30%), 0.01~0.08% crosslinking agent (e.g., 0.01%, 0.03%, 0.05% or 0.08%), 0.1~0.3% azo initiator (e.g., 0.1%, 0.2% or 0.3%), and 6~6.5% water (e.g., 6%, 6.1%, 6.2%, 6.3%, 6.4% or 6.5%).

[0040] According to some preferred embodiments, in step (3), the initiation temperature is 83~85℃ (for example, it can be 83℃, 84℃ or 85℃), and the reaction is held at 90~95℃ (for example, it can be 90℃, 92℃, 93℃ or 95℃) for 60~90min (for example, it can be 60min, 70min, 80min or 90min).

[0041] In this embodiment of the invention, in the third step of the reaction, a mixture of acrylamide, cationic monomer, crosslinking agent, and water is first dissolved in the polymer backbone formed in the second step for polymerization. The reaction rate is increased by adding an azo initiator. Simultaneously, the addition of the cationic monomer allows for charge complementarity with the hydroxyethyl methacrylate branches. Its electrostatic effect effectively inhibits entanglement between branches, promoting the formation of a spatial structure where the main chain is extended and the branches are evenly distributed. This structure ensures sufficient contact area between the hydroxyl groups of the grafted hydroxyethyl methacrylate and the pulp fibers, while fundamentally preventing molecular chain aggregation, thus significantly improving the dispersion stability of the dry strength agent aqueous solution. Furthermore, by strictly controlling the content of the cationic monomer during the reaction, disordered copolymerization or excessive crosslinking with other active components in the system due to excessively high local concentrations is effectively avoided, which helps ensure the regularity of the final polymer structure.

[0042] According to some preferred embodiments, in step (1), before adding the initiator to initiate the polymerization reaction, the reaction system is further subjected to nitrogen to remove oxygen and the pH of the reaction system is adjusted to 3.0~3.5; in step (3), before adding the initiator to initiate the polymerization reaction, the reaction system is further subjected to the step of adjusting the pH of the reaction system to 3.0~3.5; in step (3), after the reaction is completed, the reaction system is further subjected to the step of adding a terminator; the terminator is at least one of hydroquinone, hydroquinone methyl ether or tert-butylcatechol; in step (3), based on the mass of acrylamide, the terminator is 0.3~0.5% (for example, it can be 0.3%, 0.4% or 0.5%).

[0043] In this embodiment of the invention, before the polymer reaction, nitrogen is introduced into the reaction system to remove oxygen, and the pH of the reaction system is adjusted to 3.0~3.5, thereby inhibiting monomer hydrolysis and creating a suitable environment for subsequent reactions. After the reaction is completed, a terminator is added to stop the polymerization reaction and precisely control the molecular weight, thereby ensuring the stability of product performance.

[0044] According to some preferred embodiments, the effective content of the amphoteric polyacrylamide dry strength agent aqueous solution is 13-17%, and the apparent viscosity is 1000-3000 MPa·s.

[0045] This invention also provides an aqueous solution of amphoteric polyacrylamide dry strength agent prepared by any of the above-described preparation methods.

[0046] This invention provides a method for producing an aqueous solution of dry strength agent suitable for the papermaking industry. It is prepared by grafting hydroxyethyl methacrylate onto amphoteric polyacrylamide. The aqueous solution of dry strength agent obtained by this method has a moderate solid content, excellent solubility, and the heat of reaction during polymerization is easy to control, resulting in high production stability. Compared with traditional cationic or ordinary amphoteric polyacrylamide, this dry strength agent can more effectively improve the tensile strength and burst strength of cultural paper, packaging paper, and specialty paper.

[0047] To more clearly illustrate the technical solution and advantages of the present invention, the following examples provide a detailed description of an aqueous solution of amphoteric polyacrylamide dry strength agent and its preparation method.

[0048] Example 1: (1) Acrylamide and deionized water were mixed to form a 10% mass concentration mixed solution. Then, based on the mass of acrylamide, 1% of active monomer (N-hydroxymethylacrylamide), 7% of anionic monomer (fumaric acid), 0.2% of chelating agent (disodium ethylenediaminetetraacetate), and 0.6% of chain transfer agent (isopropanol) were added and stirred until homogeneous. The pH of the mixture was then adjusted to 3.0 with sulfuric acid. The mixture was then transferred to a glass reactor, a digital thermometer was inserted, and high-purity nitrogen was introduced into it for 30 min. The temperature was then raised to 65°C using a water bath. 0.15% of the mass of acrylamide initiator (ammonium persulfate-sodium bisulfite in a mass ratio of 1:1) was added to the mixture to initiate the polymerization reaction. After the mixture reacted to 90°C, it was kept at that temperature for 20 min to obtain the first reaction mixture. (2) Keep the temperature above 85℃, add the grafted monomer (hydroxyethyl methacrylate) dropwise to the first reaction mixture at a rate of 0.5 mL / min and react for 10 min to obtain the second reaction mixture; wherein, the grafted monomer is 4% of the mass of acrylamide in step (1); (3) Mix acrylamide, 20% cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% crosslinking agent (ethylene glycol diacrylate) and 6.5% deionized water to form a mixed solution. Then adjust the pH of the mixed solution to 3.2 with sulfuric acid. Pour it into the second reaction mixture in step (2) and mix well. Add 0.1% azo initiator (azobisisobutyl cyanide hydrochloride) and initiate the polymerization reaction at 85°C. Keep the reaction at 90°C for 80 min. After the solution viscosity reaches 1000~3000 mPa·s, add 0.3% terminator (hydroquinone) to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of azo initiator, cationic monomer, deionized water and crosslinking agent are based on the mass of acrylamide in step (3). The amount of acrylamide added in step (3) is 15% of the mass of acrylamide in step (1).

[0049] Example 2: (1) Acrylamide and deionized water were mixed to form a 10% mass concentration mixed solution. Then, based on the mass of acrylamide, 1% of active monomer (N-hydroxymethylacrylamide), 7% of anionic monomer (fumaric acid), 0.2% of chelating agent (disodium ethylenediaminetetraacetate), and 0.5% of chain transfer agent (isopropanol) were added and stirred until homogeneous. The pH of the mixture was then adjusted to 3.0 with sulfuric acid. The mixture was then transferred to a glass reactor, a digital thermometer was inserted, and high-purity nitrogen was introduced into it for 30 min. The temperature was then raised to 65°C using a water bath. 0.15% of the mass of acrylamide initiator (ammonium persulfate-sodium bisulfite in a mass ratio of 1:1) was added to the mixture to initiate the polymerization reaction. After the mixture reacted to 90°C, it was kept at that temperature for 20 min to obtain the first reaction mixture. (2) Keep the temperature above 85℃, add the grafted monomer (hydroxyethyl methacrylate) dropwise to the first reaction mixture at a rate of 0.5 mL / min and react for 10 min to obtain the second reaction mixture; wherein, the grafted monomer is 6% of the mass of acrylamide in step (1); (3) Mix acrylamide, 20% cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% crosslinking agent (ethylene glycol diacrylate) and 6.5% deionized water to form a mixed solution. Then adjust the pH of the mixed solution to 3.2 with sulfuric acid. Pour it into the second reaction mixture in step (2) and mix well. Add 0.1% azo initiator (azobisisobutyl cyanide hydrochloride) and initiate the polymerization reaction at 85°C. Keep the reaction at 90°C for 80 min. After the solution viscosity reaches 1000~3000 mPa·s, add 0.3% terminator (hydroquinone) to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of azo initiator, cationic monomer, deionized water and crosslinking agent are based on the mass of acrylamide in step (3). The amount of acrylamide added in step (3) is 15% of the mass of acrylamide in step (1).

[0050] Example 3: (1) Acrylamide and deionized water were mixed to form a 10% mass concentration mixed solution. Then, based on the mass of acrylamide, 1% of active monomer (N-hydroxymethylacrylamide), 7% of anionic monomer (fumaric acid), 0.2% of chelating agent (disodium ethylenediaminetetraacetate), and 0.5% of chain transfer agent (isopropanol) were added and stirred until homogeneous. The pH of the mixture was then adjusted to 3.0 with sulfuric acid. The mixture was then transferred to a glass reactor, a digital thermometer was inserted, and high-purity nitrogen was introduced into it for 30 min. The temperature was then raised to 65°C using a water bath. 0.15% of the mass of acrylamide initiator (ammonium persulfate-sodium bisulfite in a mass ratio of 1:1) was added to the mixture to initiate the polymerization reaction. After the mixture reacted to 90°C, it was kept at that temperature for 20 min to obtain the first reaction mixture. (2) Keep the temperature above 85℃, add the grafted monomer (hydroxyethyl methacrylate) dropwise to the first reaction mixture at a rate of 0.5 mL / min and react for 10 min to obtain the second reaction mixture; wherein, the grafted monomer is 8% of the mass of acrylamide in step (1); (3) Mix acrylamide, 20% cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% crosslinking agent (ethylene glycol diacrylate) and 6.5% deionized water to form a mixed solution. Then adjust the pH of the mixed solution to 3.2 with sulfuric acid. Pour it into the second reaction mixture in step (2) and mix well. Add 0.1% azo initiator (azobisisobutyl cyanide hydrochloride) and initiate the polymerization reaction at 85°C. Keep the reaction at 90°C for 80 min. After the solution viscosity reaches 1000~3000 mPa·s, add 0.3% terminator (hydroquinone) to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of azo initiator, cationic monomer, deionized water and crosslinking agent are based on the mass of acrylamide in step (3). The amount of acrylamide added in step (3) is 15% of the mass of acrylamide in step (1).

[0051] Example 4: (1) Acrylamide and deionized water were mixed to form a 10% mass concentration mixed solution. Then, based on the mass of acrylamide, 1% of active monomer (N-hydroxymethylacrylamide), 8% of anionic monomer (fumaric acid), 0.2% of chelating agent (disodium ethylenediaminetetraacetate), and 0.6% of chain transfer agent (isopropanol) were added and stirred until homogeneous. The pH of the mixture was then adjusted to 3.5 with sulfuric acid. The mixture was then transferred to a glass reactor, a digital thermometer was inserted, and high-purity nitrogen was introduced into it for 30 min. The temperature was then raised to 63°C using a water bath. 0.15% of the mass of acrylamide initiator (ammonium persulfate-sodium bisulfite in a mass ratio of 1:1) was added to the mixture to initiate the polymerization reaction. After the mixture reacted to 93°C, it was kept at that temperature for 20 min to obtain the first reaction mixture. (2) Keep the temperature above 85℃, add the grafted monomer (hydroxyethyl methacrylate) dropwise to the first reaction mixture at a rate of 0.5 mL / min and react for 10 min to obtain the second reaction mixture; wherein, the grafted monomer is 4% of the mass of acrylamide in step (1); (3) Mix acrylamide, 20% cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% crosslinking agent (ethylene glycol diacrylate) and 6.5% deionized water to form a mixed solution. Then adjust the pH of the mixed solution to 3.5 with sulfuric acid and pour it into the second reaction mixture in step (2) and mix well. Add 0.1% azo initiator (azobisisobutyl cyanide hydrochloride) and initiate the polymerization reaction at 85°C. After reacting to 90°C and holding for 80 min, add 0.3% terminator (hydroquinone) after the solution viscosity reaches 1000~3000 mPa·s to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of azo initiator, cationic monomer, deionized water and crosslinking agent are based on the mass of acrylamide in step (3). The amount of acrylamide added in step (3) is 15% of the mass of acrylamide in step (1).

[0052] Example 5: Example 5 is basically the same as Example 1, except that in step (1), the chain transfer agent is 0.7% based on the mass of acrylamide.

[0053] Example 6: Example 6 is basically the same as Example 1, except that in step (1), the active monomer (N-hydroxymethylacrylamide) is 3% based on the mass of acrylamide.

[0054] Example 7: Example 7 is basically the same as Example 1, except that in step (2), the grafted monomer (hydroxyethyl methacrylate) is 12% based on the mass of acrylamide in step (1).

[0055] Example 8: Example 8 is basically the same as Example 1, except that in step (2), the dripping rate of the grafted monomer is 0.8 mL / min.

[0056] Example 9: Example 9 is basically the same as Example 1, except that in step (3), the cationic monomer (methacryloyloxyethyltrimethylammonium chloride) is 35% based on the mass of acrylamide in step (3).

[0057] Comparative Example 1 (1) Acrylamide and deionized water were mixed to form a 10% mass concentration mixed solution. Then, based on the mass of acrylamide, 1% of active monomer (N-hydroxymethylacrylamide), 7% of anionic monomer (fumaric acid), 0.2% of chelating agent (disodium ethylenediaminetetraacetate), and 0.6% of chain transfer agent (isopropanol) were added and stirred until homogeneous. The pH of the mixture was then adjusted to 3.0 with sulfuric acid. The mixture was then transferred to a glass reactor, a digital thermometer was inserted, and high-purity nitrogen was introduced into it for 30 min. The temperature was then raised to 65°C using a water bath. 0.15% of the mass of acrylamide initiator (ammonium persulfate-sodium bisulfite in a mass ratio of 1:1) was added to the mixture to initiate the polymerization reaction. After the mixture reacted to 90°C, it was kept at that temperature for 20 min to obtain the first reaction mixture. (2) Acrylamide, 20% cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% crosslinking agent (ethylene glycol diacrylate) and 6.5% deionized water are stirred and mixed to form a mixed solution. The pH of the mixed solution is then adjusted to 3.2 with sulfuric acid. The solution is then poured into the first reaction mixture in step (1) and mixed. 0.1% azo initiator (azobisisobutyl cyanide hydrochloride) is added to initiate the polymerization reaction at 85°C. The reaction is carried out at 90°C for 80 minutes. After the solution viscosity reaches 1000~3000 mPa·s, 0.3% terminator (hydroquinone) is added to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of azo initiator, cationic monomer, deionized water and crosslinking agent are based on the mass of acrylamide in step (2). The amount of acrylamide added in step (2) is 15% of the mass of acrylamide in step (1).

[0058] Comparative Example 2 Comparative Example 2 is basically the same as Example 1, except that the raw materials in steps (1), (2), and (3) are mixed and reacted in one step. Specifically, the first acrylamide and deionized water are mixed to form a mixed solution with a mass concentration of 10%. Then, based on the mass of the first acrylamide, 4% of graft monomer (hydroxyethyl methacrylate), 1% of active monomer (N-hydroxymethylacrylamide), 7% of anionic monomer (fumaric acid), 0.2% of chelating agent (disodium ethylenediaminetetraacetate), 0.6% of chain transfer agent (isopropanol), second acrylamide, 20% of cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% of crosslinking agent (ethylene glycol diacrylate), and 6.5% of deionized water are stirred and mixed. Then, the pH of the mixture is adjusted to 3.5 with sulfuric acid, and then it is transferred to... A digital thermometer was inserted into a glass reactor, and high-purity nitrogen gas was introduced for 30 minutes. Then, the temperature was raised to 65°C using a water bath. 0.15% of an oxidation-reduction initiator (ammonium persulfate-sodium bisulfite by mass ratio of 1:1) was added to the mixture to initiate the polymerization reaction. After the mixture reacted to 90°C, it was kept at that temperature for 20 minutes. Then, 0.1% of an azo initiator (azobisisobutyronitrile hydrochloride) was added, and the reaction was kept at that temperature for 60 minutes. When the solution viscosity reached 1000~3000 mPa·s, 0.3% of a terminator (hydroquinone) was added to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of the azo initiator, cationic monomer, and crosslinking agent were all based on the mass of the second acrylamide, and the amount of the second acrylamide added was 15% of the mass of the first acrylamide.

[0059] Comparative Example 3 Comparative Example 3 is basically the same as Example 1, except that the grafting monomer in step (2) is added in step (3), that is: (3) acrylamide, 20% cationic monomer (methacryloyloxyethyltrimethylammonium chloride), 0.03% crosslinking agent (ethylene glycol diacrylate) and 6.5% deionized water are stirred and mixed to form a mixed solution, and then the pH of the mixed solution is adjusted to 3.2 with sulfuric acid. It is then poured into the first reaction mixture in step (1) with the grafting monomer (hydroxyethyl methacrylate) and mixed. 0.1% azo dye is added. The initiator (azobisisobutyl cyanide hydrochloride) initiates the polymerization reaction at 85°C. The reaction is held at 90°C for 80 minutes. After the solution viscosity reaches 1000~3000 mPa·s, 0.3% of the terminator (hydroquinone) is added to obtain an aqueous solution of amphoteric polyacrylamide dry strength agent. The contents of azo initiator, cationic monomer and crosslinking agent are based on the mass of acrylamide in step (3). The amount of acrylamide added in step (3) is 15% of the mass of acrylamide in step (1), and the grafted monomer is 4% of the mass of acrylamide in step (1).

[0060] Comparative Example 4 Comparative Example 4 is basically the same as Example 1, except that in step (3), no cationic monomer is added.

[0061] The performance of the amphoteric polyacrylamide dry strength agent aqueous solutions provided in the examples and comparative examples was tested, and the test results are shown in Table 1 below: The testing method is as follows: (a) Determination of apparent viscosity of aqueous solution: Turn on and preheat the Brookfield viscometer, calibrate it, set the test rotor type to LV-04 and the speed to 30 rpm, and install the test rotor correctly. Weigh approximately 200 g of the amphoteric polyacrylamide dry strength agent aqueous solution sample into a 250 mL beaker, place it on the lifting platform, adjust the height so that the liquid level covers the rotor groove, and measure the reading at a temperature of 23~25℃. This reading is the apparent viscosity of the aqueous solution.

[0062] (b) Paper bursting strength test Using an aqueous sample of amphoteric polyacrylamide dry strength agent as a papermaking dry strength agent, the aqueous sample of amphoteric polyacrylamide dry strength agent was prepared into an aqueous solution with a mass fraction of 0.15% by water. During the papermaking process, the diluted solution of the aqueous solution was added to the mixed pulp for papermaking. After the paper was formed, the tensile strength of the paper was measured.

[0063] Place the sample in the center of the lower clamp and press the start button on the touchscreen to begin the measurement. The instrument's real-time display shows all parameters. When the sample is punctured, the instrument emits a long beep, the diaphragm automatically resets, the upper clamp automatically snaps up, and the test result is displayed in the window. Four dots are marked alternately on both sides of the paper; this completes the measurement for one sample. Read the bursting strength data displayed on the interface; this is the bursting strength of the sample, in N·m / g. (C) Determination of paper tensile strength Amphoteric polyacrylamide dry strength agent aqueous solution (effective content 13~17%) was used as the papermaking dry strength agent. During the papermaking process, it was added to the mixed pulp (papermaking was carried out at 30 kg / t (per ton of oven-dry pulp)). After paper formation, the tensile strength of the paper was measured. Accurate samples were cut using a standard paper cutter. The cut samples were placed flat on the test preparation table, and the samples were pushed into the clamps with both hands, pulling them flat to ensure they were level on the same plane. The pneumatic clamps automatically clamped the samples and performed the automatic test. After the test, the clamps automatically reset, released, and the next sample was placed, until the test was completed. The tensile strength data displayed on the interface was read, which is the tensile strength of the sample, in units (N·m / g).

[0064] Table 1 Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing an aqueous solution of amphoteric polyacrylamide dry strength agent, characterized in that, The preparation method includes the following steps: (1) Mix acrylamide, active monomer, anionic monomer, chelating agent, chain transfer agent and water, then add redox initiator to initiate polymerization reaction to obtain the first reaction mixture; (2) The grafted monomer is added dropwise to the first reaction mixture, and a second reaction mixture is obtained after the reaction; wherein the grafted monomer is an unsaturated ester compound containing hydroxyl groups; (3) Mix the mixed solution of acrylamide, cationic monomer, crosslinking agent and water with the second reaction mixture, and initiate the polymerization reaction with an azo initiator to obtain the aqueous solution of the amphoteric polyacrylamide dry strength agent.

2. The preparation method according to claim 1, characterized in that, In step (1), the active monomer is N-hydroxymethylacrylamide; The anionic monomer is at least one of fumaric acid, maleic acid, citric acid or itaconic acid; The chelating agent is at least one of disodium ethylenediaminetetraacetate, triethanolamine, or trisodium aminotriacetate. The chain transfer agent is at least one of isopropanol, methanol, thiol, or sodium formate; and / or The redox initiator is a group consisting of ammonium persulfate-sodium bisulfite, tert-butyl hydroperoxide-pyrosulfite, or potassium persulfate-thiosulfate.

3. The preparation method according to claim 1, characterized in that, In steps (1) and (2), based on the mass of acrylamide, the amounts of each reaction raw material are as follows: active monomer 0.5~2%, anionic monomer 5~10%, chelating agent 0.1~0.3%, chain transfer agent 0.2~0.8%, redox initiator 0.1~0.3%, grafted monomer 2~10%, and water 7~9%.

4. The preparation method according to claim 1, characterized in that, In step (1), the initiation temperature is 63~65℃, and the reaction is kept at 87~93℃ for 20~30 minutes. In step (2), the grafting monomer is hydroxyethyl methacrylate; Preferably, the dropping rate of the grafted monomer is 0.3~0.5 mL / min, and the reaction time is 10~15 min.

5. The preparation method according to claim 1, characterized in that, In step (3), the cationic monomer is at least one of methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, dimethylaminoethyl acrylate, or dimethylaminoethyl methacrylate. The crosslinking agent is at least one of ethylene glycol diacrylate, N,N-dimethylacrylamide, or methylenebisacrylamide; and / or The azo initiator is at least one of azobisisobutyronitrile hydrochloride, azobisisobutyronitrile, azobisisoheptanenitrile, or azobisisobutylamidine hydrochloride.

6. The preparation method according to claim 1, characterized in that, The mass ratio of acrylamide in step (3) to acrylamide in step (1) is 100:(15~20); and / or In step (3), based on the mass of acrylamide, the amounts of each reaction raw material are as follows: 10-30% cationic monomer, 0.01-0.08% crosslinking agent, 0.1-0.3% azo initiator, and 6-6.5% water.

7. The preparation method according to claim 1, characterized in that, In step (3), the initiation temperature is 83~85℃, and the reaction is kept at 90~95℃ for 60~90 minutes.

8. The preparation method according to claim 1, characterized in that, In step (1), before adding the initiator to initiate the polymerization reaction, the steps also include purging the reaction system with nitrogen to remove oxygen and adjusting the pH of the reaction system to 3.0~3.5; In step (3), before adding the initiator to initiate the polymerization reaction, the pH of the reaction system is adjusted to 3.0~3.5; Preferably, in step (3), after the reaction is completed, a step of adding a terminator to the reaction system is further included; preferably, the terminator is at least one of hydroquinone, hydroquinone methyl ether, or tert-butylcatechol; More preferably, in step (3), the terminating agent is 0.3-0.5% based on the mass of acrylamide.

9. The preparation method according to claim 1, characterized in that, The effective content of the aqueous solution of the amphoteric polyacrylamide dry strength agent is 13-17%, and the apparent viscosity is 1000-3000 MPa·s.

10. An aqueous solution of an amphoteric polyacrylamide dry strength agent, characterized in that, It is prepared by any one of claims 1 to 9.

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