Water-in-oil emulsion type nano cross-linked copolymer papermaking retention aid and preparation method thereof

Abstract

The invention relates to the technical field of papermaking, in particular to a water-in-oil emulsion type nano cross-linked copolymer papermaking retention aid and a preparation method thereof. The water-in-oil emulsion type nano cross-linked copolymer papermaking retention aid comprises the following components: a water phase, an oil phase and a phase inversion agent, the mass ratio of the water phase to the oil phase is (7.2-7.5): (2.5-2.8), and the water phase comprises the following components in parts by weight: 10-15 parts of a template polymer, 300-320 parts of acrylamide, 80-120 parts of a cationic monomer, 0.01-0.04 part of a structure regulator, 200-300 parts of water and 0.1-0.5 part of an initiator; the oil phase comprises the following components in parts by weight: 220-260 parts of white oil and 25-35 parts of an emulsifier; the molecular weight of the template polymer is 500,000-1,000,000, and the retention rate of the filler and the strength of finished paper can be remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, and in particular to an oil-in-water emulsion type nano-crosslinked copolymer papermaking retention aid and its preparation method. Background Technology

[0002] With the development of paper machines towards higher speeds, larger sizes, and neutral papermaking processes, the traditional retention and filtration systems used to control the flocculation of fibers and other components in the pulp to improve retention and filtration performance are no longer satisfactory. Meanwhile, the large-scale use of waste paper and deinked pulp in recent years has led to significant problems with filtration due to the abundance of fine fibers in the waste paper pulp, resulting in poor fiber retention, reduced paper strength, lower quality, and deteriorated paper machine performance. All of these factors make retention and filtration technologies in the wet end of papermaking increasingly important.

[0003] To improve the physical properties of paper and enhance papermaking workability, it is necessary to reduce the amount of additives and increase efficiency. However, increased papermaking speed is accompanied by problems such as increased shear force and rising water temperature. Improved closed-loop white water circulation and fiber recycling rates lead to increased inorganic salt content in the white water, which in turn increases conductivity. Traditional retention aids are easily damaged by shear force, causing floc disintegration, and are also susceptible to interference from salt ions in the papermaking system, resulting in charge neutralization failure. Therefore, it is difficult to simultaneously achieve high retention rates and high paper physical strength.

[0004] Therefore, to address the above shortcomings, a novel retention aid is needed. Summary of the Invention

[0005] The technical problem to be solved by this invention is that existing retention aids have low retention rates and it is difficult to achieve both high retention rates and physical strength. In view of the defects in the prior art, this invention provides an oil-in-water emulsion type nano-crosslinked copolymer paper retention aid and its preparation method.

[0006] To address the aforementioned technical problems, this invention provides a water-in-oil emulsion type nano-crosslinked copolymer paper retention aid. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid comprises the following components: an aqueous phase, an oil phase, and a phase-transfer agent. The mass ratio of the aqueous phase to the oil phase is (7.2~7.5):(2.5~2.8). The aqueous phase comprises the following components in parts by weight: 10~15 parts of template polymer (e.g., 10, 11, 12, 13, 14, or 15 parts, etc.), 300~320 parts of acrylamide (e.g., 300, 305, 310, 315, or 320 parts, etc.), 80~120 parts of cationic monomer (e.g., 80, 90, 100, 110, or 120 parts, etc.), and 0.01~0.04 parts of structure modifier (e.g., 0.01, 0.02, 0.03 parts, etc.). The oil phase comprises the following components in parts by weight: white oil 22 parts (e.g., 0.04 parts, etc.), water 200-300 parts (e.g., 200 parts, 210 parts, 220 parts, 230 parts, 240 parts, 250 parts, 260 parts, 270 parts, 280 parts, 290 parts, or 300 parts, etc.), and initiator 0.1-0.5 parts (e.g., 0.1 parts, 0.2 parts, 0.3 parts, 0.4 parts, or 0.5 parts, etc.); the oil phase comprises the following components in parts by weight: white oil 22 The template polymer has a molecular weight of 0 to 260 parts (e.g., 220, 230, 240, 250, or 260 parts, etc.), 25 to 35 parts (e.g., 25, 26, 28, 30, 33, or 35 parts, etc.), and 22 to 28 parts (e.g., 22, 23, 24, 25, 26, 27, or 28 parts, etc.).

[0007] To address the aforementioned problems, this invention synthesizes a water-in-oil emulsion type nano-crosslinked copolymer paper retention aid. Through the synergistic effect of the "nano-SiO2-organic crosslinked network", it achieves a balance between high retention rate and paper strength. It has high stability, easy dilution, and rapid compatibility with the wet end system of papermaking, significantly improving filler retention rate and paper strength.

[0008] Preferably, the template polymer is prepared by the following method, comprising: (1) Disperse nano-SiO2 in a mixed solution of ethanol and water, add silane coupling agent KH570 to react, and dry to obtain modified nano-SiO2; (2) Modified nano-SiO2, cationic monomer and water are reacted in the presence of an azo thermal decomposition initiator to obtain the template polymer.

[0009] Preferably, the volume ratio of ethanol to water in step (1) is 3:1.

[0010] Preferably, the mass of the silane coupling agent KH570 in step (1) is 5% to 10% of the mass of nano-SiO2, for example, it can be 5%, 6%, 7%, 8%, 9% or 10%, etc.

[0011] Preferably, the reaction temperature in step (1) is 50~70℃ and the reaction time is 3~5h.

[0012] Preferably, the mass parts of the modified nano-SiO2, cationic monomer, and water are: 5-8 parts of modified nano-SiO2 (e.g., 5, 6, 7, or 8 parts), 12-17 parts of cationic monomer (e.g., 12, 13, 14, 15, 16, or 17 parts), and 78-82 parts of water (e.g., 78, 79, 80, 81, or 82 parts); and the mass parts of the azo thermal decomposition initiator are 0.005-0.01 parts (e.g., 0.005, 0.006, 0.007, 0.008, 0.009, or 0.01 parts).

[0013] Preferably, the cationic monomer in step (2) includes any one or a combination of at least two of methacryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide, preferably methacryloyloxyethyltrimethylammonium chloride.

[0014] Preferably, the azo thermal decomposition initiator in step (2) is azobisisobutyramidine hydrochloride.

[0015] Preferably, the reaction temperature in step (2) is 60~80℃ and the reaction time is 6~8h.

[0016] Preferably, the cationic monomer comprises any one or a combination of at least two of methacryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide, and is preferably methacryloyloxyethyltrimethylammonium chloride.

[0017] Preferably, the structure modifier is a mixture of a chain transfer agent and a crosslinking agent, wherein the mass ratio of the chain transfer agent to the crosslinking agent is 1:(0.1~0.3), for example, it can be 1:0.1, 1:0.2, 1:0.3, etc. Preferably, the chain transfer agent comprises sodium hypophosphite and / or sodium formate; sodium hypophosphite is preferred.

[0018] Preferably, the crosslinking agent is any one of N,N'-methylenebisacrylamide, polyethylene glycol 600 diacrylate, polyethylene glycol 600 dimethacrylate, N-vinyl-N-methyl-2-acrylamide, and N-allyl methacrylamide, and is more preferably N,N'-methylenebisacrylamide.

[0019] Preferably, the initiator includes redox initiators and azo initiators; preferably, the redox initiator includes a combination of potassium persulfate and sodium bisulfite, a combination of ammonium persulfate and sodium bisulfite, a combination of hydrogen peroxide and sodium bisulfite, and most preferably a combination of ammonium sulfate and sodium bisulfite.

[0020] Preferably, the azo initiator includes any one or a combination of at least two of azobisisobutyramidine hydrochloride, azobisisobutyramidine imidazoline hydrochloride, azobisisobutyronitrile, and azobisisobutyronitrile, with azobisisobutyramidine hydrochloride being the most preferred.

[0021] Preferably, the emulsifier includes Span and Tween; the mass ratio of Span to Tween is 1:(0.2~0.6), for example, it can be 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, etc., preferably 1:0.35; Preferably, the span includes any one or a combination of at least two of span 20, span 80, or span 85.

[0022] Preferably, the Tween includes any one or a combination of at least two of Tween 20, Tween 60, or Tween 80.

[0023] Preferably, the Span is Span 80 and the Tween is Tween 80.

[0024] Preferably, the phase inversion agent is any one or a combination of at least two of isotrimethylene ether, nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, dodecylphenol ether, or dinonylphenol ether, and is preferably isotrimethylene ether.

[0025] On the other hand, the present invention provides a method for preparing the water-in-oil emulsion type nano-crosslinked copolymer papermaking retention aid as described above, the preparation method comprising: (1) Mix white oil and emulsifier to obtain an oil phase; (2) Mix water, template polymer, cationic monomer, acrylamide and structure modifier, adjust the pH to 4.5~5, and add initiator to obtain aqueous phase; (3) Mix the oil phase and water phase until the viscosity is 1000~2000cps and cool to 16~18℃; (4) Then add a reducing agent, heat to 40~45℃ to react, cool and add a phase inversion agent to obtain the oil-in-water emulsion type nano-crosslinked copolymer paper retention aid.

[0026] Implementing this invention has the following beneficial effects: By introducing crosslinking agents into the polymer backbone to construct a three-dimensional network structure, its unique three-dimensional network structure exhibits superior shear resistance, electrical conductivity, and fine component capture capabilities compared to traditional linear polymers, while maintaining stable performance under complex wet-end chemical conditions. Structure modifiers can adjust intermolecular crosslinking to obtain a three-dimensional network structure, resulting in flocculants with a network structure. This allows the flocculant molecular chains to maintain their original shape after high-speed shearing or recover their original shape when the shear force decreases, thus maintaining good flocculation and aggregation performance even after high shear. Furthermore, its unique network molecular structure enables it to capture suspended fine particles more effectively than other types, resulting in lower suspended solids content in the supernatant.

[0027] The synergistic effect of the "nano-SiO2-organic cross-linked network" achieves a balance between high retention rate and paper strength. After the modified nano-SiO2 is covalently embedded in the polymer backbone, the exposed -SiOH groups on its surface form hydrogen bonds with the -OH groups on the surface of fillers (such as CaCO3 and kaolin), while simultaneously forming localized high charge density regions in the fiber interstices. This "inorganic-organic hybrid patch" can selectively adsorb fine components, improving filler retention rate by 15%-30%.

[0028] Network structure enhancement: Nano-SiO2 acts as a crosslinking point, and its rigid structure inhibits the coiling of polymer chains. It can still maintain a three-dimensional network under shear force, thereby improving the shear strength of fiber-filler flocs in pulp suspension. Detailed Implementation

[0029] 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 only some, not all, of the embodiments of the present invention. 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.

[0030] Example 1 This embodiment provides a water-in-oil emulsion type nano-crosslinked copolymer papermaking retention aid and its preparation method. Preparation of template polymers: Preparation of template polymer: 6 parts of modified nano-SiO2, 15 parts of acryloyloxyethyltrimethylammonium chloride and 79 parts of deionized water were mixed evenly and poured into a polymerization reactor equipped with a stirrer and a thermometer. The temperature was raised to 50°C, and nitrogen gas was blown for 60 min. Then 0.008 parts of azobisisobutyramidine hydrochloride were added and reacted at a constant temperature of 70°C for 7 h. The resulting free-flowing viscous liquid was the template polymer.

[0031] The preparation steps for surface modification of nano-SiO2 are as follows: In an apparatus equipped with an electric stirrer, thermometer, and nitrogen protection device, 600 ml of ethanol and 200 ml of deionized water were added sequentially and stirred for 5 min. Then, 10 g of nano-SiO2 was added, and the mixture was sonicated for 30 min (40 kHz). Next, 0.8 g of KH570 was added, and the pH was adjusted to 4 (acetic acid). After purging with nitrogen for 20 min to remove oxygen, the mixture was heated to 60 °C and reacted for 4 h. After centrifugation and washing, the mixture was vacuum dried at 60 °C to obtain modified nano-SiO2.

[0032] The preparation method of paper retention aid includes the following steps: (1) Add 235g of white oil, 20g of Span 80 and 9g of Tween 80 to a beaker and stir until well mixed to obtain the oil phase; (2) In another beaker, add 300g of deionized water, 12g of template polymer, 320g of acrylamide, 120g of acryloyloxyethyltrimethylammonium chloride, 0.01g of sodium hypophosphite, and 0.003g of N,N'-methylenebisacrylamide (MBA) in sequence. Adjust the pH of the mixture to 4 with hydrochloric acid. After stirring and mixing evenly, add 0.04g of ammonium persulfate and 0.06g of azobisisobutyramidine hydrochloride to form the aqueous phase. (3) Add the oil phase and water phase to the blender, emulsify for 8 minutes and test the emulsion viscosity. If the viscosity is greater than 1000 cps, pour it into the polymerization reactor and blow nitrogen for 60 minutes, then cool to 17°C. (4) Use a micro-injection pump to inject a 1% sodium bisulfite aqueous solution, control the reaction temperature, raise the temperature by 0.1℃ in about 10-20s, and raise the temperature to 42℃ in 2h; (5) After the reaction continues at a constant temperature for 1 hour, an initiator is added until the temperature no longer changes. After the reaction is complete, the mixture is cooled to 25°C and 25g of isomeric tridecyl alcohol polyoxyethylene ether is added to obtain the water-in-oil emulsion type nano-crosslinked copolymer paper retention aid.

[0033] Example 2 This embodiment provides a water-in-oil emulsion type nano-crosslinked copolymer papermaking retention aid and its preparation method. Preparation of template polymers: Preparation of template polymer: 6 parts of modified nano-SiO2, 15 parts of acryloyloxyethyltrimethylammonium chloride and 79 parts of deionized water were mixed evenly and poured into a polymerization reactor equipped with a stirrer and a thermometer. The temperature was raised to 50°C, and nitrogen gas was blown for 60 min. Then 0.008 parts of azobisisobutyramidine hydrochloride were added and reacted at 80°C for 6 h. The resulting fluid and viscous liquid was the template polymer.

[0034] The preparation steps for surface modification of nano-SiO2 are as follows: In an apparatus equipped with an electric stirrer, thermometer, and nitrogen protection device, 600 ml of ethanol and 200 ml of deionized water were added sequentially and stirred for 5 min. Then, 10 g of nano-SiO2 was added, and the mixture was sonicated for 30 min (40 kHz). Next, 0.8 g of KH-570 was added, and the pH was adjusted to 5 (acetic acid). After purging with nitrogen for 20 min to remove oxygen, the mixture was heated to 60 °C and reacted for 4 h. After centrifugation and washing, the mixture was vacuum dried at 60 °C to obtain modified nano-SiO2.

[0035] The preparation method of paper retention aid includes the following steps: (1) Add 230g white oil, 20g Span 80 and 7g Tween 80 to a beaker and stir until well mixed to obtain the oil phase; (2) In another beaker, add 300g deionized water, 12g template polymer, 320g acrylamide, 100g methacryloyloxyethyltrimethylammonium chloride, 0.01g sodium hypophosphite, and 0.003g N,N'-methylenebisacrylamide (MBA) in sequence. Adjust the pH of the mixture to 4 with hydrochloric acid. After stirring and mixing evenly, add 0.04g ammonium persulfate and 0.06g azobisisobutyramidine hydrochloride to form the aqueous phase. (3) Add the oil phase and water phase to the blender, emulsify for 5 minutes and test the emulsion viscosity. If the viscosity is greater than 1000 cps, pour it into the polymerization reactor and blow nitrogen for 60 minutes, then cool to 18℃. (4) Use a micro-injection pump to inject a 1% sodium bisulfite aqueous solution, control the reaction temperature, raise the temperature by 0.1℃ in about 10s, and raise the temperature to 42℃ in 2h; (5) After the reaction continues at a constant temperature for 1 hour, an initiator is added until the temperature no longer changes. After the reaction is complete, the mixture is cooled to 25°C and 25g of isomeric tridecyl alcohol polyoxyethylene ether is added to obtain the water-in-oil emulsion type nano-crosslinked copolymer paper retention aid.

[0036] Example 3 This embodiment provides a water-in-oil emulsion type nano-crosslinked copolymer papermaking retention aid and its preparation method. Preparation of template polymers: Preparation of template polymer: 8 parts of modified nano-SiO2, 12 parts of acryloyloxyethyltrimethylammonium chloride and 80 parts of deionized water were mixed evenly and poured into a polymerization reactor equipped with a stirrer and a thermometer. The temperature was raised to 50°C, and nitrogen gas was blown for 60 min. Then 0.008 parts of azobisisobutyramidine hydrochloride were added and reacted at a constant temperature of 70°C for 7 h to obtain a free-flowing viscous liquid, which is the template polymer.

[0037] The preparation steps for surface modification of nano-SiO2 are as follows: In an apparatus equipped with an electric stirrer, thermometer, and nitrogen protection device, 600 ml of ethanol and 200 ml of deionized water were added sequentially and stirred for 5 min. Then, 10 g of nano-SiO2 was added, and the mixture was sonicated for 30 min (40 kHz). Next, 0.8 g of KH-570 was added, and the pH was adjusted to 4-5 (acetic acid). After purging with nitrogen for 20 min to remove oxygen, the mixture was heated to 60 °C and reacted for 4 h. After centrifugation and washing, the mixture was vacuum dried at 60 °C to obtain modified nano-SiO2.

[0038] The preparation method of paper retention aid includes the following steps: (1) Add 240g white oil, 20g Span 80 and 9g Tween 80 to a beaker and stir until well mixed to obtain the oil phase; (2) In another beaker, add 290g deionized water, 10g template polymer, 310g acrylamide, 110g acryloyloxyethyltrimethylammonium chloride, 0.01g sodium hypophosphite, and 0.003g N,N'-methylenebisacrylamide (MBA) in sequence. Adjust the pH of the mixture to 4.0-5.0 with hydrochloric acid. After stirring and mixing evenly, add 0.04g ammonium persulfate and 0.06g azobisisobutyramidine hydrochloride to form the aqueous phase. (3) Add the oil phase and water phase to the blender, emulsify for 5-10 minutes and test the emulsion viscosity. If the viscosity is greater than 1000 cps, pour it into the polymerization reactor and blow nitrogen for 60 minutes, then cool to 18℃. (4) Use a micro-injection pump to inject a 1% sodium bisulfite aqueous solution, control the reaction temperature, raise the temperature by 0.1℃ in about 20s, and raise the temperature to 42℃ in 2h; (5) After the reaction continues at a constant temperature for 1 hour, an initiator is added until the temperature no longer changes. After the reaction is complete, the mixture is cooled to 25°C and 25g of isomeric tridecyl alcohol polyoxyethylene ether is added to obtain the fracturing thickener.

[0039] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the paper retention aid emulsion in Comparative Example 1 did not contain a template polymer, meaning that no template polymer was added during the preparation of this invention. Everything else was the same as in Example 1. Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that no structure modifier was added to the paper retention aid emulsion in Comparative Example 1, meaning no structure modifier was added during the preparation of this invention. Everything else is the same as in Example 1. Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that the template polymer of the paper retention aid emulsion in Comparative Example 1 is replaced with template polymer 2, and the rest is the same as in Example 1.

[0040] Preparation of template polymer 2: 15 parts of acryloyloxyethyltrimethylammonium chloride and 85 parts of deionized water were mixed evenly and poured into a polymerization reactor equipped with a stirrer and a thermometer. The temperature was raised to 50°C, and nitrogen gas was blown for 60 min. Then 0.008 parts of azobisisobutyramidine hydrochloride were added, and the reaction was carried out at 70°C for 7 h. The resulting fluid viscous liquid was the template polymer.

[0041] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that only sodium hypophosphite was added to the paper retention aid emulsion structure modifier in Comparative Example 1, i.e. N,N'-methylenebisacrylamide (MBA) was removed, while the rest were the same as in Example 1.

[0042] The products obtained above will be subjected to performance testing: 1. Determination of molecular weight Determination method: First, wash the paper retention aid emulsion several times with a 1:1 volume ratio ethanol / water mixture, then vacuum dry at 40℃ for 24 hours to obtain a white solid polyacrylamide (PAM). Grind it into powder, and determine the intrinsic viscosity using an Ubbelohde viscometer according to GB / T31246—20145.2 Determination of relative molecular weight. Calculate the molecular weight according to the formula.

[0043] 2. Determination of retention rate The total retention rate and ash retention rate were tested using a dynamic water filter.

[0044] 3. Water filtration performance Test the water filtration rate in 30 seconds using a dynamic water filter.

[0045] 4. Determination of retention rate and filtration performance after shearing The paper retention aid emulsions prepared in the same mass example and the paper retention aid emulsions prepared in the comparative example were dissolved in paper recycling water (conductivity 3000-4000 μS / CM) at a certain concentration (stirred at 600 r / min for 5 min). After dissolution, the emulsions were sheared for 30 min with a high-speed stirrer at 6000 r / min. The total retention rate, ash retention rate and filtration performance were tested using a dynamic water filter.

[0046] The test results are shown in Tables 1 and 2.

[0047] Table 1 Table 2 shows the measurement results obtained after high-speed shearing. As can be seen from the data in Tables 1 and 2, the water-in-oil emulsion type nano-crosslinked copolymer prepared by the present invention has a significant retention and filtration aid effect, and still has a high retention and filtration aid effect after high-speed shearing in high-conductivity circulating water.

[0048] 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 water-in-oil emulsion type nano-crosslinked copolymer paper retention aid, characterized in that: The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid comprises the following components: an aqueous phase, an oil phase, and a phase-transfer agent, wherein the mass ratio of the aqueous phase to the oil phase is (7.2~7.5):(2.5~2.8). The aqueous phase comprises the following components in parts by weight: 10~15 parts template polymer, 300~320 parts acrylamide, 80~120 parts cationic monomer, 0.01~0.04 parts structure modifier, 200~300 parts water, and 0.1~0.5 parts initiator. The oil phase comprises the following components in parts by weight: 220~260 parts white oil and 25~35 parts emulsifier. The phase inversion agent is 22-28 parts, and the molecular weight of the template polymer is 500,000 to 1,000,000.

2. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 1, characterized in that: The template polymer is prepared by the following methods, including: (1) Disperse nano-SiO2 in a mixed solution of ethanol and water, add silane coupling agent KH570 to react, and dry to obtain modified nano-SiO2; (2) Modified nano-SiO2, cationic monomer and water are reacted in the presence of an azo thermal decomposition initiator to obtain the template polymer.

3. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 2, characterized in that: The volume ratio of ethanol to water in step (1) is 3:1; Preferably, the mass of the silane coupling agent KH570 in step (1) is 5% to 10% of the mass of nano-SiO2; Preferably, the reaction temperature in step (1) is 50~70℃ and the reaction time is 3~5h.

4. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 2, characterized in that: The mass fractions of the modified nano-SiO2, cationic monomer, and water are: 5-8 parts of modified nano-SiO2, 12-17 parts of cationic monomer, and 78-82 parts of water; the mass fractions of the azo thermal decomposition initiator are 0.005-0.01 parts. Preferably, the cationic monomer in step (2) includes any one or a combination of at least two of methacryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide, preferably methacryloyloxyethyltrimethylammonium chloride; Preferably, the azo thermal decomposition initiator in step (2) is azobisisobutyramidine hydrochloride; Preferably, the reaction temperature in step (2) is 60~80℃ and the reaction time is 6~8h.

5. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 1, characterized in that: The cationic monomer includes any one or a combination of at least two of methacryloyloxyethyltrimethylammonium chloride, acryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, diallyldimethylammonium chloride, and dimethylaminopropylacrylamide, preferably acryloyloxyethyltrimethylammonium chloride.

6. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 1, characterized in that: The structure modifier is a mixture of a chain transfer agent and a crosslinking agent, wherein the mass ratio of the chain transfer agent to the crosslinking agent is 1:(0.1~0.3); Preferably, the chain transfer agent comprises sodium hypophosphite and / or sodium formate; sodium hypophosphite is preferred. Preferably, the crosslinking agent is any one of N,N'-methylenebisacrylamide, polyethylene glycol 600 diacrylate, polyethylene glycol 600 dimethacrylate, N-vinyl-N-methyl-2-acrylamide, and N-allyl methacrylamide, and is more preferably N,N'-methylenebisacrylamide.

7. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 1, characterized in that: The initiator includes redox initiators and azo initiators; preferably, the redox initiator includes a combination of potassium persulfate and sodium bisulfite, a combination of ammonium persulfate and sodium bisulfite, a combination of hydrogen peroxide and sodium bisulfite, and more preferably a combination of ammonium sulfate and sodium bisulfite. Preferably, the azo initiator includes any one or a combination of at least two of azobisisobutyramidine hydrochloride, azobisisobutyramidine imidazoline hydrochloride, azobisisobutyronitrile, and azobisisobutyronitrile, with azobisisobutyramidine hydrochloride being the most preferred.

8. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 1, characterized in that: The emulsifier includes Span and Tween; the mass ratio of Span to Tween is 1:(0.2~0.6), preferably 1:0.35; Preferably, the spitting includes any one or a combination of at least two of spitting 20, spitting 80, or spitting 85; Preferably, the Tween includes any one or a combination of at least two of Tween 20, Tween 60, or Tween 80; Preferably, the Span is Span 80 and the Tween is Tween 80.

9. The water-in-oil emulsion type nano-crosslinked copolymer paper retention aid according to claim 1, characterized in that: The phase inversion agent is any one or a combination of at least two of isotrimethylene ether, nonylphenol polyoxyethylene ether, octylphenol polyoxyethylene ether, dodecylphenol ether, or dinonylphenol ether, preferably isotrimethylene ether.

10. The method for preparing the water-in-oil emulsion type nano-crosslinked copolymer papermaking retention aid according to any one of claims 1 to 9, characterized in that: The preparation method includes: (1) Mix white oil and emulsifier to obtain an oil phase; (2) Mix water, template polymer, cationic monomer, acrylamide and structure modifier, adjust the pH to 4.5~5, and add initiator to obtain aqueous phase; (3) Mix the oil phase and water phase until the viscosity is 1000~2000cps and cool to 16~18℃; (4) Then add a reducing agent, heat to 40~45℃ to react, cool and add a phase inversion agent to obtain the oil-in-water emulsion type nano-crosslinked copolymer paper retention aid.

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