A high cationic degree branched structure dry strength agent based on bio-based monomers and a preparation method thereof

Abstract

The application discloses a high cationic degree branched structure dry strength agent based on a bio-based monomer and a preparation method thereof. A starch skeleton is cationized and grafted by using CHPTAC to obtain a high cationic degree. In addition, a crosslinking agent is added and a crosslinking kinetics model is established to control the branching degree to be 0.3-0.7. A composite initiation system (redox+auxiliary initiator) realizes low-temperature polymerization at <=60 DEG C. The dry strength agent molecule prepared in the application has a high cationic density, can be quickly adsorbed to the surface of the negative pulp fiber through electrostatic action, and at the same time, the branched or micro-crosslinked structure on the molecular chain provides multiple adsorption points to form a stronger 'fiber-polymer-fiber' bridge, thereby significantly improving the binding strength between fibers. The moderate branched structure has a more relaxed conformation than the traditional linear polymer and covers more fiber surface areas.

Description

Technical Field

[0001] This invention relates to the field of papermaking technology, specifically to a highly cationic branched dry strength agent based on bio-based monomers and its preparation method. Background Technology

[0002] Dry strength agents are key chemicals used in the papermaking industry to enhance the physical strength of paper. Traditional dry strength agents are mainly polyacrylamide copolymers, whose raw materials, such as acrylamide, are derived from petrochemical routes. These agents require high synthesis temperatures (80-95℃), large amounts of initiators, high energy consumption, and high carbon emissions. Although there have been research reports on bio-based dry strength agents in recent years, they still have the following drawbacks:

[0003] Low cationicity and insufficient fiber binding capacity: Existing bio-based dry strength agents (such as starch graft copolymers) have low cationicity and low adsorption efficiency for anionic pulp fibers, resulting in limited reinforcing effect.

[0004] The molecular structure is simple and mostly linear: the lack of reasonable design of branched or cross-linked structures leads to uncontrollable molecular weight distribution, which affects the bridging efficiency of dry strength agents between fibers.

[0005] Polymerization temperature is still too high: most bio-based polymerization still requires 60~80℃, failing to fully utilize the advantages of low temperature and energy saving.

[0006] Crosslinking reaction is difficult to control: introducing crosslinking agents can easily lead to gelation or excessive crosslinking, resulting in poor product stability.

[0007] Organic solvent or excessive initiator residue: Some processes use organic solvents or excessive initiators, which does not meet the requirements of green environmental protection. Summary of the Invention

[0008] This invention provides a method for preparing a high-cationic branched dry strength agent based on bio-based monomers. Starting from the molecular structure source, a bio-based polymer with high cationicity and moderate branching structure is designed, and a low-temperature aqueous phase polymerization process is developed to achieve green preparation.

[0009] The present invention adopts the following technical solution:

[0010] A method for preparing a highly cationic branched dry strength agent based on a bio-based monomer includes the following steps:

[0011] (1) Prepare a cationic starch solution and adjust the pH to neutral;

[0012] (2) Add bio-based acrylamide, crosslinking agent and deionized water to the cationic starch solution and adjust the pH to 4.5~6.5;

[0013] (3) After nitrogen purging to remove oxygen, add the redox initiation system and auxiliary initiator, control the reaction temperature ≤60℃, and polymerize for 2~5 hours;

[0014] (4) After polymerization, heat to 65~70℃ and keep warm for 1 hour to eliminate residual monomers, then cool to room temperature, filter and discharge to obtain the high cationic branched dry strength agent based on bio-based monomers.

[0015] Preferably, in step (1), the preparation of the cationic starch solution includes:

[0016] The modified starch was mixed with deionized water and gelatinized at 85-90°C for 30 minutes. After cooling to 40-50°C, the cationic etherifying agent CHPTAC was added, and the pH was adjusted to 11-12 with NaOH. The mixture was then reacted at 50-60°C for 2-3 hours to obtain the cationic starch solution.

[0017] Preferably, in step (2), the crosslinking agent includes at least one of N,N'-methylenebisacrylamide, epichlorohydrin, and sodium tripolyphosphate.

[0018] Preferably, in step (3), the redox initiation system is a system composed of ammonium persulfate and sodium bisulfite.

[0019] Preferably, in step (3), the auxiliary initiator is tetramethylethylenediamine or ascorbic acid.

[0020] Preferably, in step (3), the reaction temperature is controlled at 45~55℃.

[0021] Preferably, by weight, the bio-based acrylamide is 20-50 parts; the modified starch is 10-40 parts; the cationic etherifying agent CHPTAC is 5-20 parts; the crosslinking agent is 0.05-1.0 parts; the total amount of the composite initiation system is 0.05-0.5 parts, the composite initiation system includes a redox initiator and the auxiliary initiator; and the deionized water is 40-70 parts.

[0022] Preferably, the cationic etherifying agent CHPTAC is 10-15 parts.

[0023] Preferably, the modified starch is gelatinized corn starch or tapioca starch.

[0024] Key process parameter optimization:

[0025] A crosslinking reaction kinetic model was adopted: by adjusting the amount of crosslinking agent (0.05~0.5 wt% monomer) and the reaction temperature (45~55℃), the degree of branching was controlled to avoid gelation. The kinetic model is based on the free radical copolymerization rate constant, and establishes the relationship between the crosslinking point density and the reaction time, so as to achieve controllable molecular weight and distribution.

[0026] The graft copolymerization behavior of the cationic etherifying agent CHPTAC with the starch backbone: the grafting rate increases with the increase of CHPTAC dosage, but the grafting efficiency decreases when the CHPTAC dosage exceeds 25 parts. This invention determines that the optimal CHPTAC dosage is 10-15 parts, with a grafting rate ≥85%.

[0027] The present invention also provides a highly cationic branched dry strength agent based on a bio-based monomer, prepared by the preparation method described above.

[0028] The dry strength agent molecules prepared in this invention have a high cationic density, enabling them to rapidly adsorb onto the negatively charged pulp fiber surface via electrostatic interactions. Simultaneously, the branched or micro-crosslinked structures on the molecular chains provide multiple adsorption sites, forming stronger "fiber-polymer-fiber" bridges and significantly improving the bonding strength between fibers. The moderately branched structure exhibits a more extended conformation than traditional linear polymers, covering a larger area of ​​the fiber surface.

[0029] The dry strength agent of this invention has the following molecular structure characteristics: Main chain: composed of bio-based acrylamide and modified starch (such as gelatinized corn starch or cassava starch); Cationic side chain: grafted onto the starch backbone or acrylamide chain by a cationic etherifying agent (CHPTAC, 3-chloro-2-hydroxypropyltrimethylammonium chloride), imparting high cationicity (charge density ≥1.5 meq / g); Moderately branched structure: by adding crosslinking agents (such as N,N'-methylenebisacrylamide, epichlorohydrin, sodium tripolyphosphate, etc.) during polymerization, a controllable branched or micro-crosslinked network is formed, with the branching index controlled between 0.3 and 0.7 to avoid excessive crosslinking leading to gelation; Controllable molecular weight: the molecular weight is adjustable in the range of 500,000 to 5,000,000 Daltons, with a distribution index (PDI) ≤2.5.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The preparation method of the present invention uses CHPTAC to cationically graft the starch skeleton to obtain high cationicity. In addition, a crosslinking agent is added and a crosslinking kinetic model is established to control the degree of branching at 0.3~0.7. The composite initiation system (redox + auxiliary initiator) achieves low-temperature polymerization at ≤60℃, all-aqueous phase, solvent-free, and direct discharge.

[0032] (2) High cationicity of the dry strength agent in this invention: Through CHPTAC grafting modification, the cationicity is ≥1.5 meq / g, which increases the adsorption rate of pulp fiber by more than 30%, and the strengthening effect is better than that of traditional dry strength agents. Moderate branching structure: The degree of branching is controlled by crosslinking agent and kinetic model to avoid gelation, while improving crosslinking efficiency and increasing tensile index by 15~25%. Low temperature polymerization (≤60℃): Compared with the traditional 80~95℃, the reaction temperature is reduced by more than 30%, resulting in significant energy saving. No organic solvents, zero VOC emissions: All-aqueous phase reaction, direct discharge after post-treatment, environmentally friendly. High bio-based content: Bio-based acrylamide + modified starch, bio-based carbon content ≥50%, in line with the "dual carbon" strategy. Controllable molecular weight: Through kinetic model adjustment of crosslinking agent and initiator, the molecular weight distribution is narrow (PDI≤2.5), and the product quality is stable. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0034] Example 1

[0035] This embodiment describes a method for preparing a dry strength agent (low-temperature aqueous phase polymerization process), which includes the following steps:

[0036] Raw material composition (parts by weight):

[0037] Bio-based acrylamide: 20-50 parts;

[0038] Modified starch (cationic or nonionic): 10-40 parts;

[0039] Cationic etherifying agent CHPTAC: 5-20 parts;

[0040] Crosslinking agent: 0.05~1.0 parts (selected from N,N'-methylenebisacrylamide, epichlorohydrin, sodium tripolyphosphate);

[0041] Composite initiation system: 0.05~0.5 parts in total, including redox initiator (ammonium persulfate / sodium bisulfite) and auxiliary initiator (such as tetramethylethylenediamine or ascorbic acid).

[0042] Deionized water: 40-70 parts;

[0043] pH adjuster: appropriate amount (adjust to 4.5~6.5).

[0044] Step 1: Starch gelatinization and cationization pretreatment

[0045] The modified starch was mixed with deionized water and gelatinized at 85-90℃ for 30 minutes, then cooled to 40-50℃. CHPTAC was added, and the pH was adjusted to 11-12 with NaOH. The mixture was reacted at 50-60℃ for 2-3 hours to obtain a cationic starch solution. The pH was then adjusted to neutral with acid.

[0046] Step 2: Monomer mixing and crosslinking agent addition

[0047] Add bio-based acrylamide, crosslinking agent and remaining deionized water to the above cationic starch solution, stir to dissolve, and adjust the pH to 4.5~6.5.

[0048] Step 3: Low-temperature initiation of polymerization

[0049] After purging with nitrogen to remove oxygen, a redox initiation system (ammonium persulfate / sodium bisulfite) and an auxiliary initiator (ascorbic acid) are added. The reaction temperature is controlled at ≤60℃ (preferably 45~55℃), and the polymerization reaction is carried out for 2~5 hours. During the polymerization process, the temperature is maintained by the exothermic reaction itself, and jacket cooling is used if necessary.

[0050] Step 4: Maturation and Discharging

[0051] After polymerization, the temperature is raised to 65-70℃ and held for 1 hour to eliminate residual monomers. Then, it is cooled to room temperature and filtered out. No organic solvents are added throughout the process. The product is a milky white viscous liquid with a solid content of 15-25%.

[0052] Key process parameter optimization:

[0053] A crosslinking reaction kinetic model was adopted: by adjusting the amount of crosslinking agent (0.05~0.5 wt% monomer) and the reaction temperature (45~55℃), the degree of branching was controlled to avoid gelation. The kinetic model is based on the free radical copolymerization rate constant, and establishes the relationship between the crosslinking point density and the reaction time, so as to achieve controllable molecular weight and distribution.

[0054] The graft copolymerization behavior of the cationic etherifying agent CHPTAC with the starch backbone: the grafting rate increases with the increase of CHPTAC dosage, but the grafting efficiency decreases when the CHPTAC dosage exceeds 25 parts. This invention determines that the optimal CHPTAC dosage is 10-15 parts, with a grafting rate ≥85%.

[0055] The dry strength agent prepared in this embodiment has the following molecular structure characteristics:

[0056] Main chain: composed of bio-based acrylamide and modified starch (such as gelatinized corn starch and tapioca starch);

[0057] Cationic side chains: grafted onto the starch backbone or acrylamide chain via a cationic etherifying agent (CHPTAC, 3-chloro-2-hydroxypropyltrimethylammonium chloride) to impart high cationicity (charge density ≥1.5 meq / g).

[0058] Moderately branched structure: By adding crosslinking agents (such as N,N'-methylenebisacrylamide, epichlorohydrin, sodium tripolyphosphate, etc.), a controllable branched or micro-crosslinked network is formed during the polymerization process. The branching index is controlled between 0.3 and 0.7 to avoid excessive crosslinking leading to gelation. Controllable molecular weight: The molecular weight is adjustable in the range of 500,000 to 5,000,000 Daltons, and the distribution index (PDI) is ≤2.5.

[0059] In this invention, the bio-based acrylamide-modified starch copolymer backbone has cationic side chains grafted with the cationic etherifying agent CHPTAC, and branched or micro-crosslinked structures introduced by the crosslinking agent; the dry strength agent has a cationicity ≥1.5 meq / g and a branching index of 0.3~0.7.

[0060] The composite initiation system is a combination of a redox initiator and an auxiliary initiator. The redox initiator is ammonium persulfate / sodium bisulfite, and the auxiliary initiator is tetramethylethylenediamine or ascorbic acid. The total amount of initiator is 0.05% to 0.5% of the total mass of the monomer.

[0061] The amount of crosslinking agent and the reaction time are controlled by a crosslinking reaction kinetic model to make the molecular weight distribution index ≤2.5. The kinetic model is based on the free radical copolymerization rate equation.

[0062] The above-mentioned dry strength agent achieves the following effects:

[0063] High cationicity: Through CHPTAC grafting modification, the cationicity is ≥1.5 meq / g, which increases the adsorption rate of pulp fibers by more than 30% and has a better reinforcing effect than traditional dry strength agents.

[0064] Moderately branched structure: The degree of branching is controlled by crosslinking agents and kinetic models to avoid gelation, while improving crosslinking efficiency and increasing the tensile index by 15-25%.

[0065] Low-temperature polymerization (≤60℃): Compared with the traditional 80~95℃, the reaction temperature is reduced by more than 30%, resulting in significant energy savings.

[0066] No organic solvents, zero VOC emissions: all-aqueous phase reaction, direct discharge after post-treatment, environmentally friendly.

[0067] High bio-based content: Bio-based acrylamide + modified starch, bio-based carbon content ≥50%, in line with the "dual carbon" strategy.

[0068] Controllable molecular weight: By adjusting the crosslinking agent and initiator through a kinetic model, the molecular weight distribution is narrow (PDI≤2.5), resulting in stable product quality.

Claims

1. A method for preparing a highly cationic branched dry strength agent based on a bio-based monomer, characterized in that, Includes the following steps: (1) Prepare a cationic starch solution and adjust the pH to neutral; (2) Add bio-based acrylamide, crosslinking agent and deionized water to the cationic starch solution and adjust the pH to 4.5~6.5; (3) After nitrogen purging to remove oxygen, add the redox initiation system and auxiliary initiator, control the reaction temperature ≤60℃, and polymerize for 2~5 hours; (4) After polymerization, heat to 65~70℃ and keep warm for 1 hour to eliminate residual monomers, then cool to room temperature, filter and discharge to obtain the high cationic branched dry strength agent based on bio-based monomers.

2. The preparation method according to claim 1, characterized in that, In step (1), the preparation of the cationic starch solution includes: The modified starch was mixed with deionized water and gelatinized at 85-90°C for 30 minutes. After cooling to 40-50°C, the cationic etherifying agent CHPTAC was added, and the pH was adjusted to 11-12 with NaOH. The mixture was then reacted at 50-60°C for 2-3 hours to obtain the cationic starch solution.

3. The preparation method according to claim 1, characterized in that, In step (2), the crosslinking agent includes at least one of N,N'-methylenebisacrylamide, epichlorohydrin, and sodium tripolyphosphate.

4. The preparation method according to claim 1, characterized in that, In step (3), the redox initiation system is a system composed of ammonium persulfate and sodium bisulfite.

5. The preparation method according to claim 1, characterized in that, In step (3), the auxiliary initiator is tetramethylethylenediamine or ascorbic acid.

6. The preparation method according to claim 1, characterized in that, In step (3), the reaction temperature is controlled at 45~55℃.

7. The preparation method according to any one of claims 2 to 6, characterized in that, By weight, the bio-based acrylamide is 20-50 parts; the modified starch is 10-40 parts; the cationic etherifying agent CHPTAC is 5-20 parts; the crosslinking agent is 0.05-1.0 parts; the total amount of the composite initiation system is 0.05-0.5 parts, the composite initiation system includes a redox initiator and the auxiliary initiator; and the deionized water is 40-70 parts.

8. The preparation method according to claim 7, characterized in that, The cationic etherifying agent CHPTAC is 10-15 parts.

9. The preparation method according to claim 7, characterized in that, The modified starch is gelatinized corn starch or tapioca starch.

10. A highly cationic branched dry strength agent based on a bio-based monomer, prepared by the preparation method according to any one of claims 1 to 9.