Porous composite water-retaining agent based on xanthan gum and clay minerals and active extrusion preparation method of porous composite water-retaining agent

Abstract

The invention discloses a porous composite water-retaining agent based on xanthan gum and clay minerals and an active extrusion preparation method of the porous composite water-retaining agent. According to the method, the characteristic that xanthan gum forms a high-viscosity three-dimensional network through molecular chain stretching under the acidic pretreatment condition is utilized, the strong shearing effect of twin-screw extrusion is combined, air is involved in and stably wrapped, and uniform micro-nano bubbles are formed to serve as a foaming template; meanwhile, through physical filling and interface bonding of clay minerals, a stable porous polymer network is constructed in the water-retaining agent. The structure effectively overcomes the salt shielding effect in the salt solution. The specific process comprises the following steps: pretreating xanthan acid, mixing the pretreated xanthan acid with a polymeric monomer, clay minerals, an initiator and the like, and performing twin-screw extrusion to complete blending and microbubble shaping; and performing short-time polymerization on the prepolymer in a steam environment, and then crushing and drying. According to the invention, active extrusion and thermal initiation polymerization are fused, the process is green, the energy consumption is low, and the obtained water-retaining agent has high water absorption rate and excellent salt tolerance, and is suitable for agricultural water saving and saline-alkali soil improvement.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, specifically providing a method for preparing porous composite water-retaining agents based on a synergistic pore-forming strategy. This method utilizes the unique chemical properties of xanthan gum, combined with the efficient shearing and mixing advantages provided by active extrusion technology, to successfully construct a stable porous structure within the water-retaining agent. This effectively suppresses the "shielding" effect caused by salt concentration, significantly improving the material's water absorption and retention performance in saline environments. Background Technology

[0002] Water-retaining agents are highly absorbent polymers containing numerous hydrophilic groups such as hydroxyl, carboxyl, and amide groups, capable of rapidly absorbing and retaining hundreds to thousands of times their own weight in water. This property is of great significance in arid and water-scarce regions, significantly improving water resource utilization efficiency. However, the performance of traditional water-retaining agents deteriorates significantly in high-salt environments, with water absorption capacity decreasing by more than 80% due to the "salt shielding effect." Therefore, developing water-retaining agents that combine high water absorption capacity with excellent salt tolerance has become an urgent need to address the dual challenges of water scarcity and soil salinization.

[0003] The water absorption and retention capacity of water-retaining agents originates from the abundant hydrophilic groups (such as carboxyl, sulfonic acid, hydroxyl, and amino groups) in their three-dimensional cross-linked network. After ionization, these groups not only strongly bind water molecules through hydrogen bonds but also act as polyelectrolytes, creating osmotic pressure inside and outside the gel, driving continuous water penetration. Simultaneously, the electrostatic repulsion between mobile cations within the network and fixed anions on the polymer chains promotes network expansion, further enhancing water absorption. However, in high-salt environments, external salt ions neutralize the charge on the polymer network, severely inhibiting the water absorption and swelling process dominated by osmotic pressure and electrostatic repulsion, leading to a sharp decline in water absorption performance and sustained-release function.

[0004] To improve the salt resistance of water-retaining agents, current research mainly proceeds in two directions: First, introducing other functional monomers, such as 2-acrylamido-2-methylpropanesulfonic acid (AMPS) containing sulfonic acid groups (see patent CN106220789A), or developing novel salt-resistant monomers (such as CN202510398981.8, CN116925279B, etc.). However, acrylic acid (AA) and acrylamide (AM) remain the main raw materials for large-scale production due to their mature technology, low cost, and balanced overall performance; while the synthesis routes of novel salt-resistant monomers are complex and costly, and large-scale industrial applications have not yet been realized. Second, utilizing solvents or porogens to construct porous structures (such as patent ZL201110422624.9), providing additional water transport pathways through capillary action, bypassing the ion-driven water absorption mechanism, thereby mitigating the salt ion shielding effect. While this strategy has shown promise in the laboratory, its core challenge lies in the precise control of the porous structure (such as pore size, distribution, and connectivity), which must be highly matched with the polymer gelation process in both time and space. This places extremely stringent demands on the control of reaction kinetic parameters (such as time, temperature, and concentration). The high cost and difficulty of implementing such complex processes in existing industrial continuous production hinders their practical application.

[0005] Therefore, balancing cost and industrial feasibility, the key to overcoming current technological bottlenecks lies in developing a simple, efficient, and easily scalable method to construct a stable porous structure within water-retaining agents, thereby physically enhancing their water absorption rate and salt resistance. Xanthan gum is an extracellular polysaccharide produced by the fermentation of *Xanthomonas auricula-judae*. Its main chain consists of rigid β-1,4-glycosidic bonds, and its side chains contain stable chemical groups, thus exhibiting good thermal and chemical stability, as well as typical acid and alkali resistance. Furthermore, xanthan gum solutions exhibit typical pseudoplastic fluid behavior, showing a rheological behavior of "viscous at low shear and thinning at high shear," providing unique advantages for its application in specific processing techniques. Summary of the Invention

[0006] The purpose of this invention is to utilize the unique physicochemical properties of xanthan gum, through the efficient shearing and blending effects generated during the active extrusion process, to entrain and stably encapsulate air, forming uniform micro-nano bubbles. Simultaneously, by combining the physical filling and interfacial bonding effects of clay minerals, a stable porous polymer network is constructed within the water-retaining agent. This invention features a simple process, is easily scaled up for industrial applications, and produces a water-retaining agent with a rich pore structure, significantly improved water absorption and retention properties, and enhanced salt resistance, demonstrating excellent application potential in areas such as saline-alkali land improvement.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: An active extrusion preparation method for a salt-resistant composite water-retaining agent based on xanthan gum and clay minerals involves pretreating xanthan gum with acid for 4-24 hours, then mixing it with polymeric monomers, clay minerals, crosslinking agents, initiators, and water. The mixture is then extruded 2-6 times using a twin-screw extruder to complete the blending and microbubble shaping. The resulting prepolymer is then polymerized in a steam environment at 70-100°C for 15-30 minutes. The polymerized product is then pulverized and dried to obtain the finished salt-resistant composite water-retaining agent.

[0008] The raw materials, by percentage of total mass in the formula, are as follows: water 45%~55%, xanthan gum 8%~12%, clay minerals 10%~15%, initiator 0.8%~1.2%, polymeric monomers 22%~30%, and crosslinking agent 1.5%~2.5%.

[0009] The polymer monomer is selected from two of acrylamide, acrylic acid, methacrylic acid, 2-acrylamide-2-methylpropanesulfonic acid, and polyvinyl alcohol.

[0010] The crosslinking agent is N,N One of '-methylenebisacrylamide, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, and divinylbenzene.

[0011] The initiator is selected from at least one of potassium persulfate, ammonium persulfate, and azobisisobutylamidine hydrochloride.

[0012] The clay mineral is at least one of attapulgite, montmorillonite, kaolinite, and illite, and its particle size ranges from 100 to 600 mesh.

[0013] The acid used in the acid pretreatment is one of acetic acid, propionic acid, citric acid, or oxalic acid, with a concentration of 0.1~1.0 mol / L.

[0014] The conditions for the twin-screw active extrusion are as follows: barrel temperature maintained at 20~45℃, screw speed at 80~200rpm, screw length-to-diameter ratio at 20~30:1, and barrel working pressure at 1.5~3.0 MPa.

[0015] The concept of this invention is as follows: 1. Activation of Xanthan Gum and Construction of Bubble Templates: Xanthan gum was pretreated with acid to allow its molecular chains to fully extend in an acidic environment, constructing a high-viscosity three-dimensional network structure. This step aims to significantly enhance its ability to form and stabilize bubbles during subsequent processing. Simultaneously, the excellent structural and chemical stability of xanthan gum itself ensures that this high-viscosity network can be effectively maintained under subsequent high-intensity extrusion shearing, providing a crucial foundation for the stable encapsulation of micro and nanobubbles.

[0016] 2. High-efficiency pore formation through active extrusion: The strong shear field and dynamic pressure changes during twin-screw extrusion provide a unique physical environment for bubble formation, dispersion, and shaping. Under high shear, air is efficiently entrained and broken into microbubbles. Acid-pretreated xanthan gum molecular chains entangle and adsorb onto the bubble interface, forming a stable protective layer that prevents bubble coalescence or rupture. Simultaneously, the material transport and mixing during extrusion ensures uniform bubble distribution within the material, achieving initial control of the porous structure.

[0017] 3. Synergistic Stabilizing Effect of Clay Minerals: The addition of clay minerals not only physically reinforces the polymer matrix through their nanoscale layered structure or fibrous morphology, but more importantly, their abundant surface active groups, such as hydroxyl groups, can form various interfacial interactions with xanthan gum molecular chains and polymer monomers, including hydrogen bonds and electrostatic attraction. This interfacial interaction further stabilizes the bubble template, reducing bubble escape during subsequent processing. Furthermore, clay mineral particles can act as nodes in the porous structure, improving the mechanical strength and structural stability of the porous network and reducing structural collapse during water absorption and swelling.

[0018] 4. Structural Fixation through Rapid Steam Polymerization: The steam environment provides a mild yet efficient heating method, enabling rapid heating and uniform polymerization of prepolymers. Compared to traditional oven heating, steam heating is faster, decomposing the initiator and triggering the polymerization reaction in a short time, rapidly solidifying and shaping the porous structure. This effectively avoids the expansion, rupture, migration, or merging of bubbles caused by prolonged heating, ensuring the integrity and uniformity of the prepared porous structure.

[0019] The present invention has the following advantages over the prior art: 1. This invention regulates the interfacial activity and solution behavior of xanthan gum through acid pretreatment, and combines this with the dynamic physical field of twin-screw extrusion to achieve efficient and stable dispersion of micro- and nano-bubbles with air as the internal phase. This in-situ bubble template method eliminates the need for additional chemical porogens or organic solvents, reducing the introduction of chemicals at the source and aligning with the trend of green chemistry. Furthermore, bubble formation and shaping are completed in one step during extrusion, greatly simplifying the process, reducing production costs, and providing favorable conditions for continuous industrial production.

[0020] 2. This invention ingeniously utilizes the interfacial regulation effect of xanthan gum and the multi-scale synergistic effect of clay minerals to construct a composite water-retaining agent with a hierarchical porous structure. This structure not only possesses high porosity and specific surface area, which is beneficial for rapid water adsorption and storage, but also the strong interfacial bonding force between the clay minerals and the polymer matrix endows the material with good structural stability. In a salt solution environment, the hierarchical porous structure can provide more water transport channels, reduce the accumulation of salt ions on the material surface, thereby effectively alleviating the "salt shielding effect" and significantly improving the water absorption performance and long-term water retention capacity of the water-retaining agent in high-salt environments. Attached Figure Description

[0021] Figure 1 The image shows a SEM image of the salt-resistant composite water-retaining agent prepared in Example 3.

[0022] Figure 2 The swelling properties of the salt-resistant composite water-retaining agent (a) and blank control (b) prepared in Example 3 were investigated in salt solutions of different concentrations.

[0023] Figure 3 The swelling properties of the salt-resistant composite water-retaining agent prepared in Example 3 and the blank control in pure water (a) and 0.9% NaCl solution (b). Detailed Implementation

[0024] The following examples illustrate in detail the preparation method and performance of the water-retaining agent of the present invention. These examples are only used to further explain the technical solution of the present invention and are not intended to limit the scope of protection of the present invention.

[0025] Example 1: 10.0 g of xanthan gum was weighed and added to 100 mL of 0.5 mol / L acetic acid solution, and pretreated at room temperature for 12 h with stirring. Then, 18.04 g of potassium hydroxide was added and stirred until homogeneous to form the xanthan gum pretreated solution. 28.0 g of acrylic acid, 7.0 g of acrylamide, 2.0 g of N,N'-methylenebisacrylamide, and 15.0 g of attapulgite (300 mesh) were added sequentially and mixed thoroughly. 1.0 g of ammonium persulfate was dissolved in a small amount of water and added to the system, and stirred until homogeneous. The mixture was transferred to a twin-screw extruder for active extrusion under the following conditions: barrel temperature 35℃, screw speed 150 r / min, length-to-diameter ratio 25:1, and barrel pressure 2.0 MPa, with four repeated extrusions. The resulting prepolymer was subjected to heat treatment polymerization at 90℃ for 20 min. The product was pulverized, dried at 105℃ for 6 h, and passed through a 200-mesh sieve to obtain a salt-resistant composite water-retaining agent. Test results show that the product has a swelling degree of 420 g / g in deionized water and 72 g / g in 0.9% NaCl solution.

[0026] Example 2: 12.0 g of xanthan gum was weighed and added to 100 mL of 0.2 mol / L citric acid solution. The mixture was pretreated at room temperature for 24 h with stirring. Then, 17.02 g of potassium hydroxide was added and stirred until homogeneous to form the xanthan gum pretreated solution. 5.0 g of 2-acrylamido-2-methylpropanesulfonic acid, 1.8 g of N,N'-methylenebisacrylamide, and 13.0 g of montmorillonite (400 mesh) were added sequentially and mixed thoroughly. 1.2 g of potassium persulfate was dissolved in a small amount of water and added to the system. After stirring until homogeneous, the mixture was transferred to a twin-screw extruder. The extrusion conditions were set as follows: barrel temperature 30℃, screw speed 120 r / min, length-to-diameter ratio 28:1, and barrel pressure 1.8 MPa. The extrusion was repeated 5 times. The extruded prepolymer was polymerized at 95℃ in a steam environment for 25 min, then pulverized, dried at 105℃ for 5 h, and passed through a 200-mesh sieve to obtain the finished product. The swelling degree of this water-retaining agent is 450 g / g in deionized water and 85 g / g in 0.9% NaCl solution.

[0027] Example 3: 9.0 g of xanthan gum was weighed and added to 110 mL of 0.8 mol / L oxalic acid solution. The mixture was stirred at room temperature for 8 h for pretreatment. Then, 18.15 g of potassium hydroxide was added and stirred until homogeneous to form a xanthan gum pretreatment solution. 15.0 g of acrylamide, 1.5 g of trimethylolpropane triacrylate, and 14.0 g of kaolinite (500 mesh) were added sequentially and mixed thoroughly. 0.9 g of azobisisobutylamidine hydrochloride was dissolved in a small amount of water and added to the system. After stirring until homogeneous, the mixture was fed into a twin-screw extruder. The extrusion parameters were: barrel temperature 40℃, screw speed 180 r / min, length-to-diameter ratio 22:1, and barrel pressure 2.5 MPa. The extrusion was repeated 3 times. The resulting prepolymer was polymerized in steam at 85℃ for 30 min, pulverized, dried at 105℃ for 7 h, and passed through a 200-mesh sieve to obtain a water-retaining agent. Its swelling performance was tested and found to be 380 g / g in deionized water and 98 g / g in 0.9% NaCl solution.

[0028] Example 4: 11.0 g of xanthan gum was weighed and added to 95 mL of 0.3 mol / L propionic acid solution. The mixture was stirred at room temperature for 16 h for pretreatment. Then, 12.50 g of potassium hydroxide was added and stirred until homogeneous to form the xanthan gum pretreatment solution. 10.0 g of polyvinyl alcohol, 2.2 g of ethylene glycol dimethacrylate, and 12.0 g of illite (600 mesh) were added sequentially and mixed thoroughly. 1.1 g of ammonium persulfate and potassium persulfate (mass ratio 1:1) were dissolved in a small amount of water and added to the system. After stirring until homogeneous, the mixture was transferred to a twin-screw extruder. The extrusion conditions were: barrel temperature 25℃, screw speed 100 r / min, length-to-diameter ratio 30:1, barrel pressure 1.5 MPa, and the extrusion was repeated 6 times. The extruded prepolymer was polymerized at 70℃ in a steam environment for 30 min, then pulverized, dried at 105℃ for 8 h, and passed through a 200-mesh sieve to obtain the finished product. The swelling degree of this water-retaining agent is 350 g / g in deionized water and 60 g / g in 0.9% NaCl solution.

[0029] Example 5: 15.0 g of xanthan gum was weighed and added to 90 mL of 1.0 mol / L acetic acid solution, and pretreated at room temperature for 4 h with stirring. Then, 17.03 g of potassium hydroxide was added and stirred until homogeneous to form the xanthan gum pretreated solution. 8.0 g of methacrylic acid, 2.5 g of divinylbenzene, and 15.0 g of attapulgite (100 mesh) were added sequentially and mixed thoroughly. 1.5 g of ammonium persulfate was dissolved in a small amount of water and added to the system, stirred until homogeneous, and then fed into a twin-screw extruder. The extrusion parameters were: barrel temperature 45℃, screw speed 200 r / min, length-to-diameter ratio 20:1, and barrel pressure 3.0 MPa, with repeated extrusion twice. The resulting prepolymer was polymerized in a 100℃ steam environment for 15 min, pulverized, dried at 105℃ for 4 h, and passed through a 200-mesh sieve to obtain the final product. The swelling degree of this water-retaining agent in deionized water was 480 g / g, and the swelling degree in 0.9% NaCl solution was 70 g / g.

[0030] Morphology and performance characterization of composite water-retaining agents: 1. SEM analysis: Figure 1SEM images of the composite water-retaining agent prepared in Example 3 at different magnifications are presented. The results show that the material contains a rich porous channel structure. The formation of this structure originates from the properties of xanthan gum and the strong mechanical shearing effect of active extrusion, which generates and stabilizes a large number of micro- and nano-bubbles in the prepolymer. These bubbles decompose and release gas during the subsequent steam-heated polymerization process, leaving a porous network in the polymer matrix. This structure not only enables the water-retaining agent to swell rapidly upon contact with water, but also effectively alleviates the surface "salt shielding" effect commonly seen in traditional water-retaining agents in salt water, thereby significantly improving the water absorption ratio and water absorption rate of the material in various solution environments. This study further quantitatively measured the swelling and water retention properties of the composite water-retaining agent.

[0031] 2. Determination of swelling and water retention properties of composite water-retaining agents: Figure 2 The swelling behavior of the composite water-retaining agent (a) prepared in Example 3 and the blank sample (b) without polymer activation was compared in different salt solutions. The results showed that the composite water-retaining agent treated with polymer activation exhibited significantly better swelling capacity than the blank sample in both pure water and various salt solutions. This significant improvement in salt resistance is mainly attributed to the porous structure introduced into the material. Traditional water-retaining agents typically have a dense structure, with water gradually penetrating into the polymer network mainly through osmotic pressure; however, this composite water-retaining agent, with its continuously open pores, allows water to quickly enter the material, fully utilizing the water absorption function of the internal hydrophilic groups, thereby greatly enhancing its swelling capacity in salt water.

[0032] Figure 3 The water absorption kinetics of the composite water-retaining agent of this invention and the blank sample were further compared in pure water and 0.9 wt% sodium chloride solution. It can be seen that the water absorption rate of the composite water-retaining agent in both media is much higher than that of the blank sample, reaching saturation within 20 minutes, while the blank sample requires more than 40 minutes. This indicates that its porous structure not only accelerates the process of water penetration and diffusion but also effectively alleviates the inhibitory effect of salt ions on hydrophilic groups, thus significantly shortening the time to reach saturation while maintaining high water absorption capacity, demonstrating excellent comprehensive water absorption performance.

Claims

1. A process for the active extrusion preparation of a salt-tolerant composite water-retaining agent based on xanthan gum and clay minerals, characterized in that, The method comprises the following steps: The xanthan gum is acid pretreated for 4-24 hours, and then mixed with a polymerization monomer, a clay mineral, a crosslinking agent, an initiator and water, and then blended and micro-bubble shaped by double screw extrusion for 2-6 times, and then the obtained pre-polymer is polymerized in a steam environment at 70-100 DEG C for 15-30 minutes, and then the polymerization product is crushed and dried to obtain a salt-resistant composite water-retaining agent product.

2. The method of claim 1, wherein, The raw materials are as follows in percentage of total formula mass: water 45-55%, xanthan gum 8-12%, clay mineral 10-15%, initiator 0.8-1.2%, polymerization monomer 22-30%, and crosslinking agent 1.5-2.5%.

3. The method of claim 1, wherein, The polymerization monomer is selected from two of acrylamide, acrylic acid, methacrylic acid, 2-acrylamide-2-methylpropane sulfonic acid and polyvinyl alcohol.

4. The method of claim 1, wherein, The crosslinking agent is N,N one of N,N'-methylenebisacrylamide, ethylene glycol dimethacrylate, trimethylolpropane triacrylate, divinylbenzene.

5. The method of claim 1, wherein, The initiator is selected from at least one of potassium persulfate, ammonium persulfate and azobis isobutyl amidine hydrochloride.

6. The method of claim 1, wherein, The clay mineral is at least one of attapulgite, montmorillonite, kaolinite and illite, and the particle size range is 100-600 mesh.

7. The method of claim 1, wherein, The acid used in the acid pretreatment is one of acetic acid, propionic acid, citric acid and oxalic acid, and the concentration is 0.1-1.0 mol / L.

8. The method of claim 1, wherein, The double screw extrusion conditions are as follows: the barrel temperature is kept at 20-45 DEG C, the screw rotation speed is 80-200 rpm, the screw length-diameter ratio is 20-30:1, and the barrel working pressure is 1.5-3.0 MPa.

9. The salt-tolerant composite water retaining agent prepared by the method of any one of claims 1 to 8, characterized in that, The product has a hierarchical porous structure formed by micro-nano bubble templates, and the water absorption ratio in 0.9% NaCl solution is ≥65 g / g and the water absorption ratio in pure water is ≥350 g / g.

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