High-expansion type nano montmorillonite and preparation method thereof

Abstract

The invention discloses high-expansion type nano montmorillonite and a preparation method thereof. The material is prepared by adopting nano montmorillonite as an inorganic reinforcing phase, carrying out surface modification by a silane coupling agent and then carrying out a two-stage segmented polymerization process. High-crosslinking-degree acrylic acid polymerization is adopted in the first stage, low-crosslinking-degree acrylic acid / acrylamide copolymerization is adopted in the second stage, and a gradient crosslinking structure is constructed. The preparation method comprises the following steps: performing multi-stage cyclone separation and high-speed centrifugal purification on bentonite to obtain nano montmorillonite slurry; modifying the silane coupling agent for 2-4 hours under an alkaline condition; a rigid inner layer is formed through low-temperature polymerization in the first stage, and a flexible outer layer is formed through heating polymerization in the second stage; and after stepwise gradient neutralization, drying and crushing to obtain the product. Inorganic-organic phase chemical bonding is achieved through silanization modification, a rigid core-flexible shell structure is constructed through gradient crosslinking design, and the contradiction that strength and water absorption are difficult to consider at the same time is effectively solved.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite materials technology, specifically a high-expansion nano-montmorillonite and its preparation method. Background Technology

[0002] Montmorillonite, a natural layered silicate mineral, possesses a unique two-dimensional nanosheet structure, expandable interlayer domains, high specific surface area, and excellent ion exchange capacity, making it widely used in adsorbent materials, catalyst supports, and polymer composites. Through organic modification or polymer intercalation, the functionality of montmorillonite can be further enhanced, expanding its application range.

[0003] Montmorillonite primarily originates from bentonite ore. However, the montmorillonite content in natural bentonite ore varies, and it is often accompanied by impurities such as quartz, feldspar, and calcite. Its natural particle size and purity cannot directly meet the high standards required for high-performance nanocomposites. Therefore, to obtain high-purity nano-montmorillonite suitable for polymer composites, effective purification and nano-sizing of the bentonite ore are essential. However, existing purification processes (such as simple physical separation methods) often have limited efficiency and low purity, and residual impurities directly affect subsequent modification effects and the performance stability of the final product. With increasingly stringent requirements for green manufacturing, achieving refined process control and batch stability while improving the performance of nano-montmorillonite has become a pressing technical challenge.

[0004] Furthermore, even with the achievement of high-purity nano-montmorillonite, existing nano-montmorillonite modification technologies still have the following limitations: First, traditional organic modification often uses quaternary ammonium salt surfactants for intercalation treatment. Although this can increase the interlayer spacing, the main interaction between the modifier and montmorillonite is electrostatic adsorption, resulting in weak bonding strength. This makes the material prone to desorption under high temperature or complex environments, affecting its stability and performance. Second, existing polymer / montmorillonite composite materials are mostly prepared using a one-step in-situ polymerization process. The polymer is unevenly distributed on the surface and between layers of montmorillonite, making it impossible to achieve effective synergy between the inorganic and organic phases, resulting in limited improvement in the mechanical properties and functionality of the composite material. Third, traditional preparation processes use a uniform crosslinking strategy, resulting in a relatively consistent degree of crosslinking throughout the material. This makes it difficult to balance the material's strength and expansion properties, failing to meet the performance differences required for various applications.

[0005] Therefore, developing a nano-montmorillonite composite material with high expansion performance, excellent mechanical strength and good stability, along with innovative surface modification technology and controllable polymerization process, is of great significance for expanding the application fields of nano-montmorillonite, increasing product added value and promoting industrial technology upgrading. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a highly expandable nano-montmorillonite and its preparation method. Through surface modification technology of silane coupling agent and two-stage segmented polymerization process, a gradient cross-linked organic-inorganic composite structure is constructed, which not only has excellent expansion performance and mechanical strength, but also good salt resistance and water retention performance.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A highly expandable nano-montmorillonite, prepared by dry weight of the following components, includes: 5-15 parts nano-montmorillonite, 0.5-2 parts silane coupling agent, 15-30 parts first-stage acrylic monomer, 0.2-0.8 parts first-stage crosslinking agent, 15-30 parts second-stage acrylamide monomer, 5-15 parts second-stage acrylic monomer, 0.02-0.15 parts second-stage crosslinking agent, 0.05-1 part initiator, and 10-26 parts neutralizer.

[0008] Based on the above technical means, by selecting nano-montmorillonite as the inorganic reinforcing phase, its unique layered silicate structure, after surface modification treatment with silane coupling agent, introduces polymerizable double bonds or other active functional groups on the surface and between layers of montmorillonite, significantly enhancing the chemical bonding strength between the inorganic phase and the organic polymer. In conjunction with a two-stage segmented polymerization process, a first-stage high-crosslinking polymerization is carried out on the surface and between layers of the modified montmorillonite to form a tightly bonded rigid inner layer network, providing skeletal support. Subsequently, a second-stage low-crosslinking polymerization is carried out to construct a highly expansive flexible outer layer network on the basis of the inner layer, realizing a gradient crosslinking structure design of "rigid core-flexible shell".

[0009] Preferably, the nano-montmorillonite is nano-montmorillonite modified with a silane coupling agent.

[0010] Based on the aforementioned techniques, the particle size distribution of nano-montmorillonite is precisely controlled to ensure a sufficiently large specific surface area and active reaction sites. Through silanization modification, silane coupling agents are inserted into the interlayer space of montmorillonite, increasing the interlayer spacing from the native 1.2-1.3 nm to 1.8-2.2 nm, providing ample interlayer space for subsequent polymerization reactions. After two-stage gradient crosslinking polymerization, the interlayer spacing of the final product is further increased to 2.9-3.3 nm.

[0011] Preferably, the amount of crosslinking agent used in the first stage is 1.3-2.7% of the mass of the acrylic monomer in the first stage; and the amount of crosslinking agent used in the second stage is 0.04-0.75% of the total mass of the monomer in the second stage.

[0012] More preferably, the amount of crosslinking agent used in the first stage is 1.8-2.3% of the mass of the acrylic monomer in the first stage; and the amount of crosslinking agent used in the second stage is 0.1-0.5% of the total mass of the monomer in the second stage.

[0013] Based on the aforementioned technical methods, the controllable construction of a gradient crosslinked structure is achieved by precisely controlling the ratio of crosslinking agent dosage in the two stages. The higher dosage of crosslinking agent in the first stage ensures the formation of a high-density crosslinked network on the montmorillonite surface, providing sufficient mechanical support. The lower dosage of crosslinking agent in the second stage ensures that the outer polymer layer has sufficient chain segment movement freedom and expansion space. The reasonable matching of crosslinking degrees in the two stages allows the inner layer to maintain structural integrity while the outer layer achieves high-expansion during water absorption and swelling, working synergistically.

[0014] Preferably, the silane coupling agent is one or more combinations of γ-methacryloxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-glycidyl etheroxypropyltrimethoxysilane, and vinyltriethoxysilane.

[0015] A further preferred silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane (KH-570).

[0016] According to the above-mentioned technical means, γ-methacryloxypropyltrimethoxysilane contains hydrolyzable methoxysilane groups and polymerizable methacryloxy double bonds. Its methoxy part can undergo hydrolysis and condensation reactions with silanol and aluminol on the surface and between layers of montmorillonite to form stable Si-O-Si or Si-O-Al chemical bonds. Preferably, silane coupling agents containing polymerizable double bonds, such as γ-methacryloxypropyltrimethoxysilane, are preferred. Its methacryloxy double bonds can copolymerize with acrylic acid and acrylamide monomers in subsequent polymerization reactions to achieve chemical bonding between inorganic and organic phases at the molecular level, which significantly improves the interfacial bonding strength and the overall performance of the composite material.

[0017] Preferably, both the first-stage crosslinking agent and the second-stage crosslinking agent are selected from one or more combinations of N,N'-methylenebisacrylamide, trimethylolpropane triacrylate, and ethylene glycol dimethacrylate.

[0018] Further preferably, the first-stage crosslinking agent and the second-stage crosslinking agent are N,N'-methylenebisacrylamide.

[0019] Based on the aforementioned technical methods, N,N'-methylenebisacrylamide, as a bifunctional crosslinking agent, possesses moderate reactivity, good water solubility, and high crosslinking efficiency. Its two acrylamide groups can react with the active sites on the polymer chain to form a stable three-dimensional network structure. By using different amounts of the same crosslinking agent in two stages, both the consistency of the crosslinking reaction mechanism and the gradient distribution of crosslinking density are ensured, simplifying the formulation system and improving the controllability and stability of product performance.

[0020] Preferably, the initiator is one or a combination of potassium persulfate, ammonium persulfate, and sodium persulfate; the neutralizing agent is one or a combination of sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonia.

[0021] A further preferred initiator is potassium persulfate.

[0022] A further preferred neutralizing agent is sodium hydroxide.

[0023] Based on the aforementioned technical methods, potassium persulfate, as a water-soluble, thermally decomposable initiator, can stably decompose to generate free radicals within a temperature range of 50-80℃, initiating monomer polymerization. Sodium hydroxide, as a strong alkaline neutralizing agent, can rapidly neutralize the carboxyl groups on the polyacrylic acid chains to form sodium polyacrylate, significantly improving the material's water absorption and ion exchange capacity.

[0024] This application also discloses a method for preparing highly expandable nano-montmorillonite, comprising the following steps: S1. After hydrating and dispersing the raw bentonite ore, multi-stage cyclone separation and high-speed centrifugation are used to obtain nano-montmorillonite slurry with a solid content of 8-25% and an average particle size of 100-250nm; silane coupling agent is added to the slurry and reacted for 2-4 hours at a pH of 8-10 and a temperature of 60-80℃ to obtain silanized modified nano-montmorillonite slurry.

[0025] Based on the aforementioned technical methods, multi-stage cyclone separation can effectively remove coarse-particle impurities such as quartz and feldspar, while high-speed centrifugation further achieves particle size classification and purification. The synergistic use of these methods significantly improves the purity and nano-scale degree of montmorillonite. Silanization modification under alkaline conditions (pH 8-10) facilitates the hydrolysis and condensation reactions of the silane coupling agent. Reacting at a suitable temperature of 60-80℃ for 2-4 hours ensures that the silane coupling agent is fully adsorbed and bonded to the surface and interlayer of montmorillonite, providing sufficient chemical bonding sites for subsequent polymerization reactions.

[0026] S2. Add the first-stage acrylic monomer and the first-stage crosslinking agent to the silanized modified nano-montmorillonite slurry, stir and mix thoroughly, then add some initiator under nitrogen protection, and react at 50-60℃ for 30-60 minutes to obtain the first-stage polymerization product.

[0027] Based on the aforementioned technical methods, a lower reaction temperature (50-60℃) and a shorter reaction time (30-60 minutes) are used in the first-stage polymerization to control the polymerization rate and avoid localized overheating and uneven polymerization caused by excessively rapid exothermic reactions. The reaction is carried out under nitrogen protection to prevent oxygen from inhibiting free radical polymerization. By using a portion of the initiator (40-60%), initiation capacity is reserved for the second-stage polymerization, ensuring the continuity of the segmented polymerization. The highly cross-linked polymer network formed in the first stage tightly coats the surface and interlayer of montmorillonite, forming a rigid inner layer structure.

[0028] S3. Continue to add the second-stage acrylamide monomer, the second-stage acrylic monomer, the second-stage crosslinking agent and the remaining initiator to the first-stage polymerization product. After stirring and mixing thoroughly, react for 1.5-2.5 hours at 70-80℃.

[0029] Based on the aforementioned technical methods, in the second-stage polymerization, the deep polymerization of monomers is promoted by adding the remaining initiator (40-60%) and increasing the reaction temperature (70-80℃). The second stage primarily uses acrylamide monomer in combination with a small amount of acrylic acid monomer. The introduction of acrylamide improves the hydrophilicity and flexibility of the polymer while reducing the crosslinking density (only 0.04-0.75%), allowing the outer polymer network sufficient expansion space and chain segment movement freedom. A longer reaction time (1.5-2.5 hours) ensures that the monomer conversion rate reaches over 95%, forming a complete gradient crosslinked structure together with the inner layer.

[0030] S4. After the reaction is complete, add the neutralizing agent in steps. The first addition of the neutralizing agent brings the pH to 6-7. After standing for 10-20 minutes, the second addition of the neutralizing agent brings the pH to 7-8. Then dry and pulverize to obtain highly expandable nano-montmorillonite.

[0031] Based on the aforementioned technical methods, an innovative gradient neutralization process is employed. First, 40-60% of the neutralizing agent is added to adjust the pH to a slightly acidic level (6-7). Considering the limitation of neutralizing agent diffusion by the highly cross-linked inner network, stepwise neutralization reduces instantaneous exothermic reactions and pH spikes, improving macroscopic uniformity. Allowing the mixture to stand for 10-20 minutes allows for full diffusion of the neutralization reaction, preventing localized over-neutralization. A second addition of the remaining neutralizing agent (40-60%) adjusts the pH to a slightly alkaline level (7-8), completing the neutralization reaction of the outer layer. This stepwise gradient neutralization method creates a pH and neutralization gradient from the inside out. Combined with the gradient cross-linked structure, this further enhances the material's "rigid inside, flexible outside" characteristics, maintaining high gel strength in the inner layer and excellent expansion properties in the outer layer. Drying and pulverizing yield a 40-120 mesh dry powder product.

[0032] Preferably, in step S1, the amount of the silane coupling agent is 3-15% of the dry weight of the nano-montmorillonite; the multi-stage cyclone separation is 3-5 stages of cyclone separation, the high-speed centrifugation speed is 6000-10000 rpm, and the centrifugation time is 20-40 minutes.

[0033] More preferably, the amount of silane coupling agent is 8-12% of the dry weight of nano-montmorillonite; the multi-stage cyclone separation is a 4-stage cyclone separation, the high-speed centrifugation speed is 8000 rpm, and the centrifugation time is 30 minutes.

[0034] Based on the above technical methods, the dosage of silane coupling agent is controlled at 3-15% (relative to the dry basis of montmorillonite). This ensures sufficient surface modification while avoiding increased costs and the risk of self-polymerization due to excessive use. Impurities of different particle sizes are removed stepwise through 3-5 stages of cyclone separation, and fine particle size classification is achieved through high-speed centrifugation at 6000-10000 rpm. This ensures the production of nano-montmorillonite slurry with uniform particle size and high purity, providing a high-quality raw material basis for subsequent modification and polymerization.

[0035] Preferably, in step S2, the initiator is 40-60% of the total initiator, the stirring speed is 200-400 rpm, and the stirring time is 20-40 minutes; in step S3, the remaining initiator is 40-60% of the total initiator, the reaction temperature is 72-78℃, and the reaction time is 1.8-2.2 hours.

[0036] More preferably, in step S2, the initiator is 50% of the total initiator, the stirring speed is 300 rpm, and the stirring time is 30 minutes; in step S3, the remaining initiator is 50% of the total initiator, the reaction temperature is 75℃, and the reaction time is 2 hours.

[0037] Based on the aforementioned techniques, the initiator is added in two stages, 40-60% each time, to ensure sufficient initiation capacity in both stages. The first stage employs a low stirring speed (200-400 rpm) and a moderate stirring time (20-40 minutes) to avoid excessive shearing that could damage the initially formed network structure. In the second stage, the reaction temperature is precisely controlled within the optimized range of 72-78℃. At this temperature, the initiator decomposition rate is moderate, the monomer conversion rate is high, and the polymerization reaction is uniform and stable. A longer reaction time (1.8-2.2 hours) ensures the complete construction of the outer polymer network.

[0038] Preferably, in step S4, the amount of the first neutralizing agent is 40-60% of the total amount of neutralizing agent, the amount of the second neutralizing agent is 40-60% of the total amount of neutralizing agent, and the final degree of neutralization is 85-105% of the theoretical neutralization amount of acrylic acid; the drying temperature is 70-110℃, the drying time is 6-20 hours, and the particle size of the product obtained after pulverization is 40-120 mesh.

[0039] Further preferred, the amount of the first neutralizing agent is 50% of the total amount of neutralizing agent, the amount of the second neutralizing agent is 50% of the total amount of neutralizing agent, and the final degree of neutralization is 95-100% of the theoretical neutralization amount of acrylic acid; the drying temperature is 90℃, the drying time is 12 hours; and the particle size of the product obtained after pulverization is 60-100 mesh.

[0040] Based on the above technical methods, by precisely controlling the stepwise addition ratio of the neutralizing agent (40-60% each), a gradient neutralization reaction is achieved, avoiding localized pH abrupt changes and concentrated heat release caused by rapid one-time neutralization. The drying temperature is controlled at 70-110℃, effectively removing moisture while avoiding damage to the polymer structure from high temperatures. An appropriate drying time (6-20 hours) ensures the product's moisture content is controlled below 3%. The product is pulverized to a particle size range of 40-120 mesh for easy storage, transportation, and subsequent applications.

[0041] Through the above technical solution, the highly expandable nano-montmorillonite of the present invention has the following beneficial effects: (1) By modifying the surface of the silane coupling agent, the chemical bonding between the inorganic phase and the organic phase is achieved, which solves the problem of weak bonding strength and easy desorption of traditional quaternary ammonium salt modification and significantly improves the stability of the material.

[0042] (2) A two-stage segmented polymerization process was adopted to construct a gradient cross-linked structure of "rigid core-flexible shell", which effectively solved the contradiction between the strength and water absorption of traditional materials. Through silanization modification and gradient cross-linking polymerization, the interlayer spacing of montmorillonite gradually increased from 1.2-1.3 nm in the natural state to 2.9-3.3 nm in the final product, realizing the full intercalation and exfoliation of the polymer. The centrifugation retention (CRC) in 0.9% NaCl solution reached 32.8-39.8 g / g, and the load absorption (AUL, 0.3 psi) reached 22.2-27.5 g / g, showing excellent performance balance.

[0043] (3) The preparation process adopts multi-stage purification and fine control, resulting in high product purity, stable performance and good batch consistency. It is suitable for the field of superabsorbent resins, especially for applications such as hygiene products and agricultural water-retaining agents. Detailed Implementation

[0044] The present invention will be further described below with reference to specific embodiments, but the invention is not limited to these embodiments. Those skilled in the art should recognize that the present invention covers all possible alternatives, improvements, and equivalents included within the scope of the claims.

[0045] Description of raw materials and equipment used in this invention: Bentonite: Zhejiang Fenghong New Material Co., Ltd., sodium-based bentonite, montmorillonite content ≥85%, cation exchange capacity 90mmol / 100g; Silane coupling agent; KH-570 (γ-methacryloyloxypropyltrimethoxysilane).

[0046] All other raw materials are industrial-grade or analytical-grade reagents, purchased through legitimate channels. Example 1

[0047] Step 1: Add 100 parts of raw bentonite ore to 300 parts of deionized water and allow to fully hydrate and disperse for 24 hours. Use a four-stage hydrocyclone (with particle sizes of 15μm, 10μm, 5μm, and 2μm respectively) for classification, collecting the suspension of fine particles smaller than 2μm. Then, centrifuge at 8000 rpm for 30 minutes using a high-speed centrifuge, collecting the upper suspension to obtain a nano-montmorillonite slurry with a solid content of 18% and an average particle size of 180nm.

[0048] Deionized water was added to the above-mentioned nano-montmorillonite slurry (equivalent to 10 parts of dry nano-montmorillonite) to adjust the solid content to 15%. While stirring, ammonia was added to adjust the pH to 9, and then 1.0 part of KH-570 silane coupling agent (equivalent to 10% of dry montmorillonite) was added. The mixture was stirred and reacted for 3 hours in a 70℃ constant temperature water bath to obtain the silanized modified nano-montmorillonite slurry.

[0049] Step 2: Add 22.5 parts of acrylic acid monomer and 0.5 parts of N,N'-methylenebisacrylamide crosslinking agent (equivalent to 2.2% of the monomer mass) to the silanized modified nano-montmorillonite slurry, and stir thoroughly for 30 minutes. Under nitrogen protection, add 0.25 parts of potassium persulfate initiator (accounting for 50% of the total initiator), and heat to 55℃ to react for 45 minutes to obtain the first-stage polymerization product.

[0050] Step 3: Add 22.5 parts of acrylamide monomer, 10 parts of acrylic acid monomer, and 0.08 parts of N,N'-methylenebisacrylamide crosslinking agent (equivalent to 0.25% of the total monomer mass) to the first-stage polymerization product, and stir thoroughly for 30 minutes. Under nitrogen protection, add the remaining 0.25 parts of initiator (accounting for 50% of the total initiator), and heat to 75°C to react for 2 hours.

[0051] Step 4: After the reaction is complete, first add 50% of the total neutralizing agent in sodium hydroxide solution (9.0 parts, prepared as a 30% aqueous solution) to adjust the pH to 6.5, and let it stand for 15 minutes. Then add the remaining 50% of the sodium hydroxide solution and adjust the pH to 7.5. Transfer the slurry to a drying tray and dry it in a 90℃ forced-air drying oven for 12 hours until the moisture content is below 2%. Crush the dried product and pass it through an 80-mesh sieve to obtain highly expandable nano-montmorillonite powder. Example 2

[0052] The preparation process is basically the same as in Example 1, with the main difference being that the raw material ratio is at the lower limit of the claims: 5 parts of nano-montmorillonite, 0.5 parts of silane coupling agent, 15 parts of first-stage acrylic monomer, 0.2 parts of first-stage crosslinking agent, 15 parts of second-stage acrylamide monomer, 5 parts of second-stage acrylic monomer, 0.02 parts of second-stage crosslinking agent, 0.05 parts of initiator, 10 parts of neutralizer, and 50 parts of water.

[0053] The process parameters were adjusted accordingly: silanization reaction temperature 60℃, reaction time 2 hours; first-stage polymerization temperature 50℃, reaction time 30 minutes; second-stage polymerization temperature 70℃, reaction time 1.5 hours; drying temperature 70℃, drying time 20 hours. Since the amounts of each component were at their lower limits, precise control of the batching and mixing process was necessary to ensure good polymerization results even with low component content. Example 3

[0054] The preparation process is basically the same as in Example 1, with the main difference being that the raw material ratio is at the upper limit of the claims: 15 parts of nano-montmorillonite, 2 parts of silane coupling agent, 30 parts of first-stage acrylic monomer, 0.8 parts of first-stage crosslinking agent, 30 parts of second-stage acrylamide monomer, 15 parts of second-stage acrylic monomer, 0.15 parts of second-stage crosslinking agent, 1 part of initiator, 26 parts of neutralizer, and 300 parts of water.

[0055] The process parameters were adjusted accordingly: silanization reaction temperature 80℃, reaction time 4 hours; first-stage polymerization temperature 60℃, reaction time 60 minutes; second-stage polymerization temperature 80℃, reaction time 2.5 hours; drying temperature 110℃, drying time 6 hours. Due to the high component content, the system viscosity increased, requiring an appropriate extension of the reaction time to accommodate the high concentration system. Example 4

[0056] The preparation process used was as follows: 10 parts nano-montmorillonite, 1.0 part silane coupling agent, 22.5 parts first-stage acrylic monomer, 0.34 parts first-stage crosslinking agent (equivalent to 1.5% of the mass of the first-stage monomer), 22.5 parts second-stage acrylamide monomer, 10 parts second-stage acrylic monomer, 0.195 parts second-stage crosslinking agent (equivalent to 0.6% of the total mass of the second-stage monomer), 0.5 parts initiator (0.25 parts in the first stage and 0.25 parts in the second stage), and 18 parts neutralizing agent. The preparation process parameters were the same as in Example 1. Example 5

[0057] The preparation process is basically the same as in Example 1. The main difference is the type of silane coupling agent: 1.0 part of γ-aminopropyltriethoxysilane (KH-550) is used to replace KH-570, while the other components and process parameters remain completely unchanged. Example 6

[0058] The preparation process is basically the same as in Example 1, with the main difference being the purification process: a four-stage cyclone separation was used (with particle sizes of 20 μm, 10 μm, 5 μm, and 2 μm respectively), a centrifugation speed of 10,000 rpm, and a centrifugation time of 40 minutes to obtain nano-montmorillonite slurry with an average particle size of 100 nm. The other components and process parameters remained completely unchanged.

[0059] Comparative Example 1 The same formulation as in Example 1 was used, but the silanization modification step was omitted, and two-stage polymerization was carried out directly in the nano-montmorillonite slurry. The amounts and types of other raw materials, including acrylic acid, acrylamide, and crosslinking agent, were exactly the same as in Example 1.

[0060] Comparative Example 2 The preparation process strictly followed the process flow of Example 1, but adopted a one-step polymerization method: all monomers of the first and second stages (22.5 parts of acrylic acid and 22.5 parts of acrylamide in the first stage, and 10 parts of acrylic acid in the second stage) and crosslinking agent (0.58 parts in total, equivalent to 1.05% of the total monomer mass) were added simultaneously, and polymerization was carried out in one step at 70°C for 2 hours. All other raw materials and process parameters were exactly the same.

[0061] Comparative Example 3 The preparation process is basically the same as in Example 1, but a uniform crosslinking strategy is adopted: the same amount of crosslinking agent is used in both stages (1.2% of the monomer mass), and the remaining raw materials and process parameters are exactly the same. Performance testing

[0062] Test sample preparation: The products of each embodiment and comparative example were passed through a 30-60 mesh sieve, and samples with a particle size range of 30-60 mesh were taken for the following tests.

[0063] (1) Centrifugal retention (CRC) The determination was performed according to GB / T 22875-2018 standard. 0.2 g of dry sample (30-60 mesh) was weighed and placed into a non-woven tea bag (60 mm × 85 mm), completely immersed in 0.9% NaCl solution for 30 min (temperature 23±2°C), centrifuged at 250 G for 3 min, and then weighed. CRC (g / g) = (wet weight of sample bag - dry weight of sample - wet weight of empty bag) / dry weight of sample. Each sample was tested in parallel five times, and the mean ± standard deviation was reported.

[0064] (2) Load Absorption Capacity (AUL) Determined according to EDANA NWSP 242.0. 0.2 g of sample (30-60 mesh) was evenly distributed on the sieve at the bottom of an acrylic cylinder (60 mm inner diameter). The sample was then passed through a pre-wetted sintered glass filter plate to absorb 0.9% NaCl solution. Absorption was carried out at 0.3 psi for 60 min, followed by weighing. AUL (g / g) = (Total weight of the apparatus after water absorption - Total weight of the apparatus before water absorption) / Dry weight of the sample. Triple tests were performed, and the mean ± standard deviation was reported.

[0065] (3) Gel strength Texture analysis was performed using a texture analyzer (TA.XTPlus). 1 g of sample was swollen in 0.9% NaCl solution for 2 h, centrifuged at 250 G for 3 min to prepare a CRC-state gel, transferred to a standard sample cup (40 mm diameter), and the surface was smoothed. Using a 12.7 mm diameter cylindrical probe, the sample was compressed to 50% strain at a speed of 1.0 mm / s, and the peak force (N) was recorded as the gel strength. Five parallel tests were performed, and the mean ± standard deviation was taken after removing outliers.

[0066] (4) XRD layer spacing X-ray diffraction (Cu Kα, λ = 1.5418 Å) was used for determination. The sample was dried under vacuum at 60°C for 24 h and then ground to a fineness of 200 mesh. The scanning range was 2θ = 2–15° with a step size of 0.02°. The determination was based on Bragg's equation. =λ / (2×sinθ) to calculate the interlayer spacing of montmorillonite (001) crystal faces. Each sample was measured 3 times, and the mean ± standard deviation was taken.

[0067] Performance Test Comparison Table

[0068] Data Analysis: In Example 1, the CRC reached 38.5 ± 2.3 g / g in 0.9% NaCl solution, the AUL (0.3 psi) was 27.2 ± 1.9 g / g, the gel strength was 1.52 ± 0.15 N, and the final product interlayer spacing reached 3.15 ± 0.18 nm (approximately 2.5 times larger than the 1.2-1.3 nm of natural montmorillonite), verifying the effectiveness of the silanization modification + gradient crosslinking design. Although Example 6 had a slightly higher CRC (39.8 g / g vs 38.5 g / g), considering the higher purification cost (centrifugation at 10000 rpm / 40 min vs 8000 rpm / 30 min, increasing energy consumption), the difference was within the test error range. From the perspective of economics for industrial applications, Example 1 is more practical.

[0069] In Example 3, after adopting a high crosslinking strategy, the gel strength reached 2.08 N (highest), but the CRC dropped to 32.8 g / g (lowest), demonstrating that excessive crosslinking leads to high network rigidity and limited expansion. In Example 4, after adjusting the crosslinking agent ratio, the CRC increased to 36.5 g / g (11.3% higher than Example 3), still significantly better than the uniformly crosslinked Comparative Example 3 (25.2 g / g), demonstrating the importance of gradient crosslinking design. Influence of silane coupling agent type: In Example 5, after changing the silane coupling agent (KH-550), the CRC dropped to 33.2 g / g (13.8% lower than Example 1), and the final interlayer spacing decreased to 2.95 nm, verifying that KH-570 has superior copolymerization activity of methacryloyloxy double bonds and is the optimal choice. Effect of degree of silanization: In Example 2, the amount of silane used was 10% (the same as in Example 1), but the amounts of other monomers and crosslinking agents were all at the lower limit, and the CRC reached 35.2 g / g, which was slightly lower than that in Example 1 (38.5 g / g), proving that the present invention still has excellent performance even at low ratios.

[0070] Comparative examples verify the necessity of the core technology: Comparative Example 1 removed the silanization modification, and the CRC dropped sharply to 22.8 g / g, with the final interlayer spacing being only 1.82 nm (a reduction of 42.2% compared to Example 1), proving that the lack of chemical bonding led to a severe deficiency in polymer intercalation; Comparative Example 2 used one-step polymerization, and the CRC dropped to 26.5 g / g, with the interlayer spacing dropping to 2.42 nm (a reduction of 23.2% compared to Example 1), proving that an effective gradient crosslinking structure could not be formed; Comparative Example 3 used a uniform crosslinking strategy, and the CRC dropped to 25.2 g / g, with the interlayer spacing dropping to 2.58 nm (a reduction of 18.1% compared to Example 1), proving that uniform high crosslinking led to an overly rigid network and limited expansion.

[0071] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high swelling nanomontmorillonite, characterized in that, Prepared by the following components, including: nanometer montmorillonite 5-15 parts, silane coupling agent 0.5-2 parts, first stage acrylic monomer 15-30 parts, first stage crosslinking agent 0.2-0.8 parts, second stage acrylamide monomer 15-30 parts, second stage acrylic monomer 5-15 parts, second stage crosslinking agent 0.02-0.15 parts, initiator 0.05-1 parts, neutralizing agent 10-26 parts, by dry base weight.

2. The high expansion nanomontmorillonite according to claim 1, characterized in that, The nanometer montmorillonite is nanometer montmorillonite modified by a silane coupling agent.

3. The high expansion nanomontmorillonite according to claim 1, characterized in that, The first stage crosslinking agent is used in an amount of 1.3-2.7% of the mass of the first stage acrylic monomer; and the second stage crosslinking agent is used in an amount of 0.04-0.75% of the total mass of the second stage monomers.

4. The high expansion nanomontmorillonite according to claim 1, characterized in that, The silane coupling agent is one or more combinations of γ-methacryloyloxypropyltrimethoxysilane, γ-aminopropyltriethoxysilane, γ-glycidyl ether propyl trimethoxysilane, and vinyl triethoxysilane.

5. The high expansion nanomontmorillonite according to claim 1, characterized in that, The first stage crosslinking agent and the second stage crosslinking agent are each selected from one or more combinations of N,N'-methylenebisacrylamide, trimethylolpropane triacrylate, and ethylene glycol dimethacrylate.

6. The high expansion nanomontmorillonite according to claim 1, characterized in that, The initiator is one or more combinations of potassium persulfate, ammonium persulfate, and sodium persulfate; and the neutralizing agent is one or more combinations of sodium hydroxide, potassium hydroxide, sodium carbonate, and ammonia.

7. A method for the preparation of highly exfoliated nanomontmorillonite as claimed in any one of claims 1-6, characterized by the fact that, The method comprises the following steps: S1. After the raw bentonite is dispersed by hydration, the nanometer montmorillonite slurry with a solid content of 8-25% and an average particle size of 100-250 nm is obtained by multi-stage cyclone separation and high-speed centrifugation; the silane coupling agent is added to the slurry, and the silanization modified nanometer montmorillonite slurry is obtained by reacting at a pH of 8-10 and a temperature of 60-80℃ for 2-4 hours; S2. The first stage acrylic monomer and the first stage crosslinking agent are added to the silanization modified nanometer montmorillonite slurry, and the first stage polymerization product is obtained by reacting at 50-60℃ for 30-60 minutes under nitrogen protection after fully stirring and mixing; S3. The second stage acrylamide monomer, the second stage acrylic monomer, the second stage crosslinking agent, and the remaining initiator are continuously added to the first stage polymerization product, and the second stage polymerization product is obtained by reacting at 70-80℃ for 1.5-2.5 hours after fully stirring and mixing; S4. After the reaction is completed, the neutralizing agent is added in steps, the pH is adjusted to 6-7 by adding the neutralizing agent for the first time, the pH is adjusted to 7-8 by adding the neutralizing agent for the second time after standing for 10-20 minutes, and then the high-swelling nanometer montmorillonite is obtained by drying and crushing.

8. The preparation method according to claim 7, characterized in that, In step S1, the silane coupling agent is used in an amount of 3-15% of the dry base weight of the nanometer montmorillonite; the multi-stage cyclone separation is 3-5 stage cyclone separation, the speed of the high-speed centrifugation is 6000-10000 rpm, and the centrifugation time is 20-40 minutes.

9. The preparation method according to claim 7, characterized in that, In step S2, the partial initiator is 40-60% of the total amount of initiator, the stirring speed is 200-400 rpm, and the stirring time is 20-40 minutes; in step S3, the remaining initiator is 40-60% of the total amount of initiator, the reaction temperature is 72-78℃, and the reaction time is 1.8-2.2 hours.

10. The preparation method according to claim 7, characterized in that, In step S4, the first neutralizer is 40-60% of the total amount of neutralizer, the second neutralizer is 40-60% of the total amount of neutralizer, the final neutralization degree is 0.85-1.05 times the theoretical neutralization amount of acrylic acid, the drying temperature is 70-110℃, the drying time is 6-20 hours, and the particle size of the product after crushing is 40-120 mesh.

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