Method for preparing high-viscosity sodium carboxymethyl cellulose

Abstract

The invention discloses a method for preparing high-viscosity sodium carboxymethyl cellulose in the field of high polymer materials and inorganic nano composite materials. The key point of the method is that a special crystal face regulation and control calcium sulfate nanosheet is added in advance before an alkalization step. The nanosheet is prepared by a multi-step process: firstly synthesizing an organic-inorganic hybrid precursor by a hydrothermal method, then stripping a template through ion exchange, then selectively etching a specific crystal face by using a complexing agent to regulate and control surface characteristics, and finally strengthening by hydrothermal crystallization to obtain a final product. When being introduced into the synthesis of sodium carboxymethyl cellulose, the sodium carboxymethyl cellulose can effectively promote alkalization and etherification reactions, is used as an anchor point to improve the reaction uniformity, and constructs an enhanced network structure in a final product, thereby synergistically improving the solution viscosity, substitution degree and stability of the product. The process is simple, raw materials are easy to obtain, and the performance of the obtained product is obviously better than that of a traditional method.

Description

Technical Field

[0001] This invention relates to the field of polymer materials and inorganic nanocomposite materials, specifically to a method for preparing high-viscosity sodium carboxymethyl cellulose. Background Technology

[0002] Sodium carboxymethyl cellulose (CMC), a crucial water-soluble cellulose ether derivative, is widely and deeply applied in numerous fields, including the food industry, oil extraction, pharmaceutical processes, daily chemicals, and emerging battery electrode binders, due to its excellent thickening, suspending, emulsifying, film-forming, and water-retaining abilities. Its core performance characteristics, especially the apparent viscosity of its aqueous solution, directly determine its effectiveness and reliability in specific applications. The mainstream industrial technology for large-scale production of high-viscosity CMC is the slurry process. This method uses organic solvents such as isopropanol as the dispersion medium, allowing cellulose to swell fully in a strongly alkaline environment to form alkali cellulose, which then undergoes an etherification substitution reaction with chloroacetic acid. Although the solution viscosity can be increased to a high level through continuous optimization of parameters such as solvent composition, the ratio of alkali to etherifying reagents, and reaction temperature and duration, this technical approach still has several inherent drawbacks. For example, the entire process is quite complex, involving the use and subsequent recycling of a large amount of organic solvents, which increases production costs and environmental burden; at the same time, further improvements in product performance, especially in terms of salt resistance, acid resistance and long-term storage stability, seem to have approached the theoretical limits that traditional methods can achieve, encountering significant technical bottlenecks.

[0003] To overcome the performance limitations of traditional processes, researchers have attempted to introduce various inorganic nanomaterials into the synthesis system or finished product of sodium carboxymethyl cellulose (CMC) in order to improve its overall performance through composite modification. Common inorganic fillers include calcium carbonate, silica, and montmorillonite. However, these conventional inorganic particles present inherent interfacial compatibility problems with the organic molecular chains of CMC. Untreated inorganic particles are prone to agglomeration in the polymer matrix, forming stress defects that may actually degrade the material's performance. To address the agglomeration problem, complex organic surface modification of the inorganic fillers is usually required, such as grafting organic functional groups onto their surface using silane coupling agents. This additional step not only increases the complexity of the process and the cost of raw materials, but the introduced organic interfacial layer may also age, hydrolyze, or degrade under long-term use or harsh environments, leading to a decline in the modification effect and affecting the long-term stability of the composite material's performance. Therefore, developing a novel inorganic modifier that can efficiently and stably bind to intrinsic cellulose molecules without relying on a vulnerable organic interlayer has become an urgent and important research direction in this field.

[0004] In recent years, the cutting-edge theory of "crystal plane engineering" in materials science has provided revolutionary insights for designing novel interface materials. This theory posits that by precisely controlling the types and proportions of dominant crystal planes exposed during the growth of inorganic crystals, the atomic arrangement, electron density distribution, and chemical reactivity of their surfaces can be fundamentally altered, thereby achieving targeted tailoring of the material's surface physicochemical properties. This means it is possible to design inorganic nanomaterials that do not require any organic encapsulation, yet whose specific exposed crystal planes possess extremely strong intrinsic affinity for the hydroxyl and sodium carboxymethyl cellulose functional groups. Applying this advanced concept to the synthesis and modification of cellulose ethers holds promise for directly solving the scientific challenge of inorganic-organic interfacial compatibility, achieving a robust molecular-level bond between the modifier and the matrix, and opening a new path for the preparation of next-generation high-performance, high-stability sodium carboxymethyl cellulose products. This invention was conceived in this context, aiming to put the design concept of crystal plane engineering into practice and create a novel modified material and process. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing high-viscosity sodium carboxymethyl cellulose, which solves the technical problems of limited viscosity improvement of the product aqueous solution, complex production process, and difficulty in stable reinforcing effect of conventional inorganic fillers due to poor compatibility with cellulose when using the traditional slurry method to prepare sodium carboxymethyl cellulose.

[0006] The present invention achieves the above objectives through the following technical solutions:

[0007] A method for preparing high-viscosity sodium carboxymethyl cellulose includes the following steps:

[0008] S1, by weight, add 1500-2500 parts of isopropanol to a three-necked flask, add 80-120 parts of cellulose at room temperature and with stirring, then add 5-15 parts of crystal-faceted calcium sulfate nanosheets, stir, add 120-180 parts of aqueous solution containing sodium hydroxide, react at 34-36℃ to obtain alkali cellulose slurry containing nanosheets;

[0009] S2, 100-160 parts of chloroacetic acid are pre-dissolved in 200-400 parts of isopropanol to obtain an etherifying agent solution; the etherifying agent solution is added dropwise to an alkali cellulose slurry containing nanosheets; after the addition is complete, the temperature is raised to 54-56℃ to react and obtain a reaction solution;

[0010] S3. Allow the reaction solution to cool naturally to 38-42℃, then continue stirring to obtain a mixture.

[0011] S4. Add an aqueous ethanol solution to the mixture and adjust the pH value to 7.0-7.5. Obtain a solid product by filtration. Wash the solid product with an aqueous ethanol solution to obtain a washed solid product. Dry the washed solid product in a vacuum drying oven at 78-82℃, and then crush and sieve it.

[0012] In this invention, calcium sulfate nanosheets with regulated crystal surfaces serve as a key additive in the preparation of sodium carboxymethyl cellulose. Their mechanism of action spans the entire process from alkalization to final product formation, representing a multi-stage, dynamic interface engineering approach. During the alkalization stage, the nanosheets are mixed with cellulose fibers, isopropanol medium, and alkali solution. Their unique surface crystal faces generate strong physical adsorption and hydrogen bonding with the hydroxyl groups on the cellulose macromolecular chains. This interaction facilitates faster and more uniform penetration of the alkali solution into the interior of the cellulose microcrystals, promoting full swelling of cellulose and the formation of alkali cellulose, creating a superior starting state for the subsequent etherification reaction. The role of the nanosheets is even more crucial during the etherification reaction. The highly active surfaces of the nanosheets, uniformly dispersed in alkali cellulose, can act as multifunctional "nanoanchors." On the one hand, they can combine with the forming sodium carboxymethyl cellulose molecular chains through electrostatic interactions and ionic coordination (calcium ions on the nanosheet surface and carboxymethyl anions), which to some extent stabilizes the molecular chains and inhibits their excessive coiling or aggregation. On the other hand, this anchoring effect may locally alter the chemical microenvironment and accessibility of the hydroxyl groups on the glucose ring of cellulose, thereby affecting the nucleophilic substitution reaction pathway of chloroacetic acid. This helps to promote a more uniform etherification reaction on the cellulose chain, reduce side reactions, and ultimately obtain a higher and more uniform degree of substitution. In the system after the reaction and in the final product, the nanosheets are connected to numerous sodium carboxymethyl cellulose molecular chains through the aforementioned various non-covalent bond forces, effectively constructing a strengthened three-dimensional supramolecular network structure with nanosheets as cross-linking nodes on a macroscopic scale. When the product dissolves in water, this network can more effectively bind water molecules and resist damage caused by changes in ionic strength or acidity / base, thus exhibiting exceptionally high solution viscosity, excellent salt resistance, and storage stability on a macroscopic scale. In summary, the addition of nanosheets is not a simple physical doping process, but rather a deep involvement in the chemical reaction process. By constructing an enhanced microstructure, it fundamentally improves the overall performance of the product.

[0013] According to a preferred embodiment of the present invention, in step S1, the reaction time at 34-36°C is 1.5-2 hours.

[0014] According to a preferred embodiment of the present invention, in step S2, the reaction time at 54-56°C is 2-4 hours.

[0015] According to a preferred embodiment of the present invention, in step S3, the stirring reaction is continued for 1-2 hours.

[0016] According to a preferred embodiment of the present invention, in step S4, the washed solid product is dried in a vacuum drying oven at 78-82°C for 6-8 hours.

[0017] According to a preferred embodiment of the present invention, the preparation steps of the crystal-plane-controlled calcium sulfate nanosheets include:

[0018] A1, by weight, 80-120 parts of calcium sulfate dihydrate and 40-60 parts of sodium dodecyl sulfate are dispersed in deionized water and stirred to obtain a suspension; the suspension is transferred to a high-pressure reactor and hydrothermally reacted at 175-185℃; after natural cooling, the precipitate is collected by centrifugation, and the precipitate is washed alternately with deionized water and ethanol, and dried under vacuum at 58-62℃ to obtain an organic-inorganic hybrid precursor;

[0019] A2. After grinding the organic-inorganic hybrid precursor, add 300-500 parts of sodium nitrate in a mixed solution of ethanol and water, stir and react at 78-82℃. After the reaction is completed, collect the solid by centrifugation, wash the solid with deionized water until neutral, and obtain the middle of the exfoliated nanosheet.

[0020] A3. The exfoliated nanosheets were redispersed in an acetate-sodium acetate buffer solution with a pH of 4.8-5.2. 4-8 parts of disodium ethylenediaminetetraacetate were added, and the mixture was stirred at a constant temperature of 48-52℃ under ultrasonic assistance. After the reaction was completed, the mixture was separated by high-speed centrifugation to obtain a solid product. The solid product was washed with deionized water to obtain the washed wet nanosheets.

[0021] A4. The washed wet nanosheets are dispersed in deionized water, transferred to a hydrothermal reactor, and subjected to hydrothermal treatment at 118-122℃ to obtain the product, which is then freeze-dried.

[0022] In this invention, the preparation of calcium sulfate nanosheets with crystal facet control is a precisely designed and controllable synthesis process based on the theory of "crystal facet engineering." Its purpose is to break the isotropic natural growth habit of calcium sulfate crystals and, through a series of continuous chemical reaction steps, actively and selectively shape the dominant crystal faces that are ultimately exposed, thereby obtaining two-dimensional nanomaterials with specific surface physicochemical properties. First, calcium sulfate dihydrate and sodium dodecyl sulfate are used as starting materials, and the reaction is carried out under a high-temperature and high-pressure hydrothermal environment. The long-chain alkyl hydrophobic ends of sodium dodecyl sulfate adsorb onto specific crystal faces of calcium sulfate, acting as a structural guiding template to guide the crystal growth along a confined direction, initially forming an organic-inorganic hybrid precursor with layered characteristics. This step lays the morphological and structural foundation for subsequent crystal facet control. Subsequently, through a long-term ion exchange treatment, sodium ions in a sodium nitrate solution gradually replace and remove sodium dodecyl sulfate molecules embedded in the interlayer. This process not only removes the organic template but, more importantly, triggers the transformation of the crystal structure from the hydrated layered precursor to anhydrous calcium sulfate, while also causing partial crystal exfoliation, resulting in the initial nanosheet structure. Next, selective etching of crystal faces is performed. The obtained nanosheet intermediates are dispersed in a weakly acidic buffer system, and disodium ethylenediaminetetraacetate (EDTA) is introduced as a complexing etchant. Because calcium atoms on different crystallographic faces have different coordination environments and reactivity, EDTA preferentially undergoes strong complexation with calcium ions on high surface energy, highly reactive crystal faces, thus selectively dissolving and etching these crystal faces. Specific crystal faces that are thermodynamically more stable and less reactive are relatively preserved and further reconstructed and grown under continuous, mild conditions. By precisely controlling the pH, temperature, and duration of the etching environment, the exposure ratio of the target crystal faces can be finely controlled. Finally, the crystallinity of the etched nanosheets is repaired and strengthened through a secondary hydrothermal treatment, eliminating lattice defects that may be caused by chemical etching, making the crystal structure of the nanosheets more complete and stable, and then freeze-drying to obtain the final product. The calcium sulfate nanosheets obtained through this series of processes have surfaces dominated by crystal faces rich in specific atomic arrangements and electronic states, which theoretically have a high affinity for the functional groups in cellulose derivatives.

[0023] According to a preferred embodiment of the present invention, in step A1, the hydrothermal reaction is carried out at 175-185°C for 24-30 hours.

[0024] According to a preferred embodiment of the present invention, in step A2, the stirring reaction time at 78-82°C is 48-50 h.

[0025] According to a preferred embodiment of the present invention, in step A3, the stirring time at a constant temperature of 48-52°C is 36-40 hours.

[0026] According to a preferred embodiment of the present invention, in step A4, the hydrothermal treatment at 118-122°C is carried out for 6-8 hours.

[0027] The beneficial effects of this invention are as follows:

[0028] The technical solution provided by this invention, by introducing self-designed crystal-plane-controlled calcium sulfate nanosheets and making key adjustments to the synthesis process, has produced significant and multifaceted beneficial technical effects, mainly reflected in three aspects: the excellent improvement of the final product performance, the efficient simplification of the production process, and the comprehensive enhancement of the advantages of integrated application.

[0029] First, breakthroughs have been achieved in the core performance indicators of the product. The most significant effect of the sodium carboxymethyl cellulose prepared by this invention is a substantial increase in the viscosity of its aqueous solution. Under the same concentration conditions, its viscosity value can be several times higher than that of products prepared by traditional optimal processes. This is directly attributed to the unique role played by the crystal-faceted nanosheets in the reaction system. These nanosheets are uniformly dispersed among the cellulose fibers during the alkalization stage. Their specifically exposed crystal faces form strong interactions with the hydroxyl groups on the cellulose molecular chains, which not only promotes the penetration and swelling of the alkali solution, making the subsequent etherification reaction more complete, but also acts as anchoring points during the etherification process, guiding the carboxymethyl groups to be substituted more uniformly and efficiently on the cellulose glucose units, thereby obtaining a higher and more uniform degree of substitution. Furthermore, the nanosheets are dispersed at the nanoscale in the final product and bond with the sodium carboxymethyl cellulose molecular chains through ionic coordination and hydrogen bonding, forming a more robust and durable three-dimensional network structure in solution. This not only contributes to the ultra-high viscosity but also significantly improves the product's salt and acid resistance, enabling it to maintain excellent viscosity stability in complex ionic environments or over a wide range of pH levels, thus expanding its application boundaries.

[0030] Secondly, a clever balance and optimization of complexity and efficiency is achieved at the production process level. The innovation of this invention lies in placing the complex and precise material design work of "crystal plane engineering" pre-processed, independent of the main synthesis process of sodium carboxymethyl cellulose. While the preparation of crystal-plane-controlled calcium sulfate nanosheets is a rigorous process, it can be stored and used as a raw material after a single synthesis. In the main process, it only needs to be added as a solid additive at the beginning of alkalization. Subsequent steps are highly compatible with the traditional slurry method, eliminating the need for harsh conditions such as high temperature and pressure, inert gas protection, or complex in-situ reactions. It also completely avoids the cumbersome secondary treatment using organic surface modifiers such as silane coupling agents. This design concept of "pre-made modifiers and simplified main process" makes the entire production process easy to operate, requires no special equipment, and allows for easy control of the production rhythm. It significantly reduces process complexity and operational difficulty, which is highly conducive to large-scale scaling and stable production. Furthermore, since the main raw materials are all commercially available common chemicals, production costs are effectively controlled.

[0031] Finally, this invention brings outstanding comprehensive application and environmental benefits. From a material perspective, the calcium sulfate nanosheets used are inorganic mineral materials, widely available, inexpensive, and non-toxic, aligning with the development direction of green chemistry. The interfacial bonding between the nanosheets and sodium carboxymethyl cellulose is based on stable physicochemical interactions, avoiding the degradation and aging problems that may exist in organic modification layers, thus giving the final product a longer shelf life and service life, and significantly reducing performance degradation. In summary, this invention, through a cleverly conceived composite modification strategy, not only successfully prepared a high-viscosity sodium carboxymethyl cellulose product with performance far exceeding existing levels, but also provided a feasible, cost-controllable, and environmentally friendly efficient production path, possessing significant industrial application value and market competitive potential. Detailed Implementation

[0032] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0033] Example 1

[0034] Preparation of calcium sulfate nanosheets with crystal facet regulation: A1, 100.0 g of calcium sulfate dihydrate and 50.0 g of sodium dodecyl sulfate were added to 2000 mL of deionized water and stirred continuously at 500 rpm for 60 min at room temperature (25 °C) to form a homogeneous suspension. This suspension was transferred to a 5 L polytetrafluoroethylene-lined high-pressure reactor, sealed, and placed in a forced-air drying oven. The temperature was increased to 180 °C at a rate of 2 °C / min, and the hydrothermal reaction was carried out continuously at this temperature for 24 h. After the reaction, the reactor was allowed to cool naturally to room temperature in an oven. The contents were then removed and centrifuged at 8000 rpm for 10 min to obtain the precipitate. The precipitate was washed three times alternately with 1000 mL of deionized water and 500 mL of anhydrous ethanol, and centrifuged under the same conditions after each wash. The washed precipitate was then transferred to a vacuum drying oven and dried at 60°C and -0.1 MPa for 12 hours to obtain a dry white powder of the organic-inorganic hybrid precursor.

[0035] A2. The obtained precursor powder was ground thoroughly in an agate mortar for 10 minutes, and then completely added to 2500 mL of an ethanol-water mixture. This mixture was prepared by dissolving 400.0 g of sodium nitrate in 1250 mL of anhydrous ethanol and 1250 mL of deionized water. This mixture was placed in an 80°C constant temperature oil bath and stirred under reflux at 300 rpm for 48 hours. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 10 minutes to collect the solid. The solid was then washed three times with 1000 mL of deionized water until the washings were neutral according to pH paper, yielding the exfoliated nanosheet intermediate.

[0036] A3. All intermediates were redispersed in 1600 mL of sodium acetate-acetate buffer solution at pH 5.0, and 6.0 g of disodium ethylenediaminetetraacetate was added. The dispersion was then subjected to ultrasonic cleaning at 40 kHz and 300 W for 10 min, followed by transfer to a 50 °C constant temperature water bath and continuous stirring at 350 rpm for 36 h. After the reaction was complete, the solid product was separated by high-speed centrifugation at 10000 rpm for 15 min, and then washed three times with 800 mL of deionized water by centrifugation to obtain wet nanosheets.

[0037] A4. All the washed wet nanosheets were redispersed in 1000 mL of deionized water and sonicated for 5 min to ensure uniform dispersion. Then, the mixture was transferred to a 2 L hydrothermal reactor and placed in an oven for a second hydrothermal treatment at 120 °C for 6 h. After treatment, the resulting slurry was quickly transferred to a freeze dryer cold trap and pre-frozen at -50 °C for 4 h, followed by freeze-drying at -50 °C and 10 Pa for 24 h to obtain the final white, fluffy powder of crystal-faceted regulated calcium sulfate nanosheets.

[0038] Preparation of high-viscosity sodium carboxymethyl cellulose: S1, 2000 g of isopropanol was added to a 5 L three-necked flask equipped with a mechanical stirrer, thermometer, and reflux condenser. At room temperature (25 °C) and with continuous stirring at 300 rpm, 100.0 g of refined cotton cellulose and 10.0 g of the prepared crystal-faceted calcium sulfate nanosheets were added sequentially to the flask, and stirring was continued for 30 min to ensure thorough dispersion. Subsequently, 300 mL of a 45% sodium hydroxide aqueous solution containing 150.0 g of sodium hydroxide was slowly added dropwise to the flask over 15 min using a constant-pressure dropping funnel, with the system temperature controlled to not exceed 35 °C during the addition process using an ice-water bath. After the addition was complete, the oil bath temperature was set to 35 °C, and the reaction was carried out at this temperature with constant stirring at 300 rpm for 1.5 h to obtain an alkali cellulose slurry containing nanosheets.

[0039] S2, 130.0 g of chloroacetic acid was pre-dissolved in 300 g of isopropanol to prepare an etherifying agent solution. Over 30 minutes, this etherifying agent solution was slowly added dropwise to the above-mentioned alkali cellulose slurry using a constant-pressure dropping funnel, maintaining the oil bath temperature at 35°C during the addition. After the addition was complete, the oil bath temperature of the reaction system was raised to 55°C, and the reaction was continued at this temperature with stirring at 300 rpm for 2 hours.

[0040] S3. After the reaction is complete, stop heating, remove the oil bath, and allow the reaction solution to cool naturally to 40°C in air. Continue stirring at this temperature for 1 hour to obtain the reaction mixture.

[0041] S4. To terminate the reaction, 500 mL of 70% (w / w) aqueous ethanol solution was added to the reaction mixture. Then, 20% (w / w) aqueous glacial acetic acid solution was slowly added dropwise to adjust the pH of the mixture to 7.2, with real-time pH monitoring using a pH meter. The solid product was obtained by vacuum filtration through a Buchner funnel. This solid product was then washed five times with 500 mL of 80% (w / w) aqueous ethanol solution, with filtration performed after each wash, until 5 mL of the filtrate was added to 1 mL of 0.1 mol / L silver nitrate solution and no white silver chloride precipitate was formed. The washed filter cake was placed in a vacuum drying oven and dried at 80℃ and -0.1 MPa for 6 hours. Finally, it was pulverized using a universal pulverizer and passed through an 80-mesh standard sieve to obtain a white powdery high-viscosity sodium carboxymethyl cellulose product.

[0042] Example 2

[0043] The specific implementation method is the same as in Example 1, except that the preparation of calcium sulfate nanosheets with crystal facet regulation is as follows: A1, 90.0 g of calcium sulfate dihydrate and 45.0 g of sodium dodecyl sulfate were added to 1800 mL of deionized water and stirred continuously at 500 rpm for 60 min at room temperature (25°C) to form a uniform suspension. This suspension was transferred to a 5 L polytetrafluoroethylene-lined high-pressure reactor, sealed, and placed in a forced-air drying oven. The temperature was increased to 178°C at a rate of 2°C / min, and the hydrothermal reaction was carried out continuously at this temperature for 26 h. After the reaction, the reactor was allowed to cool naturally to room temperature in an oven. The contents were then removed from the reactor and centrifuged at 8000 rpm for 10 min to obtain the precipitate. The precipitate was washed three times alternately with 900 mL of deionized water and 450 mL of anhydrous ethanol, and centrifuged under the same conditions after each wash. The washed precipitate was then transferred to a vacuum drying oven and dried at 59°C and -0.1 MPa for 13 hours to obtain a dry white powder of organic-inorganic hybrid precursor.

[0044] A2. The obtained precursor powder was ground thoroughly in an agate mortar for 10 minutes, and then completely added to 2200 mL of an ethanol-water mixture. This mixture was prepared by dissolving 350.0 g of sodium nitrate in 1100 mL of anhydrous ethanol and 1100 mL of deionized water. This mixture was placed in a 79°C constant temperature oil bath and stirred under reflux at 300 rpm for 49 hours. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 10 minutes to collect the solid. The solid was then washed three times with 900 mL of deionized water until the washings were neutral according to pH paper, yielding the exfoliated nanosheet intermediate.

[0045] A3. All intermediates were redispersed in 1500 mL of sodium acetate-acetate buffer solution with a pH of 4.9, and 5.0 g of disodium ethylenediaminetetraacetate was added. The dispersion was then subjected to ultrasonic cleaning at 40 kHz and 300 W for 10 min, followed by transfer to a constant temperature water bath at 49 °C and continuous stirring at 350 rpm for 37 h. After the reaction was complete, the solid product was separated by high-speed centrifugation at 10000 rpm for 15 min, and then washed three times with 750 mL of deionized water by centrifugation to obtain wet nanosheets.

[0046] A4. All the washed wet nanosheets were redispersed in 1000 mL of deionized water and sonicated for 5 min to ensure uniform dispersion. Then, the mixture was transferred to a 2 L hydrothermal reactor and placed in an oven for a second hydrothermal treatment at 119 °C for 7 h. After treatment, the resulting slurry was quickly transferred to a freeze dryer cold trap and pre-frozen at -50 °C for 4 h, followed by freeze-drying at -50 °C and 10 Pa for 24 h to obtain the final white, fluffy powder of crystal-faceted regulated calcium sulfate nanosheets.

[0047] Preparation of high-viscosity sodium carboxymethyl cellulose: S1, 1800 g of isopropanol was added to a 5 L three-necked flask equipped with a mechanical stirrer, thermometer, and reflux condenser. At room temperature (25 °C) and with continuous stirring at 300 rpm, 90.0 g of refined cotton cellulose and 8.0 g of the prepared crystal-faceted calcium sulfate nanosheets were added sequentially, and stirring was continued for 30 min to ensure thorough dispersion. Subsequently, 270 mL of a 45% sodium hydroxide aqueous solution containing 135.0 g of sodium hydroxide was slowly added dropwise to the flask over 15 min using a constant-pressure dropping funnel, with the system temperature controlled to not exceed 34 °C during the addition process using an ice-water bath. After the addition was complete, the oil bath temperature was set to 34 °C, and the reaction was carried out at this temperature with constant stirring at 300 rpm for 1.8 h to obtain an alkali cellulose slurry containing nanosheets.

[0048] S2, 120.0 g of chloroacetic acid was pre-dissolved in 250 g of isopropanol to prepare an etherifying agent solution. Over 30 minutes, this etherifying agent solution was slowly added dropwise to the above-mentioned alkali cellulose slurry using a constant-pressure dropping funnel, maintaining the oil bath temperature at 34°C during the addition. After the addition was complete, the oil bath temperature of the reaction system was raised to 54°C, and the reaction was continued at this temperature with stirring at 300 rpm for 3 hours.

[0049] S3. After the reaction is complete, stop heating, remove the oil bath, and allow the reaction solution to cool naturally to 39°C in the air environment. Continue stirring at this temperature for 1.2 hours.

[0050] S4. To terminate the reaction, 450 mL of 70% (w / w) aqueous ethanol solution was added to the reaction mixture. Then, 20% (w / w) aqueous glacial acetic acid solution was slowly added dropwise to adjust the pH of the mixture to 7.1, with real-time pH monitoring using a pH meter. The solid product was obtained by vacuum filtration through a Buchner funnel. This solid product was then washed five times with 450 mL of 80% (w / w) aqueous ethanol solution, with filtration performed after each wash, until 5 mL of the filtrate was added to 1 mL of 0.1 mol / L silver nitrate solution and no white silver chloride precipitate was formed. The washed filter cake was placed in a vacuum drying oven and dried at 79°C and -0.1 MPa for 7 hours. Finally, it was pulverized using a universal pulverizer and passed through an 80-mesh standard sieve to obtain a white powdery high-viscosity sodium carboxymethyl cellulose product.

[0051] Example 3

[0052] The specific implementation method is the same as in Example 1, except that the preparation of calcium sulfate nanosheets with crystal facet regulation is as follows: A1, 110.0 g of calcium sulfate dihydrate and 55.0 g of sodium dodecyl sulfate were added to 2200 mL of deionized water and stirred continuously at 500 rpm for 60 min at room temperature (25°C) to form a uniform suspension. This suspension was transferred to a 5 L polytetrafluoroethylene-lined high-pressure reactor, sealed, and placed in a forced-air drying oven. The temperature was increased to 182°C at a rate of 2°C / min, and the hydrothermal reaction was carried out continuously at this temperature for 28 h. After the reaction, the reactor was allowed to cool naturally to room temperature in an oven. The contents were then removed from the reactor and centrifuged at 8000 rpm for 10 min to obtain the precipitate. The precipitate was washed three times alternately with 1100 mL of deionized water and 550 mL of anhydrous ethanol, and centrifuged under the same conditions after each wash. The washed precipitate was then transferred to a vacuum drying oven and dried at 61°C and -0.1 MPa for 11 hours to obtain a dry white powder of organic-inorganic hybrid precursor.

[0053] A2. The obtained precursor powder was ground thoroughly in an agate mortar for 10 minutes, and then completely added to 2800 mL of an ethanol-water mixture. This mixture was prepared by mixing 1400 mL of anhydrous ethanol and 1400 mL of deionized water, and 450.0 g of sodium nitrate was pre-dissolved in it. This mixture was placed in an 81°C constant temperature oil bath and stirred under reflux at 300 rpm for 49.5 h. After the reaction was completed, the mixture was centrifuged at 8000 rpm for 10 minutes to collect the solid, and then washed repeatedly with 1100 mL of deionized water and centrifuged three times until the washing solution was neutral when tested with pH paper, yielding the exfoliated nanosheet intermediate.

[0054] A3. All intermediates were redispersed in 1700 mL of sodium acetate-acetate buffer solution with a pH of 5.1, and 7.0 g of disodium ethylenediaminetetraacetate was added. The dispersion was then subjected to ultrasonic cleaning at 40 kHz and 300 W for 10 min, followed by transfer to a constant temperature water bath at 51 °C and continuous stirring at 350 rpm for 38 h. After the reaction was complete, the solid product was separated by high-speed centrifugation at 10000 rpm for 15 min, and then washed three times with 850 mL of deionized water by centrifugation to obtain wet nanosheets.

[0055] A4. All the washed wet nanosheets were redispersed in 1000 mL of deionized water and sonicated for 5 min to ensure uniform dispersion. Then, the mixture was transferred to a 2 L hydrothermal reactor and placed in an oven for a second hydrothermal treatment at 121 °C for 7.5 h. After treatment, the resulting slurry was quickly transferred to a freeze dryer cold trap and pre-frozen at -50 °C for 4 h, followed by freeze-drying at -50 °C and 10 Pa for 24 h to obtain the final white, fluffy powder of crystal-faceted regulated calcium sulfate nanosheets.

[0056] Preparation of high-viscosity sodium carboxymethyl cellulose: S1, 2200 g of isopropanol was added to a 5 L three-necked flask equipped with a mechanical stirrer, thermometer, and reflux condenser. At room temperature (25 °C) and with continuous stirring at 300 rpm, 110.0 g of refined cotton cellulose and 12.0 g of the prepared crystal-faceted calcium sulfate nanosheets were added sequentially to the flask, and stirring was continued for 30 min to ensure complete dispersion. Subsequently, 330 mL of a 45% sodium hydroxide aqueous solution containing 165.0 g of sodium hydroxide was slowly added dropwise to the flask over 15 min using a constant-pressure dropping funnel, with the system temperature controlled to not exceed 36 °C during the addition process using an ice-water bath. After the addition was complete, the oil bath temperature was set to 35.5 °C, and the reaction was carried out at this temperature with constant stirring at 300 rpm for 1.6 h to obtain an alkali cellulose slurry containing nanosheets.

[0057] S2, 150.0 g of chloroacetic acid was pre-dissolved in 350 g of isopropanol to prepare an etherifying agent solution. Over 30 minutes, this etherifying agent solution was slowly added dropwise to the above-mentioned alkali cellulose slurry using a constant-pressure dropping funnel, maintaining the oil bath temperature at 35.5 °C during the addition. After the addition was complete, the oil bath temperature of the reaction system was raised to 55.5 °C, and the reaction was continued at this temperature with stirring at 300 rpm for 2.5 hours.

[0058] S3. After the reaction is complete, stop heating, remove the oil bath, and allow the reaction solution to cool naturally to 41°C in the air environment. Continue stirring at this temperature for 1.5 hours.

[0059] S4. To terminate the reaction, 550 mL of 70% (w / w) aqueous ethanol solution was added to the reaction mixture. Then, 20% (w / w) aqueous glacial acetic acid solution was slowly added dropwise to adjust the pH of the mixture to 7.4, with real-time pH monitoring using a pH meter. The solid product was obtained by vacuum filtration through a Buchner funnel. This solid product was then washed five times with 550 mL of 80% (w / w) aqueous ethanol solution, with filtration performed after each wash, until 5 mL of the filtrate was added to 1 mL of 0.1 mol / L silver nitrate solution and no white silver chloride precipitate was formed. The washed filter cake was placed in a vacuum drying oven and dried at 81 °C and -0.1 MPa for 6.5 h. Finally, it was pulverized using a universal pulverizer and passed through an 80-mesh standard sieve to obtain a white powdery high-viscosity sodium carboxymethyl cellulose product.

[0060] Comparative Example 1

[0061] The specific implementation method is the same as in Example 1, except that in step S1, after adding isopropanol and cellulose to the three-necked flask, no crystal-faceted calcium sulfate nanosheets are added, and the subsequent alkalization, etherification, neutralization, washing and drying steps are carried out directly.

[0062] Comparative Example 2

[0063] The specific implementation method is the same as in Example 1, except that in step S1, anhydrous calcium sulfate analytical grade powder is used to replace the same mass of crystal-faceted calcium sulfate nanosheets, and is added together with cellulose to isopropanol for dispersion.

[0064] Comparative Example 3

[0065] The specific implementation method is the same as in Example 1, except that in step S1, fumed silica nanopowder is used instead of calcium sulfate nanosheets with the same mass of crystal facet control, and is added together with cellulose to isopropanol for dispersion.

[0066] Performance testing

[0067] The high-viscosity sodium carboxymethyl cellulose prepared in Examples 1-3 and Comparative Examples 1-3 were subjected to performance testing according to the following method, which included the following steps: all samples were tested in a constant temperature and humidity laboratory environment.

[0068] Viscosity Test: Accurately weigh 6.00 g of dried sample powder using an analytical balance and place it in a 500 mL beaker. Measure 294 mL of deionized water, preheat it in a 25.0℃ constant temperature water bath, and pour it into the beaker. Place the beaker in the 25.0℃ water bath and continuously stir at 800 rpm for 120 min using a magnetic stirrer to ensure complete dissolution and hydration of the sample, forming a homogeneous solution with a mass fraction of 2.0%. After stirring, allow it to stand for 10 min to defoam. Measure using a Brookfield rotational viscometer, calibrating the instrument with standard silicone oil before testing. During measurement, pour an appropriate amount of sample solution into a 200 mL dedicated test cup, connect rotor #3, and place the test cup in a 25.0℃ constant temperature circulating water bath jacket for equilibration for 5 min. First, read the stable viscosity value at 6.0 rpm, recording it as η1; then switch the rotation speed to 60.0 rpm and read the stable viscosity value, recording it as η2. Each measurement was repeated three times and the average value was taken. The viscosity unit is expressed in mPa·s.

[0069] Substitution degree test: The ashing-acid-base titration method was used. Accurately weigh 1.500 g of dried, constant-weight sample and place it in a porcelain crucible pre-heated to constant weight in a muffle furnace at 800 °C. Place the crucible in a temperature-controlled muffle furnace and heat to 700 °C at a rate of 10 °C / min, then ignite at this temperature for 240 min to completely ashing the sample. After cooling, transfer all the white ash to a 250 mL Erlenmeyer flask using a small amount of deionized water. Accurately add 50.00 mL of 0.1000 mol / L hydrochloric acid standard solution using a pipette. Place the Erlenmeyer flask on a hot plate and gently reflux for 10 min to expel dissolved carbon dioxide. After cooling to room temperature, add 2-3 drops of 1.0% phenolphthalein ethanol solution as an indicator. Titrate with 0.1000 mol / L sodium hydroxide standard solution until the solution turns pale pink and does not fade within 30 seconds. A blank experiment was performed simultaneously. The degree of substitution of sodium carboxymethyl cellulose is calculated based on the volume of hydrochloric acid standard solution consumed by the sample.

[0070] Salt tolerance test: Prepare a 2.0% sample solution according to the viscosity test method described above. Weigh 3.00g of solid sodium chloride and slowly add it to 100.0g of the prepared sample solution while stirring, ensuring complete dissolution of the sodium chloride to obtain a test solution containing 1.0% sodium chloride. Continue stirring this test solution at 25.0℃ for 30min, and then measure its viscosity at 6.0rpm according to the viscosity test method described above, recording it as η1-salt. Calculate the ratio of this viscosity to the original solution viscosity η1, i.e., the salt viscosity ratio.

[0071] Acid resistance test: Prepare a 2.0% sample solution according to the viscosity test method described above. Slowly add 0.1 mol / L hydrochloric acid solution using a dropper, and monitor the pH value of the solution precisely to 3.00 while stirring magnetically and monitoring in real time with a pH meter. Allow the acidified solution to stand at 25.0℃ for equilibration for 60 min, avoiding vigorous shaking during this period. After equilibration, measure its viscosity at 6.0 rpm according to the viscosity test method described above, and record it as η1-acid. Calculate the ratio of this viscosity to the original solution viscosity η1, i.e., the acid-viscosity ratio.

[0072] Test results:

[0073] Table 1: Test results of each embodiment and comparative example

[0074] Test Project Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Comparative Example 3 η1 viscosity / mPa·s 7150 6320 6680 1850 2100 2250 η2 viscosity / mPa·s 3250 2980 3100 920 1050 1100 Substitution degree (DS) 1.65 1.59 1.62 1.41 1.44 1.45 Salt viscosity ratio (1% NaCl) 0.82 0.79 0.80 0.55 0.58 0.61 Acid-viscosity ratio (pH=3.0) 0.76 0.72 0.74 0.48 0.52 0.50

[0075] As can be seen from Table 1, Examples 1-3 of the present invention comprehensively and effectively solve the three major technical problems in the prior art—limited improvement of product aqueous solution viscosity, complex production process, and poor compatibility of conventional inorganic fillers—by introducing crystal plane-controlled calcium sulfate nanosheets.

[0076] First, in terms of improving product viscosity, the sodium carboxymethyl cellulose prepared in Examples 1-3 has a low shear viscosity η1 of 6320-7150 mPa·s in a 2% aqueous solution, which is more than 3.4 times the viscosity of the conventional product of Comparative Example 1 (1850 mPa·s), and far exceeds the products of Comparative Example 2 (2100 mPa·s) and Comparative Example 3 (2250 mPa·s) with added ordinary calcium sulfate or silica, achieving a significant breakthrough. This breakthrough improvement stems directly from the multiple key roles played by the crystal-plane-controlled nanosheets during the synthesis process: in the alkalization stage, they promote the uniform swelling of cellulose; in the etherification stage, they act as efficient anchoring points to guide a more complete and uniform substitution reaction, as evidenced by the high degree of substitution (DS) of 1.59-1.65 in the example products, which is much higher than the 1.41-1.45 of the comparative products; in the final product, the nanosheets and sodium carboxymethyl cellulose molecular chains are combined through ionic coordination and hydrogen bonding to construct a denser and more stable three-dimensional supramolecular network structure, thus exhibiting extremely high viscosity on a macroscopic scale.

[0077] Secondly, in terms of simplifying the production process, this invention successfully decouples the complex "crystal plane engineering" material preparation from the relatively simple sodium carboxymethyl cellulose slurry synthesis. Although the preparation process of nanosheets involves multiple steps, they can be added in one step of the main synthesis process as a modifier, without changing the core equipment and processes of the traditional process, or introducing complex steps such as in-situ reactions and coupling agent treatment. This achieves the goal of "high-performance modifier and simplified main process," fundamentally simplifying the operational complexity required to obtain high-performance products.

[0078] Finally, the comparison results between the examples and Comparative Examples 2 and 3 are crucial in addressing the issue of poor compatibility of inorganic fillers. Comparative Example 2 used ordinary calcium sulfate powder without crystal facet control, and its product performance was only slightly improved compared to the blank control, Comparative Example 1, demonstrating that the effect of simple physical doping is extremely limited. Comparative Example 3 used conventional silica nanoparticles, and the improvement effect was similarly weak. This highlights the technical bottleneck of conventional inorganic fillers, which are difficult to exert a reinforcing effect due to their weak interfacial affinity with the cellulose matrix. In contrast, the calcium sulfate nanosheets in the examples, specially constructed through crystal facet engineering, significantly enhanced the intrinsic interaction with cellulose hydroxyl and carboxymethyl anions through their surface atomic arrangement and electronic states, achieving a strong bond and uniform dispersion at the nanoscale. This fully transforms the reinforcing potential of inorganic fillers into a leap in product performance, completely solving the core problem of interfacial compatibility.

[0079] In summary, the test data fully demonstrates that this invention not only significantly improves the viscosity and overall performance of the product, but also achieves a balance between efficient modification process and excellent application performance through ingenious material and process design.

[0080] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A method for preparing high-viscosity sodium carboxymethyl cellulose, characterized in that, Includes the following steps: S1, by weight, add 1500-2500 parts of isopropanol to a three-necked flask, add 80-120 parts of cellulose at room temperature and with stirring, then add 5-15 parts of crystal-faceted calcium sulfate nanosheets, stir, add 120-180 parts of aqueous solution containing sodium hydroxide, react at 34-36℃ to obtain alkali cellulose slurry containing nanosheets; S2, 100-160 parts of chloroacetic acid are pre-dissolved in 200-400 parts of isopropanol to obtain an etherifying agent solution; the etherifying agent solution is added dropwise to an alkali cellulose slurry containing nanosheets; after the addition is complete, the temperature is raised to 54-56℃ to react and obtain a reaction solution; S3. Allow the reaction solution to cool naturally to 38-42℃, then continue stirring to obtain a mixture. S4. Add an aqueous ethanol solution to the mixture and adjust the pH value to 7.0-7.

5. Obtain a solid product by filtration. Wash the solid product with an aqueous ethanol solution to obtain a washed solid product. Dry the washed solid product in a vacuum drying oven at 78-82℃, and then crush and sieve it.

2. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 1, characterized in that, In step S1, the reaction time is 1.5-2 hours at 34-36°C.

3. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 1, characterized in that, In step S2, the reaction time is 2-4 hours after heating to 54-56℃.

4. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 1, characterized in that, In step S3, the stirring reaction continues for 1-2 hours.

5. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 1, characterized in that, In step S4, the washed solid product is dried in a vacuum drying oven at 78-82℃ for 6-8 hours.

6. The method for preparing high-viscosity sodium carboxymethyl cellulose according to any one of claims 1-5, characterized in that, The preparation steps of the crystal-plane-tuned calcium sulfate nanosheets include: A1, by weight, 80-120 parts of calcium sulfate dihydrate and 40-60 parts of sodium dodecyl sulfate are dispersed in deionized water and stirred to obtain a suspension; the suspension is transferred to a high-pressure reactor and hydrothermally reacted at 175-185℃; after natural cooling, the precipitate is collected by centrifugation, and the precipitate is washed alternately with deionized water and ethanol, and dried under vacuum at 58-62℃ to obtain an organic-inorganic hybrid precursor; A2. After grinding the organic-inorganic hybrid precursor, add 300-500 parts of sodium nitrate in a mixed solution of ethanol and water, stir and react at 78-82℃. After the reaction is completed, collect the solid by centrifugation, wash the solid with deionized water until neutral, and obtain the middle of the exfoliated nanosheet. A3. The exfoliated nanosheets were redispersed in an acetate-sodium acetate buffer solution with a pH of 4.8-5.

2. 4-8 parts of disodium ethylenediaminetetraacetate were added, and the mixture was stirred at a constant temperature of 48-52℃ under ultrasonic assistance. After the reaction was completed, the mixture was separated by high-speed centrifugation to obtain a solid product. The solid product was washed with deionized water to obtain the washed wet nanosheets. A4. The washed wet nanosheets are dispersed in deionized water, transferred to a hydrothermal reactor, and subjected to hydrothermal treatment at 118-122℃ to obtain the product, which is then freeze-dried.

7. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 6, characterized in that, In step A1, the hydrothermal reaction is carried out at 175-185℃ for 24-30 hours.

8. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 6, characterized in that, In step A2, the reaction is stirred at 78-82℃ for 48-50 hours.

9. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 6, characterized in that, In step A3, the stirring time at a constant temperature of 48-52℃ is 36-40 hours.

10. The method for preparing high-viscosity sodium carboxymethyl cellulose according to claim 6, characterized in that, In step A4, the hydrothermal treatment at 118-122℃ takes 6-8 hours.