A method for preparing high aspect ratio rod-shaped cellulose nanocrystals

Abstract

This invention discloses a method for preparing high aspect ratio rod-shaped cellulose nanocrystals, belonging to the field of cellulose material preparation technology. Using natural cellulose as raw material, this invention employs a step-by-step oxidation process under alkaline conditions, combined with halogen salt catalysis, to add hypochlorite solution in stages, thereby preparing high aspect ratio rod-shaped cellulose nanocrystals with diameters of 10-50 nm, lengths of 1-10 μm, and aspect ratios of 100-200. This invention effectively solves the problems of excessive fiber degradation, uneven functional group distribution, and easy product agglomeration in traditional sodium hypochlorite oxidation methods through a step-by-step oxidation strategy, achieving precise control of the aspect ratio of cellulose. The resulting cellulose exhibits excellent aspect ratio and dispersibility, and the process conditions are mild and environmentally friendly, providing a reliable pathway for the industrial production of high-performance cellulose materials.

Description

Technical Field

[0001] This invention belongs to the field of cellulose material preparation technology, and particularly relates to a method for preparing high aspect ratio rod-shaped cellulose nanocrystals. Background Technology

[0002] Cellulose nanocrystals, with their excellent mechanical properties and high crystallinity, have shown broad application prospects in composite materials, biomedicine, and other fields. As one-dimensional rigid rod-shaped nanounits, the aspect ratio is the core structural parameter that determines their macroscopic functional properties, directly affecting the material's reinforcement efficiency, dispersibility, and functionality.

[0003] Currently, the mainstream preparation technology for cellulose nanocrystals is acid hydrolysis. However, the traditional "one-time acid addition" process is violent and has poor controllability, easily causing excessive breakage of cellulose molecular chains. The aspect ratio of the product is usually below 20, which is difficult to meet the requirements of high-end applications. Although chemical modification combined with mechanical methods can rapidly produce high aspect ratio cellulose nanofibers, the fibers are flexible and have poor dispersibility in hydrophobic matrices, limiting their application scenarios.

[0004] Compared to flexible cellulose nanofibers, high aspect ratio rod-shaped cellulose nanocrystals combine the advantages of both rigid rod-like structures and high aspect ratios, integrating the strengths of nanocrystals and nanofibers: the rigid structure significantly improves their dispersibility in hydrophobic matrices, while the high aspect ratio facilitates the construction of continuous networks, achieving significant mechanical reinforcement even at low addition levels, and endowing composite materials with excellent barrier properties, thermal dimensional stability, and optical modulation properties. Therefore, developing controllable preparation techniques for high aspect ratio cellulose nanocrystals is of significant strategic value for promoting their industrial application in high-performance composite material reinforcing phases, flexible electronic device substrates, and other high-end fields.

[0005] However, existing preparation methods generally suffer from insufficient ability to control the aspect ratio. Related studies mostly focus on local optimization of reaction conditions or post-processing, making it difficult to achieve precise control at the reaction process level and thus failing to consistently obtain rod-shaped products with high aspect ratios and high dispersibility. Summary of the Invention

[0006] To address the aforementioned technical bottlenecks, this invention proposes a method for preparing high aspect ratio rod-shaped cellulose nanocrystals. This invention presents a three-stage, step-feed oxidation process, with selective oxidation as its core. Through segmented temperature control and batch-addition of the oxidant, the feed ratio and reaction process are used as the core means to regulate the aspect ratio, precisely controlling the degree of oxidative degradation under mild conditions. This process differs from the traditional single-feed method, achieving a balance between high aspect ratio, high yield, and process simplicity, providing a novel technical pathway for the large-scale, stable, and controllable preparation of high aspect ratio rod-shaped cellulose nanocrystals.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing high aspect ratio rod-shaped cellulose nanocrystals includes the following steps: (1) Crush the natural cellulose raw material, add deionized water and stir to obtain pretreated cellulose slurry; (2) Add an alkaline solution and a halogen salt powder to the cellulose slurry obtained in step (1), stir, and obtain a mixture; (3) Add hypochlorite solution to the mixture obtained in step (2) in three stages. The first stage is 30-50% of the total mass of hypochlorite solution, and the mixture is stirred at 20-30℃ for 1-3 hours. The second stage is 30-50% of the total mass of hypochlorite solution, and the mixture is stirred at 60-80℃ for 2-6 hours. The third stage is 10-20% of the total mass of hypochlorite solution, and the mixture is stirred at 60-80℃ for 2-6 hours. (4) Add a reaction terminator to the reaction solution in step (3), stir, then cool, filter, and wash to obtain high aspect ratio rod-shaped cellulose nanocrystals.

[0008] This invention introduces a halide salt into a hypochlorite oxidation system, utilizing it to generate hypohalates with stronger oxidizing power in situ. A phased, gradient addition of sodium hypochlorite is employed during the reaction to precisely control the oxidation process, ultimately yielding functionalized cellulose materials with higher oxidation levels and richer surface oxygen-containing functional groups.

[0009] Further, in step (1), the pulverization is to pulverize the natural cellulose raw material to 50-200 mesh; the stirring time is 1-2 hours.

[0010] Further, in step (1), the natural cellulose raw material is selected from one or more of short-staple cotton, wood pulp, bamboo pulp, straw, bagasse and bacterial cellulose, and the crystallinity of the raw material is ≥55%.

[0011] Furthermore, the mass ratio of the natural cellulose raw material to the alkaline solution is 1:4; the alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution, with a mass fraction of 1-5%.

[0012] Furthermore, the amount of halogen salt powder added is 5-20% of the mass of the natural cellulose raw material; the halogen salt powder is selected from one or more of sodium chloride, sodium bromide, potassium bromide and potassium chloride.

[0013] Furthermore, in step (2), the stirring temperature is 25-35℃ and the stirring time is 1-2h.

[0014] Furthermore, the mass ratio of the natural cellulose raw material to the hypochlorite solution is 1:8; the mass concentration of chlorine in the hypochlorite solution is 5-15%.

[0015] Furthermore, the mass ratio of the natural cellulose raw material to the terminating agent is 5:4; the terminating agent is one or more of hydrogen peroxide, sodium bisulfite, and dilute hydrochloric acid.

[0016] Furthermore, in step (4), the stirring time is 1-2 hours.

[0017] The present invention also provides a high aspect ratio rod-shaped cellulose nanocrystal prepared by the above preparation method, which has an aspect ratio of 100-200, a diameter of 10-50 nm, and a length of 1-10 μm.

[0018] This invention also provides the application of high aspect ratio rod-shaped cellulose nanocrystals in the reinforcing phase of high-performance composite materials.

[0019] Compared with the prior art, the present invention has the following advantages and technical effects: (1) The method of the present invention is simple and fast, can efficiently break the chemical bonds in the cellulose molecular chain, and can achieve large-scale processing of cellulose in a short time, making it suitable for industrial continuous production. (2) The production process of this invention is mature, the cost is low, and the economic benefits are high; (3) The method of the present invention can effectively degrade natural cellulose to the required molecular weight level by adopting a stepwise oxidation strategy of hypochlorite concentration, thereby achieving controllable degradation. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 SEM images of the cellulose nanocrystals prepared in Example 1 are shown; where a and b represent different magnifications. Figure 2 Here are SEM images of the cellulose nanocrystals prepared in Example 2; where a and b represent different magnifications. Figure 3 SEM image of the cellulose nanocrystals prepared in Comparative Example 1; Figure 4 SEM image of the cellulose nanocrystals prepared in Comparative Example 2 Detailed Implementation Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0021] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0022] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0023] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0024] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0025] This invention uses natural cellulose as raw material and, under alkaline conditions, employs a step-by-step oxidation process combined with halogen salt catalysis, adding hypochlorite solution in stages to prepare high aspect ratio rod-shaped cellulose nanocrystals with diameters of 10-50 nm, lengths of 1-10 μm, and aspect ratios of 100-200. This invention effectively solves the problems of excessive fiber degradation, uneven functional group distribution, and easy product agglomeration in traditional sodium hypochlorite oxidation methods through a step-by-step oxidation strategy, achieving precise control over the aspect ratio of cellulose. The resulting cellulose exhibits excellent aspect ratio and dispersibility, and the process conditions are mild and environmentally friendly, providing a reliable pathway for the industrial production of high-performance cellulose materials.

[0026] This invention provides a method for preparing high aspect ratio rod-shaped cellulose nanocrystals, comprising the following steps: (1) Crush the natural cellulose raw material, add deionized water and stir to obtain pretreated cellulose slurry; (2) Add an alkaline solution and a halogen salt powder to the cellulose slurry obtained in step (1), stir, and obtain a mixture; (3) Add hypochlorite solution to the mixture obtained in step (2) in three stages. The first stage is 30-50% (e.g., 50%) of the total mass of hypochlorite solution, and the mixture is stirred at 20-30℃ (e.g., 30℃) for 1-3 hours (e.g., 1 hour). The second stage is 30-50% (e.g., 30%) of the total mass of hypochlorite solution, and the mixture is stirred at 60-80℃ (e.g., 70℃) for 2-6 hours (e.g., 4 hours). The third stage is 10-20% (e.g., 20%) of the total mass of hypochlorite solution, and the mixture is stirred at 60-80℃ (e.g., 70℃) for 2-6 hours (e.g., 4 hours). (4) Add a reaction terminator to the reaction solution in step (3), stir, then cool, filter, and wash to obtain high aspect ratio rod-shaped cellulose nanocrystals.

[0027] In some optional embodiments of the present invention, step (1) involves pulverizing the natural cellulose raw material to 50-200 mesh (e.g., 100 mesh); the stirring time is 1-2 hours (e.g., 1 hour).

[0028] In step (1) of some optional embodiments of the present invention, the natural cellulose raw material is selected from one or more of short-staple cotton, wood pulp, bamboo pulp, straw, bagasse, and bacterial cellulose, and the crystallinity of the raw material is ≥55%. Exemplarily, in the following preferred embodiments of the present invention, the natural cellulose raw material is short-staple cotton.

[0029] In step (2) of some optional embodiments of the present invention, the mass ratio of natural cellulose raw material to alkaline solution is 1:4; the alkaline solution is sodium hydroxide solution or potassium hydroxide solution, with a mass concentration of 1-5% (e.g., a mass concentration of 1% or 5%).

[0030] In step (2) of some optional embodiments of the present invention, the amount of halogen salt powder added is 5-20% (e.g., 6.2%) of the mass of the natural cellulose raw material; the halogen salt powder is selected from one or more of sodium chloride, sodium bromide, potassium bromide, and potassium chloride. Exemplarily, in the following preferred embodiments of the present invention, the halogen salt powder is sodium bromide powder.

[0031] In step (2) of some optional embodiments of the present invention, the stirring temperature is 25-35°C (e.g., the stirring temperature is 30°C) and the time is 1-2 hours (e.g., stirring for 1 hour).

[0032] In step (3) of some optional embodiments of the present invention, the mass ratio of natural cellulose raw material to hypochlorite solution is 1:8; the mass concentration of chlorine in hypochlorite solution is 5-15% (e.g., 10%).

[0033] In step (4) of some optional embodiments of the present invention, the mass ratio of natural cellulose raw material to terminating agent is 5:4; the terminating agent is one or more of hydrogen peroxide, sodium bisulfite and dilute hydrochloric acid, with a mass concentration of 30%. Exemplarily, in the following preferred embodiments of the present invention, the terminating agent is hydrogen peroxide.

[0034] In some optional embodiments of the present invention, step (4) involves stirring for 1-2 hours (e.g., stirring for 1 hour).

[0035] Using the above preparation method, a high aspect ratio rod-shaped cellulose nanocrystal can be prepared with an aspect ratio of 100-200, a diameter of 10-50 nm, and a length of 1-10 μm.

[0036] These high aspect ratio rod-shaped cellulose nanocrystals can be used as reinforcing phases in high-performance composite materials.

[0037] Unless otherwise specified, "room temperature" in this invention refers to 25±2℃.

[0038] All raw materials used in this invention were purchased commercially. The chemical reagents and raw materials used in the following embodiments of this invention are of analytical grade or higher purity.

[0039] The technical solution of the present invention will be further illustrated by the following embodiments.

[0040] Example 1 A method for preparing high aspect ratio rod-shaped cellulose nanocrystals includes the following steps: (1) Crush 50g of natural cellulose raw material (short lint cotton) to 100 mesh, add 600mL of deionized water and stir for 1h to obtain pretreated cellulose slurry; (2) Add 200g of 1% sodium hydroxide solution and 3.1g of sodium bromide powder to the cellulose slurry obtained in step (1), and stir at 30°C for 1 hour to obtain a mixture; (3) Add a total of 400g of hypochlorite solution (the mass concentration of chlorine in the hypochlorite solution is 10%) to the mixture obtained in step (2) in three stages. In the first stage, add 160g of sodium hypochlorite solution and stir at 30°C for 1 hour; in the second stage, add 160g of sodium hypochlorite solution and stir at 70°C for 1 hour; in the third stage, add 80g of sodium hypochlorite solution and stir at 70°C for 4 hours. (4) Add 40g of hydrogen peroxide with a mass concentration of 30% to the reaction solution of step (3), continue stirring for 1h, then cool to room temperature, filter and wash to obtain high aspect ratio rod-shaped cellulose nanocrystals.

[0041] Example 2 A method for preparing high aspect ratio rod-shaped cellulose nanocrystals includes the following steps: (1) Crush 50g of natural cellulose raw material (short lint cotton) to 100 mesh, add 600mL of deionized water and stir for 1h to obtain pretreated cellulose slurry; (2) Add 200g of 1% sodium hydroxide solution and 3.1g of sodium bromide powder to the cellulose slurry obtained in step (1), and stir at 30°C for 1 hour to obtain a mixture; (3) Add a total of 500g of hypochlorite solution (the mass concentration of chlorine in the hypochlorite solution is 10%) to the mixture obtained in step (2) in three stages. In the first stage, add 200g of sodium hypochlorite solution and stir at 30°C for 1 hour; in the second stage, add 200g of sodium hypochlorite solution and stir at 70°C for 1 hour; in the third stage, add 100g of sodium hypochlorite solution and stir at 70°C for 4 hours. (4) Add 40g of 30% hydrogen peroxide to the reaction solution of step (3), continue stirring for 1h, then cool to room temperature, filter and wash to obtain high aspect ratio rod-shaped cellulose nanocrystals.

[0042] Comparative Example 1 A method for preparing cellulose nanocrystals includes the following steps: (1) Crush 50g of natural cellulose raw material (short lint cotton) to 100 mesh, add 600mL of deionized water and stir for 1h to obtain pretreated cellulose slurry; (2) Add 200g of 1% sodium hydroxide solution and 3.1g of sodium bromide powder to the cellulose slurry obtained in step (1), and stir at 30°C for 1 hour to obtain a mixture; (3) Add 500g of hypochlorite solution (with a chlorine mass concentration of 10%) to the mixture obtained in step (2) and stir at 70°C for 6 hours. (4) Add 40g of 30% hydrogen peroxide to the reaction solution of step (3), continue stirring for 1h, then cool to room temperature, filter and wash to obtain cellulose nanocrystals.

[0043] Comparative Example 2 A method for preparing high aspect ratio rod-shaped cellulose nanocrystals includes the following steps: (1) Crush 50g of natural cellulose raw material (short lint cotton) to 100 mesh, add 600mL of deionized water and stir for 1h to obtain pretreated cellulose slurry; (2) Add 200g of 1% sodium hydroxide solution and 3.1g of sodium bromide powder to the cellulose slurry obtained in step (1), and stir at 30 °C for 1 h to obtain a mixture; (3) Add a total of 400g of hypochlorite solution (the mass concentration of chlorine in the hypochlorite solution is 10%) to the mixture obtained in step (2) in two stages. In the first stage, add 200g of sodium hypochlorite solution and stir at 30°C for 2 hours. In the second stage, add 200g of sodium hypochlorite solution and stir at 70°C for 4 hours. (4) Add 40g of hydrogen peroxide with a mass concentration of 30% to the reaction solution of step (3), continue stirring for 1h, then cool to room temperature, filter and wash to obtain high aspect ratio rod-shaped cellulose nanocrystals.

[0044] The cellulose nanocrystals prepared in Examples 1-2 and Comparative Examples 1-2 were scanned using field emission scanning electron microscopy. The results are shown in the figure. Figure 1-4 ,in Figure 1 a and b in the image are SEM images of the cellulose nanocrystals prepared in Example 1. Figure 2 a and b in the image are SEM images of the cellulose nanocrystals prepared in Example 2. Figure 3 , Figure 4 The images shown are SEM images of the cellulose nanocrystals prepared in Comparative Examples 1 and 2, respectively. Figure 1 , Figure 2 As can be seen, the cellulose nanocrystals in Examples 1 and 2 exhibit a slender fibrous structure with a high aspect ratio. Figure 3 The product obtained in Comparative Example 1 was short rod-shaped, with severe damage to the fiber structure. Figure 4 In Comparative Example 2, most of the products were longer than those in Comparative Example 1, but a small amount of shorter fibers were also present. This indicates that the feeding method has a significant impact on the product morphology. Comparative Example 1 used a one-time feeding method, with a large amount of sodium hypochlorite entering the system instantly, triggering a violent oxidation reaction. This resulted in excessive erosion of the cellulose crystalline region, extensive breakage of molecular chains, and the formation of short rod-shaped products. Comparative Example 2 used a two-time feeding method, which alleviated the violent oxidation caused by the first feeding to some extent. However, due to the high oxidant concentration peak still forming in the system during the second feeding, some of the initially oxidized cellulose chains underwent secondary over-cutting, resulting in some products being too short. In contrast, Examples 1 and 2, by adding sodium hypochlorite solution in three separate feedings, maintained the reaction system at a low instantaneous sodium hypochlorite concentration range. The oxidation reaction proceeded in a "gradual peeling" mode, effectively controlling the oxidation reaction rate and avoiding excessively high local concentrations. The mild reaction conditions concentrated the oxidation mainly in the amorphous region, allowing the cellulose crystalline region to be completely preserved, thus maintaining the slender fiber morphology.

[0045] The above are merely preferred embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing high aspect ratio rod-shaped cellulose nanocrystals, characterized in that, Includes the following steps: (1) Crush the natural cellulose raw material, add deionized water and stir to obtain cellulose slurry; (2) Add an alkaline solution and a halogen salt powder to the cellulose slurry obtained in step (1), stir, and obtain a mixture; (3) Add hypochlorite solution to the mixture obtained in step (2) in three stages. The first stage is 30-50% of the total mass of hypochlorite solution, and the mixture is stirred at 20-30℃ for 1-3 hours. The second stage is 30-50% of the total mass of hypochlorite solution, and the mixture is stirred at 60-80℃ for 2-6 hours. The third stage is 10-20% of the total mass of hypochlorite solution, and the mixture is stirred at 60-80℃ for 2-6 hours. (4) Add a reaction terminator to the reaction solution in step (3), stir, then cool, filter, and wash to obtain high aspect ratio rod-shaped cellulose nanocrystals.

2. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 1, characterized in that, In step (1), the crushing refers to crushing the natural cellulose raw material to 50-200 mesh.

3. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 2, characterized in that, The natural cellulose raw material is selected from one or more of short-staple cotton, wood pulp, bamboo pulp, straw, sugarcane bagasse and bacterial cellulose, and the crystallinity of the raw material is ≥55%.

4. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 1, characterized in that, The mass ratio of the natural cellulose raw material to the alkaline solution is 1:4; the alkaline solution is a sodium hydroxide solution or a potassium hydroxide solution with a mass concentration of 1-5%.

5. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 1, characterized in that, The amount of halogen salt powder added is 5-20% of the mass of the natural cellulose raw material; the halogen salt powder is selected from one or more of sodium chloride, sodium bromide, potassium bromide and potassium chloride.

6. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 1, characterized in that, In step (2), the stirring temperature is 25-35℃ and the stirring time is 1-2h.

7. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 1, characterized in that, The mass ratio of the natural cellulose raw material to the hypochlorite solution is 1:(8-10); the mass concentration of chlorine in the hypochlorite solution is 5-15%.

8. The method for preparing high aspect ratio rod-shaped cellulose nanocrystals according to claim 1, characterized in that, The mass ratio of the natural cellulose raw material to the terminating agent is 5:4; the terminating agent is one or more of hydrogen peroxide, sodium bisulfite, and dilute hydrochloric acid.

9. A high aspect ratio rod-shaped cellulose nanocrystal prepared by the preparation method according to any one of claims 1-8, characterized in that, The high aspect ratio rod-shaped cellulose nanocrystals have an aspect ratio of 100-200, a diameter of 10-50 nm, and a length of 1-10 μm.

10. The application of the high aspect ratio rod-shaped cellulose nanocrystals as described in claim 9 in the reinforcing phase of high-performance composite materials.

Images