Method for improving reaction activity of pulp through freeze thawing pretreatment

Abstract

The invention provides a method for improving reaction activity of pulp through freeze-thaw pretreatment, which comprises the following steps: fully dipping the pulp in an alkaline aqueous solution with a certain concentration, freezing the pulp into a solid at low temperature, and heating and thawing the solid to obtain activated pulp; according to the method, a low-concentration alkaline aqueous solution is used as a medium, and the reaction activity of the pulp can be remarkably improved through freeze thawing treatment. The method is mild in condition and simple to operate, few cellulose molecular chains are degraded in the treatment process, few byproducts are dissolved out, and the problems of cellulose degradation and byproduct dissolution in the pulp pretreatment process are solved.

Description

Technical Field

[0001] This invention belongs to the field of cellulose modification, specifically relating to a method for improving the reactivity of pulp by using freeze-thaw pretreatment. Background Technology

[0002] Pulp is a cellulose-rich fibrous material made from plant fibers through chemical or mechanical processes to remove most of the non-fibrous components. It is a fundamental raw material for the production of many important products. Cellulose's molecular structure consists of glucose units linked by β-1,4-glycosidic bonds, exhibiting high crystallinity and a strong hydrogen bond network, severely reducing its accessibility. This low accessibility has become a major technical bottleneck for the efficient utilization of cellulose resources. In the chemical modification process of pulp, the reaction rate is slow, the conversion rate is low, and the reagent consumption is high, often requiring harsh reaction conditions (such as high temperature, high pressure, high concentration of acid or alkali, and long reaction time). This not only leads to high energy consumption and high costs but may also cause cellulose degradation, a decrease in molecular weight, affecting the performance of the final product and causing a series of environmental problems. How to improve the activity of pulp cellulose without destroying its main chain structure is one of the key technical problems in current cellulose modification research.

[0003] In existing technologies, the main methods for improving the reactivity of pulp include mechanical pretreatment, chemical activation, and solvation. Mechanical pretreatment, such as ball milling, ultrasonic treatment, and microwave treatment, can disrupt the crystalline regions of cellulose to a certain extent, increasing its specific surface area and thus improving reaction efficiency. However, these methods are energy-intensive and may cause cellulose degradation. Solvation methods (such as ionic liquids, N-methylmorpholine-N-oxide, and deep eutectic solvent systems) can achieve swelling or dissolution of cellulose chains at the molecular level, significantly improving the uniformity and efficiency of the chemical reaction. However, these solvents are usually expensive, difficult to recycle, and pose potential safety and environmental problems, which are not conducive to the scale-up of green chemical industries. Chemical activation is another important way to improve the reactivity of pulp. Common chemical activation treatments include acid hydrolysis, oxidative modification, and activation before esterification or etherification. Acid hydrolysis acts on the amorphous regions of cellulose, relaxing the local structure and increasing the reaction rate. However, prolonged use or high acid concentration conditions can easily lead to molecular chain degradation and structural loss, affecting the subsequent modification effect. Oxidation or carboxymethylation can introduce new functional groups onto the cellulose surface, increasing hydrophilicity and the number of reaction sites. However, these processes require stringent conditions and precise control. Among various chemical methods, alkali treatment is widely used due to its simplicity and rapid reaction rate. Traditional alkali treatment typically uses a high-concentration sodium hydroxide solution (18%–28%) as the medium. The competition between the alkali solution and the hydroxyl groups in the cellulose molecules causes the molecular chains to expand and partially decrystallize, thereby increasing reactivity. This process is often referred to as mercerization. However, during high-concentration alkali treatment, the cellulose molecular chains are prone to breakage, leading to a significant reduction in the molecular weight of the product and affecting the mechanical properties of the final product. Furthermore, high-concentration alkali treatment of pulp generates large amounts of alkaline waste liquid and leaches byproducts, increasing the environmental burden. Therefore, relying on high-concentration alkali treatment to improve pulp reactivity makes it difficult to balance the enhancement of cellulose reactivity with structural stability.

[0004] Based on this, this invention proposes a method to improve the reactivity of pulp by using a low-concentration alkaline aqueous solution as a medium and employing freeze-thaw treatment. This method involves freeze-thawing the pulp in a low-concentration alkaline aqueous solution, allowing the alkaline environment and the freeze-thaw process to work synergistically. This enhances the solvent's competitiveness for hydrogen bonding while avoiding the degradation of cellulose molecular chains and the dissolution of byproducts caused by high-concentration alkali. Compared to traditional mercerization or high-concentration alkali activation, this invention offers milder treatment conditions and a more stable system, effectively improving the reactivity of the pulp while maintaining the integrity of the cellulose backbone, thus facilitating subsequent esterification and etherification modifications. This method is simple, energy-efficient, and requires no organic solvents, providing a new approach for the efficient modification of pulp. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the reactivity of pulp through freeze-thaw pretreatment using a low-concentration alkaline aqueous solution as the medium. The main technical problem to be solved is how to enhance the reactivity of pulp without significantly reducing the molecular weight of cellulose and minimizing the dissolution of byproducts. This method uses a low-concentration alkaline aqueous solution to freeze-thaw pulp, which effectively disrupts intermolecular hydrogen bonds, thereby significantly improving the accessibility and reactivity of cellulose. This invention offers a simple process, is applicable to various pulp raw materials, significantly improves the accessibility of pulp raw materials, and provides an efficient and controllable pretreatment route for their chemical modification, functionalization, and the preparation of derivative products.

[0006] The objective of this invention and the technical problem it solves are achieved by the following technical solutions.

[0007] This invention provides a method for improving the reactivity of pulp through freeze-thaw pretreatment, comprising the following steps: immersing the pulp in an alkaline aqueous solution of a certain concentration, then freezing the mixture into a solid at a low temperature, and finally heating it to thaw, thereby obtaining highly reactive pulp.

[0008] In some embodiments, in the aforementioned processing method, the pulp is at least one of wood pulp, hemp pulp, and cotton pulp.

[0009] In some embodiments, in the aforementioned processing method, the alkali is a strong alkali containing hydroxide ions, including but not limited to sodium hydroxide and potassium hydroxide.

[0010] In some embodiments, in the aforementioned treatment method, the concentration of the alkaline aqueous solution is 0.5-10%.

[0011] In some embodiments, in the aforementioned processing method, the freezing temperature is between -5°C and -45°C.

[0012] By employing the above technical solution, the method for improving the reactivity of pulp through freeze-thaw pretreatment according to the present invention has at least the following advantages:

[0013] (1) The freeze-thaw activation process is carried out in a low-temperature and low-concentration alkaline medium. During the process, the dissolution of hemicellulose by-product is significantly reduced, and a higher yield of activated pulp can be obtained.

[0014] (2) Compared with traditional mercerizing treatment, the pulp fiber swelling degree is high and the product reactivity is higher during the freeze-thaw treatment.

[0015] (3) The freeze-thaw activation conditions are mild, the cellulose molecular chain is less degraded, and the cellulose can maintain a high molecular weight. The mechanical properties and stability of the pulp prepared after freeze-thaw treatment are better than those of traditional mercerization treatment.

[0016] (4) This method is simple to operate. It only requires immersing the pulp in an alkaline solution for low-temperature freezing and then thawing. The product can be directly subjected to subsequent chemical modification or functionalization without separation. Attached Figure Description

[0017] Figure 1 This is the XRD pattern of the activated cotton pulp in Example 3.

[0018] Figure 2 This is the XRD pattern of cotton pulp in Comparative Example 1. Detailed Implementation

[0019] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following detailed description, in conjunction with preferred embodiments, details the specific implementation, structure, features, and effects of a method for improving the reactivity of pulp through freeze-thaw treatment according to the present invention. It should be noted that the following embodiments are exemplary and are not intended to limit the scope of the present invention. Those skilled in the art, after reading the contents of this invention, can make various modifications or alterations to the invention, and these equivalent forms also fall within the scope defined by the claims herein.

[0020] Unless otherwise specified, all materials and reagents mentioned below are commercially available products well known to those skilled in the art; unless otherwise specified, all methods described are methods known in the art. Unless otherwise defined, the technical or scientific terms used should have the ordinary meaning understood by those skilled in the art to which this invention pertains.

[0021] This invention provides a method for improving the reactivity of pulp through freeze-thaw pretreatment. The preparation method includes the following steps: immersing the pulp in an alkaline aqueous solution of a certain concentration, then freezing the mixture into a solid at a low temperature, and finally heating it to thaw, thereby obtaining highly reactive pulp.

[0022] The aforementioned pulp is at least one of wood pulp, hemp pulp, and cotton pulp.

[0023] The aforementioned bases are strong bases containing hydroxide ions, including but not limited to sodium hydroxide and potassium hydroxide.

[0024] The concentration of the aforementioned alkaline aqueous solution is 0.5% to 10%.

[0025] The concentration of alkaline aqueous solution significantly affects the reactivity of pulp and the degradation of cellulose molecular chains. If the concentration is too low, the swelling of the cellulose material during freeze-thaw cycles is limited, which is detrimental to improving its reactivity. If the concentration is too high, reagent consumption increases, cellulose degradation intensifies, and the dissolution of byproducts increases. Therefore, the concentration of alkaline aqueous solution needs to be controlled between 0.5% and 10%.

[0026] The aforementioned freezing temperature is between -5℃ and -45℃.

[0027] To accurately and reproducibly characterize the properties of the cellulose materials in the embodiments of the present invention, the following test methods are uniformly described before each embodiment. The reactivity of the cellulose materials involved in this application was tested by a modified Fock method (Tian Chao et al. Determination of the reactivity of dissolving pulp for viscose fiber using a modified Fock test. Proceedings of the 16th Annual Academic Conference of China Paper Industry Association. 2014: 451-456.).

[0028] Example 1

[0029] Add 20 mL of 0.5% sodium hydroxide solution and 1 g of cotton pulp to a 30 mL sample bottle, shake to completely submerge, freeze at -18℃, remove and heat to thaw, to obtain highly reactive cotton pulp.

[0030] X-ray diffraction analysis revealed that the activated pulp was of type I crystal with a crystallinity of 80.03%. The reactivity of the activated pulp, determined using the Fock method, was 42.18%.

[0031] Example 2

[0032] The difference from Example 1 is that the sodium hydroxide solution concentration is 1% and the freezing temperature is -5°C.

[0033] X-ray diffraction analysis revealed that the activated pulp was of type I crystal with a crystallinity of 78.51%. The reactivity of the activated pulp, determined using the Fock method, was 44.07%.

[0034] Example 3

[0035] The difference from Example 1 is that the concentration of the sodium hydroxide solution is 2%.

[0036] X-ray diffraction analysis revealed that the active pulp was of cellulose type II with a crystallinity of 71.26%. The reactivity of the active pulp was 76.33% as determined by the Fock method.

[0037] Example 4

[0038] The difference from Example 1 is that the concentration of the sodium hydroxide solution is 4%.

[0039] X-ray diffraction analysis revealed that the active pulp was of cellulose type II with a crystallinity of 62.18%. The reactivity of the active pulp was 77.43% as determined by the Fock method.

[0040] Example 5

[0041] The difference from Example 1 is that the alkali used is potassium hydroxide, and the concentration of the potassium hydroxide solution is 6%.

[0042] X-ray diffraction analysis revealed that the active pulp was of cellulose type II with a crystallinity of 53.19%. The reactivity of the active pulp was determined to be 81.78% using the Fock method.

[0043] Example 6

[0044] The difference from Example 1 is that the concentration of the sodium hydroxide solution is 8%.

[0045] X-ray diffraction analysis revealed that the active pulp was of cellulose type II with a crystallinity of 48.55%. The reactivity of the active pulp was determined to be 84.21% using the Fock method.

[0046] Example 7

[0047] The difference from Example 1 is that the cellulose material used is wood pulp, and the sodium hydroxide solution concentration is 9%.

[0048] X-ray diffraction analysis revealed that the active pulp was of cellulose type II with a crystallinity of 52.73%. The reactivity of the active pulp was determined to be 80.92% using the Fock method.

[0049] Example 8

[0050] The difference from Example 1 is that the sodium hydroxide solution concentration is 10% and the freezing temperature is -45°C.

[0051] X-ray diffraction analysis revealed that the active pulp was of cellulose type II with a crystallinity of 52.15%. The reactivity of the active pulp was 79.31% as determined by the Fock method.

[0052] Comparative Example 1

[0053] The difference from Example 1 is that the solution used is a pure aqueous solution.

[0054] X-ray diffraction analysis revealed that the pulp was of cellulose type I with a crystallinity of 80.12%. The reactivity of the pulp, determined by the Fock method, was 40.86%.

[0055] Comparative Example 2

[0056] The difference from Example 1 is that the sodium hydroxide solution concentration is 4%, and there is no freeze-thaw process.

[0057] X-ray diffraction analysis revealed that the pulp's crystal type was cellulose I, with a crystallinity of 78.15%. The Fock method determined the pulp's reactivity to be 50.85%.

[0058] Comparative Example 3

[0059] The difference from Example 1 is that the sodium hydroxide solution concentration is 8%, and there is no freeze-thaw process.

[0060] X-ray diffraction analysis revealed that the pulp was of cellulose type I with a crystallinity of 62.06%. The reactivity of the pulp was 54.49% as determined by the Fock method.

[0061] Comparative Example 4

[0062] The difference from Example 1 is that the sodium hydroxide solution concentration is 12%, and there is no freeze-thaw process.

[0063] X-ray diffraction analysis revealed that the pulp was of cellulose type I with a crystallinity of 68.78%. The reactivity of the pulp was 56.97% as determined by the Fock method.

[0064] Comparative Example 5

[0065] The difference from Example 1 is that the sodium hydroxide solution concentration is 16%, and there is no freeze-thaw process.

[0066] X-ray diffraction analysis revealed that the pulp's crystal type was cellulose II with a crystallinity of 54.05%. The Fock method was used to determine the pulp's reactivity, which was 73.51%.

[0067] Comparative Example 6

[0068] The difference from Example 1 is that the sodium hydroxide solution concentration is 20%, and there is no freeze-thaw process.

[0069] X-ray diffraction analysis revealed that the pulp was of cellulose type II with a crystallinity of 49.84%. The reactivity of the pulp was 72.95% as determined by the Fock method.

[0070] The experimental data of Examples 1-8 and Comparative Examples 1-6 are summarized in Table 1.

[0071] Table 1. Summary of experimental data for Examples 1-8 and Comparative Examples 1-6

[0072] Alkali concentration Crystallinity (CrI / %) Crystal form Reactivity (%) Example 1 0.5wt% 80.03 Ⅰ 42.18 Example 2 1wt% 78.51 Ⅰ 44.07 Example 3 2wt% 71.26 Ⅱ 76.33 Example 4 4wt% 62.18 Ⅱ 77.43 Example 5 6wt% 53.19 Ⅱ 81.78 Example 6 8wt% 48.55 Ⅱ 84.21 Example 7 9wt% 52.73 Ⅱ 80.92 Example 8 10wt% 52.15 Ⅱ 79.31 Comparative Example 1 0wt% 80.12 Ⅰ 40.86 Comparative Example 2 4wt% 78.15 Ⅰ 50.85 Comparative Example 3 8wt% 62.06 Ⅰ 54.49 Comparative Example 4 12wt% 68.78 Ⅰ 56.97 Comparative Example 5 16wt% 54.05 Ⅱ 73.51 Comparative Example 6 20wt% 49.84 Ⅱ 72.95

[0073] As can be seen from the above examples and comparative examples, the method proposed in this invention uses a low-concentration alkaline aqueous solution as a medium, and through freeze-thaw treatment, it can significantly reduce the alkali concentration required for pulp crystal transformation. Compared with traditional high-concentration alkali treatment (above 16%), which is required to significantly improve the reactivity of pulp, this method can significantly improve the reactivity of pulp at a lower alkali concentration.

[0074] The technical features in the claims and / or specification of this invention can be combined, and the combination is not limited to the combinations obtained through reference in the claims. Technical solutions obtained by combining the technical features in the claims and / or specification are also within the scope of protection of this invention.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for improving the reactivity of pulp through freeze-thaw pretreatment, characterized in that, Includes the following steps: The pulp is soaked in an alkaline aqueous solution of a certain concentration, then the mixture is frozen into a solid at a low temperature, and finally heated to melt it, resulting in highly reactive pulp.

2. The method according to claim 1, characterized in that, The pulp is at least one of wood pulp, hemp pulp, and cotton pulp.

3. The method according to claim 1, characterized in that, The alkali mentioned is a strong alkali containing hydroxide ions, including but not limited to sodium hydroxide and potassium hydroxide.

4. The method according to claim 1, characterized in that, The mass concentration of the alkaline aqueous solution is 0.5% to 10%.

5. The method according to claim 1, characterized in that, The freezing temperature is between -5℃ and -45℃.

6. A method for improving the reactivity of pulp through freeze-thaw pretreatment, characterized in that, It is prepared by any one of the preparation methods described in claims 1 to 5.

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