Method for dissolving cellulose in ternary system, cellulose solution, regenerated cellulose and preparation method of regenerated cellulose

Abstract

The invention relates to the field of high polymer materials, and discloses a method for dissolving cellulose in a ternary system, a cellulose solution of the method, regenerated cellulose and a preparation method of the regenerated cellulose. The method comprises the following steps: dissolving N-methylmorpholine-N-oxide in water, mixing with gamma-valerolactone, heating and stirring; adding cellulose, and uniformly stirring to obtain a cellulose suspension; after reducing the water content, heating and stirring until the cellulose is completely dissolved to obtain a cellulose solution; the cellulose solution is subjected to film casting or spinning, is solidified and regenerated in solidification liquid, and is washed and dried to obtain regenerated cellulose. The average cellulose dissolution rate can reach 99.1%, and the highest cellulose dissolution rate can reach 99.7%. The content of NMMO degradation products is obviously reduced, and the minimum content of N-methylmorpholine and the minimum content of morpholine are only 0.66 g / kg and 0.05 g / kg respectively. The method is widely applicable to various cellulose raw materials, and the dissolution rate is 98% or above.

Description

Technical Field

[0001] The invention relates to the field of polymer materials, and more specifically to a method for dissolving cellulose using a ternary system, a cellulose solution thereof, and regenerated cellulose and a preparation method thereof. Background Art

[0002] Cellulose is the most widely distributed, abundant, renewable and biodegradable polysaccharide in nature. Nature can provide about 100 billion tons of cellulose each year, accounting for more than 50% of the carbon content in the plant kingdom. It is the most valuable natural renewable resource for mankind. Cellulose chemistry and industry began more than 160 years ago and was the main research object during the birth and development of polymer chemistry. The study of cellulose has made a significant contribution to the establishment and development of polymer physical and mechanical chemistry.

[0003] At room temperature, cellulose is neither soluble in water nor in general organic solvents, such as alcohol, ether, acetone, benzene, etc. Therefore, cellulose is stable at room temperature because hydrogen bonds exist within and between cellulose molecules, which makes cellulose molecules polar, making it difficult for solvent molecules to bond with cellulose macromolecules. As a highly polar, hydrogen-bond acceptor solvent, N-methylmorpholine-N-oxide (NMMO) can form an excellent solvent system for cellulose with water, and is also the most successful cellulose solvent system in industrialization and the highest product added value. However, due to the large molecular weight of cellulose, the molecular bonding ability of NMMO and its molecular bonding ability with cellulose hydroxyl groups is very strong, so the obtained cellulose solution often has a high viscosity and poor rheological properties, so the dissolution temperature is high and the dissolution time is long; this leads to severe degradation of NMMO, and the generated N-methylmorpholine and morpholine are easy to mix with air in the gas phase and explode, so it is necessary to add antioxidants quantitatively to prevent NMMO degradation and safety accidents, which leads to the high cost of cellulose dissolution in this system.

[0004] Gamma-Valerolactone (GVL) is an organic compound that is a colorless to pale yellow liquid with an aroma similar to vanilla or cream. Due to its good solubility and relatively low toxicity, gamma-valerolactone is often used as an industrial solvent for various chemical reactions and material extraction. Compared with other traditional organic solvents, such as dimethyl sulfoxide (DMSO) or N,N-dimethylformamide (DMF), gamma-valerolactone has lower toxicity, which makes it safer for use in food, cosmetics and pharmaceuticals. Gamma-valerolactone shows good stability during heating and can be used in various chemical environments. It is not easily decomposed or reacted even at high temperatures.

[0005] Prior art CN116120597A adds cellulose raw material to NMMO alcohol solution for activation and swelling, and then dissolves cellulose rapidly at a relatively low temperature under vacuum, heating and stirring conditions. Although the dissolution rate is increased by alcohol organic solvents, there are still problems such as high NMMO decomposition rate and high dissolution temperature, and the degradation damage to cellulose is also relatively large. Prior art CN103774479B uses a mixed solvent obtained by mixing NMMO and DMSO to dissolve the cellulose contained in the pretreated biomass powder in the process of preparing biomass cellulose, but DMSO has a large damage to cellulose degradation, and is highly corrosive to equipment, and the dissolution process is complicated and inefficient. Prior art CN108822304A uses a binary solvent composed of γ-valerolactone and water to grade lignin with high molecular weight dispersibility to obtain different molecular weight lignin fractions with high molecular weight uniformity, without involving the dissolution of cellulose. Summary of the invention

[0006] The present invention provides a method for dissolving cellulose with a ternary system to overcome the defects of the prior art in the dissolution process, such as high NMMO decomposition rate, high dissolution temperature, large degradation damage to cellulose, high viscosity of the obtained cellulose solution and poor rheological properties.

[0007] Another object of the present application is to provide a cellulose solution;

[0008] Another object of the present application is to provide a method for preparing regenerated cellulose;

[0009] Another object of the present application is to provide a regenerated cellulose.

[0010] Further reduce the degradation degree of NMMO and improve the rheological properties of cellulose solution. This patent aims to provide excellent cellulose solution for the subsequent preparation of Lyocell fiber and cellulose membrane, which will have broad application prospects.

[0011] In order to solve the above technical problems, the technical solution of the present invention is as follows:

[0012] A ternary system for dissolving cellulose, characterized in that it includes N-methylmorpholine-N-oxide, water, and γ-valerolactone; preferably, the mass ratio of γ-valerolactone to N-methylmorpholine-N-oxide is 0.05-0.5; preferably, the ternary system is obtained by dissolving N-methylmorpholine-N-oxide in water, mixing with γ-valerolactone, and heating and stirring.

[0013] A method for dissolving cellulose using a ternary system, comprising the following steps:

[0014] S1, dissolving N-methylmorpholine-N-oxide in water, mixing with γ-valerolactone, and heating and stirring;

[0015] S2, adding cellulose and stirring evenly to obtain a cellulose suspension;

[0016] S3. After reducing the water content, heating and stirring are performed until the cellulose is completely dissolved to obtain a cellulose solution.

[0017] Preferably, the water content is reduced using evaporation under reduced pressure.

[0018] Furthermore, the mass ratio of γ-valerolactone to N-methylmorpholine-N-oxide in S1 is 0.05 to 0.5.

[0019] Furthermore, after the water content of S3 is reduced, the ratio of the mass of water to the total mass of N-methylmorpholine-N-oxide and water is 13% to 15%.

[0020] Furthermore, the temperature of heating and stirring in S3 is 70 to 120°C.

[0021] Furthermore, in the cellulose solution obtained in S3, the mass fraction of cellulose is 1% to 15%.

[0022] Furthermore, the raw material of the cellulose comes from cotton fiber or dissolving pulp.

[0023] A cellulose solution is prepared by the method of dissolving cellulose using the ternary system.

[0024] The invention discloses a method for preparing regenerated cellulose. The cellulose solution is subjected to film casting or spinning, and after coagulation and regeneration in a coagulation liquid, the regenerated cellulose is washed and dried to obtain the regenerated cellulose.

[0025] A regenerated cellulose is prepared by the regenerated cellulose preparation method.

[0026] Preferably, the regenerated cellulose comprises Lyocell fiber and cellulose film.

[0027] The combined effect of NMMO and GVL effectively promoted the dissolution of cellulose. The addition of GVL significantly reduced the viscosity of the NMMO / H2O system. The reduced viscosity helps the solute molecules move more freely, thereby promoting the dissociation and dissolution of cellulose molecules. In a solution with lower viscosity, cellulose molecules are more likely to diffuse and fully contact with solvent molecules, thereby accelerating the dissolution process and reducing the required dissolution temperature. The addition of GVL improves the rheological properties of the NMMO / H2O system, allowing the solution to maintain good fluidity and stability at lower temperatures. This property makes the distribution of cellulose molecules in the solution more uniform, reduces the local overheating that may occur during the dissolution process, and thus reduces the dissolution temperature overall. GVL can stabilize the NMMO solution and reduce the degradation of NMMO at high temperatures through its polar aprotic properties. The degradation of NMMO will generate some byproducts (such as N-methylmorpholine and morpholine), which may react adversely with cellulose molecules and reduce the dissolution efficiency. The addition of GVL reduces the dissolution temperature and reduces the degradation of NMMO, thereby improving the efficiency and stability of the dissolution process.

[0028] As a hydrogen bond acceptor, NMMO can form hydrogen bonds with the hydroxyl groups in cellulose, destroying the hydrogen bonds between and within cellulose molecules. The addition of GVL significantly reduces the viscosity of the dissolving solution, making it easier for cellulose molecules to move and dissolve in the solution. GVL lowers the required dissolution temperature, reduces the thermal degradation of NMMO, and improves the stability and safety of the dissolution process. NMMO dissolves cellulose by destroying the hydrogen bonds of cellulose, while GVL optimizes the dissolution conditions by reducing the viscosity and melting point of the dissolving solution. The synergistic effect of the two ensures efficient dissolution of cellulose at lower temperatures, reduces the degradation products of NMMO, and improves the dissolution efficiency and safety.

[0029] Compared with the prior art, the technical solution of the present invention has the following beneficial effects:

[0030] 1. High cellulose dissolution rate: The present invention utilizes the polar aprotic properties of GVL and adds it to the NMMO / H2O solvent system to significantly reduce the viscosity, melting point, and freezing point of the cellulose solution, and significantly shorten the cellulose dissolution time, which not only improves the rheological properties of the cellulose solution, but also greatly increases the cellulose dissolution rate. The average cellulose dissolution rate of the present invention can reach 99.1%, and the highest can reach 99.7%.

[0031] 2. Reduce NMMO decomposition: GVL is a polar aprotic solvent with no proton donor in its molecular structure, which avoids excessive interaction with the hydroxyl group in NMMO and reduces the degradation of NMMO. As the amount of GVL increases, the required dissolution temperature gradually decreases. The stabilizing effect of GVL on NMMO and the decrease in dissolution temperature significantly reduce the content of NMMO degradation products in the solution. The minimum contents of N-methylmorpholine and morpholine are only 0.66g / kg and 0.05g / kg, respectively. This is conducive to improving the safety and stability of production and reducing the problem of increased production costs caused by NMMO degradation.

[0032] 3. Widely applicable to various cellulose raw materials: The method for dissolving cellulose by the ternary system of the present invention can be used to dissolve cotton pulp, coniferous wood dissolving pulp, broadleaf wood dissolving pulp and bamboo dissolving pulp. Although the cellulose dissolution rates of different types of raw materials are slightly different, the overall dissolution rates are all above 98%, indicating that the method for dissolving cellulose by the ternary system of the present invention is widely applicable to various cellulose raw materials. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with specific examples, but the examples do not limit the present invention in any form. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.

[0034] Unless otherwise specified, the reagents and materials used in the following examples are commercially available.

[0035] Example 1

[0036] 1. Ternary system dissolving cellulose

[0037] (1) Accurately weighing 391.5 g of a 50% NMMO aqueous solution and 25 g of GVL, adding them to a dissolution tank, heating (60° C.) and stirring evenly, the mass ratio of GVL to NMMO being 0.13, to obtain a ternary system for dissolving cellulose;

[0038] (2) adding a cellulose sample (hardwood dissolving pulp, 30 g absolute dry) into the above solution and continuing to stir to obtain a cellulose suspension;

[0039] (3) The dissolution tank was decompressed and evaporated to remove the water of the solvent system until the NMMO: (NMMO + H2O) was accurately measured to be 13.4%. At the same time, the mixture was heated to 90°C and stirred until the cellulose was completely dissolved (about 2.5 hours). At this time, the cellulose content was 10.5%.

[0040] 2. Effect detection

[0041] 50 mL of the dissolved cellulose solution was taken, filtered at high temperature, and then subjected to two-step acid hydrolysis and ion chromatography to determine the content of the insoluble components. The dissolution rate of the dissolved pulp added in this system was calculated. The contents of N-methylmorpholine and morpholine, the main degradation products of NMMO in this reaction system, were measured by capillary electrophoresis.

[0042] 3. Preparation of regenerated cellulose

[0043] The cellulose solution is cast into a film or spun, coagulated and regenerated in a coagulation liquid, and then washed and dried to obtain regenerated cellulose, which are cellulose film and Lyocell fiber respectively.

[0044] Embodiments 2 to 6

[0045] W*，％ G* Cellulose category Cellulose concentration*, % Dissolving temperature, °C Example 2 13.1 0.1 Cotton pulp 10 100 Example 3 14.5 0.1 Softwood Dissolving Pulp 10 105 Example 4 15.0 0.2 Hardwood dissolving pulp 10 90 Example 5 13.3 0.05 Bamboo dissolving pulp 10 120 Example 6 14.6 0.5 Softwood Dissolving Pulp 1 70 Example 7 13.0 0.2 Softwood Dissolving Pulp 15 95

[0046] *W is the ratio of water to NMMO aqueous solution; G is the mass ratio of GVL to NMMO; cellulose concentration is the mass fraction of cellulose after reduced pressure evaporation.

[0047] Comparative Example 1

[0048] The technical solution of Comparative Example 1 is similar to that of Example 1, except that GVL is replaced by an equal amount of water.

[0049] Comparative Example 2

[0050] The technical solution of Comparative Example 2 is similar to that of Example 1, except that GVL is replaced by DMSO.

[0051] Comparative Example 3

[0052] The technical solution of Comparative Example 3 is similar to that of Example 1, except that GVL is replaced by ethanol.

[0053] Comparative Example 4

[0054] The technical solution of Comparative Example 4 is similar to that of Example 1, except that the mass ratio of GVL to NMMO is 0.03 and the dissolution temperature is 140°C.

[0055] Comparative Example 5

[0056] The technical solution of Comparative Example 4 is similar to that of Example 1, except that the mass ratio of GVL to NMMO is 0.8 and the dissolution temperature is 60°C.

[0057] Results Analysis

[0058] Table 1 Cellulose dissolution rate and NMMO degradation product content

[0059] N-methylmorpholine, g / kg Morpholine, g / kg Cellulose dissolution rate, % Example 1 1.9 0.26 98.3 Example 2 1.2 0.11 99.7 Example 3 1.9 0.16 98.5 Example 4 0.91 0.09 99.3 Example 5 5.3 0.65 98.8 Example 6 0.66 0.05 99.4 Example 7 1.07 0.10 97.9 Comparative Example 1 5.7 0.75 68.6 Comparative Example 2 6.0 0.70 77.8 Comparative Example 3 6.3 0.72 63.2 Comparative Example 4 5.5 0.68 81.3 Comparative Example 5 1.5 0.22 89.4

[0060] From the data in Table 1, it can be seen that the highest dissolution rate in the examples is 99.7% (Example 2), the lowest dissolution rate is 97.9% (Example 7), and the cellulose dissolution rates of most examples are above 98%, which fully demonstrates the effectiveness of the ternary system cellulose dissolution method of the present invention. The high dissolution rate is due to the combined effect of NMMO and GVL, which effectively destroys the hydrogen bonds between cellulose molecules and fully dissolves the cellulose molecules.

[0061] The main degradation products of NMMO are N-methylmorpholine and morpholine, and the proportion of GVL has a significant effect on the stability of NMMO and the cellulose dissolution performance. The data in Table 1 show that the degradation product contents of Examples 4 and 6 are relatively low, especially in Example 6, where the contents of N-methylmorpholine and morpholine are 0.66 g / kg and 0.05 g / kg, respectively, indicating that at a higher proportion of GVL and a lower temperature, the degradation product content of NMMO can be significantly reduced while maintaining a higher cellulose dissolution rate. As a polar aprotic solvent, GVL can effectively stabilize NMMO molecules, reduce their degradation at high temperatures, and effectively reduce the viscosity and melting point of the cellulose dissolving solution, thereby increasing the cellulose dissolution rate. In Example 5, the proportion of GVL is relatively low, and the cellulose dissolution rate is still relatively high, but it will lead to the degradation of NMMO at high temperatures, resulting in an increase in the contents of N-methylmorpholine and morpholine.

[0062] Temperature has an important influence on the degradation of NMMO and the dissolution of cellulose. Under the ternary system of the present invention, low temperature (Example 6) helps to reduce the degradation of NMMO while maintaining a high cellulose dissolution rate. However, although high temperature (Example 5) can accelerate the dissolution of cellulose, it will also accelerate the degradation of NMMO, resulting in an increase in the content of N-methylmorpholine and morpholine. It can be seen that temperature control is crucial for improving dissolution efficiency and reducing degradation products. In Examples 2 and 3, when the ratio of GVL to NMMO is the same, the dissolution temperature increases from 100°C to 105°C, resulting in an increase in the content of N-methylmorpholine and morpholine (from 1.2g / kg and 0.11g / kg to 1.9g / kg and 0.16g / kg), while the cellulose dissolution rate decreases slightly (from 99.7% to 98.5%). This shows that even slight temperature changes can significantly affect the degradation of NMMO and the dissolution of cellulose.

[0063] In the examples, as the amount of GVL increases, the required dissolution temperature gradually decreases. The stabilizing effect of GVL on NMMO and the decrease in the dissolution temperature significantly reduce the content of NMMO degradation products (N-methylmorpholine and morpholine) in the solution, which is beneficial to improving the safety and stability of production and reducing the problem of increased production costs caused by NMMO degradation.

[0064] GVL is a polar aprotic solvent with no proton donor in its molecular structure, which avoids excessive interaction with the hydroxyl groups in NMMO and reduces the degradation of NMMO. GVL effectively reduces the viscosity of the cellulose solution, improves the rheological properties of the solution, and helps the uniform dissolution of cellulose. At lower temperatures, GVL can still maintain high solubility performance while significantly reducing the degradation products of NMMO. In Comparative Example 1, GVL was not added, and NMMO and GVL could not play a synergistic role. The cellulose dissolution rate was only 68.6%, which was 31.2% lower than that of Example 2.

[0065] When GVL is replaced by DMSO (Comparative Example 2), DMSO is a highly polar solvent that can effectively dissolve cellulose, but its proton donor property is strong and it is easy to interact with the hydroxyl group in NMMO. At high temperatures, DMSO will produce by-products, affecting the stability of the solution system and the dissolution efficiency of cellulose, and the dissolution rate is less than 77.8%.

[0066] In Comparative Example 3, the monohydroxyl functional group of ethanol cannot effectively reduce the degradation of NMMO, and has less interaction with NMMO, causing NMMO to be more easily degraded and produce a large amount of by-products, with N-methylmorpholine and morpholine contents of 6.3 g / kg and 0.72 g / kg, respectively. The increase in by-product content not only makes the subsequent purification of the cellulose solution more difficult, increases the cost and time of the purification step when preparing regenerated cellulose, but also the degradation products may remain in the regenerated cellulose, affecting its mechanical properties and chemical stability. At the same time, at lower concentrations, the dissolving power of ethanol is insufficient, resulting in extremely low dissolution rate and stability, with a cellulose dissolution rate of only 63.2%.

[0067] GVL can effectively reduce the melting point and solidification point of the solution, thereby maintaining good solubility at a relatively low temperature. In comparative example 4, when the mass ratio of GVL to NMMO is too low, the viscosity of the solution is high, and cellulose dissolution requires a higher temperature. A higher dissolution temperature will aggravate the degradation of NMMO, resulting in an increase in degradation products and a reduction in dissolution efficiency, which is only 81.3%. In comparative example 5, the excessively high proportion of GVL will increase the viscosity of the solution, making it more difficult to stir the solution evenly, thereby affecting the dissolution efficiency, and the cellulose dissolution rate is reduced to 89.4%. Higher viscosity will also affect subsequent spinning or film-forming processes. In addition, the temperature may be too low to provide enough heat energy to overcome the hydrogen bonds between cellulose molecules, thereby slowing down the dissolution process, resulting in the inability of cellulose to be completely dissolved, and undissolved cellulose particles may exist in the final solution. Incompletely dissolved cellulose is prone to forming an uneven structure in the subsequent regeneration process, resulting in a decrease in the mechanical properties (such as strength and toughness) of the regenerated cellulose.

[0068] By rationally controlling the mass ratio of GVL to NMMO and the dissolution temperature, the cellulose dissolution rate can be maximized and the degradation products of NMMO can be reduced, thus achieving an efficient and stable cellulose dissolution process.

[0069] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the embodiments of the present invention. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the embodiments here. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the claims of the present invention.

Claims

1. A ternary system for dissolving cellulose, characterized in that: It comprises N-methylmorpholine-N-oxide, water and γ-valerolactone; preferably, the mass ratio of γ-valerolactone to N-methylmorpholine-N-oxide is 0.05-0.5; preferably, the ternary system is obtained by dissolving N-methylmorpholine-N-oxide in water, mixing with γ-valerolactone, and heating and stirring.

2. A method for dissolving cellulose using a ternary system, characterized in that: The following steps are involved: S1, dissolving N-methylmorpholine-N-oxide in water, mixing with γ-valerolactone, and heating and stirring; S2, adding cellulose and stirring evenly to obtain a cellulose suspension; S3. After reducing the water content, heating and stirring are performed until the cellulose is completely dissolved to obtain a cellulose solution.

3. The method for dissolving cellulose using a ternary system according to claim 2, characterized in that: The mass ratio of γ-valerolactone to N-methylmorpholine-N-oxide in S1 is 0.05 to 0.

5.

4. The method for dissolving cellulose using a ternary system according to claim 2, characterized in that: The temperature of heating and stirring in S3 is 70 to 120°C.

5. The method for dissolving cellulose using a ternary system according to claim 2, characterized in that: After the water content of S3 is reduced, the ratio of the mass of water to the total mass of N-methylmorpholine-N-oxide and water is 13% to 15%.

6. The method for dissolving cellulose using a ternary system according to claim 2, characterized in that: In the cellulose solution obtained in S3, the mass fraction of cellulose is 1% to 15%.

7. The method for dissolving cellulose using a ternary system according to claim 2, characterized in that: The raw material of the cellulose comes from cotton fiber or dissolving pulp.

8. A cellulose solution, characterized in that The cellulose dissolving method is prepared by the ternary system method according to any one of claims 2 to 7.

9. A method for preparing regenerated cellulose, characterized in that: The cellulose solution according to claim 8 is subjected to film casting or spinning, coagulated and regenerated in a coagulation liquid, and then washed and dried to obtain regenerated cellulose.

10. A regenerated cellulose, characterized in that: The regenerated cellulose is prepared by the method for preparing regenerated cellulose according to claim 9.