A method for preparing cellulose-based carbon fiber

Abstract

The invention discloses a method for preparing cellulose-based carbon fibers, which comprises the steps of preparing a spinning solution, obtaining a precursor fiber, and then carbonizing the finished product. The preparation of the spinning solution comprises the derivatization and modification process of cellulose, specifically, dissolving cellulose in a solvent, heating the solution to obtain cellulose phosphonate, and then adding olefin nitrile to obtain a cyanated cellulose phosphonate spinning solution, wherein the solvent comprises a reactive phosphate ionic liquid. The invention is characterized in that cellulose phosphonate is derived from cellulose in a phosphate ionic liquid, and the cellulose phosphonate is further cyanated, thereby changing the thermal decomposition pathway of the cellulose precursor fiber, reducing carbon-containing volatiles and structural defects, and increasing the yield and mechanical properties of the cellulose-based carbon fibers.

Description

Technical Field

[0001] The invention relates to the field of carbon fiber preparation, and in particular to a method for preparing cellulose-based carbon fiber. Background Art

[0002] Cellulose is the oldest known carbon fiber precursor material. Although significant research was conducted on cellulose precursors in the 1950-1970s, research in this area was shelved due to disadvantages such as high production costs and low yields, as well as the emergence of polyacrylonitrile (PAN) precursors. However, carbon fibers derived from petroleum-based raw materials such as pitch or polyacrylonitrile are disadvantageous in terms of sustainability and resource conservation. Today, there is a renewed interest in the production of carbon fibers from renewable raw materials such as lignin or cellulose, but naturally grown cellulose fibers are not suitable for the production of carbon fibers due to the impurities, porous structure and low degree of orientation of lignin and hemicellulose. In contrast, cellulose fibers produced by the viscose or lyocell process and by ionic liquid technology have a well-defined fiber diameter and high purity, and when processed into continuous tows, they are considered promising precursors for high-performance carbon fibers.

[0003] One challenge of cellulose-based carbon fibers is that the maximum theoretical carbon yield of cellulose carbonization is 44.4 wt%, corresponding to the loss of five water molecules per anhydrous glucose repeating unit. However, due to degradation reactions and the formation of volatile carbon-containing compounds, the total mass loss can be as high as 90%, and the mechanical and morphological properties of the resulting carbon fibers are directly affected. Although other strategies have been developed to achieve higher carbonization yields of cellulose, such as low heating rates during pyrolysis, oxidative pretreatment, blending with other precursor materials and the application of carbonization aids, including dehydration catalysts such as ammonium phosphate and sulfate, the resulting carbon fibers have poor performance. This is because there are phase separation and compatibility issues of the polymers, as well as uneven distribution of carbonization aids in the fiber material (fiber diameter, crystalline / amorphous regions), resulting in defects in the precursor fibers, accompanied by crystallization or agglomeration of these compounds during the drying process, which may also cause damage to the carbon fiber structure. These shortcomings can be overcome if the reactive components are fixed by chemical functionalization of cellulose. Summary of the invention

[0004] To solve the above problems, the purpose of the present invention is to provide a method for preparing cellulose-based carbon fibers, specifically to provide a method for deriving and modifying cellulose precursors on a molecular scale, thereby ultimately improving the yield and performance of cellulose-based carbon fibers.

[0005] The present invention is achieved through the following technical solutions:

[0006] A method for preparing cellulose-based carbon fibers includes, in sequence, the preparation of a spinning solution, obtaining a precursor fiber, and then carbonizing to obtain a finished product. The preparation of the spinning solution includes a cellulose derivatization and modification process, specifically, dissolving cellulose in a solvent, heating to react to obtain cellulose phosphonate, and then adding olefin-based nitrile to react to obtain a cyanated cellulose phosphonate spinning solution. The solvent includes a reactive phosphate ionic liquid, such as 1,3-dimethylimidazolium methylphosphite ([DMIM]MHP), 1-ethyl-3-methylimidazolium methylphosphite ([EMIM]MHP), etc.; the olefin-based nitrile includes acrylonitrile and crotononitrile, etc.

[0007] Different from the prior art, the present invention uses reactive phosphate ionic liquid as a solvent to dissolve cellulose to derive cellulose phosphonate, which is further cyanated to change the thermal decomposition pathway of the cellulose precursor fiber, reduce carbon-containing volatiles and structural defects, and increase the yield and mechanical properties of cellulose-based carbon fibers.

[0008] The solvents are reactive phosphate ionic liquid and N,N-dimethylacetamide (DMAc).

[0009] The volume ratio of the reactive phosphate ionic liquid to N,N-dimethylacetamide DMAc is 70-95:5-30.

[0010] The heating reaction temperature is 80-150°C.

[0011] The heating reaction temperature was 120°C.

[0012] The carbonization process to obtain the finished product includes: (1) gradually and slowly heating the precursor fiber to 200-300°C for oxidation stabilization, and (2) carbonizing at 800-1500°C.

[0013] Oxidative stabilization is performed by heating the precursor fiber to 250°C at a rate of 1°C / min in an air atmosphere and maintaining the temperature for 2 hours for thermal stabilization.

[0014] The carbonization step is as follows: the precursor fiber is heated to 1200° C. at a rate of 2° C. / min in an inert atmosphere and maintained for 1 hour for carbonization.

[0015] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0016] The invention is characterized in that cellulose is derived into cellulose phosphonate in phosphate ionic liquid, and the cellulose phosphonate is further cyanated, thereby changing the thermal decomposition pathway of cellulose precursor fibers, reducing carbon-containing volatiles and structural defects, and increasing the yield and mechanical properties of cellulose-based carbon fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings described herein are used to provide a further understanding of the embodiments of the present invention, constitute a part of this application, and do not constitute a limitation of the embodiments of the present invention. In the drawings:

[0018] Figure 1 is a reactive phosphate ionic liquid 1,3-dimethylimidazolium methylphosphite [DMIM] MHP and (b) a non-reactive phosphate ionic liquid 1,3-dimethylimidazolium dimethylphosphate [DMIM] DMP;

[0019] Figure 2 It is a modification process of cellulose precursor. DETAILED DESCRIPTION

[0020] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with embodiments and drawings. The exemplary embodiments of the present invention and their description are only used to explain the present invention and are not intended to limit the present invention.

[0021] Example 1

[0022] (1) 10 g of cellulose was dissolved in 100 mL of a mixed solvent of [DMIM]MHP / DMAc (95 / 5, v / v), heated to 120° C., and reacted for 1 hour to obtain cellulose phosphonate, and then 3 g of acrylonitrile was added and the reaction was continued for 1 hour to obtain a cyanoethylated cellulose phosphonate spinning solution;

[0023] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0024] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0025] The yield of the obtained cellulose-based carbon fibers was 40.1% and the tensile strength was 1.8 GPa.

[0026] When the volume ratio of [DMIM]MHP in the mixed solvent is relatively high, the final carbon fiber yield is higher but the strength is lower.

[0027] Example 2

[0028] (1) 10 g of cellulose was dissolved in 100 mL of a mixed solvent of [DMIM]MHP / DMAc (90 / 10, v / v), heated to 120° C., and reacted for 1 hour to obtain cellulose phosphonate, and then 3 g of acrylonitrile was added and the reaction was continued for 1 hour to obtain a cyanoethylated cellulose phosphonate spinning solution;

[0029] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0030] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0031] The yield of the obtained cellulose-based carbon fibers was 38.5% and the tensile strength was 2.3 GPa.

[0032] When the volume ratio of [DMIM]MHP in the mixed solvent is 90%, the final carbon fiber yield is higher and the strength is the highest.

[0033] Example 3

[0034] (1) 10 g of cellulose was dissolved in 100 mL of a mixed solvent of [DMIM]MHP / DMAc (80 / 20, v / v), heated to 120° C., and reacted for 1 hour to obtain cellulose phosphonate, and then 3 g of acrylonitrile was added and the reaction was continued for 1 hour to obtain a cyanoethylated cellulose phosphonate spinning solution;

[0035] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0036] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0037] The yield of the obtained cellulose-based carbon fibers was 37.2% and the tensile strength was 2.0 GPa.

[0038] When the volume ratio of [DMIM]MHP in the mixed solvent is low, the final carbon fiber yield and strength will decrease.

[0039] Example 4

[0040] (1) 10 g of cellulose was dissolved in 100 mL of a mixed solvent of [DMIM]MHP / DMAc (70 / 30, v / v), heated to 120° C., and reacted for 1 hour to obtain cellulose phosphonate, and then 3 g of acrylonitrile was added and the reaction was continued for 1 hour to obtain a cyanoethylated cellulose phosphonate spinning solution;

[0041] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0042] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0043] The yield of the obtained cellulose-based carbon fibers was 36.4% and the tensile strength was 1.7 GPa.

[0044] When the volume ratio of [DMIM]MHP in the mixed solvent is low, the final carbon fiber yield and strength will decrease.

[0045] Example 5

[0046] (1) 10 g of cellulose was dissolved in 100 mL of a solvent [DMIM]MHP, heated to 120° C., and reacted for 1 hour to obtain cellulose phosphonate, and then 3 g of acrylonitrile was added and the reaction was continued for 1 hour to obtain a cyanoethylated cellulose phosphonate spinning solution;

[0047] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0048] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0049] The yield of the obtained cellulose-based carbon fibers was 34.7% and the tensile strength was 1.4 GPa.

[0050] When the cellulose precursor was treated with only the reactive solvent [DMIM]MHP, the yield and strength of the carbon fibers were lower than those of the example using the mixed solvent.

[0051] Comparative Example 1

[0052] (1) dissolving 10 g of cellulose in 100 mL of a mixed solvent of [DMIM]MHP / DMAc (90 / 10, v / v), heating to 120° C., and reacting for 1 hour to obtain a cellulose phosphonate spinning solution;

[0053] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0054] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0055] The yield of the obtained cellulose-based carbon fibers was 33.3% and the tensile strength was 1.2 GPa.

[0056] When the precursor cellulose is not cyanoethylated, the final carbon fiber yield and strength are low.

[0057] Comparative Example 2

[0058] (1) dissolving 10 g of cellulose in 100 mL of a solvent [DMIM]MHP, heating to 120° C., and reacting for 1 hour to obtain a cellulose phosphonate spinning solution;

[0059] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0060] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0061] The yield of the obtained cellulose-based carbon fibers was 30.5% and the tensile strength was 1.0 GPa.

[0062] When the cellulose precursor is treated with only the reactive solvent [DMIM]MHP and the precursor cellulose is not cyanoethylated, the yield and strength of the final carbon fibers are greatly reduced.

[0063] Comparative Example 3

[0064] (1) Dissolving 10 g of cellulose in 100 mL of 1,3-dimethylimidazolium dimethyl phosphate [DMIM] DMP solvent and heating to 120° C., no reaction occurs, and the cellulose is dissolved to obtain a spinning solution;

[0065] (2) coagulating the spinning solution into fibers in ethanol by dry-wet spinning, and obtaining precursor fibers by stretching, washing, and drying;

[0066] (3) The precursor fiber was heated to 250°C at a rate of 1°C / min in an air atmosphere and maintained for 2 hours for thermal stabilization. The precursor fiber was further heated to 1200°C at a rate of 2°C / min in an inert atmosphere and maintained for 1 hour for carbonization, and finally a cellulose-based carbon fiber was obtained.

[0067] The yield of the obtained cellulose-based carbon fibers was 20.3% and the tensile strength was 0.5 GPa.

[0068] When the cellulose precursor is treated with non-reactive ionic liquid [DMIM] DMP and the cellulose precursor is not cyanoethylated, the final carbon fiber yield and strength are the lowest.

[0069] Table 1 shows the results of the above experiments. Figure 1 The difference between reactive and non-reactive ionic liquids is demonstrated. Figure 2 The modification process of cellulose precursor is demonstrated.

[0070] Table 1 Effects of different solvents and modified monomers on the strength and yield of cellulose-based carbon fibers

[0071]

[0072] As can be seen from Table 1, different solvent ratios and modified monomers have a great influence on the strength and yield of cellulose-based carbon fibers. As can be seen from Examples 1-4, as the volume ratio of [DMIM]MHP decreases, the yield of carbon fibers decreases and the strength is also affected, but when the volume ratio of [DMIM]MHP is 90%, the strength of carbon fibers is the highest. As can be seen from Examples 1-5 and Comparative Examples 1-2, when only the reactive solvent [DMIM]MHP is used to treat the cellulose precursor, or the cellulose precursor is not cyanoethylated, or only the reactive solvent [DMIM]MHP is used to treat the cellulose precursor without cyanoethylation, the yield and strength of carbon fibers will decrease.

[0073] From Examples 1-5 and Comparative Example 3, it can be seen that when the non-reactive ionic liquid [DMIM] DMP is used to treat the cellulose precursor without cyanidation treatment, the yield and strength of the carbon fiber are the lowest (20.3% and 0.5 GPa). The above results also show that the use of reactive solvents and cyanidation treatment has a synergistic effect on the yield and strength of carbon fiber.

[0074] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing cellulose-based carbon fiber, comprising the steps of preparing a spinning solution, obtaining a precursor fiber, and carbonizing the fiber to obtain a finished product. It is characterized in that The preparation of the spinning solution includes the derivatization and modification process of cellulose, specifically: dissolving cellulose in a solvent, heating the solution to obtain cellulose phosphonate, and then adding olefin nitrile to obtain cyanide cellulose phosphonate spinning solution, wherein the solvent includes reactive phosphate ionic liquid 1,3-dimethylimidazole methyl phosphite, and the olefin nitrile is acrylonitrile.

2. The preparation method according to claim 1, It is characterized in that The solvents are reactive phosphate ionic liquid and N,N-dimethylacetamide (DMAc).

3. The preparation method according to claim 2, It is characterized in that The volume ratio of the reactive phosphate ionic liquid to N,N-dimethylacetamide DMAc is 70-95:5-30.

4. The preparation method according to claim 1, It is characterized in that The heating reaction temperature is 80-150°C.

5. The preparation method according to claim 4, It is characterized in that The heating reaction temperature was 120°C.

6. The preparation method according to claim 1, It is characterized in that The carbonization process to obtain the finished product includes: (1) gradually and slowly heating the precursor fiber to 200-300°C for oxidation stabilization, and (2) carbonizing at 800-1500°C.

7. The preparation method according to claim 6, It is characterized in that The oxidation stabilization was performed by heating the precursor fiber to 250° C. at a rate of 1° C. / min in an air atmosphere and maintaining the temperature for 2 hours for thermal stabilization.

8. The preparation method according to claim 6, It is characterized in that The carbonization step is as follows: the precursor fiber is heated to 1200° C. at a rate of 2° C. / min in an inert atmosphere and maintained for 1 hour for carbonization.

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