Carbon fiber converted from plant fiber to achieve densification modification and preparation method thereof

Abstract

The invention relates to a carbon fiber converted from plant fibers to achieve densification modification and a preparation method thereof, and belongs to the technical field of preparation of carbon fibers from plant fibers. The method comprises the following steps: soaking plant fibers in alkali liquor for boiling treatment, and then boiling the plant fibers in deionized water to obtain delignified plant fibers; immersing the delignified plant fiber into a Tris-Cl solution with dopamine hydrochloride, carrying out a modification reaction at room temperature, and washing the obtained fiber product in deionized water to obtain a modified plant fiber; fixing one end of the modified plant fiber, applying tension to the other end of the modified plant fiber, and naturally drying at room temperature to obtain dried modified plant fiber; and sequentially carrying out pre-oxidation and carbonization treatment on the dried and modified plant fiber to obtain the modified plant fiber-based carbon fiber. The method provided by the invention is short in period and low in cost, and the obtained carbon fiber has more excellent tensile strength.

Description

Technical Field

[0001] The invention belongs to the technical field of preparing carbon fibers from plant fibers, and particularly relates to a carbon fiber obtained by converting plant fibers to achieve densification modification and a preparation method thereof. Background Art

[0002] Carbon fiber is a high-performance fiber with carbon as the main component. It is famous for its structure that mainly contains carbon elements, and its carbon content is generally higher than 90%. This material is particularly favored in various industrial fields, such as aviation, aerospace, new energy, automobiles and sports equipment, because of its excellent specific strength, high specific modulus, stability to high temperatures, corrosion resistance, wear resistance and radiation resistance. At present, most of the carbon fibers on the market are made of organic polymer fibers (polyacrylonitrile fibers) by carbonization, which leads to relatively high manufacturing costs, thus limiting their application in the daily civilian market. In addition, polyacrylonitrile-based carbon fibers based on petrochemical products and their high pollution and high carbon emission production processes are also in urgent need of change. In the production cost of carbon fiber, the cost of precursor fibers accounts for a considerable part, about more than 40%. Therefore, if innovative and more environmentally friendly precursor fibers with lower costs can be developed, it will help to significantly reduce the overall production cost of carbon fiber and further promote its widespread application in the civilian field.

[0003] Compared with polyacrylonitrile and other synthetic polymers derived from non-renewable petrochemical products, cellulose-based natural fiber materials are regarded as carbon fiber precursors with great potential due to their high yield, renewability, and environmental friendliness. In fact, the earliest commercialized carbon fiber was converted from regenerated cellulose fiber (viscose fiber). Its excellent anti-ablation and thermal insulation properties make it a strategic resource that cannot be replaced by other types of carbon fiber. However, due to the low density of viscose-based carbon fiber and the poor orientation of graphite crystallites, it is difficult to improve the mechanical properties (tensile strength 0.4-0.8GPa), and its current output accounts for only about 1% of the world's total carbon fiber output. In the document “Cho D, Kim JM, Song IS, Hong I, Effect of alkali pre-treatment of jute on the formation of jute-based carbon fibers, MATER LETT 2011, 65: 1492-1494”, the authors used NaOH solution to treat natural jute fibers and carbonized the fibers in an inert environment. Although the alkali treatment process eliminated the pore structure characteristic of natural fibers that had an adverse effect on the mechanical properties of the fibers, the surface defects of the prepared carbon fibers were still significant, and the document did not report on their mechanical properties. The existing Chinese patent with application number CN201310105838.2 applies pre-oxidation treatment and fiber surface defect repair treatment to natural jute fibers while performing alkali treatment, thereby improving the carbon yield and mechanical properties of the fibers. However, this method still does not effectively solve the problem of poor mechanical properties of cellulose-based carbon fibers. Summary of the invention

[0004] In view of the above problems, the present invention proposes a method for preparing carbon fiber by converting plant fiber to achieve densification modification, which overcomes the problems of low carbon yield of cellulose-based carbon fiber produced by prior art, as well as poor mechanical properties due to low density and poor orientation of graphite crystallites.

[0005] The first object of the present invention is to provide a method for preparing carbon fiber by converting plant fiber to achieve densification modification, comprising:

[0006] The plant fiber is immersed in an alkali solution and boiled, and then boiled in deionized water to obtain delignified plant fiber;

[0007] The delignified plant fiber is immersed in a Tris-Cl solution containing dopamine hydrochloride, and a modification reaction is carried out at room temperature, and then the obtained fiber product is washed in deionized water to obtain a modified plant fiber;

[0008] Fix one end of the modified plant fiber, apply tension to the other end, and dry naturally at room temperature to obtain dry modified plant fiber;

[0009] The dried modified plant fiber is subjected to pre-oxidation and carbonization treatment in sequence to obtain modified plant fiber-based carbon fiber.

[0010] Further, the alkali solution is Na 2 SO 3 , NaOH and water to form a solution, wherein the boiling treatment time in the alkali solution is 2-12 hours.

[0011] Furthermore, in the alkali solution, Na 2 SO 3 The concentration of HCl is 0.5-1M, and the concentration of NaOH is 0.5-3M.

[0012] Furthermore, in the Tris-Cl solution containing dopamine hydrochloride, the dosage ratio of dopamine hydrochloride to Tris-Cl solution is 0.4-0.8 g:1 L, and the concentration of the Tris-Cl solution is 5-15 mM.

[0013] Furthermore, the pH of the modification reaction is 7.5 to 8.5, and the time of the modification reaction is 12-36 hours.

[0014] Furthermore, the pulling force is 5-30 MPa.

[0015] Further, the specific operation of the pre-oxidation is as follows:

[0016] Fix one end of the dried modified plant fiber, apply 5-30MPa tension on the other end, heat it to the pre-oxidation temperature in air atmosphere, and treat it for 2-4h; the heating rate is 2-5℃ / min; the pre-oxidation temperature is 250-340℃.

[0017] Furthermore, the atmosphere of the carbonization treatment is a nitrogen atmosphere, the heating rate of the carbonization treatment is 2-5°C / min, the temperature of the carbonization treatment is 1000-1500°C, and the time of the carbonization treatment is 0.5-3h.

[0018] Furthermore, the plant fiber material includes any one of flax fiber, jute fiber, sisal fiber, bamboo fiber, wood fiber and cotton fiber.

[0019] The second object of the present invention is to provide a plant fiber conversion to achieve densification modified carbon fiber, comprising the following steps:

[0020] The plant fiber is immersed in an alkali solution and boiled, and then boiled in deionized water to obtain delignified plant fiber;

[0021] The delignified plant fiber is immersed in a Tris-Cl solution containing dopamine hydrochloride, and a modification reaction is carried out at room temperature, and then the obtained fiber product is washed in deionized water to obtain a modified plant fiber;

[0022] Fix one end of the modified plant fiber, apply tension to the other end, and dry naturally at room temperature to obtain dry modified plant fiber;

[0023] The dried modified plant fiber is subjected to pre-oxidation and carbonization treatment in sequence to obtain modified plant fiber-based carbon fiber.

[0024] Beneficial effects of the present invention:

[0025] (1) A novel "top-down" process is used to remove components other than CNF (carbon nanofiber) in plant fibers, eliminate the original pore structure of plant fibers, and obtain micron-sized precursor fibers basically composed of CNF. Compared with viscose fibers, another carbon fiber precursor made of cellulose that has long been commercialized, the cellulose fibers prepared by the "top-down" process are mainly composed of natural CNF, and their preferred orientation along the fiber axis and the denser Cellulose I crystal structure are retained. Therefore, the carbon fibers obtained after carbonization have better tensile strength.

[0026] (2) The present invention applies tensile stress along the fiber axis during the drying process of delignified plant fibers, further promoting the preferential orientation of CNF along the fiber axis, and effectively improving the problem of low tensile strength caused by poor orientation of graphite crystallites in cellulose-based carbon fibers.

[0027] (3) The present invention uses PDA to modify the chemical structure of cellulose while densifying delignified plant fibers, thereby overcoming the shortcomings of low carbon yield and density of cellulose-based carbon fibers produced by the prior art, and ultimately improving the tensile strength of carbon fibers.

[0028] (4) The present invention has a novel concept, short cycle and low cost, and provides a new method for preparing cellulose-based carbon fibers, which is helpful to develop high value-added applications of natural fibers. High-performance carbon fiber composite materials have good application prospects and economy in the fields of new energy, aerospace, automobiles, sports, etc.

[0029] Other features and advantages of the present invention will be described in the following description, and partly become apparent from the description, or understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures pointed out in the description, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0031] Figure 1 The cross-sectional SEM image of the flax fiber in Example 1 of the present invention is shown;

[0032] Figure 2 The cross-sectional SEM image of jute fiber in Example 2 of the present invention is shown;

[0033] Figure 3 The cross-sectional SEM image of sisal fiber in Example 2 of the present invention is shown;

[0034] Figure 4 The cross-sectional SEM image of the delignified flax fiber in Example 1 of the present invention is shown;

[0035] Figure 5 The cross-sectional SEM image of delignified jute fiber according to Example 2 of the present invention is shown;

[0036] Figure 6 The cross-sectional SEM image of the delignified sisal fiber in Example 3 of the present invention is shown;

[0037] Figure 7 The cross-sectional SEM image of the modified flax fiber-based carbon fiber in Example 1 of the present invention is shown;

[0038] Figure 8 The cross-sectional SEM image of the modified jute fiber-based carbon fiber according to Example 2 of the present invention is shown;

[0039] Fig. 9 The cross-sectional SEM image of the modified sisal fiber-based carbon fiber in Example 3 of the present invention is shown;

[0040] Fig.10 The relationship between the tensile strength and the tensile strain of the modified flax fiber-based carbon fiber in Example 1 of the present invention is shown;

[0041] Fig.11 The relationship between the tensile strength and the tensile strain of the modified jute fiber-based carbon fiber in Example 2 of the present invention is shown;

[0042] Fig.12 The relationship between the tensile strength and the tensile strain of the modified sisal fiber-based carbon fiber in Example 3 of the present invention is shown;

[0043] Fig.13The relationship between the tensile strength and tensile strain of the flax fiber-based carbon fiber not modified by PDA in Comparative Example 1 of the present invention is shown;

[0044] Fig.14 The relationship between the tensile strength and the tensile strain of the modified flax fiber-based carbon fiber dried without applying tension in Comparative Example 2 of the present invention is shown. DETAILED DESCRIPTION

[0045] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0046] A method for preparing carbon fiber by converting plant fiber to achieve densification modification according to an embodiment of the present invention is characterized by comprising:

[0047] Step 1: soaking the plant fiber in an alkali solution and boiling it, and then boiling it in deionized water to obtain delignified plant fiber;

[0048] Step 2: immersing the delignified plant fiber in a Tris-Cl solution containing dopamine hydrochloride, performing a modification reaction at room temperature, and then washing the obtained fiber product in deionized water to obtain a modified plant fiber;

[0049] Step 3, fixing one end of the modified plant fiber, applying tension to the other end, and drying naturally at room temperature to obtain dried modified plant fiber;

[0050] Step 4: pre-oxidation and carbonization treatment are performed on the dried modified plant fiber in sequence to obtain modified plant fiber-based carbon fiber.

[0051] In step 1, the alkali solution is a NaOH aqueous solution, or Na 2 SO 3 , NaOH and water mixed to form a solution, preferably Na 2 SO 3 , a solution formed by mixing NaOH and water;

[0052] Among them, the Na 2 SO 3 In the solution formed by mixing NaOH and water, Na 2 SO 3 The concentration of Na 2 SO 3The specific concentrations used are 0.5M, 0.51M, 0.52M, 0.53M, 0.54M, 0.55M, 0.56M, 0.57M, 0.58M, 0.59M, 0.6M, 0.61M, 0.62M, 0.63M, 0.64M, 0.65M, 0.66M, 0.67M, 0.68M, 0.69M, 0.7M, 0.71M, 0.72M, 0.73M, 0.74M, 0.76M, Any value among 0.75M, 0.76M, 0.77M, 0.78M, 0.79M, 0.8M, 0.81M, 0.82M, 0.83M, 0.84M, 0.85M, 0.86M, 0.87M, 0.88M, 0.89M, 0.9M, 0.91M, 0.92M, 0.93M, 0.94M, 0.95M, 0.96M, 0.97M, 0.98M, 0.99M, 1M;

[0053] The concentration of NaOH is 0.5-3M, and the concentration of NaOH is specifically 0.55M, 0.6M, 0.65M, 0.7M, 0.75M, 0.8M, 0.85M, 0.9M, 0.95M, 1M, 1.05M, 1.1M, 1.15M, 1.2M, 1.25M, 1.3M, 1.35M, 1.4M, 1.45M, 1.5M, 1.55M, 1.6M, 1.6 Any value among 5M, 1.7M, 1.75M, 1.8M, 1.85M, 1.9M, 1.95M, 2M, 2.05M, 2.1M, 2.15M, 2.2M, 2.25M, 2.3M, 2.35M, 2.4M, 2.45M, 2.5M, 2.55M, 2.6M, 2.65M, 2.7M, 2.75M, 2.8M, 2.85M, 2.9M, 2.95M, and 3M.

[0054] Wherein, the boiling treatment time in the alkali solution is 2-12h, and the specific treatment time is any value of 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, 8h, 8.5h, 9h, 9.5h, 10h, 10.5h, 11h, 11.5h, and 12h.

[0055] In step 1, the boiling treatment in deionized water is performed 2-3 times.

[0056] The purpose of step 1 is to remove most of the lignin, hemicellulose, wax, pectin and other components contained in the plant fiber to obtain delignified plant fiber basically composed of cellulose nanofibers;

[0057] In step 2, in the Tris-Cl solution containing dopamine hydrochloride, the dosage ratio of dopamine hydrochloride to Tris-Cl solution is 0.4-0.8 g:1L, the concentration of the Tris-Cl solution is 5-15 mM, and the concentration of the Tris-Cl solution is specifically any value of 5 mM, 6 mM, 7 mM, 8 mM, 9 mM, 10 mM, 11 mM, and 12 mM.

[0058] In step 2, the pH of the modification reaction is 7.5 to 8.5, and the modification reaction time is 12-36 hours. The modification reaction time is specifically 12 hours, 12.5 hours, 13 hours, 13.5 hours, 14 hours, 14.5 hours, 15 hours, 15.5 hours, 16 hours, 16.5 hours, 17 hours, 17.5 hours, 18 hours, 18.5 hours, 19 hours, 19.5 hours, 20 hours, 20.5 hours, 21 hours, 21.5 hours. h, 22h, 22.5h, 23h, 23.5h, 24h, 24.5h, 25h, 25.5h, 26h, 26.5h, 27h, 27.5h, 28h, 28.5h, 29h , 29.5h, 30h, 30.5h, 31h, 31.5h, 32h, 32.5h, 33h, 33.5h, 34h, 34.5h, 35h, 35.5h, 36h any value.

[0059] In step three, the magnitude of the pulling force is 5-30 MPa, and the pulling force is specifically any value among 5 MPa, 8 MPa, 10 MPa, 13 MPa, 15 MPa, 17 MPa, 20 MPa, 23 MPa, 25 MPa, 28 MPa, and 30 MPa.

[0060] In step 4, the specific operation of pre-oxidation is as follows:

[0061] Fix one end of the dried modified plant fiber, apply 5-30MPa tension to the other end, heat it to the pre-oxidation temperature in an air atmosphere, and treat it for 2-4h;

[0062] Wherein, the heating rate is 2-5°C / min; the pre-oxidation temperature is 250-340°C, and the pre-oxidation temperature is specifically any value among 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, and 350°C.

[0063] In step 4, the atmosphere of the carbonization treatment is a nitrogen atmosphere, the heating rate of the carbonization treatment is 2-5°C / min, the temperature of the carbonization treatment is 1000-1500°C, and the time of the carbonization treatment is 0.5-3h;

[0064] The temperature of the carbonization treatment may be any one of 1000°C, 1100°C, 1150°C, 1200°C, 1250°C, 1300°C, 1350°C, 1400°C, 1450°C, and 1500°C.

[0065] In an embodiment of the present invention, the plant fiber material includes any one of flax fiber, jute fiber, sisal fiber, bamboo fiber, wood fiber, and cotton fiber.

[0066] Example 1

[0067] Method for preparing carbon fiber by converting plant fiber to achieve densification modification:

[0068] First, flax fibers are soaked in Na 2 SO 3 / NaOH aqueous solution (NaOH concentration range is 0.5M, Na 2 SO 3 The concentration range is 0.5 M) and the mixture is boiled for 12 h, and then the product is boiled in deionized water for 3 times to obtain delignified flax fiber;

[0069] Dissolve 0.6 g of dopamine hydrochloride in 1 liter of 5 mM Tris-Cl solution and adjust the pH to 7.5 to 8.5; immerse the delignified flax fiber prepared in step 1 in the solution and react at room temperature for 12 hours; then wash the obtained fiber product in deionized water for 5-30 minutes, then fix one end of the modified flax fiber, apply a tensile force of 15 MPa to the other end, and dry it naturally at room temperature to obtain a dried modified flax fiber;

[0070] The dried modified flax fiber was placed in a pre-oxidation furnace, one end of the fiber was fixed, and a tensile force of 15 MPa was applied to the other end. The modified flax fiber was heated to 300°C at a heating rate of 3°C / min in an air atmosphere for pre-oxidation treatment for 4 hours; then the obtained pre-oxidized fiber was transferred to a high-temperature cracking furnace, and the pre-oxidized fiber was heated to 1000°C at a heating rate of 4°C / min from room temperature in high-purity nitrogen for carbonization treatment for 3 hours; then it was cooled to room temperature with the furnace to obtain modified flax fiber-based carbon fiber.

[0071] Example 2

[0072] Method for preparing carbon fiber by converting plant fiber to achieve densification modification:

[0073] First, soak the jute fibers in Na 2 SO 3 / NaOH aqueous solution (NaOH concentration range is 2M, Na 2SO3 concentration range is 0.7M) for 2 h, and then the product is boiled in deionized water for 4 times to obtain delignified jute fiber;

[0074] Dissolve 0.8 g of dopamine hydrochloride in 1 liter of 15 mM Tris-Cl solution and adjust the pH to 7.5 to 8.5; immerse the delignified jute fiber prepared in step 1 in the solution and react at room temperature for 12 hours; then wash the obtained fiber product in deionized water for 5-30 minutes, then fix one end of the modified plant fiber, apply a tensile force of 5 MPa to the other end, and dry it naturally at room temperature to obtain a dried modified jute fiber;

[0075] The dried modified jute fiber is placed in a pre-oxidation furnace, one end of the fiber is fixed, and a tensile force of 5-30 MPa is applied to the other end. The modified plant fiber is heated to 250°C at a heating rate of 5°C / min in an air atmosphere for pre-oxidation treatment for 4 hours; the obtained pre-oxidized fiber is then transferred to a high-temperature cracking furnace, and the pre-oxidized fiber is heated to 1500°C at a heating rate of 5°C / min from room temperature in high-purity nitrogen for carbonization treatment for 0.5 hours; and then cooled to room temperature with the furnace to obtain modified jute fiber-based carbon fiber.

[0076] Example 3

[0077] Method for preparing carbon fiber by converting plant fiber to achieve densification modification:

[0078] First, sisal fiber is soaked in Na 2 SO 3 / NaOH aqueous solution (NaOH concentration range is 3M, Na 2 SO3 concentration range is 0.55M) for 6 hours, and then the product is boiled in deionized water for 4 times to obtain delignified sisal fiber;

[0079] Dissolve 0.8g of dopamine hydrochloride in 1 liter of 10mM Tris-Cl solution and adjust the pH to 7.5 to 8.5; immerse the delignified sisal fiber prepared in step 1 in the solution and react at room temperature for 12-36h; then wash the obtained fiber product in deionized water for 5-30min, then fix one end of the modified plant fiber, apply 20MPa tension to the other end, and dry naturally at room temperature to obtain dry modified sisal fiber;

[0080] The dried modified sisal fiber is placed in a pre-oxidation furnace, one end of the fiber is fixed, and a tensile force of 5-30 MPa is applied to the other end. The modified plant fiber is heated to 300°C at a heating rate of 5°C / min in an air atmosphere for pre-oxidation treatment for 4 hours; then the obtained pre-oxidized fiber is transferred to a high-temperature cracking furnace, and the pre-oxidized fiber is heated to 1300°C at a heating rate of 2-5°C / min from room temperature in high-purity nitrogen for carbonization treatment for 2 hours; then it is cooled to room temperature with the furnace to obtain modified sisal fiber-based carbon fiber.

[0081] Process testing and performance testing:

[0082] The cross sections of the raw flax fibers, jute fibers and sisal fibers of Examples 1-3 were observed by scanning electron microscopy, and the obtained electron microscope images are as follows: Figure 1-Figure 3 As shown, Figure 1 The corresponding SEM image is of the cross section of flax fiber. Figure 2 The corresponding SEM image of the jute fiber cross section is shown below. Figure 3 The corresponding one is the SEM image of the cross section of sisal fiber;

[0083] The cross sections of the delignified flax fibers, delignified jute fibers, and delignified sisal fibers obtained in Examples 1-3 were observed by scanning electron microscopy. The obtained electron microscopy images are as follows: Figure 4-Figure 6 As shown, Figure 4 The corresponding SEM image is of the cross section of delignified flax fiber. Figure 5 The corresponding SEM image of the cross section of delignified jute fiber is shown in Figure 1. Figure 6 The corresponding one is the SEM image of the cross section of delignified sisal fiber;

[0084] The cross sections of the dried modified flax fiber, dried modified jute fiber and dried modified sisal fiber obtained in Examples 1-3 were observed by scanning electron microscope, and the obtained electron microscope images are as follows: Figure 7-Figure 9 As shown, Figure 7 The corresponding SEM image is of the cross section of the dried modified flax fiber. Figure 8 The corresponding SEM image of the cross section of the dried modified jute fiber is shown in Figure 2. Fig. 9 The corresponding SEM image is the cross section of the dried modified sisal fiber;

[0085] from Figure 1-Figure 6 It can be seen that flax fiber, jute fiber and sisal fiber have original pore structure and low density;

[0086] from Figure 7-Figure 9 and Figure 3-6 From the comparison of fiber types one by one, it can be seen that the modification and drying steps of PAD (polydopamine) eliminate the original pore structure of plant fibers, improve the disadvantage of low density of plant fibers, and obtain micron-sized precursor fibers basically composed of CNF.

[0087] The modified flax fiber-based carbon fiber, modified jute fiber-based carbon fiber and modified sisal fiber-based carbon fiber obtained in Examples 1-3 were subjected to tensile property tests (ASTM D3379-75), and the relationship between the tensile strength and the tensile strain obtained is shown in the figure below: Figure 10-12 As shown, Fig.10 The corresponding figure is the relationship between the tensile strength and tensile strain of the modified flax fiber-based carbon fiber. Fig.11 The corresponding graph is the relationship between the tensile strength and tensile strain of the modified jute fiber-based carbon fiber. Fig.12 The corresponding figure is the relationship between the tensile strength and tensile strain of the modified sisal fiber-based carbon fiber.

[0088] from Figure 10-12 It can be seen that the tensile strength of modified flax fiber-based carbon fiber is as high as 1252MPa, the tensile strength of modified jute fiber-based carbon fiber is as high as 1196MPa, and the tensile strength of modified sisal fiber-based carbon fiber is as high as 1202MPa.

[0089] Comparative Example 1:

[0090] Method for preparing carbon fiber by converting plant fiber to achieve densification modification:

[0091] First, flax fibers are soaked in Na 2 SO 3 / NaOH aqueous solution (NaOH concentration range is 0.5M, Na 2 SO3 concentration range of 0.5M) for 12 h, and then the product was boiled in deionized water for 3 times to obtain delignified flax fiber;

[0092] Fix one end of the delignified flax fiber, apply a tensile force of 15 MPa to the other end, and dry it naturally at room temperature to obtain dry delignified flax fiber;

[0093] The dried delignified flax fiber is placed in a pre-oxidation furnace, one end of the fiber is fixed, and a tensile force of 15 MPa is applied to the other end. The modified flax fiber is heated to 300°C at a heating rate of 3°C / min in an air atmosphere for pre-oxidation treatment for 4 hours; then the obtained pre-oxidized fiber is transferred to a high-temperature cracking furnace, and the pre-oxidized fiber is heated from room temperature to 1000°C at a heating rate of 4°C / min in high-purity nitrogen for carbonization treatment for 3 hours; then it is cooled to room temperature with the furnace to obtain modified flax fiber-based carbon fiber.

[0094] The modified flax fiber-based carbon fiber obtained in Comparative Example 1 was subjected to a tensile performance test (ASTM D3379-75), and the relationship between the tensile strength and the tensile strain obtained is shown in the figure below: Fig.13 As shown, from Fig.13It can be seen that the tensile strength of the modified flax fiber-based carbon fiber obtained in Comparative Example 1 is 613 MPa, which is much lower than the tensile strength of the modified flax fiber-based carbon fiber obtained in Example 1.

[0095] Comparative Example 2:

[0096] Method for preparing carbon fiber by converting plant fiber to achieve densification modification:

[0097] First, flax fibers are soaked in Na 2 SO 3 / NaOH aqueous solution (NaOH concentration range is 0.5M, Na 2 SO3 concentration range of 0.5M) for 12 h, and then the product was boiled in deionized water for 3 times to obtain delignified flax fiber;

[0098] Dissolve 0.6 g of dopamine hydrochloride in 1 liter of 5 mM Tris-Cl solution and adjust the pH to 7.5 to 8.5; immerse the delignified flax fiber prepared in step 1 in the solution and react at room temperature for 12 hours; then wash the obtained fiber product in deionized water for 5-30 minutes to obtain modified flax fiber; and dry it naturally at room temperature without applying tension;

[0099] The modified flax fiber is placed in a pre-oxidation furnace, one end of the fiber is fixed, and a tensile force of 15 MPa is applied to the other end. The modified flax fiber is heated to 300°C at a heating rate of 3°C / min in an air atmosphere for pre-oxidation treatment for 4 hours; then the obtained pre-oxidized fiber is transferred to a high-temperature cracking furnace, and the pre-oxidized fiber is heated from room temperature to 1000°C at a heating rate of 4°C / min in high-purity nitrogen for carbonization treatment for 3 hours; then it is cooled to room temperature with the furnace to obtain modified flax fiber-based carbon fiber.

[0100] The modified flax fiber-based carbon fiber obtained in Comparative Example 2 was subjected to a tensile performance test (ASTM D3379-75), and the relationship between the tensile strength and the tensile strain obtained is shown in the figure below: Fig.14 As shown, from Fig.14 It can be seen that the tensile strength of the modified flax fiber-based carbon fiber obtained in Comparative Example 1 is 223 MPa, which is much lower than the tensile strength of the modified flax fiber-based carbon fiber obtained in Example 1, indicating that the tensile stress applied along the fiber axis during the drying process can effectively retain and promote the preferential orientation of CNF along the fiber axis, thereby improving the problem of low tensile strength caused by poor orientation of graphite crystallites in cellulose-based carbon fibers (reason: applying tensile stress along the fiber axis can effectively retain the preferential orientation of natural CNF along the fiber axis, which may further promote the orientation of CNF along the fiber axis).

[0101] Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent substitutions for some of the technical features therein; and these modifications or substitutions do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing carbon fiber by converting plant fiber to achieve densification modification, It is characterized in that include: The plant fiber is immersed in an alkali solution and boiled, and then boiled in deionized water to obtain delignified plant fiber; The delignified plant fiber is immersed in a Tris-Cl solution containing dopamine hydrochloride, and a modification reaction is carried out at room temperature, and then the obtained fiber product is washed in deionized water to obtain a modified plant fiber; Fix one end of the modified plant fiber, apply tension to the other end, and dry naturally at room temperature to obtain dry modified plant fiber; The dried modified plant fiber is subjected to pre-oxidation and carbonization treatment in sequence to obtain modified plant fiber-based carbon fiber.

2. A method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 1, It is characterized in that The alkali solution is Na 2 SO 3 , NaOH and water to form a solution, wherein the boiling treatment time in the alkali solution is 2-12 hours.

3. A method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 2, It is characterized in that In the alkali solution, Na 2 SO 3 The concentration of HCl is 0.5-1M, and the concentration of NaOH is 0.5-3M.

4. The method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 1, It is characterized in that In the Tris-Cl solution containing dopamine hydrochloride, the dosage ratio of dopamine hydrochloride to Tris-Cl solution is 0.4-0.8 g:1 L, and the concentration of the Tris-Cl solution is 5-15 mM.

5. The method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 1, It is characterized in that The pH of the modification reaction is 7.5 to 8.5, and the time of the modification reaction is 12 to 36 hours.

6. The method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 1, It is characterized in that The magnitude of the pulling force is 5-30 MPa.

7. The method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 1, It is characterized in that The specific operation of the pre-oxidation is as follows: Fix one end of the dried modified plant fiber, apply 5-30MPa tension on the other end, heat it to the pre-oxidation temperature in air atmosphere, and treat it for 2-4h; the heating rate is 2-5℃ / min; the pre-oxidation temperature is 250-340℃.

8. The method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to claim 1, It is characterized in that The atmosphere of the carbonization treatment is a nitrogen atmosphere, the heating rate of the carbonization treatment is 2-5°C / min, the temperature of the carbonization treatment is 1000-1500°C, and the time of the carbonization treatment is 0.5-3h.

9. A method for preparing carbon fiber by converting plant fiber into densified modified carbon fiber according to any one of claims 1 to 8, It is characterized in that The plant fiber material includes any one of flax fiber, jute fiber, sisal fiber, bamboo fiber, wood fiber and cotton fiber.

10. A plant fiber converted to achieve densified modified carbon fiber, It is characterized in that Prepared by the preparation method described in any one of claims 1 to 9.

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