Carbon fiber surface treatment method

Abstract

The invention relates to a carbon fiber surface treatment method, which improves the interfacial compatibility of carbon fibers and a matrix material, improves the interfacial shear strength by 70-90%, and is simple and convenient to operate, easy to control and suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber materials, and specifically relates to a carbon fiber surface treatment method. Background Technology

[0002] Polyacrylonitrile-based carbon fiber possesses advantages such as high strength, high modulus, corrosion resistance, and fatigue resistance, and is widely used in aerospace, wind power generation, and other fields. Carbon fibers are classified into high-strength standard modulus, high-strength medium modulus, high modulus, and high-strength high modulus based on their strength, modulus, and other properties. Fiber diameter is a crucial parameter affecting the strength and modulus of carbon fibers. To obtain higher-strength carbon fibers, the fiber diameter is typically reduced to achieve a more uniform radial structure. For example, T800 grade carbon fiber, with a strength of approximately 5.5–5.8 GPa and a modulus of 294 GPa, generally has a diameter of around 5–5.5 μm. However, reducing the carbon fiber diameter can lead to imbalances in the tensile-compression ratio of the composite material. Increasing the carbon fiber diameter while maintaining the existing performance indicators of high-strength medium modulus carbon fibers has significant positive implications for improving carbon fiber production efficiency, reducing production costs, and enhancing the compressive properties of composite materials.

[0003] The preparation of polyacrylonitrile-based carbon fibers includes processes such as polymerization, spinning, pre-oxidation, carbonization, and surface treatment. Surface treatment is necessary after carbonization because the carbonized fibers have fewer surface-active functional groups and higher inertness, resulting in poor bonding with the resin matrix during composite formation and affecting the performance of the composite material.

[0004] Large-diameter, high-strength, intermediate-modulus carbon fibers have higher carbonization temperatures, typically exceeding 1400℃, resulting in a higher degree of surface graphitization, higher surface inertness, and weaker bonding with the resin interface. Therefore, optimizing interfacial properties through efficient surface treatment is particularly important. After surface treatment, oxygen- and nitrogen-containing active functional groups are introduced into the carbon fiber surface, enhancing surface wettability and increasing roughness. This strengthens the physical intercalation and chemical bonding with the resin matrix, ultimately improving the performance of carbon fiber composites.

[0005] Currently, there are various surface treatment methods for carbon fibers, including anodizing, surface coating, plasma treatment, and vapor deposition. However, the application of these methods in actual production is limited. For example, controlling the thickness and uniformity of the coating is difficult with surface coating methods; while plasma treatment yields good results, it has high equipment costs and energy consumption; and vapor deposition typically has a slow deposition rate, especially when preparing thicker coatings or films, requiring a long time. Among various surface treatment methods, anodizing is currently a relatively mature method for carbon fiber surface treatment. Its advantages include continuous production, ease of operation, and mild, easily controllable processing conditions, and it has been widely used in industry.

[0006] Chinese patent CN114427109B discloses a carbon fiber anodizing surface treatment apparatus and method. This method uses ammonium bicarbonate to anodize carbon fibers and studies the effect of electrolyte concentration on the interfacial shear strength of the composite material and the tensile strength of the carbon fiber itself. It was found that the interfacial shear strength of the treated carbon fiber reached a maximum of over 70 MPa. These studies indicate that anodizing treatment can significantly improve the interfacial properties of carbon fiber composites. However, most studies focus on low-modulus carbon fibers and are usually based on a single electrolyte system, making it difficult to simultaneously achieve both oxidation effect and fiber strength maintenance.

[0007] Chinese patent CN106436274A discloses a method for anodizing surface treatment of carbon fiber. This method uses dilute nitric acid pretreatment under ultrasonic conditions to activate the surface groups of carbon fiber, improve the wettability of carbon fiber, and enhance its adhesion to the resin matrix. However, the dilute nitric acid pretreatment time is too long, resulting in low efficiency and difficulty in continuous production, which affects its industrial applicability.

[0008] For low-modulus carbon fibers, the current anodizing process and equipment are relatively mature; however, for large-diameter high-strength medium-modulus carbon fibers, due to their stable chemical properties and stronger surface inertness, they are more difficult to be oxidized to introduce active groups, so there is an urgent need for further exploration and improvement of the anodizing process and equipment. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a method for surface treatment of carbon fiber.

[0010] This invention provides a carbon fiber surface treatment method, comprising:

[0011] (1) Prepare an electrolyte solution by mixing ammonium salt electrolyte with deionized water;

[0012] (2) Add the electrolyte from step (1) to an imidazole ionic liquid to obtain a mixed electrolyte;

[0013] (3) Using the mixed electrolyte from step (2), carbon fiber is used as the anode and graphite plate is used as the cathode to perform anodic gradient electrolytic oxidation treatment to obtain carbon fiber with surface treatment.

[0014] The preferred embodiment of the above preparation method is as follows:

[0015] The ammonium salt electrolyte in step (1) includes one or more of ammonium borate, ammonium citrate, ammonium formate, and ammonium acetate.

[0016] In step (1), the electrolyte mass percentage concentration is 2-5%.

[0017] In step (2), the imidazole ionic liquid is one or more of 1-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-dodecyl-3-methylimidazolium tetrafluoroborate, and 1-hexadecyl-3-methylimidazolium tetrafluoroborate.

[0018] In step (2), the molar ratio of imidazole ionic liquid to ammonium salt electrolyte is 1:1.2 to 1:2.

[0019] In step (3), the anode carbon fiber is a large-diameter high-strength intermediate-modulus carbon fiber with a diameter of 6.5 to 7.5 μm, a tensile strength of 5.5 to 7.0 GPa, and a tensile modulus of 270 to 310 GPa.

[0020] Furthermore, the anodic carbon fiber is a large-diameter, high-strength, intermediate-modulus carbon fiber that has not undergone surface treatment after carbonization.

[0021] In step (3), the anodic gradient electrolytic oxidation treatment temperature is 20–40 °C, and the electrolytic oxidation charge is 5–50 C·g. -1 .

[0022] The anodic gradient electrolytic oxidation treatment in step (3) includes: setting up at least 3 electrolytic cells, each containing a mixed electrolyte, wherein the first cell has a treatment charge of 5-10 C·g. -1 The final tank processing capacity is 25–50 C·g. -1 The charge in each electrolytic cell increases progressively, with a charge difference of 5–10 C·g between adjacent cells. -1 .

[0023] The number of electrolytic cells is 3-5.

[0024] Furthermore, the oxidation treatment time in each electrolytic cell is 10–15 seconds.

[0025] This invention provides a surface-treated carbon fiber prepared by the method described above.

[0026] This invention provides an application of the surface-treated carbon fiber in pressure vessels, aerospace, and rail transportation.

[0027] The present invention provides a surface-treated carbon fiber that significantly improves its interfacial properties with the matrix material, enabling carbon fiber composite materials to play a core role in many fields with stringent material performance requirements, such as pressure vessels, aerospace, and rail transportation.

[0028] In this invention, an ammonium salt solution is used as the base electrolyte, and imidazole ionic liquid is added as an electrolyte additive to perform anodic electrolytic oxidation treatment on large-diameter high-strength intermediate-modulus carbon fibers, so as to improve the surface properties of the carbon fibers and enhance their interfacial bonding strength with the matrix material.

[0029] This invention uses an ammonium salt solution as the base electrolyte and adds imidazole ionic liquid as an electrolyte additive, significantly improving the wettability and stability of the electrolyte. By controlling conditions such as the flux and the number of electrolytic cells, uniform oxidation treatment of the surface of large-diameter high-strength intermediate-modulus carbon fibers is achieved, significantly improving the surface activity of the carbon fibers and the interfacial bonding strength with the matrix material. The treated large-diameter high-strength intermediate-modulus carbon fibers, while introducing oxygen-containing functional groups, also form a uniform groove structure on the fiber surface, enabling uniform modification of the large-diameter high-strength intermediate-modulus carbon fibers and increasing the interfacial shear strength by 70-90%.

[0030] Beneficial effects

[0031] (1) Low current flux breaks the chemical inertness of the carbon fiber surface, preferentially oxidizes the defect sites at the edge of graphite microcrystals, and generates basic oxygen-containing functional groups, but does not damage the fiber body strength; high current flux provides higher energy to penetrate the energy barrier, achieve deep oxidation, form highly active groups such as hydroxyl (-OH) and carbonyl (C=O), and increase the functional group density.

[0032] (1) The surface activity is improved in this invention. The treated carbon fiber surface is introduced with oxygen-containing functional groups, which significantly improves the surface activity and hydrophilicity.

[0033] (2) This invention improves the interfacial compatibility between carbon fiber and matrix material, increases the interfacial shear strength by 70-90%, and is easy to operate and control, making it suitable for large-scale industrial production.

[0034] (3) The electrolyte and ionic liquid of the present invention work together to form a uniform groove structure on the surface of carbon fiber while introducing oxygen-containing functional groups, and ensure the uniform modification of large-diameter high-strength intermediate-modulus carbon fiber. Detailed Implementation

[0035] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0036] The large-diameter, high-strength, intermediate-modulus carbon fibers used in the examples and comparative examples, which were carbonized and untreated, had a diameter of 6.5–7.5 μm, a tensile strength of 5.5–7.0 GPa, and a tensile modulus of 270–310 GPa.

[0037] Example 1

[0038] (1) Take 0.5 mol of 1-methylimidazolium tetrafluoroborate ionic liquid and 2500 g of ammonium borate aqueous solution and stir thoroughly at room temperature, then pour into an electrolytic cell; the mass fraction of the ammonium borate aqueous solution is 5%.

[0039] (2) Use large-diameter high-strength intermediate-modulus carbon fibers that have not been surface treated after carbonization as the anode and a graphite plate as the cathode.

[0040] (3) There are 3 electrolytic cells, and the electrolysis charge of the first cell is 7 C·g. -1 The intermediate groove is 15 C·g -1 The final temperature is 25°C·g. -1 .

[0041] (4) Perform anodic electrolytic oxidation treatment at 30℃, with each electrolytic cell taking 10s.

[0042] Example 2

[0043] (1) Take 0.3 mol of 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid and 2500 g of ammonium citrate aqueous solution and stir thoroughly at room temperature, then pour into an electrolytic cell; the mass fraction of the ammonium citrate aqueous solution is 5%.

[0044] (2) Use large-diameter high-strength intermediate-modulus carbon fibers that have not been surface treated after carbonization as the anode and a graphite plate as the cathode.

[0045] (3) There are 3 electrolytic cells, and a DC power supply is applied. The electrolysis charge of the first cell is 7 C·g. -1 The intermediate groove is 15 C·g -1 The final temperature is 25°C·g. -1 .

[0046] (4) Perform anodic electrolytic oxidation treatment at 30℃, with each electrolytic cell taking 10s.

[0047] Example 3

[0048] (1) Take 0.5 mol of 1-dodecyl-3-methylimidazolium tetrafluoroborate ionic liquid and 2500 g of ammonium formate aqueous solution and stir thoroughly at room temperature, then pour into an electrolytic cell; the mass fraction of the ammonium formate aqueous solution is 3.5%.

[0049] (2) Use large-diameter high-strength intermediate-modulus carbon fibers that have not been surface treated after carbonization as the anode and a graphite plate as the cathode.

[0050] (3) There are 3 electrolytic cells, and the electrolysis charge of the first cell is 7 C·g. -1 The intermediate groove is 15 C·g -1 The final temperature is 25°C·g. -1 .

[0051] (4) Perform anodic electrolytic oxidation treatment at 30℃, with each electrolytic cell taking 10s.

[0052] Example 4

[0053] (1) Take 0.5 mol of 1-hexadecyl-3-methylimidazolium tetrafluoroborate ionic liquid and 2500 g of ammonium acetate aqueous solution and stir thoroughly at room temperature, then pour into an electrolytic cell; the mass fraction of the ammonium acetate aqueous solution is 3.5%.

[0054] (2) Use large-diameter high-strength intermediate-modulus carbon fibers that have not been surface treated after carbonization as the anode and a graphite plate as the cathode.

[0055] (3) There are 3 electrolytic cells, and the electrolysis charge of the first cell is 7 C·g. -1 The intermediate groove is 15 C·g -1 The final temperature is 25°C·g. -1 .

[0056] (4) Perform anodic electrolytic oxidation treatment at 30℃, with each electrolytic cell taking 10s.

[0057] Example 5

[0058] This embodiment is basically the same as Embodiment 1, except that no ionic liquid is added to the electrolyte.

[0059] Example 6

[0060] This embodiment is basically the same as Embodiment 2, except that no ionic liquid is added to the electrolyte.

[0061] Example 7

[0062] This embodiment is basically the same as Embodiment 3, except that no ionic liquid is added to the electrolyte.

[0063] Example 8

[0064] This embodiment is basically the same as Embodiment 4, except that no ionic liquid is added to the electrolyte.

[0065] Example 9

[0066] This embodiment is basically the same as Embodiment 1, except that there are 4 electrolytic cells, each electrolytic cell has a processing time of 10 seconds, and the electrolysis charge of the first cell is 7 C·g. -1 The charge of the second electrolytic cell is 17 C·g. -1 The charge of the third electrolytic cell is 23 C·g. -1 The final temperature is 32°C·g. -1 .

[0067] Example 10

[0068] This embodiment is basically the same as Embodiment 1, except that there are 5 electrolytic cells, each electrolytic cell has a processing time of 10 seconds, and the electrolysis charge of the first cell is 5 C·g. -1 The charge of the second electrolytic cell is 12 C·g. -1 The charge of the third electrolytic cell is 18 C·g. -1 The charge of the fourth electrolytic cell is 24 C·g. -1 The final trough is 32C·g -1 .

[0069] Comparative Example 1

[0070] This comparative example is basically the same as Example 1, except that only an ionic liquid was added, and no electrolyte was added.

[0071] Comparative Example 2

[0072] This comparative example is basically the same as Example 1, except that no electrolyte or ionic liquid was added.

[0073] Comparative Example 3

[0074] This comparative example is basically the same as Example 1, except that there is no electrolytic cell and the electrolysis charge is 20 C·g. -1 The processing time is 30 seconds.

[0075] Table 1 Interfacial shear strength of different samples

[0076]

[0077]

[0078] In Comparative Example 2, neither electrolyte nor ionic liquid was added, and the interfacial shear strength after surface treatment was only 37.8 MPa. In Example 5, only electrolyte was added for surface treatment, and the interfacial shear strength after treatment was 53.5 MPa, which was 41.5% higher than that of Comparative Example 2. In Example 1, ionic liquid and electrolyte were mixed and then used to treat carbon fibers, and the interfacial shear strength after treatment was 71.4 MPa, which was 88.9% higher than that of Comparative Example 2.

[0079] Comparing the data from Examples 1-8, it can be seen that the interfacial shear strength after surface treatment with a mixture of ionic liquid and electrolyte is 1.3 to 1.5 times that of surface treatment with electrolyte alone.

[0080] Comparing the data from Examples 1, 9, 10 and Comparative Example 3, it can be seen that, under the same conditions, the surface treatment effect achieved by using 3 to 5 electrolytic cells is 1.17 to 1.31 times that of treatment using only 1 electrolytic cell.

[0081] Comparing the data of Example 5, Comparative Example 1 and Comparative Example 2, it can be seen that in Comparative Example 1, only ionic liquid was added for surface treatment, and the interfacial shear strength after treatment was 43.4 MPa. Its treatment effect was only slightly improved by 15% compared with Comparative Example 2. Therefore, electrolyte additives such as ionic liquids cannot replace electrolytes for surface treatment of carbon fibers.

[0082] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for surface treatment of carbon fiber, comprising: (1) Prepare an electrolyte solution by mixing ammonium salt electrolyte with deionized water; (2) Add the electrolyte from step (1) to an imidazole ionic liquid to obtain a mixed electrolyte; (3) Using the mixed electrolyte from step (2), carbon fiber is used as the anode and graphite plate is used as the cathode to perform anodic gradient electrolytic oxidation treatment to obtain carbon fiber with surface treatment.

2. The method according to claim 1, characterized in that, The ammonium salt electrolyte in step (1) includes one or more of ammonium borate, ammonium citrate, ammonium formate, and ammonium acetate; In step (1), the electrolyte mass percentage concentration is 2-5%.

3. The method according to claim 1, characterized in that, In step (2), the imidazole ionic liquid is one or more of 1-methylimidazolium tetrafluoroborate, 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-dodecyl-3-methylimidazolium tetrafluoroborate, and 1-hexadecyl-3-methylimidazolium tetrafluoroborate.

4. The method according to claim 1, characterized in that, In step (2), the molar ratio of imidazole ionic liquid to ammonium salt electrolyte is 1:1.2 to 1:

2.

5. The method according to claim 1, characterized in that, In step (3), the anode carbon fiber is a large-diameter high-strength intermediate-modulus carbon fiber with a diameter of 6.5 to 7.5 μm, a tensile strength of 5.5 to 7.0 GPa, and a tensile modulus of 270 to 310 GPa.

6. The method according to claim 1, characterized in that, In step (3), the temperature of the anodic gradient electrolytic oxidation treatment is 20-40℃.

7. The method according to claim 1, characterized in that, The anodic gradient electrolytic oxidation treatment in step (3) includes: setting up at least 3 electrolytic cells, each containing a mixed electrolyte, wherein the first cell has a treatment charge of 5-10 C·g. -1 The final tank processing capacity is 25–50 C·g. -1 The charge in each electrolytic cell increases progressively, with a charge difference of 5–10 C·g between adjacent cells. -1 .

8. The method according to claim 7, characterized in that, The number of electrolytic cells is 3-5; the oxidation treatment time in each electrolytic cell is 10-15 seconds.

9. A surface-treated carbon fiber prepared by the method of claim 1.

10. An application of the surface-treated carbon fiber of claim 9 in the fields of pressure vessels, aerospace, and rail transportation.

Images