Surface treatment method of carbon fiber, surface modified carbon fiber and composite material

Abstract

The invention relates to a surface treatment method of carbon fibers, surface modified carbon fibers and a composite material, and belongs to the technical field of carbon fiber surface modification. The surface treatment method comprises the following steps: carrying out first-stage electrochemical treatment on carbon fibers by adopting a first electrolyte, wherein the pH value of the first electrolyte is greater than or equal to 13; carrying out second-stage electrochemical treatment on the carbon fibers subjected to the first-stage electrochemical treatment by adopting a second electrolyte, wherein the pH value of the second electrolyte is less than or equal to 2; third-stage electrochemical treatment is conducted on the carbon fibers subjected to second-stage electrochemical treatment through third electrolyte, the third electrolyte comprises ammonium salt and an amino compound, and the amino compound comprises an amino-containing polymer and / or an amino-containing monomer. The carbon fibers are subjected to surface modification by adopting three-stage stepped electrochemical treatment, and abundant carboxyl and amino functional groups are introduced into the surfaces of the carbon fibers, so that the interface bonding performance of the carbon fibers is effectively improved, and the interlaminar shear strength of the composite material is further improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of carbon fiber surface modification, and particularly relates to a carbon fiber surface treatment method, surface modified carbon fiber and composite material. BACKGROUND

[0002] Carbon fibers are mainly composed of graphite microcrystals, and have low surface chemical activity and poor wettability, which leads to weak physical / chemical bonding with resin matrix materials. In the application process, due to the characteristics of smooth surface, strong inertness and few active functional groups of carbon fibers, it is difficult to form effective bonding with the resin matrix. When the composite material is stressed, the poor interfacial bonding easily causes problems such as delamination and crack propagation, which significantly reduces the overall performance of the material (such as interlaminar shear strength). Therefore, before the carbon fiber is combined with the resin matrix, the carbon fiber is often subjected to electrochemical surface treatment, and the active oxygen atoms generated by anode are used to electrochemically etch the surface of the carbon fiber, so as to activate the surface of the carbon fiber and improve the interfacial bonding performance between the carbon fiber and the resin matrix.

[0003] However, the existing carbon fiber surface treatment process usually only uses acidic and / or alkaline electrolyte to perform one-stage or two-stage electrochemical treatment on the carbon fiber, and the surface modification effect is not good, and the interfacial performance of the carbon fiber needs to be improved. SUMMARY

[0004] The present application is proposed based on the above problems, and aims to provide a carbon fiber surface treatment method, surface modified carbon fiber and composite material, so as to solve the problem that the existing surface treatment technology cannot effectively improve the interfacial performance of the carbon fiber, and thus cannot effectively improve the interlaminar shear strength of the composite material.

[0005] In a first aspect, the present application provides a carbon fiber surface treatment method, comprising the following steps: The first electrolyte is used to perform first-stage electrochemical treatment on the carbon fiber, and the pH of the first electrolyte is greater than or equal to 13; The second electrolyte is used to perform second-stage electrochemical treatment on the carbon fiber subjected to the first-stage electrochemical treatment, and the pH of the second electrolyte is less than or equal to 2; The third electrolyte is used to perform third-stage electrochemical treatment on the carbon fiber subjected to the second-stage electrochemical treatment, and the third electrolyte comprises an ammonium salt and an amino compound, and the amino compound comprises an amino-containing polymer and / or an amino-containing monomer.

[0006] In the technical solution, the carbon fiber is surface-modified by three-stage electrochemical treatment, wherein the first-stage electrochemical treatment controls the electrolyte to be strong alkaline (pH≥13), which can effectively remove impurities on the surface of the carbon fiber and oxidize and etch the surface to introduce oxygen-containing active groups (such as hydroxyl groups); the second-stage electrochemical treatment controls the electrolyte to be strong acid (pH≤2), which can increase the surface roughness of the carbon fiber and introduce a large number of carboxyl groups; and the third-stage electrochemical treatment uses a third electrolyte containing ammonium salt, amino-containing polymer and / or amino-containing monomer, wherein the ammonium salt serves as an electrolyte to provide a conductive carrier to promote electrochemical oxidation reaction on the surface of the carbon fiber, and the amino-containing polymer and / or amino-containing monomer react with the carboxyl groups on the surface of the carbon fiber during the electrochemical oxidation reaction to effectively graft more reactive amino functional groups on the surface of the carbon fiber. The surface active group density of the carbon fiber treated by the method is significantly improved, which can effectively improve the interfacial bonding performance of the carbon fiber and further improve the interlaminar shear strength of the composite material. In some embodiments, the amino-containing polymer includes at least one of polyethyleneimine or polyacrylamide, the amino-containing monomer includes at least one of polyethylene polyamine or diethylene triamine, and / or the ammonium salt includes at least one of ammonium citrate, ammonium oxalate, ammonium tartrate or cerium nitrate.

[0007] In some embodiments, the third electrolyte includes ammonium citrate and polyethyleneimine.

[0008] Further, the weight average molecular weight of the polyethyleneimine is 200 g / mol-600 g / mol.

[0009] In some embodiments, in the third electrolyte, the mass percentage of the ammonium salt is 3wt%-20wt%, and the mass percentage of the amino compound is 0.1wt%-0.5wt%.

[0010] Further, the solvent of the third electrolyte is water.

[0011] In some embodiments, the electrolyte in the first electrolyte includes at least one of sodium hydroxide, potassium hydroxide or calcium hydroxide.

[0012] In some embodiments, the mass percentage of the electrolyte in the first electrolyte is 3wt%-12wt%.

[0013] In some embodiments, the electrolyte in the second electrolyte includes at least one of sulfuric acid, nitric acid, phosphoric acid, oxalic acid or acetic acid.

[0014] In some embodiments, the mass percentage of the electrolyte in the second electrolyte is 15wt%-50wt%.

[0015] In some embodiments, the conditions of the first-stage electrochemical treatment include: a surface treatment electric quantity of 60C / g-100C / g, a temperature of 50-90℃, and a time of 20-90s; and / or, the conditions of the second-stage electrochemical treatment include: a surface treatment electric quantity of 10C / g-30C / g, a temperature of 40-80℃, and a time of 20-70s; and / or, the conditions of the third-stage electrochemical treatment include: a surface treatment electric quantity of 10C / g-30C / g, a temperature of 30-70℃, and a time of 15-50s.

[0016] In some embodiments, after the third-stage electrochemical treatment, the method further includes: water washing and drying.

[0017] Further, the water washing includes multiple ultrasonic cleaning.

[0018] Further, the drying is performed at a temperature of 80-120℃ for a time of 2-4h. In a second aspect, the embodiments of the present application provide a surface-modified carbon fiber.

[0019] In the above technical solution, the surface-modified carbon fiber provided by the present application has abundant carboxyl and amino groups on the surface, has good interface bonding performance, can form effective adhesion with a resin matrix, and improves the interlaminar shear strength of the composite material.

[0020] In a third aspect, the embodiments of the present application provide a composite material, which includes a resin matrix and the above surface-modified carbon fiber.

[0021] In the above technical solution, the composite material in the present application effectively improves the overall performance, especially the mechanical strength, by using the surface-modified carbon fiber to be combined with the resin matrix, and can be widely applied in the fields of aerospace, new energy vehicles, etc. BRIEF DESCRIPTION OF DRAWINGS In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments, and it should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope, and for those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0022] Figure 1 A process flow chart of the surface treatment method of the carbon fiber provided by the embodiments of the present application. DETAILED DESCRIPTION

[0023] In the following, the embodiments of the surface treatment method of the carbon fiber, the surface-modified carbon fiber and the composite material in the present application will be specifically described with appropriate reference to the drawings, but there will be cases of omitting unnecessary detailed description.

[0024] The ranges disclosed herein are intended to be "open" ranges, i.e., the upper and lower limits of the range are not included. The ranges are also intended to include any and all sub-ranges of the same, wherein any statement of a range can be read to include any and all sub-ranges covering the same. For example, a statement of 60 to 120 can include 60 to 120, 60 to 80, 80 to 110, 80 to 100, 80 to 90, 90 to 120, 100 to 120, 110 to 120, 60 to 70, 70 to 80, 70 to 90, 70 to 100, 70 to 110, 70 to 120, 90 to 100, 90 to 110, 90 to 120, 100 to 110, 100 to 120, 110 to 120, and the like.

[0025] All the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions, if not particularly stated. All the technical features and optional technical features of the present application can be combined with each other to form new technical solutions, if not particularly stated. All the steps of the present application can be sequentially performed or randomly performed, preferably sequentially performed, if not particularly stated.

[0026] Due to the characteristics of smooth surface, strong inertness and few active functional groups of carbon fibers, it is difficult to form effective bonding with the resin matrix. In order to improve the interfacial bonding performance of carbon fibers and improve the performance of carbon fiber / resin composite materials, the carbon fibers are often subjected to electrochemical surface treatment before being combined with the resin matrix. The active oxygen atoms generated by anode are used to electrochemically etch the surface of the carbon fibers to activate the surface of the carbon fibers.

[0027] However, the existing carbon fiber surface treatment process usually only uses acidic and / or alkaline electrolyte to perform one-stage or two-stage electrochemical treatment on the carbon fibers, and the surface modification effect is not good, and the interfacial performance of the carbon fibers needs to be improved.

[0028] Based on this, the first aspect of the embodiments of the present application proposes a surface treatment method of carbon fibers, comprising the following steps: using a first electrolyte to perform first-stage electrochemical treatment on the carbon fibers, the pH of the first electrolyte is greater than or equal to 13; using a second electrolyte to perform second-stage electrochemical treatment on the carbon fibers subjected to the first-stage electrochemical treatment, the pH of the second electrolyte is less than or equal to 2; using a third electrolyte to perform third-stage electrochemical treatment on the carbon fibers subjected to the second-stage electrochemical treatment, the third electrolyte contains an ammonium salt and an amino compound, and the amino compound includes an amino-containing polymer and / or an amino-containing monomer.

[0029] In the present application, the surface of the carbon fibers is modified by using three-stage electrochemical treatment, which significantly improves the interfacial bonding performance of the carbon fibers. The first-stage electrochemical treatment controls the electrolyte to be strong alkaline (pH≥13), which can effectively remove the inert impurities (e.g., amorphous carbon formed in the carbonization process) on the surface of the carbon fiber and perform oxidation etching on the surface of the carbon fiber to improve the surface roughness. The rough surface can form a "mechanical embedding" (similar to an "anchoring effect") with the subsequent resin matrix, which can greatly enhance the physical bonding force of the interface. At the same time, the active groups containing oxygen (e.g., hydroxyl groups) are introduced during the etching, which solves the chemical inertness problem of the original carbon fiber.

[0030] The second-stage electrochemical treatment controls the electrolyte to be strong acid (pH≤2), which can neutralize the residual alkali in the first-stage electrochemical treatment process to ensure the cleanliness of the surface. The rough surface of the carbon fiber is further oxidized and etched to improve the surface roughness and uniformity, increase the O content on the surface of the carbon fiber, and promote the conversion of hydroxyl groups into carboxyl groups, thereby effectively increasing the carboxyl content on the surface of the carbon fiber and improving the hydrophilicity of the surface to better react with the subsequent amino-containing polymer or amino-containing monomer.

[0031] In the third-stage electrochemical treatment, an ammonium salt is used as the electrolyte and an amino compound is added. The ammonium salt is used to provide a conductive carrier to promote the electrochemical oxidation reaction on the surface of the carbon fiber. The amino-containing polymer and / or the amino-containing monomer are rich in amino groups, which can undergo an amidation reaction with the carboxyl groups generated in the first-stage electrochemical treatment / second-stage electrochemical treatment during the electrochemical oxidation reaction to form a firm graft, thereby introducing a large number of amino functional groups on the surface of the carbon fiber. Compared with carboxyl groups, amino groups have higher reactivity with the resin matrix and form more stable chemical bonds (e.g., ring-opening addition reaction with epoxy resin to form a stable C-N covalent bond; amide bond (-CONH-) with carboxyl-containing resin), which can effectively improve the interfacial bonding performance of the carbon fiber. However, if only the first-stage electrochemical treatment and the second-stage electrochemical treatment are performed, there are lack of active sites, and the amino functional groups are difficult to achieve firm and stable grafting, which limits the improvement of the interfacial performance of the carbon fiber. Moreover, the use of ammonium salt instead of strong acid or strong base in the third-stage electrochemical treatment provides a relatively mild electrochemical treatment environment, which can avoid the over-damage of the carbon fiber caused by the use of acid or base, i.e., it is beneficial to ensure the mechanical strength of the modified carbon fiber.

[0032] The density of the surface active groups (amino groups and carboxyl groups) of the carbon fiber treated by the method is significantly improved, which can effectively improve the interfacial bonding performance of the carbon fiber and further improve the interlaminar shear strength of the composite material. The surface treatment method of the carbon fiber provided in the embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0033] Figure 1 The process flow chart of the surface treatment method of the carbon fiber provided in the embodiments of the present application is shown in FIG. 1. Please refer to FIG. 1. Figure 1The surface treatment method of the carbon fiber provided by the embodiments of the present application comprises the following steps: S1: performing first-stage electrochemical treatment on the carbon fiber by using a first electrolyte, wherein the pH of the first electrolyte is greater than or equal to 13.

[0034] For example, the pH of the first electrolyte is 13, 13.2, 13.5, 13.8, 14, or a range between any two of the above values.

[0035] In some embodiments, the electrolyte in the first electrolyte comprises at least one of sodium hydroxide, potassium hydroxide, or calcium hydroxide. Preferably, the electrolyte in the first electrolyte is sodium hydroxide.

[0036] Further, the solvent in the first electrolyte is water.

[0037] In some embodiments, the mass percentage of the electrolyte in the first electrolyte is 3wt% to 12wt%. For example, the mass percentage of the electrolyte in the first electrolyte is 3wt%, 5wt%, 6wt%, 8wt%, 10wt%, 12wt%, or a range between any two of the above values.

[0038] Preferably, the mass percentage of the electrolyte in the first electrolyte is 5wt% to 10wt%.

[0039] In some embodiments, the conditions of the first-stage electrochemical treatment comprise: a surface treatment electric quantity of 60C / g to 100C / g, a temperature of 50°C to 90°C, and a time of 20s to 90s.

[0040] For example, the surface treatment electric quantity of the first-stage electrochemical treatment is 60C / g, 70C / g, 80C / g, 90C / g, 100C / g, or a range between any two of the above values; the temperature is 50°C, 60°C, 70°C, 80°C, 90°C, or a range between any two of the above values; and the time is 20s, 30s, 50s, 80s, 90s, or a range between any two of the above values.

[0041] Preferably, the surface treatment electric quantity of the first-stage electrochemical treatment is 70C / g to 80C / g, the temperature is 60°C to 80°C, and the time is 30s to 60s.

[0042] S2: performing second-stage electrochemical treatment on the carbon fiber subjected to the first-stage electrochemical treatment by using a second electrolyte, wherein the pH of the second electrolyte is less than or equal to 2.

[0043] For example, the pH of the second electrolyte is 0.5, 1, 1.5, 2, or a range between any two of the above values.

[0044] In some embodiments, the electrolyte in the second electrolyte includes at least one selected from sulfuric acid, nitric acid, phosphoric acid, oxalic acid, or acetic acid. Preferably, the electrolyte in the second electrolyte is sulfuric acid.

[0045] Furthermore, the solvent in the second electrolyte is water.

[0046] In some embodiments, the mass percentage of the electrolyte in the second electrolyte is 15 wt% to 50 wt%. As an example, the mass percentage of the electrolyte in the second electrolyte is within any two of the following values: 15 wt%, 20 wt%, 30 wt%, 40 wt%, 45 wt%, 50 wt%, or more.

[0047] Preferably, the mass percentage of the electrolyte in the second electrolyte is 20wt% to 30wt%.

[0048] In some embodiments, the conditions for the second-stage electrochemical treatment include: a surface treatment charge of 10C / g to 30C / g, a temperature of 40℃ to 80℃, and a time of 20s to 70s.

[0049] As an example, the surface treatment charge of the second-stage electrochemical treatment is within the range of any two values ​​between 10C / g, 15C / g, 20C / g, 25C / g, 30C / g or above; the temperature is within the range of any two values ​​between 40℃, 50℃, 60℃, 70℃, 80℃ or above; and the time is within the range of any two values ​​between 20s, 30s, 40s, 60s, 70s or above.

[0050] Preferably, the surface treatment charge for the second-stage electrochemical treatment is 15C / g to 20C / g, the temperature is 50℃ to 70℃, and the time is 30s to 60s.

[0051] S3: The carbon fibers that have undergone the second-stage electrochemical treatment are subjected to a third-stage electrochemical treatment using a third electrolyte. The third electrolyte includes ammonium salts and amino compounds, and the amino compounds include amino-containing polymers and / or amino-containing monomers.

[0052] In some embodiments, the amino-containing polymer includes at least one of polyethyleneimine or polyacrylamide, and the amino-containing monomer includes at least one of polyethylenepolyamine or diethylenetriamine.

[0053] In some embodiments, the ammonium salt includes at least one of ammonium citrate, ammonium oxalate, ammonium tartrate, or cerium ammonium nitrate. In some embodiments, the third electrolyte includes ammonium citrate and polyethyleneimine. Polyethyleneimine has a polyamino structure, with each molecule containing dozens of amino groups (primary, secondary, and tertiary amines), providing a large number of reaction sites, thus allowing for the introduction of more amino groups onto the carbon fiber surface and achieving more robust grafting. Ammonium citrate, as an electrolyte, provides H+ through its carboxyl group (-COOH).+ Promotes the protonation of the amino group in polyethyleneimine (-NH2→-NH3). + This enhances the electrostatic adsorption of polyethyleneimine to the negatively charged surface of carbon fibers; at the same time, the carboxyl groups of ammonium citrate can also undergo an "amidation reaction" with the amino groups of polyethyleneimine, further promoting the grafting of polyethyleneimine onto the surface of carbon fibers.

[0054] Furthermore, the solvent in the third electrolyte is water.

[0055] Furthermore, the weight-average molecular weight of polyethyleneimine is 200 g / mol to 600 g / mol. Using low molecular weight polyethyleneimine allows for complete dissolution in water without aggregation, enabling uniform contact with the carboxyl sites on the carbon fiber surface. Additionally, its shorter molecular chain results in higher reaction efficiency.

[0056] As an example, the weight-average molecular weight of polyethyleneimine is in the range of any two values ​​between 200 g / mol, 300 g / mol, 400 g / mol, 500 g / mol, 600 g / mol, or above. In some embodiments, the mass percentage of ammonium salt in the third electrolyte is 3 wt% to 20 wt%, and the mass percentage of amino-containing polymer or amino-containing monomer is 0.1 wt% to 0.5 wt%. By controlling the mass percentages of ammonium salt and amino-containing polymer or amino-containing monomer within a suitable range, it is beneficial to maintain a stable reaction environment, promote the protonation of amino-containing polymer or amino-containing monomer, and facilitate the grafting of more amino groups onto the carbon fiber surface.

[0057] As an example, in the third electrolyte, the mass percentage of ammonium salt is in the range of any two values ​​between 3 wt%, 8 wt%, 10 wt%, 12 wt%, 15 wt%, 20 wt%, or more; and the mass percentage of amino-containing polymer or amino-containing monomer is in the range of any two values ​​between 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, or more.

[0058] Preferably, in the third electrolyte, the mass percentage of ammonium salt is 5wt% to 15wt%, and the mass percentage of amino-containing polymer or amino-containing monomer is 0.3wt% to 0.5wt%.

[0059] In some embodiments, the conditions for the third-level electrochemical treatment include: a surface treatment charge of 10C / g to 30C / g, a temperature of 30℃ to 70℃, and a time of 15s to 50s.

[0060] As an example, the surface treatment charge of the third-level electrochemical treatment is within the range of any two values ​​between 10C / g, 15C / g, 20C / g, 25C / g, 30C / g or above; the temperature is within the range of any two values ​​between 30℃, 40℃, 50℃, 60℃, 70℃ or above; and the time is within the range of any two values ​​between 15s, 20s, 30s, 40s, 50s or above.

[0061] Preferably, the surface treatment charge for the third-stage electrochemical treatment is 15C / g to 20C / g, the temperature is 40℃ to 60℃, and the time is 20s to 40s.

[0062] In some embodiments, the third-stage electrochemical treatment further includes: washing with water and drying.

[0063] Furthermore, the water washing includes multiple ultrasonic cleaning processes, which helps to completely remove residual electrolytes.

[0064] Furthermore, the drying temperature is 80℃~120℃, and the time is 2h~4h. As an example, the drying temperature is within the range of any two values ​​of 80℃, 90℃, 100℃, 110℃, 120℃ or above, and the time is within the range of any two values ​​of 2h, 3h, 4h or above.

[0065] Preferably, the drying method is vacuum drying, with a pressure ≤10 kPa.

[0066] In addition, this application embodiment also provides a surface-modified carbon fiber, which is prepared by the above-described surface treatment method.

[0067] The surface-modified carbon fiber provided in this application has abundant carboxyl and amino groups on its surface, exhibiting excellent interfacial bonding properties. It can form an effective bond with the resin matrix, thereby improving the interlaminar shear strength of the composite material. Specifically, the surface-modified carbon fiber has a carboxyl content of not less than 20%, an amino content of not less than 15%, and a tensile strength greater than 5700 MPa.

[0068] In addition, this application also provides a composite material, including a resin matrix and the above-mentioned surface-modified carbon fiber.

[0069] The composite material in this application, by combining surface-modified carbon fiber with a resin matrix, effectively improves the overall performance, especially the mechanical strength, with an interlaminar shear strength of not less than 90 MPa, and can be widely used in aerospace, new energy vehicles and other fields.

[0070] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0071] Example 1 This embodiment provides a surface treatment method for carbon fiber, including the following steps: (1) First-stage electrochemical treatment Carbon fibers were immersed in a first electrolytic cell containing an aqueous sodium hydroxide solution (pH 13.7), with the sodium hydroxide mass percentage controlled at 5 wt%, the surface treatment charge at 80 C / g, the treatment temperature at 60 ℃, and the treatment time at 30 s.

[0072] (2) Second-stage electrochemical treatment The carbon fibers that have undergone the first stage of electrochemical treatment are immersed in a second electrolytic cell containing sulfuric acid aqueous solution (pH 0.6). The mass percentage of sulfuric acid is controlled at 30 wt%, the surface treatment charge is 15 C / g, the treatment temperature is 50 ℃, and the treatment time is 30 s.

[0073] (3) Third-stage electrochemical treatment The carbon fibers that have undergone the third stage of electrochemical treatment are immersed in a third electrolytic cell containing a mixed aqueous solution of ammonium citrate and polyethyleneimine. The mass percentage of ammonium citrate is 5 wt%, the mass percentage of polyethyleneimine is 0.5 wt%, the surface treatment charge is 15 C / g, the treatment temperature is 50℃, and the treatment time is 30 s.

[0074] (4) Post-processing The carbon fibers that have undergone the third stage of electrochemical treatment were subjected to three ultrasonic water washes and dried at 100°C for 3 hours in a vacuum drying oven to obtain surface-modified carbon fibers.

[0075] Example 2 This embodiment provides a surface treatment method for carbon fiber, which differs from Embodiment 1 in that: In step (3), the mass percentage of polyethyleneimine is 0.4 wt%.

[0076] Example 3 This embodiment provides a surface treatment method for carbon fiber, which differs from Embodiment 1 in that: In step (1), the mass percentage of sodium hydroxide is 8 wt%, and the pH is 14.

[0077] Example 4 This embodiment provides a surface treatment method for carbon fiber, which differs from Embodiment 1 in that: In step (2), the mass percentage of sulfuric acid is 20 wt%, and the pH is 0.7.

[0078] Comparative Example 1 This comparative example provides a carbon fiber without surface treatment.

[0079] Comparative Example 2 This comparative example provides a surface treatment method for carbon fiber, which differs from Example 1 in that: No polyethyleneimine is added in step (3).

[0080] Comparative Example 3 This comparative example provides a surface treatment method for carbon fiber, which differs from Example 1 in that: (1) First-stage electrochemical treatment Carbon fibers were immersed in a second electrolytic cell containing sulfuric acid aqueous solution (pH 0.6), with the mass percentage of sulfuric acid controlled at 30 wt%, the surface treatment charge at 15 C / g, the treatment temperature at 50 ℃, and the treatment time at 30 s.

[0081] (2) Second-stage electrochemical treatment The carbon fibers that have undergone the first stage of electrochemical treatment are immersed in the first electrolytic cell of sodium hydroxide aqueous solution (pH 14), with the mass percentage of sodium hydroxide controlled at 5 wt%, the surface treatment charge at 80 C / g, the treatment temperature at 60℃, and the treatment time at 30 s.

[0082] Comparative Example 4 This comparative example provides a surface treatment method for carbon fiber, which differs from Example 1 in that: Steps (1) and (2) are not included.

[0083] Some surface treatment parameters in Examples 1-4 and Comparative Examples 1-4 are shown in Table 1.

[0084] Table 1. Some surface treatment parameters in the examples and comparative examples.

[0085] Test case The surface-modified carbon fibers provided in the examples and comparative examples are combined with epoxy resin matrix to prepare carbon fiber / resin composite materials. The preparation method of the composite material is as follows: (1) The carbon fiber experimental filament is wound on a wooden board covered with release paper to a fixed number of turns; (2) It is mixed with AG80 resin and curing agent in a certain proportion, and evenly coated on the wound filament in the previous step, impregnated, and kneaded; (3) It is placed in a mold and pressed into a fixed cross section (constant width and thickness), heated and cooled to make an interlayer shear strip (one-way plate).

[0086] The properties of surface-modified carbon fibers and composite materials were tested, and the test results are shown in Table 2. The specific test methods are as follows: (1) Carboxyl / amino content: X-ray photoelectron spectroscopy (XPS) was used to detect the carboxyl and amino content on the carbon fiber surface.

[0087] (2) Tensile strength: Tested according to GB / T3362 test standard.

[0088] (3) Interlaminar shear strength (IFSS): Tested according to ASTM D2344 standard.

[0089] Table 2 Performance test results of surface-modified carbon fibers and composite materials

[0090] As shown in Table 2, compared with Comparative Examples 1-4, the surface-modified carbon fibers obtained by surface treatment in Examples 1-4 of this application are all rich in carboxyl and amino groups, while maintaining high tensile strength. The resulting composite materials have good interlaminar shear strength. Specifically, the carboxyl content is not less than 20%, the amino content is not less than 12%, and the tensile strength is greater than 5700 MPa; the corresponding interlaminar shear strength of the composite material is not less than 90 MPa.

[0091] A comparison of the results of Example 1 and Comparative Example 1 shows that the three-stage step electrochemical treatment of carbon fibers in this application effectively increases the content of carboxyl and amino groups on the surface of carbon fibers, while maintaining no significant reduction in tensile strength, and significantly improves the interlaminar shear strength of the composite material.

[0092] A comparison of the results of Example 1 and Comparative Example 2 shows that without the addition of polyethyleneimine, more amino groups cannot be introduced. Although the composite material contains more carboxyl groups, the interlaminar shear strength of the composite material is only slightly improved.

[0093] A comparison of the results of Example 1 and Comparative Example 3 shows that using a strong acid for the first stage of electrochemical treatment followed by a strong base for the second stage of electrochemical treatment cannot produce enough carboxyl groups, which leads to unstable grafted amino groups, resulting in a decrease in the amino grafting content and a relatively small increase in the interlaminar shear strength of the composite material.

[0094] A comparison of the results of Example 1 and Comparative Example 4 shows that without the first and second stage electrochemical treatments, even with the addition of polyethyleneimine to the electrolyte, the amino groups cannot be firmly grafted, resulting in poor stability. Therefore, the amino and carboxyl group content on the carbon fiber surface cannot be effectively increased, and consequently, the interlaminar shear strength of the composite material cannot be effectively improved.

[0095] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A surface treatment method for carbon fiber, characterized in that, Includes the following steps: The carbon fiber is subjected to the first stage of electrochemical treatment using a first electrolyte solution with a pH ≥ 13. A second electrolyte is used to perform a second electrochemical treatment on the carbon fibers that have undergone the first stage of electrochemical treatment, wherein the pH of the second electrolyte is ≤2; A third electrolyte is used to perform a third-stage electrochemical treatment on the carbon fibers that have undergone the second-stage electrochemical treatment. The third electrolyte includes ammonium salts and amino compounds, and the amino compounds include amino-containing polymers and / or amino-containing monomers.

2. The surface treatment method according to claim 1, characterized in that, The amino-containing polymer includes at least one of polyethyleneimine or polyacrylamide, and the amino-containing monomer includes at least one of polyethylenepolyamine or diethylenetriamine; And / or, the ammonium salt includes at least one of ammonium citrate, ammonium oxalate, ammonium tartrate, or cerium ammonium nitrate.

3. The surface treatment method according to claim 1, characterized in that, The third electrolyte includes ammonium citrate and polyethyleneimine; Preferably, the weight-average molecular weight of the polyethyleneimine is 200 g / mol to 600 g / mol.

4. The surface treatment method according to claim 1, characterized in that, In the third electrolyte, the ammonium salt has a mass percentage of 3wt% to 20wt%, and the amino compound has a mass percentage of 0.1wt% to 0.5wt%. Preferably, the solvent of the third electrolyte is water.

5. The surface treatment method according to any one of claims 1 to 4, characterized in that, The electrolyte in the first electrolyte solution includes at least one of sodium hydroxide, potassium hydroxide, or calcium hydroxide; Preferably, the mass percentage of the electrolyte in the first electrolyte solution is 3wt% to 12wt%.

6. The surface treatment method according to any one of claims 1 to 4, characterized in that, The electrolyte in the second electrolyte includes at least one of sulfuric acid, nitric acid, phosphoric acid, oxalic acid, or acetic acid; Preferably, the mass percentage of the electrolyte in the second electrolyte solution is 15wt% to 50wt%.

7. The surface treatment method according to any one of claims 1 to 4, characterized in that, The conditions for the first-stage electrochemical treatment include: surface treatment charge of 60C / g to 100C / g, temperature of 50℃ to 90℃, and time of 20s to 90s; And / or, the conditions for the second-stage electrochemical treatment include: surface treatment charge of 10C / g to 30C / g, temperature of 40℃ to 80℃, and time of 20s to 70s; And / or, the conditions for the third-level electrochemical treatment include: surface treatment charge of 10C / g to 30C / g, temperature of 30℃ to 70℃, and time of 15s to 50s.

8. The surface treatment method according to any one of claims 1 to 4, characterized in that, The third-stage electrochemical treatment also includes: water washing and drying; Preferably, the water washing includes multiple ultrasonic cleaning processes; Preferably, the drying temperature is 80℃~120℃ and the time is 2h~4h.

9. A surface-modified carbon fiber, characterized in that, It is prepared by the surface treatment method as described in any one of claims 1 to 8.

10. A composite material, characterized in that, It includes a resin matrix and surface-modified carbon fibers as described in claim 9.

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