PA sizing chopped carbon fiber and preparation method thereof

Abstract

The invention discloses a PA sizing chopped carbon fiber and a preparation method thereof, and belongs to the technical field of treating fiber products by using carbon or compounds thereof. The preparation method comprises the steps of carbon fiber desizing, carbon fiber surface modification, carbon nanotube deposition and sizing in a PA sizing agent. The surface modification of the carbon fiber comprises the following steps: performing surface treatment through atmospheric pressure plasma jet, introducing active oxygen and active nitrogen groups on the surface of the carbon fiber, and selectively nucleating and growing nano particles with catalytic activity on the carbon fiber. The PA sizing chopped carbon fiber prepared by the invention has good comprehensive performance, and a new method is provided for the treatment of carbon fiber products.

Description

Technical Field

[0001] The present invention relates to the technical field of treating fiber products with carbon or its compounds, and particularly relates to a PA-sized short carbon fiber and a preparation method thereof. Background Art

[0002] Carbon fiber material is an artificial material with both light weight and high mechanical strength, generally made of carbonized fibers treated with epoxy coating and graphite pressed weaving. Carbon fiber materials are widely used in many fields such as aerospace, automotive industry, sports and leisure.

[0003] Chinese Patent CN104532548A discloses a method for in-situ growth of carbon nanotubes on the surface of carbon fibers. The carbon fibers are controllably surface-treated by an electrochemical method to control the number and distribution of active sites on the surface of the carbon fibers; the catalyst is loaded by a solution impregnation method to achieve uniform distribution of the catalyst on the surface of the carbon fibers. The method for in-situ growth of CNTs on the surface of carbon fibers involved in the present invention includes carbon fiber surface treatment, catalyst loading, catalyst reduction and CNTs growth processes. The carbon fiber surface treatment uses an electrochemical oxidation method, with the electrolyte concentration being 1-10 wt%; the electrolysis current being 0.1-1.5 A, and the electrolysis time being 1-15 min. This method has mild treatment conditions, simple operation and is easy to control. The fiber strength loss is small, the loss rate of the multifilament tensile strength is less than 10%, the CNTs on the surface of the carbon fibers are uniformly and densely distributed, the loading amount and distribution density are controllable, and the interlaminar shear strength of the composite material is increased by more than 13%. It is applicable to the processing field of high-performance carbon fiber reinforced resin matrix composites, and particularly applicable to the processing field of carbon fiber reinforced resin matrix composites with high requirements for interlaminar bonding performance.

[0004] Chinese Patent CN108625159A provides a device and method for uniformly growing carbon nanotubes on the surface of carbon fiber cloth, including the following steps: Step 1: Winding the carbon fiber cloth loaded with the catalyst precursor solution after desizing around a bracket and placing it in a vacuum reaction chamber; Step 2: Heating the vacuum reaction chamber to the reduction temperature in an inert gas atmosphere; Step 3: Introducing hydrogen into the vacuum reaction chamber in Step 2 for catalyst reduction; Step 4: After the reduction is completed, heating the vacuum reaction chamber to the growth temperature in an inert gas atmosphere; The present invention can achieve uniform growth of carbon nanotubes on the surface of carbon fiber cloth and achieve the industrialization purpose of large-scale production.

[0005] Chinese Patent CN109763319A discloses a process for catalytically growing carbon nanotubes on the surface of PAN-based carbon fibers using a sulfur-doped modified catalyst, which includes the following steps: Step 1: Desizing the PAN-based carbon fibers; Step 2: Surface oxidation treatment of the desized carbon fibers by electrochemical anodic oxidation; Step 3: Preparing an ethanol solution of cobalt nitrate hexahydrate and thiourea with a molar ratio of 5:2 as the catalyst precursor, immersing the surface-oxidized carbon fibers in the precursor for 5 - 30 min, and then drying them in an oven; Step 4: Placing the carbon fibers in a tube furnace, heating them to 600 - 800 °C at a rate of 5 - 10 °C / min under nitrogen protection, introducing a H2 / C2H2 mixed gas with a flow ratio of 4:1 - 1:1, holding for 5 - 20 min, and taking out the sample after cooling to room temperature. This invention can improve the surface properties of carbon fibers, increase the surface roughness of the fibers, and effectively improve the interfacial properties of carbon fiber reinforced composites.

[0006] The surface energy of carbon fibers is low and the number of active groups is small, resulting in insufficient wettability between the catalyst and the carbon fibers, and it is difficult to uniformly load on the surface of the fibers. Therefore, during the chemical vapor deposition process, it is difficult to catalytically grow regular carbon nanotubes. The prior art uses chemical or electrochemical methods to oxidize the surface of carbon fibers, and improves the adhesion ability of the catalyst on the surface of carbon fibers by increasing the specific surface area through oxidation, thereby enhancing the deposition effect of carbon nanotubes on the surface of carbon fibers. Using chemical methods such as acid or alkali treatment has a poor modification effect on carbon fibers and is prone to uneven treatment, resulting in uneven growth of carbon nanotubes. While using electrochemical methods for treatment, although it has better uniformity, the electrochemical etching has a high dependence on current. When the current is small, the morphology change of the loaded catalyst is not significant, and when the current is increased, it will cause damage to the mechanical properties of the carbon fibers. Generally speaking, increasing the specific surface area through oxidation damages the structural integrity of the carbon fibers, and may cause a decrease in the performance of the carbon fiber matrix in actual applications. Summary of the Invention

[0007] In view of the above-mentioned defects of the prior art, the problem to be solved by the present invention is to provide a PA-sized short carbon fiber with excellent mechanical properties and its preparation method.

[0008] The present invention provides a method for preparing PA-sized short carbon fibers, which includes carbon fiber desizing, carbon fiber surface modification, depositing carbon nanotubes, and sizing by immersing in a PA sizing agent; the carbon fiber surface modification includes surface treatment by atmospheric pressure plasma jet to introduce active oxygen and active nitrogen groups on the surface of the carbon fibers; and the selective nucleation and growth of catalytically active nanoparticles on the carbon fibers.

[0009] The present invention uses an atmospheric pressure plasma jet to treat carbon fibers, generating functional groups containing active oxygen and active nitrogen. And due to the internal heating of the carbon fibers caused by microwave radiation, it promotes the subsequent nucleation of catalytically active cobalt compounds on the surface of the carbon fibers rather than in the reaction medium, optimizing the deposition effect of carbon nanotubes.

[0010] After treatment with an atmospheric pressure plasma jet, the surface roughness of the carbon fibers does not increase like that of oxidation treatment. Instead, it only modifies the surface at the nanoscale without changing the crystal structure of the carbon fibers. During the treatment process, the reactants can react with the structural defects on the surface of the carbon fibers, such as vacancies, cavities, grain boundaries, etc., and dope nitrogen atoms at these defects to generate graphitized nitrogen or pyrrole and pyridine structures, which has a certain repair effect on the surface defects and amorphous regions, making the carbon fibers have better mechanical properties.

[0011] Since the introduction of nitrogen atoms forms new groups, it changes the degree of interfacial polarization of the carbon fibers. Compared with the untreated carbon fibers, the change in the polarizability leads to an increase in the local surface temperature during the microwave treatment of the carbon fibers. The resulting dangling bonds can promote the transformation of pyrrole nitrogen atoms or pyridine nitrogen atoms into graphitized nitrogen atoms; in addition, the increase in temperature will promote the nucleation and growth of cobalt compounds on the surface of the carbon fibers. Compared with the way of nucleating in the reaction medium and then attaching to the surface of the carbon fibers or directly adsorbing, the catalytic substances in the present invention have stronger binding to the substrate, wider distribution of the catalysts, and higher catalytic activity, making the carbon nanotubes have excellent deposition effects on the carbon fibers.

[0012] A preparation method of PA-sized short carbon fibers comprises the following steps:

[0013] S1. Immerse the carbon fibers in absolute ethanol, heat under reflux to remove the organic coating on the surface, dry, then wash successively with chloroform and acetone, and after re-drying, obtain desized carbon fibers for standby;

[0014] S2. Place the desized carbon fibers in a plasma surface treatment device, and perform surface treatment on the desized carbon fibers through an atmospheric pressure plasma jet to obtain surface-treated carbon fibers for standby;

[0015] S3. Mix ammonium chloride, cobalt chloride hexahydrate and deionized water evenly, then add concentrated ammonia water to obtain a reaction solution; add hydrogen peroxide to the reaction solution and react. After the reaction ends, cool the product, filter and collect the precipitate; mix the precipitate with deionized water evenly, filter and collect the filtrate. After the filtrate is cooled to room temperature, add concentrated hydrochloric acid to it, mix evenly and then cool, filter and collect the filter cake. The filter cake is washed with absolute ethanol and dried to obtain a precursor for standby;

[0016] S4. Take another portion of the precursor and mix it evenly with deionized water to obtain a precursor solution; immerse the surface-treated carbon fiber in the precursor solution, perform microwave treatment under closed conditions, conduct a cooling treatment after the microwave treatment is completed, then recover the carbon fiber, wash it with absolute ethanol, and dry it to obtain modified carbon fiber for standby;

[0017] S5. Place the modified carbon fiber in a deposition device and deposit carbon nanotubes on its surface by chemical vapor deposition to obtain carbon nanotube-reinforced carbon fiber for standby;

[0018] S6. Immerse the carbon nanotube-reinforced carbon fiber in a PA sizing agent for treatment, then pull out the carbon fiber in bundles, dry it, and cut it into short lengths to obtain PA-sized short carbon fibers.

[0019] Specifically, a method for preparing PA-sized short carbon fibers includes the following steps, in parts by weight:

[0020] S1. Immerse the carbon fiber in absolute ethanol, perform heat reflux treatment to remove the organic coating on the surface, dry it, then wash it successively with chloroform and acetone, and dry it again after washing to obtain desized carbon fiber for standby;

[0021] S2. Place the desized carbon fiber in a plasma surface treatment device and perform surface treatment on the desized carbon fiber through an atmospheric pressure plasma jet to obtain surface-treated carbon fiber for standby;

[0022] S3. Mix 2.35 - 3.10 parts of ammonium chloride, 2.95 - 3.85 parts of cobalt chloride hexahydrate with 5.0 - 10.0 parts of deionized water evenly, then add 7.5 - 10.0 parts of concentrated ammonia water to obtain a reaction solution; add 10.0 - 15.0 parts of hydrogen peroxide to the reaction solution and react, cool the product to 0 - 4 °C after the reaction is completed, filter and collect the precipitate; mix the precipitate evenly with 30 - 40 parts of deionized water at 85 - 100 °C, filter and collect the filtrate, add 4.75 - 6.20 parts of concentrated hydrochloric acid to the filtrate after it is cooled to room temperature, mix evenly and then cool to 0 - 4 °C, filter and collect the filter cake, wash the filter cake with absolute ethanol and dry it to obtain a precursor for standby;

[0023] S4. Take another 0.15 - 0.20 parts of the precursor and mix it evenly with 50 - 75 parts of deionized water to obtain a precursor solution; immerse 1.65 - 2.15 parts of the surface-treated carbon fiber in the precursor solution, perform microwave treatment under closed conditions, conduct a cooling treatment after the microwave treatment is completed, then recover the carbon fiber, wash it with absolute ethanol, and dry it to obtain modified carbon fiber for standby;

[0024] S5. Place the modified carbon fiber in a deposition device, and deposit carbon nanotubes on its surface by chemical vapor deposition to obtain carbon nanotube-reinforced carbon fiber for standby.

[0025] S6. Immerse the carbon nanotube-reinforced carbon fiber in a PA sizing agent for treatment, then pull out the carbon fiber in bundles, and after drying and cutting, obtain PA-sized short carbon fibers.

[0026] Preferably, in step S1, the temperature of the heat reflux treatment is 50 - 70 °C, and the treatment time is 6 - 18 h.

[0027] Preferably, in step S2, the gas for surface treatment is nitrogen, the gas flow rate is 15 - 30 L / min, the treatment power is 115 - 140 W, the treatment rate is 50 - 100 cm / min, and the distance between the reactor nozzle and the desized carbon fiber is 15 - 30 mm.

[0028] Preferably, in step S3, the concentration of the concentrated ammonia water is 25 - 28 wt%.

[0029] Preferably, in step S3, the concentration of the hydrogen peroxide is 6 - 8 wt%.

[0030] Preferably, in step S3, the temperature of the reaction is 50 - 65 °C, and the reaction time is 0.5 - 2.0 h.

[0031] Preferably, in step S3, the concentration of the concentrated hydrochloric acid is 36 - 38 wt%.

[0032] Preferably, in step S4, the temperature of the microwave treatment is 165 - 185 °C, the treatment time is 2 - 6 h, and the microwave power is 550 - 650 W.

[0033] Preferably, in step S4, the cooling treatment is carried out in three stages. First, cool at a rate of 2.0 - 2.5 °C / min to 145 - 150 °C and maintain at this temperature for 5 - 15 min; then cool at a rate of 2.0 - 2.5 °C / min to 110 - 120 °C and maintain at this temperature for 5 - 15 min; finally, cool at a rate of 3.0 - 5.0 °C / min to room temperature.

[0034] Preferably, in step S5, the carbon source gas for chemical vapor deposition is acetylene or ethylene; the inert gas is nitrogen or argon; the volume ratio of the carbon source gas, inert gas, and hydrogen is 1:2:1; the deposition temperature is 450 - 550 °C, and the pressure during the deposition process is 0.01 - 0.02 MPa.

[0035] Chemical vapor deposition is not limited to the above preferred parameters, and the general chemical vapor deposition method in the art can also be used to deposit carbon nanotubes on the surface of the modified carbon fiber.

[0036] Preferably, the treatment time of the PA sizing agent on the carbon nanotube-reinforced carbon fiber in step S6 is 5-15 min.

[0037] Based on common general knowledge in the art, the above preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0038] The introduction and functions of some raw materials in the formula of the present invention are as follows:

[0039] Carbon fiber: Carbon fiber is a special fiber mainly composed of carbon elements, and its carbon content varies with different types, generally above 90%. Carbon fiber has the characteristics of general carbon materials, such as high temperature resistance, friction resistance, electrical conductivity, thermal conductivity, and corrosion resistance. However, different from general carbon materials, its shape has significant anisotropy, is soft, can be processed into various fabrics, and shows high strength along the fiber axis direction. Carbon fiber has a small specific gravity, so it has a high specific strength.

[0040] PA sizing agent: The PA sizing agent is a thin layer of polyamide resin evenly covering the surface of carbon fiber, which plays an important role in the performance of carbon fiber, the preparation of its woven fabric and prepreg, and the performance of composite materials. The primary function of the sizing agent is to bundle a large number of single filaments into a bundle to prevent the fibers from fluffing and loosening, and at the same time, it also provides the possibility for carbon fiber cloth and three-dimensional braids.

[0041] The beneficial effects of the present invention:

[0042] Compared with the prior art, the present invention uses atmospheric pressure plasma jet to treat carbon fiber, modifies the surface of carbon fiber at the nanoscale, and its surface roughness will not increase like oxidation treatment, and the crystal structure of carbon fiber is not changed. During the treatment process, the reactants react with the structural defects on the surface of carbon fiber, such as vacancies, cavities, grain boundaries, etc., and nitrogen atoms are doped at these defects to produce graphitized nitrogen or pyrrole and pyridine structures, which repair the surface defects in the amorphous region and improve the mechanical properties of carbon fiber.

[0043] Compared with the prior art, the present invention introduces active groups on the surface of carbon fiber, changes the degree of interfacial polarization of carbon fiber. When carbon fiber is treated by microwave, the change in the polarization rate is beneficial to the increase of the local surface temperature. The dangling bonds formed thereby can promote the transformation of pyrrole nitrogen atoms or pyridine nitrogen atoms into graphitized nitrogen atoms; in addition, the increase in temperature also promotes the nucleation and growth of cobalt compounds on the surface of carbon fiber. Compared with the way of nucleating in the reaction medium and then attaching to the surface of carbon fiber or directly adsorbing, the catalytic substance of the present invention has a stronger binding property with the matrix, a wider distribution of the catalyst, and higher catalytic activity.

[0044] Compared with the prior art, after the catalytic substance nucleates and grows, the present invention conducts a staged cooling treatment, avoiding the peeling of the catalytic substance caused by different thermal expansion coefficients, maintaining the high catalytic activity on the surface of the modified carbon fiber, and being conducive to improving the deposition effect of carbon nanotubes during the chemical vapor deposition process. Detailed implementation mode

[0045] The present invention will be further described below by way of examples, but the present invention is not limited to the scope of the described examples. For the experimental methods without specific conditions in the following examples, they are carried out according to conventional methods and conditions, or selected according to the product specifications.

[0046] The parameters of some raw materials in the comparative examples and examples of the present invention are as follows:

[0047] Carbon fiber, model: T300B-1000-40B, provided by Shenzhen Turing Evolution Technology Co., Ltd.; concentrated ammonia water, commercially available, concentration 28wt%; hydrogen peroxide, commercially available, concentration 6wt%; concentrated hydrochloric acid, commercially available, concentration 36wt%; PA sizing agent, model: Hydrosize PA845, provided by Michelman Company, USA.

[0048] Example 1

[0049] A kind of PA-sized short carbon fiber is prepared by the following method:

[0050] S1. Immerse the carbon fiber in absolute ethanol, and perform a heat reflux treatment to remove the organic coating on the surface. The temperature of the heat reflux treatment is 60°C, the treatment time is 12h, dry it, and then wash it successively with chloroform and acetone. After washing, dry it again to obtain desized carbon fiber for standby;

[0051] S2. Place the desized carbon fiber in a plasma surface treatment device, and perform surface treatment on the desized carbon fiber through an atmospheric pressure plasma jet. The gas for surface treatment is nitrogen, the gas flow rate is 25L / min, the treatment power is 125W, the treatment rate is 75cm / min, and the distance between the reactor nozzle and the desized carbon fiber is 20mm to obtain surface-treated carbon fiber for standby;

[0052] S3. Mix 2.35 kg of ammonium chloride, 2.95 kg of cobalt chloride hexahydrate and 5.0 kg of deionized water evenly, then add 7.5 kg of concentrated ammonia water to obtain a reaction solution; add 10.0 kg of hydrogen peroxide to the reaction solution and react. The reaction temperature is 60 °C and the reaction time is 1 h; after the reaction, cool the product to 0 °C and filter to collect the precipitate; mix the precipitate evenly with 30 kg of deionized water at 95 °C, filter to collect the filtrate, add 4.75 kg of concentrated hydrochloric acid to the filtrate after cooling to room temperature, mix evenly and then cool to 0 °C, filter to collect the filter cake, wash the filter cake with absolute ethanol and dry it to obtain a precursor for standby;

[0053] S4. Take another 0.15 kg of the precursor and mix it evenly with 50 kg of deionized water to obtain a precursor solution; immerse 1.65 kg of the surface-treated carbon fiber in the precursor solution and perform microwave treatment under closed conditions. The microwave treatment temperature is 175 °C, the treatment time is 4 h, and the microwave power is 600 W; after the microwave treatment, perform a cooling treatment, which is carried out in three stages. First, cool to 150 °C at a rate of 2.0 °C / min and maintain at this temperature for 10 min; then cool to 115 °C at a rate of 2.5 °C / min and maintain at this temperature for 10 min; finally, cool to room temperature at a rate of 4.0 °C / min, then recover the carbon fiber, wash it with absolute ethanol and dry it to obtain modified carbon fiber for standby;

[0054] S5. Place the modified carbon fiber in a deposition device and deposit carbon nanotubes on its surface by chemical vapor deposition. The carbon source gas for chemical vapor deposition is acetylene, the inert gas is argon, and the volume ratio of acetylene, argon and hydrogen is 1:2:1. The flow rate of the mixed gas is 24 L / min, the deposition temperature is 500 °C, the deposition time is 10 min, and the pressure during the deposition process is 0.01 MPa to obtain carbon nanotube-reinforced carbon fiber for standby;

[0055] S6. Immerse the carbon nanotube-reinforced carbon fiber in the PA sizing agent for 5 min. The mass ratio of the carbon nanotube-reinforced carbon fiber to the PA sizing agent is 3:1. Then pull out the carbon nanotube-reinforced carbon fiber in bundles, dry it and cut it short to obtain PA-sized short carbon fibers.

[0056] Example 2

[0057] A kind of PA-sized short carbon fiber is prepared by the following method:

[0058] S1. Immerse the carbon fiber in absolute ethanol and perform heat reflux treatment to remove the organic coating on the surface. The heat reflux treatment temperature is 60 °C and the treatment time is 12 h. After drying, wash it successively with chloroform and acetone, and then dry it again to obtain de-sized carbon fiber for standby;

[0059] S2. Place the desized carbon fiber in a plasma surface treatment device, and perform surface treatment on the desized carbon fiber through an atmospheric pressure plasma jet. The gas for surface treatment is nitrogen, the gas flow rate is 25 L / min, the treatment power is 125 W, the treatment rate is 75 cm / min, and the distance between the reactor nozzle and the desized carbon fiber is 20 mm to obtain surface-treated carbon fiber for standby;

[0060] S3. Mix 2.35 kg of ammonium chloride, 2.95 kg of cobalt chloride hexahydrate and 5.0 kg of deionized water evenly, and then add 7.5 kg of concentrated ammonia water to obtain a reaction solution; add 10.0 kg of hydrogen peroxide to the reaction solution and carry out the reaction. The reaction temperature is 60 °C and the reaction time is 1 h; after the reaction is completed, cool the product to 0 °C and filter to collect the precipitate; mix the precipitate evenly with 30 kg of deionized water at 95 °C, filter to collect the filtrate, add 4.75 kg of concentrated hydrochloric acid to the filtrate after cooling to room temperature, mix evenly and then cool to 0 °C, filter to collect the filter cake, and wash and dry the filter cake with absolute ethanol to obtain a precursor for standby;

[0061] S4. Take another 0.15 kg of the precursor and mix it evenly with 50 kg of deionized water to obtain a precursor solution; immerse 1.65 kg of the surface-treated carbon fiber in the precursor solution and carry out microwave treatment under closed conditions. The microwave treatment temperature is 175 °C, the treatment time is 4 h, and the microwave power is 600 W; after the microwave treatment is completed, cool it naturally to room temperature, then recover the carbon fiber, wash and dry it with absolute ethanol to obtain modified carbon fiber for standby;

[0062] S5. Place the modified carbon fiber in a deposition device, and deposit carbon nanotubes on its surface through chemical vapor deposition. The carbon source gas for chemical vapor deposition is acetylene, the inert gas is argon, and the volume ratio of acetylene, argon and hydrogen is 1:2:1. The flow rate of the mixed gas is 24 L / min, the deposition temperature is 500 °C, the deposition time is 10 min, and the pressure during the deposition process is 0.01 MPa to obtain carbon nanotube-reinforced carbon fiber for standby;

[0063] S6. Immerse the carbon nanotube-reinforced carbon fiber in a PA sizing agent for 5 min. The mass ratio of the carbon nanotube-reinforced carbon fiber to the PA sizing agent is 3:1. Then pull out the carbon nanotube-reinforced carbon fiber in bundles, dry and cut it short to obtain PA-sized short-cut carbon fiber.

[0064] Example 3

[0065] A kind of PA-sized short-cut carbon fiber is prepared by the following method:

[0066] S1. Immerse carbon fiber in absolute ethanol, and perform heat reflux treatment to remove the organic coating on the surface. The temperature of the heat reflux treatment is 60 °C, the treatment time is 12 h. After drying, wash it successively with chloroform and acetone, and then dry it again to obtain desized carbon fiber for standby;

[0067] S2. Mix 2.35 kg of ammonium chloride, 2.95 kg of cobalt chloride hexahydrate and 5.0 kg of deionized water evenly, then add 7.5 kg of concentrated ammonia water to obtain a reaction solution; add 10.0 kg of hydrogen peroxide to the reaction solution and react. The reaction temperature is 60 °C, and the reaction time is 1 h; after the reaction, cool the product to 0 °C, filter and collect the precipitate; mix the precipitate evenly with 30 kg of deionized water at 95 °C, filter and collect the filtrate. After the filtrate is cooled to room temperature, add 4.75 kg of concentrated hydrochloric acid to it, mix evenly and then cool to 0 °C, filter and collect the filter cake. The filter cake is washed with absolute ethanol and dried to obtain a precursor for standby;

[0068] S3. Take another 0.15 kg of the precursor and mix it evenly with 50 kg of deionized water to obtain a precursor solution; immerse 1.65 kg of the desized carbon fiber in the precursor solution, and perform microwave treatment under closed conditions. The temperature of the microwave treatment is 175 °C, the treatment time is 4 h, and the microwave power is 600 W; after the microwave treatment is completed, cool it naturally to room temperature, then recover the carbon fiber, wash it with absolute ethanol and dry it to obtain modified carbon fiber for standby;

[0069] S4. Place the modified carbon fiber in a deposition device, and deposit carbon nanotubes on its surface by chemical vapor deposition. The carbon source gas for chemical vapor deposition is acetylene, the inert gas is argon, and the volume ratio of acetylene, argon and hydrogen is 1:2:1. The flow rate of the mixed gas is 24 L / min, the deposition temperature is 500 °C, the deposition time is 10 min, and the pressure during the deposition process is 0.01 MPa to obtain carbon nanotube reinforced carbon fiber for standby;

[0070] S5. Immerse the carbon nanotube reinforced carbon fiber in PA sizing agent for 5 min. The mass ratio of the carbon nanotube reinforced carbon fiber to the PA sizing agent is 3:1. Then pull out the carbon nanotube reinforced carbon fiber in bundles, dry and cut it short to obtain PA sized short carbon fiber.

[0071] Example 4

[0072] A kind of PA sized short carbon fiber is prepared by the following method:

[0073] S1. Immerse the carbon fiber material in absolute ethanol, and perform heat reflux treatment to remove the organic coating on the surface. The temperature of the heat reflux treatment is 60 °C, the treatment time is 12 h. After drying, wash it successively with chloroform and acetone, and then dry it again to obtain desized carbon fiber for standby;

[0074] S2. Place the desized carbon fiber in a plasma surface treatment device, and perform surface treatment on the desized carbon fiber through an atmospheric pressure plasma jet. The gas for surface treatment is nitrogen, the gas flow rate is 25 L / min, the treatment power is 125 W, the treatment rate is 75 cm / min, and the distance between the reactor nozzle and the desized carbon fiber is 20 mm to obtain surface-treated carbon fiber for standby;

[0075] S3. Mix 3.10 kg of ammonium chloride, 3.85 kg of cobalt chloride hexahydrate with 10.0 kg of deionized water evenly, then add 10.0 kg of concentrated ammonia water to obtain a reaction solution; add 15.0 kg of hydrogen peroxide to the reaction solution and react. The reaction temperature is 60 °C and the reaction time is 1 h; after the reaction is completed, cool the product to 0 °C and filter to collect the precipitate; mix the precipitate with 40 kg of deionized water at 95 °C evenly, filter to collect the filtrate, add 6.20 kg of concentrated hydrochloric acid to the filtrate after cooling to room temperature, mix evenly and then cool to 0 °C, filter to collect the filter cake, and wash and dry the filter cake with absolute ethanol to obtain a precursor for standby;

[0076] S4. Take another 0.20 kg of the precursor and mix it evenly with 75 kg of deionized water to obtain a precursor solution; immerse 2.15 kg of the surface-treated carbon fiber in the precursor solution and perform microwave treatment under closed conditions. The microwave treatment temperature is 175 °C, the treatment time is 4 h, and the microwave power is 600 W; after the microwave treatment is completed, perform a cooling treatment. The cooling treatment is carried out in three stages. First, cool it to 150 °C at a rate of 2.0 °C / min and maintain it at this temperature for 10 min; then cool it to 115 °C at a rate of 2.5 °C / min and maintain it at this temperature for 10 min; finally, cool it to room temperature at a rate of 4.0 °C / min, then recover the carbon fiber, wash and dry it with absolute ethanol to obtain modified carbon fiber for standby;

[0077] S5. Place the modified carbon fiber in a deposition device and deposit carbon nanotubes on its surface by chemical vapor deposition. The carbon source gas for chemical vapor deposition is acetylene, the inert gas is argon, and the volume ratio of acetylene, argon, and hydrogen is 1:2:1. The flow rate of the mixed gas is 24 L / min, the deposition temperature is 500 °C, the deposition time is 10 min, and the pressure during the deposition process is 0.01 MPa to obtain carbon nanotube-reinforced carbon fiber for standby;

[0078] S6. Immerse the carbon nanotube-reinforced carbon fiber in the PA sizing agent for 5 min. The mass ratio of the carbon nanotube-reinforced carbon fiber to the PA sizing agent is 3:1. Then pull out the carbon nanotube-reinforced carbon fiber in bundles, dry and cut it short to obtain PA-sized short carbon fiber.

[0079] Control Example 1

[0080] This control example is carbon fiber without any treatment.

[0081] Test Example 1

[0082] The tensile properties of PA-sized chopped carbon fibers were tested according to the specific methods and procedures in Standard GB / T 3362-2017 "Test Method for Tensile Properties of Carbon Fiber Multifilaments". Ten specimens were measured for each group of tests. According to the requirements in the above standard, the specimens were made by impregnating carbon fiber multifilaments with epoxy resin adhesive. The formula of the resin adhesive is shown in Appendix A of this standard. After the carbon fiber multifilaments were impregnated with the adhesive and cured, they were cut according to the standard specimen size. The specimens should be evenly impregnated with the adhesive, smooth, straight, and defect-free. During the test, the loading rate was 10 mm / min, and the test environment conformed to the requirements in GB / T 1446-2005 "General Rules for Test Methods of Fiber Reinforced Plastics". The results were taken as the arithmetic mean as required. The test results of the tensile properties of PA-sized chopped carbon fibers are shown in Table 1.

[0083] Table 1:

[0084] Item Tensile strength (GPa) Example 1 4.18 Example 2 3.95 Example 3 3.77 Control Example 1 3.64

[0085] Test Example 2

[0086] The interlaminar shear strength of PA-sized chopped carbon fibers was characterized by making a composite material of carbon fibers and resin. The test was carried out according to the specific methods and procedures in JC / T 773-2010 "Determination of Interlaminar Shear Strength of Fiber Reinforced Plastics by Short Beam Method". The specimens were rectangular strips with a uniform thickness of 2 mm, a length of 20 mm, and a width of 10 mm. The number of specimens in each group was 10, and the test speed was 1 mm / min during the test. The results were taken as the arithmetic mean as required. The test results of the interlaminar shear strength are shown in Table 2.

[0087] Table 2:

[0088] Item Interlaminar shear strength (MPa) Example 1 114.7 Example 2 96.8 Example 3 83.5 Control Example 1 73.6

[0089] Test Example 3

[0090] The roughness of Example 1 of the present invention was measured using an atomic force microscope. The surface roughness of the original carbon fiber, the surface roughness of the carbon fiber after treatment with an atmospheric pressure plasma jet, the surface roughness after catalyst nucleation and after carbon nanotubes were deposited were measured respectively. The methods and steps in the test reference (Han Xianwu, Chen Xiaoan, Yang Xueheng, et al. Nanoscale surface roughness measurement based on atomic force microscope [J]. Journal of Chongqing University (Natural Science Edition), 2007, 30(2): 5-8. DOI: 10.3969 / j.issn.1000-582X.2007.02.002.) were followed. The test results of the surface roughness of PA-sized short carbon fibers at different treatment stages are shown in Table 3.

[0091] Table 3:

[0092] Stage Average surface roughness Ra (nm) Original carbon fiber 6.82 Atmospheric pressure plasma jet treatment 6.54 Catalyst nucleation 17.36 Deposition of carbon nanotubes 95.38

[0093] From the test results of the above test examples, it can be seen that the PA-sized short carbon fibers prepared by the present invention have excellent mechanical properties. The present invention uses an atmospheric pressure plasma jet to treat the carbon fiber, generating functional groups containing active oxygen and active nitrogen, and due to the internal heating of the carbon fiber caused by microwave radiation, it promotes the subsequent nucleation of the catalytically active cobalt compound on the surface of the carbon fiber rather than in the reaction medium, optimizing the deposition effect of carbon nanotubes. After treatment with an atmospheric pressure plasma jet, the surface roughness of the carbon fiber does not increase like that of oxidation treatment, but only modifies its surface at the nanoscale without changing the crystal structure of the carbon fiber. During the treatment process, the reactants can react with the structural defects on the surface of the carbon fiber, such as vacancies, cavities, grain boundaries, etc., doping nitrogen atoms at these defects to generate graphitic nitrogen or pyrrole and pyridine structures, which has a certain repair effect on the surface defects and amorphous regions, making the carbon fiber have better mechanical properties.

[0094] Since the introduction of nitrogen atoms forms new groups, the degree of interfacial polarization of the carbon fiber changes. Compared with the untreated carbon fiber, the change in the polarizability leads to an increase in the local surface temperature during microwave treatment of the carbon fiber. The resulting dangling bonds can promote the transformation of pyrrole nitrogen atoms or pyridine nitrogen atoms into graphitic nitrogen atoms; in addition, the increase in temperature will promote the nucleation and growth of cobalt compounds on the surface of the carbon fiber. Compared with the way of nucleating in the reaction medium and then attaching to the surface of the carbon fiber or directly adsorbing, the catalytic substance of the present invention has a stronger binding property with the matrix, a wider distribution of the catalyst, and higher catalytic activity, making the carbon nanotubes have excellent deposition effects on the carbon fiber.

Claims

1. A preparation method of PA-sized short carbon fibers, comprising carbon fiber desizing, carbon fiber surface modification, carbon nanotube deposition, and sizing by immersion in a PA sizing agent, characterized in that: The surface modification of the carbon fiber includes surface treatment by an atmospheric pressure plasma jet to introduce active oxygen and active nitrogen groups on the surface of the carbon fiber; and the selective nucleation and growth of catalytically active nanoparticles on the carbon fiber.

2. The preparation method of the PA-sized chopped carbon fiber according to claim 1, characterized in that, The following steps are included, by weight: S1. Immerse the carbon fiber in absolute ethanol, perform heat reflux treatment to remove the organic coating on the surface, after drying, wash it successively with chloroform and acetone, and after re-drying, obtain desized carbon fiber for standby; S2. Place the desized carbon fiber in a plasma surface treatment device, and perform surface treatment on the desized carbon fiber by an atmospheric pressure plasma jet to obtain surface-treated carbon fiber for standby; S3. Mix 2.35 - 3.10 parts of ammonium chloride, 2.95 - 3.85 parts of cobalt chloride hexahydrate and 5.0 - 10.0 parts of deionized water evenly, then add 7.5 - 10.0 parts of concentrated ammonia water to obtain a reaction solution; add 10.0 - 15.0 parts of hydrogen peroxide to the reaction solution and react. After the reaction is completed, cool the product to 0 - 4 °C, filter and collect the precipitate; mix the precipitate evenly with 30 - 40 parts of deionized water at 85 - 100 °C, filter and collect the filtrate. After the filtrate is cooled to room temperature, add 4.75 - 6.20 parts of concentrated hydrochloric acid to it, mix evenly and then cool to 0 - 4 °C, filter and collect the filter cake. The filter cake is washed with absolute ethanol and dried to obtain a precursor for standby; S4. Take another 0.15 - 0.20 parts of the precursor and mix it evenly with 50 - 75 parts of deionized water to obtain a precursor solution; Immerse 1.65 - 2.15 parts of the surface-treated carbon fiber in the precursor solution, perform microwave treatment under closed conditions, perform cooling treatment after the microwave treatment is completed, then recover the carbon fiber, wash it with absolute ethanol and dry it to obtain modified carbon fiber for standby; S5. Place the modified carbon fiber in a deposition device, and deposit carbon nanotubes on its surface by chemical vapor deposition to obtain carbon nanotube-reinforced carbon fiber for standby; S6. Immerse the carbon nanotube-reinforced carbon fiber in a PA sizing agent for treatment, then pull out the carbon fiber in bundles, and after drying and short-cutting, obtain PA-sized short-cut carbon fiber.

3. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: In step S1, the temperature of the heat reflux treatment is 50 - 70 °C, and the treatment time is 6 - 18 h.

4. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: In step S2, the gas for the surface treatment is nitrogen, the gas flow rate is 15 - 30 L / min, the treatment power is 115 - 140 W, the treatment rate is 50 - 100 cm / min, and the distance between the reactor nozzle and the desized carbon fiber is 15 - 30 mm.

5. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: In step S3, the concentration of the concentrated ammonia water is 25 - 28 wt%; the concentration of the hydrogen peroxide is 6 - 8 wt%; the concentration of the concentrated hydrochloric acid is 36 - 38 wt%.

6. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: In step S3, the temperature of the reaction is 50 - 65 °C, and the reaction time is 0.5 - 2.0 h.

7. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: In step S4, the temperature of the microwave treatment is 165 - 185 °C, the treatment time is 2 - 6 h, and the microwave power is 550 - 650 W.

8. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: The cooling treatment described in step S4 is carried out in three stages. First, it is cooled at a rate of 2.0 - 2.5 °C / min to 145 - 150 °C and maintained at this temperature for 5 - 15 min. Subsequently, it is cooled at a rate of 2.0 - 2.5 °C / min to 110 - 120 °C and maintained at this temperature for 5 - 15 min. Finally, it is cooled to room temperature at a rate of 3.0 - 5.0 °C / min.

9. The preparation method of the PA-sized chopped carbon fiber according to claim 2, characterized in that: The carbon source gas for chemical vapor deposition described in step S5 is acetylene or ethylene; the inert gas is nitrogen or argon; the volume ratio of the carbon source gas, inert gas, and hydrogen is 1:2:1; the deposition temperature is 450 - 550 °C, and the pressure during the deposition process is 0.01 - 0.02 MPa.

10. A PA-sized chopped carbon fiber, characterized in that: It is prepared by using the method according to any one of claims 1 - 9.