A sizing agent for carbon fibers based on sequential in-situ polymerization, a preparation method and applications thereof

Abstract

The application discloses to the technical field of carbon fiber composite material, and specifically discloses a kind of carbon fiber sizing agent based on sequential in-situ polymerization construction gradient interface and preparation method and application.The sizing agent includes monomer A and monomer B of different reactivity, and initiation system capable of initiating polymerization in stages.Monomer A contains anchoring group that can be reacted with carbon fiber surface and polymerizable group, and monomer B contains functional group highly compatible with nylon matrix.Through two-stage heat treatment process of low temperature first and then medium temperature, in-situ polymerization of monomer A and monomer B is triggered in turn, so as to form an interface layer with gradient change in chemical composition and modulus between carbon fiber surface and nylon matrix.The application effectively solves the technical problem that interfacial bonding strength of carbon fiber and nylon matrix is difficult to consider both strength and toughness, and significantly improves interlaminar shear strength and impact resistance of composite material.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber reinforced composite materials technology, specifically to a carbon fiber sizing agent based on sequential in-situ polymerization, its preparation method, and its application. Background Technology

[0002] Carbon fiber reinforced nylon composites have broad application prospects in the automotive, aerospace, and electronics industries due to their advantages such as lightweight, high strength, and chemical resistance. However, the inertness of the carbon fiber surface and its poor compatibility with the polar nylon matrix result in weak interfacial bonding strength, making it difficult to fully realize the mechanical properties of the composite material. Furthermore, the significant modulus difference between the carbon fiber and the nylon matrix easily leads to stress concentration at the interface, causing interfacial debonding and material failure.

[0003] Traditional sizing agents are mostly pre-synthesized polymer emulsions (such as epoxy resins and polyurethane emulsions), which are physically adsorbed onto the fiber surface. The interfacial layer structure formed by this method is simple and often fails to meet the requirements of high strength and toughness simultaneously. High-strength interfacial layers usually have high modulus and high brittleness, which easily leads to stress concentration; while high-toughness interfacial layers often have low modulus and cannot effectively transfer loads.

[0004] In the prior art, patent CN111074543A discloses a method to improve interfacial properties by grafting carbon nanotubes (CNTs) onto the surface of carbon fibers and further anionicly polymerizing nylon 6. Although this method has achieved certain results, it has the following obvious defects: (1) The process is extremely complex: it requires more than ten steps, including carbon fiber oxidation, reduction, CNT oxidation, esterification grafting, nylon end-capping, and anionic polymerization. The process is long, energy consumption is high, and production costs are huge. (2) It depends on nanomaterials: its core technology must introduce carbon nanotubes (CNTs) as an intermediate layer. CNTs are expensive, their dispersion is difficult to control, and the grafting process is complex and has poor reproducibility, which seriously restricts industrial application. (3) The interface structure is limited: the formed "carbon fiber-CNTs-nylon" structure focuses more on physical and mechanical interlocking. The chemical gradient design of the interface layer is insufficient, which limits the synergistic improvement of strength and toughness.

[0005] Therefore, developing a novel sizing agent capable of actively constructing a gradient, multifunctional interface layer to achieve a synergistic improvement in interface strength and toughness is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies in which it is difficult to simultaneously achieve the interfacial bonding strength and toughness of carbon fiber / nylon composite materials. This invention provides a carbon fiber sizing agent based on sequential in-situ polymerization to construct a gradient interface, as well as its preparation method and application.

[0007] To achieve the above objectives, the present invention provides the following technical solution: I. Carbon Fiber Sizing Agent Component Design: The sizing agent of this invention achieves interface gradient construction by rationally selecting monomers, initiators, and emulsifiers and utilizing a "dual initiator sequential polymerization" mechanism. The selection criteria and functions of each component are as follows: 1. Monomer A (5-30 parts by weight): (1) Function: As an "anchor layer monomer", it forms a strong chemical bond with the carbon fiber surface through polymerization reaction, providing "root support" for the gradient interface.

[0008] (2) Structural requirements: The molecule must contain both polymerizable carbon-carbon double bonds (for free radical polymerization to form polymer chains) and reactive functional groups. The reactive functional groups are selected from at least one of epoxy, acid anhydride, oxazoline, amino, hydroxy or carboxyl groups, and are used to react chemically with the hydroxyl and carboxyl groups on the carbon fiber surface.

[0009] (3) Preferred types: at least one or a mixture thereof, of glycidyl methacrylate (GMA, the epoxy group can react with the hydroxyl group on the carbon fiber surface), ethyl isocyanate methacrylate (ICEMA, the isocyanate group can react with hydroxyl and amino groups), maleic anhydride (MAH, the anhydride group can react with hydroxyl groups), and 2-vinyl-2-oxazoline (VOX, the oxazoline group can react with carboxyl groups).

[0010] (4) Dosage control: If the dosage is less than 5 parts, the anchoring layer thickness is insufficient and the interface bonding strength is low; if it is more than 30 parts, it is easy to cause the interface layer to be too thick and the brittleness to increase, and the impact toughness to decrease.

[0011] 2. Monomer B (10-50 parts by weight) (1) Function: As a “compatibility layer monomer”, its polymerization product has good compatibility with nylon resin, and can copolymerize with unreacted monomer A to form a transition layer.

[0012] (2) Selection criteria: The molecule must contain a polar functional group that is compatible with the nylon matrix. The polar functional group is selected from at least one of amide group, lactam ring, pyrrolidone ring, and pyridine ring. It also contains polymerizable carbon-carbon double bonds for free radical polymerization.

[0013] (3) Preferred species: at least one of N-vinylpyrrolidone (NVP, the pyrrolidone ring forms hydrogen bonds with nylon), acrylamide (AM, the amide group forms hydrogen bonds with nylon), N-vinylcaprolactam (NVCL, the caprolactam ring has good compatibility with nylon 6 / 12), N-isopropylacrylamide (NIPAM, amide group) or a mixture thereof.

[0014] (4) Dosage control: If the dosage is less than 10 parts, the compatibility layer is insufficient and the bonding with the nylon matrix is ​​weak; if it is more than 50 parts, it is easy to cause monomer B to agglomerate and the polymerization uniformity is poor.

[0015] 3. Initiator I (0.1-5 parts by weight) and Initiator II (0.1-5 parts by weight) (1) Core design: Utilize the difference in thermal decomposition temperature between the two initiators to achieve a sequential reaction of “first initiating the polymerization of monomer A, then initiating the polymerization of monomer B”.

[0016] (2) Initiator I (low-temperature initiator): thermal decomposition temperature 25-50℃, used to initiate the polymerization of monomer A during the first stage of low-temperature heat treatment. Redox initiation systems are preferred, including but not limited to: hydrogen peroxide and ascorbic acid combination (H2O2-Vc), sodium bisulfite (NaHSO3), copper sulfate and sodium bisulfite combination (CuSO4-NaHSO3), ferrous ions and hydrogen peroxide combination (Fe²⁺). + At least one of the following: (-H2O2). These redox systems have extremely slow reaction rates below 25°C, but can effectively initiate polymerization in the range of 25-50°C, and their activity can be controlled by pH adjustment or complexation. Water-soluble azo initiators with a thermal decomposition temperature of 40-50°C, such as azobisisobutyramidine hydrochloride (AIBA·HCl), can also be selected.

[0017] (3) Initiator II (Medium-temperature initiator): The thermal decomposition temperature is within the range of 70-90℃. It is used to initiate the polymerization of monomer B during the second stage of medium-temperature heat treatment. Water-soluble persulfates are preferred, including at least one of potassium persulfate (KPS) and ammonium persulfate (APS); oil-soluble peroxides or azo initiators, such as benzoyl peroxide (BPO) and azobisisobutyronitrile (AIBN), can also be used, but they need to be solubilized by emulsifiers to ensure uniform dispersion in the emulsion system.

[0018] (4) Dosage control: When the dosage of each initiator is less than 0.1 parts, the polymerization conversion rate is low; when it is more than 5 parts, too many free radicals are easily generated, resulting in excessive polymer crosslinking and increased brittleness.

[0019] 4. Emulsifier (0.5-5 parts by weight) (1) Function: Reduces the interfacial tension between monomer (oil phase) and deionized water (aqueous phase), forming a stable water-in-oil emulsion, ensuring that the sizing agent uniformly coats the carbon fiber surface.

[0020] (2) Preferred types: anionic (sodium dodecyl sulfate SDS, sodium dodecylbenzene sulfonate SDBS, with high emulsification efficiency), nonionic (OP-10, AEO-9, with good compatibility with monomers and strong emulsion stability), or anionic-nonionic compound (to improve the long-term stability of the emulsion).

[0021] (3) Dosage control: When the dosage is less than 0.5 parts, the emulsion is prone to separation; when the dosage is more than 5 parts, the residual emulsifier will reduce the interfacial bonding strength.

[0022] 5. Deionized water (50-200 parts by weight) (1) Function: As a dispersion medium, it adjusts the solid content of the sizing agent (usually controlled at 10-40% to facilitate impregnation and uniform coating). The dosage is adjusted according to the target solid content. If the dosage is too low, the emulsion viscosity will be high; if it is too high, the solid content will be insufficient and the coating amount will be small.

[0023] II. Preparation method of sizing agent: 1. Emulsifier Dissolution: Add the emulsifier from the formulation to 50-70% deionized water and stir at 15-25℃ (500-1000 rpm) until completely dissolved to form an aqueous phase. At this temperature, the decomposition rate of initiator I is extremely low, which can effectively avoid premature polymerization during the preparation process.

[0024] 2. Pre-emulsification: Transfer the aqueous phase to a high-speed shear press and start shearing (2000-4000 rpm). Slowly add monomer A, monomer B, initiator I, and initiator II sequentially. After each component is added, continue shearing for 5-10 minutes to ensure uniform dispersion. The temperature is controlled at 15-25℃ throughout the pre-emulsification process, and the low temperature is maintained by jacket cooling.

[0025] 3. Emulsion Stabilization: After all components are added, continue high-speed shearing for 15-30 minutes to form a stable emulsion with uniform particle size distribution (100-500nm). The temperature is still controlled at 15-25℃.

[0026] 4. Solid content adjustment: Add the remaining 30-50% of deionized water, stir (500-1000 rpm) for 10-15 minutes, and adjust the solid content of the emulsion to 10-40% to obtain the carbon fiber sizing agent. The resulting emulsion can be stored at low temperature (e.g., 5-10℃) to maintain stability.

[0027] 5. Key process control: High-speed shearing is the core to ensure emulsion stability. Too low a speed can easily lead to monomer agglomeration, while too high a speed may generate too much foam. Low temperature control (15-25℃) throughout the preparation process is the key to avoid premature decomposition of initiator I and ensure the storage stability of the sizing agent.

[0028] III. Carbon Fiber Processing Method: A gradient interface is constructed on the carbon fiber surface through a process of "sizing removal → impregnation → two-stage heat treatment → short cutting". Specific steps are as follows: Step S1: Desizing the carbon fibers with the original sizing agent on the surface by heat treatment in air or inert atmosphere at 400-600℃ for 1-10 minutes, or by plasma treatment (power 50-100W, 1-5min). Step S2: Impregnate or roll the treated carbon fiber in the sizing agent to ensure that the carbon fiber surface is uniformly coated with the sizing agent emulsion. The coating amount is controlled at 2-5% of the fiber weight (if the coating amount is too low, the interface layer will be insufficient; if it is too high, it will easily agglomerate).

[0029] Step S3: Perform the first stage of heat treatment: treat at 25-50℃ (preferably 40-50℃) for 5-15 minutes (preferably 8-12 minutes). Initiator I decomposes at this temperature to generate free radicals, which initiate the carbon-carbon double bond polymerization of monomer A to form polymer chains. At the same time, the reactive functional groups of monomer A react chemically with the hydroxyl and carboxyl groups on the carbon fiber surface to "anchor" the polymer chains to the carbon fiber surface, forming an anchoring layer.

[0030] Step S4: Perform the second stage of heat treatment: treat at 70-90℃ (preferably 80-85℃) for 5-15 minutes (preferably 8-12 minutes). Initiator II decomposes at this temperature, initiating the polymerization of monomer B to form a polymer layer compatible with nylon resin. The unreacted monomer A (residual double bond) undergoes a copolymerization reaction with monomer B to form a "monomer A-monomer B" copolymer, which serves as a transition layer between the anchoring layer and the compatibility layer. Finally, a gradient interface structure is formed from the inside out: "anchoring layer (carbon fiber side, high bonding strength) → transition layer (middle, gradient connection) → compatibility layer (nylon side, high compatibility)".

[0031] Step S5: The pretreated carbon fibers are chopped to obtain short carbon fibers with a length of 3-12mm (preferably 6-8mm). If the length is too short (<3mm), the reinforcing effect will be weak, and if it is too long (>12mm), the dispersion in the resin will be poor and it will easily form agglomerates.

[0032] IV. Applications of chopped carbon fiber: The treated chopped carbon fibers are melt-blended with nylon resin (such as nylon 6, nylon 66, and nylon 12) using a twin-screw extruder. Specific parameters are as follows: extrusion temperature: 220-260℃ (adjusted according to the type of nylon, such as 230-250℃ for nylon 6 and 250-260℃ for nylon 66); screw speed controlled at 50-350 rpm; fiber addition amount controlled at 10-30 wt% (adjusted according to the mechanical property requirements of the composite material).

[0033] This product is designed to meet the core requirements of various industries for "high strength, low weight, and easy molding," and is mainly used in the automotive or electronics fields.

[0034] The beneficial effects of this invention are: 1. This invention designs monomers A and B with different reactivity, and a staged triggering initiation system, using sequential in-situ polymerization technology to sequentially form an anchoring layer, a transition layer, and a compatibility layer highly compatible with the nylon matrix on the carbon fiber surface, thereby constructing an interface structure with gradient changes in chemical composition and modulus. This gradient interface can effectively alleviate stress concentration while achieving high interfacial bonding strength and toughness.

[0035] 2. The present invention has a simple process and low cost. It adopts a sequential in-situ polymerization process of "one-step pre-emulsification + two-stage heat treatment", which does not require complex pretreatment or expensive materials. The process is shorter and the raw material cost is lower. It can be directly adapted to existing carbon fiber processing production lines and is easy to industrialize.

[0036] 3. This invention achieves true low-temperature first-stage polymerization and medium-temperature second-stage polymerization through a combination of redox initiation and thermal initiation systems, avoiding the adverse effects of high temperatures on monomer stability. It offers a wide process window and strong controllability. Simultaneously, low-temperature control during the preparation process ensures the storage stability of the sizing agent. Attached Figure Description

[0037] Figure 1 This is a schematic cross-sectional view of the carbon fiber sizing agent after in-situ polymerization according to the present invention. Detailed Implementation

[0038] The technical solution of the present invention will be described in detail below with reference to various embodiments.

[0039] Example 1 1. Preparation of sizing agent: (1) Components (parts by weight): 15 parts of monomer A (glycidyl methacrylate GMA), 30 parts of monomer B (N-vinylpyrrolidone NVP), 0.5 parts of initiator I (0.5 parts of hydrogen peroxide H2O2 + 0.3 parts of ascorbic acid Vc), 1.5 parts of initiator II (potassium persulfate KPS), 2 parts of emulsifier (sodium dodecyl sulfate SDS), and 100 parts of deionized water.

[0040] (2) Preparation steps: Dissolve 2 parts of emulsifier in 50 parts of deionized water (stirring at 20°C), transfer to a high-speed shear press (3000 rpm), control the system temperature at 20±2°C, add GMA, NVP, H2O2, Vc and KPS in sequence, shear for 8 minutes at each step, and finally shear for 20 minutes to form a stable emulsion. Add 50 parts of deionized water to adjust the solid content to 30%. Store the obtained emulsion at 5°C for later use.

[0041] 2. Carbon fiber treatment: Step S1: Desizing: PAN-based carbon fiber (T700) is heat-treated in air at 500°C for 4 minutes; Step S2: Impregnation: Pad-on method, coating amount 3%; Step S3: First stage heat treatment: 45℃ for 15 minutes (H2O2-Vc redox system initiates GMA polymerization and anchors, KPS stabilizes); Step S4: Second stage heat treatment: 85℃ for 12 minutes (KPS decomposition initiates NVP polymerization, which copolymerizes with the residual double bonds in the anchoring layer to form a chemical transition layer). Step S5: Short cut: Cut to 6mm.

[0042] 3. Application and performance testing: The fiber was melt-blended and granulated with nylon 6 resin (melting point 225℃) at a fiber addition of 30wt% in a twin-screw extruder (temperature 230-250℃, speed 300rpm), and injection-molded into standard specimens. The interfacial shear strength, flexural strength and impact toughness were then tested.

[0043] Example 2 1. Preparation of sizing agent: (1) Components (parts by weight): 12 parts of monomer A (GMA), 25 parts of monomer B (acrylamide AM), 0.8 parts of initiator I (ammonium persulfate-sodium bisulfite redox system, APS:NaHSO3=1:1), 1.2 parts of initiator II (ammonium persulfate APS), 1.5 parts of emulsifier (sodium dodecylbenzenesulfonate SDBS), and 80 parts of deionized water.

[0044] (2) Preparation steps: Dissolve 1.5 parts of SDBS in 40 parts of deionized water (dissolve at 20℃), perform high-speed shearing (2500rpm) while controlling the temperature at 20±2℃, add GMA, AM, NaHSO3 and APS, shear for 6 minutes at each step, for a total of 15 minutes, and add 40 parts of deionized water to adjust the solid content to 28%. Store at 5℃.

[0045] 2. Carbon fiber treatment: Step S1: Desizing: PAN-based carbon fiber (T700) is heat-treated in a nitrogen atmosphere at 450℃ for 5 minutes; Step S2: Immersion: Soaking method, soak for 8 minutes, coating amount 2.5%; Step S3: First stage heat treatment: 40℃ for 15 minutes (NaHSO3 initiates GMA polymerization); Step S4: Second stage heat treatment: 80℃ for 12 minutes (APS initiates AM polymerization, copolymerizing with the anchoring layer); Step S5: Short cut: Cut to 6mm.

[0046] 3. Application and performance testing: Add 25wt% of Nylon 66 resin (melting point 260℃) and perform twin-screw extrusion (temperature 250-260℃, speed 280rpm) to test injection molded specimens.

[0047] Example 3 1. Preparation of sizing agent: (1) Components (parts by weight): 15 parts of monomer A (ethyl isocyanate methacrylate ICEMA, using end-capped isocyanate, added immediately before use to avoid long-term contact with water), 25 parts of monomer B (N-vinylcaprolactam NVCL), 0.1 parts of initiator I (0.1 parts of CuSO4 + 0.6 parts of NaHSO3), 1.2 parts of initiator II (benzoyl peroxide BPO), 2 parts of emulsifier (SDS + OP-10 compound), and 100 parts of deionized water.

[0048] (2) Preparation steps: Dissolve the emulsifier in 50 parts of deionized water (20℃), and add ICEMA, NVCL, CuSO4, NaHSO3 and BPO (pre-dissolved with a small amount of monomer) in sequence under shearing. Shear for 8 minutes at each step, for a total of 20 minutes. Adjust the solid content to 30% and store at 5℃.

[0049] 2. Carbon fiber treatment: Step S1: Desizing: Same as in Example 1; Step S2: Impregnation: Coverage amount 3%; Step S3: First stage heat treatment: 35℃ for 20 minutes; Step S4: Second stage heat treatment: 85℃ for 12 minutes; Step S5: Short cut: 6mm.

[0050] 3. Application testing: Same as Example 1.

[0051] Example 4 1. Preparation of sizing agent: (1) Components (parts by weight): 10 parts of monomer A (maleic anhydride MAH), 20 parts of monomer B (acrylamide AM), 0.2 parts of initiator I (FeSO4·7H2O + 0.6 parts of H2O2), 1.0 part of initiator II (azobisisobutyronitrile AIBN), 1.5 parts of emulsifier (SDBS), and 80 parts of deionized water.

[0052] (2) Preparation steps: Same as in Example 2, prepared at 20°C and stored at 5°C.

[0053] 2. Carbon fiber treatment: Step S1: Desizing: Same as in Example 1; Step S2: Impregnation: Coating amount 2.5%; Step S3: First stage heat treatment: 30℃ for 20 minutes; Step S4: Second stage heat treatment: 80℃ for 15 minutes; Step S5: Short cut: 6mm.

[0054] 3. Application testing: Blending test with Nylon 6.

[0055] Example 5 Similar to Example 1, but monomer A was replaced with a mixture of 10 parts GMA and 5 parts HEMA, while other conditions remained unchanged.

[0056] Example 6 Similar to Example 1, but monomer B was replaced with a mixture of 20 parts NVP and 5 parts NIPAM, while other conditions remained unchanged.

[0057] Comparative Example 1 Untreated PAN-based carbon fiber (T700) was directly chopped to 6 mm and then blended with nylon 6 (30 wt% fiber). The blending and testing conditions after chopping were the same as in Example 1.

[0058] Comparative Example 2 The carbon fiber was desizing, and then sizing was performed on the carbon fiber using commercially available epoxy resin emulsion (30% solid content) (sizing coating amount 3%). The carbon fiber was cured at 120°C for 10 minutes, chopped to 6 mm, and then blended with nylon 6 (30 wt% fiber). The blending and testing conditions after chopping were the same as in Example 1.

[0059] Comparative Example 3 The carbon fiber was desizing, and then sizing was performed on the carbon fiber using commercially available nylon 6 resin emulsion (30% solid content) (sizing coating amount 3%). The carbon fiber was cured at 120°C for 10 minutes, chopped to 6 mm, and then blended with nylon 6 (30 wt% fiber). The blending and testing conditions after chopping were the same as in Example 1.

[0060] Comparative Example 4 (Monomer A only, no monomer B) Sizing agent: 45 parts GMA, 1.2 parts H2O2-Vc, 2 parts SDS, 100 parts water, prepared at 20℃. Carbon fiber treatment: treated at 45℃ for 15 minutes (GMA polymerization only), then chopped and blended with nylon 6.

[0061] Comparative Example 5 (Monomer B only, no monomer A) Sizing agent: 45 parts NVP, 1.5 parts KPS, 2 parts SDS, and 100 parts water, prepared at 20℃. Carbon fiber treatment: directly treated at 85℃ for 15 minutes (no low-temperature stage), then chopped and blended with nylon 6.

[0062] Comparative Example 6 (one-step heat treatment, no temperature gradient) Similar to Example 1, but the carbon fiber treatment was performed directly at 85°C for 20 minutes in the second stage, without the low-temperature 45°C stage.

[0063] The test data from the above embodiments and comparative examples are summarized, and the results are shown in the table below.

[0064]

[0065] The test results of the above examples and comparative examples verify that the carbon fiber sizing agent of the present invention, which constructs a gradient interface through sequential in-situ polymerization, can significantly improve the interfacial bonding performance between carbon fiber and nylon resin. The interfacial shear strength (82-87 MPa), flexural strength (275-288 MPa), and impact toughness (18.2-19.2 kJ / m²) of Examples 1-6 are significantly better than those of the comparative examples. The performance of Comparative Examples 4 (without monomer B) and 5 (without monomer A) is significantly lower than that of the examples, proving that the synergistic effect of the anchoring layer and the compatibility layer is indispensable; the performance of Comparative Example 6 (without temperature gradient) is also lower than that of the examples, proving that two-stage sequential polymerization is crucial for constructing the gradient interface.

[0066] This invention successfully constructed a gradient interface structure of "anchoring layer → chemical copolymerization transition layer → compatibility layer" on the surface of carbon fiber through reasonable reagent matching and temperature gradient design, which effectively alleviated stress concentration and achieved a synergistic improvement in interfacial bonding strength and toughness.

[0067] Those skilled in the art will appreciate that various modifications to the above embodiments can be made without departing from the overall spirit and concept of the present invention. All such modifications fall within the protection scope of the present invention. The protection scheme of the present invention is defined by the appended claims.

Claims

1. A carbon fiber sizing agent based on sequential in-situ polymerization, characterized in that, By weight, it includes the following components: Monomer A: 5-30 parts; Monomer B10-50 parts; Initiator I: 0.1-5 parts; Initiator II 0.1-5 parts; Emulsifier 0.5-5 parts; 50-200 parts deionized water; Wherein, the thermal decomposition temperature of initiator I is lower than that of initiator II; initiator I is a low-temperature initiator with a thermal decomposition temperature in the range of 25°C to 50°C; and initiator II is a medium-temperature initiator with a thermal decomposition temperature in the range of 70°C to 90°C.

2. The sizing agent according to claim 1, characterized in that, The monomer A is: A compound whose molecular structure contains both polymerizable carbon-carbon double bonds and anchoring functional groups; wherein the anchoring functional group is selected from at least one of epoxy group, isocyanate group, acid anhydride group, oxazoline group, amino group, hydroxyl group or carboxyl group.

3. The sizing agent according to claim 2, characterized in that, The monomer A is selected from at least one of glycidyl methacrylate, ethyl isocyanate methacrylate, maleic anhydride, 2-vinyl-2-oxazoline, hydroxyethyl methacrylate, p-vinylaniline, and methacrylic acid.

4. The sizing agent according to claim 1, characterized in that, The monomer B is a compound whose molecular structure contains both polymerizable carbon-carbon double bonds and polar functional groups compatible with the nylon matrix. The polar functional groups are selected from at least one of amide groups, lactam rings, pyrrolidone rings, and pyridine rings.

5. The sizing agent according to claim 4, characterized in that, The monomer B is selected from at least one of N-vinylpyrrolidone, acrylamide, N-vinylcaprolactam, N-isopropylacrylamide, 2-vinylpyridine, and N-vinylformamide.

6. The sizing agent according to claim 1, characterized in that, Initiator I is selected from at least one redox initiation system selected from the combination of hydrogen peroxide and ascorbic acid, sodium bisulfite and oxidant, copper sulfate and sodium bisulfite, and ferrous ion and hydrogen peroxide, or a water-soluble azo initiator with a thermal decomposition temperature of 40-50℃; Initiator II is selected from at least one water-soluble persulfate selected from potassium persulfate and ammonium persulfate, or at least one selected from benzoyl peroxide and azobisisobutyronitrile solubilized by an emulsifier.

7. The sizing agent according to claim 1, characterized in that, The emulsifier is selected from at least one of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkylphenol polyoxyethylene ether, and fatty alcohol polyoxyethylene ether.

8. A method for preparing the sizing agent according to any one of claims 1-7, characterized in that, Includes the following steps: The emulsifier is dissolved in a portion of deionized water and stirred at 15-25°C until completely dissolved to form an aqueous phase. The aqueous phase is then transferred to a high-speed shearing machine, where monomer A, monomer B, initiator I, and initiator II are added sequentially at 15-25°C. After each component is added, shearing is continued for 5-10 minutes, and finally, shearing is continued for 15-30 minutes to form a stable emulsion. The remaining deionized water is added to adjust the solid content, thus obtaining the carbon fiber sizing agent.

9. A method for treating carbon fibers using the sizing agent according to any one of claims 1-7, characterized in that, Includes the following steps: Step S1: Desizing treatment is performed on the carbon fibers with the original sizing agent on the surface: Heat treatment in air or an inert atmosphere at 400-600℃ for 1-10 minutes; or Plasma treatment method was used; Step S2: Impregnate the treated carbon fiber in the sizing agent; Step S3: Perform the first stage of heat treatment: treat at 25-50℃ for 5-15 minutes to allow monomer A to polymerize under the action of initiator I to form an anchoring layer; Step S4: Perform the second stage of heat treatment: Treat at 70-90℃ for 5-15 minutes to allow monomer B to polymerize under the action of initiator II. At the same time, unreacted monomer A and monomer B will also undergo copolymerization to form a gradient transition zone. The polymerization between monomers B is equivalent to forming a compatible layer on the anchoring layer. From the inside out, "anchoring layer → transition layer → compatible layer" is formed, thus constructing a gradient interface. Step S5: The pretreated carbon fiber is chopped to obtain short carbon fiber with a length of 3-12mm.

10. An application of the short-cut carbon fiber prepared by the method of claim 8, characterized in that, Shortened carbon fibers and nylon resin are melt-blended and granulated using a twin-screw extruder, and then injection-molded into standard test strips.

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