Sizing method of carbon fiber with high interface bonding strength

Abstract

The invention provides a sizing method for carbon fibers with high interface bonding strength, which belongs to the technical field of carbon fibers and comprises the following steps: activating carbon fibers subjected to high-temperature carbonization; immersing the activated carbon fiber in the sizing liquid, and performing ultrasonic treatment to obtain the impregnated carbon fiber; wherein the sizing liquid is prepared from a sizing agent, and the sizing agent is prepared from diethanol amine modified novolac epoxy resin, a diepoxy terminated catechol-phenylboronic acid ester cross-linking agent, an amino-functionalized multi-walled carbon nanotube, amino-terminated polyether and water; and sequentially carrying out first high-temperature treatment and second high-temperature treatment on the impregnated carbon fiber to obtain the carbon fiber with high interface bonding strength. The sizing method provided by the invention effectively solves the problems of weak interface bonding force, easy debonding, brittle fracture and the like caused by chemical inertness of the surface of the carbon fiber, and obviously improves the interface shear strength, the interface stripping toughness and the overall cycle fatigue life of the carbon fiber composite material.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber technology, specifically relating to a sizing method for carbon fibers with high interfacial bonding strength. Background Technology

[0002] Carbon fiber reinforced polymer composites have wide applications in structural materials due to their low density, high specific strength, and high specific stiffness. The interfacial region between the carbon fiber and the polymer matrix is ​​a key factor affecting the macroscopic mechanical properties, environmental stability, and service life of the composite. However, due to the high degree of graphitization on the carbon fiber surface, it exhibits significant chemical inertness, low surface energy, and a lack of reactive functional groups. These intrinsic physicochemical characteristics make it difficult for effective chemical bonding to form between the carbon fiber and most polymer matrices, relying mainly on van der Waals forces or mechanical interlocking effects for stress transfer. Under complex stress, this weak interfacial layer is prone to microcracks that evolve into interfacial debonding, leading to interlaminar shear failure or fracture of the composite.

[0003] To improve interfacial properties, existing technologies typically employ surface oxidation combined with coating with polymeric sizing agents (such as bisphenol A type epoxy resin emulsions). However, conventional sizing agents lack covalent anchoring points with the carbon fiber surface, and free emulsifiers in the system easily form weak boundary layers at the interface. Furthermore, some studies have attempted to introduce rigid nanofillers such as carbon nanotubes to increase interfacial roughness and mechanical interlocking effects, but such rigid modified layers easily induce modulus abrupt changes between the fiber and resin, forming new stress concentration sources. Simultaneously, under destructive loads, the dense three-dimensional covalent network of traditional thermosetting resin interfaces cannot undergo segment slippage; the accumulated elastic energy can only be released through irreversible breakage of the main chain, resulting in brittle interfacial debonding.

[0004] In summary, the technical problem that needs to be solved in the existing technology is: how to provide a sizing method for carbon fibers to solve the problems of weak interfacial bonding caused by the chemical inertness of the carbon fiber surface, easy formation of weak boundary layers, and abrupt changes in modulus between the fiber and the resin. Summary of the Invention

[0005] In view of the above situation and to overcome the defects of the prior art, the purpose of the present invention is to provide a sizing method for carbon fibers with high interfacial bonding strength, so as to at least partially solve the problems mentioned in the background art.

[0006] The technical solution adopted in this invention is as follows: This invention proposes a sizing method for carbon fibers with high interfacial bonding strength, the method comprising the following steps: Activation treatment is performed on carbonized carbon fibers at high temperature. The activated carbon fibers are immersed in a sizing solution and ultrasonically treated to obtain impregnated carbon fibers. The sizing solution contains a sizing agent, which includes: diethanolamine-modified phenolic epoxy resin, biepoxy-terminated catechol-phenylboronic acid ester crosslinking agent, amino-functionalized multi-walled carbon nanotubes, amino-terminated polyether, and water. The impregnated carbon fibers are subjected to a first high-temperature treatment and a second high-temperature treatment in sequence to obtain carbon fibers with high interfacial bonding strength.

[0007] In some embodiments of the present invention, the activation treatment is performed using atmospheric pressure air plasma treatment, the radio frequency power of the plasma treatment is 150W, and the treatment time is 60 seconds.

[0008] In some embodiments of the present invention, the sizing liquid is prepared by diluting the sizing agent with water to a solid content of 2-3 wt%.

[0009] In some embodiments of the present invention, the sizing agent comprises the following components, by mass percentage: Diethanolamine-modified phenolic epoxy resin: 35.0-55.0%; Biepoxide-terminated catechol-phenylboronic acid ester crosslinking agent 10.0-25.0%; Amino-functionalized multi-walled carbon nanotubes 1.0-8.0%; Amino-terminated polyethers 3.0-12.0%; and Water content: 15.0-35.0%; The sum of the mass percentages of all components in the sizing agent is 100%.

[0010] In some embodiments of the present invention, the method for preparing the diethanolamine-modified phenolic epoxy resin includes: Phenolic epoxy resin was heated to 85°C, and diethanolamine was added. A ring-opening addition reaction was carried out at 85°C for 2 hours. Then, the temperature was lowered to 55°C and glacial acetic acid was added for protonation neutralization to obtain diethanolamine-modified phenolic epoxy resin. The mass ratio of phenolic epoxy resin to diethanolamine was (5-10):1.

[0011] In some embodiments of the present invention, the preparation method of the bisepoxy-terminated catechol-phenylboronic acid ester crosslinking agent includes: 4-Carboxyphenylboronic acid was reacted with epichlorohydrin in the presence of a phase transfer catalyst and an acid-binding agent at a reaction temperature of 85°C for 10 hours. After purification, 4-(glycidoxycarbonyl)phenylboronic acid intermediate was obtained. Dopamine hydrochloride was reacted with triethylamine under ice bath conditions to release the amino group, and a free amino intermediate was obtained. The free amino intermediate was then subjected to a ring-opening addition reaction with 1,4-butanediol diglycidyl ether at a reaction temperature of 20-30°C for 2-4 hours to obtain an epoxy-terminated intermediate. The 4-(glycidyloxycarbonyl)phenylboronic acid intermediate was mixed with the epoxy-terminated intermediate and heated under dehydration conditions at 75-80°C for 24 hours to carry out a dehydration esterification condensation reaction. After the reaction was completed, the product was collected and dried to obtain a diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent.

[0012] In some embodiments of the present invention, the molar ratio of 4-carboxyphenylboronic acid to epichlorohydrin is 1:(1.2-1.5); the phase transfer catalyst is tetrabutylammonium bromide; and the acid-binding agent is potassium carbonate.

[0013] In some embodiments of the present invention, the molar ratio of dopamine hydrochloride to triethylamine is 1:1.1; the molar ratio of dopamine hydrochloride to 1,4-butanediol diglycidyl ether is 1:1.

[0014] In some embodiments of the present invention, the molar ratio of the 4-(glycidyloxycarbonyl)phenylboronic acid intermediate to the epoxy-terminated intermediate is 1:1.

[0015] In some embodiments of the present invention, the temperature of the first high-temperature treatment is 70-95°C and the time is 1-5 minutes; the temperature of the second high-temperature treatment is 140-180°C and the time is 2-8 minutes.

[0016] The beneficial effects achieved by this invention are as follows: The sizing method of the present invention involves immersing activated carbon fibers in a sizing agent containing diethanolamine-modified phenolic epoxy resin, a bis-epoxy-terminated catechol-phenylboronic acid ester crosslinking agent, amino-functionalized multi-walled carbon nanotubes, amino-terminated polyethers, and water. The synergistic effect of the components in the sizing agent is used to improve the interfacial structure of the carbon fiber composite material, thereby effectively solving the problems of weak interfacial bonding, easy debonding, and brittle fracture caused by the chemical inertness of the carbon fiber surface. This significantly improves the interfacial shear strength, interfacial peel toughness, and overall cyclic fatigue life of the carbon fiber composite material. Detailed Implementation

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods and materials similar to or equivalent to those described herein may be applied to this invention. The preferred embodiments and materials described herein are for illustrative purposes only and do not limit the scope of this invention.

[0019] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0020] To address the problems mentioned in the background art, this invention provides a sizing method for carbon fibers with high interfacial bonding strength, comprising the following steps: Activation treatment is performed on carbonized carbon fibers at high temperature. The activated carbon fibers are immersed in a sizing solution and ultrasonically treated to obtain impregnated carbon fibers. The sizing solution contains a sizing agent, which includes: diethanolamine-modified phenolic epoxy resin, biepoxy-terminated catechol-phenylboronic acid ester crosslinking agent, amino-functionalized multi-walled carbon nanotubes, amino-terminated polyether, and water. The impregnated carbon fibers were subjected to a first high-temperature treatment and a second high-temperature treatment in sequence to obtain carbon fibers with high interfacial bonding strength.

[0021] The sizing method of this invention involves immersing activated carbon fibers in a sizing agent comprising diethanolamine-modified phenolic epoxy resin, a biepoxy-terminated catechol-phenylboronic acid ester crosslinking agent, amino-functionalized multi-walled carbon nanotubes, amino-terminated polyethers, and water. The synergistic effect of these components improves the interfacial structure of the carbon fiber composite material. Specifically, the catechol groups in the crosslinking agent can physically adsorb onto the carbon fiber surface and transform into o-benzoquinone structures under subsequent high-temperature treatment, irreversibly covalently binding with active sites on the fiber surface, thereby enhancing the interfacial bonding force between the carbon fiber and the sizing layer. Simultaneously, the borate ester bonds in the crosslinking agent construct a dynamic covalent network, which can undergo topological rearrangement through transesterification under load, achieving adaptive relaxation of interfacial stress and healing of microcracks, converting concentrated stress into internal energy dissipation. Furthermore, the high-modulus amino-functionalized multi-walled carbon nanotubes and the flexible long-chain amino-terminated polyethers synergistically form a gradient modulus transition layer from rigid to flexible between the fiber and the resin matrix, mitigating modulus abrupt changes at the material interface and reducing stress concentration. The epoxy groups at both ends of the crosslinking agent molecule can also copolymerize with the resin matrix during subsequent composite molding to form an interpenetrating polymer network and eliminate phase separation.

[0022] On the other hand, this sizing agent uses diethanolamine-modified phenolic epoxy resin and water as the dispersion medium. It achieves spontaneous dispersion of the aqueous phase by relying on the resin's own polar groups, avoiding the weak boundary layer defects caused by the addition of small-molecule emulsifiers in traditional water-based systems. In terms of process, the ultrasonic treatment during impregnation utilizes the cavitation effect to reduce surface tension, promoting the penetration of sizing agent nanomicelles into the fiber bundles and achieving uniform coating of the monofilaments. Subsequent high-temperature treatments, specifically the first and second high-temperature treatments, systematically complete water evaporation, deep oxidative crosslinking of catechols, and phase separation of polymer segments, firmly attaching the aforementioned gradient interface layer to the carbon fiber surface.

[0023] Therefore, the method of the present invention can effectively solve the problems of weak interfacial bonding, easy debonding and brittle fracture caused by the chemical inertness of carbon fiber surface, and significantly improve the interfacial shear strength, interfacial peel toughness and overall cyclic fatigue life of carbon fiber composite materials.

[0024] In some embodiments, the activation treatment employs atmospheric pressure air plasma treatment with a radio frequency power of 150W and a treatment time of 60 seconds. This combination of plasma activation parameters exhibits excellent surface modification effects. Under these specific power and time conditions, the plasma not only effectively cleans microscopic impurities from the carbon fiber surface but also generates high-density oxygen- and nitrogen-containing polar groups in situ on the originally chemically inert carbon fiber surface. Simultaneously, this process increases the surface roughness by approximately 20%-30% without damaging the carbon fiber's bulk lattice structure and mechanical properties. This moderate roughening of the physical morphology and the synergistic enhancement of surface chemical activity provide numerous active binding sites for the strong covalent grafting of the bis-epoxy-terminated catechol-phenylboronic acid ester crosslinking agent in the subsequent sizing agent. These effects complement each other, fundamentally improving the interfacial wettability and reactivity of the carbon fiber, thereby ensuring a high-strength covalent anchorage and physical adsorption between the sizing layer and the carbon fiber.

[0025] In some embodiments, the sizing solution is prepared by diluting the sizing agent with water to a solid content of 2-3 wt%. If the solid content is too high, the viscosity of the emulsion system increases, making it difficult to fully penetrate into the internal structure of the carbon fiber bundle. This can easily lead to an excessively thick outer layer coating of the bundle while the inner monofilaments leak sizing, resulting in interface defects. If the solid content is too low, a single impregnation cannot deposit a sufficient amount of polymer resin and multiple dynamic covalent monomers on the fiber surface, making it difficult to construct a continuous and appropriately thick gradient modulus interface layer. By controlling the solid content within the range of 2-3 wt%, and in conjunction with the ultrasonic cavitation effect during the impregnation process, the nanomicelles of the sizing agent can be effectively encouraged to penetrate deeply into the tiny gaps between the monofilaments inside the bundle. This step not only achieves uniform coating of each carbon fiber monofilament but also helps to precisely control the final sizing amount on the carbon fiber surface within an optimal range during subsequent drying and heat treatment steps.

[0026] In some embodiments, the sizing agent comprises the following components, by weight percentage: Diethanolamine-modified phenolic epoxy resin: 35.0-55.0%; Biepoxide-terminated catechol-phenylboronic acid ester crosslinking agent 10.0-25.0%; Amino-functionalized multi-walled carbon nanotubes 1.0-8.0%; Amino-terminated polyethers 3.0-12.0%; and Water content: 15.0-35.0%; The sum of the mass percentages of all components in the sizing agent is 100%.

[0027] This formulation achieves a high degree of synergy in the microstructure and mechanical properties of its components. Specifically, 35.0-55.0% of diethanolamine-modified phenolic epoxy resin serves as a self-emulsifying matrix, providing a suitable density of hydrophilic sites to ensure spontaneous dispersion and long-lasting microemulsion stability in a 15.0-35.0% aqueous continuous medium, meeting environmental requirements for zero volatile organic solvent (VOC) emissions, while simultaneously forming a high-strength polymer backbone after curing. 10.0-25.0% of bis-epoxy-terminated catechol-phenylboronic acid ester crosslinking agent ensures sufficient anchoring sites, dynamic stress relaxation, and microcrack self-healing capabilities at the interface. 1.0-8.0% of amino-functionalized multi-walled carbon nanotubes serve as an interface rigidity-reinforcing phase, forming a reasonable rigidity-flexibility ratio with 3.0-12.0% of terminal amino polyethers. An appropriate amount of rigid carbon nanotubes can create a nanoscale mechanical interlocking effect, alleviating the severe modulus jump between the fiber and the resin; while an appropriate amount of flexible end-amino polyethers are embedded in the epoxy curing network, reducing network brittleness and providing free volume for conformational adjustment of the interfacial macromolecular chains under load, thereby enhancing the peel toughness of the interface. These components, synergistically mixed within a defined mass percentage range, can spontaneously rearrange during the drying and curing process, forming an interfacial transition layer with gradient modulus characteristics.

[0028] In some embodiments, the method for preparing diethanolamine-modified phenolic epoxy resin includes: Phenolic epoxy resin was heated to 85°C, and then diethanolamine was added. A ring-opening addition reaction was carried out at 85°C for 2 hours. The temperature was then lowered to 55°C and glacial acetic acid was added for protonation neutralization to obtain diethanolamine-modified phenolic epoxy resin. The mass ratio of phenolic epoxy resin to diethanolamine was (5-10):1.

[0029] Under specific temperature (85℃) and time (2 hours) conditions, diethanolamine can undergo a highly efficient ring-opening addition reaction with the epoxy groups of phenolic epoxy resin, thereby introducing polar hydroxyl and secondary amine groups into the resin backbone. Subsequently, the temperature is lowered to 55℃ and glacial acetic acid is added for protonation neutralization. This cooling operation effectively avoids premature crosslinking or side reactions that may occur at high temperatures, and forms water-soluble cationic centers within the molecule through protonation. Limiting the mass ratio of phenolic epoxy resin to diethanolamine to (5-10):1 precisely balances the hydrophilicity and crosslinking activity of the resin. If the modification ratio of diethanolamine is too low, insufficient hydrophilic groups are introduced into the resin structure, making it difficult to impart spontaneous dispersion and long-term stability to the system in the aqueous phase; if the modification ratio is too high, it will excessively consume the highly active epoxy groups, leading to a significant decrease in the crosslinking density and mechanical strength of the polymer backbone after final curing. This preparation process endows the base resin with an excellent self-emulsification mechanism, fundamentally eliminating the need for added small molecule emulsifiers in traditional water-based sizing agent systems. It effectively eliminates the weak boundary layer defects formed by the aggregation of free emulsifiers at the carbon fiber interface, which are extremely easy to damage, thereby ensuring the high strength and high stability of the macroscopic interface of the composite material.

[0030] In some embodiments, the preparation method of the diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent includes: 4-Carboxyphenylboronic acid was reacted with epichlorohydrin in the presence of a phase transfer catalyst and an acid-binding agent at a reaction temperature of 85°C for 10 hours. After purification, 4-(glycidoxycarbonyl)phenylboronic acid intermediate was obtained. Dopamine hydrochloride was reacted with triethylamine under ice bath conditions to release the amino group, and a free amino intermediate was obtained. The free amino intermediate was then subjected to a ring-opening addition reaction with 1,4-butanediol diglycidyl ether at a reaction temperature of 20-30℃ for 2-4 hours to obtain an epoxy-terminated intermediate. The 4-(glycidoxycarbonyl)phenylboronic acid intermediate was mixed with the epoxy-terminated intermediate and heated under dehydration conditions at 75-80°C for 24 hours to carry out a dehydration esterification condensation reaction. After the reaction was completed, the product was collected and dried to obtain the diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent.

[0031] In the first step, a phase transfer catalyst and an acid-binding agent are used at 85°C to promote the reaction of 4-carboxyphenylboronic acid with epichlorohydrin, thereby introducing an end-epoxy group and a phenylboronic acid core structure into the molecular structure. This condition ensures the yield and purity of the intermediate. In the second step, the amino group of dopamine is released under ice bath conditions using triethylamine, which effectively protects the highly reactive catechol groups from damage. Subsequently, a ring-opening addition is carried out at a mild temperature of 20-30°C, precisely introducing a biomimetic adhesion group and another end-epoxy group, preventing thermal degradation of the structure or premature cross-linking of the reactive groups. Finally, the mixture is heated under reflux at 75-80°C for 24 hours to remove reaction byproducts through a physical dehydration mechanism, thereby breaking the chemical equilibrium and driving a complete dehydration esterification condensation reaction between the boric acid group of the phenylboronic acid intermediate and the cis-o-dihydroxyl group of the epoxy-terminated intermediate. This preparation method offers controllable conditions, enabling the precise and complete assembly within a macromolecular framework of catechol adhesion domains responsible for interfacial anchoring, dynamic covalent cores of borate esters responsible for stress relaxation, and biepoxide end groups responsible for chemical network interpenetration with the matrix. The successful synthesis of this specific structure lays the material foundation for achieving high-strength bonding and microcrack self-healing in the composite material interfacial layer.

[0032] In some embodiments, the molar ratio of 4-carboxyphenylboronic acid to epichlorohydrin is 1:(1.2-1.5); the phase transfer catalyst is tetrabutylammonium bromide; and the acid-binding agent is potassium carbonate. By controlling the molar ratio of 4-carboxyphenylboronic acid to epichlorohydrin to be 1:(1.2-1.5), epichlorohydrin is kept in a moderate excess. This ratio allows epichlorohydrin to act as both a reactant and a trace solvent in the reaction system, thereby ensuring the complete reaction and conversion of 4-carboxyphenylboronic acid while avoiding side reactions caused by excessive reagent excess or increasing the difficulty of subsequent desolventizing and purification.

[0033] In some embodiments, the molar ratio of dopamine hydrochloride to triethylamine is 1:1.1; the molar ratio of dopamine hydrochloride to 1,4-butanediol diglycidyl ether is 1:1. In this step, setting the molar ratio of dopamine hydrochloride to triethylamine to 1:1.1 allows triethylamine to primarily neutralize and neutralize the free amino groups. This slightly excess ratio design ensures the complete release of the free amino groups in dopamine hydrochloride, thereby guaranteeing the conversion rate of the subsequent ring-opening addition reaction. Simultaneously, strictly limiting the excess of triethylamine effectively avoids excessively strong local alkalinity in the reaction system, which could lead to unintended oxidation of catechol groups or other side reactions within the system. On the other hand, controlling the molar ratio of dopamine hydrochloride to 1,4-butanediol diglycidyl ether at 1:1, this precise equimolar ratio aims to achieve quantitative epoxy end-capping or protection of the active amino groups. This ratio effectively prevents the occurrence of multiple substitutions or excessive cross-linking of amino groups, ensuring that the required number of epoxy groups are precisely introduced into the molecular structure, while ensuring that the catechol adhesion groups on the side chains remain intact and are not adversely affected by steric hindrance.

[0034] In some embodiments, the molar ratio of 4-(glycidyloxycarbonyl)phenylboronic acid intermediate to epoxy-terminated intermediate is 1:1. This 1:1 ratio control offers significant advantages in terms of process and material performance: First, it ensures efficient conversion of reactants, minimizing the residue of unreacted precursor molecules, thereby improving the synthesis yield of the target crosslinking agent monomer and the purity of the final product; second, this precise ratio effectively avoids the formation of single-sided end-capping or complex oligomeric byproducts caused by an excess of a certain precursor, ensuring that the reaction product strictly possesses the complete molecular configuration of biepoxy end groups, monoboronate ester, and monocatechin. This highly homogeneous and creatively functional monomer lays a rigorous molecular chain foundation for the subsequent precise construction of a dynamic covalent network with adaptive stress relaxation and microcrack closure repair capabilities in the sizing layer.

[0035] In some embodiments, the first high-temperature treatment is performed at 70-95°C for 1-5 minutes; the second high-temperature treatment is performed at 140-180°C for 2-8 minutes. In the first high-temperature treatment stage, the temperature of 70-95°C and the time of 1-5 minutes promote the slow and uniform evaporation of moisture in the sizing agent system. This gentle pre-drying process effectively avoids microscopic physical defects such as pinholes or bubbles inside the sizing coating caused by rapid boiling and escaping of moisture, thus ensuring the density and continuity of the sizing film. In the second high-temperature treatment stage, the temperature of 140-180°C and the time of 2-8 minutes not only completely remove residual moisture from the coating, but more importantly, trigger deep oxidative crosslinking of the catechol groups in the crosslinking agent in this specific high-temperature aerobic environment. Simultaneously, this specific thermodynamic process drives phase separation of polymer segments within the sizing agent, promoting spontaneous rearrangement of the system and ultimately fixing a gradient modulus interface layer rich in dynamic bonds. This step-by-step heat treatment process precisely controls the film-forming kinetics and cross-linking reaction rate of the sizing layer, ultimately ensuring that the carbon fiber surface is firmly coated with a structurally complete, dense, and tough interface layer, providing a process guarantee for improving the overall mechanical properties of the composite material.

[0036] The present invention will be described below through specific embodiments. It should be noted that these embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0037] Example 1: (1) Carbon fiber activation treatment: Carbon fibers that have undergone high-temperature carbonization are activated by atmospheric pressure air plasma. The radio frequency power of the plasma treatment is 150W and the treatment time is 60 seconds to obtain activated carbon fibers.

[0038] (2) Preparation of diethanolamine-modified phenolic epoxy resin: Multifunctional phenolic epoxy resin (F-51) was heated to 85°C, and diethanolamine was added. A ring-opening addition reaction was carried out at 85°C for 2 hours. The temperature was then lowered to 55°C, and glacial acetic acid was added for protonation neutralization, yielding diethanolamine-modified phenolic epoxy resin. The mass ratio of phenolic epoxy resin to diethanolamine was 7.5:1.

[0039] (3) Preparation of biepoxide-terminated catechol-phenylboronic acid ester crosslinking agent: In N,N-dimethylformamide, 4-carboxyphenylboronic acid was reacted with epichlorohydrin (molar ratio 1:1.35) in the presence of a phase transfer catalyst (5 mol% tetrabutylammonium bromide) and an acid-binding agent (potassium carbonate) at a reaction temperature of 85°C for 10 hours. After purification, 4-(glycidoxycarbonyl)phenylboronic acid intermediate was obtained. In anhydrous methanol, dopamine hydrochloride was reacted with triethylamine (molar ratio 1:1.1) under ice bath conditions to release an amino group, which was then reacted with 1,4-butanediol diglycidyl ether (molar ratio 1:1) in a ring-opening addition reaction at 25°C for 3 hours to obtain an epoxy-terminated intermediate. The above-mentioned 4-(glycidoxycarbonyl)phenylboronic acid intermediate was mixed with the epoxy-terminated intermediate in a 1:1 molar ratio and heated under dehydration conditions at 78°C for 24 hours to carry out a dehydration esterification condensation reaction. After the reaction was completed, the product was collected and dried to obtain the diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent.

[0040] (4) Preparation of sizing agent: The following components were mixed by mass percentage: 45.0% diethanolamine-modified phenolic epoxy resin, 18.0% diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent, 4.5% amino-functionalized multi-walled carbon nanotubes (aspect ratio > 1000), 7.5% amino-terminated polyether (Jeffamine D-230), and 25.0% water. The mixture was then subjected to ultrasonic high-shear dispersion at 40°C for 1 hour to obtain the sizing agent.

[0041] (5) Impregnation and high-temperature treatment: The above sizing agent was diluted with deionized water to a solid content of 2.0 wt% to obtain a working sizing solution. The activated carbon fibers were immersed in the working sizing solution and ultrasonically treated at a frequency of 40 kHz to obtain impregnated carbon fibers. Finally, the impregnated carbon fibers were subjected to a first high-temperature treatment (85°C, 2 minutes) and a second high-temperature treatment (160°C, 4 minutes) to obtain carbon fibers with high interfacial bonding strength.

[0042] Example 2: Consistent with Example 1, except that: Sizing agent formulation: 35.0% diethanolamine modified phenolic epoxy resin, 25.0% diepoxy terminal catechol-phenylboronic acid ester crosslinking agent, 8.0% amino-functionalized multi-walled carbon nanotubes, 12.0% amino-terminated polyether, and 20.0% water.

[0043] Parameter substitution: The mass ratio of phenolic epoxy resin to diethanolamine is 5:1. The first high-temperature treatment is performed at 70°C for 1 minute; the second high-temperature treatment is performed at 140°C for 2 minutes. The sizing agent is diluted with water to a solid content of 3.0 wt%.

[0044] Example 3: Consistent with Example 1, except that: Sizing agent formulation: 55.0% diethanolamine modified phenolic epoxy resin, 10.0% diepoxy terminal catechol-phenylboronic acid ester crosslinking agent, 1.0% amino-functionalized multi-walled carbon nanotubes, 3.0% amino-terminated polyether, and 31.0% water.

[0045] Parameter substitution: The mass ratio of phenolic epoxy resin to diethanolamine is 10:1. The first high-temperature treatment is performed at 95°C for 5 minutes; the second high-temperature treatment is performed at 180°C for 8 minutes.

[0046] Example 4: Consistent with Example 1, except that: In preparing the crosslinking agent, the molar ratio of 4-carboxyphenylboronic acid to epichlorohydrin was 1:1.2. The ring-opening addition reaction of dopamine hydrochloride with 1,4-butanediol diglycidyl ether was carried out at 20°C for 2 hours. The reflux temperature for the dehydration esterification condensation reaction was 75°C.

[0047] Example 5: Consistent with Example 1, except that: In preparing the crosslinking agent, the molar ratio of 4-carboxyphenylboronic acid to epichlorohydrin was 1:1.5. The ring-opening addition reaction of dopamine hydrochloride with 1,4-butanediol diglycidyl ether was carried out at 30°C for 4 hours. The reflux temperature for the dehydration esterification condensation reaction was 80°C.

[0048] Example 6: Consistent with Example 1, except that: Sizing agent formulation: 40.0% diethanolamine modified phenolic epoxy resin, 20.0% diepoxy terminal catechol-phenylboronic acid ester crosslinking agent, 5.0% amino-functionalized multi-walled carbon nanotubes, 10.0% amino-terminated polyether, and 25.0% water.

[0049] Comparative Example 1: The difference between this comparative example and Example 1 is that the sizing agent does not contain a bis-epoxy-terminated catechol-phenylboronic acid ester crosslinking agent, amino-functionalized multi-walled carbon nanotubes, or amino-terminated polyethers. Only a 2.0 wt% ordinary bisphenol A type epoxy resin aqueous emulsion was used to ultrasonically impregnate the activated carbon fibers and perform the same heat treatment.

[0050] Comparative Example 2: The difference between this comparative example and Example 1 is that the sizing agent formulation removes the diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent and replaces it with an equal mass of ordinary multifunctional epoxy resin.

[0051] Comparative Example 3: The difference between this comparative example and Example 1 is that the amino-functionalized multi-walled carbon nanotubes are removed from the sizing agent formulation.

[0052] Test method: To verify the beneficial effects of the present invention, the carbon fibers treated in Examples 1-6 and Comparative Examples 1-3 were respectively prepared into composite laminates by vacuum-assisted resin transfer molding with aerospace-grade bisphenol A epoxy resin (AG-80), and their performance was tested according to the following standards.

[0053] 1. Interfacial shear strength of monofilament (IFSS): The single-fiber microdroplet embedding and debonding method is adopted. A tensile force is applied by a microdroplet debonding measuring instrument to debond the resin microdroplets from the fiber surface. At the same time, the maximum force value during the debonding process is recorded. Combined with the embedding length of the microdroplets and the fiber diameter, the interfacial shear strength can be calculated.

[0054] 2. Interlaminar shear strength (ILSS): The test was conducted in accordance with the standard GB / T30969-2014 Test Method for Shear Strength of Short Beams of Polymer-Based Composite Materials.

[0055] 3. Self-healing efficiency (based on post-impact compressive strength CAI): Referring to the standard GB / T21239-2007 "Fibrous Reinforced Plastic Composites - Drop Weight Impact Damage Test Method", a quantitative amount of internal impact micro-damage was first caused to a standard laminate using a drop weight, and the compressive strength (CAI) after initial damage was measured. d Take another set of samples that have been subjected to the same impact, place them at 150℃ under slight lateral static pressure for 2 hours to trigger self-healing, and measure the compressive strength (CAI) after repair. h The self-repair efficiency (η) is calculated using the formula: η = (CAI) h / CAI d ) × 100%.

[0056] The data from the above tests were analyzed, and the results are shown in Table 1.

[0057] Table 1

[0058] Analysis of the test results in Table 1 shows that, compared with Comparative Example 1, which only used conventional epoxy sizing agent, the interfacial shear strength (IFSS) of Example 1 jumped from 58.4 MPa to 115.6 MPa, and the interlaminar shear strength (ILSS) increased from 67.5 MPa to 108.4 MPa, nearly doubling. This proves that the catechol groups of the crosslinking agent in the sizing agent successfully achieved strong and irreversible covalent anchoring on the carbon fiber surface; at the same time, the diepoxy groups at the other end of the crosslinking agent deeply crosslinked with the matrix resin, forming a penetrating polymer network, which completely solved the problem of easy interfacial debonding caused by the surface inertness of carbon fibers.

[0059] Comparative Example 2 removed the crosslinking agent containing dynamic borate ester bonds. Data showed that Comparative Example 2 not only had significantly lower mechanical strengths than Example 1, but also completely lost its self-healing ability. Example 1 of this invention, with its high self-healing efficiency of up to 86.5%, strongly demonstrates the high inventiveness of the dynamic covalent core of borate esters in interface engineering: under destructive impact, borate ester bonds can undergo topological rearrangement through reversible transesterification, absorbing and dissipating enormous fracture energy; under certain thermal stimulation, the broken chemical bonds can spontaneously recouple, welding and closing microcracks in situ, greatly extending the fatigue life of the composite material.

[0060] Comparative Example 3, without the addition of a rigid reinforcing phase (amino-functionalized carbon nanotubes), showed significantly lower IFSS, ILSS, and self-healing efficiency compared to Example 1. This indicates that relying solely on a dynamic resin network (soft phase) still results in a severe modulus step between the carbon fiber (ultra-high modulus) and the polymer matrix (low modulus), making it prone to stress concentration under stress wave impact. This invention, by introducing an appropriate amount of carbon nanotubes, together with flexible polyether segments, constructs a rigid-flexible nano-micron transition layer at the interface, truly achieving smooth stress transmission from the fiber to the matrix and avoiding irreversible tearing of the interface due to instantaneous large deformation.

[0061] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0062] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the protection scope of the present invention.

Claims

1. A sizing method for high interfacial bonding strength carbon fibers, characterized in that, The method includes the following steps: Activation treatment is performed on carbonized carbon fibers at high temperature. The activated carbon fibers are immersed in a sizing solution and ultrasonically treated to obtain impregnated carbon fibers. The sizing solution contains a sizing agent, which includes: diethanolamine-modified phenolic epoxy resin, biepoxy-terminated catechol-phenylboronic acid ester crosslinking agent, amino-functionalized multi-walled carbon nanotubes, amino-terminated polyether, and water. The impregnated carbon fibers are subjected to a first high-temperature treatment and a second high-temperature treatment in sequence to obtain carbon fibers with high interfacial bonding strength.

2. The method according to claim 1, characterized in that, The activation process employs atmospheric pressure air plasma treatment, with a radio frequency power of 150W and a treatment time of 60 seconds.

3. The method according to claim 1, characterized in that, The sizing solution is prepared by diluting the sizing agent with water to a solid content of 2-3 wt%.

4. The method according to claim 3, characterized in that, The sizing agent comprises the following components by weight percentage: Diethanolamine-modified phenolic epoxy resin: 35.0-55.0%; Biepoxide-terminated catechol-phenylboronic acid ester crosslinking agent 10.0-25.0%; Amino-functionalized multi-walled carbon nanotubes 1.0-8.0%; Amino-terminated polyethers: 3.0-12.0%; as well as Water content: 15.0-35.0%; The sum of the mass percentages of all components in the sizing agent is 100%.

5. The method according to claim 1, characterized in that, The preparation method of the diethanolamine-modified phenolic epoxy resin includes: Phenolic epoxy resin was heated to 85°C, and diethanolamine was added. A ring-opening addition reaction was carried out at 85°C for 2 hours. Then, the temperature was lowered to 55°C and glacial acetic acid was added for protonation neutralization to obtain diethanolamine-modified phenolic epoxy resin. The mass ratio of phenolic epoxy resin to diethanolamine was (5-10):

1.

6. The method according to claim 1, characterized in that, The preparation method of the bisepoxy-terminated catechol-phenylboronic acid ester crosslinking agent includes: 4-Carboxyphenylboronic acid was reacted with epichlorohydrin in the presence of a phase transfer catalyst and an acid-binding agent at a reaction temperature of 85°C for 10 hours. After purification, 4-(glycidoxycarbonyl)phenylboronic acid intermediate was obtained. Dopamine hydrochloride was reacted with triethylamine under ice bath conditions to release the amino group, and a free amino intermediate was obtained. The free amino intermediate was then subjected to a ring-opening addition reaction with 1,4-butanediol diglycidyl ether at a reaction temperature of 20-30°C for 2-4 hours to obtain an epoxy-terminated intermediate. The 4-(glycidyloxycarbonyl)phenylboronic acid intermediate was mixed with the epoxy-terminated intermediate and heated under dehydration conditions at 75-80°C for 24 hours to carry out a dehydration esterification condensation reaction. After the reaction was completed, the product was collected and dried to obtain a diepoxy-terminated catechol-phenylboronic acid ester crosslinking agent.

7. The method according to claim 6, characterized in that, The molar ratio of 4-carboxyphenylboronic acid to epichlorohydrin is 1:(1.2-1.5); the phase transfer catalyst is tetrabutylammonium bromide; and the acid-binding agent is potassium carbonate.

8. The method according to claim 6, characterized in that, The molar ratio of dopamine hydrochloride to triethylamine is 1:1.1; the molar ratio of dopamine hydrochloride to 1,4-butanediol diglycidyl ether is 1:

1.

9. The method according to claim 6, characterized in that, The molar ratio of the 4-(glycidoxycarbonyl)phenylboronic acid intermediate to the epoxy-terminated intermediate is 1:

1.

10. The method according to claim 1, characterized in that, The first high-temperature treatment is performed at a temperature of 70-95℃ for 1-5 minutes; the second high-temperature treatment is performed at a temperature of 140-180℃ for 2-8 minutes.

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