Carbon Fiber Modification

I. Purpose of Carbon Fiber Modification

Improving the compatibility between carbon fiber and the matrix: Enhancing the mechanical properties of composite materials and strengthening the mechanical interlocking, physical adhesion, and chemical bonding between the fiber surface and the matrix.

Improving interfacial bonding: During manufacturing, carbon fibers undergo high-temperature carbonization treatment above 1000℃, resulting in a smooth surface lacking active functional groups. This leads to surface inertness, poor adhesion to polymers, and weak interfacial bonding, directly affecting the interlaminar shear strength of the composite material.

Enhancing surface activity: This allows for effective stress load transfer between the carbon fiber and the matrix material, thereby increasing the value of the fiber material in industrial applications.

Improving fiber properties: This includes improving temperature resistance and oxidation resistance, which can be achieved by introducing trace amounts of elements such as P, B, and Zn onto the fiber surface or by coating with metallic or non-metallic layers.

II. Mechanism Analysis of Modification

1. Physical Modification Mechanism: The physical modification of carbon fibers mainly achieves interfacial reinforcement by increasing surface roughness and specific surface area:

Increasing surface roughness: Methods such as gas-phase oxidation and plasma treatment can significantly increase the surface roughness of carbon fibers. "Atmospheric pressure argon plasma treatment can increase the oxygen content on the carbon fiber surface by 22.5%, reduce the water contact angle to 45.1°, and maintain the tensile strength at 3.23 GPa after 300 seconds of treatment." AFM testing showed that the surface roughness (Ra) increased from 0.31 μm to 0.47 μm.

Surface etching and activation: Electrochemical oxidation treatment, through a "combined process of layer-by-layer oxidation etching and functional group changes," creates micropores and grooves on the carbon fiber surface, increasing the mechanical interlocking effect.

Surface morphology improvement: "Plasma treatment removes contaminants through physical bombardment and introduces hydroxyl/carboxyl active groups, significantly improving interlayer shear strength."

2. Chemical modification mechanism

The chemical modification of carbon fibers mainly achieves interfacial enhancement by introducing active functional groups:

Introduction of oxygen-containing functional groups: Liquid-phase oxidation (using concentrated nitric acid, concentrated sulfuric acid, hydrogen peroxide, etc. as oxidants) and electrochemical oxidation can significantly increase the types and numbers of oxygen-containing functional groups (such as hydroxyl and carboxyl groups) on the carbon fiber surface. "Electrolytic potentiometric treatment can increase the oxygen content on the carbon fiber surface from 9.36% to 18.04%, reduce the contact angle from 90.2° to 62.4°, and increase the interlaminar shear strength by up to 56%."

Chemical Bond Formation: "DA or polydopamine (PDA) mainly achieves chemical grafting modification by reacting the -NH₂ in the molecule with the -C=O and -COO- functional groups on the carbon fiber surface through a Schiff base reaction, forming stable chemical bonds on the carbon fiber surface."

Surface Grafting Reaction: The surface grafting method involves "placing the carbon fiber in an atmosphere of active monomers, where, under the action of an initiator, the monomers react with the active groups or edge carbon atoms on the fiber."

Special Modification Method: "In NH₄HCO₃ solution, the fiber surface mainly undergoes an electrolytic oxygen release reaction of water and an electrochemical oxidation reaction of some electroactive substances; the content of various oxygen-containing functional groups on the fiber surface changes continuously with the extension of treatment time, and the reaction of NH₄⁺ with the functional groups on the fiber surface introduces a large number of amide groups into the fiber surface." Coupling Agent Modification: "An aminosilane coupling agent (KH550) was used to treat the surface of carbon fibers, forming a chemically bonded interface layer.

After modification: the number of active functional groups increased: the O-C=O content increased by 95.24%, and the C=O content surged by 508.45%, forming more resin bonding sites."

III. Comprehensive Performance of Modification Effects

After modification, the surface polarity of carbon fibers significantly improved, the contact angle decreased, and the wettability enhanced, thereby effectively improving the interfacial properties of the composite material. "Surface modification technology enhances the surface activity of carbon fibers, strengthens the interfacial properties between carbon fibers and the matrix material, and improves their adhesion to the matrix."

In practical applications, the interfacial shear strength between modified carbon fibers and the resin matrix significantly improved. "The IFSS of DA-modified carbon fibers and epoxy resin E51 increased to 65.32 MPa, a 47.35% increase compared to unmodified carbon fibers."

In summary, carbon fiber modification effectively improves the interfacial properties between carbon fibers and the matrix through both physical and chemical mechanisms, thereby significantly improving the overall performance of the composite material.

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