Polymer coatings applied via methods such as electron-induced polymerisation, polymer surface media, and surface coating introduce a thin polymer film onto the carbon fibre surface, thereby achieving compatibility with the matrix resin.
Plasma constitutes a collective of matter comprising a sufficient quantity of positively and negatively charged particles with approximately equal charge numbers. The modification of fibre surfaces via plasma oxidation refers to the process whereby non-polymerising gases exert physical and chemical effects upon material surfaces. Treating carbon fibre surfaces with low-temperature plasma or microwave plasma is also a proven method. This approach features gas-solid reactions, is pollution-free, and requires minimal processing time—typically achieving the desired effect within seconds. The plasma gas may be reactive (oxygen, ammonia, carbon monoxide, etc.) or inert (helium, nitrogen, argon, etc.). Commonly employed oxygen plasma exhibits high energy and strong oxidising properties. Upon colliding with the carbon fibre surface, it oxidises carbon-carbon double bonds at microcrystalline edges, corners, and defects into oxygen-containing active groups. However, plasma generation necessitates a vacuum environment, requiring complex equipment, and presents challenges for continuous, stable, and prolonged processing.
Electrolytic oxidation, also known as anodic oxidation, involves subjecting carbon fibre to electrolytic oxidation treatment by applying a direct current electric field in an aqueous electrolyte solution, with the carbon fibre serving as the anode and a graphite plate as the cathode. This process generates active functional groups on the carbon fibre surface. The electrochemical oxidation reaction conditions are mild, with short processing times and relatively simple process equipment. This enables seamless integration and alignment with carbon fibre production lines for industrial-scale manufacturing. By controlling process parameters such as electrolysis temperature, electrolyte concentration, and current density, selective control over the oxidation degree and surface functional groups of the carbon fibre can be achieved. Electrolytic oxidation is currently a widely adopted method in the industrial production of carbon fibre. During anodic oxidation surface treatment, as the carbon fibre itself serves as the anode, oxygen anions in the electrolyte migrate towards the fibre under the influence of an electric field when a current is applied. These anions discharge at the fibre surface, generating nascent oxygen species that subsequently oxidise the fibre. This process yields oxygen-containing functional groups such as hydroxyl, carboxyl, and carbonyl groups. Concurrently, the carbon fibre undergoes a degree of etching, altering its inherent surface microstructure. When employing electrochemical oxidation, the rational selection of the electrochemical oxidation apparatus is a prerequisite for ensuring effective surface treatment of the carbon fibre. Factors to consider when selecting the apparatus include the cathode material, electrolyte, and current choice. The cathode material must be both conductive and corrosion-resistant. Graphite plates, possessing excellent conductivity and corrosion resistance, are widely utilised in industrial production. Electrolytes may comprise acids, bases, or salts, such as nitric acid, sulphuric acid, phosphoric acid, potassium hydroxide, sodium hydroxide, potassium phosphate, potassium nitrate, ammonium carbonate, ammonium bicarbonate, or ammonium dihydrogen carbonate. In acidic electrolytes, oxygen atoms generated by water electrolysis are adsorbed by unsaturated carbon atoms on the carbon fibre surface. These adsorbed oxygen atoms interact with adjacent carbon atoms, producing carbon dioxide and thereby etching the graphite microcrystals. The reduction in carbon atoms at edges and corners significantly increases surface functional groups. For alkaline electrolytes, hydroxide ions are adsorbed by active carbon atoms on the carbon fibre surface. These interact with adjacent carbon atoms bearing adsorbed hydroxide ions to produce oxygen, thereby increasing the number of surface active carbon atoms. Anodic surface treatment typically employs direct current, though alternating current is also utilised. Effective treatment results can be achieved with relatively low electrical charges.
he most direct effect of surface treatment on carbon fibers is an enhanced interfacial bonding between the fibers and resin, leading to a significant improvement in the shear strength of composite materials. Typically, following surface treatment—particularly through physicochemical etching—the strength of carbon fibers diminishes, with a marked reduction observed at higher treatment levels. Conversely, under appropriate surface treatment, the etching process may reduce the size of surface defects, resulting in a certain increase in the tensile strength of carbon fibers. Surface treatment of carbon fibre generally has little effect on its modulus.