Carbon fiber, as an advanced material characterised by high strength and modulus, typically requires composite processing with other matrix materials to form composite structures for practical application. Due to the high-temperature treatment exceeding 1300°C that carbon fibre undergoes, comprising over 90% carbon content with minimal surface active functional groups, it exhibits significant inertness. This results in poor adhesion when combined with matrix materials such as polymer resins, hindering the expression of the fiber’s superior mechanical properties and ultimately compromising the composite material’s performance. Consequently, surface treatment is typically required during carbon fibre production to introduce reactive groups onto its surface, thereby enhancing bonding with resin matrices.
As surface treatment of carbon fibers enhances the performance of composite materials, research into surface treatment methods constitutes a key focus within carbon fiber production technology. Through years of investigation, scientists have developed multiple techniques for treating carbon fiber surfaces, including vapour phase oxidation, liquid oxidation, polymer coating, plasma oxidation, and electrolytic oxidation. Among these methods, electrolytic oxidation is currently the primary technique employed in industrial production.
The vapour-phase oxidation method involves exposing carbon fibers to vapour-phase oxidants (air, oxygen) under specific conditions such as heating and catalysis, thereby oxidising their surfaces to generate reactive functional groups (hydroxyl and phenolic groups). This treatment significantly enhances the interfacial shear strength between carbon fibers and the matrix. For instance, oxidising carbon fibers in air at 450°C for 10 minutes improves both shear and tensile strength in the resulting composite material. Continuous introduction of ozone at concentrations of 0.5–15 mg/L into surface treatment furnaces yields interfacial shear strengths of 78–105 MPa. Employing halogens or sulphur dioxide as inhibitors within an oxygen atmosphere can also effectively modify surface properties. The advantages of the gas-phase oxidation method include convenient online integration and rapid processing speed. Disadvantages include suboptimal uniformity in carbon fibre treatment, stringent process conditions, challenging control, potential significant damage to mechanical properties, and substantial environmental impact due to the use of toxic and hazardous gases.
The liquid-phase oxidation method employs strongly oxidising liquids or solutions, such as nitric acid, potassium dichromate, sodium hypochlorite, hydrogen peroxide, or potassium persulphate, to treat the surface of carbon fibers. This process generates oxygen-containing functional groups—including carboxyl, hydroxyl, and carbonyl groups—on the fibre surface, thereby enhancing interfacial bonding with resins. As the liquid-phase oxidation method is milder than its vapour-phase counterpart, the degree of oxidation is more readily controlled, reducing the risk of excessive oxidation that could compromise the fiber’s mechanical properties. Consequently, it is one of the more extensively researched techniques. However, the extended processing time makes it difficult to integrate with carbon fibre production lines, typically confining its use to batch surface treatment. Furthermore, the highly oxidising liquids cause severe corrosion to equipment and are challenging to remove completely from the carbon fibers.