Gas-phase oxidation is typically carried out at low or high temperatures using air, O2, or oxygen-containing gases (such as ozone and CO2). It is characterized by simple equipment, low cost, and low pollution. Compared to liquid-phase oxidation, gas-phase oxidation is easier to implement industrially by strictly controlling conditions such as time and temperature. However, with increasing time and temperature, oxidation etches carbon fibers, increasing their surface area but also affecting their tensile strength. Therefore, precise control of processing time and temperature is necessary to avoid excessive damage to the carbon fibers and a decrease in the mechanical properties of the composite material.
Liquid-phase oxidation involves placing carbon fibers in a liquid reaction system, allowing the oxidant in the liquid system to react with the carbon fibers, thereby etching the carbon fiber surface. HNO3, KMnO4, NaOCl, and NaClO3 are commonly used oxidants. Compared to gas-phase treatment, liquid-phase treatment is generally milder, does not cause excessive pitting, and has a longer processing time, making it suitable for studying the mechanisms of surface treatment.
Anodizing, also known as electrochemical oxidation, is a treatment method that uses carbon fiber as the anode and applies a potential to the carbon fiber to generate oxygen on its surface. Nascent oxygen is highly reactive and can oxidize and etch carbon fiber. The electrolytes used in anodizing mainly include HNO3, H2SO4, NaCl, KNO3, NaOCl, NH4OH, NH4HCO3, NH4HS, and NaOH. In actual production, NH4HCO3 solution is widely used due to its high reaction rate and weak corrosiveness. Anodizing offers mild reaction conditions, high operability, and fast and uniform oxidation, making it suitable for large-scale production and thus widely used in the treatment of commercial carbon fibers. In practical applications, operators often adjust the final properties of the carbon fiber to meet production requirements by changing the type of electrolyte or adjusting the current.
