Carbonization is a crucial process in which PAN fibres undergo a transformation from organic polymers to inorganic carbon structures. The carbonization of PAN fibres typically comprises two stages: low-temperature carbonisation and high-temperature carbonisation. The low-temperature carbonisation process generally occurs at temperatures ranging from 300 to 1000°C, while high-temperature carbonisation typically takes place at temperatures between 1000 and 1600°C. Carbonisation involves subjecting pre-oxidised PAN fibres to further high-temperature treatment under high-purity inert gas protection. During this process, the linear molecules within the pre-oxidised PAN fibres and the cyclic molecules formed during pre-oxidation undergo additional cross-linking, cyclisation, and polycondensation. This transforms the resulting cyclic and aromatic structures into a two-dimensional aromatic layer structure, with the content of elements such as N, H, and O gradually decreasing while the carbon content increases. Ultimately, the carbon content reaches over 90%. The trapezoidal structure formed during PAN fibre preoxidation undergoes partial transformation into a folded, disordered graphite structure following low- and high-temperature carbonisation. Concurrently, the fibre diameter diminishes, density increases, and both strength and modulus are substantially enhanced. The final properties of carbon fibre are closely tied to the carbonisation process, with the maximum treatment temperature exerting the most significant influence on fibre strength, modulus, and other characteristics.
PAN precursor filaments undergo preoxidation to form insoluble, infusible preoxidised fibres with a heat-resistant trapezoidal structure. These are subsequently carbonised in low-temperature and high-temperature carbonisation furnaces protected by high-purity inert gas (typically nitrogen). During carbonisation, key process factors influencing the final properties of carbon fibres include temperature, duration, and tension. Furthermore, as the carbon content of the fibres increases from approximately 63% in the pre-oxidised state to over 90% during carbonisation, extensive pyrolysis reactions occur. The emissions from these pyrolysis fumes significantly impact the performance characteristics of the resulting carbon fibres.
Low-temperature carburising is typically conducted in a furnace with 3 to 6 gradually increasing temperature zones. The initial zone’s starting temperature is generally 300–350°C, subsequently rising in 100–200°C increments to reach 700–900°C. Low-temperature carbonisation seldom exceeds 1000°C. High-temperature carbonisation follows low-temperature carbonisation, typically conducted in a dedicated high-temperature carbonisation furnace separate from the low-temperature unit. This high-temperature furnace comprises 1-5 temperature zones, with the central zone usually operating at the highest temperature while the outer zones maintain relatively lower temperatures to sustain the central high-temperature environment. For producing general-purpose carbon fibres, the maximum carbonisation temperature generally ranges from 1200-1400°C. Conducting low-temperature and high-temperature carbonisation in separate furnaces serves dual purposes. Firstly, the significant temperature range requirements across both stages necessitate distinct furnace heating systems and insulation materials. Separate furnaces facilitate tailored design and construction to meet these specific thermal characteristics. Secondly, as pre-oxidised fibres generate substantial pyrolysis products around 600°C, dual-furnace carbonisation facilitates efficient waste gas collection and centralised treatment.
Compared to pre-oxidation, the residence time of fibres during the carbonisation stage is considerably shorter. Typically, the low-temperature carbonisation phase lasts 2–10 minutes, while the high-temperature carbonisation phase spans 20–120 seconds. Although extending carbonisation time enhances carbon fibre performance, it simultaneously incurs substantially higher equipment manufacturing and operational maintenance costs.
During the low-temperature and high-temperature carbonisation stages, the removal of substantial non-carbon constituents from the fibres causes significant shrinkage, subjecting them to considerable stress. Generally, the shrinkage rate of PAN pre-oxidised fibres remains stable at approximately 10% across varying temperatures. Although the carbonisation stage exhibits higher shrinkage rates, the initial low temperatures of the low-temperature carbonisation phase permit the application of 2%-5% tensile stress to the fibres. During the high-temperature carbonisation stage, the fibres exhibit significantly increased rigidity owing to the preliminary formation of a lamellar structure during low-temperature carbonisation, making stretching difficult. To ensure smooth process execution, a controlled shrinkage of approximately -4% is typically applied during this stage to maintain suitable fibre tension.
Atmosphere control during the carbonisation stage is another critical factor in carbon fibre production. Whether employing low-temperature or high-temperature carbonisation, the process is typically conducted within a high-purity nitrogen atmosphere. The oxygen and moisture content within this nitrogen gas significantly impacts the final properties of the carbon fibre. Oxygen levels are generally maintained below 5 parts per million, whilst moisture content is controlled by monitoring the atmosphere’s dew point, which should typically be kept below -60°C. Beyond protecting the fibres from oxidation at elevated temperatures, the nitrogen employed during carbonisation also serves to carry away pyrolysis by-products. Consequently, the gas flow dynamics within the furnace during carbonisation further influence the final properties of the carbon fibre.