The pre-oxidation process of PAN precursor filaments represents a pivotal stage in carbon fibre production, facilitating the transition from organic to inorganic structures. From a structural transformation perspective, it may also be termed a thermal stabilisation process. Typically conducted in an air atmosphere, this involves prolonged heat treatment of PAN precursor filaments within a controlled thermal gradient, thereby imparting a thermally stable structure to the resulting fibres. Generally, the temperature range employed for preoxidation varies between 180–300°C depending on the copolymer composition of the precursor. This treatment induces a partial transformation of the PAN precursor’s molecular structure from linear macromolecules into a crosslinked network, thereby establishing the structural foundation for subsequent carbonisation. During PAN precursor preoxidation, both the composition and structure of the fibre undergo changes under thermally coupled external conditions. The fibre colour transitions through stages from white to pale yellow, yellowish brown, dark brown, and finally black. Although the mechanism behind this colour change remains unclear, it is generally attributed to the formation of trapezoidal ring structures during preoxidation. Chemical reactions occurring during preoxidation include cyclisation, dehydrogenation, aromatisation, oxidation, and crosslinking. The enhanced thermal stability of the post-preoxidised fibre is attributed to the cyclisation of nitrile groups forming ladder-type structures. Concurrently with these chemical reactions and compositional changes, structural alterations in the aggregated state of the PAN precursor occur, such as the disappearance of crystalline structures and transformations in orientation patterns.
The pre-oxidation process represents a crucial intermediate stage in the transformation of PAN precursor filaments from chain-like organic polymers into inorganic carbon materials with graphite-like structures, significantly influencing the mechanical and other properties of the final carbon fibres. Initially, Akio Kondo of the Osaka Technical Research Institute in Japan successfully produced carbon fibres using PAN as raw material. However, the mechanical properties of these fibres proved unsatisfactory. Building upon Kondo’s methodology, the Royal Research Institute in the United Kingdom subsequently developed high-performance carbon fibres at commercially viable prices. The most critical technical advancement involved controlling the tensile stress of the carbon fibres during the pre-oxidation process. This underscores the pivotal role of pre-oxidation in the preparation and production of high-performance carbon fibres.
In practical research and production processes, the pre-oxidation of PAN typically employs two methods: batch processing and continuous processing. During batch pre-oxidation, fibres are generally heated using either a fixed tension or fixed length approach. The fixed tension method involves applying a specific weight to one end of the filament bundle before thermal treatment, whereas the fixed length method entails securing or winding the bundle onto a frame. The batch method offers advantages in process flexibility and ease of operation, allowing precise control over heating rates, atmosphere composition, and preoxidation duration. Its disadvantages include limited continuous length, which hinders large-scale production. Furthermore, managing tension variations caused by fibre shrinkage during processing proves challenging, making it difficult to achieve optimal orientation structures. Additionally, tow strands are prone to damage, adversely affecting the final carbon fibre properties. In industrial production, the batch method finds little practical application, typically being reserved for general scientific research or the simple preparation of pre-oxidised fibres where mechanical properties are not a requirement. With the deepening development of research in the field of high-performance carbon fibres, the batch method is generally only employed for foundational experiments involving principle verification.
The continuous pre-oxidation process employs drive rollers to subject PAN precursor filaments to continuous pre-oxidation treatment within the pre-oxidation furnace. The tension of the filament bundles and the duration of pre-oxidation can be readily controlled by adjusting the transmission speed of the drive rollers. During continuous pre-oxidation, the PAN precursor passes horizontally or vertically through a furnace comprising multiple temperature zones. The furnace atmosphere typically consists of hot air, with temperatures increasing in a gradient from low to high. Forced ventilation within the furnace dissipates the reaction heat and decomposition products generated during the pre-oxidation of the filament bundle. Characterised by excellent thermal stability and strong process operability, the continuous method is widely adopted in the industrial production of high-performance carbon fibres.
For the pre-oxidation reaction process, whether employing batch or continuous methods, the distinction lies solely in the provision of thermal fields or tensile environments for pre-oxidation. The factors that genuinely influence the reaction progression are the pre-oxidation process conditions—namely, the rational determination and coordination of pre-oxidation temperature, duration, heating rate, tensile stress, and atmosphere.