Determination of the Critical Residence Time in Industrial Continuous Two-Stage Carbonization of PAN-Based Carbon Fibers

Shortening the carbonization residence time offers a direct route to increase industrial carbon fiber throughput and reduce cost. To enable this, time-dependent structural evolution was systematically investigated by precisely varying the residence times of low- and high-temperature carbonization (LTC and HTC) on a pilot-scale continuous production line. Comparative analysis of the chemical composition and physical structure established a time-resolved evolution model. A critical residence time of ∼1 min was identified for both LTC and HTC. Before the critical residence time, heteroatom removal was pronounced and structural rearrangement initiated rapid formation of carbon microcrystallites; as the critical residence time was approached, a turbostratic carbon network was established and the tensile strength and Young’s modulus reached ∼4.9 and ∼250 GPa, respectively. Beyond the critical residence time, structural evolution continued to progress, but the growth trend slowed, and further improvements in mechanical properties were marginal.