From PAN precursor to carbon fiber: Multi-stage stretching-mediated structure evolution and performance enhancement

The performance of polyacrylonitrile based carbon fibers is highly dependent on the optimization of the stretching process during precursor fiber preparation. This study systematically analyzes the effects of stretching processes in wet spinning, dry-jet wet spinning, and gel spinning on fiber orientation degree, crystallinity, and mechanical properties, while exploring the critical role of stretching across the entire manufacturing process, including spinning, pre-oxidation, and graphitization. Overall observations from the literature indicate that in the coagulation bath stage, balancing negative stretching and total stretching ratio is essential to suppress surface defects and enhance molecular chain alignment. Hot-water stretching significantly eliminates internal defects via thermo-solvent synergistic plasticization, and under specific process conditions, a two-stage total stretching ratio of 2 × 2.5 achieves a balance between fiber performance and process stability. High-pressure steam stretching further optimizes molecular chain arrangement but requires careful control to avoid fracture risks. During pre-oxidation, applied stretching inhibits thermal shrinkage and disorientation. In contrast graphitization stretching significantly improves modulus by regulating crystal dimensions and orientation. However, current processes face challenges such as insufficient multi-stage parameter synergy, defect control difficulties, high energy consumption, and limited mechanistic understanding. Future advancements should integrate intelligent optimization, low-energy process innovations, and cross-scale characterization technologies to drive the industrial production of high-performance carbon fibers. This review provides theoretical foundations and technical directions for optimizing the stretching process and enhancing the performance of carbon fibers.