The effect of processing on the structure and properties of carbon fibers

Even though the same three process steps (fiber formation, stabilization, and carbonization) are used to produce both polyacrylonitrile-based (PAN-based) and pitch-based carbon fibers, their final properties differ significantly. This is a direct result of the precursors used to produce these two types of carbon fibers (polymeric versus liquid-crystalline). Liquid-crystalline materials readily orient during fiber formation, creating fibers with a high degree of molecular orientation, whereas polymers form fibers with less ordered, fibrillar structures. Carbon fibers with high degrees of molecular orientation exhibit high moduli and thermal conductivities. By contrast, carbon fibers with more discontinuous and less ordered, fibrillar structures tend to develop higher tensile strengths. Thus, it is not surprising that PAN-based carbon fibers have become the preferred reinforcement for high-strength composites. However, recent studies have proven that, by disrupting molecular orientation during fiber formation, the strengths of pitch-based carbon fibers can be improved significantly. Alternatively, linearizing the molecular orientation during fiber formation can yield pitch-based fibers with enhanced thermal conductivities. Researchers now realize that understanding and controlling structure during the fiber formation step is critical if the properties of carbon fibers are to be optimized. Controlling the structure during fiber formation can also permit milder conditions to be used during subsequent process steps. As a result, research into precursor fiber formation offers the best opportunity for improving properties and reducing production costs for both PAN-based and pitch-based carbon fibers. (C) 1998 Elsevier Science Ltd. All rights reserved.