Pretreatment and Pyrolysis of Rayon-based Precursor for Carbon Fibers

In this work, two rayon fibers were investigated as carbon fiber precursors. A detailed consideration has been applied to a domestically produced cellulose fiber to carbon fiber (CF) transition. This transition of precursor to carbon fiber can be subdivided into two stages: pyrolysis (thermal decomposition) of cellulose in air and high temperature treatment in an inert atmosphere. The specific objectives were to investigate the stabilization stage of the produced rayon with respect to changes taking place during thermal decomposition, and to evaluate the effects on the properties of the carbonized fiber. Changes taking place during the conversion process of the domestic precursor are compared with respect to the commercial rayon fiber.

Phosphoric acid was used as a catalyst and a flame retardant during stabilization. It was observed that the acid plays a multirole of heat absorption, catalytic dehydration by lowering the required temperatures, and acts as a protection during carbonization. The effects of time and temperature during stabilization were studied systematically. The temperature affects the structural changes taking place, and the time required for completion of stabilization reactions. The thermal behaviors of rayon fibers were analysed by Thermogravimetry Analysis (TGA) and Differential Scanning Calorimetry (DSC). The results showed that the phosphoric acid treated fibers underwent pyrolysis under lower temperatures and over a wider temperature range. Wide Angle X-ray Diffraction (WAXD) was used to analyse the degree of cristallinity of the precursor and the subsquent carbon fibers. The highly ordered and oriented precursor becomes totally amorphous after pyrolysis. The crystallite order was reinduced during carbonization under tension. Fourier Transform Infrared Spectroscopy (FTRI) was utilized to investigate the chemical transition during the heat treatment. The intensity of peaks corresponding to chemical groups present in the precursor decreased by the end of low temperature pyrolysis and disapeared during carbonization indicating the fibers were mostly carbon. The mechanical properties, morphology and structure of the precursor and the obtained carbon fibers were studied by Scanning Electron Microscopy (SEM). An increase in applied tension during carbonization increased the carbon content slightly leading to better quality fibers.