A procedure for precise determination of thermal stabilization reactions in carbon fiber precursors

Thermal stabilization of polyacrylonitrile (PAN) fibers is an important step in production of carbon fibers. Understanding the onset and temperature range of the stabilization reactions is a key for adjusting processing parameters such as tension, stretching, etc. However, stabilization reactions are very complex and overlap. In order to separate the stabilization reactions, we combined the results of FTIR (Fourier Transform Infrared Spectroscopy), DSC (Differential Scanning Calorimetry), TGA (Thermogravimetry analysis), TMA (Thermomechanical analysis), and densitometry. It was shown that combination of DSC and TGA allowed separation of reactions regardless of the composition of initial PAN fibers. FTIR, TMA and densitometry results verified the validity of proposed method. Accordingly, three special and commercial grade PAN fibers with different chemical composition were studied. FTIR results indicated that during thermal stabilization of PAN fibers chemical reactions including cyclization, oxidation and dehydrogenation occurred in the fibers and a ladder polymer was formed. According to DSC and TGA curves, initiation temperature, temperature range and order of occurrence of these reactions were a function of chemical composition of initial fibers. In fibers containing itaconic acid plus methyl acrylate comonomers, oxidation reactions already started at 175 °C. Cyclization started above 210 °C, and reactions occurring above 250 °C were mainly dehydrogenation. In fibers containing only itaconic acid cyclization initiated above 210 °C, dehydrogenation started after 242 °C and oxidation occurred only after 284 °C. In fibers containing vinyl acetate comonomers, the initial reactions above 240 °C were attributed to cyclization. Oxidation occurred below 290 °C and dehydrogenation started above 290 °C.