Effect Of Low-temperature Stabilization And Cyclization On The Structure And Property Of Polyacrylontrile-based Carbon Fibers

Theoretically, perfect graphite can have elastic modulus and tensile strength about 1020 and 180 GPa, respectively. However, the tensile strength of commercial carbon fibers is far lower than the theoretical value. The highest tensile strength of carbon fibers manufactured in laboratory reported to be 9.03 GPa, which is only 5% of the theoretical value. It is believed that the defects such as microvoids, crack, skin-core structure and misorientation of crystallites are responsible for the large reduction in the strength. The low char yield of commercial carbon fibers is another reason. It is recognized that ladder-structure carbon is formed through cyclization and crosslinking reaction during stabilization and carbonization. However, oxidative chain pyrolysis usually competes with cyclization and the extents of cyclization and crosslinking are limited to a certain level because some acrylontrile mono mers and oligomeric products are formed through thermal chain scission along the original polymer chain at high temperatures. As a result, the char yield in a typical commercial fiber is lower than 50 %. In contrast, the theoretical char yield of a PAN precursor is 68 %. This is due to weight loss of thermal chain scission along the polymer chains forming monomers and olimogers. Suppression of weigh loss during stabilization and carbonization stage is expected to increase the char yield and tensile strength of carbon fibers. Herein, we report a low-temperature thermal stabilization method for the fabrication of PAN-based carbon fibers with improved mechanical properties and char yield. Fourier transform infrared spectroscopy(FTIR), thermal gravimetry(TG), differential scanning calorimetry(DSC), X-ray diffraction(XRD), small angle X-ray scattering(SAXS) and scanning electron microscopy(SEM) were used to characterize the chemical structure, thermal property and crystal structure of PAN fibers during low-temperature thermal stabilization, and the influence of low-temperature thermal stabilization on the stabilization and carbonization. We have made the following progress.(1) The structure evolution of PAN fibers during the low-temperature thermal stabilization at 120 oC and 130 oC were studied, respectively. Thermal-stable oxygenous pyridine structures were obtained owing to cyclization and oxidization reactions in amorphous region, which restricted the disorientation of PAN molecules. The weight-loss rate of heated treated fibers was slower compared with that of precursor fibers from the result of thermal gravimetry study. As a result, a relatively higher char yield was obtained.(2) The structure evolution during stabilization of precursor fibers P0 and treated fibers P1 were studied. During the first stage of stabilization(before 250 oC), the cyclization reaction of fibers P1 was slower than that of P0 due to the partial cyclization at the low-temperature thermal stabilization of 120 oC. During the second stage of stabilization(after 250 oC), the cyclization degree of fibers P1 was larger than that of P0. Therefore, we can set a relative high initial temperature of stabilization, decreasing zone of stabilization and accelerating stabilization rate. The tensile strength and microvoids defect during stabilization were studied. The result shows that the tensile strength decreases during stabilization both for fibers P0 and P1. However, the tensile strength of fiber P1 is higher than that of fiber P0, owing to the less defects of microvoids.(3) The structure evolution during low-temperature carbonization of precursor fibers P0 and treated fibers P1 were studied. Carbon basal plane structures were obtained by intermolecular crosslinking reaction during low-temperature carbonization. The resultant low-temperature carbonized fibers of P1 had a larger polycyclic aromatic layers, higher preferred orientation and smaller interlayer spacing. The high temperature carbonization results show that the tensile property of carbon fibers was best when the carbonization temperature was 1400 oC. The resultant carbon fibers had a higher graphitization degree, less microvoids defect and higher preferred orientation, which gave rise to considerable increases in tensile strengt. Low-temperature thermal stabilization method was of important significance for the preparation of high performance carbon fibers.