| The thermal properties of isotropic pitch(IP), mesophase pitch(MP) and single filament of pitch fibers have been investigated using the thermalmechanical analysis(TMA) and by other methods such as differential thermal analysis(DTA), differential scanning calorimetry(DSC), thermogravimetry(TG), X-ray diffraction(XRD). The present study focus on the difference of the thermal performance of pitch before and after melt spinning process, the thermal properties of the green pitch fibers and the thermal behavior of mesophase pitch fibers during the stabilization.Thermal characteristic parameters such as glass transition temperature(Tg) and softening temperature(Ts) of pitch and pitch fiber were determined by TMA. The Tg and Ts of IP sample were108℃ and 173℃, while it was determined for MP sample as Tg of157℃ and Ts of 222℃. It was found that the glass transition temperature of pitch fiber is lower than that of its raw pitch. It was mainly due to the internal stresses formed during spinning process, the relative larger external force acted on pitch fiber and faster heat conduction in the thin fibers than in bulk materials.Single filament-based TMA method has been proved to be useful and reliable in characterization of pitch fibers. Glass transition temperature (Tg) and the linear coefficient of thermal expansion below and above Tg (CLTE1 and CLTE2) were determined for the pitch fibers with higher producibility. Different loads and heating rates were applied to examine their effects on measurement results. A load-free CLTE below Tg can be extracted from the intercept of CLTE-stress curve, showing a constant value of 63.3μm/(m·℃) for the pitch fibers spun from different spinning speeds. On the other hand, the free CLTE above Tg showed a remarkable trend decreased from positive to negative for pitch fibers produced with accelerated spinning speed. The isotropic pitch fibers can be partially oriented to fiber axis under strong elongation, and the limited degree of orientation is confirmed by X-ray diffraction. Thus, the free CLTE above Tg can serve as an excellent indication of degree of preferred orientation for pitch fibers.Initial oxidation temperature(110℃) of sample MPF fibers determined by TMA was found to be lower than that by TG. The oxidation process can be divided into three phases according to the TMA curve of MPF fibers. Stage I,110-170℃, oxidation of MPF fiber starts slowly with low expansion ratio and no obvious oxidation weight gain. Stage Ⅱ,170-240 ℃, the sharp shrinkage recorded by TMA starts right above 170℃ and reaches the maximum at 210℃. Afterwards the MPF fibers begin to expand again. Stage III, above 240℃, the expand ratio MPF fiber slows down since a certain degree of oxidation had occurred in the fiber. |
