Thermal Decomposition Behavior of a Heterocyclic Aramid Fiber

Heterocyclic aramid fibers are one of the high-performance fibers with excellent mechanical and thermal properties. In this article, the thermal decomposition behaviors of a type of the fibers were studied in nitrogen and air by pyrolysis/gas chromatographymass spectrometry (Py/GC-MS), thermal gravimetric analysisdifferential thermal analysis/Fourier transform infrared spectroscopy (TGA-DTA/FTIR), and thermal gravimetric analysisdifferential thermal analysis/mass spectrometry (TGA-DTA/MS). The results showed that under nitrogen atmosphere, the thermal decomposition mainly happened between 520 degrees C and 580 degrees C, the temperature of the maximum weight loss rate was 550 degrees C, and the weight remaining at 800 degrees C was 58%. HCN, NH3, NO2, NO, CO2, CO, H2O, and some other compounds containing benzene rings were detected by the TGA-DTA/FTIR. Among these released chemicals, the intensity of the absorption peak assigned to CO2 was the strongest. These chemicals were also identified by the TGA-DTA/MS. The Py-GC/MS analysis revealed that the number of chromatographic peaks increased with the increase of temperature. Most of the pyrolysis products were produced between 550 degrees C and 600 degrees C, which represented the major pyrolysis process. Moreover, the detection of benzene ring containing compound fragments reflected the process of the molecular chain scission. In air atmosphere, the thermal decomposition mainly happened between 500 degrees C and 680 degrees C. The maximum weight loss rate was observed at 600 degrees C, and almost 100% weight was lost at 900 degrees C. NH3, NO2, CO2, and H2O were detected by the TGA-DTA/MS, and the ion current intensity of CO2 was again the strongest with a strong oxidation reaction at around 670 degrees C. It was speculated that the thermal decomposition began with the breaking of the bonds between PPTA (poly-p-phenylene terephthalamide) blocks and heterocyclic blocks at high temperature. Then, with the increase of temperature, the chemical bonds inside the PPTA blocks and heterocyclic blocks were broken. In this process, free radicals that led to restructuring and new breakages to produce micromolecular products were introduced.