Comparative studies of the thermal characteristics of aromatic fibres (Arselon, Arselon-S, Fenilon, Armos, Arimid PM) in air and nitrogen medium were conducted with TGA and DTA. TGA and DTA showed that the heterocyclic fibers Arimid PM and Armos have the highest thermal stability in air and nitrogen medium and Arselon and Arselon-S fibres are slightly inferior to them, but they in turn have much higher thermal characteristics than Fenilon. In nitrogen medium, all of the investigated aromatic fibres form coke residue, attaining 60% for Arimid, 57-62% for Armos and Fenilon, and 46-53% for Arselon and Arselon-S.Exposure to high temperatures causes a complicated set of chemical transformations which are a function of the environment in thermostable aromatic fibres. Thermooxidative processes take place in oxidizing medium (in air), thermal degradation reactions predominate in neutral medium (in nitrogen or a vacuum), while pyrolysis takes place at higher temperatures in neutral medium with formation of coke residue.In previous publications [1-3], thermooxidative degradation of different kinds of thermostable aromatic fibres/yarns polyoxadiazole, polyimide, polyamide in air medium was compared. The research was continued to compare thermal degradation both in air and in inert medium (nitrogen) to more completely understand the mechanism of the thermal transformations in these fibres, as this would allow evaluating the role of atmospheric oxygen in the reactions that take place. Studies using thermogravimetric analysis in a vacuum were conducted for polyoxadiazole fibres in [4], but we were unable to find any published comparative analysis with other thermostable fibres.We investigated and compared the thermal properties of aromatic thermostable fibres/yarns in air and in nitrogen medium. The following samples of thermostable fibres were selected for the studies: polyoxadiazole Arselon (previously called Oxalon) and Arselon-S (light-stabilized), para-aramid Armos, meta-aramid Fenilon, and polyimide Arimid PM. The fundamental physicomechanical properties of the samples are reported in Table 1.The temperature characteristics of the fibres were investigated by thermogravimetric (TGA) and differential thermal (DTA) analysis.The thermal analysis was conducted with a PaulikPaulikErdey system Q-1500 D derivatograph. The initial temperature of the tests was 23°C. The final temperature in the test in air was determined in burning or completion of pyrolysis of the samples. In testing in nitrogen, it attained 700°C (carbonization is essentially completed below this temperature and no weight change is observed in the samples). The temperature elevation rate was 5°C/min.In assessing the thermal transformations in the fibres investigated, the following characteristic points were determined with the TGA and DTA data (Fig. 1): onset of intensive decomposition based on TGA curves (point of intersection of tangents to curve segments before and after inflection) T 1 ; temperature of completion of degradation in air based on DTA curves T 2...

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