Thermogravimetry in an oxidizing atmosphere

The thermogravimetric oxidation tests were performed using air as oxidizing agent. Information trials using oxygen had disclosed very fast oxidation reactions, sometimes so fast that effects are overtaken and concealed which are clearly visible in air.With regard to 

Most of the aromatics from Table 3-2, up to and including anthracene, which were tested, evaporate or sublimate below the temperature of 225 °C in the flow of inert gas through the thermobalance. Most of the aromatics also evaporate or sublimate in the flow of air, due to their high oxidation stability. Fig. 3-26 demonstrates that in experiments with n-hexylpyrene, a deviation of the oxidation from the evaporation curve first occurs above 300 °C. In the oxidation experiment a small portion of coke is deposited, which will bum at temperatures above 450 °C. 

Even relatively bulky molecules such as adamantane, evaporate or sublimate completely and thus present identical TGA and DTG curves both in argon and in air. The DTG curve exhibits one peak only at a temperature of 140 C. 

Experiments in air using different heating rates have demonstrated not only a parallel shift of the TGA curves to higher temperatures at faster heating rates, but also a change of the appearance of the curve, as shown in Fig. 3-27 for n-hexacontane at heating rates from 5 up to 100 K/min. Whereas the curve at )3= 10 K/min is more or less a parallel displacement of the curve at j8= 5 K/min, the curve at 3= 20 K/inin shows quite another form. 

The date for the TGA and DTG temperatures in Table 3-9 demonstrate, especially in the case of the DTG maximum temperatures, that at fast heating rates effects are taken over, and obscured, which are still present at slower heating rates. At heating rates exceeding 50 K/min only one sharp peak appears at 344 °C (50 K/min) or 383 °C (100 K/min) 

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