Thermogravimetry isothermal experiments

Non-isothermal thermogravimetry experiments where the sample is also completely destroyed to volatiles and carbon residue have the highest potential to reveal the mutual link between degradation of polyolefin at lower temperatures and its tendency to ignition and burning. 

The thermochemical approach to kinetic studies is in the stage of development, as compared to the commonly used approaches. This explains the insufficiency and, in some cases, a rather low reliability of the obtained data to answer the above listed questions and to draw final conclusions on the decomposition mechanisms for certain substances. Independent verification and/or comparative experiments are required to confirm or disprove the results obtained. Information presented in Part III of this book may be helpful for further development of such studies. Part III is devoted to the technique of isothermal thermogravimetry and contains the primary results of kinetic measurements, and their detailed discussion, for a few dozens of substances. Results of these measurements have been partly discussed in a generalized form in Part II. 

Figures la and lb show the TGA-mass spectrometry data for water and NMP on the as-received poly(amide-imide), indicating that neither liquid was removed until five minutes into the run, which corresponded to approximately 220 C. The thermogravimetry showed that there was a total of approximately 2 weight percent of water and NMP in the polymer. Isothermal thermogravimetric experiments were conducted on the as-received sample between 145 and 165 C. Temperatures above 155X were enough to dry the sample in 3 hours. Although, further drying of the samples were done at 190 C, an intermediate value between 155 and 220°C. Even this simple experiment showed that there were strong associations of the water and NMP to the poly(amide-imide).

Storage conditions of commercial plasters often result in partial hydration of the hemihydrate material. Dynamic or isothermal thermogravimetry can be used to estimate the primary phases and impurities present in commercial plaster. In dynamic TG experiments at low heating rates, the hemihydrate and dihydrate contents can be estimated from separate mass change steps of the respective dehydration reactions shown in the resulting TG curves. 

Temperature-programmed thermogravimetry in inert gas gives a quick survey of the evaporation behavior, as shown in Fig. 4-144 for some polysiloxanes. For comparison, the TGA curves of a polyalphaolefm (sample 1) and of a perfluoroalkylether (sample 29) are also plotted. Those substances evaporate at considerably lower temperatures than the polysiloxanes despite their substantially higher viscosities. Similar behavior was observed in the isothermal gravimetry. For comparison the oven-aged samples (120 days at 220 "C both with and without wood chips) were also tested by thermogravimetry. The index numbers from those experiments are listed in Table 4-185. 

The experiments are carried out in isothermal isobaric conditions on metal plates whose only the two principal faces react. We use thermogravimetry to measure the increase in mass of the solid, Am. The kinetic law observed can be represented by a hne in coordinates (Am, t). Far from the equilibrium conditions, the measnred reactivity varies as the vapor pressure of sulfur to the power 1/6. 

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