Influence of temperature on the thermo-oxidative degradation of polyamide 6 films

The thermo-oxidative degradation of polyamide 6 (PA6) was studied at relative high temperatures (between 120 and 170 °C) using oxygen uptake and hydroperoxide determination methods, chemiluminescence, FT-IR and UV-VIS spectroscopy as well as solution viscosity and tensile property measurements.The relation between the results of the different analytical techniques and influence of temperature on these relations was determined. Arrhenius plots of the degradation determined with the different methods are linear; however the activation energies determined from these plots depend on the analytical method used. For oxygen uptake measurements and changes in UV absorbance (at 280 nm) and solution viscosity an activation energy of about 120 kJ/mol was calculated, for the increase in carbonyl index of about 80 kJ/mol and for the decrease in elongation at break of about 150 kJ/mol.The changes in oxygen uptake UV absorbance and solution viscosity are probably due to the same chemical process. The lower activation energy from changes in the carbonyl index is attributed to the formation of gaseous products, which play a larger role at higher temperatures. The higher activation energy from the elongation at break measurements was ascribed to the contribution of physical changes that play the largest role at the highest temperatures.     

Thermo-oxidative degradation of novel epoxy containing silicon and phosphorous nanocomposites

Modified epoxy nanocomposites containing silicon and phosphorous was prepared and compared with pure epoxy. The study of thermo-oxidative degradation of modified epoxy nanocomposites and pure epoxy has been utilized by thermal analysis. The thermal stability of modified epoxy nanocomposites is not superior to that of the pure epoxy at low temperature, however, the char yield of modified epoxy nanocomposites is higher than that of the pure epoxy at 800 °C in air atmosphere. The modified epoxy nanocomposites possess better thermal stability at high temperature range. The values of the limiting oxygen index of pure epoxy and modified epoxy nanocomposites are 24 and 32, respectively. This indicates that modified epoxy nanocomposites possesses better flame retardance.By the Kissinger’s method, the activation energies of thermo-oxidative degradation for epoxy nanocomposites are less than those of thermo-oxidative degradation for pure epoxy in first stage of thermo-oxidative degradation. However, the activation energies of thermo-oxidative degradation for epoxy nanocomposites are more than those of thermo-oxidative degradation for pure epoxy in second stage of thermo-oxidative degradation.     

The regression of isothermal thermogravimetric data to evaluate degradation Ea values of polymers: A comparison with literature methods and an evaluation of lifetime prediction reliability

The thermooxidative degradation of four well known polymers, polyethylene (PE), polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA), was carried out in a thermogravimetric (TG) analyser, at various temperatures (in the 473–533 K range), in isothermal heating conditions. The resulting set of experimental TG data was used to determine the apparent activation energy (E_a) of degradation through two isothermal literature methods, as well as through a very simple method we set up, based on the direct regression of the experimental mass loss data, in order to verify the general applicability of our method to various polymers. The results from different methods were in good agreement. Degradation experiments in dynamic heating conditions, which were also performed, gave E_a values in good agreement with those in isothermal heating conditions for PS, PC and PMMA, while for PE a large discrepancy was observed, which was discussed and interpreted. The results suggested the general applicability of our method to all polymers, independently on their structure and degradation mechanism. A long-term (about 13 months) isothermal degradation experiment was also carried out with the same polymers at relatively low temperature (423 K). Only PE and PS evidenced appreciable mass loss in the investigated period, but the experimental data were not in agreement with those from the short-term degradations at higher temperatures, thus suggesting different degradation kinetics, and a low reliability of the lifetime predictions for polymers in service based on experiments at higher temperatures.     

The regression of isothermal thermogravimetric data to evaluate degradation Ea values of polymers: A comparison with literature methods and an evaluation of lifetime predictions reliability. Part II

The kinetics of the isothermal degradation in static air atmosphere of four well known polymers, polyethylene (PE), polystyrene (PS), polycarbonate (PC) and poly(methyl methacrylate) (PMMA) was studied by both a long-term (more than three years) experiment at relatively low temperature (423 K) and a set of short-term experiments at higher temperatures. The activation energy (E_a) values of degradation were determined by both the MacCallum and Wilkinson literature methods, and were compared with those obtained through a new very simple method we set up, based on the direct regression of TG mass loss data. About two years ago we published the results concerning PE and PS because their mass losses during long-term experiments were sufficiently high. The long-term degradation experiments were continued until now and in this second part we report the results concerning PC and PMMA. The degradation E_a values calculated from short-term experimental data through the three different methods were in good agreement with each other for both PC and PMMA, thus confirming the general applicability of our simple method for the determination of E_a. The experimental data at lower temperature of PC were not in agreement with those at higher temperatures, thus confirming the low reliability of the kinetic parameters (and then of lifetime predictions) at low temperature determined by experiments at higher temperatures. Partially disagreeing results were obtained for PMMA, which were discussed and interpreted.