Lifetime prediction for polymer coatings via thermogravimetric analysis

Polymer coatings, when brought to elevated temperatures may experience thermal decomposition, leading to failure of their protective properties. The process of thermal decomposition can be followed by thermogravimetry (TG), which allows quantitative analysis. Applying the right theoretical model, the TG data can be extrapolated to a broader temperature range for evaluating the coating's lifetime. The paper provides a thorough analysis of the current-state experimental and theoretical approaches in this area. As an example, thermal decomposition in nitrogen, air, and oxygen of dual polymer coatings on two different optical fibers is studied via isothermal and non-isothermal TG. For one of the coatings, the isothermal mass loss behavior resembles an n-th order kinetics function. For the other coating, the TG curves exhibit a more complex behavior, suggesting presence of an antioxidant in the chemical composition. From the non-isothermal TG data, using isoconversional Flynn-Wall-Ozawa, Kissinger-Akahira-Sunose and advanced Vyazovkin, Farjas-Roura and Budrugeac approaches, the activation energies are determined, and the isothermal mass loss functions are simulated. For several fiber/gas combinations, a significant discrepancy is observed between the experimentally obtained isothermal TG curves and those simulated from the non-isothermal data. The noted disagreement is analyzed in a view of miscellaneous assumptions of the advanced simulation methods, including the basic isoconversion principle. It is concluded that the isoconversional approaches are not applicable to the studied complex systems, and that the isothermal TG method should be used for determining the coating lifetime at elevated temperatures.