Thermal stability of optical fiber coatings: comparison of experimental thermogravimetric approaches

Thermal decomposition of poly(methyl methacrylate) (PMMA) and four polymer coatings on optical fibers (single acrylate, dual acrylate, polyimide and a hard silicone) was studied via thermogravimetry (TG). Of main interest was the upper use temperature of the fibers, which can be estimated for a given continuous use time, assuming a certain failure criterion. Arrhenius parameters of the decomposition reaction (the activation energy and the pre-exponential factor) were determined via several model-free approaches: isothermal, Modulated TG (MTG) and four non-isothermal isoconversional approaches. The isothermal approach was considered as a benchmark for the other approaches. The MTG results disagreed markedly with the isothermal ones, and the discrepancy appears unrelated to the choice of the decomposition kinetics model. The non-isothermal isoconversion methods provide closer agreement with the isothermal data. Still, in a few examples, substantial discrepancies between the isothermal and non-isothermal data were observed. It is concluded that the isothermal approach is the best one to use for model-free determination of the Arrhenius parameters from TG. The temperatures for observation of the isothermal kinetics should be selected such that the preheating time is short in comparison with the decomposition time of the material under investigation.