Thermal stability of optical fiber coatings: comparison of experimental thermogravimetric approaches
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Thermal stability of optical fiber coatings: comparison of experimental thermogravimetric approaches Andrei A. Stolov1 · Debra A. Simoff1 Received: 6 February 2020 / Accepted: 31 July 2020 © Akadémiai Kiadó, Budapest, Hungary 2020
Abstract 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 nonisothermal 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. Keywords Thermogravimetric analysis · Optical fiber · Coating · Thermal stability · Longevity · Acrylate · Silicone · Polyimide
Introduction Most optical fibers use a silica-based waveguide for transmitting optical signals. The light is guided inside a doped silica core that has a higher refractive index than surrounding doped or pure silica cladding [1]. The silica waveguide is coated with a polymer material to protect it from potential fracture since silica glass is brittle by nature. In addition, some polymer coatings limit potential microbends of the fiber and in this way support the light transmission [2]. Certain specialty applications of optical fibers demand their continuous use at elevated temperatures, sometimes in aggressive environments [3–6]. Brought to a harsh condition, polymer coatings degrade chemically, and over time lose their protective functionalities. Thus, there is a need
* Andrei A. Stolov [email protected] 1
OFS, Avon, CT, USA
for evaluating lifetimes of optical fibers in different environments and at various temperatures. The concept of “lifetime” is dictated by specifics of the fiber application. The fiber fails if one of its critical features falls out of required specification. For instance, fiber attenuation may exceed the dynamic budget of an interrogator. In another example, the fiber mechanical strength may deteriorate, so that fracture becomes inevitable. Optical and mechanical degradation of
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