Assessment of a color measurement-based method for the characterization of polymer thermo-oxidation

Abstract

Epoxy resins are used in various applications where environmental factors can interact and degrade the material. Thermo-oxidation is one of the degradation processes that can lead to both mechanical and chemical changes. This work aims to present a technique for characterizing thermo-oxidative degradation based on color analysis. The UV–vis spectroscopy reveals the direct link between the chemical modification and the color variation. The color difference $\Delta E_{ab}^{*}$ between a virgin and an aged sample (in CIELAB color space) provides an excellent indicator of oxidation degree. Calibration correlations have been developed based on reference samples aged under known conditions of temperature and pressure, translating color differences into an oxidation equivalent duration, represented as an equivalent time $t^{*}$ or to directly estimate mechanical properties. The $t^{*}$ parameter is the time that the sample should be exposed to the reference conditions to undergo the same oxidation level (equivalent to the same color difference change and then, degradation). Nanoindentation measurements were performed to validate the color measurement method. Some limitations were identified, including the poor correlation under non-equivalent time-temperature-pressure conditions, poor relevance for assessing high oxidation levels, and the impact of light scattering in areas with strong color gradients. The spatial resolution of color measurement is ten times higher than nanoindentation. Furthermore, the color measurement is non-destructive, can be conducted in situ, and is suitable for monitoring the aging of industrial components.