Acceleration and Amplification of Physical Aging in Thermo-Oxidized Epoxy Networks

Abstract

The durability of civil engineering constructions is enhanced by protecting the concrete structures with epoxy coatings, which sustainability for decades of service is also conditioned by their aptitude to keep their dimensional stability under physical aging. This phenomenon, involving structural relaxation in the glassy state, is generally slow far from the glass transition, but has long-term consequences that may be detrimental for the macroscopic properties. Based on the premise that the structural relaxation rate is modified by the chemical modifications affecting the molecular mobility, this study investigates the effect of thermo-oxidative degradation on the physical aging kinetics. In a first step, the degradation by chain scission is suggested by results from thermal analyses, evidencing a decrease of the glass transition, an increase of the heat capacity step, and a lower thermal stability. Then, the kinetics of physical aging are compared between neat and thermo-oxidized epoxy networks. Using fast scanning calorimetry (FSC), the structural relaxation is followed for various aging temperatures, showing that physical aging proceeds faster while its intensity is amplified. Eventually, the relaxation functions fitted according to the Tool–Narayanaswamy–Moynihan (TNM) equation reveal lower activation energy in the oxidized networks in consistence with the chain scission scenario.