Parallel measurements and engineering simulations of conversion, shear modulus, and internal stress during ambient curing of a two-component epoxy coating

Macroscopic crack initiation and propagation, as a result of internal stress, poses a threat to the performance of protective coatings. In demanding environments, such as corner geometries and saltwater exposure, the cracks may accelerate substrate corrosion and potentially lead to collapse of infrastructure. The present work is focused on the underlying mechanisms of curing-induced internal stress and investigates the dynamics of the parallel processes of crosslinking reactions and evolution of mechanical properties for a solvent and pigment-free epoxy resin cured with a diamine hardener. Experimental techniques, applied at room temperature and constant relative humidity, include attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, advanced rheometry, dynamic mechanical thermal analysis (DMTA), and a beam deflection device. When taking post-vitrification mobility control into account, the reaction kinetics behind the crosslinking and network development could be simulated with a modified Kamal–Sourour model. In addition, engineering models for simulation of coating modulus, volumetric shrinkage, and internal stress as a function of conversion were proposed and found to be in good agreement with experimental data. This allowed, by comparing the magnitudes of the modulus and the internal stress, for evaluation of whether premature cracks are expected to initiate. Furthermore, we show that the curing-induced internal stress development is strongly influenced by the current coating elastic modulus and film thickness, highlighting the effect of coating property transients. The experimental techniques and engineering modeling tools collected can be used to evaluate, without demanding computational complexities, the simultaneous development of coating modulus and curing-induced internal stress.