Interlayer molecular migration and reaction in an epoxy-polyurethane coating system: Implications for the system hardness

Curing of coatings under solvent evaporation-suppressed conditions, can lead to trapping of solvents and subsequent solvent migration into mid- or topcoats in a multilayer coating system. Such migrations may have adverse effects on the coating mechanical properties, hardness in particular. Using a two-layer epoxy-polyurethane (PU) coating system with a strong practical significance for the heavy industry, the aim of the present work was to quantify the migration-reaction phenomenon for selected alcohols (co-solvents), and map the underlying mechanisms. In the experimental series undertaken, the evaporation rate of solvents was measured gravimetrically, and the urethane group formation quantified by the use of Fourier-transform infrared spectroscopy (FTIR). Simultaneously, the transient hardness development of the epoxy-PU system was followed and the coating surface morphology studied with a three-dimensional profilometer. In addition, to estimate the required coating curing time for dry-to-handle conditions, high-pressure compression measurements were conducted. As expected, due to solvent build-up in a closed exposure chamber, the amount of residual solvents in the epoxy-PU coating system increased under the evaporation-suppressed conditions. In particular, the residual 1-butanol in the epoxy primer migrated to the PU topcoat, where it reacted with the isocyanate reactants. The reduced cross-linking density, as a consequence of this undesired reaction, and the simultaneous suppressed solvent evaporation, resulted in an insufficient hardness development of the PU topcoat, despite continuous evaporation of solvents and consumption of isocyanates were indeed observed. To elucidate the underlying mechanisms, a kinetic study on the reactivity of 1-butanol with the isocyanate crosslinker was also conducted, and it showed a two orders of magnitude higher reaction rate than for the polyol reactant in the PU system, which explains the negative effects of the 1-butanol migration. In comparison, 2-butanol was less reactive towards isocyanates, and could substitute 1-butanol, whereby, under the evaporation-suppressed conditions, the hardness development of the epoxy-PU coating system was significantly improved and the curing time required for high-pressure dry-to-handle conditions reduced from more than eight to four days. The results of this work provide insight and guidelines for how to optimize a formulation to obtain adequate curing of the epoxy-PU coating system under evaporation-suppressed conditions.