Curing-induced internal stress in epoxy coatings: Effects of epoxy binder, curing agent, filler, initial solvent concentration, curing temperature, and relative humidity

Curing-induced internal stresses in epoxy coatings are highly influenced by the type and concentration of product ingredients and the conditions applied. In this work, the effects on the curing-dependent modulus and the internal stress development of the epoxy/crosslinker chemistry, curing temperature, relative humidity, filler conditions, and initial solvent concentration, are studied. Analytical methods include the attenuated total reflection-Fourier transform infrared (ATR-FTIR) technique, dynamic mechanical thermal analysis (DMTA), a 3D optical profilometer, and beam deflection. An elevated curing temperature (35 or 45 degrees C) resulted in a smaller elastic modulus and, despite an accelerated curing reaction and a higher final reactant conversion, only a slight increase, around 0.2 MPa, in the average internal stress. An increased relative humidity (from 35 to 60 %), also resulted in a smaller elastic modulus and less volumetric shrinkage and internal stress. However, at 90 % relative humidity, the internal stress, due to an enhanced final reactant conversion, was higher than observed at 60 % RH. The presence of either BaSO4 or CaCO3 filler in the formulation reduced the final reactant conversion and volumetric shrinkage, but resulted in a higher elastic modulus and internal stress. When the solvent concentration increased from zero to 20 vol%, the final conversion, although extra volumetric shrinkage was introduced by the solvent evaporation, increased from 0.72 to 0.85, while the internal stress decreased from 1.25 to 0.6 MPa. Guidelines for how to optimize coating formulations and curing conditions, to avoid internal stress, are presented.