Detection and Quantification of Premature Crack Formation in Curing Epoxy Coatings

Cracks and defects, as a result of internal stress during curing, can accelerate the degradation and failure of protective epoxy coatings for essential infrastructures. To reduce, or eliminate, this so-called premature crack formation, it is essential to understand the underlying mechanisms. The present work investigated the simultaneous development in internal stresses, the degree of conversion of reactants (i.e., cross-linking), the volumetric shrinkage, and the elastic modulus of a solvent-based novolac epoxy cured with a cycloaliphatic amine. In addition, the crack susceptibility of the coating was quantified from a stress-strain curve (tensile mode), and the effects of film thickness, solvent content, pigment volume concentration, and flat versus 90-degree angle geometry were mapped. Finally, digital microscopy, in combination with a nondestructive scanning acoustic microscope (SAM) analysis, proved efficient for characterization of the crack morphology. Residual solvent, due to plasticization, promotes a low crack susceptibility. Later in the process, however, the solvent plasticization contributes to a high reactant conversion, and when the solvent finally evaporates, an increased internal stress is established. This, in turn, leads to an associated higher probability for crack initiation and growth. The crack susceptibility method provides insight and input to guidelines on how to modify formulations and curing conditions.

Automated summary

This study investigates the simultaneous evolution of internal stresses, conversion degree, shrinkage, and elastic modulus in a solvent-based novolac epoxy cured with a cycloaliphatic amine. The crack susceptibility of the coating was quantified, and the effects of film thickness, solvent content, pigment volume concentration, and geometry were examined. Digital microscopy and nondestructive scanning acoustic microscopy were efficient techniques for characterizing crack morphology.