An evaluation of coil coating paints for self-stratification using Hansen solubility parameters

Abstract

Strip steel products are painted via a sequential, multistep application of various paint layers. This imparts properties, such as adhesion, corrosion protection and aesthetics, to the final finished product. However, various techniques, including slide coating, allow for simultaneous deposition of multiple paint layers. Here, we examine whether a similar system could be used for steel coil coating, without the requirement for expensive production-scale testing. Hansen solubility parameters (HSPs) were used to test the applicability of a typical, three-layer coil coating paint system for gradient self-stratification in wet-on-wet paint application techniques. As expected, the paint components are found to layer when placed on top of each other, remaining that way for some time. HSP analysis suggests, however, that the interlayer compatibility of some of the layers could be improved by solvent addition or substitution to produce a more graduated self-stratifying coating suitable for use in wet-on-wet techniques. Adhesion testing has subsequently indicated that cohesive failure occurs within the midcoat layer of the paint system, but adhesive failures are possible between the midcoat and topcoat layers. Using HSP-tailored paint systems in wet-on-wet paint application techniques, it may be possible to improve coating layer adhesion by “designing-in” gradient stratification during paint deposition.