Self-stratifying coatings: a review

A self-stratifying coating is an economical coating containing multiresinous components with different functional groups, which spontaneously stratify after application to the substrate. These coating systems could be composed of two or more different layers to protect substrates against corrosion, by first layer, and simultaneously to create a desirable appearance, by second layer, with decorative properties. Conventional multilayer coating systems encounter some problems such as poor interfacial adhesion, application, and labor costs and also lengthy processing time. The concentration gradient of two layers would eliminate the inter-coating boundary which can be the point of failure in conventional coatings. In this paper, the surface tension and solubility theory regarding these coatings are discussed. In addition, the effects of different factors on the pigment location into the coating systems are studied. The main factors, including curing mechanisms, substrate effects, thickness, viscosity, kinetics of reaction, evaporation rate of solvents, dispersing agents, and surface properties of pigments have been reviewed. The models for prediction of self-stratifying coatings such as UNIFAC and computer simulation have also been addressed and taken into consideration. The prospect of these coatings and their application in different industries is presented.