TESTING THE ADHESION OF PAINT FILMS TO METALS BY SWELLING IN N-METHYL PYRROLIDONE

A new test for estimating the adhesion of paints to metal substrates is presented. Small painted disks are immersed in N-methyl pyrrolidone (NMP) at 60-degrees-C and the time for the paint film to delaminate completely and intact from the substrate is recorded. This time, termed the NMP paint retention time or NMPRT, is shown to be sensitive to changes in the metal pretreatment, the type of paint, the cure conditions, and the presence of water at the interface. As the strong swelling of the paint in NMP induces interfacial shear stresses, the NMPRT value is believed to be a measure of the number or strength of the interfacial bonds. Two examples are discussed in some detail: (i) delamination of automotive epoxy-based electrocoat systems from phosphated cold-rolled steel (CRS) and electrogalvanized steel (EGS), and (ii) modification of the interface between stainless steels (types 301 and 409) and epoxy or polyester powder paint systems by means of organofunctional silanes. In both examples it is shown how the NMP method can be used to optimize paint performance on metals.