Effects of highly absorbing pigments on near infrared cured polyester/melamine coil coatings

In order to expand available colour range for an industrial coil coating line a range of 25 μm polyester melamine coatings were applied to galvanised steel substrate and rapidly cured using near infrared (NIR) radiative curing. The purpose was to improve understanding of this relatively new curing technology and identify any problems associated with differing absorption of a range of coloured coatings. It has been suggested that in order to increase efficiency of NIR cure, NIR absorbers should be added to the coating formulation. UV/Vis/NIR spectroscopy was used to deduce the parts that coating and substrate absorption play in topcoat cure and lab scale trials were run on coatings throughout the colour range with their heating profiles and surface finish being recorded and assessed. The results showed that in this particular application having a coating that absorbs too strongly in the NIR region can actually result in solvent boil defects due to cross linking and film formation occurring prior to solvent removal.