Elongational flow behavior of automotive coatings and its relation to atomization and mottling

The elongational flow properties of waterborne and solvent borne automotive basecoats have been characterized using opposing jet and contraction flow techniques. The ratio of elongational to shear viscosity varies between less than 20 and more than 300. showing that the behavior of such fluids in complex flows (atomization, filtration, impingement of droplets on substrates) cannot be deduced from shear viscosity alone. A clear correlation is found between the elongational viscosity of the fresh material and its sprayability as well as the optical appearance of the final coating in the case of the waterborne formulations. The opposing jet technique resolves these differences much better than contraction how experiments. No such correlation is found for the solvent borne systems presumably since their rheological properties undergo a much stronger change during application. (C) 2001 Published by Elsevier Science B.V.