Rheology parameters controlling spray atomization and roll misting behavior of waterborne coatings

Formation of small coating droplets is a phenomenon common to spray and roll application of coatings. Generation of small droplets, referred to as atomization, is the mechanism of controlling the quality and thickness of the applied coating layer in spray technology. Ln roll coating application, small coating droplets are formed at the nip-exit region under certain conditions. Because of the high-shear rates at roll nips and spray nozzles, it is common to relate high-sheer viscosity with atomization and roll misting behavior of coatings. This paper will review the relative importance of shear and extensional flaws, and the response of coatings to such flows, in affecting misting and atomization behavior. Experimental efforts to quantify the effect of shear and extensional rheological parameters have clearly established a lack of correlation of high-shear viscosities with both spray atomization and roll misting behavior of coatings. However, extensional viscosity is shown to have a strong correlation. Finally, how rheology modifiers can be selected to circumvent misting problems in roll coating applications and atomization problems in spray applications is discussed. (C) 2000 Published by Elsevier Science S.A.