INFLUENCE OF THE ELONGATIONAL FLOW RESISTANCE AND PIGMENTATION OF COATING FLUIDS ON HIGH-SPEED ROTARY BELL ATOMIZATION

Abstract

High-speed rotary bell atomization is the key coating technology in the automotive industry. We investigated the atomization behavior of shear-thinning fluids emphasizing the contributions of elon-gational flow resistance and the presence of anisotropic particles. Mixtures of two commercial acrylic thickener solutions allowed for a variation of elongational relaxation time λe by almost two orders of magnitude. Suspending different fractions of highly anisotropic, flake-shaped particles in such thickener solutions resulted in a fourfold increase of λe. In both series of model fluids, shear viscosity remained essentially unchanged. Light-scattering techniques were used to determine the droplet size. The length of the ligaments formed at the bell edge during an important intermediate step preceding droplet formation was obtained from high-speed videos in combination with a customized image analysis code. For the pure thickener solutions, an increase in the elongational relaxation time resulted in an increase in the ligament length but did not affect the droplet size, since drops were not only formed from primary ligaments but were also formed after further fragmentation downstream. Suspended glass flakes accelerated ligament disintegration despite the increasing λe but, again, did not affect droplet size. The flake-shaped particles appear to act as predetermined breaking points disturbing the flow inside the ligaments. This phenomenon was verified using an industrial automotive basecoat including different amounts of aluminum flakes. These new insights regarding the high-speed atomization of complex fluids may support the targeted formulation of coating fluids.