Spray applications: Part V. Influence of high solid compositions on coatings sprayability

Abstract

The air-spray patterns of high solid coatings were examined using a Particle Image Velocimeter (PIV) to determine the affects of varying the composition (oligomeric acrylic, oligomeric ester, and melamine resin) and molecular weight of high solid resins in coating formulations on their sprayability. High molecular weight tails (e.g., in the acrylic) are known to increase the elasticity of the fluid, and their presence imparted poor spray behavior (i.e., less misting, presence of ligaments, narrower exit fanning, and higher velocities farther from the nozzle) to the high solid coating. The Sauter Mean Diameter (SMD) of the droplets increased with increasing viscosity, which is dependent on weight average molecular weight of the total components of the blends. High solids stored at 60°C resulted in poorer spray with both increasing storage time and increasing levels of catalyst stored for 2 h. The high solid coatings were Newtonian fluids with no elastic functions associated with shear deformations. The elastic behavior associated with the high molecular weight tails in the acrylic oligomers is related, through their similar variation in spray behavior with HEUR thickened latex coatings in Part IV of this series, to dynamic uniaxial extensional viscosities.