Air Entrainment in Coatings

A series of experiments that clarify how air bubbles become entrained into coatings are described. The contact line dynamics at the air-liquid interface surrounding a fiber is characterized for typical coating dies operating at atmospheric and under pressurized conditions. Glycerin and other viscous liquids are used to reveal that a critical fiber speed exists at which air entrainment begins. The observations confirm that the critical capillary number Ca sub c depends on the physical properties of the coating material. When the liquid supply is pressurized, the experiments demonstrate that adjusting the pressure can stabilize the free surface interface at a prescribed location. Controlling the meniscus in this way eliminates air entrainment. The threshold occurs when the applied pressure balances the shear exerted on the coating by the moving fiber.