Drying air-induced disturbances in multi-layer coating systems

Abstract

A range of new experimental techniques is developed to quantify drying-air induced disturbances on low viscosity single and multi-layer coating systems. Experiments on prototype slide-bead coating systems show that the surface disturbances take the form of a wavelike pattern and quantify precisely how its amplitude increases rapidly with wet thickness and decreases with viscosity. Heat transfer measurements show that the redistribution of water to form an additional lower viscosity carrier layer while increasing the solids concentration of the upper layer or layers enables the maximum drying rate, for which drying-air induced surface disturbances are acceptably small, to be increased with significant commercial benefits.

Heat Transfer and Thermal Air Management in the Electronics and Process Industries

“…Another important problem is particularly prevalent during the manufacture of high quality, multi-layer coatings, where drying air induced disturbances to the free surface of coated films can destroy product quality. A recent study into coated product robustness to drying-air induced disturbances has shown that an effective strategy to overcome this problem is to redistribute solvent from the upper layers to the lower layers so that the uppermost layers are more viscous and hence resistant to drying-air induced disturbances (Ikin & Thompson, 2007). For products where this redistribution of solvent is not possible, for example in the wide variety of single-layer coating systems, alternative hot air jet drying methods may be preferable.…”

“…This in turn leads to greater problems with surface disturbances when drying air emerges from an array of nozzles arranged perpendicular to the machine direction and disturbs the surface of the wet coating. A recent study into surface disturbances in multilayer coated products has shown how carefully redistributing solvent so as to increase the viscosity of the upper layers can significantly improve robustness to drying-air induced disturbances, leading to important commercial benefits in terms of reduced drying load and increased drying rates (Ikin & Thompson, 2007). Baking is also a complex process of simultaneous heat, water and water vapour transport within the product where the heat is supplied by a variety of indirect-fired and direct-fired forced convection ovens (Zareifard et al, 2006).…”

“…They are used, for example, to dry coated products, paper and textiles (Ikin & Thompson, 2007), or to bake a wide variety of food products (Norton & Sun, 2006), by directing hot air jets towards a target product in order to transfer energy into it. The effect of parameters such as jet velocity, jet diameter, nozzle to chip spacing, nozzle geometry, turbulence level and fluid properties on the effective heat transfer coefficients have been reviewed in detail by several authors, see for example Martin (1977), Webb & Ma (1995), Lienhard (1995) and Garimella (2000).…”