Choosing a specific precision coating method depends on a variety of factors, including the desired final product characteristics, anticipated processing steps, and the nature of the material being coated. There is a highly interdependent relationship between coating techniques and fluids, making consideration of both at the same time very important.
“Fluid design” is influenced by the desired optimal final coating thickness, the number of layers, the expected curing and handling of the material post coating, the degree of accuracy required, and many other physical, chemical and optical properties. The list of interrelated considerations in the design of fluid rheology includes viscosity, dispersability and stability of solids, Newtonian/non-Newtonian/ thixotropic/ shear thinning characteristics, surface tension of fluid relative to base and adjoining layers, adhesion requirements, ability to wet a surface, and many others.
The creation of an optical quality antireflective film with a precise “quarter wavelength” thick coating is an example where fluid and coating choices are interrelated. To avoid optical distortion, coating requires extreme thickness control both across and down the web. Two commonly used methods that can deliver this level of quality are high-precision gravure and slot-die coating. Both work well with very low solids concentration (typically a polymer) at low viscosity (due to the solvent), allowing self-leveling and uniform shrinking in the “z” direction during drying. If the rest of the system is optimized with chatter-free drives, high-efficiency filtration, tension control and zoned-temperature drying, virtually defect-free coatings can be achieved.
Finding an experienced high-precision, quality coating partner with diverse past process expertise saves a great amount of time in product scale up. Evaluating alternative combinations of both fluid design and coating methods at the early stages of product definition will allow optimal selection for successful manufacturing at large scale with good economics and high quality.