In all coating processes there is an operating region in which a uniform, stable defect free coating can be made. Outside of that region the coating is not stable and defective coatings will be made will be produced. There are several variables, solution flow rate & wet thickness, rheology, substrate, the specific coating method that control the optimum operating range. The basic limitations to increased coating speed are:
• air entrainment in the coating which results in many bubbles in the coating),
• ribbing or rivulets severe, regular streaks that persist in the machine direction.
•incomplete wetting of the substrate surface
• unstable edges
There is also a minimum speed limitation. below which uniform coating cannot be attained. The minimum and maximum speeds for uniform coating generally vary with other process parameters (e.g. vacuum level in slot or slide coating) and frequently there are settings of these process parameters above and below which uniform coating cannot be attained.
Collectively these limitations define what is called the coating window. The coating window defines the optimum operating region for the particular coating process and usually depends on many parameters (e.g., coating thickness, rheology, geometry, etc.), and as a result the “ coating window” is actually a multidimensional region of complex shape.
A generalized coating window for the slot die process is attached. It shows the quality level that can be achieved at various line speed and vacuum levels for a specific solution and coating process variables. The area areas of defect free coating are shown along with the type of defects that can be encountered outside of this region. It also shows that at low speeds a coating cannot be made and as the speed increases the defect free operating region is increased. Thus, if there is drying capacity, it shows the maximum speed that can be used to coat defect free film.
In order to select a coating method for a new product or to insure a reproducible, high productivity, high quality product; it is essential that the coating window be determined and used to select the optimum operating conditions. Also, if there two possible coating techniques that can be used, comparing the coating windows will help define the best process to use.
The procedure for generating a coating window is to begin with the initial formulation, set up the coater to run in a nominally operable regime and then vary process setting to define the range of stable and defective operation as a function of coating speed. The formulation and coating weight can also be used for more extensive characterizations. The samples should be dried for the best quality evaluation. If this is not possible visual observation at he coating station is effective.
Computer coating simulations can also generate coating windows. This technique is useful if the desired range of variables can not be obtained in a coater or potential new coating methods.