Preventing distortion is often a significant concern when coating a thin film substrate with a solvent, particularly an aqueous solution. The risk of defects caused by drying water-based coatings is high for thick layers and/or higher-viscosity solutions. Careful consideration of the coating set-up parameters and solution properties of the coating itself is critical to prevent defects. Target dry thickness, wet lay-down, line speed, coating width and drying dynamics are a few of the elements that must be taken into account when designing and executing the coating process. For example, the thicker the coating, the longer the drying process, and therefore, the greater the likelihood of dryer defects.
Contract coaters need multiple drying alternative capabilities to satisfy any duration and temperature requirements, and ensure coatings are completely dry to specifications before film is finished and rolled. Contract coaters draw upon a strong body of knowledge about “temperature profile” in coating systems, where problems with escape of solvents – including water exiting the coating solution – are avoided.
Solution properties such as viscosity, density, surface tension, Newtonian vs. non-Newtonian behavior, etc., are also key to the slot die coating process. High-viscosity fluids will require a thicker shim, and in many cases a corresponding wider gap, and can be troublesome to coat. Conversely, a sheer thinning solution is often an advantage, because it will wet out the film surface more evenly and form a uniform layer across the substrate. Either way, an accurate understanding of the percent solids in the solution is needed to correctly assign oven dry time, temperature, and airflow for a defect-free, finished product.
Coaters put each of these factors into a coating model to determine potential issues, and then adjust set-up parameters, coating method or, if possible, solution composition to quickly arrive at an initial process plan. A high-viscosity solution, for example, might require dilution with additional solvent to coat and dry evenly, while a coating solution with a high surface-energy can be improved by adding a surfactant to enable proper wetting of the substrate. These are simple solutions, but many others are attainable based on experience.
Coating machinery presents another crucial factor. Anything in contact with a water-based solution runs a risk of oxidation or rust; so utilizing high-grade stainless steels, polymeric compounds, or other materials that do not corrode, is critical to maintaining equipment performance and reliability. Companies looking for a contract coater should evaluate their experience in process development and their manufacturing assets to ensure streamlined product development from prototyping to manufacturing.