There are a plethora of factors to consider when applying aqueous and solvent based coatings to flexible substrates. Whether the final product is for a display, battery or photovoltaic component, imaging product, cast film or membrane – quality is paramount. Each step of the coating process must be conducted with the utmost detail and technical expertise, including the drying process. Effective drying procedures can have a substantial impact on product quality and functionality, which makes it critical to partner with an experienced contract coater.
The wet thickness, viscosity, solvent type and percent solids of coated layers are important factors when determining the best way to dry a product. One often must balance the desire for higher linespeeds with quality requirements. Hotter drying is not always faster. Dryers with zoned temperature control enable an experienced contract coater to both increase drying rate and improve final quality by tailoring the drying conditions to achieve desired properties in the coated layer.
Drying processes to drive off water or organic solvent can vary in nature from simple ovens that convey the coated material through a box of heated air to more sophisticated multi-zone air flotation systems like those employed by Carestream Contract Manufacturing. While hot boxes may be adequate to dry some materials, their poor heat transfer limits evaporation rate and drying speed. The drying ovens used in Carestream’s Oregon and Colorado facilities feature air flows that enhance heat transfer rates on the coated side and/or the backside of the web. These ovens also use air flotation in some areas to reduce roller contact and further improve ventilation.
To minimize defects related to airborne material, drying ovens must use a clean air supply. Without the use of a clean air source, contaminants can stick to the liquid surface to create point defects and other quality issues. The HEPA filtration in Carestream dryers eliminates dirt, dust and other contaminates thus enabling higher product quality.
The Carestream Difference: Zone Controlled Drying
Zone controlled drying optimizes the evaporation rate as the film moves through the dryer. The ability to vary temperature, pressure and air velocity at different times in the drying process also affects defect formation and final product performance. A typical drying process can be divided into four stages, including:
- Initial evaporation and solution concentration – constant rate drying
- Diffusion controlled drying – falling rate drying
In Stage 1, constant rate drying, there is plenty of solvent (water or other) at the surface of the coated layer. As a result, heat and mass transfer at the coating surface control the rate of solvent removal. In this stage, controlling the rate of solvent removal is critical since defects ranging from soft final films to coating layers with unique drying-related patterns can form if care is not taken. With some materials, a Stage 0 is employed to chill or heat set the coating prior to solvent removal. The goal in this stage is to use the unique rheological properties of certain coated materials to effectively solidify the coated layer prior to removing solvent. One is then able to impinge air more forcefully without defect formation once in the dryers to remove solvent more quickly.
In Stage 2, falling rate drying, solvent at the surface of the coated layer is depleted and the solvent diffusion rate through the solidifying material becomes the rate limiting factor. The rate of drying decreases in this stage as the coated layer continues to solidify. As the layer solidifies, higher temperatures and air velocities can be used to accelerate heat transfer and diffusion without risking airflow induced defects. Care must be taken not to overheat the coating in this stage. Depending on coated layer properties, high coating temperatures above the melting point of the coating’s solids-solvent mixture could result in non-desirable patterns and thickness non-uniformities.
Removing most of the original solvent is critical for some products like cast films and membranes, especially when organic solvents with lower odor thresholds are involved. In Stage 3, the conditioning stage, certain temperature and/or humidity is applied to tweak out as much solvent as possible and/or to modify certain coating properties. Even higher temperature or simply additional time is used to drive off the remaining few percent of solvent. In some cases, the conditioning stage is used to enhance adhesion, improve gloss or drive reactions that impart functionality to the coating.
Some dried layers are soft at the higher temperatures used in Stage 3, so it is important to use Stage 4, the cooling stage, to cool the web back to near room temperature in the last zone of the dryer. The cooler web prevents impressions in a warm soft coated layer during winding. If this is not done, other wound roll defects like ferrotyping, sticking or blocking may occur.
Examples where zone drying is important include:
- Inkjet receiver media: Zone drying in combination with robust formulation enables lower binder levels than would normally be employed to avoid layer cracking during drying.
- Free standing membranes: Membranes with unique microstructure are used in a variety of industries. A zone drying process allows for manipulation of solvent removal in ways that enable control of microstructure formation during the drying process. Hence, unique features are imparted to these membranes by simply drying and conditioning a given formulation differently.
Carestream Contract Manufacturing professionals employ zone controlled drying on a daily basis with the company’s aqueous and solvent coaters. Drying models are used to help predict coating layer temperature and solvent concentration throughout the dryers to ensure layers are dry and conditioned before ever running an experiment. At Carestream, the ability to predict and control drying in different stages saves development time, enhances productivity and dramatically improves final product quality.