The web-coating process replaces the air at the surface of a substrate with one or more coated layers to give a new structure with different functional properties. As a result, it can manufacture many diverse coated products with a wide range of compositions, thickness and functional properties.
The coating solution properties are important and must be controlled because they can influence:
- The range of coated products that can be manufactured on a coating line.
- The ability of the coating applicator to uniformly and reproducibly coat defect-free coatings, with uniform thickness.
- The yield and operability of the coating machine during a production run.
- The distortion of the wet coating during drying in the drying process.
- The even distribution of coating solution on the substrate.
The coating solution properties also determine the appropriate coating applicator that should be used for a specific product. Table 1shows that each coating method has operating ranges for viscosity and wet thickness, which is controlled by percent solids. Each coating applicator has a specific solution requirement for it to operate successfully. Operation outside of these limits in manufacturing will lead to defective product.
Measure, control, optimize
During manufacturing, variation of the solution properties can result in a defective coating layer, which does meet product quality standards and results in reduced coating-line and operating yields. When this occurs, none of the subsequent process steps can correct these deficiencies, and so a defective product will result.
Therefore, it is essential to measure, control and optimize solution properties to meet all requirements, so that the manufacturing process can reproducibly turn out quality product in the volumes needed. These properties need to be measured during product development to optimize formulation and during the coating process to ensure reproducible quality and coating-line operation.