Solution properties that need to be measured are described in the following section.
Viscosity: Viscosity measures the resistance of a solution to flow under mechanical stress and is defined as the ratio of the shear stress to the shear rate. The viscosity is the slope of the shear stress vs. shear rate curve. There are several commercially available instruments to measure viscosity. Standard procedures to measure viscosity are available from ASTM.
The standard viscosity measurement is at low shear rates and is adequate for most needs. However, the coating hardware that processes the coating solution has higher shear rates. Therefore, the viscosity also should be measured at shear rate similar to those in the plant to ensure that they are stable during solution handling. Process control limits should be established for each product because operating outside these limits can result in coating defects such as ribbing, streaks, and mottle.
Solids concentration: The solution solids concentration is a key variable in determining the dry coating thickness and is a variable that influences viscosity. Variations in the solids level will cause variability in the dry coating weight, physical quality and drying performance because of the variability in the solvent level in the dryer.
A uniform solids content is needed to ensure constant viscosity, maintain coating weight, and ensure that the coating can be dried at optimum line speed. The dry coating weight is a function of the applied wet thickness and the solids content of that wet layer. Individual coatings can have the same thickness of wet layer, but dry the coating varies depending on the solid content. If the solids content varies, then the solvent level will vary, which can lead to variations in drying rate and line speeds. This is measured by a simple gravimetric, weight-loss measurement.