Solution properties that need to be measured are described in the following section.
Surface tension: Surface tension is a property of the coating solution in which the exposed surface tends to contract to the smallest possible area because of unequal molecular cohesive forces near the surface. It is important because it controls the wettability of a coating solution on a substrate. To obtain a coating that uniformly wets the entire substrate, the surface tension should be at least 5 dynes/cm lower than the surface tension of the substrate. In addition, surface tension is important for the leveling of the wet coating after application and in the formation of defects such as cells, craters, dryer bands, fat edges, adhesive failure, and delaminating
Surface tension units is force per unit length, such as dynes/cm, and it can be measured by several techniques. Two of these are the Wilhelmy plate method, in which the force necessary to withdraw a thin platinum plate from the solution is measured, and the duNuoy ring method, in which a ring made out of platinum wire is substituted for the Wilhelmy plate and bubble techniques.
Aging stability: A batch of coating solution may not be completely consumed during a coating campaign, or there may be a delay in coating after it is prepared. Therefore, a coating solution has to be stable and not change properties or solution uniformity. This can be measured by accelerated-aging oven testing or by retaining samples in the laboratory and visually observing solution or dispersion changes as the material ages.
Temperature: Solution viscosity is a function of the solution temperature. Therefore, viscosity measurements should be taken at a constant temperature and plant-coating temperatures measured and controlled in plant coatings.
Dispersion uniformity: A coating dispersion contains a solvent and non-soluble particles are separated from each other and uniformly distributed. During aging and under plant processing conditions, they can agglomerate, and the final coated product may not perform as required. Uniformity can be measured by light-scattering techniques and examining coated film with digital image analysis.
Coating solution bubbles: Bubbles in the solution that is being coated may result in small pockets of gas trapped in the final coated film and can prevent the coating from wetting the substrate. The bubble level can be measured in the coating flow lines using ultrasonic detectors and corrective action taken when they are found.
Test methods: There are commercially available laboratory instruments to measure all of these solution properties. In addition, ASTM has issued many standard procedures that can be used to develop local operating procedures. In addition, there are on-line instruments that can measure viscosity, temperature, and bubble content during the coating process.