The coating solution must uniformly wet the entire substrate surface and form a continuous wet film with no visible discontinuities such as patches clear areas spots. In order to accomplish this, the surface tension1 of the coating solution must be at least 5 dynes/cm less then substrate that is to be coated so that the solution will uniformly wet the substrate surface. incomplete wetting of the surface will result in a variety of defects, craters, patches, spots, repellents and unstable edges.
The surface tension of the coating solution can be directly measured by a variety of commercial instruments:
• du Nouy Ring
• Wilhemly Plate
• Spinning drop
• Pendant drop
• Bubble Pressure
In addition a goniometer can be used to measure contact angle of solution on a substrate.
The surface tension of the substrate can be measured by using Dyne Pens, which contain fluid with known surface tension solution in them. They are available in individual pens with range of 32-56 dyne/cm in 2 dyne/cm increments. The surface tension is determined by drawing lines with pens of varying surface tension across the width of the substrate. If the ink from the pen is stable does not break up the dyne level of solution is close to that of the substrate. This method can help determine surface tension uniformity in the substrate. Calibrated surface tension solutions are also available to test surface.
Additional surface properties requirements are
• A clean surface with not contamination from dirt oil, etc.
• No visible surface defects in substrate or previously applied coatings
• Not affected by in ambient air coating room conditions
• Are stable
1. Definition: Surface tension is a physical property equal to the amount of force per unit area necessary to expand the surface of a