#8
Author: Edward Cohen, AIMCAL
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Subject: A potential source for the observed bubbles is poor wetting of the meyer rod.
A potential source for the observed bubbles is poor wetting of the meyer rod when the coating is added into the applicator. As the solution is added, it flows around the rod and does not completely wet it. This results in trapped air between the rod, which can either lead to streaks in the coating if the bubble is stagnant or bubbles, as parts of the entrapped air break of and go with the coating.
A way to avoid this is to wet the meyer rod before the coating is added and the base with the same solvent as used in the coating solution. This will insure that there is no entrapped air. Adjusting the viscosity and surface tension can also help. Base wetting can also be a factor, if the surface tension of the base is higher then the caoting solution. This will encourage non-wetting and air entrapment. Check the surface tension of base and coating. The base should be 3-5 dynes/cm2 lower then the coating solution.
Dissolved air in the coating solution can also lead to bubbles. To avoid this cause the coating solution can be deaerated by drawing a vacuum on it. The bubbles can also be allowed to rise in a narrow container. Air tends to accumulate at top surface. Therefore, the coating should be drawn from the bottom of the container, where bubbles are at a minimum.
It is also possible to reduce bubbles in the coating by slowing down the drying and allowing them to rise to the surface.
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