Web Coating

Blisters are drying defects that are often encountered in the web-coating process. They can either be a small, dome-like protrusions or a small recessed in the dried coating film. They tend to occur all over the surface of the coating. While they appear as two different types, protrusions and depressions (see Figure 1 below), the drying mechanism that creates them is the same for both types.

They are caused by bubbles, water vapor in aqueous coating solutions or solvents, being trapped between the coating solution and the surface of the coating. This causes the surface to bulge, creating the blister. If the surface is relatively dry it will solidify the bulge. If the surface is still fluid then the surface will collapse causing the depressions.  

The vapor bubble results from the boiling of the coating solution. This boiling will occur when the drying film temperature is close to the boiling point of the solvent. High film temperatures at the end of the dryer will also cause the blisters. Rapid drying can also be a factor by retarding diffusion of the solvent to the surface.

Knowing the boiling point of the solvent and insuring controlling drying temperatures so they are well above the solvent boiling point can eliminate blisters. Reducing drying rate and lowering drying air dry bulb temperatures will also help.