Epoxy Coating Failure : Non-Osmotic Blistering

Non-Osmotic Bubbling

In our previous article, we took a close look at osmotic blistering and the toll it takes on coated surfaces. In this article, we’ll examine the other common mechanism of coating failure: non-osmotic bubbling.

Beyond Moisture Damage

As we previously explained, osmotic blistering arises from situations in which coated surfaces are immersed in water or subject to continual high humidity. Water, sometimes driven by temperature gradients, gets under the coating surface, vaporizes and causes the damage we call osmotic blistering. On the other hand, coating damage from causes other than osmotic blistering we call bubbling, though the results of the two are similar.

Environmental Conditions

In many locations, industrial coatings can be applied only during certain seasons when the climate isn’t too extreme. We call these “blue sky” days, and they usually occur in late spring, summer and early fall. However, certain conditions can have a negative impact on coatings. For example, bubbling can result from applying coatings in direct sunlight, as the hot sun dries the coating surface faster than the coating’s inner body. A non-permeable skin develops that trap solvents below. As the sun raises the temperature of these trapped solvents, vapor pressure builds to the point where bubbles form.

Applying a coating in cooler temperatures or high humidity can also take a toll by slowing down drying and curing. Under these circumstances, the coating doesn’t properly release its solvents. If you then apply another coating layer, the first layer’s solvents become trapped, eventually leading to bubbling when the temperature warms up and volatizes the solvents.

These defects are exacerbated by applying coating layers too thickly.

Porous Substrates

Formed concrete and concrete block have porous surfaces that can lead to bubbling by trapping air or moisture beneath the film. The crevices, cracks and joints also enable water penetration. If you apply a non-breathable coating, it will seal in the moisture that will cause bubbling when higher temperatures or pressures occur.

Coating Over Moisture

Moisture-cured urethane (MCU) and aliphatic polyurethane (AP) can form bubbles within the coating or on its backside. When applying MCU in very moist conditions, curing occurs too quickly and creates CO2 outgassing. The CO2 becomes trapped in the film, where the vapor pressure builds and eventually produces bubbles. AP cures through polymerization of isocyanate and polyol. However, isocyanate can react with moisture and release CO2 that gets trapped on the lower layers or backside of the film, and on the borders of multiple layers of coating. Often, the bubbles formed this way are tiny and lead to a phenomenon called foaming. When multiple layers are applied, bubbling can reduce adhesion and cause peeling.

Newer formulations, such as thick-film coatings with 100 percent solids, form thick, flexible films that are better able to withstand bubbling and blistering without failing. You might not have to repair these until the end of their normal service life, especially if the bubbles remain unbroken.