Types of Blisters
Blisters fall into two main categories.
1) Osmotic Blisters
Osmotic Blisters are, put simply, caused by an accumulation or concentration of moisture (for instance) at various points within a coated film, usually through:
- Solvent or water entrapment when over coating too soon.
- Permeation of the lining system by moisture.
- Surface contamination between coats.
- Surface contamination due to water soluble salts.
- Thermal effects due to expansion and contraction during service.
When soluble salts or solvents become trapped, moisture will attempt to penetrate the coated film in an attempt to equalise the concentration of salt or solvent on either side of the film.
Where significant thermal gradients (thermal differences) are present, moisture on the warmer side will attempt to condense on the cooler side of the coated film.
When the osmotic pressure overcomes the paint’s own adhesive force, a blister will form.
2) Non-Osmotic Blistering
Non-Osmotic Blisters are usually associated with the substrate or environmental conditions during the painting operation itself. Some common types are due to:
- Excessive application of coating resulting in over thickness.
- Application of coating over porous surfaces resulting in air pockets.
- Application of coating during fluctuating high and low temperatures.
- Application of moisture cured urethanes during high humidity or wet surfaces.
For example, if a substrate is painted in a lower temperature environment and then later moved to a warmer environment, any trapped solvents, moisture or air will attempt to expand, causing a blister.
Some coatings such as moisture cured urethanes react with humidity / moisture and produce gas – again causing blisters.
Blisters are a sign that the coating is failing. Once this protection has gone, the plant or equipment being afforded the protection is vulnerable to damage, which could result in costly repairs or even shutdown.
When coating, always ensure the substrate and environmental conditions are free from contamination, excessive humidity and moisture. If the paint data sheet specifies a maximum surface salt level, ensure the cleaned substrate is checked by using an adequate test method (ISO 8502-6 Bresle Method for instance).
To prevent high thermal gradients, use insulation or apply a paint system with a higher tolerance to thermal gradients.
To avoid these problems, work with a professional blast cleaning / surface preparation and coatings contractor like KUE Group. Please contact us for additional information by emailing [email protected].