Blisters in insulated metal panels, and industrial coatings in general, can have various causes. The formation of blisters often relates to the presence of contaminants or conditions during the application process of the coating. Here are some of the common causes:
Contamination of the Steel Substrate: Water-soluble salts like chlorides, sulfates, and nitrates on the steel surface can lead to osmotic blistering. These contaminants are typically invisible and require specific analytical methods to detect their presence in the liquid within blisters.
Trapped Water-Soluble Solvents: Solvents that remain trapped within the applied coating can cause blisters. The presence of these solvents is sometimes noticeable due to their odor and requires laboratory methods such as gas chromatography/mass spectroscopy for confirmation.
Thermal Gradients: Temperature differences across the coated surface, known as the “cold wall effect,” can lead to blistering when warmer water molecules penetrate the coating film and condense at a cooler interface, accumulating enough pressure to form liquid-filled blisters.
Improper Application Conditions: Coating applied in direct sunlight or thicker than recommended can bubble due to rapid surface drying. This prevents solvent from escaping from the lower levels, causing vapor pressure and bubble formation. Similarly, cooler temperatures or high relative humidity during application can slow the drying and curing, trapping solvents and later forming bubbles when conditions warm up.
Porous Substrates: Applying coatings on porous materials like concrete can trap air or moisture, which expands under heat, increasing pressure and causing bubbles.
Internal Bubbling: Bubbles can also form within the coating or on the backside, often in moisture-cured urethane coatings or aliphatic polyurethanes that react with moisture during application, leading to trapped CO2 gas and a Swiss cheese-like appearance when viewed microscopically.
Reaction Between Components: In two-part aliphatic urethanes, the reaction between polyol and isocyanate in the presence of moisture can cause foaming, with fine bubbles forming at the interface of the coating layers, which can disrupt adhesion.
Addressing these issues typically involves proper surface preparation, controlling environmental conditions during application, and using appropriate application techniques to ensure the integrity of the coating and to avoid blistering.