In addition blistering can occur within one coat. This is due to entrapped air or solvent (or both) and is not as frequently occurring as the types above.
Blistering can be caused by a number of different conditions:
· Soluble pigments in the primer have often caused blistering failures. Soluble pigments absorb moisture vapour passing through the coating, creating a rather concentrated solution. At that point, the phenomenon of osmosis occurs. Osmosis is the transfer of moisture through the coating in the direction of the most concentrated solution. When there are soluble materials on or within the coating, blistering is almost inevitable. The blisters formed will be liquid filled from the osmotic action. The same phenomenon can occur if there are soluble salts contaminating the substrate or contaminating the surface between coats.
· Blistering can also be caused by contamination of the surface by materials (e.g. oils, waxes, dust, etc.) that will not allow proper adhesion of the coating. The moisture vapour tends to concentrate in these areas of low adhesion. In this case, the blisters are so-called “dry” blisters.
· An improper shop-primer can also cause blistering. This can often be the case when a shop-primer is used on a surface, which is later over-coated with a high-performance coating. The primer does not have the necessary adhesion or physical properties to provide adequate adhesion for the high-performance coatings. Also, under some circumstances, the strong solvents in the high-performance materials can cause the shop-primer to disbond from the surface.
· Finally, blistering failure can cause poor or inadequate solvent release from the coating. Entrapped solvents (depending on the water sensitivity of the solvent) can increase the water absorption and moisture vapour transmission of the coating and lead to blistering. Solvent odour is usually connected with retained solvents, both within the coating and in the liquid (mixture of water and solvent) inside the blisters.
The intensity, quality and size of the blisters are evaluated according to ISO 4628/2.