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Blistering formation in highperformance industrial coatings is a common phenomenon. Understanding why blisters form is sometimes very simple and in other instances quite complex. For example, understanding and describing the process by which moisture migrates through a coating film at a molecular level to achieve equilibrium can be very complex, especially to the layman. On the other hand, understanding why blisters form as a result of coatings application on surfaces with elevated temperatures or on porous substrates (such as concrete) is much simpler.
Frequently in the coating industry, the terms "blistering" and "bubbling" are used synonymously. Technically, this is not correct. When blisters form by an osmotic mechanism, the term "blistering" is more common and correct. Flowever, when they are caused by nonosmotic methods, the term "blistering" is somewhat misleading and the term "bubbling" is more accurate. Blister formation is typically a result of increased pressure from moisture accumulation at certain points in a coating film, while bubbles are typically formed as a result of gas and vapor pressures within the coating film or the substrate. Despite this, these terms will likely continue to be used interchangeably.
This article addresses both osmotic and other non-osmotic methods by which blisters and bubbles form and identifies the roles that environmental conditions, substrate type, generic coating type and surface contamination can play in their formation. Because carbon steel and formed concrete are the most common substrates to which industrial coatings are applied, those substrates will also be the focus of this article.
Osmotic Blistering
Osmosis is the process by which moisture molecules are transferred through a semi-permeable membrane. In this case, the coating film is the semi-permeable membrane.
Osmotic blistering (Figs. 1 and 2) is probably the most recognized type of blistering that occurs in coatings applied to carbon steel that is subject to immersion service or prolonged exposure to high-moisture environments. The period of time that industrial coating systems are exposed to moisture in normal atmospheric service environments is typically not sufficient to produce osmotic blistering.
There are some well-known mechanisms or driving forces that fuel osmotic blister formation. In simple terms, the actions of these forces (discussed in greater detail later in this column) result in the accumulation or concentration of moisture at specific points within...