Osmotic Blistering

Osmotic Blistering
Osmosis is the process by which water molecules migrate through a semi-permeable membrane; in a coating system, the paint film acts as this semi-permeable membrane. Osmotic blistering  is the most common type of blistering defect in coatings applied to carbon steel substrates, typically occurring in service conditions involving prolonged water immersion or high-humidity environments. Under conventional atmospheric conditions, the duration of moisture exposure is usually insufficient to trigger osmotic blistering in industrial coating systems.
Several known mechanisms or driving forces can induce osmotic blistering in coatings. Simply put, these forces (discussed in detail later in this article) cause moisture to accumulate and concentrate at specific locations within the paint film. Key driving forces include: the presence of water-soluble salt contaminants on the steel substrate surface; residual water-soluble solvents within the paint film after application; and temperature gradients (temperature differences) across the painted surface.
Osmotic Blistering Caused by Water-Soluble Salts and Residual Solvents
Water-soluble salt contaminants on the steel substrate or residual water-soluble solvents trapped within the paint film after application can both trigger osmotic blistering. Water-soluble salts—such as chlorides, sulfates, and nitrates—are typically invisible to the naked eye; specialized analytical techniques, such as ion chromatography, are required to detect and confirm their presence in the fluid accumulated within the blisters. Residual solvents can often be preliminarily identified by the characteristic odor of the fluid inside the blisters (which are usually filled with liquid), though precise verification ultimately requires laboratory methods such as gas chromatography-mass spectrometry (GC-MS). When a coating is exposed to abundant moisture for an extended period (e.g., during immersion) and a concentration gradient of soluble salts or solvents exists across the paint film, hydrophilic soluble salts or residual solvents can induce osmotic blistering. Once a concentration gradient forms across the paint film—which functions as a semi-permeable membrane—osmotic pressure is generated, driving water molecules to slowly permeate the film's molecular structure. Upon permeation, the moisture migrates toward and accumulates in areas with higher concentrations of salt solution or solvent. Osmotic forces continuously drive moisture through the paint film in an attempt to balance the pressure on both sides and achieve osmotic equilibrium. Depending on the concentration gradient of soluble contaminants across the osmotic barrier, internal pressure can reach extremely high levels (reportedly exceeding 15,000 psi). The greater the concentration difference of soluble salt contaminants across the film, the more free moisture accumulates, resulting in larger and more densely distributed blisters. Blistering defects occur when this internal pressure exceeds the adhesion strength between the coating and the substrate.
Osmotic Blistering Induced by Temperature Gradients
This phenomenon is widely known in the coatings industry as the "cold wall effect." A temperature gradient arises when the temperature of the metal or steel substrate—in submerged areas of equipment such as storage tanks or vessels—is lower than that of the stored liquid medium. Warmer water molecules from the stored liquid penetrate the coating film and condense at the cooler interface within the lining or at the interface between the lining and the substrate; the resulting accumulation of liquid generates internal pressure, causing the formation of liquid-filled blisters. To mitigate this issue in conditions involving temperature differentials, thermal insulation can be applied to the tank's exterior wall to minimize the temperature gradient.
Non-Osmotic Blistering: Bubbling
While most blistering defects occur in coating systems exposed to immersion or long-term high-humidity environments, blistering can also be triggered by other mechanisms. This type of non-osmotic blistering (referred to here as "bubbling") is often closely linked to the inherent properties of the substrate or the environmental conditions during application.
Bubbling Caused by Application in High or Low Temperatures
In most regions, coating application is restricted to periods with favorable environmental conditions—typically late spring, summer, and early autumn—creating a limited application window that necessitates efficient work during fair weather. Although suitable application conditions significantly reduce the risk of defects, potential issues remain. For instance, applying coatings to substrates exposed to direct sunlight or applying the coating at a thickness exceeding specifications can lead to bubbling. Heat from direct sunlight causes the surface layer of the coating to cure or dry much faster than the interior, creating a hard, dry surface film that seals off internal pathways and prevents solvents in the underlying layer from evaporating normally. Residual solvents inside the coating vaporize and expand when heated, generating internal vapor pressure that ultimately leads to blistering. Blistering can also occur when application temperatures are too low or relative humidity is too high. Low temperatures and high humidity significantly slow down the drying and curing process, hindering solvent evaporation. If a subsequent coat is applied too soon under these conditions, solvents in the base layer become trapped, leading to blisters. These blisters often do not appear immediately; instead, they emerge later as the ambient temperature rises and the trapped solvents vaporize.
Blistering caused by application on porous substrates
Applying coatings to porous substrates—such as cast-in-place concrete or hollow concrete blocks—is also prone to causing blistering. These concrete substrates have an inherently porous structure that typically traps air or moisture. In non-vacuum environments, air naturally occupies the void spaces within the pores, while moisture can infiltrate the substrate from either the exterior or the interior of the structure. External moisture generally enters through natural pores, cracks, fissures, or expansion joints, whereas internal moisture arises from vapor permeation (such as ambient humidity and condensation). If an airtight, impermeable coating is applied to such porous substrates, the internal air and moisture become completely sealed in. When the surface is subsequently exposed to heat—such as from sunlight—the trapped air expands and the moisture vaporizes, causing a sharp rise in internal pressure. This pressure exerts continuous force on the underside of the coating, eventually resulting in blisters.
Blistering caused by moisture in the substrate
Blisters do not always appear on the coating surface; they can also form within the coating film or at the interface between the coating and the substrate. For instance, coating systems such as moisture-cured polyurethane (MCU)—which relies on a reaction with atmospheric moisture to cure—and aliphatic polyurethanes containing water-sensitive components are prone to blistering when applied under wet conditions.
Blistering in Moisture-Cured Polyurethane Coatings
When moisture-cured polyurethane coatings are applied under humid conditions (e.g., residual moisture on the substrate surface, excessive ambient humidity, or contact of the uncured film with condensation or rain), the coating reacts with moisture and cures according to its design mechanism. However, in the presence of excess moisture, the curing reaction rate accelerates sharply, generating carbon dioxide gas—a phenomenon commonly known as "outgassing." While excess moisture accelerates curing, the generated carbon dioxide becomes trapped within the paint film, raising the internal vapor pressure and ultimately causing blisters to form inside the coating. Microscopic examination of the coating cross-section reveals a honeycomb-like porous structure resembling Swiss cheese. Figure 3: Blistering on the underside of a polyurethane paint film.
Blistering in Aliphatic Polyurethane Coatings
Two-component aliphatic polyurethanes cure through the polymerization reaction between polyol and isocyanate components. However, when applied to a damp substrate surface, blistering occurs on the underside of the polyurethane film (see Figure 3). The cause is a reaction between the isocyanate component in the coating and moisture—following the same reaction principle as the moisture-cured polyurethane mentioned earlier—which generates carbon dioxide gas. This gas becomes trapped at the base of the polyurethane film and at the interface where moisture is present; the accumulation of gas creates pressure, eventually leading to blister formation. These blisters are characterized by extremely fine bubbles that are usually invisible to the naked eye. Under microscopic examination, the fine bubbles appear as a foam-like structure; consequently, this defect is often referred to as "foaming." Whether in moisture-cured or aliphatic polyurethanes, the outgassing resulting from these reactions compromises the inter-coat adhesion of the coating system. As for other types of blistering, unless the blisters are densely packed or severe, they generally do not significantly compromise the coating's adhesion.
Conclusion
In summary, blistering and bubbling are extremely common defects in industrial coatings, arising from a wide variety of causes. This article has detailed the osmotic mechanism behind coating blistering and outlined the causes of various non-osmotic blistering phenomena. Currently, there are differing views within the industry regarding the impact of blistering. Some argue that if the blisters remain intact and unbroken, repair is unnecessary, and they need not be classified as functional coating defects. For instance, modern 100% solids, high-build coating systems—such as elastomeric polyurethanes, polyurethane hybrids, and polyureas—form dense, monolithic, and highly flexible protective films upon application. Even if blistering occurs, the monolithic nature and flexibility of these coatings prevent cracking or rupture, ensuring the underlying substrate remains undamaged. Furthermore, under submerged conditions, the weight of the contained medium and the external water pressure can compress the monolithic coating, ensuring a tight fit against the substrate. In such scenarios, the necessity of removing and repairing blisters requires careful assessment based on actual operating conditions. Even with the presence of blisters, these monolithic, flexible, high-build coatings retain sufficient structural integrity to meet design service life requirements.