Blister formation is a common phenomenon in high performance industrial coatings. Understanding why blisters form is sometimes very simple and in other cases very complicated. For example, understanding and describing the process by which moisture migrates through a coating film at the molecular level to achieve equilibrium can be complex, especially for a layman. On the other hand, it is simpler to understand why blisters form when a coating is applied to a hot surface or a porous substrate such as concrete.
Often in the coatings industry, the terms "blister" and "blister" are used synonymously. Technically, this is incorrect. The term "blister" is more common and correct when blisters are formed by an osmotic mechanism. However, the term "blistering" is somewhat misleading when they are caused by non-permeable methods, the term "foaming" is more accurate. Blister formation is usually due to increased pressure due to moisture accumulation at certain points in the coating film, while gas bubbles are usually formed due to gas and vapor pressure within the coating film or substrate. Nonetheless, the terms may continue to be used interchangeably.
This article discusses infiltration and other non-infiltration methods by which blisters and gas bubbles are formed, and determines the role that environmental conditions, substrate type, general coating type, and surface contamination can play in their formation. Since carbon steel and formed concrete are common substrates to which industrial coatings are applied, these substrates will also be the focus of this article.
Osmotic Blistering
Osmosis is the process of the transfer of water molecules through a semipermeable membrane. In this case, the coated film is a semipermeable film.
Penetrant blistering (Figures 1 and 2) is probably a common type of blistering that occurs in coatings applied to carbon steel that are prone to immersion or prolonged exposure to high humidity. Industrial coating systems are generally not exposed to moisture long enough to produce penetrating blistering in normal atmospheric pressure service environments.
Figure 1: Dense Osmotic Blister
There are a few well-known mechanisms or drivers that drive the formation of osmotic blisters. In simple terms, the action of these forces (discussed in more detail later in this column) results in the accumulation or concentration of moisture at specific points within the coating film. These drivers are:
Contamination of steel substrates with water soluble salts;
Water-soluble solvents that are applied to the coating film; and
The thermal gradient (temperature difference) across the surface of the coating.
Figure 2: Blistering with spot penetrability
Osmotic Blistering Caused by Water-Soluble Salts and Entrained Solvents
Permeable blisters may form due to water soluble salt contamination on the coated steel surface or water soluble solvents retained or "trapped" in the applied coating. Because water-soluble salts such as chlorides, sulfates, and nitrates are generally invisible, detection of these contaminants in blister fluids requires specific analytical testing methods such as ion chromatography (IC) to confirm their presence . Although retained or trapped solvents can often be detected by the solvent odor of fluid removed from the blisters (vesicles are usually liquid-filled), solvent detection using laboratory analytical methods, such as gas chromatography/mass spectrometry (GCMS), can detect they exist. ).
Osmotic blisters arise due to the presence of hydrophilic soluble salts or retained solvents when there is sufficient moisture in contact with the coated film (ie, the coating in immersion service), and when there are differences between each side of the film. The concentration of dissolved salt or solvent. This relative difference on each side of the semi-permeable coating creates osmotic pressure and allows water molecules to slowly penetrate the molecular infrastructure of the coating. As moisture penetrates, it migrates and accumulates towards the more concentrated salt solution or where the solvent is present. Osmotic forces accelerate the transport of water through the coating in an attempt to equalize the pressure on both sides of the coating (equilibrium). Pressures can reach high levels (reportedly in excess of 15,000 psi) depending on the concentration of soluble contaminants on both sides of the permeation cell. When higher concentrations of soluble salt contaminants are present on either side of the film, more free moisture builds up and blisters can be larger and more concentrated. When these pressures exceed the adhesive bond of the coating to the substrate, blisters form.
Osmotic Blistering Caused by Thermal Gradients
This phenomenon is commonly referred to as the "cold wall effect" in the coatings industry. Thermal gradients occur when the metal or steel substrate in the submerged zone of a tank or vessel is cooler than the liquid contained in the tank. Bubbles form when the hotter water molecules in the stored liquid penetrate the coating film and then condense at the cooler interface within the liner or at the liner/substrate interface. Eventually, enough fluid builds up to create pressure that causes fluid-filled blisters to form in the coating. One way to minimize thermal gradients is to use external insulation on the tank, which otherwise could create a sufficient temperature difference.
Impermeable Blistering: Bubbles
While many blistering problems are often associated with coatings and high sustained moisture exposure in dip service, blisters can and do form through other mechanisms. These impermeable blisters, which we call foam, are often related to substrate properties or environmental conditions during coating application.
Coating applications at high-low temperatures
In many locations, the coating application season is limited to times when environmental conditions are favorable (ie, typically in late spring, summer, and early fall). Those "blue sky" days need to be productive. The environmental conditions given to us these days are usually favorable; however, they also have pitfalls. For example, applying a coat in direct sunlight or applying a thicker coat than recommended can cause bubbles to form. Bubbles usually form because the heat from the sun causes the surface of the applied coating to dry faster than the bulk of the film. This rapid surface drying process creates a rigid "skin" surface layer that prevents the lower levels of solvent in the film from escaping. As the underlying solvent is heated, it volatilizes and expands creating vapor pressure within the coating film. Vapor pressure is what causes bubbles to form.
In other cases, air bubbles can form in the coating due to lower temperatures or higher relative humidity during application. Drying and curing of the coating film is significantly slower when the application is performed at cooler temperatures or higher humidity. As a result, the solvent is not properly released from the membrane. Therefore, if the next coat is applied too soon, the solvent in the underlying film is trapped and bubble formation may occur. However, bubbles may not occur immediately, and their appearance may be delayed until ambient conditions become warm enough to volatilize the entrained solvent.
Coating Application on Porous Substrates
Coating applications on porous substrates such as shaped concrete and concrete blocks (CMU) can also lead to blistering. In this case, the inherent porosity of the concrete substrate often contains trapped air or moisture. In this regard, air is present as it will occupy any open space that is not under vacuum, and moisture enters from the outside or inside of the structure. External moisture typically enters through the natural porosity of the concrete substrate and along cracks, fissures, or control joints, and internal moisture can be generated by "steam drive" (i.e., moisture and condensed moisture) from within the structure. When applying an air impermeable coating over these porous substrates, air and moisture are often "sealed" within the substrate. Therefore, any conditions (i.e., solar energy that causes the air to warm while moisture evaporates causes expansion and pressure to build up within the concrete. The increased pressure on the backside of the coating often causes air bubbles to form.
Application of paint on moisture
Bubbles do not always appear on the coated surface. Bubbles sometimes form within the coating or on the backside of the film. For example, moisture-cure polyurethane (MCU) coatings that dry and cure by reaction with atmospheric moisture and other coating types formulated with moisture-sensitive components (i.e., aliphatic polyurethanes) can also foam.
Moisture Cured Polyurethane Bubbles
在潮湿条件下应用MCU涂层时，例如表面残留水分，相对湿度过高或冷凝水分或雨水接触未固化的MCU表面，涂层按设计运行，易与水分反应以实现固化。不幸的是，当水分充足时，反应迅速发生，并产生二氧化碳气体（CO 2）（通常称为“排气”）作为该反应的副产物。当过量的水分加速固化时，CO 2经常被捕获在涂膜中，并且由此产生的蒸气压增加会在涂层内产生气泡。事实上，当用显微镜观察涂层的横截面时，它通常具有瑞士奶酪般的外观。
图3：聚氨酯漆膜背面冒泡
脂肪族聚氨酯鼓泡
两部分脂族氨基甲酸酯通过两种组分（多元醇和异氰酸酯）的聚合而固化。但是，当涂在水分上时，会在聚氨酯薄膜的背面形成气泡（图3）。发生这种情况是因为氨基甲酸酯制剂的异氰酸酯组分与水分反应。与前面讨论的MCU反应一样，形成了CO2气体。气体通常被捕获在聚氨酯涂层膜的下层中，并且也存在于任何先前施加的涂层的界面处，其中存在水分。同样，由气体形成产生的压力导致气泡形成。与这种形成过程的一个不同之处在于气泡可以非常精细并且肉眼并不总是可见。当在显微镜下观察横截面时，细小气泡通常具有泡沫状外观。出于这样的原因，这种现象通常被称为“发泡”。在使用MCU和脂肪族氨基甲酸酯配方的气泡的情况下，层之间发生的除气会破坏粘附。通常，除非非常强烈和浓缩，否则其他形式的起泡对涂层附着几乎没有或没有不利影响。
结论
In conclusion, blistering and blistering are common coating problems that occur for several different reasons. Although this article describes and discusses the osmotic mechanisms that produce blisters, there are several causes of blisters. Opinions vary on the possible damage from blisters and air bubbles. One view is that if they remain uninterrupted, they may not be considered coating defects requiring repair. For example, some recently developed 100% solids, thick-film coating formulations such as elastomeric polyurethanes, urethane hybrids, and polyureas are commonly used to form thick, integral, and flexible protective films. While blistering and blistering can still occur in these films, the bulk and flexible nature of the films prevents them from cracking and the underlying substrate generally remains undamaged. Additionally, when used in dipping service, the actual weight and pressure of the liquid contained within the container can be used to hold the monolithic film in place on the protected substrate. In this case, whether there is any benefit in cutting and repairing the blister needs to be carefully considered. Even in the presence of gas bubbles or gas bubbles, the flexible and integral film usually has sufficient structural integrity of its own, which allows the coating to achieve its originally intended service life.
