Principles of bubble formation and bursting
In water-based coating systems, the introduction and stabilization of hydrophobic substances such as emulsion molecules, pigments and fillers into the water-based system is achieved through surface-active substances. The emulsifier ensures the stability of the emulsion resin molecules in the aqueous phase, and the pigments and fillers can be mixed into the aqueous medium under the action of wetting agents and dispersants. All surface-active substances in water-based systems can generate and stabilize foam.
The foam-stabilizing effect of surface-active molecules is the main factor in system foaming. Other foaming factors such as formula components, production and construction methods, and types of substrates all contribute to the formation of foam and increase or decrease the efficiency of defoaming agents.
In pure liquids without surfactants, such as water, bubbles rise to the surface and then burst. The interfacial tension between air and liquid is too high and bubbles cannot exist stably. However, if the system contains surfactants, the bubbles can be stabilized as the hydrophobic end of the surfactant. These surfactant molecules have the characteristics of hydrophilic and hydrophobic end groups, and can form a layer around the bubbles, with the hydrophobic end facing the bubbles and the hydrophilic end facing the water. The resulting reduced interfacial tension between bubbles and liquid stabilizes the presence of bubbles. When the bubbles rise to the surface of the liquid, a stable double layer is formed including the surfactant layer on the bubbles and the liquid surfactant because there are also surface-active molecules between the air and liquid interfaces. This stable double layer consists of a surfactant monolayer at the air-liquid interface and a surfactant monolayer at the liquid-air interface.
Stable air bubbles in surfactant-containing water
According to the foam formation mechanism, the bubble cells will form a tight spherical ring. According to the water seepage process of drainage between bubbles, the water between the bubble interfaces will be displaced and concentrated in the gaps between the bubbles. Because of this drainage, the narrow spacing between bubbles promotes the formation of octahedral foam spheres. This is what is known as a foam aggregate consisting of tightly packed hexagonal foam.
Bubble deformation caused by hydrophobic effect
Two other effects also stabilize the foam. The Marangoni effect is an indication of an optimal energy efficiency state. For example, the lack of surfactant molecules on the surface of a liquid or bubble makes the surface tension of this part different from that of other parts. The system promotes surfactant migration to equalize different surface tensions (self-healing effect).
Approximately octahedral deformed spherical layer
The second effect is the electrostatic repulsion of plasma on two monolayers of different surface-active molecules. Due to the hydrophobic effect, the distance between the two layers is continuously narrowed, and finally the layer ruptures, resulting in bubble breakage. The electrostatic repulsion will prevent the continuous narrowing between the layers and regulate the monolayer of surfactant molecules to maintain a balanced distance.
In fact, in the production and use of paints and coatings, the extremely stable foam polymer phenomenon described above is rare. Air bubbles can cause production and application problems and surface defects in the mid-coat.
Narrowing of interlayer spacing and electrostatic repulsion
Through the study of foaming and defoaming mechanisms, modern defoaming technology provides an effective solution to overcome foam problems. Due to the large number of available coating raw materials and the different compositions of coating formulations (e.g. resins, emulsifiers, wetting agents, dispersants, solvents, thickeners) it has not been possible to accurately predict which type of defoamer will provide the best defoaming for the required system. agent. In addition, the influence of system composition on defoaming agents makes defoaming agents more unpredictable. Extensive laboratory work and defoaming application experience can effectively help find the appropriate defoaming agent for the system.
Foam formation during production
During the mixing and dispersing process, air is introduced into the coating system. The high-speed stirring during the grinding and dispersion process allows a large amount of air to be introduced and stably present in the system under the action of surfactants. At the same time as the grinding stage, the surface of the pigments and fillers is moistened by water and wetting agents, and the air is released. At this stage of production, it is very important to use an efficient defoamer that can turn small individual bubbles into large bubbles and expel them from the system.
What kind of defoaming agent is needed in production is determined based on the characteristics of the coating system and the production process. The high shear force and rapid stirring during the grinding process introduce a large amount of air, so it is necessary to choose a defoaming agent that is efficient, resistant to shear force and difficult to emulsify. Because it does not contain or contains a small amount of emulsifier, this type of defoaming agent is relatively difficult to emulsify in the system medium. Therefore, the particle size of its active ingredients can remain relatively stable under shearing force. The difference is that the particle size of the active ingredients of defoaming agents that are easily emulsified can easily become smaller under shearing force.
Systems with low viscosity or low pigment content cannot use defoaming agents that are difficult to emulsify. Such defoamers can cause delamination of the coating’s surface problematic system. Here, defoaming agents that are easily emulsifiable are usually chosen. The above is a very rough classification, and defoaming agents are often determined based on specific systems.
From an economic perspective, efficient defoaming agents are also important. The generation of foam during production will increase production time. Large amounts of air during the grinding process will reduce the efficiency of shear force transfer. The air acts like a bladder between the mixer and the paint. At this time, the production time to achieve the same grinding quality is much longer than without bubbles. Also, the required packaging volume is non-reproducible due to the air contained. During the later painting stages, the agitation rate decreases as temperature- and shear-sensitive emulsion resins are added. The defoaming agent added at this time is to remove the air in the system and prevent the bubbles from being stabilized. The emulsifier contained in the emulsion resin stabilizes the foam.
Foam formation during pumping and packaging
During pumping, packaging, and transportation, air is also introduced into the system due to the movement of the air/liquid media surface. In printing, curtain coating, dip coating and other construction methods, paint will also be pumped and transported. Not only the foam formed during the production process, but also the bubbles generated during later pumping and packaging need to be eliminated. Therefore, defoaming agents that have already played a role in the production process must also be able to effectively remove foam or prevent foam formation.
To meet quality management requirements, the density of each can of paint must be measured. Visual requirements are also important because consumers do not like to see foam on the paint surface. Therefore, the defoaming agent still works during storage.
Foam formation during construction
Construction technology and substrate type have an important impact on the tendency of foam formation during construction. Foam is more likely to form on porous water-permeable substrates such as walls, wallpaper, and untreated wood. Application tools such as brushes, rollers, and spray guns show just as important an impact as the underlying layer. They may promote or prevent foam formation.
When using the spraying process, the paint leaving the nozzle is fully saturated with air, so a highly efficient defoamer must be used. Such defoamer must be able to effectively eliminate microbubbles formed on the substrate.
