Reasons for the formation of paint bubbles
During the grinding stage of coatings, a large number of bubbles may be generated, so defoamers are often used as components of grinding slurries. Considering the activity of some surfactants at the air or water interface, bubbles are often generated before coating mixing and in the grinding chamber of dispersion equipment, and the bubbles are stabilized. The generated bubbles slow down the dispersion process and affect the moisture resistance of the coating. The use of silane defoamers may cause defects on the surface of the paint film.
During the production and use of coatings, bubbles can be introduced into the coating, and raw materials such as surfactants and dispersants can also generate bubbles. In the production process of the vast majority of coatings, it is easy for air to be drawn in and form bubbles. Therefore, in the production process of coatings, selecting appropriate mixing equipment and conditions can avoid the entrapment of air. At the same time, allowing the coating to air dry for a period of time as much as possible can also prevent air entrapment.
Bubbles may also be generated to some extent during the application process of coatings, which mainly depends on the method of coating application. For example, curtain coating can continuously draw air into the coating, and airless spraying can also easily draw air in. Spraying construction under relatively low humidity conditions or high temperature conditions can easily generate bubbles.
Bubble formation in latex paint
Latex paint is stabilized with surfactants, so bubbles are easily generated during the mixing process. In the production and storage process of latex paint and to ensure good construction performance, it is necessary to remove bubbles. The foam can be reduced by optimizing the parameters of the spray gun, or the viscosity and solid content of the coating can be changed by adjusting the formula of the coating to reduce the foam.
Foam is a relatively large volume of gas dispersed in a smaller volume of liquid. Gases have a certain degree of solubility in liquid media and are also affected by temperature. When the paint begins to dry, dissolved gases detach from the paint film in the form of bubbles, which exist in a spherical shape and require the lowest surface energy. Large bubbles rise faster than small bubbles and gather on the surface. They are often covered by a film of surfactants or other additives on the surface of the coating, where bubbles accumulate as densely as possible. In some coating systems, bubbles accumulate tightly on the surface and even remain on the surface after the paint film has cured. Thick paste coatings are like plasticized sols.
DefoamerThe impact
Defoamer can reduce or eliminate foam in paint. In the water-based paint system, it is almost impossible to completely remove all foam. Appropriate foam control agent can inhibit the formation of foam, but more importantly, it can make the dry film free of bubbles and defects of the paint film caused by bubbles in the dry film.
A good defoamer not only has excellent defoaming ability, but also can maintain defoaming for a long time under room temperature and heating conditions. A good defoamer needs to be insoluble with the foam system. If it has good compatibility with the system, it will help to produce foam. Defoamers must have excellent dispersibility throughout the entire system. Spreadability refers to the uniform and smooth diffusion of defoamer on the surface, which can cover foam and eliminate it. Its working mechanism is to reduce the surface tension of the liquid around foam, so that small foam can gather into large foam, and finally foam will burst.
How to choose defoamers?
For water-based systems, the vast majority of foam control agents contain carriers, hydrophobic active substances and additives that can improve their spreading, compatibility, stability and other properties.
There are generally three types of active agents: hydrophobic silica; Hydrophobic organosilicon; Hydrophobic and oleophilic organic substances. Many foam control agents are mixtures of the above active agents. In addition, other additives can include surfactants, cosolvents, and thickeners.
This type of defoamer can be a suspension of alcohols, oils, or composite organic silicone oil in fine silica particles. Defoamer must be able to spread rapidly on the surface of foam and penetrate rapidly to make foam burst rapidly. Organosilicon defoamers are usually of the polysiloxane type. For example, polydimethylsiloxane containing acrylate functional groups and polyether modified polydimethylsiloxane. In the future, the process of foam production in the system, the compatibility and concentration of the system, temperature and viscosity must be taken into account when selecting appropriate defoamers. Each of the above factors will have a direct impact on the selection of defoamers.
The evaluation of foam control agent mainly considers the following aspects: spreading rate; Compatibility with the system; Stability and cost-effectiveness of defoaming. However, the above factors are often contradictory in a formula, for example, the defoamer with the best compatibility with the system often has the worst defoaming stability; The ones with the worst compatibility often have the fastest deployment speed.
Many defoamers are colloidal suspensions of particles that can be used as seeds for foam aggregation.
Some foam control agents have an impact on the gloss of the paint film, but not all of them have an impact, so this must be taken into account when designing the formula. The color of latex paint is very important for latex paint, so the negative impact of defoamers on color must be considered.
