Selection of defoaming agents in water-based paints
In water-based paint, it is easy to foam. Because the high surface tension of water (70+) must be blended into the low surface tension of paint (around 30), low surface energy materials are essential.
Defoaming agents are a carefully selected class of materials that generally:
①. The surface tension of the defoaming agent should be smaller than the surface tension of the system to be defoamed;
②. The solubility in the system to be defoamed should be as small as possible;
③. Must have good penetration and spreading capabilities;
④. It is stable to the system and will not react with other components in the system;
⑤. Try to have no or very few other negative effects, such as low odor, low toxicity, and no significant impact on the production, storage, and construction performance of the paint.
A good defoamer should not have good water solubility, because surfactants such as defoamer must be distributed at the gas-liquid interface to reduce the interfacial tension, and surfactants that are easily soluble in water reduce the The aggregation concentration at the gas-liquid interface is not conducive to defoaming or may even cause foaming instead of defoaming. It can be seen that the solubility or miscibility with water determines the defoaming ability of a surfactant.
Although low molecular weight surfactants that are too miscible cannot defoam and may even foam and stabilize foam, completely immiscible surfactants can cause paint film defects such as shrinkage cavities. A truly good defoaming agent should be at a certain balance point between good miscibility and immiscibility in the water system. Excessive solubility and insolubility will not produce ideal defoaming effects. This is also the reason for choosing defoaming agents. The main reason why foaming agent is difficult.
The structure of the defoamer determines its surface activity and solubility in aqueous systems. Taking silicone defoamer as an example, it can be changed by changing the number of dimethylsiloxane in the molecule and the number of polyether hydrophilic groups. Type and length to adjust its miscibility and defoaming ability.
Due to the complexity and variability of water-based paint formulations, there is no universal defoamer suitable for all water-based paints. The type and amount of defoamer selected will vary depending on the emulsion and formulation components used. Water-based paints should try various defoaming agents for each system to find the most effective ones, including the defoaming effect during the production process, the defoaming effect during discharging and the storage stability effect.
The long-term storage stability effect of defoaming agents is particularly important. It can often be observed that a defoaming agent has an excellent defoaming effect when the paint liquid is just discharged, but it loses or significantly reduces the defoaming effect after three to five months of storage. Defoaming agent with unstable defoaming effect obviously cannot be used in formulations.
In addition, the defoaming effect is often reduced under high shear forces, that is, defoaming is good when stationary, but poor defoaming under construction conditions. These are all things you should pay attention to when choosing defoaming agents.
