Bubble control is necessary in many industrial areas, such as paper, water treatment, pharmaceuticals, dyes and coatings. With the continuous upgrading of national environmental regulations and the continuous improvement of residents' environmental awareness, water-based coatings have made great progress.




The emulsifying agent and wetting dispersant present in the formulation of water-based coatings reduce the surface tension of the system and easily stabilize the bubbles in the system. The existence of bubbles will have an adverse effect on the production and coating of coatings.


In the grinding process, the "air bag" formed by bubbles around the filler reduces the transfer efficiency of shear force, making the grinding time increase. After coating, the dry bubbles remaining on the surface will not only affect the beauty of the coating film, but also become the center of corrosion and reduce the durability of the coating film.




In order to eliminate these problems, almost all water-based coatings need to add defoamer. The active components of defoamer can achieve the purpose of defoamer by disturbing and destroying the stabilizing effect of bubbles. Different types of defoamer act in different ways, so understanding the mechanism of action of defoamer helps to systematically adjust the key physical and chemical parameters, so that the main steps in the mechanism can be controlled to achieve the best defoamer efficiency.






1, the stability of the bubble


In the production process of water-based coatings, mechanical mixing is easy to bring air into the system. In the process of coating construction, brush coating, roll coating and spraying are easy to bring gas into the wet film. Other porous substrates, such as wood and cement, release gases into the film as the paint wets and penetrates. These gases brought into the coating system are surrounded by the liquid phase and form bubbles, which are stabilized by electrostatic action and surface tension gradient.






1.1 Electrostatic Effect


The formulation of water-based coatings contains a variety of surfactant substances, which are characterized by a molecule containing a polar or charged hydrophilic end group and a hydrophobic hydrocarbon chain. This unique molecular structure makes it easy for surfactants to form directional micelles at the gas-liquid interface, as shown in Figure 1, with the hydrophilic end towards the water phase and the hydrophobic end towards the air.


= Because the density of air is less than that of paint, bubbles will float to the paint interface once they are generated. According to Stokes' law, the speed at which the bubble rises depends on the radius of the bubble and the viscosity of the paint. The larger the bubble radius, the lower the viscosity of the paint, the faster the bubble rise. Microbubbles usually present in coatings have a large specific surface area, resulting in a high surface free energy.


From a thermodynamic point of view, microbubbles are unstable and spontaneously fuse with each other to become macrobubbles with smaller surface area and lower free energy. When the macrobubble rises to the liquid level, the hydrophilic group at the gas-liquid interface of the bubble and the hydrophilic group at the gas-liquid interface of the paint produce mutual repulsion, so that the bubble is in a stable state and is not easy to break.






1.2 Marangoni effect


When there is no surfactant in the pure water system, the bubbles produced by external force rise from the main body of the liquid phase to the liquid surface. Due to the action of gravity, the liquid in the upper part of the bubble film will flow down the gas-liquid boundary on the bubble film to the main body of the liquid phase, resulting in a gradual reduction in the thickness of the bubble film. When the liquid film thickness is less than 10nm, the bubble will burst.


In the presence of a surfactant, as shown in Figures 2(a) and 2(b), the surfactant molecules in the upper part of the bubble will also decrease as the liquid drains down, resulting in a higher surface tension on the upper part than on both sides of the bubble. Surface tension is an energy state that always tends to flow from low surface tension to high surface tension. In this way, the liquid on both sides of the bubble will re-flow to the upper part of the high surface tension, creating a force opposite to the gravitational drainage, as shown in Figure 2(c), this reverse flow of liquid is called the Malangani flow. When these two forces reach an equilibrium state before the critical thickness of the bubble, the stable bubble shown in Figure 2(d) will be produced.






=


In this bidirectional flow process, the bubble film will have a certain stretching process. If the bubble has a certain elasticity, it is more conducive to the stability of the bubble. According to the Gibbs elasticity theory shown in equation (1), where F is the elastic parameter, A is the surface area of the bubble, and γ is the surface tension of the liquid phase. In order for the bubble to have a certain elasticity, the surface tension of the liquid needs to change with the surface area of the bubble, so that the value of dγ/dA is greater than 0.
= During the contraction and extension of the bubble, if the bubble cannot change its surface tension, the bubble will burst due to high rigidity. The surface tension of pure water hardly changes, so bubbles cannot exist stably.






2 Mechanism of action of defoamer


Defoamer is an additive that destroys the double electric layer of surfactant around the bubble, causing the bubble wall to become unstable and rupture, thus releasing the encapsulated gas. The typical defoamer is mainly composed of carrier oil, active solid particles and emulsifier. The carrier oil can be mineral oil, silicone oil, natural oil, white oil, etc., which can quickly transport active solid particles such as hydrophobic silica, paraffin or metal soap to the bubble film to play a role.


The emulsifier is used to regulate the compatibility of the defoamer with the main phase of the coating. The principle of selecting the best defoamer is to find a balance between its defoamer efficiency and compatibility with the system. Highly incompatible defoamer has a very high efficiency in the system, but because it cannot be integrated into the system and migrates to the gas-liquid interface, it is very easy to produce surface defects.


However, highly compatible defoaming agents are quickly integrated into the coating system and are not sufficient to provide efficient defoaming efficiency. The mechanism of action of defoamer in waterborne coatings can be divided into three categories: bridge-dewetting, bridge-stretching and fluid spreading entrainment.


No matter how the defoamer acts, the first condition is that the defoamer can enter the thin layer of the bubble film. Thermodynamic permeability coefficient E is used to represent the difficulty of the defoamer entering the thin layer of the bubble, and its expression is shown in equation (2) :


= Where, γAW, γOW and γOA respectively represent the surface tension of the liquid, the interfacial tension of the liquid and the defoamer, and the surface tension of the defoamer. When E > 0, it indicates that the defoamer can enter the thin layer of the bubble and connect with its double-layer film to form a bridge, and the stability of this bridge effect also affects the action efficiency of the defoamer molecule, which is expressed in thermodynamic terms by the bridge coefficient B, as shown in equation (3) :


= Unstable, and can further play a defoaming role. When B < 0, a stable bridge is formed, and the bubble is stable and not easy to break. When E < 0, it indicates that the defoamer cannot enter the bubble bilayer film, but is repelled to the adjacent Platonic channel [9]. Only when the capillary pressure gradually increases due to the gravity drainage of the bubble and the Platonic channel film Narrows, can the defoamer be forced to enter the bubble film for spreading. At this time, the efficiency of the defoamer is closely related to the spreading coefficient S of the carrier oil, as shown in equation (4) :


S = γ AW-γ OW-γ OA (4)


The study shows that the carrier oil with higher spreading coefficient S has higher defoaming efficiency than the non-spreading carrier oil defoaming agent. Therefore, the permeability coefficient E, bridging coefficient B and spreading coefficient S play a decisive role in the defoaming process. FIG. 3 shows the schematic diagram of the action mechanism of the defoamer under different conditions.


=
2.1 Bridge-dewetting effect


When the permeability coefficient E of the defoamer is > 0, as shown in FIG. 3(a) and FIG. 3(b), the defoamer enters the bubble film, and when the bridging coefficient B is > 0, the active solid particles in the defoamer form a bridge with the bubble double film, and the liquid on the film layer is dehumidified due to the strong hydrophobicity of the surface of the solid particles, as shown in FIG. 3(c) and FIG. 3(d). Eventually, the bubble membrane is punctured and the bubble bursts.


Similarly, the carrier oil of defoamer with a hydrophobic surface also has the effect of dewetting. Different from solid particles, oil droplets have the ability to decompose and deform. After entering the bubble film, the defoamer oil droplets decompose and deform into prisms, and no obvious deformation occurs, as shown in FIG. 3(e) and FIG. 3(f). At this time, dewetting occurs.


When the components of the defoamer contain both carrier oil and hydrophobic solid particles, the synergistic effect makes the defoamer more efficient, because the presence of solid particles makes the bubble film more powerful, that is, it has a higher permeability coefficient, so that oil droplets are easier to enter the bubble film layer to play a role.






2.2 Bridging and stretching


When oil droplets enter the bubble film layer to form the bridging effect, when the spread coefficient of carrier oil S > 0, the spread and diffusion of oil droplets leads to the formation of oil drop surfaces with different curvations at the oil-water interface and gas-water interface, as shown in FIG. 3(g).


At this time, due to the non-equilibrium capillary pressure action, the oil droplets gradually stretch and thin, as shown in Figure 3(h), until the rupture results in the rupture of the bubble. Silicone defoamer benefits from the high spreading coefficient of silicone oil and acts mainly through the bridging and stretching mechanism. When the bridging coefficient B < 0, bubbles stable in FIG. 3(i) and FIG. 3(j) will be formed.






2.3 Fluid entrainment


As mentioned above, when the permeability coefficient B < 0, the defoamator is repelled to the Platonic channel near the bubble film, as shown in Figure 3(k), and enters the bubble film under non-equilibrium capillary pressure, as shown in Figure 3(m) and Figure 3(n).


When the defoamer molecule reaches the second layer of bubble bilayer film, the surfactant is gradually replaced by adsorption because of its strong spreading ability. With the movement of Malangani fluid, the carrier oil moves with the bubble film, causing the local bubble film to gradually thin and eventually break. The prerequisite for the fluid entrainment mechanism is that the carrier oil has a good spreading ability, that is, S > 0. Some defoaming agents that do not contain hydrophobic solid particles mostly rely on this defoaming mechanism.






3 Classification of defoamer






3.1 Mineral oil defoamer


The composition of typical mineral oil defoamer is shown in Table 1. =
Among them, mineral oil as a carrier of defoamer, mainly aromatic or aliphatic mineral oil, and aromatic mineral oil is easy to make paint yellowing risk, and harmful to human physiology, has rarely been used. The main hydrophobic particles are silica, paraffin, metal soap or polyurea. A small amount of emulsifier in the defoamer can disperse the hydrophobic particles in the carrier oil well, and can also improve the compatibility of the defoamer and the system.
For environmental and health reasons, the traditional APEO emulsifiers have been replaced by linear or branched fatty alcohols. Mineral oil defoamer is mainly used in matte and semi-gloss latex paints. For high quality waterborne industrial coatings, the introduction of mineral oil defoamer can easily lead to the risk of oil separation and gloss reduction on the surface. The main action mechanism of mineral oil dispersants is fluid entrainment.






3.2 Silicone defoamer


The composition of silicone defoamer is shown in Table 2. =
Silicone oil is used as the carrier of defoamer, and the main component is polysiloxane or polymethicone. The SI-O bonds in polysiloxane polymers are quite flexible, the silico-oxygen skeleton provides a high spread coefficient, while the methyl group provides hydrophobicity and low surface tension. These properties make polysiloxane defoamer very efficient. And polysiloxane can also be modified to improve its compatibility, such as the use of polyether chain modification can improve the hydrophilicity of polydimethylsiloxane, so improve its compatibility in the polar system.
Because of their silicone content, this type of defoamer is more expensive than mineral oils and is often used in high-end coatings. Silicone defoamer can also improve the dispersion and defoamer performance of silicone oil by combining with hydrophobic particles such as polyurea and silica.


Compared with mineral oils, the main advantage of silicone defoamer is that it will not cause the gloss of the highlight system to decrease, nor will it affect the compatibility of the color paste of the system. The chemical structure of polysiloxane makes it have a better effect on reducing surface tension than non-silicon defoamer, so it can play a role because of its better penetration and spreading coefficient. The action of silicone defoamer is mainly based on bridging and stretching mechanism.






3.3 Molecular defoamer


Molecular defoamer is essentially a kind of non-ionic surfactant, which competes for adsorption and substitution with foam stable surfactants on the bubble thin layer. The wetting dispersant and emulsifier in the coating formulation mainly stabilize the bubbles through the interionic force, hydrogen bond and van der Waals force, while the molecular defoamer destroys these forces at the molecular level to achieve the defoamer effect.


Conventional defoamer is mainly through incompatibility with the system to achieve defoamer performance, but it will also produce some side effects, such as surface defects, poor recoating and storage performance decline. As a surface active agent, molecular defoamer has good compatibility with most systems and can also provide wetting effect. In general, molecular defoamer achieves a good balance in defoamer efficiency and compatibility, and is highly effective in controlling both microbubbles and macrobubbles. Molecular defoamer is suitable for low viscosity, high gloss and low PVC paints and varnishes.






4. Development direction of water-based coating defoamer


As the scale of water-based coatings continues to grow, the demand for efficient defoamer is also increasing. The research on defoamer in China has been going on for nearly 20 years. The first generation of defoamer mainly uses animal and plant oil as carrier. In the 1960s, the second generation of defoamer based on polyether chain epoxide blocks was born. The representative of the third generation defoamer is polydimethylsiloxane as the main active substance, which is also the most widely used defoamer.


Based on the current application status of defoamer in the coating industry, the future development of defoamer will mainly focus on the following four aspects:


(1) Improve the mechanical stability and storage stability of the existing active components to ensure high efficiency and durability in the coating system. The compatibility and defoaming efficiency can be balanced by chemical modification of existing active components or by exploring new active components.
(2) For different coating systems, develop special types of defoamer, from the traditional general type to the special type, to maximize the function of customized defoamer.
(3) Replace the traditional products with single components and poor economic benefits, and develop high-efficiency synergistic defoamer products.
(4) From the perspective of environmental resources, the development of multi-functional new molecular defoamer, such as both wetting function, reduce the minimum film forming temperature of the coating, reduce or discard the use of film forming additives, reduce VOC emissions.




As a necessary additive in water-based coatings, the efficiency of defoamer is not only affected by other components in the coating formulation, but more importantly, the physical and chemical properties of the components of the defoamer itself, such as surface hydrophilicity, interfacial tension, rheological properties, etc. A deep understanding of the mechanism of defoamer is conducive to the selection of the best defoamer solution in practical applications, and the opening of new efficient defoamer