Action mechanism and development trend of antifoam

Action mechanism and development trend of defoamers for water-based coatings

action mechanism and development trend of defoamers for water-based coatings

June 12, 2019

bubble control is necessary in many industrial fields, such as paper industry, water treatment, medicine, dye and coating industry. With the continuous upgrading of national environmental protection regulations and the continuous improvement of residents' awareness of environmental protection, water-based coatings have made great progress

the emulsifier and wetting dispersant in the formulation of water-based coating reduce the surface tension of the system and are easy to stabilize the bubble coating in the system. The existence of bubbles will have an adverse impact on the production and coating of coatings

the trade name is purasorb reg; PLGA of PLG is a semi crystalline copolymer pigment. During the grinding process, the "air bag" formed by bubbles around the pigments and fillers reduces the transmission efficiency of shear force and increases the grinding time. After coating, the dry bubbles left on the surface will not only affect the beauty of the film, but also become the center of corrosion and reduce the durability of the film

in order to eliminate these problems, almost all water-based coatings need to add defoamers. The active components of defoamer can achieve the purpose of defoaming by interfering with and destroying the stability effect of bubbles. Different types of defoamers have different action modes, so understanding the action mechanism of defoamers is helpful to systematically adjust the key physical and chemical parameters, so that the main steps in the mechanism can be controlled to achieve the best defoaming efficiency

1. Stability of bubbles

in the production process of water-based coatings, mechanical mixing is easy to bring air into the system. In the process of coating construction, brushing, roller coating and spraying are easy to bring gas into the wet film. Other porous substrates, such as wood and cement, will release gas into the film with the wetting and penetration of the coating. These gases brought into the coating system are wrapped by the liquid phase, which can improve the thermal stability of the diaphragm; 2. It can improve the wettability of the diaphragm to the electrolyte to form bubbles, and maintain stability by relying on electrostatic action and surface tension gradient

1.1 electrostatic action

the formulation of waterborne coatings contains a variety of surfactant substances, which are characterized by the fact that the molecules contain 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, the hydrophilic end faces the aqueous phase and the hydrophobic end faces the air

as the density of air is less than that of paint, bubbles will float up to the paint interface once they are generated. According to Stokes' law, the rising speed of bubbles depends on the radius of bubbles and the viscosity of paint. The larger the bubble radius is, the lower the viscosity of the coating is, and the faster the bubble rises. Generally, the specific surface area of microbubbles in coatings is large, resulting in high surface free energy

from the perspective of thermodynamics, microbubbles are very unstable and will spontaneously fuse with each other to become macrobubbles with smaller specific surface area, that is, lower free energy. When the macro bubble rises to the liquid level, the hydrophilic group at the gas-liquid interface on the bubble and the hydrophilic group at the gas-liquid interface of the coating repel each other, making the bubble in a stable state and not easy to burst

1.2 Marangoni effect

when there is no surfactant in the pure water system, bubbles generated by external force rise from the main body of the liquid phase to the liquid level. Due to the effect of gravity, the liquid above the bubble film will flow down the gas-liquid interface on the bubble film back to the main body of the liquid phase, causing the thickness of the bubble liquid film to gradually decrease. When the thickness of the liquid film is lower than 10nm of the critical film thickness, bubbles will burst

in the presence of surfactant, as shown in Fig. 2 (a) and Fig. 2 (b), the surfactant molecules on the upper part of the bubble will also decrease with the liquid draining down, resulting in higher surface tension on the upper part than on both sides of the bubble. Surface tension is an energy state, which always tends to flow from low surface tension to high surface tension. In this way, the liquid on both sides of the bubble will flow back to the upper part with high surface tension, producing a force opposite to gravity drainage, as shown in Figure 2 (c). The reverse flow of this liquid is called Marangoni flow. When these two forces reach an equilibrium state before reaching the critical thickness of bubbles, stable bubbles in Figure 2 (d) will be produced

in this two-way flow process, the bubble film will undergo 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 formula (1), where f is the elasticity parameter and a is the surface area of the bubble, γ Is the surface tension of liquid phase. In order to make the bubble obtain a certain elasticity, the surface tension of the liquid needs to change with the surface area of the bubble, so that D γ/The value of Da is greater than 0

in the process of bubble contraction and expansion, if the bubble cannot change its surface tension, the bubble will rupture due to its high rigidity. The surface tension of pure water hardly changes, so bubbles cannot exist stably

2 antifoaming agent action mechanism

antifoaming agent is an auxiliary agent that destroys the surfactant double-layer around the bubble, making the bubble wall unstable and rupture, thus releasing the wrapped gas. Typical defoamers are mainly composed of carrier oil, active solid particles and emulsifiers. 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 membrane to play a role

emulsifier is used to adjust the compatibility between defoamer and main phase of coating. The principle of choosing the best defoamer is to find a balance between its defoaming efficiency and its compatibility with the system. Highly incompatible defoamers have high efficiency in the system, but they can not be fused into the system and migrate to the gas-liquid interface, which is very easy to produce surface defects

however, defoamers with high compatibility are quickly integrated into the coating system, which is not enough to provide efficient defoaming efficiency. The action mechanism of Defoamers in waterborne coatings can be divided into three categories: bridging dewetting, bridging stretching and fluid spreading entrainment

no matter how the defoamer acts, the first condition is that the defoamer can enter the bubble film thin layer. Thermodynamically, the permeability coefficient e is used to express the difficulty of the defoamer entering the bubble thin layer, and its expression is as follows (2):

, where, γ AW、 γ OW、 γ OA represents the surface tension of liquid, the interfacial tension between liquid and defoamer, and the surface tension of defoamer, respectively. When E> 0, it indicates that the defoamer can enter the bubble thin layer 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. In thermodynamics, it is expressed by the bridge coefficient b, and its expression is as follows: (3):

is unstable, which can further play the defoaming role. When B <0, a stable bridge is formed, and the bubbles are stable and not easy to burst. When e <0, it indicates that the defoamer cannot enter the bubble bilayer film, but is excluded to the adjacent Plato channel [9]. Until the capillary pressure gradually increases due to the gravity drainage of bubbles, which narrows the Platonic channel membrane, the defoamer can be forced into the bubble membrane for spreading. At this time, the defoamer efficiency is closely related to the spreading coefficient s of carrier oil, and the expression is as follows (4):

s = γ AW － γ OW － γ OA (4)

research shows that the defoaming efficiency of carrier oil with high spreading coefficient s is significantly higher than that of non spreading carrier oil defoamer. Therefore, the permeability coefficient e, bridging coefficient B and spreading coefficient s play a decisive role in the defoaming process. Figure 3 shows the action mechanism of defoamer under different conditions

2.1 bridging dewetting effect

when the permeability coefficient e of the defoamer is> 0, as shown in Figure 3 (a) and figure 3 (b), the defoamer enters the bubble membrane, and when the bridging coefficient B is> 0, the active solid particles in the defoamer form a bridge with the bubble double-layer membrane, and due to the strong hydrophobicity of the surface of the solid particles, the liquid on the membrane is dewetting, as shown in Figure 3 (c) and figure 3 (d), and finally the bubble membrane is punctured and the bubble bursts

similarly, defoamer carrier oil with hydrophobic surface also has the function of dewetting. Unlike solid particles, oil droplets have the ability to decompose and deform. After entering the bubble film, the defoamer oil droplets decompose and deform into prismatic shape, and no obvious deformation will occur, as shown in Figure 3 (E) and figure 3 (f). At this time, the dewetting effect occurs

when the antifoam component contains carrier oil and hydrophobic solid particles at the same time, its synergistic effect makes the antifoam efficiency higher. This is because the existence of solid particles makes the puncture effect of bubble membrane stronger, that is, it has a higher permeability coefficient, making it easier for oil droplets to enter the bubble membrane layer to play a role

2.2 bridging tensile action

when the oil drops enter the bubble film to form a bridging effect, when the spreading coefficient s of the carrier oil is> 0, the spreading and diffusion of the oil drops leads to the formation of oil drop liquid levels with different curvature at the oil-water interface and the gas-water interface, as shown in figure 3 (g)

at this time, due to the non-equilibrium capillary pressure, the oil droplets gradually stretch and thin, as shown in Figure 3 (H), until they break and cause the bubble to burst. Silicone defoamer benefits from the high spreading coefficient of silicone oil, mainly through the bridging tensile mechanism. When the bridging coefficient B is less than 0, stable bubbles in Figure 3 (I) and figure 3 (J) will be formed

2.3 fluid entrainment

as mentioned above, when the permeability coefficient B is less than 0, the defoamer is repelled to the Plato channel near the bubble membrane, as shown in Figure 3 (k), and enters the bubble membrane under the action of non-equilibrium capillary pressure, as shown in Figure 3 (m) and figure 3 (n)

when the defoamer molecules reach the second layer of the bubble bilayer membrane, the surfactant is gradually replaced by adsorption due to its strong spreading ability. With the movement of Marangoni fluid, the bubble film entrained by carrier oil moves, resulting in the gradual thinning of local bubble film and finally rupture. The prerequisite for the occurrence of fluid entrainment mechanism is that the carrier oil has a good spreading ability, that is, s> 0. Some defoamers without hydrophobic solid particles mostly rely on this defoaming mechanism

3 defoamer classification

3.1 mineral oil defoamer

typical mineral oil defoamer composition is shown in Table 1

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

3.2 organosilicon defoamer

the composition of organosilicon defoamer is shown in Table 2

silicone oil is used as the carrier of defoamer, and the main component is poly