Discussion on Antifoaming Agent Mechanism and Application (I)

Abstract : This article starts with the definition and concept of foam, and it briefly introduces the role of defoamers and the substances that can play a role in ink coatings, and focuses on the application and testing methods of defoamers.

Keywords: defoamer, waterborne, paint, test

In the production of many inks and coatings, it is necessary to use surface-active substances to achieve certain special effects. For example, in the production of water-based paints, in the production of water-insoluble binders, water-insoluble binders need to be treated with emulsifiers. At the same time, we use additives to improve the substrate's wettability and pigment dispersion.

The most important characteristic of surfactants is their tendency to accumulate at the interface, for example at the air/water interface. The arrangement of surfactants at the interface depends on their solubility and chemical structure. It will reduce the surface tension of the interface and thus stabilize the system. But surfactants also bring about side effects that people don't want to see --- stabilizing the air that is introduced into the coatings during production and construction, and forming stable foams. This is especially common in waterborne paint systems.

First, the definition and concept of bubbles

Foam can be defined as a stable dispersion of air bubbles in a liquid medium. If general air is introduced into the liquid, the resulting bubbles can be assumed to be spherical. Since the density of air is less than that of liquid, it will migrate to the surface. When the gas reaches the surface of the liquid containing no surfactant, the air bubbles will be broken, the air in the bubble will dissipate, and the liquid around the air bubbles will fuse together again. Therefore, pure Liquids (liquids without surfactants) are free of foam problems.

In a surfactant-containing liquid, spherical bubbles rise and become stable macrobubbles (bulbs). Although these air bubbles rise toward the air interface, they are surrounded by a layer of surfactant molecules. There is also a surface film of surfactant molecules on the air interface, so when the bubbles rise to the surface, a double film layer is formed. Double film thickness up to several microns, these small surface bubbles are very stable and difficult to eliminate. The bubbles rise to the surface and form a bubble. As the liquid drains from the double layer, the microbubbles will convert into macroscopic bubbles that are kinetically stable, and the foam double layer in the cap becomes thinner, making the foam easier to break.

The stability of the bubble is affected by several factors. One of the factors is that the stable sandwich is very thin, and the liquid in the sandwich cannot be discharged or discharged slowly. The electrostatic repulsive forces between the surfactant ions, which also occupy the interlayer, also prevent the collapse of the bubbles. Another factor that contributes to the stability of the bubble is Gibbs' elasticity. This elastic effect manifests itself by stretching the film with the surface active substance. In this case, the surface of the double membrane layer is expanded, resulting in a local decrease in the concentration of the surfactant and an increase in the surface tension. The tensioned film layer is like a piece of elastic skin, so that the surface tension is as low as possible.

Waterborne coatings and inks will produce foam during manufacture or construction. Mixing, dispersing or filtering operations during the manufacturing process will allow the foam crown to form and rise, which will increase production time and reduce the system's effective volume. During the construction process, the stable foam that accumulates on the air interface will produce common surface defects after the coating film is dried. The foam destroys the migration of the ink during the printing process and in the worst case even causes the ink to overflow from the working tank.

Second, the need for defoamer to play a role

The current theory of bubble formation is different between defoaming and defoaming. The defoamer acts to trap air at the interface. The function of the defoaming agent is to lift the tiny air (microbubbles) scattered in the coating film to the surface quickly after the coating process and the coating film are completed. However, in practice, this distinction is often not clearly made. Defoamers can also eliminate microbubbles to some extent. The defoamer's defoaming mechanism in aqueous coatings and inks is discussed below.

Defoamers must show certain results in order to show the effect. One of them is that they must be able to destroy the above-mentioned foam stabilizing mechanism. For this reason, the defoamer must be insoluble in the system and can migrate to the air interface. Of course, serious side effects such as shrinkage cavities due to the use of defoamers cannot occur.

In order to be able to defoam, the defoamer emulsified into tiny droplets in the paint must be combined with the foam-stabilizing surfactant layer and penetrate into the bilayer film of the foam. The defoamer must then spread quickly across the cracked surfactant layer. The film at this time has a significant reduction in elasticity compared to the previously stable sandwiched surfactant film. Finally, this instability leads to the breakdown of the bilayer, which leads to a defoaming effect. Therefore, incompatibility, high spreadability, and low surface tension are important properties that each defoamer must possess.

The addition of fine hydrophobic particles, such as silica, to the defoamer will effectively defoam the foam. Their mechanism of action can be interpreted as a dewetting process. The stable bilayer of the surfactant does not wet the hydrophobic solid particles, resulting in instability of the surface tension in the local area, and the rupture of the bilayer film.

Third, the water-based paint and ink defoaming substances

For aqueous systems, the two largest types of defoamers are petroleum hydrocarbons (mineral oils) and silicones. Traditionally, aromatic and aliphatic petroleum derivatives have been used as active spreading substances in the defoamer composition. In the early days, aromatic oils were widely used. However, their health and environmental hazards hinder their application. Aliphatic oils are less toxic, but have lower compatibility in aqueous media and can cause severe loss of light in medium to high gloss coatings.

A series of very efficient defoamers can be obtained with hydrophobic or partially hydrophilic polyether modified silicones. The modification function makes the defoamer have a very strong spreading property, it does not affect the gloss of the coating, and has high compatibility with the system. It is widely used in modern water-based paint systems and ink systems.

The polyether-modified silicone combines a polyether segment and a siloxane segment together in a bonded form of an SI-O-C bond and an SI-C bond. The siloxane segments provide surface activity, and the degree of compatibility is determined by the nature of the polyether segments. The chemical nature of the defoamer system limits the chemistry of the defoamer selected. Due to the complexity of the aqueous system formulation, the relationship between the structure of the defoamer and the range of its effect is difficult to elucidate clearly. Since the requirements are different, it cannot be expected that there is a method that can be used to solve these defoaming problems. Due to the high spreading force of certain polyether-modified silicone materials, it does not add hydrophobic solid particles and also has excellent defoaming ability.

Fourth, add antifoam dosage and steps

The amount and effectiveness of defoamers depends on the formulation of the coating or ink, especially the type of polymer, the pH, the amount of pigment, and the chemical structure of the other auxiliaries used. Similarly, the manufacturing process and construction process also require the type and amount of defoamer.

Discussing the method of addition, two types of defoamer products, defoamer emulsion and defoamer concentrate, are to be distinguished. The emulsion droplets of the defoamer emulsion products have been dispersed to an ideal particle size distribution in the product delivery state. It can be added in the millbase or during the paint-making process and it is easy to mix evenly in the paint or ink. For defoamers that are free of hydrophobic solids, they can even be added with stirring in the paint.

The defoamer concentrate containing 100% active material has a direct influence on the defoamer's effectiveness. The particle size distribution obtained during the addition phase determines the defoaming effect. For this reason, the defoamer concentrate is usually added to the grinding base. In the material. Anti-foaming agents are added to the system using high shear forces, but excessively high shear forces can reduce the defoaming effect. The excessive dispersion and poor dispersion of the defoamer are not conducive to the performance of the defoamer. When the defoamer is added, a relatively low shear force is used, and the initial defoaming effect may be good, but after the storage stage, defoaming The effect of the agent may be reduced.

Conventional defoamers are designed to be used at the time of delivery. If the shear force is not sufficient when added, the 100% active defoamer can be pre-diluted with an alcohol or coalescent. The dilution ratio ranges from 1:1 to 1:9 for defoamer dilution to facilitate the addition and improve the reasonable distribution of defoamer particle size. This helps to improve the defoaming efficiency and reduce the tendency of shrinkage. Pre-dilution of anti-foaming agents in general will reduce the long-term stability of the defoamer active, so care should be taken in the post-dilute storage stability of the anti-foaming agent.