Waterborne Coatings Defoamer Introduction and Classification
One. Brief introduction of coating defoamer
The development of modern science and technology has put forward high-quality and high-efficiency requirements for the coatings industry. That is, water-based coatings and solvent-based coatings should use various auxiliaries in the formulation. Among them, there are auxiliaries that help pigments wet out and disperse; there are auxiliaries that improve the storage stability of finished paint products; Some additives to achieve special functions. Most of the above-mentioned additives are surfactants, which can change the surface tension of the coating, so that the coating itself has internal factors that are easy to foam or stabilize the foam. In the coating manufacturing process, various high-speed mixers are used, such as three-roller mills, sand mills, and ball mills. Various methods and methods used in coating application, such as air spray, airless spray, roller coating, flow coating and spray coating. In these processes, the free energy of the coating system is increased to varying degrees, helping to generate foam, which is an external factor that generates foam.
The foam problem of water-based latex coatings is the most prominent in the coatings industry, which is caused by its special formula and special production process.
(1) Latex paint uses water as a diluent. In emulsion polymerization, a certain amount of emulsifier must be used in order to obtain a stable aqueous dispersion. The use of emulsifiers causes the surface tension of the emulsion system to be greatly reduced, which is the main reason for generating foam.
(2) Wetting agents and dispersants for dispersing pigments in latex paints are also substances that reduce the surface tension of the system, which is helpful for the generation and stability of foam.
(3) Latex paint with low viscosity is not easy to apply. After the thickener is used, the film wall of the foam is thickened and its elasticity is increased, making the foam stable and difficult to eliminate.
(4) Extraction of free monomers during the production of emulsions; adjustment of dispersion and stirring when preparing latex paints; spraying, brushing, and roller operations during the construction process. All of these can change the free energy of the system to varying degrees and promote bubbles.
The foam problem of latex paint makes the production operation difficult. The air in the foam not only hinders the dispersion of pigments or fillers, but also makes the utilization of the equipment insufficient and affects the output. It can be filled several times due to the foam during canning. The air bubbles left in the paint film during construction cause surface defects, which not only damage the appearance, but also affect the corrosion resistance and weather resistance of the paint film.
two. Classification of defoamers
Defoamers are classified in different ways in different reference books. Generally divided, it can be divided into aqueous and solvent; or silicon-containing, silicon-free. Then we can generally think that there are four major categories of markets that are more common.
1.Lower alcohols and esters
Including isopropanol, butanol, tributyl phosphate, and so on. Because it has certain toxicity and VOC, and the defoaming effect is not obvious, the foam suppression is not good, so it has gradually faded out.
2.Organic polar compounds
It mainly refers to some surfactants with low HLB value, such as polyether surfactants, polyethylene glycol fatty acid esters, etc. These are rare in the market due to HLB.
3. Mineral oils
In addition to the defoaming effect of mineral oil complexes, it also contains some hydrophobic particles, such as metal stearic acid soap and polyurea. Also has a certain efficiency, the market is common.
4, organic silicon
Hydrophobic silicone oil is mainly used as the active ingredient, and some other carrier surfactants are used in combination to achieve better results, but the added amount needs to be considered. The market is common.
Generally speaking, three or four types are more common in the market. Do a good job of matching to ensure the integration of the defoamer and the entire system, and play the role of defoaming and foam suppression. As for the specific types, there are countless kinds of defoamers in the Chinese market. Only by carrying out specific experiments in your own laboratory can you see if they are suitable.
three. Brief introduction of defoamer for water-based coatings
Bubbles in water-based coatings have always been a headache for production and users. Here we first talk about the generation and stability of bubbles. In simple terms, the lower the surface tension, the smaller the free energy required for the system to form foam, and the easier it is to produce foam. However, such foams are not necessarily stable. Only when the system contains surface-active substances that can form elastic films, such as dispersants, thickeners, leveling agents, etc., can the foam be stable.
Knowing the basic principle of foam stabilization, it is easy to defoam. First of all, pay attention to whether there are ingredients for foam stabilization in the system, and then choose a suitable defoamer. Generally, the effect of silicon-containing defoamers will be better, but Be aware that other performance will be affected. It should also be noted that different systems use different types of defoamers, such as acrylic emulsions, polyurethane dispersions, water-soluble resins, etc. The types of defoamers applicable to each system are different, and they must be verified by experiments. There are many defoamers that have a strong ability to break the foam, but the foam suppressing ability is poor, and it has little effect on the bubbles generated during use. At this time, two or more defoamers should be considered for mixed use and should be divided Add twice, add antifoaming agent with good foam suppression effect when grinding, and add antifoaming agent with good foam breaking effect when paint is finished.
Of course, the defoaming is also related to the viscosity of the system. The smaller the viscosity, the faster the defoaming.
In addition, the defoaming is also related to the volatility of the system. The paint liquid that volatilizes quickly will dry up on the surface. When the air bubbles float on the surface before it breaks, it will be fixed and will accumulate more.
1. Selection of defoamer
There are many types of defoaming agents, which are roughly organic, silica, and silicone, and are divided into emulsified and non-emulsified. When choosing a defoamer for water-based coatings, you should pay attention to: (1) strong defoaming ability; (2) good stability; (3) does not affect gloss; (4) there is no obstacle such as recoatability.
It is recommended that you use a high-speed stirring method to check the defoaming ability of the defoamer. Fix the speed, stirring time, dosage, viscosity and other parameters, and then compare the height of the foam and the time to eliminate it.
Generally, high-viscosity latex paints have difficulty in defoaming and have many factors for stabilizing the foam. The amount of addition is slightly higher, generally 0.3-1.0%. Low-viscosity latex paints or water-soluble paints, especially water-soluble paints, can contain a certain amount of co-solvent, so the amount can be appropriately reduced, usually 0.01-0.2%. Other water-based paints or resins are generally about 0.1%. Anyway, the higher the amount, the better, it will cause shrinkage, oil paint and other lacquer diseases. There are more silicon-containing defoamers and it will affect the recoatability.
Most defoamers cannot be added directly to the diluted water-based coatings. Generally, they must be added when the viscosity of the resin or coating is high, and they have good dispersion. In addition, it is best to add it in two, one in the abrasive, and the other in the paint, each half.