I. Overview
As one of the raw materials of coatings, the amount of additives is usually very small (generally about 1% of the total formulation), but the effect is great. The addition of it can not only avoid many coating defects and film defects, but also make the production and construction process of the coating easy to control, and the addition of certain additives can endow the coating with some special functions. Therefore, additives are an important part of coatings.
2. Classification of additives
Commonly used additives for coatings include organic anti-settling agents, thickeners, leveling agents, foam control agents, adhesion promoters, wetting and dispersing agents, etc.
3. Performance and application of additives
(1) Organic anti-settling agent
Most of these products are based on polyolefins, dispersed in some solvent, sometimes modified with a castor oil derivative. These additives come in three forms: liquid, paste, and powder.
1. Rheological properties:
The main rheological function of organic anti-settling agents is to control the suspension of pigments – that is, to prevent hard settling or to avoid settling altogether, which is their typical application. But in practice, it causes an increase in viscosity and also some degree of sag resistance, especially in industrial coatings. Organic anti-settling agents will dissolve due to elevated temperature, thereby losing their effectiveness, but their rheology will recover as the system cools.
2. Application of organic anti-settling agent:
In order to make the anti-settling agent work effectively in the coating, it should be properly dispersed and activated. Specific steps are as follows:
(1) Wetting (dry powder only). The dry powder organic anti-sedimentation agent is an aggregate, in order to separate the particles from each other, it must be wetted by solvent and (or) resin. It is usually sufficient to add it to the grinding slurry with moderate agitation.
(2) Deagglomeration (only for dry powder). The aggregation force of organic anti-sedimentation agents is not very strong, and simple turbulent mixing is sufficient in most cases.
(3) Dispersion, heating, duration of dispersion (all types). All organic anti-sedimentation agents have a minimum activation temperature, and if it is not reached, no matter how large the dispersing force is, there will be no rheological activity. The activation temperature depends on the solvent used. When the minimum temperature is exceeded, the applied stress will activate the organic anti-sedimentation agent and give full play to its performance.
(2) Thickener
There are different types of thickeners used in solvent-based and water-based paints. Common types of thickeners used in waterborne coatings are: cellulose ethers, polyacrylates, associative thickeners and inorganic thickeners.
1. The most commonly used cellulose ether thickener is hydroxyethyl cellulose (HEC). Depending on the viscosity, there are different specifications. HEC is a powdery water-soluble product, which is a non-ionic thickener. It has good thickening effect, good water resistance and alkali resistance, but its disadvantages are that it is easy to grow mold, rot, and has poor leveling property.
2. The polyacrylate thickener is an acrylate copolymer emulsion with a high carboxyl content, and its biggest feature is its good resistance to mold invasion. When the pH is 8-10, this kind of thickener becomes swollen and increases the viscosity of the water phase; but when the pH is greater than 10, it dissolves in water and loses its thickening effect. Therefore, there is a greater sensitivity to pH. At present, ammonia water is the most commonly used pH adjuster for latex paints in China. Therefore, when this type of thickener is used, the pH value will decrease with the volatilization of ammonia water, and its thickening effect will also decrease.
3. Associative thickeners have different thickening mechanisms from other types of thickeners. Most thickeners bring viscosity through hydration and the formation of a weak gel structure in the system. However, associative thickeners, like surfactants, have both hydrophilic parts and mouth-friendly yellow cleansing oil parts in the molecule. The hydrophilic parts can be hydrated and swelled to thicken the water phase. The lipophilic end groups can be combined with emulsion particles and pigment particles. associate to form a network structure.
4. The inorganic thickener is represented by bentonite. Usually water-based bentonite swells when it absorbs water, and the volume after absorbing water is several times its original volume. It not only acts as a thickener, but also prevents sinking, sagging, and floating color. Its thickening effect is better than that of alkali-swellable acrylic and polyurethane thickeners in the same amount. In addition, it also has a wide range of pH adaptability, good freeze-thaw stability and biological stability. Because it does not contain water-soluble surfactants, the fine particles in the dry film can prevent water migration and diffusion, and can enhance the water resistance of the coating film.
(3) leveling agent
There are three main types of leveling agents commonly used:
1. Modified polysiloxane type leveling agent
This type of leveling agent can strongly reduce the surface tension of the coating, improve the wettability of the coating to the substrate, and prevent shrinkage; it can reduce the surface tension difference on the surface of the wet film due to solvent volatilization, improve the surface flow state, and make The paint is quickly leveled; this type of leveling agent can also form an extremely thin and smooth film on the surface of the coating film, thereby improving the smoothness and gloss of the coating film surface.
2. Long-chain resin type leveling agent with limited compatibility
Such as acrylate homopolymer or copolymer, which can reduce the surface tension of the coating and the substrate to a certain extent to improve wettability and prevent shrinkage; and can form a single molecular level on the surface of the coating film to increase the surface tension of the coating Homogenize, improve surface fluidity, inhibit solvent volatilization speed, eliminate defects such as orange peel and brush marks, and make the coating film smooth and even.
3. Leveling agent with high boiling point solvent as the main component
This type of leveling agent can adjust the volatilization rate of the solvent, so that the coating film has a more balanced volatilization rate and solvency during the drying process, and prevents the flow of the coating film from being hindered by the solvent volatilization too fast and the viscosity is too high, resulting in poor leveling disadvantages, and can prevent the shrinkage caused by the poor solubility of the base material and the precipitation caused by the solvent volatilization too fast.
(4) Foam control agent
Foam control agents are also called antifoaming agents or defoaming agents. Anti-foaming agents prevent or delay the formation of foam: Anti-foaming agents are surfactants that burst bubbles that have formed. The difference between the two is only theoretical to a certain extent, a successful defoamer can also prevent the formation of foam like an antifoam agent. Generally speaking, antifoaming agent is composed of three basic components: active compound (ie, active agent); diffusing agent (available or not); carrier.
(5) Wetting and dispersing agents
Wetting and dispersing agents may have a range of functions, but the main two functions are to reduce the time and/or energy required to complete the dispersion process while stabilizing the pigment dispersion. Wetting agents and dispersants are usually divided into the following
Five categories:
1. Anionic wetting agent
2. Cationic wetting agent
3. Electroneutral, amphoteric wetting agent
4. Bifunctional, non-electrically neutral wetting agent
5. Non-ionic wetting agent
The first four types of wetting agents and dispersants can play a wetting role and help pigment dispersion because their hydrophilic ends have the ability to form physical and chemical bonds with the pigment surface, edges, corners, etc., and move toward the Orientation of the pigment surface, usually the hydrophobic end. Nonionic wetting and dispersing agents also contain hydrophilic end groups, but they cannot form physical and chemical bonds with the pigment surface, but can combine with the adsorbed water on the surface of the pigment particles. This water binding to the pigment particle surface is unstable and leads to non-ionic absorption and desorption. The desorbed surfactant in this resin system is free and tends to cause side effects such as poor water resistance.
The wetting agent and dispersant should be added during the pigment dispersion process, so as to ensure that other surface active substances can be in close contact with the pigment to play their role before reaching the surface of the pigment particle.
Four. Summary
Coating is a complex system. As a component of the system, additives are added in a small amount, but they play a vital role in its performance. Therefore, when developing solvent-based coatings, which additives to use and their dosage should be determined through a large number of repeated experiments.
Post time: Jan-30-2023