Update:As one of the raw materials of coatings, the dosage of coating additives is usually very small (generally about 1% of th...
As one of the raw materials of coatings, the dosage of coating additives
is usually very small (generally about 1% of the total formulation), but it has a great effect. 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 a certain additive can give the coating some special functions. Therefore, additives are an important part of coatings.
Commonly used additives for coatings include organic anti-settling agents, thickeners, leveling agents, foam control agents, adhesion promoters, wetting and dispersing agents, etc.
(1) Organic anti-settling agent
Most of these products are based on polyolefins, with a solvent as the dispersing medium, sometimes modified with a castor oil derivative. There are three types of such additives: liquid, paste, and powder.
1. Rheological properties:
The main rheological function of organic antisettling agents is to control the suspension of pigments - ie to prevent hard settling or avoid settling altogether, which is their typical application. But in practice, it causes viscosity build-up and also some degree of sag resistance, especially in industrial coatings. Organic anti-settling agents lose their effectiveness due to dissolution due to elevated temperature, but their rheology can be restored as the system cools.
2. Application of organic anti-settling agent:
In order for the anti-settling agent to function effectively in the coating, it should be properly dispersed and activated. Specific steps are as follows:
(1) Wetting (for dry powder only). Dry powdered organic anti-settling agents are aggregates, and in order to separate their particles from each other, they must first be wetted by solvents and/or resins. Usually just add it to the grind with moderate agitation.
(2) Deagglomeration (only for dry powder). The aggregation force of organic anti-settling 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-settling agents have a minimum activation temperature, and if the temperature is not reached, even if a large dispersing force is applied, 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-settling agent and give full play to its performance.
Thickeners come in different types for solvent-borne and water-borne coatings. 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). There are different specifications depending on the viscosity. HEC is a powdery water-soluble product, which is a non-ionic thickener. It has good thickening effect, good water and alkali resistance, but its disadvantages are that it is easy to grow mildew, spoil, and has poor leveling.
2. The polyacrylate thickener is an acrylate copolymer emulsion with a high carboxyl group content, and its biggest feature is its good resistance to mold invasion. When the pH of this type of thickener is 8-10, it becomes a swelling state, which 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 value. At present, the most commonly used pH regulator for latex paints in China is ammonia water. Therefore, when this type of thickener is used, the pH value will decrease with the volatilization of ammonia water, and the thickening effect will also decrease.
3. Associative thickeners have different thickening mechanisms of other types of thickeners. Most of the thickeners bring viscosity through hydration and weak gel structure formed in the system. But associative thickeners, like surfactants, have both a hydrophilic part and a mouth-friendly yellow cleansing oil part in the molecule. The hydrophilic part can be hydrated and swelled to thicken the water phase, and the lipophilic end group can interact 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 with water, and the volume after absorbing water is several times the original volume. It not only has a thickening effect, but also can prevent sinking, sagging, and floating color. Its thickening effect is better than the same amount of alkali-swelling acrylic and polyurethane thickeners. In addition to this, it has wide pH adaptability, good freeze-thaw stability and biological stability. Since it does not contain water-soluble surfactants, the fine particles in the dry film can prevent the migration and diffusion of water, and can enhance the water resistance of the coating film.
(3) Leveling agent
There are three main types of leveling agents:
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 cavities; it can reduce the surface tension difference on the wet film surface due to solvent volatilization, improve the surface flow state, and make The coating 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 surface smoothness and gloss of the coating film.
2. Long-chain resin type leveling agent with limited compatibility
Such as acrylate homopolymer or copolymer, it can reduce the surface tension of the coating and the substrate to a certain extent, improve the wettability and prevent shrinkage; Homogenization, improve surface fluidity, inhibit solvent volatilization speed, eliminate defects such as orange peel and brush marks, and make the coating film smooth.
3. Leveling agent with high boiling point solvent as the main component
This type of leveling agent can adjust the volatilization speed of the solvent, so that the coating film has a more balanced volatilization speed and dissolving power during the drying process, preventing the flow of the coating film due to excessive solvent evaporation and excessive viscosity, resulting in poor leveling. It can prevent the deterioration of the solubility of the base material and the shrinkage caused by the precipitation caused by the rapid volatilization of the solvent.
(4) Foam control agent
Foam control agents are also known as antifoams or defoamers. Anti-foaming agents prevent or delay the formation of foam: Anti-foaming agents are surfactants that burst the formed foam. The difference between the two is theoretical to a certain extent, and a successful defoamer can also prevent foam formation like an antifoam. 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 two main functions are to reduce the time and/or energy required to complete the dispersion process while stabilizing the pigment dispersion. Wetting and dispersing agents are generally divided into the following five categories:
1. Anionic wetting agent
2. Cationic wetting agent
3. Electrically neutral, amphoteric wetting agent
4. Bifunctional, non-electrically neutral wetting agent
5. Nonionic wetting agent
The first four types of wetting agents and dispersing agents can wetting and help pigment dispersion because their hydrophilic ends have the ability to form physicochemical bonds with pigment surfaces, edges, corners, etc. The orientation of the pigment surface is usually the hydrophobic end. Nonionic wetting and dispersing agents also contain hydrophilic end groups, but they cannot form physicochemical bonds with the pigment surface, but can bind to the adsorbed water on the surface of the pigment particles. This binding to water on the surface of the pigment particles is unstable and leads to nonionic absorption and desorption. Desorbed surfactants in such resin systems are free and tend to have side effects such as poor water resistance.
Wetting and dispersing agents should be added during the pigment dispersion process to ensure that other surface-active substances can play their role in close contact with the pigment before other surface-active substances reach the surface of the pigment particles.
Coating is a complex system. As a component of the system, the additive amount is not much, but it plays a crucial role in its performance. Therefore, when developing solvent-based coatings, what kind of additives to use and the amount of them should be determined through a lot of repeated experiments.