Thickener is a common and most commonly used water-based additive in water-based coatings. After adding a thickener, it can increase the viscosity of the coating system, thereby preventing the relatively dense substances in the coating from settling. There will be no sagging phenomenon due to the viscosity of the paint being too thin. There are many types of thickener products, and different types of products have different thickening principles for different systems of coatings. There are roughly four types of common thickeners: polyurethane thickeners, acrylic thickeners, inorganic thickeners and thickeners for cellulose thickeners.
1. Thickening mechanism of associative polyurethane thickener
The structural characteristics of polyurethane associative thickeners are lipophilic, hydrophilic and lipophilic tri-block polymers, with lipophilic end groups at both ends, usually aliphatic hydrocarbon groups, and a water-soluble polyethylene glycol segment in the middle. As long as there is a sufficient amount of thickener in the system, the system will form an overall network structure.
In the water system, when the concentration of the thickener is greater than the critical micelle concentration, the lipophilic end groups associate to form micelles, and the thickener forms a network structure through the association of micelles to increase the viscosity of the system.
In the latex system, the thickener can not only form an association through the lipophilic terminal group micelles, but more importantly, the lipophilic terminal group of the thickener is adsorbed on the surface of the latex particle. When two lipophilic end groups are adsorbed on different latex particles, the thickener molecules form bridges between the particles.
2. Thickening mechanism of polyacrylic acid alkali swelling thickener
Polyacrylic acid alkali swelling thickener is a cross-linked copolymer emulsion, the copolymer exists in the form of acid and very small particles, the appearance is milky white, the viscosity is relatively low, and it has good stability at low pH sex, and insoluble in water. When alkaline agent is added, it transforms into a clear and highly swellable dispersion.
The thickening effect of polyacrylic acid alkali swelling thickener is produced by neutralizing the carboxylic acid group with hydroxide; when the alkali agent is added, the carboxylic acid group that is not easily ionized is immediately converted into ionized ammonium carboxylate or metal In the salt form, an electrostatic repulsion effect is generated along the anion center of the copolymer macromolecular chain, so that the cross-linked copolymer macromolecular chain expands and stretches rapidly. As a result of local dissolution and swelling, the original particle is multiplied many times and the viscosity is significantly increased. Since the crosslinks cannot be dissolved, the copolymer in the salt form can be regarded as a copolymer dispersion whose particles are greatly enlarged.
Polyacrylic acid thickeners have good thickening effect, fast thickening speed, and good biological stability, but they are sensitive to pH, poor water resistance, and low gloss.
3. Thickening mechanism of inorganic thickeners
Inorganic thickeners mainly include modified bentonite, attapulgite, etc. Inorganic thickeners have the advantages of strong thickening, good thixotropy, wide pH range, and good stability. However, since bentonite is an inorganic powder with good light absorption, it can significantly reduce the surface gloss of the coating film and act like a matting agent. Therefore, when using bentonite in glossy latex paint, attention should be paid to controlling the dosage. Nanotechnology has realized the nanoscale of inorganic particles, and also endowed inorganic thickeners with some new properties.
The thickening mechanism of inorganic thickeners is relatively complicated. It is generally believed that the repulsion between internal charges increases the viscosity of the paint. Due to its poor leveling, it affects the gloss and transparency of the paint film. It is generally used for Primer or high build paint.
4. Thickening mechanism of cellulose thickener
Cellulose thickeners have a long history of development and are also widely used thickeners. According to their molecular structure, they are divided into hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxymethyl cellulose, carboxymethyl cellulose, etc. , which is more commonly used hydroxyethyl cellulose (HEC).
The thickening mechanism of cellulose thickener is mainly to use the hydrophobic main chain on its structure to form hydrogen bonds with water, and at the same time interact with other polar groups on its structure to build a three-dimensional network structure and increase the rheological volume of the polymer. , restrict the free movement space of the polymer, thereby increasing the viscosity of the coating. When the shear force is applied, the three-dimensional network structure is destroyed, the hydrogen bonds between the molecules disappear, and the viscosity decreases. When the shear force is removed, the hydrogen bonds are re-formed, and the three-dimensional network structure is re-established, thereby ensuring that the coating can have good properties. rheological properties.
Cellulosic thickeners are rich in hydroxyl groups and hydrophobic segments in their structure. They have high thickening efficiency and are not sensitive to pH. However, due to their poor water resistance and affecting the leveling of the paint film, they are easy Affected by microbial degradation and other shortcomings, cellulose thickeners are actually mainly used for thickening latex paints.
In the process of coating preparation, the selection of thickener should comprehensively consider many factors, such as compatibility with the system, viscosity, storage stability, construction performance, cost and other factors. Multiple thickeners can be compounded and used to give full play to the advantages of each thickener, and reasonably control the cost under the condition of satisfying performance.