The amount of additives used in coatings is very small, but it can significantly improve the performance of coatings, and has become an indispensable part of coatings. The commonly used auxiliaries for waterborne coatings are film-forming auxiliaries, thickeners, dispersants, wetting agents, defoamers, plasticizers, fungicides, etc. Thickener is a rheological additive, which can not only thicken the coating and prevent the phenomenon of hanging in construction, but also give the coating excellent mechanical properties and storage stability. For waterborne coatings with low viscosity, it is a very important kind of additives. Chinese style

Classification of Thickeners for Waterborne Coatings

At present, there are many kinds of thickeners available in the market, including inorganic thickeners, cellulose, polyacrylate and associative polyurethane thickeners. Cellulose thickeners have a long history and many varieties, including methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose and so on. They have been the mainstream of thickeners, among which hydroxyethyl cellulose is the most commonly used one. Polyacrylate thickeners can be basically divided into two kinds: one is water-soluble polyacrylate; the other is acrylic or methacrylic acid homopolymer or copolymer emulsion thickening agent. The thickening agent itself is acidic and must be neutralized with alkali or ammonia to pH8~9 to achieve thickening effect, also known as acrylic acid swelling thickening agent. Polyurethane thickeners are newly developed associative thickeners in recent years. Inorganic thickener is a kind of gel mineral formed by water swelling and thixotropy. There are mainly bentonite, attapulgite, aluminium silicate and so on, among which bentonite is the most commonly used. Chinese style

Thickening mechanism_

The thickening mechanism of cellulose thickeners is that hydrophobic main chains are hydrogen bonded with surrounding water molecules, which enhances the fluid volume of the polymer itself, reduces the space for free movement of particles, and thus improves the viscosity of the system. Viscosity can also be improved by winding of molecular chains, which shows high viscosities at static and low shear conditions and low viscosities at high shear rates. This is due to the disorder of cellulose molecular chains at static or low shear rates, which results in the high viscosity of the system. At high shear rates, the molecules are arranged in an orderly manner parallel to the flow direction and are easy to slide with each other, so the viscosity of the system decreases. Chinese style

The thickening mechanism of polyacrylic acid thickeners is that the thickeners are dissolved in water, and the molecular chains are stretched from helix to rod by the same electrostatic repulsion of carboxylic acid ions, thus improving the viscosity of water phase. In addition, it also increases the viscosity of the system by bridging the latex particles and pigments to form a network structure. Chinese style

The thickening mechanism of associative polyurethane thickener A. J. Reuvers was studied in detail. The hydrophilic and hydrophobic groups were introduced into the molecular structure of these thickeners, which made them show certain surfactant properties. When the concentration of its aqueous solution exceeds a certain concentration, micelles are formed, and the micelles and polymer particles associate to form a network structure, which increases the viscosity of the system. On the other hand, a molecule with several micelles reduces the mobility of water molecules and improves the water phase viscosity. This kind of thickener not only affects the rheology of coatings, but also interacts with adjacent latex particles. If this effect is too strong, it will easily cause latex delamination. Chinese style

Inorganic thickener

Thickener bentonite is a kind of layered silicate, which swells to form flocculent substance after absorbing water. It has good suspension and dispersion. It combines with appropriate amount of water to form gel. It can release charged particles in water and increase the viscosity of the system. Chinese style

CHARACTERISTICS AND SELECTION OF THICKENING AGENTS

Cellulose thickener cellulose thickener has high thickening efficiency, especially in water phase; less restriction on the formulation of coatings, wide application; wide range of pH can be used. However, there are some shortcomings such as poor leveling, more spattering phenomena, poor stability and susceptibility to microbial degradation in roll coating. Because of its low viscosity at high shear and high viscosity at static and low shear, the viscosity increases rapidly after coating, which can prevent sagging, but on the other hand, it causes poor leveling. Some studies have shown that the relative molecular weight of thickener increases, and the spatter of latex coatings also increases. Cellulose thickeners are prone to spatter because of their high molecular weight. This kind of thickener achieves the thickening effect by "fixing water". It seldom adsorbs pigments and latex particles. The volume expansion of the thickener fills the whole water phase. The suspended pigments and latex particles are pushed aside and flocculate easily, so the stability is not good. As a natural polymer, it is vulnerable to microbial attack.

Polyacrylic acid thickeners have strong thickening and leveling properties, good biological stability, but are sensitive to pH value and poor water resistance.

The association structure of associative polyurethane thickener is destroyed under the action of shear force, and the viscosity decreases. When the shear force disappears, the viscosity can be restored, which can prevent the flow hanging phenomenon in the construction process. And its viscosity recovery has a certain lag, which is conducive to the film leveling. The relative molecular weight of polyurethane thickeners (thousands to tens of thousands) is much lower than that of the first two types of thickeners (hundreds of thousands to millions), which does not contribute to spatter. The high water solubility of cellulose thickeners will affect the water resistance of coatings, but polyurethane thickeners have both hydrophilic and hydrophobic groups on the molecule, hydrophobic groups have strong affinity with the matrix of coatings, which can enhance the water resistance of coatings. Because of latex particles