Coating additives are used in a small amount in coatings, but they can significantly improve the performance of coatings, and have become an indispensable part of coatings. Thickener is a kind of rheological additive, which can not only thicken the coating and prevent sagging during construction, but also endow the coating with excellent mechanical properties and storage stability. It is a very important class of additives for water-based paints with low viscosity.
A. Types of water-based paint thickeners
At present, there are many kinds of thickeners available in the market, mainly including inorganic thickeners, celluloses, polyacrylates and associative polyurethane thickeners. Inorganic thickener is a kind of gel mineral which absorbs water and expands to form thixotropy. There are mainly bentonite, attapulgite, aluminum silicate, etc., among which bentonite is the most commonly used. Cellulosic thickeners have a long history of use and there are many varieties, including methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxypropyl methyl cellulose, etc., which used to be the mainstream of thickeners. The most commonly used of these is hydroxyethyl cellulose. Polyacrylate thickeners can basically be divided into two types: one is water-soluble polyacrylate; the other is homopolymer or copolymer emulsion thickener of acrylic acid and methacrylic acid. It is acidic in itself, and must be neutralized with alkali or ammonia water to pH 8~9 to achieve thickening effect, also known as acrylic acid alkali swelling thickener. Polyurethane thickeners are newly developed associative thickeners in recent years.
B. Characteristics of various thickeners
1. Cellulose thickener
Cellulosic thickeners have high thickening efficiency, especially for the thickening of the water phase; they have less restrictions on coating formulations and are widely used; they can be used in a wide range of pH. However, there are disadvantages such as poor leveling, more splashing during roller coating, poor stability, and susceptible to microbial degradation. Because it has low viscosity under high shear and high viscosity under static and low shear, the viscosity increases rapidly after coating, which can prevent sagging, but on the other hand, it causes poor leveling. Studies have shown that as the relative molecular weight of the thickener increases, the spattering of latex paint also increases. Cellulosic thickeners are prone to splashing due to their large relative molecular mass. And because cellulose is more hydrophilic, it will reduce the water resistance of the paint film.
2. Acrylic thickener
Polyacrylic acid thickeners have strong thickening and leveling properties, and good biological stability, but are sensitive to pH and have poor water resistance.
3. Associative polyurethane thickener
The associative 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 phenomenon of sag in the construction process. And its viscosity recovery has a certain hysteresis, which is conducive to the leveling of the coating film. The relative molecular mass (thousands to tens of thousands) of polyurethane thickeners is much lower than the relative molecular mass (hundreds of thousands to millions) of the first two types of thickeners, and will not promote splashing. Polyurethane thickener molecules have both hydrophilic and hydrophobic groups, and the hydrophobic groups have a strong affinity with the matrix of the coating film, which can enhance the water resistance of the coating film. Since the latex particles participate in the association, there will be no flocculation, so the coating film can be smooth and have a high gloss. Many properties of associative polyurethane thickeners are superior to other thickeners, but due to its unique micelle thickening mechanism, those components in the coating formulation that affect micelles will inevitably affect the thickening properties. When using this kind of thickener, the influence of various factors on the thickening performance should be fully considered, and the emulsion, defoamer, dispersant, film-forming aid, etc. used in the coating should not be easily replaced.
4. Inorganic thickener
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.