At present, the market can choose a lot of thickener varieties, mainly inorganic thickener, cellulose, polyacrylate and association type polyurethane thickener four categories.Inorganic thickener is a kind of hydrogel mineral which expands and forms thixotropy by absorbing water.There are mainly bentonite, attapulgite and aluminum silicate, among which bentonite is the most commonly used.The use of cellulose thickener has a long history, a lot of varieties, methyl cellulose, carboxymethyl cellulose, hydroxyethyl cellulose, hydroxyethyl methyl cellulose, etc., was the main stream of thickener, among which the most commonly used is hydroxyethyl cellulose.Polyacrylate thickener can be basically divided into two kinds: one is water-soluble polyacrylate salt;The other is acrylic acid, methacrylic acid or copolymer emulsion thickener, this thickener itself is acidic, must be neutralized with alkali or ammonia water to pH8 -- 9 to achieve the thickening effect, also known as acrylic acid alkali swelling thickener.Polyurethane thickener is an associative thickener newly developed in recent years.
1 characteristics of various thickeners
Celluloid thickeners have high thickening efficiency, especially thickening of the aqueous phase; less restrictions on coating formulations, wide application; large pH range that can be used. However, there are disadvantages such as poor leveling, frequent splashing during roll coating, poor stability, and vulnerability to microbial degradation. Due to its low viscosity under high shear and high viscosity in static and low shear, the viscosity increases rapidly after coating to prevent sagging, but on the other hand, it causes poor leveling. Studies have shown that the relative molecular mass of the thickener increases and the spatteriness of the latex paint increases. Cellulose thickeners are prone to splashing due to their relatively large molecular mass. And because cellulose is hydrophilic, it will reduce the water resistance of the paint film.
2. Polyacrylic thickener
The polyacrylic thickener has strong thickening property and good leveling property, and has good biological stability, but is sensitive to pH value and poor in water resistance.
3. Associative polyurethane thickener
The associative structure of the associative polyurethane thickener is damaged by the shearing force, and the viscosity is lowered. When the shearing force disappears, the viscosity can be restored, and the sagging phenomenon during the construction process can be prevented. And its viscosity recovery has a certain hysteresis, which is conducive to the film leveling. 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 does not promote splashing. The polyurethane thickener has both hydrophilic and hydrophobic groups on the molecule, and the hydrophobic group has strong affinity with the substrate of the coating film, and can enhance the water resistance of the coating film.
Since the latex particles participate in the association, flocculation does not occur, so that the coating film can be made smooth and has a high gloss. Associative polyurethane thickeners have many properties superior to other thickeners, but due to their unique micellar thickening mechanism, those components of the coating formulation that affect the micelles necessarily have an effect on thickening. When using such thickeners, the effects of various factors on the thickening properties should be fully considered. Do not easily change the emulsion, defoamer, dispersant, filming aid, etc. used in the coating.
Inorganic thickener has the advantages of strong thickening, good thixotropy, wide pH range and good stability. However, since bentonite is an inorganic powder, it has good light absorption and can significantly reduce the surface gloss of the coating film and acts like a matting agent.
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