01 Preface
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. Thickener has the characteristics of small dosage, obvious thickening and convenient use, and is widely used in coatings, pharmaceuticals, printing and dyeing, cosmetics, food additives, oil recovery, papermaking, leather processing and other industries.

Thickeners are divided into oily and water-based systems according to different use systems, and most of the thickeners are hydrophilic polymer compounds.

At present, there are many kinds of thickeners available in the market. According to the composition and mechanism of action, they are mainly divided into four types: thickeners, cellulose, polyacrylate and associative polyurethane thickeners.

02 classification
cellulosic thickener
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.

Thickening mechanism:
The thickening mechanism of the cellulose thickener is that the hydrophobic main chain and the surrounding water molecules are associated through hydrogen bonds, which increases the fluid volume of the polymer itself and reduces the space for free movement of the particles, thereby increasing the viscosity of the system. Viscosity can also be increased through the entanglement of molecular chains, showing high viscosity at static and low shear, and low viscosity at high shear. This is because at static or low shear rates, the cellulose molecular chains are in a disordered state, making the system highly viscous; while 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 system Viscosity drops.

polyacrylic thickener

Polyacrylic acid thickener, also known as alkali swelling thickener (ASE), is generally an emulsion prepared by (meth)acrylic acid and ethyl acrylate through certain polymerization.

The general structure of alkali-swellable thickener is:

Thickening mechanism: The thickening mechanism of polyacrylic acid thickener is that the thickener dissolves in water, and through the same-sex electrostatic repulsion of carboxylate ions, the molecular chain extends from a helical shape to a rod shape, thereby increasing the viscosity of the water phase. In addition, it also forms a network structure by bridging between latex particles and pigments, increasing the viscosity of the system.

Associative Polyurethane Thickener

Polyurethane thickener, referred to as HEUR, is a hydrophobic group-modified ethoxylated polyurethane water-soluble polymer, which belongs to non-ionic associative thickener. HEUR is composed of three parts: hydrophobic group, hydrophilic chain and polyurethane group. The hydrophobic group plays an association role and is the decisive factor for thickening, usually oleyl, octadecyl, dodecylphenyl, nonylphenol, etc. The hydrophilic chain can provide chemical stability and viscosity stability, commonly used are polyethers, such as polyoxyethylene and its derivatives. The molecular chain of HEUR is extended by polyurethane groups, such as IPDI, TDI and HMDI.

Thickening mechanism:

1) The hydrophobic end of the molecule associates with hydrophobic structures such as latex particles, surfactants, and pigments to form a three-dimensional network structure, which is also the source of high shear viscosity;

2) Like a surfactant, when the current concentration is higher than the critical micelle concentration, micelles are formed, and the mid-shear viscosity (1-100s-1) is mainly dominated by it;

3) The hydrophilic chain of the molecule acts on the hydrogen bond of the water molecule to achieve the thickening result.

Inorganic thickener

Inorganic thickeners mainly include fumed white carbon black, sodium bentonite, organic bentonite, diatomaceous earth, attapulgite, molecular sieve, and silica gel.

Thickening mechanism:

Here, taking organic bentonite as an example, its rheological mechanism is as follows:

Organic bentonite usually does not exist in the form of primary particles, but is generally an aggregate of multiple particles. Primary particles can be produced through the process of wetting, dispersing and activation, forming an efficient thixotropic effect.

In the polar system, the polar activator not only provides chemical energy to help the organic bentonite to disperse, but also the water contained in it migrates to the hydroxyl group on the edge of the bentonite flakes to form. See, through the bridging of water molecules, countless bentonite The flakes form a gel structure, and the hydrocarbon chains on the flake surface thicken the system and produce thixotropic effects through their strong solvating ability. Under the action of external force, the structure is destroyed and the viscosity decreases, and the external force returns to the original state. viscosity and structure.

03 application

Cellulosic thickener Cellulosic thickener has high thickening efficiency, especially for the thickening of the water phase; it has few restrictions on coatings and is widely used; it can be used in a wide pH range. 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.

Polyacrylic acid thickener Polyacrylic acid thickener has strong thickening and leveling properties, good biological stability, but is sensitive to pH value and poor water resistance.

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. The high water solubility of cellulose thickener will affect the water resistance of the coating film, but the polyurethane thickener molecule has both hydrophilic and hydrophobic groups, and the hydrophobic group has a strong affinity with the matrix of the coating film , 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.

Inorganic thickener Water-based bentonite thickener has the advantages of strong thickening, good thixotropy, wide range of pH value adaptation, 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.