Compared to solvent-borne coatings, waterborne coating formulations are more complex. In formulation design, not only the type and properties of the waterborne resin need to be considered, but also the selection of various functional auxiliaries and their mutual interactions. Rheological additives are key components in coating formulations. The rheological properties of waterborne coatings are a complex interplay between viscosity and shear force, determining the production, storage, and application properties of the coating.

Rheological additives commonly used in waterborne coatings can be divided into two major categories according to their chemical properties: organic and inorganic. This article focuses on inorganic bentonite.

In geology, the term "bentonite" refers to a mudstone (clay rock, not clay mineral) primarily composed of montmorillonite (a clay mineral belonging to the montmorillonite group). It typically forms in marine environments through the devitrification of volcanic ash or tuff. This results in a very soft, porous rock that may contain more resistant mineral relic crystals and has a soapy or greasy feel. However, in commercial and industrial applications, the term "bentonite" is more broadly used to refer to any swelling clay primarily composed of montmorillonite-type minerals, including montmorillonite.

1. Structure of Montmorillonite

Montmorillonite minerals have been X ray diffraction spectroscopy has been used for very in-depth analysis, and the results show that they usually have the morphological structure shown in Figure 3.1-1 They consist of a three-layer sheet-like structure of silicate, with the middle layer composed of Al ₂ O ₃ octahedron, surrounded on both sides by SiO ₂ tetrahedron. These basic unit modules are stacked together, like a deck of cards. The distance between two adjacent layers is approximately 10-15 Å Å ), and the thickness of this basic layer is measured as 9.6 Å.

Its simplified structural formula is: (Al,Mg) ₂ [SiO ₁₀ ](OH) ₂ • n H ₂ O

Unlike other layered silicates, the octahedral layers of montmorillonite minerals are not all constructed in the same way. Some of the positively charged metal ions in the center are replaced by another ion of lower valence, giving the entire crystal lattice a negative charge, which is compensated by cations adsorbed on the basal surface.

In the central aluminum oxide octahedral layer, one out of every six Al³ + ions is isomorphously replaced by Mg² + ions. The negatively charged lattice is compensated by Na + ions on the basal surface. Natural montmorillonite, in addition to Na + ions, usually also contains Ca² + ions or a mixture of both. Sodium bentonite is more valuable, but calcium bentonite is more common. In fact, it is almost impossible to find pure montmorillonite, as it is almost always mixed with other minerals. Layered silicates with a high content of montmorillonite are the well-known bentonite, whose deposits are found in many parts of the world.

2. Hydration of Bentonite

An important characteristic of bentonite is its swelling behavior upon contact with water. There are two different ways of water absorption during hydration:  

– In intercrystalline crystal swelling , the absorption of excess water by interlayer cations and the surface of the clay mineral causes the basic layers to expand. This process is reversible.

– In osmotic swelling (osmic swelling) , due to differences in cation concentration, a diffuse ionic layer forms at the surface of the clay mineral and in the pore solution, causing electrostatic repulsion between particles.

In montmorillonite crystals, the hydration process is usually produced by water molecules entering between the basic layers (intercrystalline swelling). This expands the interlayer spacing; under appropriate conditions, the crystal bonds may even completely disintegrate.  

Sodium bentonite and calcium bentonite differ greatly in their hydration behavior, mainly due to the different nature of the interlayer cations. Sodium ions ( Na + ) are more easily hydrated in the adsorbed state than calcium ions ( Ca² + ), therefore absorbing more water and swelling. For this reason, the water absorption capacity of sodium bentonite is 600–700% , while that of calcium bentonite is only 200–300% 。

Sodium ions are larger than calcium ions ( Ca² + ) or magnesium ions ( Mg² + ), therefore having a smaller binding force. The absorption or release of water changes the interlayer spacing of calcium or magnesium saturated montmorillonite crystals, ranging from 10–20 Å between; even after adding sufficient water, individual silicate layers remain layered and stacked. In the case of sodium interlayer cations, the interlayer spacing can be varied to 160 Å . With further water absorption or swelling, the cohesive force between layers is lost, the crystal bonds completely disintegrate, and the montmorillonite crystals subsequently decompose into their individual basic layers.

3. Thickening mechanism of bentonite

The basic layer of each montmorillonite crystal consists of a negatively charged surface and a positively charged edge.

Due to these different charges, after the crystal bonds disintegrate into individual layers, a special structure is formed in the water, where the edge of one crystal is deposited on the surface of another. This large-volume framework formed in water is called a "card house" structure.

This unique three-dimensional structure can encapsulate water and fillers, making the system thicken and have good storage stability. When the system is subjected to strong shear, the network structure dissociates, exhibiting obvious shear thinning properties, making the coating have good leveling properties. Once the shear force disappears, the network structure is restored under the action of hydrogen bonds, resulting in good anti-sagging performance.

In the static storage state of the coating, these columnar bentonite particles are dispersed among the flaky particles, playing a supporting role. When subjected to shear force, the hydrogen bonds and weak electrostatic forces are broken, and the columnar particles are arranged in the direction of flow. After the shear force disappears, these columnar particles can quickly rearrange. Therefore, the introduction of bentonite makes the coating have better storage stability and thixotropy.