Hectorite Clay – A Rare Mineral for Multifold Applications

Inorganic rheology additives composed of hectorite and bentonite clay structures are commonly used in aqueous manifold formulations and, with special modification, provide excellent stabilization and thixotropic flow characteristics for non-aqueous systems along with efficient low-shear viscosity structure and sag resistance stability. Clay minerals are composed of the most commonly found minerals in the Earth's crust, and therefore are abundant in nature. Clay minerals are crystalline, layered-structure hydrated aluminum or magnesium silicates and therefore have two-dimensional structure building blocks. Consequently, the clay layer is a flattened particle of nanoscopic thickness that agglomerates to form a three-dimensional solid material form. Clays can generally be classified according to the composition of separate clay layers containing one or two silicate layers concentrated on a (central) magnesium or aluminum hydroxide layer. The 2:1 ratio clay layer group to which smectite clays such as hectorite and bentonite belong contains two silica layers and a central magnesium hydroxide or aluminum hydroxide layer. When a series of metal cations reach levels with higher valency cations than normal, smectite clays encounter a charge deficiency occurring in the crystal structure. While these are called isomorphic (isostructural) substitutions, they leave a net negative charge in the clay layers, neutralized by exchangeable cations positioned on their surfaces. Figure 2 presents a general overview of the mineral structure of smectite clay. Hectorite clay is a naturally occurring clay belonging to a group of smectite clays. Hectorite is obtained by the substitution of several magnesium cations with lithium in the clay layer structure. The negative charge in the layers is found in the central octahedral layer in the clay structure and is balanced with exchangeable sodium to maintain magnetic neutrality. In addition to differences created by comparison with other elements in the smectite group in the clay composition, the inherent size and shape of the layers play an important role in their performance. The size of hectorite is approximately 10 times smaller than bentonite. Since the viscosity-increasing effect is directly proportional to the edge length of the layers, the longer edge length of hectorite that emerges after activation provides a significant benefit.

Hectorite clay can be activated through delamination of moving layers by swelling in pure water with high shear dispersion. Sodium ions become hydrated, separating the layers from each other and causing the clay to swell. During dispersion, high shear forces cause the layers to separate from each other as colloidal, lightly charged particles. After shearing, double layer effect, van der Waals attraction forces and electrostatic repulsion processes, end-to-end flocculations form, which means the desired viscosity is created.

Hectorite, having a longer edge length compared to Bentone, creates much higher low-shear viscosity when formulated in water at equal concentration. The thixotropy effect is also considerably more pronounced. However, hectorite requires higher shear during delamination due to its smaller particle size.

Hectorite clays are formulated in various systems. In the paints and coatings market segment, hectorite clays are used, in addition to the aforementioned applications, in areas such as ceramics, cosmetics, pharmaceuticals, dishes, pottery, and porcelain. Another commonly performed application is its use in water treatment due to the strong adsorption power of the layer surfaces. Additionally, the layer (sheet) surfaces can be modified by reactive quaternary ammonium compounds, transforming the clay into a hydrophobic form. An organoclay used for solvent-based systems provides this. Depending on the type of quaternary ammonium species used, products can be designed for all different polarities and solvent chemistries. Hectorite-based organoclays are superior in performance compared to similar bentonite-based classes. These products are sold commercially under the registered trade names Bentone and Benaqua and are accepted as industry references.   Udo Schonhoff Technical Sales Manager Paints EMEA, Elementis Çise Karamehmetoğlu Sales Manager Fuchs Kimya Fırat Pınar Sales Manager Fuchs Kimya