In the formulation of products such as paints, coatings, adhesives, mortars, and various construction materials, rheological control is a cornerstone of performance. Whether the goal is to ensure smooth application, prevent sedimentation, improve sag resistance, or enhance storage stability, rheological additives are essential to achieving reliable and reproducible results. Among the most prominent mineral-based rheology modifiers available today are two clays: sepiolite and bentonite. Whilst both materials have found widespread use across industrial sectors, they differ significantly in both structure and functionality.
Despite occasional attempts to use them interchangeably, a deeper understanding of their fundamental nature reveals why they are best applied in distinct rheological contexts.
The structural divide: fibre vs plate
The most critical difference between sepiolite and bentonite begins at the microscopic level. Sepiolite possesses a fibrous, needle-like morphology, which contributes to its extraordinarily high surface area. Its internal channel structure allows it to absorb water without undergoing structural swelling. These characteristics lend themselves to the formation of physical networks when the clay is dispersed in liquid media, particularly under high-shear conditions. The resulting network, composed of entangled fibres, provides structure and viscosity through physical interaction rather than interlayer expansion.
Bentonite, in contrast, is composed primarily of smectite minerals and exhibits a laminar, platelet-like structure. When hydrated, bentonite swells as water enters the spaces between its layers. This expansion triggers a different gelation mechanism, wherein the platelets interact and orient to create a three-dimensional network capable of suspending particles and thickening systems. Its ion-exchange capacity is higher than that of sepiolite, due to the presence of mobile cations in its interlayer spaces.
These fundamental structural distinctions have a profound effect on how each material behaves in formulation environments, particularly in systems where water interaction, mechanical shear, and chemical compatibility play decisive roles.
Functional implications for rheology
Functionally, the rheological behaviour of sepiolite and bentonite is as distinct as their structures. Sepiolite-based thickeners form gels through fibre entanglement, rather than chemical swelling, which gives them a unique mechanical robustness. These gels are particularly resistant to shear and exhibit excellent thixotropy: they can recover quickly after deformation, making them ideal for applications that require structural integrity under movement, followed by rapid rebuilding of viscosity at rest. This behaviour is especially valuable in paint and coating formulations, where application by brush or spray demands low viscosity during movement, followed by fast sag resistance once applied.
Bentonite-based systems, on the other hand, rely on the hydration and orientation of their laminar particles to develop viscosity. Although these systems can also exhibit thixotropic behaviour, their recovery tends to be slower, and their gels are more prone to mechanical breakdown under prolonged or high shear. Moreover, bentonite is more sensitive to external variables such as pH, ionic strength, and temperature, factors that can compromise gel stability or lead to performance variability.
Dispersion represents a practical difference. Sepiolite requires high-shear dispersion equipment to fully activate its network, often exceeding 18 m/s in peripheral speed. Bentonite is easier to disperse, typically hydrating and swelling with gentle mixing. Still, once dispersed, its stability can degrade over time, especially in systems with high electrolyte concentrations or variable pH.
Application-driven clay selection
Tolsa has translated these scientific insights into actionable product design through our Pangel line of rheological additives. Our sepiolite-based Pangel S and B Series have been engineered specifically for solvent-based or waterborne coatings, adhesives, and construction systems that demand excellent sag resistance, anti-settling, and fast thixotropic recovery. These additives are especially effective in formulations where viscosity must remain stable under mechanical stress and environmental fluctuations.
Conversely, Tolsa’s bentonite-based Pangel M and OM/OMD Series are formulated to deliver stable gel structures in systems that require consistent performance at rest. These include static applications or systems processed under low shear, where ease of dispersion and cost-effectiveness may outweigh the need for extreme mechanical robustness.
Ultimately, the choice between sepiolite and bentonite is not a matter of preference, but one of rheological intent. Understanding the interaction between structure and function allows formulators to select the most appropriate clay for each specific challenge. Whether the goal is to stabilise a paint, suspend an active ingredient, or build a self-supporting mortar, matching the right clay to the right system is essential.
The most critical difference between sepiolite and bentonite begins at the microscopic level