In the coatings industry, the sedimentation of pigments and fillers has a significant impact on the properties of coatings at the point of use. Being able to accurately characterize sedimentation is crucial to the development of shelf-stable coatings – yet the majority of sedimentation testing is still carried out by visual inspection. In this article, we take a look at the fundamentals of sedimentation, why sedimentation is important in the coating industry, and how stability analyzers enable accurate optimization of coating formulations.
Sedimentation is the process by which solid particles, initially suspended in a liquid phase, settle to the bottom of a container over time. Sedimentation fundamentally occurs because of a difference in density between the two phases. Relatively dense solid particles will naturally sink to the bottom of a less dense fluid for the same reasons that a stone will sink to the bottom of a lake, or that airborne dust will eventually settle to the ground.
A basic description of sedimentation is given by Stokes’ Law, which considers spherical particles falling through a simple fluid. Stokes’ Law yields an equation for the speed (v) at which a solid particle will fall out of suspension:
v = (Δρ_p gd^2)/18η
In this equation, Δρ_p is the difference between the densities of the solid and liquid phases, g is the strength of gravity, d is the diameter of the particle in question and η is the viscosity of the liquid phase. This mathematical treatment tells us that the sedimentation rate depends on particle size, the viscosity of the liquid phase, and the difference in density between the solid and liquid phases: a stone sinks quickly in water, while construction dust can take minutes or hours to settle depending on particle size.
It is important to remember, though, that this model is just a first-order approximation. Sedimentation in paints, coatings, and other real-world products is much more complex, typically involving a concentration of particles of distributed sizes and shapes falling through complex fluids with viscoelastic properties.
Microtrac offers products for all particle size analysis technologies.
