Characterizing Sedimentation in the Coatings Industry

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.

Characterizing sedimentation

In order to limit sedimentation effectively and produce stable products, manufacturers need to be able to accurately characterize these behaviors during product development and manufacturing.

Visual inspection remains the most common method of sedimentation testing in the industry. Undoubtedly easy to implement, visual testing also requires virtually no capital expenditure – however, such methods are invariably subject to guesswork and inaccuracy. As well as being imprecise, the process can also be time-consuming since any variations must be visible to the eye to be detectable.

Turbiscan technology offers an alternative. Based on Static Multiple Light Scattering (SMLS), our stability analyzers provide fast, accurate, and quantified measurements of sedimentation and other destabilization processes in coatings.

These instruments work by rapidly sending light pulses through a sample at 20 “μm” intervals along its height. A reading head measures backscattering and transmission at each position, with each of these quantities relating to particle concentration and size at that position. By repeating these measurements throughout a specified time interval, Turbiscan systems give unparalleled insight into sedimentation behavior without any dilution or mechanical stress.

The Turbiscan technology can detect sedimentation up to 1,000 times faster than the naked eye – but, more importantly, it provides a precise, quantified profile of particle distribution along the sample’s height over time. This makes it easy for coatings producers to rapidly obtain useful measurements of sedimentation behavior. For simple comparisons, the Turbiscan Stability Index (TSI) sums all destabilizations into a single value. Many additional parameters can be automatically calculated to give a deeper understanding of sedimentation kinetics within any sample.

These measurements enable manufacturers to objectively determine the best strategy to overcome sedimentation and other destabilizing processes, thus improving coating quality and accelerating the development of new stable coatings.

Microtrac manufactures a complete range of Turbiscan systems, suitable for all applications from lab scale to high-throughput. If you are looking to improve your coatings’ stability, contact us today.