The sedimentation of pigments and fillers in the coating industry has a significant impact on the properties of coatings at their point of use. It is crucial to accurately characterize sedimentation for the development of shelf-stable coatings – but most sedimentation testing is still performed by visual inspection which can take weeks or months.
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This article examines the fundamentals of sedimentation, including why it is so significant to the coating industry and how stability analyzers facilitate the accurate characterization, accelerate shelf life testing, and thus help optimize coating formulations.1
Fundamentals of Sedimentation
Sedimentation is the name given to the process in which solid particles (initially suspended in a liquid phase) settle to the bottom of a container over time.
Fundamentally, sedimentation occurs due to a density difference between the two phases of the particles. For the same reasons that a stone will naturally sink to the bottom of a lake, or airborne dust will eventually settle on the ground, relatively dense solid particles will naturally sink to the bottom of a less-dense fluid.
Stokes’ Law gives a basic description of sedimentation: the principle considers spherical particles falling through a simple fluid, and yields an equation for the speed v at which a solid particle will fall out of suspension:
Δρp in this equation is the difference between the densities of the solid and liquid phase, while g is the strength of gravity. The diameter of the particle in question is represented by d and η is the viscosity of the liquid phase.
The equation shows how particle size, the viscosity of the liquid phase, and the difference in density between the solid and liquid phases all impact the sedimentation rate: as is widely known, bigger particles tend to sink quickly, while smaller particles may take minutes or hours to settle, depending on the particle size.2
Of course, it is essential to remember that this model is just a first-order approximation.3 In real-world products like coatings and paints, sedimentation can be much more complex, often involving particles of distributed sizes and shapes traveling through complex fluids with visco-elastic properties.
Sedimentation: Coatings
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The significance of sedimentation differs in various industrial products. For instance, in the coatings industry, pigments (non-soluble colorants) and fillers (functional additives) are inclined to sedimentation, effectively rendering them unusable.
However, it is important to remember the positive side: namely that sedimentation is often reversible. This means that, after they’ve settled, solid particles can sometimes be redispersed.
This is not a problem for certain products – particularly as many drinks, medicines and household products instruct the user to “shake well before use.” This simple prescribed act of shaking, when performed, is sufficient to fully redisperse solid material without causing any meaningful inconvenience.
However, inter-particle bonding in many coatings and other products means that settled particles can form rubbery or hard sediment, which cannot typically be redispersed. Paints and coatings generally require relatively high energy input to achieve redispersion even when it is possible (by means of mechanical stirring, for example).
Additionally, sedimentation can be minimized by decreasing particle size, increasing the viscosity of the liquid phase or by decreasing the difference in density between solid and liquid phases.
Rheology additives are commonly used in coatings, to provide desired properties to the formulation but, as they form 3D networks to support pigments and fillers they can help prevent them from falling out of suspension. It is, therefore, essential to control sedimentation and redispersion characteristics in order to proceed with the production of high-quality coatings.
Characterizing Sedimentation
To limit sedimentation effectively and continue to produce stable products, manufacturers need to accurately characterize these behaviors during the product development and manufacturing stages.
Despite the fact that visual inspection remains the most common method of sedimentation testing in industry, it has its clear drawbacks.
Though it is an easy method of testing to implement and requires virtually no capital expenditure – methods such as this are invariably subject to guesswork and, often, inaccuracy. This process is both time-consuming and imprecise, as any variations must be visible to the eye in order to be detectable.
An alternative method to this time-consuming and imprecise method is offered by Turbiscan® technology. Turbiscan is based on Static Multiple Light Scattering (SMLS), and enables accurate, fast and quantified measurements not only of sedimentation and other migratory processes in coatings but also particle size variations over time.
The Turbiscan analyzers work with a moving measurement head that sends a light pulse through the sample along its height (at 20um intervals). The signal is acquired for both backscattered and transmitted light allowing to detect the earliest stage of sedimentation for both clear and opaque samples by repeating these measurements throughout a specified time interval. Thus, the Turbiscan® systems can provide a truly unparalleled insight into sedimentation behavior without any mechanical stress or dilution.
Using Turbiscan® technology, sedimentation can be detected up to 200x faster than the naked eye. Perhaps more importantly, the technology also provides a precise and quantified mean particle size profile over time. It is, therefore, easier for coatings producers to rapidly obtain useful measurements of sedimentation behavior.
The Turbiscan Stability Index (TSI) sums all destabilizations into a single value for simple comparisons. Further parameters can be automatically calculated to give a more in depth understanding of sedimentation kinetics within a sample.
Manufacturers are therefore able to use such measurements to objectively determine the best strategy for overcoming the challenges of sedimentation and other destabilization processes, which consequently improves coating quality and accelerates the development of new stable coatings.
A complete range of Turbiscan® systems is available, suitable for all applications, ranging from lab scale to rapid throughput. Contact Formulaction if you’re looking to improve coating stability or would like to find out more.
References
- Dispersion stability and particle size in native state | Physical Stability | Stability Testing. https://www.formulaction.com/en/products-and-technologies/product-range/turbiscan-lab.
- The Dirt on Dust, Part I. Pro Remodeler http://www.proremodeler.com/dirt-dust-part-i (2016).
- Dey, S., Ali, S. Z. & Padhi, E. Terminal fall velocity: the legacy of Stokes from the perspective of fluvial hydraulics. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 475, 20190277 (2019).
This information has been sourced, reviewed and adapted from materials provided by Formulaction.
For more information on this source, please visit Formulaction.