How to improve paint and coating formulations: Zeta potential measurement

In this fourth post out of a series of five about the analytical approaches used to understand paints and coatings, we’ll be looking at how to investigate the electrostatic properties of their constituent particles, using the metric known as zeta potential.

In the world of paints and coatings, particle size is paramount, and if you’ve read our previous blog post in this series you’ll have a fair idea of its importance for determining a formulation’s macroscopic properties.

But size isn’t everything, and if you’re really wanting to understand the properties of a formulation at the molecular level, you’ll want to understand the electrostatic properties too. To achieve that, it’s necessary to use a metric that’s a bit more conceptually complex than size, but important nonetheless – zeta potential.

What is zeta potential?

When a particle is suspended in a liquid, it typically develops a charge at its surface, due to the presence of ionic or polar species. This charged surface results in an increased concentration of counterions near the surface, which move around with the particle.

Of course, this particle–counterion interaction diminishes with distance, and beyond a certain point (the slipping plane) the surface can no longer bring the counterions along with it. The zeta potential is the electrical potential

Why is zeta potential useful?

Although zeta potential is not a direct measurement of electrical charge at the surface, it has the advantage of being relatively easy to measure, and so has become widely used to understand the electrochemical properties of colloidal dispersions. These are present in many familiar products such as cosmetics, pharmaceuticals, inks and of course paints, and are essential during many technical and industrial processes, such as ceramic manufacture, analysis of microorganisms, wetting of powders, detergent-based cleaning, mineral flotation and sewage flocculation, amongst many others.

In all these applications, knowledge of zeta potential is useful because it tells us about the propensity of a particle to attract or repel other particles, and hence its ability to aggregate or disperse when in suspension. And this in turn influences the abundance and stability of aggregates, which in the paint industry have several consequences, which we’ll discuss in a moment.

But first, how is zeta potential measured? The method employed is electrophoretic light scattering (ELS), which uses the Doppler effect arising from the motion of charged particles in an electric field. This allows their speed to be measured, which is then converted into a zeta potential value (or distribution).
Low values of the zeta potential indicate that particles are likely to aggregate, whereas high values (say greater than ±30 mV) indicate that particles are likely to repel each other.

Applications of zeta potential in the paint industry

Optimizing dispersion stability

The most important application of zeta potential in the paints and coatings industry is to understand the stability of a dispersion – its propensity to aggregate over time.

This is a particular issue for paints formulated using water, which as a polar solvent can complicate the electrostatic interactions present between the components. Fortunately, this can be controlled using dispersants to prevent the pigment particles from agglomerating – and measuring how effective these dispersants are is a perfect application of zeta-potential measurements.

Understanding color performance

The color performance of a finished coating – including opacity, tinting strength and undertone – depends primarily on the size distribution of the pigment and extender particles, but surface electrochemistry has an important role to play, especially where surface-active water-soluble components are present.

By helping formulation scientists to understand the interactions between these soluble components and charged particles in the formulation, zeta potential measurements help to ensure that electrostatic effects can be accounted for, and that the formulation achieves the desired color properties once cured.

Optimizing gloss, weathering and flatting performance

And finally, there’s finish! Or rather, properties such as flatting, gloss, and its persistence under environmental conditions. In a similar way to color performance, optimizing these aspects of a formulation requires an understanding of not just particle size but also the interactions those particles have with other components – which is where zeta potential measurements can come in handy.

ELS – Enabling in-depth zeta potential measurements

From this brief overview, I hope it’s clear that alongside size measurement, the measurement of zeta potential using ELS should have a well-earned place in the analytical toolkit of formulation scientists. By making it possible to probe the complex electrostatic interactions present in paint and coating formulations, ELS helps manufacturers to produce stable formulations with brighter colors and longer-lasting performance.

Interested in finding out more about the role of analytical methods to characterize paints and coatings? Then check out our white paper: “Improving paint and coating formulations: Using nanostructural analysis to understand macroscopic properties”.

To find out more about DLS and ELS check out this whitepaper: ‘How to choose a light scattering instrument for nanoparticle size and zeta potential measurements’

• The everyday XRD instrument that can provide specialized nanoparticle analysis
• Optimize nanobubbles with nanoparticle tracking analysis
• Helping nanoparticles ease the (surface) tension with light scattering analysis
• Colloidal nanoparticles unlocked
• Understanding the inner world of catalysts

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