From Paint Viscosity to mouth feel of yogurt, how Rheology testing can help create the optimum product.

Welcome to my first blog. I’m Steve Carrington, product marketing manager at Malvern Instruments for our range of rheology instruments. Whilst I wouldn’t consider myself to be a rheologist by training, my university research was involved in assessing microstructural changes in complex fluids that ultimately give rise to the fascinating bulk deformation and flow properties that such systems exhibit. Take for example stable paints and smooth chocolate, which as well as being perfect ingredients for a weekend of decorating, are also systems of scientific interest; just two of the myriad products whose performance is in large part defined by their rheology.

In preceding posts, Paul Kippax has been exploring the links between different material characterization techniques. Here I’m going to continue that theme by examining the links between particle size and rheology, a subject that will feature regularly on the Malvern blog.

Creamy yoghurt, effective medicines and perfect paint viscosity 

I think it’s helpful to set any discussion about rheology in a practical context before getting in too deep. If you’re a scientist or engineer then your first encounter with the topic may have involved a lot of Greek, and little context, and that’s a shame because it is, for many industries, a most relevant discipline.

When we talk about rheology, we’re talking about the science of flow and deformation – extrusion of molten polymers, suspending bubbles in a shower gel, grout sticking to the wall rather than slumping, controlling ink flow through a printer head – as well as the thickness of your yoghurt, the stability of your medicine or paint formulations, and the feel of chocolate as it melts in your mouth.

While rheology has its roots in the polymer industry, the four products I’ve chosen to highlight in my headings exemplify the dispersed systems to which rheological know-how is now more routinely applied. Very many consumer products are two-phase systems – emulsions or suspensions – with rheology that is very closely controlled.

Using particle size to manipulate rheology in paint and more. 

Particle size – of particulates in a suspension or droplets in an emulsion – is just one property that formulators can vary to achieve the rheology they are looking for. We have an Inform paper that covers this topic in some detail : ‘10 ways to control rheology by changing particle properties’. Here however, by returning to my example of paint, I’m going to provide a very simple illustration of what can be done.

Paint Viscosity On Paint Performance

The viscosity of paint, which is essentially a suspension of pigment and other ingredients, is a key performance parameter. In fact, we ask a great deal from  paint formulators. We prefer paint that doesn’t drip and that remains uniform when stored in a can for some time. But, we don’t want to have to work too hard when we’re painting. so we need thick, viscous paint under certain circumstances that becomes less viscous, easier to move, under others. The viscosity profile for paint is therefore closely defined.

One trend is away from solvent-based systems towards those formulated with water – a more environmentally benign option. A disadvantage of water is that it is less volatile than organic solvents. Increasing solids loading is therefore an important strategy when producing paints that won’t take forever to dry.

Simply increasing solids loading increases the viscosity of the paint, potentially creating a mismatch between the viscosity profile we want and what we actually get. One way around this is to modify the particle size of the suspended ingredients. For a given solids loading, coarser particles, and/or a broader particle size distribution will give lower viscosity.

Using rheology and particle size measurement together it becomes easier to balance a paint formulation towards a successful outcome. And this is the case for many dispersed systems. Particle size and rheology go hand–in-hand for those formulating suspensions and emulsions for improved product performance.

Why not share your experiences of balancing particle size and rheology – or other parameters? Please leave  a comment below.