What Can Rheology Tell You About Your Coatings? Part 1

This article is the first in a 3-part series, to help you understand the relationship between rheology and paint performance. In Part 1, we discuss some characteristic rheological parameters and explain how these relate to the physical properties of paints. Parts 2 + 3 will describe the rheometry capabilities available at PRA and how you might use these to predict the behaviour of your paint.

Written by Dr Rachel Atkinson, PRA.

Rheology Introduction

Rheology is defined as the response of a material to a force. Rheological properties can be vital in determining the success of a coating. Paints are typically viscoelastic materials, which means they exhibit a combination of liquid-like (viscous) and solid-like (elastic) behaviour, depending on the conditions.

A material’s resistance to flow is known as its viscosity. In paint rheology experiments, the viscosity tends to decrease as the rate of deformation of the material (the shear rate) is increased, this is known as shear thinning behaviour. The viscosity at different shear rates can have a significant effect on the paint performance during the various stages of the painting process. Certain types of paint require a threshold force to be applied before they will flow; the minimum force required to break down the internal structure in the material is known as the yield stress. Time dependent behaviour may also be observed where, after shearing, the paint takes some time to recover to its initial state. This recovery behaviour is described as thixotropy and is observed as the time taken for the paint viscosity to return to its original value after application.

These features can be utilised by paint formulators to optimise the final rheological properties of a paint. Paints are designed to exhibit stability to sedimentation in the can, to be easily pumped for processing, to be applied to a substrate in a thin layer, to level well removing brush and roller marks, and to minimise sagging on vertical surfaces. Each process is associated with different levels of applied force and different shear rates (Figure 1). Understanding the rheological behaviour of paints across a range of conditions is key to optimising their behaviour in real-life use.

Figure 1: Typical viscosity profiles obtained via (a) shear rate-controlled and (b) shear stress-controlled experiments, for a shear thinning material with a yield stress. Common paint processes are highlighted at their approximate shear rates.

Important Rheological Properties of Paints

1)     Sag Resistance

Paint sagging is observed as drips or other unevenness in the paint film, as the paint starts to migrate downwards in response to gravity. This is of particular concern when painting vertical surfaces, where the gravitational forces on the paint are highest. The thicker the applied coating layer, the more likely it is that sagging will occur.

Important parameters controlling sagging are the viscosity, yield stress and thixotropy of the paint. A higher viscosity reduces paint flow and leads to reduced sagging. If the yield stress is sufficiently high, then the gravitational force on the paint is not sufficient to break down the internal structure and the paint will not flow. The thixotropy will also affect sagging; if the viscosity of the paint recovers quickly, then sagging will be reduced as this will prevent the flow of the paint.

Note that increasing resistance to sagging will most likely also reduce levelling.

Figure 2: Example sag charts for a paint with good sag resistance (left), showing well defined lines,  and poor sag resistance (right) where the paint has sagged into the layers below at higher film thicknesses.

2)     Flow and Levelling

Paint levelling describes the paints ability to minimise surface defects, such as brush strokes or roller marks, by flowing to form a smooth layer of uniform thickness. Paint levelling is primarily caused by surface tension effects, but is also affected by the rheology of the material. Paint sagging and levelling are related and competing parameters, meaning that an improvement in one often leads to a performance reduction in the other.

To understand and predict levelling/sagging behaviour, we need to understand how the paint behaves at very low shear stresses (i.e. gravity), following application (a high-shear process). Highly structured paints such as wall paints tend to have a high yield stress and high yield viscosities. This gives them good sagging resistance, which is preferable when painting vertical surfaces but can lead to the typical roller brush effect seen on painted walls. Paints with less internal structure, such as glossy trim paints, do not tend to be as resistant to sagging, but produce a smooth, high gloss finish.

3)     Sedimentation and storage stability

For most paints, it is preferable to have a high viscosity or solid-like structure at very low shear stresses, in order to prevent sedimentation of any solids or denser material. High yield stresses and high low-shear viscosities are therefore desirable to improve storage stability.

4)     Mixing/Pouring/Pumping Viscosity

The rheological behaviour of the paint during initial mixing and pouring can affect the consumer’s perception of the paint. Additionally, the viscosity while pumping is an important property when considering large scale paint production. During these processes, intermediate shear forces are used. A high viscosity or high yield stress may make the paint difficult to mix, pour or pump, or may prevent it from flowing smoothly.

5)     Application Viscosity

The behaviour of the paint at high shear corresponds to the paint performance on application. The viscosity of the paint during application is important for the final paint properties, and also for the perceived ease of application. A paint with a low viscosity at high shear stresses will have a greater spreading rate than a paint with a higher viscosity. This means that the paint will spread more easily with a brush/roller, but a thinner film will be obtained. Paints with particularly low viscosities at high shear can be used in spray applications.

6)     Roller spatter

An additional concern for paint application, particularly in the case of roller application of wall paints, is roller spatter. This occurs when paint fibres form between the roller and substrate during application, which stretch and break into droplets, causing the spatter effect. The roller spatter is linked to the elasticity of the paint; higher paint elasticity correlates with the formation of longer fibres and more spatter.

7)     Additional properties

As discussed, the film thickness is significantly dependant on the rheology during application. Furthermore, the film thickness is a considerable factor in the hiding performance of the paint, as well as the drying time. As a result, these parameters are also considerably influenced by the rheological properties of the paint.

For more detail on how rheology experiments can be used to measure and predict these behaviours, please see ‘What Can Rheology Tell You About Your Coatings? Part 2 (Coming Wednesday 8th May 2024)’.

Contact [email protected] for more information on how PRA can help you with investigating, understanding and optimising the rheology of your paint.

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