What Can Rheology Tell You About Your Coatings? Part 3

This article is the last in a 3-part series, to help you understand the relationship between rheology and paint performance. Parts 1 and 2 discussed the rheological characteristics of paints and introduced the rheometry capabilities available at PRA. In Part 3, we discuss specific rheometry experiments that can be used to measure and understand the paint properties of interest.

Written by Dr Rachel Atkinson, PRA.

Rheometry Experiments for Specific Paint Properties

1)     Viscosity

The viscosity is a critical property that can significantly affect the consumer’s perception of a paint. A paint must have the correct viscosity at each stage of the painting process, ranging from low shear gravitational forces during storage, up to the very high shear forces associated with roller and spray application.

A shear rate ramp experiment is commonly used to measure the viscosity of the paint over a range of shear rates. Figure 1 shows a typical viscosity profile, overlaid with some common paint processes at their approximate shear level. A high viscosity is generally favoured at low shear, to prevent sedimentation and sagging. This viscosity should be sufficiently reduced at medium shear, to allow for mixing and pumping of the paint. Finally, the viscosity should be low at very high shear, to enable the paint to spread to form a thin film on application. As discussed in Part 2, there is a more recent push towards using the shear stress ramp, which can often provide some additional low shear information such as yield stress behaviour. The viscosity also plays a role in predicting various paint behaviours, discussed below.

Figure 1: Shear rate vs viscosity profile for a wall paint. Approximate shear stresses are included as a guide, the actual relationship between shear rate and shear stress depends on the viscosity of the material.

2)     Sag Resistance and Levelling

Several parameters are important when considering sag and levelling behaviour, including low-shear viscosity, thixotropy and yield stress. As discussed in Part 1, sag resistance and flow/levelling are competing parameters, where improving one may negatively impact the other. Levelling has the added complication of being affected by the surface tension of the paint.

A high viscosity at low shear will prevent paint flow and sagging and can be measured as part of a shear sweep experiment (either rate-controlled or stress-controlled). As sagging occurs post application, the thixotropy of the material must also be considered. To obtain a more representative value, it is preferable to run a pre-shear step, to mimic the high shear application process. A 3-step thixotropy test can also be used to quantify the recovery time of the paint.

A yield stress can prevent the flow of the paint below a certain shear force, which will improve sag resistance. There are several methods for measuring the yield stress, but when considering sagging, we are most interested in the yield stress after application (where the paint has been pre-sheared). This is known as the dynamic yield stress and is described as the stress required to maintain flow. This can be modelled from a high-to-low shear rate ramp profile using the Herschel Bulkley model, which describes the behaviour of non-Newtonian fluids with a yield stress. Alternatively, a shear stress ramp can be carried out following an initial pre-shear step. The yield stress is observed as a peak in the viscosity profile.

3)     Sedimentation and storage stability

Low shear viscosity and yield stress are also important when considering sedimentation stability, but unlike sagging, there is no pre-shear involved to disrupt the internal structure of the paint. The static yield stress, or the stress required to initiate flow, is a more appropriate measurement in this case. The viscosity and yield stress are measured with no pre-shear step. A shear rate ramp or shear stress ramp can be carried out to measure the low shear viscosity. The shear stress ramp is preferable in this case, as this will also highlight any yield stress behaviour.

4)     Roller spatter

Roller spatter is primarily linked to the elasticity of the paint. The elastic modulus, obtained from oscillatory, frequency sweep measurements, is therefore an important value for predicting roller spatter. The high shear viscosity will also affect the formation of paint fibres between roller and substrate and can be found using a shear ramp experiment. Finally, the levelling ability of the paint post-application will affect how visible the roller spatter marks are in the final coating. This is primarily controlled by surface tension effects, but the experiments mentioned in the sag/levelling section above would also be relevant here.

5)     Wet film properties

The thickness of the paint film formed during application is influenced by the viscosity of the paint at high shear forces, which can be quantified via a shear ramp experiment. This also relates to the spreading rate of the paint (the volume required to cover a certain area), which can influence the customer perception of the paint performance. The film thickness can also affect several other properties, including hiding power and drying time.

6)     Formulation Monitoring

Once the rheological profile of a paint has been established, rheology measurements can be carried out on future paint batches to ensure consistency. Since there are a range of properties that can be predicted via rheometry, the measurement can be a good indicator of any variation in the formulation that may affect the paint properties.

Table 1: Summary of relationships between paint properties, rheological parameters and rheometry tests available at PRA.

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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