What Can Rheology Tell You About Your Coatings? Part 2

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

Written by Dr Rachel Atkinson, PRA.

Types of Rheological Measurements

Rheometry is used to measure the flow or deformation of a material in response to an applied force. During a rheometry experiment, a sample is sheared between two surfaces at a defined rate, and the resistance to shearing is measured by the rheometer as the torque required to deform the material. Alternatively, the sample can be sheared at a defined stress, and the resulting deformation is measured. In both cases, the results are translated into viscosity values through calculations.

Figure 1: Schematic diagram of a typical rheometer geometry setup, showing a paint layer sandwiched between a static lower plate and a rotating upper plate.

Tests can be conducted in a continuous or oscillatory rotation mode, depending on the property of interest. At PRA, we have a Netzsch Kinexus Rheometer equipped with both cone and plate (continuous rotation) and parallel plate (oscillation) geometries. We also have sandblasted geometries available if sample slip is a concern. Some popular measurements that can be carried out at PRA include:

  • Continuous rotational measurements:
  • Shear rate ramp
  • Shear stress ramp
  • 3-Step thixotropy
  • Oscillatory measurements:
  • Amplitude sweep (identification of linear viscoelastic region)
  • Frequency sweep

These methods can be tailored to your requirements in a variety of ways. Depending on the data ranges of interest, the data points can be distributed in a linear or logarithmic way. Shear ramp experiments can be performed as equilibrium (where the viscosity is allowed to stabilise before each new data point) or non-equilibrium experiments. We are able to add pre-shear steps, to better approximate post-application properties, as well carrying out up-ramps and down-ramps for shear rate and stress experiments. Tests can also be performed at custom temperatures.

1)     Continuous Rotational Methods

A popular and well-understood rheological characterisation of paints is the shear rate ramp experiment, where the shear rate is gradually increased, and the viscosity is measured. Paints are typically shear thinning materials, where the viscosity decreases with increasing shear rate (Figure 2a, blue). This decrease can be of many orders of magnitude in the case of highly structured wall paints.

However, more recent understanding of paint rheology has led to the shear stress ramp being favoured as the more representative option. The shear stress ramp applies a defined shear force rather than a defined shear rate, and plots this against the viscosity (Figure 2a, green). This method can show yield stress behaviour that may not be visible in the shear rate profile and can therefore give a more detailed picture of the material’s rheology. As shown in Figure 2a, the viscosity range observed over the full shear rate profile is covered by the >10 Pa section of the shear stress profile. Data below this point is not visible when using the shear rate-controlled method.

Since shear stress = viscosity x shear rate: the relationship between shear stress and shear rate depends on the material being tested. For shear thinning materials, an equivalent viscosity range is covered by fewer orders of magnitude when plotted as a function of shear stress (rather than shear rate).

Another desirable rotational rheometry experiment for paints is the 3-step thixotropy measurement. This is designed to test the recovery of the paint once a shear force is removed, to mimic the post application conditions. During this test, the viscosity is first measured under very low shear, then under very high shear for a period of time, and finally the viscosity rebuild with time is monitored under very low shear (Figure 2b). The time taken to reach the original viscosity (or a specified percentage) is reported.

Figure 2: (a) Example shear rate profile overlaid with equivalent section of the shear stress profile, highlighting additional data available at low shear stresses and (b) typical 3-step thixotropy profile.

2)     Oscillatory Methods

Oscillatory shear experiments can also be carried out at PRA. This type of measurement uses small amplitude, rotational oscillations to probe the microstructure of the paint without affecting its internal structure. The oscillatory motion is described in terms of the amplitude (strain) and the frequency (oscillations per second).

As discussed in Part 1, paints are viscoelastic materials, meaning that they exhibit a combination of liquid-like (viscous) and solid-like (elastic) behaviour. Oscillatory rheometry methods can be used to measure the relative elastic and viscous components of a material (known as the elastic modulus and viscous modulus).

The elastic and viscous moduli are strain independent up to a maximum value, above which the internal structure is broken down. This can be measured via an amplitude sweep experiment, which oscillates the material at a constant frequency and an increasing amplitude of strain. The elastic modulus begins to drop at the point where the internal structure is broken down and the paint starts to flow (Figure 3a). The region up to this point is known as the linear viscoelastic region (LVER), and oscillation experiments performed within this range will not affect the microstructure of the paint.

A frequency sweep test can subsequently be carried out to measure the time-dependent behaviour of a material. Low frequencies correspond to longer timescales, while high frequencies correspond to shorter timescales. The sample is oscillated at a fixed strain (within the LVER) and the elastic and viscous components are measured across a range of frequencies. The relationship between the two can provide significant information on the material’s behaviour, and the frequency sweep profile can act as a fingerprint of a material. In Figure 3b, the elastic modulus is larger than the viscous modulus, indicating that this particular paint displays solid dominant behaviour. However, the viscous modulus increases more rapidly and begins to narrow the gap at high frequencies.

Figure 3: Example rheology profiles for (a) amplitude sweep and (b) frequency sweep experiments.

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 3’. (Coming 15th 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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