Introduction
In the process of R&D, production and application of color paints, viscosity is a crucial rheological parameter. It not only affects the storage stability and construction performance of the product (such as leveling and sagging resistance), but also ultimately relates to the appearance and quality of the paint film. Traditional viscosity measurements are often performed at single or high shear rates, making it difficult to fully reflect the behavior of pigments under low shear conditions, such as pigment settling and changes in viscosity during storage. Therefore, measuring the viscosity of color paints at low shear rates using a rotational viscometer is of great practical significance for understanding their rheological properties, optimizing formulations, and predicting long-term stability.
Measurement principle
The basic measurement principle of a rotational viscometer is based on the viscous resistance of the rotating body in the fluid. When the rotor rotates at a constant speed in the fluid to be measured, the viscous resistance generated by the fluid exerts a reaction torque on the rotor. By measuring the torque required to maintain this rotation and calculating the shear stress and shear rate of the fluid based on the specific rotor geometry and measurement system parameters, the viscosity value can be obtained. Its core relationship can be expressed by the following formula:
η = τ / γ̇
Among them, η represents viscosity (Pa·s), τ represents shear stress (Pa), and γ̇ represents shear rate (s⁻¹). For low shear rate measurements, instruments often require highly sensitive torque transducers and drive systems capable of stable low-speed rotation to ensure accurate and repeatable data even with small shear forces.
Low shear rate measurement
The rheological behavior of color paint at low shear rate is closely related to its state of static or slow stress. The measurement of viscosity under this condition mainly serves the following purposes: to evaluate the suspension stability of pigments and fillers, and to predict the tendency of hard precipitation or agglomeration that may occur during storage; Analyzing the leveling process after construction, too high and low shear viscosity may lead to poor leveling and affect the smoothness of the paint film. Provides the basis for establishing a complete flow curve to more accurately simulate the performance of the product in real-world applications.
Measurement method
When measuring viscosity at low shear rates, it is important to follow a systematic operating procedure to ensure data reliability. First, the sample should be fully balanced in a constant temperature environment, and temperature control has a significant impact on viscosity results. Subsequently, an appropriate amount of sample is loaded into the measuring vessel to avoid introducing air bubbles. Depending on the estimated viscosity range and target shear rate, select the appropriate rotor and speed combination. The measurement program typically starts with a very low shear rate and gradually increases in steps, waiting for the readings to stabilize at each measurement point before recording the data. To investigate thixotropy or time dependence, measurements can be performed over long periods of time at a fixed low shear rate. After completing the test, clean the rotor and measuring system in a timely manner.
Notes:
Obtaining accurate low shear viscosity data requires multiple focuses. Temperature fluctuations are the main source of error and require the use of thermostatic baths or temperature-controlled measuring systems. The choice of rotor is crucial, with a rotor that is too large and may cause shear stress to go out of range at low speeds, while a rotor that is too small may have a weak signal. The sample load needs to be fully submerged in the rotor marking line. For thixotropic paints, shear history (e.g., pre-shear and recovery time) can greatly affect measurements at low shear rates, so sample pretreatment conditions need to be strictly defined and documented. In addition, the calibration status of the instrument itself and the lowest torque resolution are key to determining the lower limit of low shear measurement.
Application examples
After the raw data is measured, it is usually necessary to plot a flow curve of viscosity with shear rate. Data from low shear rate regions can be used to calculate zero shear viscosity or to fit parameters for specific rheological models (e.g., power law models, Carson models). The following table compares the differences in viscosity performance of two typical paints at low shear rates:
| Sample type | Viscosity at 0.1 s⁻¹ shear rate (Pa·s) |
| High solids decorative paint | Approx. 15.2 |
| Industrial protective primer | Approximately 42.7 |
Combined with formulation analysis, it can be seen that industrial protective primers usually contain a higher proportion of anti-settling agents and thickeners, which are designed to give the system a stronger network structure to resist pigment settling, which is directly reflected in its significantly higher low shear viscosity. Such data can provide a direct basis for adjusting the amount of rheological additives.
Conclusion
Measuring the viscosity of color paints at low shear rates using a rotary viscometer system is an effective means to deeply characterize their rheological properties and optimize product stability and workability. By strictly controlling the measurement conditions, reasonably selecting instrument parameters and scientifically analyzing the data, key information that reflects the static or near-static behavior of the color paint can be obtained. This technology helps guide formulation design, improve product quality control, and advance understanding of the complex rheological behavior of color coatings.
References
1. Rotational viscometer for coatings and varnishes to determine viscosity, relevant national standards.
2. Barnes, H. A., Hutton, J. F., & Walters, K. An Introduction to Rheology. Elsevier.
3. Determination of rheological properties of colored paints and varnishes, technical reports of related industries.