In the process of coating production and application, construction viscosity is a key parameter that affects coating quality, construction efficiency and final performance. As a widely used measurement tool, rotary viscometers can provide reliable data for precise control of coating viscosity. The principle of operation is based on measuring the torque subjected to rotation of the rotor in the fluid, which calculates the viscosity value of the fluid. This process typically follows a rheological model of Newtonian or non-Newtonian fluids, and standardization of measurement conditions is especially important for complex fluids such as coatings, which often exhibit non-Newtonian properties.
Measurement principle
The measurement of a rotational viscometer is based on the following basic relationship: the viscosity of the fluid (η) is directly proportional to the applied shear stress (τ) and inversely proportional to the shear rate (γ̇), i.e., τ = η × γ̇. For coatings, their viscosity tends to vary with the shear rate, so they need to be measured under simulated construction shear conditions. The main control parameters include rotor model, speed, measurement temperature and measurement time. These parameters need to be selected and standardized according to the type of coating and the intended application method (e.g., brushing, spraying, roller coating).
Measurement process
Effective viscosity control starts with a standardized measurement process. First, the sample needs to be fully balanced in a constant temperature environment to eliminate the effects of temperature fluctuations. Secondly, choose the appropriate rotor and speed combination according to the estimated viscosity range of the coating to ensure that the measured value is in the middle region of the instrument range to improve accuracy. When measuring, the data should be recorded after the reading is stable, and attention should be paid to observe the time dependence of the thixotropic coating under shear. Finally, the rotor should be thoroughly cleaned after each measurement to avoid residue affecting subsequent results.
Data interpretation
The viscosity data obtained from the measurement should be interpreted in conjunction with the rheological properties of the coating. The measurement at a single speed only represents the viscosity under specific shear conditions. A more comprehensive approach is to conduct multi-speed tests and draw flow curves to evaluate the pseudoplasticity, thixotropy, and other behaviors of the coating. If the measured viscosity deviates from the target range, it can be adjusted by adding solvents, diluents, or thickeners, but it needs to be remeasured and left for a period of time after each adjustment to ensure a uniform and stable system. The adjustment process should be gradual to avoid excessive changes to the formulation balance.
Influencing factors
The accuracy of the measurement results is influenced by several factors. Temperature is one of the most significant factors, and viscosity usually decreases with temperature, so the laboratory needs to be equipped with temperature control devices. The uniformity of the sample and the bubble content will also introduce errors, and it is necessary to defoam and stir moderately before measurement. In addition, the immersion depth of the rotor and the size of the vessel (if using an adapter) must comply with the instrument requirements to ensure the consistency of the flow field. Regular calibration of the instrument with standard viscosity solutions is fundamental to maintaining long-term reliability of measurements.
Operating specifications
To ensure consistency and comparability of measurements, it is recommended to follow widely recognized technical standards at home and abroad. These standards detail the requirements for instrument preparation, sample handling, measurement steps, and result reporting. Through standardized operations, data from different batches and laboratories can be effectively correlated, providing a solid basis for the production quality control and construction application of coatings.
Refer to the example
| Examples of paint types | Typical measurement conditions are recommended |
| Water-based architectural coatings | Medium speed, simulated roller coating shear |
| Industrial spray coatings | High rotational speed to simulate atomization shear |
| Highly thixotropic paste coating | Multi-speed scanning to assess recovery |
| varnishes and oil-based paints | Control the constant temperature and pay attention to the volatilization of solvents |
Conclusion
The systematic measurement and control of coating viscosity by rotating viscometer is an important link to achieve the expected construction performance and final coating quality. By understanding the measurement principles, standardizing the operation process, interpreting the data comprehensively, and strictly controlling the influencing factors, technicians can effectively correlate the laboratory measurement results with on-site construction conditions, thereby optimizing the coating formula and construction process, ensuring the smooth progress of the coating project and the stability and reliability of the coating quality.
References
ASTM D2196-20, Standard Test Method for Rheological Properties of Non-Newtonian Materials by Rotational Viscometer.
GB/T 9269-2009, Determination of viscosity of architectural coatings - Stormer viscometer method.
ISO 2555:2018, Plastics - Liquid or emulsion or dispersion resins - Determination of apparent viscosity by Brinell viscometer.