Overview
In the evaluation of the physical properties of color paints and related coatings, viscosity is a key parameter, which directly affects the construction performance, storage stability and final film formation quality of the product. Krebs Unit (KU) is a widely used viscosity representation method in the coatings industry and is determined by a stormer viscometer. Based on the resistance of a specific rotor rotating in the sample, the viscosity value is quantified in KU units, and the results are well correlated with the performance of the coating.
Principle of determination
The core principle of the Stormer viscometer is viscous resistance moment balancing. The instrument drives a specific shape of a propeller rotor to be immersed in the paint sample being measured and rotates at a constant speed. The viscous resistance of the sample to the rotation of the rotor exerts a reverse torque. The instrument measures the load weight required to maintain a constant rotor speed (typically 200 rpm) or directly measures torque and converts it to a Krebs viscosity value (KU) according to an empirical formula. This conversion relationship has been calibrated by a large number of experimental data, so that the KU value can intuitively reflect the consistency of the coating.
Its basic relationship can be expressed as:
KU = k × L + C
Where L is the load required to maintain the rotational speed (usually in grams), and k and C are instrumental constants determined by standard calibration oil. This formula ensures consistency between measurements across instruments.
Instruments and materials
The following main equipment and materials are required for the assay: a stormer viscometer main unit, a standard propeller rotor, a dedicated sample container, a thermostatic water bath (to control the sample temperature), a thermometer, and the paint sample to be tested. Ensuring that the instrument is calibrated with standard calibration oil before use is a prerequisite for reliable data. All parts in contact with the sample should be clean and dry to avoid contamination.
Assay steps
The assay process should be carried out in strict accordance with standard operating procedures to ensure repeatability and comparability of results.
First, a sufficient amount of bubble-free paint sample is transferred into a sample container to ensure that the rotor blade is completely submerged. Place the container in a thermostatic water bath to stabilize the sample temperature at the standard specified temperature (typically 23±0.5°C).
Subsequently, a clean paddle rotor is installed and submerged vertically into the sample to the specified depth. After the viscometer is started and the rotor speed stabilizes at 200 rpm, the Krebs viscosity value (KU) will be displayed directly or by looking up the table. It is recommended to repeat the measurement several times in the same sample and take the average as the final reported result.
Influencing factors
The KU readings directly reflect the apparent viscosity of the sample. Generally speaking, the higher the KU value, the thicker the paint; The lower the KU value, the rarer it is. This value has direct guidance for adjusting the coating formula to meet the requirements of different construction methods (such as brushing, roller coating, spraying).
Several factors can affect the accuracy of the test results and need to be controlled during testing:
Influencing factorsControl pointsThe sample temperature is strictly controlled at the standard temperature, and temperature changes can significantly change the viscosity. Sample preparation ensures that the sample is homogeneous and free of air bubbles, with moderate agitation before testing. Instrument Calibration Calibration is regularly calibrated with standard calibration oil to verify instrument constants. The rotor condition ensures that the rotor is clean, free of deformation, and immersed to the specified depth. Consistent resting or pre-shearing treatment of the sample prior to shear history testing.
In addition, the thixotropy of the paint may cause small changes in readings at the initial stage, and readings can be taken after the rotational speed has stabilized.
Application and Significance
The method of determining the viscosity value of Krebs by the stormer viscometer has become a routine testing method in the quality control of coating production, incoming material inspection and R&D formula adjustment due to its simple instrument structure, convenient operation, and good correlation with the construction feel. By monitoring KU values, production personnel can effectively achieve batch-to-batch consistency, while R&D personnel can quantify the impact of different rheological additives or formulation adjustments on product construction performance. This approach provides a common language within the industry, promoting uniformity in product quality standards.
References
ASTM D562 - Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer.
GB/T 9269 - Determination of viscosity of architectural coatings - Stormer viscometer method.
Coating Technology (4th ed.), Chemical Industry Press.