The basic principle of KU value
The Stormer Viscometer is a rotational viscosity measurement instrument widely used in the coatings industry, and its measurement results are expressed in Krebs Units (KUs). The instrument calculates the KU value by rotating a specifically shaped paddle in the sample, measuring the load weight or torque required to maintain a fixed speed (typically 200 rpm). The KU value is a relative viscosity value that is highly correlated with the fluidity and workability of the coating. Its core relationship can be expressed by the following formula:
KU = k × log₁₀(G)
where G represents the weight (grams) required to maintain the specified rotational speed, and k is the instrument constant. The higher the KU value, the greater the viscosity of the coating.
The meaning of KU value
KU values provide a fast, consistent, and repeatable quantification of viscosity for coating production and applications. During production, viscosity directly affects the dispersion stability of pigments, settling behavior during storage, and the uniformity of the final product. too low viscosity may lead to pigment settling and paint film sagging; High viscosity may cause construction difficulties, poor leveling and uneven film thickness. Therefore, controlling the KU value within the target range is key to ensuring batch-to-batch consistency and predetermined performance of the coating.
In the construction process, such as brushing, roller coating or airless spraying, the KU value of the coating needs to adapt to different construction methods and environmental conditions. By monitoring the KU value, the amount of diluent added can be scientifically guided to ensure that the coating achieves the best application viscosity at a specific temperature and humidity, so as to obtain the ideal paint film appearance and protective performance.
Measurement method
The measurement of KU values follows a series of standard test methods to ensure data comparability and reliability. Common standards include ASTM D562 (Standard Test Method: Standard Test Method for Determining the Viscosity of Coatings Using a Stormer Viscometer) and GB/T 9269 (Determination of Paint Viscosity by the Stormer Viscometer Method). Measurement requires strict control of sample temperature, instrument calibration and operation procedures to obtain accurate results. Standard methods typically specify detailed specifications for instruments, calibration procedures, and how results will be reported.
KU value reference for different coatings
Different types of coatings have different target KU values due to their different formulations and uses. The following table provides typical KU reference intervals for common coating categories, based on common industry practices and related technical data. The actual production control scope needs to be determined according to specific product specifications.
| Paint type: | Typical KU value range |
| Latex paint on the interior walls of buildings | 95 - 110 |
| Latex paint for building facades | 100 - 115 |
| Industrial protective primer | 75 - 95 |
| Wood decorative topcoat | 80 - 100 |
| Marine coatings | 70 - 90 |
Factors affecting KU value
The KU value of a coating is influenced by a variety of factors, and understanding these factors is fundamental to achieving effective viscosity control. Key influencing factors include:
1. Temperature: The viscosity of the paint usually decreases with increasing temperature. Measurement and control should be carried out at standard temperatures (e.g. 25°C) or temperature compensation.
2. Formulation composition: resin type, solids content, solvent type and ratio, thickener type and dosage all have a decisive impact on the final KU value.
3. Shear history: Most coatings are non-Newtonian fluids, and their viscosity may be affected by shear history such as stirring and pumping, so pretreatment methods need to be specified before measurement.
When KU values deviate from target, common adjustment strategies include adding thinners (reducing KU values) or adding thickeners (increasing KU values). Any adjustments should be made with caution and re-measured after thorough agitation while evaluating their potential impact on other properties of the coating (e.g., hiding power, drying time).
The relationship between the KU value and other rheological parameters
The KU value mainly reflects the viscosity characteristics of the paint at medium shear rate, which is close to the construction conditions of brush coating and roller coating. However, the complete rheological behavior of coatings is more complex and often requires examination of their viscosity at high shear rates (corresponding to spray applications) and at low shear rates (corresponding to storage and leveling). Therefore, KU values are often combined with data measured by other instruments (e.g., cone plate viscometers, ICI viscometers) to comprehensively characterize the flow curve of the coating and optimize the performance of the entire process from storage, application to film formation.
Epilogue
The KU value provided by the Stormer viscometer is a classic and practical viscosity parameter in the coatings industry, and plays a continuous and important role in product quality control, formulation development, and construction guidance. By standardizing measurements, understanding the influencing factors, and establishing reasonable control ranges, producers and constructors can effectively use the KU value tool to improve the consistency, construction performance, and final coating effect of coating products. Continuous attention to the development of rheological theory and measurement technology will help to understand viscosity control more deeply and promote technological progress in the industry.
References
1. ASTM International. ASTM D562-2018 Standard Test Method for Consistency of Paints Measuring Krebs Unit (KU) Viscosity Using a Stormer-Type Viscometer.
2. National Standardization Administration of China. GB/T 9269-2009 Determination of viscosity of coatings - Stormer viscometer method.
3. Patton, Temple C. Paint Flow and Pigment Dispersion: A Rheological Approach to Coating and Ink Technology. 2nd ed.
4. Compilation of industry technical data: coating process and testing method.