In the field of paint production, quality control and related research, the accurate determination of the viscosity of coatings is a crucial task. With its advantages of relatively simple operation and accurate measurement results, Rotational Viscometer has become a commonly used instrument for determining the viscosity of coatings. The following is a detailed description of the method of determining the viscosity of a coating using a Rotational Viscometer.
Scope of application and description
This measurement method is mainly used in the quality control of anti-corrosion coatings. In the production process of anti-corrosion coatings, viscosity is one of the key indicators affecting the performance of coatings. The appropriate viscosity can ensure that the coating has good coating performance during construction, such as uniform film formation, not easy to sag, etc., so as to ensure the anti-corrosion effect. In addition, this method is also used to measure the absolute viscosity of coatings, providing accurate data support for the development of coating formulations and the optimization of production processes. For the main agent (component A) in two-component coatings, this method is also relied on for viscosity determination, which helps to control the mixing ratio of the two-component and ensure consistent performance after curing. In the end, the test results are measured in Pa・s (Pascal seconds), which is a visual indication of the viscosity of the paint.
Instruments and materials
(1) Rotational viscometer
The commonly used Rotational Viscometer has a measurement range of 10 to 100,000 mPa・s, and is mainly composed of a motor, a variable speed system, a rotor connecting shaft, and a dial. The motor provides the power, and the variable speed system realizes different speed adjustment; The rotor connection shaft is used to install rotors of different sizes to accommodate the measurement of coatings of different viscosities. Some models of Rotational Viscometers have a higher degree of intelligence, which can automatically zero and determine the test range, making the operation more convenient. The number of rotors is available in sizes ranging from 4 to 7 and at a wide range of speeds, which greatly increases the flexibility and accuracy of measurements. At the same time, the LCD screen can clearly display many important information such as viscosity (mPa・s), rotational speed (r/min), rotor number used, test temperature, etc., so that the operator can obtain measurement data in real time. In addition, some instruments are equipped with interfaces that can be connected to printers and computers for easy data storage and analysis.
(2) Containers
A beaker or paint jar with a diameter of not less than 70mm is required as a container for the paint specimen. The appropriate size vessel is selected to ensure that the rotor is not disturbed by the vessel wall when rotating in the specimen, and to ensure the accuracy of the measurement results. If the diameter of the container is too small, the rotor will be affected by the fluid resistance of the container wall when it rotates, resulting in a high viscosity value for measurement.
(3) Thermometer
Thermometers with a range of 0 to 50°C and an index of 0.1°C are used to measure the temperature of paint specimens. The viscosity of the coating is extremely sensitive to temperature, and the viscosity changes significantly with a slight change in temperature. Therefore, accurate measurement and control of specimen temperature is the key to obtaining accurate viscosity measurement results.
(4) Stopwatch
A stopwatch with a graduation value of 0.1s is used for timing. During the measurement process, the rotation time of the rotor needs to be precisely controlled to ensure the repeatability and reliability of the measurement results.
Assay method
Preparation
Rotor Selection – Select the appropriate rotor based on the estimated viscosity of the paint sample. In general, a small rotor should be used for high-viscosity specimens, because the small rotor is relatively less resistant to rotation in a high-viscosity fluid, which can measure viscosity more accurately. For low-viscosity specimens, a large rotor is used, which can generate enough torque in low-viscosity fluids to make the measurement more stable. If you can't accurately estimate the viscosity of the sample, you can assume that the viscosity of the sample is high, start with the smallest rotor, and gradually replace the appropriate rotor.
Installing the Rotor – Screw the selected rotor onto the connecting shaft of the instrument to ensure that the rotor is securely mounted to avoid loosening or falling off during rotation. The instrument is then slowly lowered by means of a lifting device to allow the rotor to gradually submerge into the test specimen. During the immersion process, the liquid level of the sample should be carefully observed so that the liquid level of the sample just reaches the mark of the rotor shaft. This marking position is precisely designed to ensure that the rotor is immersed in the specimen to the required depth of measurement, thus guaranteeing the accuracy of the measurement results. If the liquid level is too high or too low, it will affect the fluid resistance of the rotor in the specimen, resulting in deviations in the measured viscosity value.
Temperature control
The thermometer is carefully placed in the specimen, and in order to achieve a stable temperature of the specimen and the viscometer rotor in accordance with the measurement requirements, it is generally necessary to place it in a thermostatic chamber. The temperature in the constant temperature room should be strictly controlled at (23±0.2)°C. Rest for a period of time, which can be adjusted according to the characteristics of the specimen and the ambient temperature difference, usually 15 to 30 minutes, to ensure sufficient heat exchange between the specimen and the rotor for a uniform temperature throughout the measurement system. In this process, unnecessary disturbances to the specimen and instrument should be avoided to prevent temperature fluctuations from affecting the measurement results.
Start the instrument
Press the pointer control lever on the upper part of the instrument, the function of the control lever is to lock the pointer, prevent unnecessary deflection of the pointer during the start of the instrument and speed adjustment, and ensure the accuracy of the measurement data.
Close the power switch and start the motor to power the rotor rotation.
Based on the estimated viscosity of the selected rotor and specimen, turn the variable speed knob to adjust to the desired speed. The general rule of thumb is to choose a slower rotation speed for high viscosity specimens; Faster rotational speeds are selected for low-viscosity specimens. Improper selection of rotational speed will lead to inaccurate measurement results, and excessive rotational speed may cause eddy currents in high-viscosity specimens, affecting the measured value; Slow rotational speed can lead to long measurement times and unstable measurement results for low-viscosity specimens.
Once the speed adjustment has stabilized, the pointer lever is relaxed, at which point the rotor begins to rotate steadily in the specimen. At the beginning of rotor rotation, it is necessary to closely observe the deflection of the pointer to ensure that the pointer rotates stably and that there is no abnormal jitter or offset.
Timing readings
Timing – Start the stopwatch to keep time at the same time that the rotor begins to rotate steadily. According to the characteristics of different paint samples, choose the appropriate rotation time, generally 60s or 120s. This timing was chosen to allow the rotor to reach sufficient hydrodynamic equilibrium in the specimen to ensure that the measured viscosity values accurately reflect the viscosity properties of the coating. For some specialty coatings, such as thixotropic coatings, a longer rotation time may be required to observe how the viscosity changes over time.
Reading – After a predetermined rotation time has been reached, the reading is taken when the hand is stable and no longer oscillating or when it oscillates slowly and evenly. When reading, the line of sight should be perpendicular to the dial to avoid reading errors caused by deviations in viewing angles. If the hand stays between two scales, it should be read according to the principle of rounding.
Calculation results
The reading of the hand on the dial must be multiplied by the coefficient of the rotor to arrive at the actual measured viscosity value, which is calculated as η = Ka. Among them, η represents the absolute viscosity in mPa・s; K represents the coefficient, different specifications of the rotor correspond to different coefficients, the specific value can refer to the manual or related technical data attached with the instrument; α indicates the reading that the pointer is pointing at, i.e., the angle at which the pointer is deflection. In the calculation process, to ensure the accuracy of the coefficients and readings, the error of any one data can lead to the deviation of the final calculation result.
Instrument description
There is a specific relationship between the spindle, rotational speed and measuring range of some Rotational Viscometers, and different spindle and rotational speed combinations correspond to different measuring ranges. In the actual measurement, the operator needs to choose the combination of rotor and speed according to the estimated viscosity of the specimen to ensure that the measured value is within the selected range. If the measured value is out of range, it may result in inaccurate measurement results or even damage to the instrument.
There are also Rotational Viscometers that are more intelligent and can automatically zero and determine the test range, greatly simplifying the operation process. Its wide range of rotor sizes and speed options allow it to be adapted to a wide range of viscosity ranges. The LCD display displays a number of real-time information, such as viscosity, rotational speed, rotor number, test temperature, etc., so that the operator can keep track of the key parameters during the measurement process. In addition, the instrument is equipped with an interface that can be connected to a printer and a computer, and the measurement data can be further analyzed, processed and stored through special software, providing more comprehensive data support for coating development and quality control.
Reference standards
In the process of determining the viscosity of coatings with a Rotational Viscometer, the relevant industry standards can be consulted. The standard makes detailed provisions and instructions on the use of Rotational Viscometers, operation procedures, data processing, etc., and provides guidance for ensuring the accuracy, reliability and consistency of measurement results. At the same time, in actual operation, relevant standards and methods should be flexibly used in combination with the instruction manual of the instrument and the specific characteristics of the paint sample to obtain accurate and effective measurement results.
Precautions
It is important to estimate the viscosity of the sample before the formal measurement. If it is not possible to estimate it, you can assume that the viscosity of the sample is high, and test it with a rotor from small to large (volume) and a speed from slow to fast to find the appropriate measurement conditions step by step. This avoids inaccurate measurement results or even damage to the instrument due to the selection of the wrong rotor and speed.
During the measurement, the "torque" value displayed by the instrument should be closely observed. When the value is between 15% and 80%, the read viscosity value is valid. If the torque value is too low, it means that the rotor is less resistant in the specimen, which may be due to the low viscosity of the specimen or the excessive rotor selection; If the torque value is too high, it may be that the viscosity of the specimen is high or the rotor is too small, and it is necessary to switch the measuring range in time, select the appropriate rotor and speed to re-measure.
At the end of each measurement, the rotor and container should be cleaned in a timely manner. For the rotor, it can be cleaned with a suitable solvent (such as ethanol, acetone, etc., selected according to the nature of the coating), and then wiped dry with a clean soft cloth to avoid the residual paint affecting the accuracy of the next measurement. Calibrate and maintain the instrument regularly, check whether the performance indicators of the instrument are normal, and ensure that the instrument is always in good working condition.
summary
The use of a Rotational Viscometer to determine the viscosity of a coating should be made clear that it is suitable for the quality control of anti-corrosion coatings, absolute viscosity measurement, and other scenarios. In the measurement process, from the preparation of instrument materials, the implementation of measurement methods, to the application of instrument characteristics, standard reference and precautions, each link is crucial. Select the appropriate rotor and speed, accurately control the temperature, timing and reading, calculate the results according to the formula, and observe the torque value to ensure the validity of the data. At the same time, regular maintenance of instruments and operation with reference to standards and specifications can obtain accurate and reliable coating viscosity data, which provides strong support for coating production and quality optimization.
