Under the action of external force, when the coating moves in laminar flow, it generates internal friction between its adjacent two layers of molecules, so that the coating produces motion resistance, which is called the viscosity of the fluid, and the viscosity is divided into dynamic viscosity, kinematic viscosity and conditional viscosity.
This paper mainly compares and analyzes the viscosity of coatings of different systems using different test methods for different test requirements in the laboratory.
1 Test Method
1.1 Stormer viscometer
The Stormer viscometer (Fig. 1) is a widely used instrument for measuring the viscosity of coatings in the laboratory, and can measure different systems of coatings such as acrylic topcoats, epoxy primers, chlorinated rubber paints, epoxy zinc-rich primers, and polyurethane topcoats. The measurement principle is to balance the torque generated by the weight mass and the viscosity resistance of the paint, etc., and the KU value of the paint is obtained by looking up the table, and the measurement range is 40~140 KU. Immerse the rotor paddle into the measured sample until the shaft mark, select the appropriate weight from 5~500 g weights and place it on the weight holder, release the locking knob, start the timing, and stop the measurement when the rotation speed is stable at 200 r/min. Due to the upgrade of hardware and software, the degree of automation of the instrument continues to improve, the STM-KU2 viscometer (Fig. 2) can directly display the viscosity KU value and CP value digitally, which facilitates the work and improves the efficiency. Compared to the viscous moment of the rotating rotor when measured by a rotating viscometer, the test results of the viscosity resistance of the measured fluid on the propeller of the Stormer viscometer are more repeatable. The advantage of Stormer viscometers is that they are easy to operate, and after the measurement, the paddles can be removed and cleaned, which can facilitate the simultaneous viscosity determination of large quantities of products. However, the structure of the Stormer viscometer is precise, and the environment is relatively demanding, and the vibration and humid environment have a direct impact on the accuracy of the instrument's measurement, and will also shorten the service life of the instrument.
Figure 1 QNZ Stormer viscometer
Fig. 1 QNZ Stormer Viscometer
Fig.2 STM-KU2 Stormer viscometer
Fig. 2 STM-KU2 Stormer Viscometer
1.2 En's viscosity method (Tu-1 viscometer method)
The En's viscosity method is applicable to GB/T 266, ASTMD1665 and IP212 standards. The principle is the ratio of the time (s) required for 200 mL of liquid outflow from the Engela viscometer (Fig. 3) to the outflow time (s) of distilled water under the same conditions at 20 °C, measured in Engela degrees. En's viscosity is the relative conditional viscosity. According to the test requirements, set up the test instrument, adjust the level of the viscometer, take the tangent between the three horizontal nails in the inner pot of the viscometer and the liquid level, pour the measured liquid, and keep the temperature to 20 °C. At the beginning of the test, the temperature of the digital thermostat can be set slightly higher, and the stirring in the outer pot can be turned at the same time to speed up the thermostatic process. At the same time as the stopper is removed, the stopwatch is pressed to stop when the 200 mL scale of the receiving bottle is reached. At 20 °C, the standard water value is (51±1) s, and the thermometer index is 0.5 °C. The temperature control system of En's viscometer can be set to different temperatures according to the needs of the test to measure the viscosity of the coating, but its disadvantage is that the test device is more complicated to build, and the cleaning is relatively troublesome, especially the products with greater viscosity are not suitable for En's viscosity method.
Fig.3. WIN-1A En's viscometer
Fig. 3 Engler Viscometer
1.3 Tu-4 viscometer method
The coating-4 viscometer method is suitable for coating products with an outflow time of less than 150 s. The COATING-4 viscometer (Fig. 4) has a capacity of 100 mL, and when the coating fills the container, the time it takes to flow out of the standard hole at the bottom is used to determine the viscosity of the coating in s, multiplied by the correction factor K, which is the conditional viscosity of the coating. The error of two parallel tests should not exceed 0.5 s. After the test, the residual liquid should be cleaned, especially the outflow hole, to ensure the finish of the inner wall of the cup. The coat-4 viscometer method is commonly used to measure low-viscosity coating products such as floor varnishes and wood coatings. The advantage of the TU-4 viscometer is its simple structure and easy operation. The test data obtained by the Tu-4 viscometer can be converted with the results of the En's viscometer. However, if cleaning is not effective, the outflow hole is easy to clog, which can lead to inaccurate measurement results.
Fig.4. XND-1 Tu-4 viscometer
Fig. 4 XND-1 Cup 4 Viscometer
1.4 Rotational viscometer method
The Rotational Viscometer (Fig. 5) can measure 10~100 000 mPa · The absolute viscosity of the liquid. The range is referred to (Table 1), and the Rotational Viscometer can be used to measure the viscosity of resins and the TI value of epoxy zinc-rich primers, as well as for the viscosity control of intermediate organobonite preglues. When measuring the liquid of unknown viscosity, operate according to the principle of rotor from small to large, speed from slow to fast to prevent the synchronous motor from burning, when the reading pointer is stable in the 30~90 index range, press the pointer locking lever and the motor switch, directly read out the value on the dial, and then multiply the coefficient on the coefficient table, and the result is the absolute viscosity of the measured sample (mPa · s). If the measurement range is exceeded, the measurement can be completed by changing the rotor and rotational speed, which are available in four types: 6 r/min, 12 r/min, 30 r/min, and 60 r/min. During the measurement process, the lifting screw should be fixed to prevent the motor from loosening and falling. When loading and unloading the rotor, it is necessary to be careful and careful, after the measurement, clean the rotor and store it in a special box, and install the yellow ferrule at the shaft connection at the lower part of the motor to protect the sensitivity of the spring hairspring. The rotational viscosity has a wide range of measurement range, so it is widely used in many fields such as coatings, oils, foods, medicines, dyes, inks, adhesives, cosmetics, etc. Rotational viscometers have high requirements for the finish of the rotor of the working part, so corrosive liquids cannot be measured with it.
Fig.5. NDJ-1 Rotational Viscometer
Fig. 5 NDJ-1 Rotary Viscometer
1.5 Falling ball viscosity method
The principle is that the free fall speed of the steel ball is inversely proportional to the viscous force of the liquid, and the greater the viscosity, the longer the steel ball will fall. Because bubbles are easy to form in high-viscosity fluids, the measurement results will be biased, and parallel tests should be done to reduce the error. A falling ball viscometer (Fig. 6) is used in a laboratory to determine the viscosity of resins used in coating raw materials. The disadvantage of the falling ball viscometer is also obvious, because it is necessary to observe the state of the steel ball, and usually only transparent Newtonian liquids can be measured, and the viscosity of opaque liquids is not applicable.
Fig.6. Falling ball viscometer
Fig. 6 Multipoint Falling Ball Viscometer
1.6 Iwata Viscosity Cup method
Because of its easy to carry and simple operation, it is widely used in on-site measurement. When measuring, immerse the Iwata cup (Fig. 7) in the fluid and submerge the fluid over the top end of the Iwata cup, raise the cup, press the stopwatch at the same time, and stop the timing when the fluid breaks for the first time, and take three measurements at the same time, with the average time of fluid outflow. The Iwata method is suitable for 20~105 s outflow time fluids, and the result data should indicate the ambient temperature. The measuring principle and structure of the Iwata Viscosity Cup are exactly the same as those of the Tu-4 cup, but the size is more compact, making it one of the tools required for on-site technical service. However, the disadvantage is that it cannot measure thick-applied primer coatings with high viscosity values.
2 Conversion of viscosity units
Absolute viscosity in Pa · s，1.0 Pa · s＝10 P＝1 000 cP；
kinematic viscosity, unit: m2/s, =/ , is the density of the fluid at the same temperature (kg/m3);
En's viscosity Et, unit (°), Et=t/K20, t is the time required for the test solution to flow out of 200 mL from the En's viscometer at t °C, unit s. K20 is the water value of the viscometer at 20 °C;
Coating-4 cup viscosity, unit m2/s, = (T4-6.0)/0.223, T4 is the fluid outflow time s measured by Tu 4 cup;
The test data obtained by different viscosity determination methods can also be converted by table, see Table 2 for details.
Table 2 Conversion of various viscosity standards
Table 2 Viscosity Standards Conversion
epilogue
The measured values obtained vary depending on the measurement method, so it is necessary to agree on the test method with the supplier and customer. Temperature is an important factor affecting viscosity, and the test temperature is usually 25 °C. In addition to sending the viscometer to the local metrology bureau for compulsory inspection every year, the viscometer can also be calibrated with stable viscosity standard silicone oil, so that the viscometer is in good working condition.
