Determination of viscosity of hottest coatings

2022-08-07
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Method for measuring viscosity of coating

method for measuring viscosity of coating

march 27, 2019

when the coating moves in laminar flow under the action of external force, internal friction will be generated between its adjacent two layers of molecules, resulting in motion resistance of the coating. This characteristic is called the viscosity of fluid, and the viscosity is divided into dynamic viscosity, kinematic viscosity and conditional viscosity

according to different test requirements in the laboratory, different test methods are used to compare and analyze the viscosity of different systems of coatings

1 test method

1.1 stomer viscometer method

stomer viscometer (Fig. 1) is an instrument widely used in the laboratory to measure the viscosity of coatings. It can measure Chinese coatings of different systems, such as acrylic topcoat, epoxy primer, chlorinated rubber paint, epoxy zinc rich primer, polyurethane topcoat, etc. The measurement principle is that when the torque generated by the balance weight mass is equivalent to the viscosity resistance of the coating, the Ku value of the coating can be obtained by looking up the table, and the measurement range is 40 ~ 140 Ku. Immerse the rotor blade in the tested sample until the mark of the rotating shaft, select a suitable weight from 5 ~ 500 g weights and place it on the weight frame, loosen the locking knob, start timing, and stop measuring when the rotating speed is stable at 200 r/min. Due to the upgrading of hardware and software, the degree of automation of the instrument continuously ensures the improvement of the safety of the experiment. Stm-ku2 viscometer (Fig. 2) can directly display the viscosity Ku value and CP value, which facilitates the work and improves the efficiency. In addition, the instrument performs weight calculation and related processing on the collected data, automatically deletes the data deviating from the central area, and improves the accuracy of instrument measurement. Compared with the viscosity moment of the rotating rotor measured by the rotating viscometer, the test results of the measured fluid viscosity resistance on the rotor blade of the stomer viscometer are more repeatable. The advantage of stomer viscometer is that it is easy to operate. After measurement, just remove the blade and clean it. It is convenient to measure the viscosity of large quantities of products at the same time. However, the stomer viscometer has a precise structure and high environmental requirements. The vibration and humid environment have a direct impact on the accuracy of the instrument measurement. The automotive industry is undergoing drastic changes all over the world, and it will also shorten the service life of the instrument

1.2 Engler viscosity method (Tu-1 viscometer method)

Engler viscosity method is applicable to gb/t 266, astmd1665 and ip212 standards. The principle of the cooperation negotiation is to determine the ratio of the time (s) required for 200 ml of liquid to flow out of Engler viscometer (Fig. 3) and the time (s) of distilled water under the same conditions at the temperature of 20 ℃, and the unit is Engla degree. Engler viscosity is the relative conditional viscosity. According to the test requirements, set up the test instrument, adjust the level of the viscometer, subject to the tangency between the three horizontal support nails in the inner pot of the viscometer and the liquid level, pour in the measured liquid, and keep the constant temperature to 20 ℃. At the beginning of the test, the temperature of the digital temperature controller can be set slightly higher, and the stirring in the outer pot can be rotated at the same time to speed up the constant temperature process. While pulling out the stopper, press the stopwatch to count the time. When the 200 ml scale of the receiving bottle is reached, stop the timing. At 20 ℃, the standard water value is (51 ± 1) s, and the thermometer graduation is 0.5 ℃. The temperature control system of Engler viscometer can set different temperatures to measure the viscosity of the coating according to the test needs, but its disadvantage is that the test device is complex to build and the cleaning is relatively troublesome, especially for products with high viscosity, Engler viscosity method is not suitable

1.3 tu-4 viscometer method

tu-4 viscometer method is applicable to coating products with an outflow time of less than 150 s. Tu-4 viscometer (Figure 4) has a capacity of 100ml. When the coating is filled in the container, the viscosity of the coating is measured by the time it flows out of the bottom standard hole, in S. the measured value multiplied by the correction factor K is the conditional viscosity of the coating. The error of two parallel tests shall not exceed 0.5 s. After the test, the residual liquid, especially the outflow hole, shall be cleaned to ensure the smoothness of the inner wall of the cup. Tu-4 viscometer method is usually used to measure low viscosity coating products such as floor varnish and wood coatings. Tu-4 viscometer has the advantages of simple structure and easy operation. The test data obtained by tu-4 viscometer can be converted with the results of Engler viscometer. However, if the cleaning is not complete, the outflow hole is easy to be blocked, which will lead to inaccurate measurement results

Figure 4 xnd-1 tu-4 viscometer

1.4 rotary viscometer method

rotary viscometer (Figure 5) can measure the absolute viscosity of 10 ~ 100000 MPa · s liquid through the combination of different rotors and rotating speeds. The measuring range refers to (Table 1). The rotary viscometer can be used to measure the resin viscosity and Ti value of epoxy zinc rich primer, as well as to control the viscosity of intermediate organic bentonite pre glue. When measuring the liquid with unknown viscosity, operate according to the principle of rotor from small to large and rotating speed from slow to fast to prevent the synchronous motor from burning. When the reading pointer is stable in the 30 ~ 90 graduation range, press the pointer locking lever and motor switch, directly read the value on the dial, and multiply it by the coefficient on the coefficient table. The result is the absolute viscosity (MPA · s) of the measured sample. If the measurement range is exceeded, the measurement can be completed by changing the rotor and rotating speed. There are four rotating speeds: 6 r/min, 12 r/min, 30r/min and 60 r/min. During the measurement, the lifting screws shall be fixed to prevent the motor from loosening and falling. When loading and unloading the rotor, be careful. After the measurement, clean the rotor 9 and the testing machine according to the national standard gb/t3803 ⑵ 002 inspection of pendulum impact testing machine, put them in a special box for storage, and install the Yellow ferrule at the connection of the rotating shaft at the lower part of the motor to protect the sensitivity of the spring spring spring. The rotary viscometer has a large range, so it is widely used in many fields, such as coatings, oils, foods, pharmaceuticals, dyes, inks, adhesives, cosmetics, etc. The rotary viscometer has high requirements for the smoothness of the rotor of the working part, so corrosive liquid can not be measured with it

Figure 5 NDJ-1 rotating viscometer

1.5 falling ball viscosity method

its principle is that the free falling speed of the steel ball is inversely proportional to the viscous force of the liquid. The greater the viscosity, the longer the falling time of the steel ball. Since bubbles are easily generated in high viscosity fluid, there will be deviation in the measurement results. Parallel test shall be conducted to reduce the error. The falling ball viscometer (Fig. 6) is used in the laboratory to measure the viscosity of resin for coating raw materials. The disadvantages of falling ball viscometer are also obvious, because it is necessary to observe the state of steel ball. Usually, it can only measure the transparent Newtonian liquid, but it is not applicable to the measurement of the viscosity of opaque liquid

Figure 6 falling ball viscometer

1.6 Yantian viscosity cup method

is widely used in field measurement because of its convenient carrying and simple operation. During the measurement, immerse the Iwata cup (Fig. 7) into the fluid, and the fluid does not cross the upper end of the Iwata cup, lift the cup, and press the stopwatch to count. When the fluid has the first fracture, stop the timing, and make three measurements at the same time. The average value is the fluid outflow time. The rock field method is applicable to the fluid with an outflow time of 20 ~ 105 s, and the result data shall indicate the ambient temperature. The measuring principle and structure of Iwata viscosity cup are exactly the same as tu-4 cup, but the volume is smaller. It is one of the necessary tools for on-site technical service. But its disadvantage is that it can not measure the high viscosity value of the thick primer

2 converted absolute viscosity of viscosity unit, unit PA · s, 1.0 PA · s = 10 P = 1000 CP

kinematic viscosity, unit m2/s, =/, is the density of the fluid at the same temperature (kg/m3)

Engler viscosity et, unit (°), et = t/k20, t is the time required for the test solution to flow 200 ml from the Engler viscometer at t ℃, unit s. K20 is the water value of viscometer at 20 ℃

viscosity of coated -4 cups, unit m2/s, = (T4 - 6.0)/0.223, T4 is the measured fluid outflow time of coated -4 cups, s

the test data obtained by different viscosity measurement methods can also be converted through tables. See Table 2 for details

Table 2 conversion of various viscosity standards. Temperature is an important factor affecting viscosity. Generally, the agreed test temperature is 25 ℃. In addition to sending the viscometer to the local metrology Bureau for compulsory inspection every year, the viscosity standard silicone oil with stable properties can also be used for calibration to keep the viscometer in good working condition

Wang Fei (COSCO Kansai coating Chemical Co., Ltd., Shanghai 201907)

attachment: conversion table of common viscosity cups

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