High-viscosity paints are non-Newtonian fluids, and their viscosity will change under the action of shear force during the flow process, so their actual viscosity values cannot be measured by the outflow method. However, the method of rotating motion can measure the change of its viscosity under different shearing forces. For this purpose, the paint is made to rotate and the stress (ie shear stress) required to make it reach a fixed rotational speed (ie shear rate) is measured, and the ratio of shear force to shear rate is defined as the viscosity of the liquid. This viscosity is called dynamic viscosity. The unit of dynamic viscosity is Pa·s. The instrument used in this determination method is a Rotational Viscometer. Modern Rotational Viscometers come in many forms. In the paint industry, a rotary viscometer is commonly used to measure the viscosity of paint (see Figure 1-9).
Due to the use of professional computer technology in the instrument, it can be used to conveniently measure the stress value required to make the paint reach a certain fixed speed, and automatically convert it into a dynamic viscosity value, so as to know the viscosity of the paint under different motion states. . By measuring the viscosity at several different rotational speeds, draw a flow curve representing the relationship between shear rate and shear stress. Figure 1-10 shows the flow curves of different types of fluids.
According to the characteristics of the flow curve, not only can it be determined which type of Newtonian fluid or non-Newtonian fluid the coating belongs to, but also whether the coating has thixotropy and the size of thixotropy can be analyzed. It can be clearly seen from Figure 1-10 that: 4 is a Newtonian fluid, and the rest are non-Newtonian fluids, of which 1 is a plastic fluid, 2 is a pseudoplastic fluid, and 3 is a dilatant fluid.
The method of using a Rotational Viscometer and drawing a flow curve can not only calculate the dynamic viscosity of the coating, but also judge the size of the thixotropy of the coating. The specific method is to make the coating rotate at a low speed in the Rotational Viscometer and measure the corresponding shear The shear rate and the shear stress value, and then gradually increase the rotational speed (that is, the shear rate), change the rotational speed once at a fixed time interval, and measure the corresponding shear rate and shear stress value respectively, and the results can be drawn in Figure 1 The ABC line in -11, and then measure the corresponding shear rate and shear stress in the manner of gradually decreasing the rotation speed at the same interval time, and obtain the CA line in Figure 1-11. If the three points of ABC form a circular curve, it means that the coating has thixotropy, and the larger the area of the ring, the stronger the thixotropy.
