After a wet or fluid coating is applied to a substrate, its viscosity begins to increase. This increase is due to several factors; Figure 2.8 depicts some of the more important ones. The magnitude of the viscosity increase is typical of low solids solution coatings due to various factors shown in Figure 2.8. Of course, the relative strength of solution coatings and powder coatings with high solids content is different. In powder coatings, the main increase is due to freezing as the temperature approaches the melting point.
The measurement of viscosity increase is important because it lets us know when various phenomena take place before solidification. The leveling and sagging phenomena discussed above can only occur if the material remains in a fluid state; as the viscosity increases, these processes become less and less smooth due to the reduction in the heave and leveling velocities according to Equations 2.7 and 2.11. obvious. In fact, the setting time t used in Equation 2.8 and the leveling time used in Equation 2.11 can be estimated from the measured viscosity versus time. Generally, if the viscosity is above about 100,000 P, leveling and sagging occur to a negligible extent.
Experimentally, the viscosity increase can be monitored using the oscillation technique (see Section 2.2.2). This method is preferred because measurements can be performed under conditions of low shear amplitude, which approximate the conditions after coating application. In addition, the freezing point can be estimated by the measurement of the elastic modulus. In order to simulate the state after coating, the oscillatory measurements should be preceded by shearing at a rather high rate, corresponding to the applied method. 36 In such experiments, the average amplitude of the torque/stress wave increases with time after cessation of ramp shear. Although it is not easy to calculate viscosity change from magnitude change, it is possible to estimate. 37 Alternatively, only the magnitude of the stress can be used for the correlation. Dodge36 found a correlation between the level of viscosity after application and the degree of leveling quantified by a special technique he developed. Another method38 that has been used involves rolling spheres off a coating applied on an inclined surface. When properly calibrated with a Newtonian fluid, the velocity of the sphere can be used as an indicator of viscosity. This approach can be very misleading since flow is not viscous and does not work for non-Newtonian fluids. A more acceptable technique is to use simple shear, stretching the sheet at a constant speed over a horizontal coating.
Figure 2.8 Schematic diagram of paint viscosity during application and film formation
