At strain rate, shear viscosity is defined as the ratio of the shear ratio to the shear strain rate. Although viscosity is often quoted as a number without reference to strain rate, it is actually a function of strain rate. Strain rate dependence and, in some cases, time dependence are required to determine viscosity if a meaningful correlation of coating phenomena is desired. In the case of coatings, the shear strain rates range from about the inverse of the thousands. Seconds (eg during spraying) down to the penultimate hundredth (following applications). Various techniques are used to measure the viscosity of paint formulations. Some of them are listed in Table 2.2. Eighteen instruments with single or undefined strain rates should be avoided in the study.
If coating phenomena are to be used, viscosity, meaningful correlation. Need to measure over a wide range of strain rates. The most accepted technique for determining the strain rate dependence of viscosity is to use . Constant strain rate test in torsional shear test. This can be done with cones and plates. low rates) or concentric cylinder geometry (at higher prices). However, oscillatory, or dynamic , measurements are also commonly used for the same purpose. Assuming that the shear strain rate frequency is an equivalent quantity, the complex viscosity is equal to the steady state constant. Rate viscosity (i.e. the Cox-Melz rule is valid). However, the applicability of the Cox-Melz rule is not versatile and its validity needs to be demonstrated before dynamic measurements. instead of steady state. The capillary technique, employed by several commercial instruments - is not suitable for general coating research as it is more suitable for measuring viscosity. at higher strain rates.
