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Coatings Technology Handbook P02

The shear viscosity is defined as the ratio of the shear to the shear strain rate, at the strain rate of interest. Although the viscosity is usually quoted as a number without reference to the strain rate, it is really a function of strain rate. The strain rate dependence and, in certain situations, the time dependence, of the viscosity need to be determined if a meaningful correlation is to be made with coating phenomena. | Coating Rheology 2-3 FIGURE 2.3 Typical pendant-drop profile. de Pg-e H 2.2 where de is the maximum equatorial diameter of the pendant drop and H is a correction factor that depends on the shape of the drop H is related to a measurable shape-dependent factor S which is defined by s d de 2.3 where ds is the diameter of the pendant drop in a selected plant at a distance de from the apex of the drop see Figure 2.3 . Tables showing the values of 1 H as a function of S are available.12-14 Recently there have been a number of significant improvements in both data acquisition and analysis of the pendant-drop profiles.15-17 The photographic recording and measurement of the pendant drop are replaced by direct digitization of a video image. The ability to measure the entire drop profile has led to the development of new algorithms for the drop-profile analysis.16 17 2.2.2 Viscosity The shear viscosity is defined as the ratio of the shear to the shear strain rate at the strain rate of interest. Although the viscosity is usually quoted as a number without reference to the strain rate it is really a function of strain rate. The strain rate dependence and in certain situations the time dependence of the viscosity need to be determined if a meaningful correlation is to be made with coating phenomena. In the case of coatings the shear strain rate range of interest extends from about a few thousand reciprocal seconds during spraying for instance down to a hundredth of a reciprocal second following application . A variety of techniques is available to measure viscosity of coating formulations. Some of them are listed in Table 2.2.18 Instruments with a single or undefined strain rate should be avoided in the study 2006 by Taylor Francis Group LLC 2-4 Coatings Technology Handbook Third Edition TABLE 2.2 Some Commercially Available Rheological Instrumentation Name of Instrument Geometries Available Shear-Rate Range Modes Available Weissenberg Rheogoniometer Couette cone and plate Broad