Spraying shear rate

Figure H1.1.4 A complete flow curve for a time-independent non-Newtonian fluid. r 0 and i , are the viscosities associated with the first and second Newtonian plateaus, respectively. Regions (1) and (2) correspond to viscosities relative to low shear rates induced by sedimentation and leveling, respectively. Regions (3) and (4) correspond to viscosities relative to the medium shear rates induced by pouring and pumping, respectively. Regions (5) and (6) correspond to viscosities relative to high shear rates by rubbing and spraying, respectively.

In processes with very small liquid dimensions and high speeds, such as spraying or coating processes, shear rates can reach up to 105 1/s. If shear thinning or shear thickening materials are used, the viscosity can therefore vary greatly depending on shear rate. 

Hypochlorite hard surface and drain cleaner compositions exhibiting enhanced extensional viscosity are mentioned in U.S. Patents 5,728,665 and 5,916,859. The viscoelastic compositions are intended for use with trigger sprayers and the hexa-decyl amineoxide/organic counterion compositions provide low bleach odor and reduced spray misting. The patent contains extensional viscosity data in support of the claims. Viscosity as a function of shear rate at various Cm diphenyloxide disulfonate concentrations is shown in Figure 4.5. Examples of steady shear and extensional viscosity as a function of shear rate and extensional rate are shown in Figure 4.6 and Figure 4.7. 

In reality, few systems are Newtonian, and some of the other principal rheological profiles are also shown in Figure 5.1. In many cases a Newtonian behavior is not desirable for a formulated product. This can be illustrated by the case of a spray cleaner. A certain minimum viscosity is often required such that the material appears to be concentrated in the bottle. The visual appearance is referred to in this chapter as the apparent viscosity and is generally considered to correspond to a shear rate of the order of 10 sec-1 (reciprocal seconds). If the formulation is Newtonian, then the viscosity will remain the same even at the relatively high shear rates corresponding to spraying (Figure 5.2). This is not desirable, as the spray pattern obtained varies considerably with the viscosity of the fluid in the spray nozzle, and better atomization is observed when the viscosity is low. Consequently, an ideal profile for such a formulation is one in which the viscosity decreases as... 

The use of antifoams is of special importance for the preparation of water-based paints [202]. Although foam problems also occur in textile and paper industries, there are some special features for paints. First, foam is formed in machines with high and medium shear rates, such as high-speed mills. The presence of a considerable foam volume inhibits the process and considerably reduces the useful load volume of the machine. Besides, foam inhibits the operation of the filling equipment. Problems also occur when paints are applied to a surface, especially using effective sprays, dipping methods and foam-curtain devices. The main reason of foam formation are surfactants used to stabilize aqueous latex dispersions. Thus, nonionic surfactants, instead of anionics, are preferred as they form less foam of low stability. 

In the dry procedure, the silane is sprayed onto well-agitated filler. In order to obtain maximum efficiency, uniform silane dispersion is essential through the shear rates provided by the mixing equipment, for example, kneaders, Banbury, Hauschild, Primax, and Plowshare mixers, two-roll mills, or extruders [19b]. Most important commercial silane coating processes are continuous and have high-throughput rates. Silane addition control, dwell time, and exact temperature control within the system are essential. All parameters need adjustment depending on the type of silane employed. 

One of the problems with many anti-drift agents is their shear degradation. At the high shear rates involved in spray nozzles (which may reach several thousand s ) the polymer chain may degrade into smaller units, resulting in a considerable reduction of the viscosity. This will reduce the anti-drift effect. It is, therefore, essential to choose polymers that are stable to ffie high shear rates involved in a spraying process. 

At the outer edge of the spray cone, where large velocity gradients and thus high shear rates in the gas flow can be found, complex interactions between gas and droplets take place that may lead to the formation of characteristic cluster structures (Fig. 19.16 left). Two mechanisms that are closely related have been identified for the formation of droplet clusters in the spray. Through the first mechanism thin filamentary clusters can be formed in the vicinity of shear layer vortices by centrifuging. Clusters are formed spontaneously and the thin filamentary structures concentrate around void zones. 

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