Film leveling

Controlled Stress Viscometer. Most rotational viscometers operate by controlling the rotational speed and, therefore, the shear rate. The shear stress varies uncontrollably as the viscosity changes. Often, before the stmcture is determined by viscosity measurement, it is destroyed by the shearing action. Yield behavior is difficult to measure. In addition, many flow processes, such as flow under gravity, settling, and film leveling, are stress-driven rather than rate-driven. 

Solvents are selected such that some will escape relatively quickly from paint films to prevent excessive flow, while others will escape slowly to provide film leveling and adhesion. With typical alkyd coatings, the first 30% of solvent has been observed to evaporate as quickly as the neat solvents, essentially at a constant rate which is dependent on volatility. Later stage evaporation occurred several times more slowly and was rate-controlled by solvent diffusion to the surface of the paint film. The transition point between this behavior was defined as the resin solids content at which the evaporation rate due to volatility equaled that due to diffusion. Transition points have been observed to typically occur at a resin solids content of 40-50% v/v. Thus alkyd paints, normally formulated at 27-40% v/v resin solids, generally exhibit rapid initial solvent-release driven by volatility while high solids coatings (usually 65-75% v/v resin solids) dry solely by a diffusion-controlled process with negligible influence by solvent volatility (Ellis, 1983). 

The cellulose resin may be added at levels of 1 - 5 wt% to improve film leveling and reduce cratering,... 

As seen from Fig. 18 when the interfacially active fractions from Daqing crude oil are used as emulsi-fier, the IFPYV of the interfacial film between jet fuel and distilled water decreases rapidly when the temperature rises from 20 to 30°C, and the IFPYV of the film levels out when the temperature is higher than 30°C. When the interfacially active fractions from Jilin erode are used as emulsifier, the IFPYV of the interfacial film between jet fuel and distilled water increases as the temperature rises from 25 to 4 5°C (see Fig. 19). This phenomenon is similar to the temperature-depen-... 

Surfacers, or undercoats in decorative house painting, are highly pigmented materials containing large quantities of extender (see below). They are easily rubbed smooth with abrasive paper. They provide the body of the paint film, level out minor irregularities in the substrate and must stick well to primer and topcoat. 

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