Wetting, coalescing and hanging of paint rheological phenomena

Coalescence, wetting, leveling, cratering, sagging and slumping are processes strongly influenced by surface tension and viscoelasticity. These are again two important parameters controlling the coating quality and appearance, so their influence on the coating process is discussed in detail.

Figure 2.4 Schematic diagram of good and poor wettability

moisten

Surface tension is an important factor in determining a coating's ability to wet and adhere to a substrate. Using a solvent with a lower surface tension can improve the ability of the paint to wet the substrate. 27 Wetting can be defined quantitatively by reference to a liquid droplet in equilibrium on a solid surface (Fig. 2.4). The smaller the contact angle, the better the wettability. When θ is greater than zero, the liquid completely wets the solid on the surface at a rate that depends on the viscosity of the liquid and the surface roughness of the solid. The equilibrium contact angle of a droplet lying on a satisfactorily smooth, uniform, flat and non-deformable surface is related to various interfacial tensions by Young's equation:

where γlv is the surface tension of liquid in equilibrium with its own saturated vapor, γsv is the surface tension of solid in equilibrium with liquid saturated vapor, and γsl is the solid-liquid interfacial tension. When θ is zero and assuming that γsv is approximately equal to γs (which is usually a reasonable approximation), it follows from Equation 2.5 that for spontaneous wetting to occur, the surface tension of the liquid needs to be greater than that of the solid.

When θ is greater than zero, liquids can also spread and wet solid surfaces, but this requires the application of a force on the liquid.

coalesce

Coalescence is the fusion of molten particles to form a continuous film. This is the first step in powder coating. Factors controlling coalescence are surface tension, radius of curvature, and viscosity of the molten powder. Figure 2.5 shows a schematic diagram of molten powder coalescence. Nix and Dodge28 relate time to consolidation to these factors with the following formula:

where tc is the coalescence time and Rc is the radius of curvature (average particle radius). To minimize coalescing time so that more time can be spent in the leveling phase, low viscosity, small particles and low surface tension are required.

sag and collapse

Coating sagging and slumping are phenomena that occur on inclined surfaces, especially vertical ones. Under the influence of gravity, downward flow can occur, causing sagging or collapse, depending on the nature of the coating fluid. In the case of pure Newtonian or shear-thinning fluids, sagging (shear flow) occurs; Figure 2.6 shows "gravity-induced" flow in a vertical plane. On the other hand, materials with a yield stress exhibit slump (plug flow and shear flow).

Figure 2.5 Schematic diagram of molten powder coalescence

The physics of this phenomenon has been discussed for Newtonian fluids. It has also been extended to other types of fluids, including shear-thinning and viscoplastic fluids. The subsequent treatment is mainly based on these three sources (ie refs 29-31). The parameters of interest in the analysis are the velocity Vo of the flowing material at the fluid-air interface and the resulting depression or slump length s. For the general case of power-law fluids with exponent n, the above quantities can be calculated:

where η0 is the zero shear viscosity and h is the film thickness. The special case of Newtonian fluids is obtained by putting n = 1 into Equation 2.8. The final sag or slump length S is determined by the velocity and the time factor t, which is actually the time interval that the material remains fluid (or the time it takes for the material to solidify). Velocity v0 is inversely proportional to zero shear viscosity. Shear-thinning fluids (n < 1) will exhibit lower sag and slump velocities, all other things being equal. Thus, in general, a Newtonian or shear-thinning fluid sags or slumps under its own weight until its viscosity increases to the point where V0 becomes negligible. However, if certain conditions are met, sagging may not occur at all. One of them is the presence of yield stress. If the yield stress (σy) is greater than the gravitational force pgh, no sag will occur. However, if the coating is sufficiently thick (large h), this condition may no longer be satisfied, and if the film thickness is greater than hs, sagging and slumping may occur, given by

Between h = 0 and h = hs, droop occurs. Velocity can be obtained by substituting (h – hs) for h in Equation 2.7

When h > hs, plug flow occurs (see Figure 2.6).

Wu31 also found that if all these materials have the same zero-shear viscosity η0, the sagging tendency generally increases in the following order: shear-thinning fluid < viscoplastic fluid < Newtonian fluid < shear-thickening fluid. The significance of η0 for viscoplastic fluids is unclear, although it is used in the equations derived by Wu.

For the special case of sprayable coatings, Wu found that a shear-thinning fluid with n = 0.6 can exhibit good sag control without yield stress while maintaining sufficient sprayability.