Overflow and Floating in Paints

Flow Phenomenon:

With solvent evaporation, a layer can form on the surface of the paint film that has higher density and surface tension and is at a lower temperature than the underlying layer. Surface tension differences in particular provide the driving force needed to transport paint upward from the lower layers against the effect of gravity. According to flow cell theory, this occurs from statistically divided points with the development of a cell current as shown in Figure 1. After the upper layer with lower solvent content breaks down, solvent-rich paint rises and spreads over the surface (A-A1); As solvent evaporates, viscosity increases, the tendency to spread decreases, and the flowing mass comes under the effect of gravity and currents. In the lower layer, flow returns to the starting point. Meanwhile, another difference in surface tension develops between the surface layer and the underlying mass; The current curves again and is directed upward. As a result of solvent evaporation, the surface of the paint film will be lower in this case (B-B1). When spreading over the surface, the mass again comes under the effect of gravity and currents. This current movement will repeat until the point is reached where viscosity and flow resistance are sufficient to eliminate the forces resulting from energy differences. As previously noted, surface tension differences, supported by density differences, appear to be the main determining factor in the flow condition as described above. Flow will be rendered ineffective by existing diffusion currents. Low viscosity, thick paint films lead to the most prominent cell formation. Pigment particles are undoubtedly dragged along the cell current. If there are differences in mobility, this causes pigment separation. At the interface, accumulation of one of the pigments occurs at the edges of the hexagons. In this case, the question arises which pigment accumulates at the cell edge. The most suitable explanation is that these are locally the least mobile pigment particles. This does not necessarily mean that these are the least mobile particles when being transported to this point. If we consider the example where relative humidity in the air plays a role, it is likely that titanium dioxide is the most mobile within the paint mass, but is strongly reduced at the interface due to water vapor. Furthermore, solvent evaporation may affect one pigment's ability to move more than another. Susceptibility to flocculation is an important part of this. For this reason, it is difficult to say which pigment accumulates at the cell edge at the interface.

Prevention of Flooding and Floating

After discussing the factors affecting flooding and floating phenomena, it is clear that differences in mobility between pigments must be avoided as much as possible. Flow phenomena that increase differences in mobility should also be avoided as much as possible. In paint production methods, it is not possible to achieve all the factors discussed above in such an ideal manner. For example, in pigment selection, color, price, and outdoor durability are extremely influential and determine the selection by these factors. However, density, polarity, and often particle size are important factors. The binder must meet special requirements. If flooding or floating occurs, in most cases the formulation will need to be modified in a way that solves the problem, and all technical specifications are still met. However, this will not always be the case and can also be a very time-consuming activity. For this reason, additives become indispensable tools that can provide a quick and economical solution to an emerging problem.

The following measures can be taken to prevent flooding and floating:

1. Effect of pigment mobility through aggregation or dispersion (deflocculation) Depending on the degree of flocculation, significant effects can be achieved, and in many cases modifying the degree of flocculation appears to be the solution to the problem.

In some cases, it is important that the dispersion be carried out as thoroughly as possible. This can be achieved by the following methods:

• Selection of a resin with very good wetting properties, • When using multiple resins, selection of a compatible resin, • Use of dispersion agents that wet the pigment to the greatest extent; many auxiliary products are available in this field. Since these products are very specific, careful selection is important.   Some floating prevention agents on the market operate according to this principle. With the help of these agents, very good properties can be obtained in preventing flooding and floating. Several floating prevention agents on the market have prevented coflocculation and thereby reduced differences in mobility among various pigments. The most important point here is the correct dosage of the additive; otherwise flooding or floating can increase and multiply. This situation shows that with the use of such agents, not only coflocculation is prevented, but homo-flocculation also occurs. Pigment (binder flocculation) is an undesirable condition because it can lead to pigment accumulation in the interface layer.

2. Effect of Flow Characteristics

If floating problems are observed, flows likely developed within a Benard cell model during paint film formation. Although a small amount of acceptable flooding can occur, if floating occurs, the paint is unsuitable as a result of color differences forming on the paint film surface. If it is possible to prevent Benard cell formation, the floating risk is greatly reduced. Turbulent flows develop more slowly by reducing differences in density and surface tension in the paint layer formed at the interface, depending as much as possible on the underlying paint mass. This can be achieved by selecting solvents with the same density and surface tension as the binder. This ideal condition can never be reached, but sometimes significant improvements can be made. Modification of the solvent mixture composition can result in better balance between evaporation rate and solvent diffusion rate from the lower layer, thus preventing surface tension and density differences in the drying paint layer.

It will not always be possible to modify solvent composition to prevent the flow phenomenon. In this case, surface tension-reducing agents can generally provide a solution. Silicones can be effective from this perspective.

However, these additives must be used with necessary precautions, as problems related to cratering and coating adhesion can arise. Silicone oils have very limited compatibility with most binders. As a result, a silicone oil film forms at the interface showing low surface tension. Consequently, the result is that there is no difference in surface tension between the final coat and the underlying paint mass, and it contributes to the development of a flow movement. Solvent separates from the upper layer and is separated from the paint film. In this case, it will be possible for a current movement to develop between the layer beneath the interface layer and deeper layers. However, this does not lead to color differences in the upper layer. However, in many cases it has been found that flooding rather than floating occurs after silicone oil is applied. This leads to the assumption that depending on the path of solvent exit from the paint film, either flooding or floating will occur. If solvent exits the paint film through turbulent flow, floating occurs; if solvent spreads along the interface layer, flooding occurs. Among auxiliary products used to overcome flooding or floating are on the one hand additives that reduce surface tension and on the other hand additives that support pigment dispersion. These auxiliary products appear to be very effective in many cases. For this reason, it is not surprising that such auxiliary products are used on a large scale to prevent flooding or floating. These types of auxiliary products are provided in the form of specially surface-treated calcium carbonate. The use of calcium carbonate as a carrier ensures good distribution of active ingredients and prevents the formation of unwanted side effects. Figure 3 shows the application of such an auxiliary product.

3. Modification of Structure

Thickeners or thixotropic agents are very effective in reducing the mobility of pigment particles. Because the effects of these additives on flow and gloss cannot always be ignored, in most cases it is impossible to use them. However, in some cases, the use of these agents can be a good solution. M. Namık Kayaalp / Chemical Engineer / Ecelak Boya Kimya Ltd.Şti. References S. H. Bell, JOCCA. August 1952. pages 373-392. J. Ferguson, JOCCA, August 1959. pages 529-542. G. Marwedel. Farbe und Lack, no. 1, 1968 G. Marwedel. Farbe und Lack, no. 5, 1975