A critical part of any coatings formulation is ensuring that the coating will be free of inherent defects, including pigment flooding and floating. Waterborne formulations represent some unique challenges due to multiple factors, including the high surface tension and polarity of water that does not contribute to the wetting of most pigment and filler particles. In this article, I’ll define some important considerations in formulating waterborne paints to avoid pigment flooding and floating.
Flooding, Floating, Surface Tension, Bernard Cells, Flocculation and Agglomeration
Floating describes a mottled, splotchy appearance on the surface of a paint film. It is most apparent in coatings colored with two or more pigments and is a result of the horizontal separation of different pigments. Flooding is the phenomena observed when the surface color of an applied film is uniform but is darker or lighter than it should be. This is attributed to a vertical separation of different pigments in the film.
Surface Tension results when the force that occurs in a liquid at the interface differs from the forces within the liquid. Thus, surface tension is caused from the surface molecules having a higher free energy than those molecules in the bulk of the liquid.
Surface tension differential can cause a convection current resulting in a regular hexagonal surface pattern called Benard Cells. A hexagonal Benard Cell pattern results in smaller, more mobile pigment particles (smaller, less dense) being deposited on the perimeter and the less mobile particles (larger, more dense) remaining away from the perimeter.
Flocculation is the recombination of dispersed pigment particles that were not properly stabilized in the pigment dispersion. Flocculation is undesirable at it detracts from hiding and color development. Flocculation is reversible by applying a low degree of shear. In figure 2, the phthalocyanine (EU) blue pigment is flocculated. Upon rubbing with a finger, the deeper blue color returns.
Pigment agglomeration is defined as pigment particles that are clumped together without sufficient vehicle or wetting agents present between pigment particles. When agglomeration occurs, extensive shear and attrition forces are usually necessary to reinstate a stabilized pigment dispersion.
Remedial Actions to Overcome Floating and Flooding in Waterborne Paints
Pigment dispersion in aqueous media uses the same principles as in organic solvent media such as proper wetting, pigment dispersion and stabilization. However, the surface tension of water and high polarity make it more problematic in wetting low polarity pigments. In many cases, water interacts aggressively with the surface of the pigment, destabilizing the dispersant on the pigment surface. Overcoming flooding and floating starts with selecting pigments that are free of fines. Also, many pigment manufacturers supply surface treated pigments to avoid flooding and floating.
Secondly, ensure that the pigment dispersion is uniform and stabilized (elimination of pigment flocculation of one pigment, with the exclusion of other pigments).
Thirdly, the use of suitable wetting agents/surfactants help to ameliorate differences in polarity and surface tension between pigments that contribute to flooding and floating. Inorganic pigments such as iron oxides (EU), titanium dioxide (EU), calcium carbonate (EU) and many other filler pigments have a polar surface and are easily wet by water. However, water alone normally does not stabilize the pigment dispersion against flocculation, so they require a surfactant to wet and stabilize the dispersion. Also, many pigment manufacturers supply surface treated pigments to avoid flooding and floating. For some organic pigments that have a surface with low polarity, many manufacturers modify the surface with a layer of inorganic oxide to provide increased polarity to improve wetting in aqueous-based systems.
Fourthly, the use of an appropriate thixotrope (EU) helps to build sufficient viscosity and a network structure that discourages pigment separation. However, one must be sure that there is acceptable compatibility between the thixatrope and dispersant (EU).
Overcoming flooding and floating in solvent-borne paints primarily involves utilizing a suitable pigment dispersant and the elimination of Benard Cell formation with the addition of a surface control agent.
A final consideration affecting the stability of an aqueous paint pigment is the pH of the pigments. For example, if a low pH carbon black pigment is used in the pigment dispersion without the use of a suitable wetting agent in an anodic aqueous based paint (pH normally > 8), longer term instability can result as the neutralizing amine on the resin backbone can migrate to the acidic pigment.
Care must be taken while reducing a latex paint with water because floating can occur (slow addition of water with proper mixing and ensure against over reduction). This is due to a shift in the equilibrium between the dispersed pigment particles and water, resulting in a decrease in the amount of stabilizing dispersant on the pigment.
There are numerous suppliers listed in UL’s Prospector website with a wide variety of wetting agents (EU), pigment dispersants (EU), surfactants (EU), thixotropes (EU) and pigments to meet your requirements in both solvent and waterborne coatings.
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