Floating and flooding are among the most common film defects encountered in the production and application of coatings. They not only destroy the uniformity and decorative appearance of the coating film, but in severe cases they also weaken the protective performance of the film, directly affecting product quality and customer experience. Understanding the physical and chemical mechanisms behind these defects is the first step toward a systematic solution.
1. What Are Floating and Flooding?
Flooding (Blooming) — Horizontal Distortion
Different pigments separate and diffuse horizontally across the surface of the wet film, forming irregular streaks, spots or cloudy floral patterns that destroy the flatness and uniformity of the coating. The visual result is patchy and messy. Root cause: drying convection combined with pigment flocculation.
Floating — Vertical Migration
In multi-pigment systems, one or more pigments migrate vertically — either enriching at the film surface or settling toward the bottom — resulting in uneven color between the upper and lower layers and a surface hue that deviates from the standard (grey paint turns bluish, white paint turns yellowish, black pigment floats up). Root cause: differential rise/settle speed of pigments.
2. Physical-Chemical Mechanisms
2.1 The Core Driving Force: Bénard Cells
When the solvent in a wet coating film evaporates, the surface loses heat, becomes cooler and denser, and sinks, while the warmer liquid below rises to replace it — creating continuous circulating convection. This macroscopic convection appears as a regular honeycomb structure on the film surface, known as Bénard cells, and is the physical mechanism that triggers pigment migration.
The key influence: this process acts like an invisible “liquid conveyor belt”, driving pigment particles of different sizes and densities to move at different speeds. The faster the solvent evaporates (for example, in solvent-borne systems), the stronger the convection and the more significant the pigment separation, floating and flooding.
2.2 Root Causes: Differential Pigment Movement
Reason 1 — Disparities in particle size and density. Light, small particles (such as organic blue pigments) tend to rise with the convection to the surface and float; heavy, large particles (such as titanium dioxide) settle or lag under gravity and stay at the bottom, leading to upper/lower separation.
Reason 2 — Mismatch of surface and interfacial properties. The Marangoni effect: surface tension gradients induce directional fluid flow, carrying pigments to migrate and accumulate. HLB imbalance: when the hydrophilic-lipophilic characteristics of the pigment and the system do not match, interfacial repulsion arises and causes spontaneous separation.
2.3 Key to Stability: Pigment Dispersion and Flocculation
Under ideal conditions, dispersants anchor tightly to the pigment particle surface through anchoring groups, and their solvated chains extend outward to form a dense protective layer. This barrier — through steric hindrance and electrostatic repulsion — effectively blocks inter-particle attraction, keeping pigments uniformly suspended and the system stable.
Once dispersion fails, pigment particles lose their protection and rapidly aggregate under van der Waals forces to form flocculates. This not only changes the effective particle size and density distribution of the pigment, but also destroys the uniformity of the suspension, making pigments highly prone to separation during convection drying — eventually causing severe floating and flooding.
3. Why Water-based Coatings Are More Prone to Floating and Flooding
3.1 During Surface Drying
1. Water has a much higher surface tension than organic solvents, creating a larger surface tension gradient and stronger convection;
2. A single thick wet film leaves more space for convection;
3. The film dries slowly — both surface drying and through drying are slower;
4. Water has a high latent heat of evaporation, creating significant temperature differences inside the wet film and intensifying convection;
5. Mismatched evaporation rates of co-solvent and water: the surface loses co-solvent first, and the polarity difference between the surface and bottom layers grows, intensifying pigment separation.
3.2 During Through-Drying
1. Film formation occurs by coalescence of latex particles; the resin is not a continuous solution and binds pigments only weakly.
2. Excessive free surfactants — excess wetting agents and emulsifiers — can: 1) enlarge the surface tension gradient of the wet film and strengthen Bénard convection; 2) displace the dispersion layer on pigment surfaces and induce flocculation.
3. Emulsifiers inherent to the latex migrate to the surface during film formation, changing the local HLB and surface tension and driving pigment migration.
3.3 Rheology System
1. Low-shear viscosity is too low: the wet film remains in a readily flowable state for a long time, giving convection ample time to transport pigments;
2. Lack of rapid structure recovery: during slow water evaporation, a network structure cannot be built quickly enough to “fix” pigment positions;
3. Thickener selection conflicts: HEUR hydrophobic associative thickeners capture free surfactants from the system, disrupting the pigment surface dispersion balance and inducing flocculation and floating; alkali-swellable thickeners alone provide insufficient thixotropy, and poorly designed combinations carry high risk.
3.4 Pigment System
1. Surface polarity differences. Inorganic pigments are highly polar (hydrophilic); organic pigment backbones are hydrophobic (lipophilic). In the same water-based system, the two pigment families “prefer different environments” and therefore move differently.
2. Selective flocculation — the No. 1 culprit in tinting floating. White paints often use a single high-HLB anionic polyelectrolyte dispersant to stabilize TiO2, while organic pigment color pastes usually combine anionic and non-ionic dispersants. When mixed, the additives compete for adsorption sites on pigment surfaces: part of the dispersant desorbs from some pigment surfaces, causing slight flocculation. The flocculated particles change in size and density, their migration speed changes, and they float rapidly under convection.
3. Overall HLB imbalance. An unsuitable overall system HLB widens the wetting gap between the two pigment families: if the system HLB is too high (too hydrophilic), hydrophobic organic pigments have poor compatibility and tend to float up (blue, green, yellow floating); if the system HLB is too low (lipophilic-leaning), polar TiO2 is poorly wetted and tends to float up (white floating).
4. Systematic Solutions
4.1 Dispersant Selection and Combination — the Most Effective Measure
Do not use a high-HLB polyelectrolyte anionic dispersant alone in the base white paint. The recommended combination is an anionic dispersant (to stabilize TiO2/inorganic pigments) plus a medium-HLB block copolymer non-ionic dispersant. This narrows the hydrophilicity gap between TiO2 and organic pigments, balances the overall HLB and reduces selective flocculation.
For organic pigments (phthalocyanine, carbon black), increase the dispersant dosage moderately — an extra 0.3%–0.8% dispersant is recommended for carbon black and phthalocyanine.
Matching principle for purchased color pastes: keep the ionic system of the base paint and the color paste consistent; avoid using large amounts of highly non-ionic color paste in an anionic base paint.
4.2 Wetting Agent Adjustment of Surface Tension and HLB
If blue/green floating occurs (organic pigments floating up), the overall system HLB is too high and too hydrophilic. Adjustment: add a small amount of medium-to-low HLB non-ionic wetting agent (0.1%–0.3%); do not overdo it, to avoid reduced water resistance and foaming.
If white floating occurs (TiO2 floating up), the system HLB is too low and lipophilic-leaning. Adjustment: properly supplement hydrophilic wetting agents and reduce strongly hydrophobic additives.
Caution: adding large amounts of low-surface-tension silicone wetting agents further widens the surface tension gradient and aggravates Bénard-cell flooding.
4.3 Control the Total Amount of Free Surfactants
Excess wetting agents intensify convection and strip the dispersion layer from pigment surfaces. Keep the formulation lean on redundant surfactants.
5. Case Studies
5.1 Co-Grinding Case — Deep Grey Waterborne Acrylic Polyurethane Topcoat
Formulation: a waterborne acrylic polyurethane system in which TiO2, carbon black, iron red, iron yellow and barium sulfate filler were co-ground. Severe floating occurred, the color changed, and the color only stabilized after three days.
Solution: the combination of DPM8513 and DPM8193 (1:1 by solid content). Floating was greatly improved and storage stability was significantly enhanced.
5.2 White Paste + Purchased Color Paste — Waterborne PUD Grey Conductive Coating
Formulation: a waterborne PUD system using an in-house resin-free TiO2 white paste combined with a purchased conductive carbon black paste — severe white floating occurred.
Solution: DPM8571 was used to make the waterborne resin-free white paste, combined with the purchased conductive carbon black paste. White floating was greatly improved and the finger-rub color difference reached ΔE < 0.3.
5.3 Separate Grinding of Color Pastes
For systems in which organic pigments, carbon black and inorganic pigments are ground separately before tinting, the recommended approach is: use DPM8571 to make the resin-free white paste, use DPM8513 for grinding organic pigments and carbon black, then tint with the separately ground single-color pastes.
6. Conclusion
Floating and flooding in Water-based coatings stem from a combination of Bénard convection, differential pigment movement, selective flocculation and HLB imbalance. Systematic solutions — balanced dispersant combinations, wetting agent adjustments and tight control of free surfactants — deliver reliable results in production. Deep-Polymer’s DPM8513, DPM8193 and DPM8571 have proven effective in real production cases.
