The application experience of waterborne coatings is largely determined by the thickener. Spattering during roller application, visible brush marks, and poor leveling are often not resin or pigment issues, but a mismatch in the thickener’s rheological profile.

HEC (hydroxyethyl cellulose) and polyurethane thickeners (HEUR) are the two most common rheology modifiers in waterborne coatings. Their performance in leveling and spatter resistance is almost opposite.

Rheology Determines Application Performance

HEC is a non-associative thickener. It raises viscosity through chain entanglement and volume exclusion in the aqueous phase. Its profile: very high low-shear viscosity, low high-shear viscosity, moderate thixotropy, and excellent water retention. This means excellent anti-settling and sag resistance at rest, but insufficient viscosity drop under application shear and weak driving force for leveling.

HEUR is an associative polyurethane thickener. Hydrophobic ends adsorb onto latex particles, forming a dynamic physical crosslink network. Its profile: low low-shear viscosity, high high-shear viscosity, delivering a near-Newtonian rheological curve. This means smooth application feel, moderate viscosity recovery after shear, and strong leveling drive.

Leveling: HEUR Wins Clearly

Leveling is essentially the film’s self-flow driven by surface tension after shear is removed. This requires moderate flowability in the low-shear region, rather than being “locked” by HEC’s high static viscosity.

HEC’s high low-shear viscosity aids sag resistance, but also hinders natural leveling after application. The film has no time to spread, and brush and roller marks freeze into the surface. Technical data explicitly states that HEUR thickeners are “superior to conventional cellulosic types (such as HEC)” in leveling. HEUR’s selective high-shear thickening gives the film better flow and spread after application, resulting in a smoother surface and higher gloss.

Spatter Resistance: HEUR Also Leads, but by a Different Mechanism

Roller spatter occurs when the coating is stretched into fine filaments under high shear, which break and eject droplets. High-molecular-weight water-soluble polymers like high-viscosity HEC align and stiffen under extension, actually aggravating filament formation and breakage.

HEUR’s associative network dissociates under high shear, giving a gentler viscosity response and lower filament resistance, significantly reducing spatter. “Spatter resistance” is an explicit selling point in HEUR technical data. HEC’s spatter performance depends on molecular weight — low-MW HEC reduces spatter but sacrifices thickening efficiency. Some HEC product sheets even list “low anti-spattering properties” as a characteristic to note.

HEC Is Not Without Merit

HEUR’s advantages in leveling and spatter do not make HEC obsolete. HEC’s water retention and sag resistance are difficult for HEUR to replace. In thick-film coatings (textured finishes) and high-PVC interior paints, HEC’s low-shear structure is the foundation for anti-settling and sag resistance.

Moreover, HEUR is highly sensitive to surfactants and co-solvents, posing viscosity fluctuation risks, and costs far more than HEC. HEUR’s low-shear thickening efficiency is insufficient, leaving sag and settling performance weak when used alone.

Practical Selection and Blending

Maximum leveling and spatter control (high-gloss topcoats, clearcoats, industrial coatings): HEUR as primary thickener, 0.3%–0.8%. Pair with defoamer to control HEUR’s foam-stabilizing tendency.

Balanced sag resistance and cost (medium-to-high PVC interior/exterior paints): HEC + HEUR blend. HEC provides low-shear structure and water retention; HEUR improves leveling and spatter. Typical: HEC 0.3%–0.5% + HEUR 0.2%–0.4%. This is the most common balancing approach among formulators.

Thick-film coatings (textured finishes): HEC as primary, with bentonite. HEUR’s low-shear thickening is insufficient for the primary role.

Validation: After adjustment, measure Leneta leveling rating, spatter rating (roller method), and ICI viscosity. Improved leveling with reduced spatter indicates adequate HEUR; if sag resistance drops, increase HEC.

Conclusion

The differences between HEC and HEUR in leveling and spatter stem from fundamentally different rheological mechanisms. HEUR trades selective high-shear thickening for smooth leveling and low spatter. HEC trades high static viscosity for sag resistance and water retention. Neither alone is complete — HEC + HEUR blending is the practical answer for waterborne coating rheology.