By Wenqin (Sunny) Wang, Yujie Lu, Gerry Valdez, Linda Magni, The Dow Chemical Co. 

Dispersants can play a critical role in optimizing waterborne coatings, influencing key properties such as stain removal and scrub resistance in interior applications, as well as efflorescence resistance, dirt pick-up resistance, and gloss or color retention in exterior coatings. Consequently, the selection and optimization of dispersant chemistry is essential for achieving targeted performance outcomes in coating formulations. Dow is a leading supplier of acrylic and polycarboxylate copolymer dispersants for architectural coatings applications, offering a product portfolio ranging from polyacid, to hydrophilic copolymers and hydrophobic copolymers.  This study examines how dispersant type, loading level, and neutralizing agent affect overall coating performance in both flat and semi-gloss formulations with two different binder systems. The formulation parameters are outlined in Table 1.  Copolymer dispersants based on four different chemistries were evaluated. All dispersants were evaluated at 1% or 2% active versus pigment weight, and two selected dispersants were also evaluated at 4% loading.

In semi-gloss formulations, Binder 2 shows a clear preference for higher dispersant loading—such as 2% compared to 1%—to achieve optimal gloss development and tint strength. This behavior contrasts with the standard acrylic binder, which does not exhibit a strong dependence on dispersant concentration. The difference is likely attributable to the colloidal stability of the binder systems; Binder 2, characterized by its smaller particle size and hydrophobic nature, requires increased dispersant levels to maintain formulation stability and maximize gloss and tint strength, particularly in binder-rich semi-gloss coatings. 

Gloss retention of the semi-gloss (SG) formulation was assessed using QUV accelerated weather testing (0.89 W/m² at 340 nm, 8 hours of UV exposure followed by 4 hours of condensation). The results are shown in Figure 1. For formulations based on Binder 1, higher dispersant levels led to poorer gloss retention. Interestingly, Binder 1 can be formulated without any dispersant while still achieving good gloss development in the semi-gloss formulation. However, the grind becomes extremely thick without dispersant, which may pose handling challenges during manufacturing.

In contrast, Binder 2 formulations exhibited higher initial gloss at increased dispersant levels. The significantly lower gloss at 0% dispersant was attributed to pigment flocculation within the formulation. Overall, gloss retention for Binder 2 remained relatively consistent across dispersant levels, and Binder 2 generally demonstrated better gloss retention than Binder 1.

Figure 2 illustrates the stain removal performance of coatings formulated with Binder 1 using a series of hydrophobic and hydrophilic household stains (adapted from ASTM D3450). Among the dispersants evaluated, the hydrophobic polycarboxylate copolymer neutralized with ammonia demonstrated exceptional performance in facilitating stain removal, as evidenced by the lowest ∆E values across the test set. Notably, several dispersant chemistries exhibited a trend of enhanced stain removal with increasing dispersant concentration, suggesting that higher loading levels can further enhance cleaning performance. 

The underlying mechanisms governing stain removal are multifaceted. Binder chemistry, surface porosity and surface energy are critical factors. Coatings with higher porosity or certain surface energy may allow stains to penetrate more deeply or adhere more strongly. Dispersants can influence the quality of pigment dispersion, thereby impacting surface porosity and the ease with which stains are removed from the coating surface. In addition, dispersants used in waterborne formulations are water-soluble polymers. At elevated concentrations, they tend to increase the hydrophilicity of the coating, which can be advantageous for the removal of certain types of stains, particularly those that are water-based or polar in nature. 

Additionally, the impact of dispersants on exterior durability was evaluated through exposure studies at Dow’s Spring House, PA test fence. After 35 months of exposure, properties such as dirt pick-up resistance and color retention did not show significant differences among the various dispersants. However, rust resistance was more strongly influenced by the choice of dispersants in the formulation, especially for binder 1. 

Figure 3 shows the rust resistance of flat paints containing different dispersants applied on cold rolled steel substrates and exposed in a north-facing vertical orientation. Binder 2 exhibits excellent rust resistance independent of dispersant type, which could be attributed to the hydrophobic chemistry and composite- forming nature of binder 2.  For binder 1, dispersant type and level played an important role: increasing the dispersant level negatively impacted corrosion resistance. Ammonia neutralized dispersants performed better than those neutralized with hard bases.   Formulations neutralized with hard base (sodium or potassium) retain cations within the film. These ions facilitate water attraction, increasing the coating’s tendency to absorb moisture and potentially form conductive pathways—factors that elevate the risk of corrosion in coatings containing hard base. 

In contrast, systems neutralized with volatile ammonia may migrate or evaporate over time, generally reducing ionic conductivity within the coating film, decreasing water absorption, and enhancing resistance to rust formation. Dispersants based on styrene-acrylic copolymer and acrylic copolymer neutralized with ammonia provided the best corrosion resistance at concentrations of 1% and 2%.

Figure 4 presents a comparative assessment of rust resistance for Binder 1 formulations after 35 months of exposure, with all dispersants incorporated at 2%. The images clearly show that the dispersants neutralized with NH₄⁺ performed better than those neutralized with hard bases. The choice of dispersant is critical to exterior durability, particularly regarding water sensitivity. For light-duty industrial coatings requiring rust resistance, careful consideration of both the dispersant’s neutralizer and its hydrophobicity is essential.

In summary, the study found that dispersant type, loading level, and neutralizing agent significantly influence the performance of waterborne coatings. In semi-gloss formulations, a premium acrylic binder (Binder 2) requires higher dispersant concentrations to achieve optimal gloss and tint strength, while the standard acrylic binder (Binder 1) is less sensitive to dispersant levels. Gloss retention is generally better with Binder 2, whereas higher dispersant levels can negatively impact Binder 1’s gloss retention. For stain removal, hydrophobic polycarboxylate copolymer dispersants neutralized with ammonia performed best, particularly at higher concentrations. Exterior durability tests at Dow’s Spring House, PA external test facility showed that most dispersants provided similar performance in terms of dirt pick-up resistance and color retention; however rust resistance was strongly affected by dispersant type and neutralizer, with ammonia-neutralized dispersants offering superior protection—especially for Binder 1. The choice of dispersant and neutralizer is thus critical for optimizing both interior and exterior coating performance. CW