Phosphate Esters in Low-VOC Waterborne Architectural Paint Systems
Surfactants are often classified by the charge present in the hydrophilic head. Anionic (negatively charged) surfactants are the most commonly used type of surfactant, and they are low-cost with high efficiencies. They provide good wetting and stabilization of emulsion dispersions through electrostatic repulsion. Nonionic (no-charge) surfactants work through steric stabilization and generally provide poorer wetting with low efficiencies. They are compatible with other classes of surfactants, but their solubility in water is temperature-dependent. Cationic (positively charged) surfactants and Amphoteric (charge-dependent upon pH of media) are two classes of surfactants used in more limited coatings situations.
One type of anionic surfactant - phosphate ester - contains the phosphate moiety as the hydrophilic head. Phosphate ester surfactants are produced by the reaction of alcohols with an activated phosphoric acid derivative. Typically, phosphate ester commercial products are comprised of a mixture of monoester, diester, residual alcohol and residual phosphoric acid (Figure 1). The property of the final phosphate ester mixture is primarily defined by the starting alcohol as well as the composition of the four different species. Therefore, the property of the final phosphate ester mixture can be tailor-made by altering the alcohol used in the preparation, as well as controlling the ratio of the different components present in the final product. Phosphate esters are initially prepared in the free-acid form, but can be neutralized to the salt form using any base, including sodium hydroxide, potassium hydroxide, ammonium hydroxide or any organic amine.