Waterborne coatings, introduced in the 1960s, are complex, multiphase fluids. To control the target rheological properties during manufacture, storage and application, typically about 0.1-1.5 wt% of a rheological additive is added to the coating formulation. High-molecular-weight cellulosics, such as hydroxyethylcellulose (HEC), remain the workhorses for controlling the rheology of high-PVC (pigment volume concentration) (45-85%) waterborne coatings. However, to achieve the application properties and gloss of solventborne alkyds in low-PVC (20-40%) waterborne coatings, the use of cellulosics remains an elusive goal.1
To fill this need, during the 1980s a new class of water-soluble polymers was developed and commercialized. They are referred to as hydrophobically modified water-soluble polymers (HM-WSPs). Currently there are three classes of commercial HM-WSPs: (1) hydrophobically modified hydroxyethylcelluloses (HM-HECs); (2) hydrophobically modified nonionic synthetic polymers (HM-NSPs); and (3) hydrophobically modified alkali-soluble emulsions (HASEs).2 Chemically, HM-WSPs are water-soluble polymers that are grafted with a relatively small amount (<5%) of lipophilic groups. They are also called “associative thickeners” (ATs) as they non-covalently associate among themselves in an aqueous environment and/or associate with the dispersed particles (binders, pigment, etc.) through their hydrophobes to form a three-dimensional inter-chain network. The formation of this three-dimensional network provides enhanced viscosity, stabilization of the binder and modulation of the coating rheology under different shear rates.