Rheology of Waterborne Coatings

By R.D. Hester, D. R. Squire, Jr

Waterborne latex coatings were introduced because they are easy to use and lacked the health hazards and noxious odors of their solvent-borne counterparts. Unfortunately, aqueous-borne latex coatings do not possess the same rheological properties of solvent-borne coatings. If not correctly formulated, a latex paint can have inferior flow and leveling properties.

They may not give adequate film build, and can be plagued by spattering during application. In order to achieve the equivalent rheology of their solvent-borne counterparts, latex formulations require the addition of rheology modifiers.

Early thickeners or rheological modifiers were derived from starch and casein and were of limited utility. In addition to their biodegradability, they produced poor films and had undesirable flow properties. They were soon replaced with high molecular weight cellulosic thickeners and clays. Cellulosic additives thicken latex systems by polymer chain entanglement.

However, their viscosities during application and in drying are not ideal. They are poor in spatter resistance, do not have good flow and leveling properties, and are biodegradable. These inherent disadvantages of cellulosics resulted in the development of associative thickeners for latex coatings beginning in 1980.1,2 These thickeners have several advantages over cellulosic additives.