Rheological Changes During the Drying of a Waterborne Latex Coating

By Frank Löfflath, Matthew Gebhard

Increases in the low shear viscosity occurring during drying of a water-based latex film were examined as a function of pH, cosolvents, and base. The water and cosolvent evaporation rates were determined as a function of temperature and humidity. Water was found to evaporate with an activity coefficient of 1 and a ∆Hv equivalent to pure water. The viscosity changes were determined from a creep measurement.

The data is explained by the volume packing model of De Kruif. For several of the films, the ratio between volume and weight fraction was not a static parameter, and an abnormally large drop in the effective volume and weight fraction was observed. This was exaggerated for the samples with ethylene glycol monobutyl ether (EB) or NH4OH. The high volatility of EB explains the former, while the latter result indicates that NH4OH suppresses viscosity during drying.