Friction coefficients (called μ) versus contact pressure or confinement (maximum indentation depth over film thickness) were measured together with bulk mechanical properties of waterborne, partially formulated, acrylic coatings. It was shown that the initial, unformulated latex film was significantly plasticized by humidity in air thanks to the presence of the acrylic acid hydrophilic monomer, resulting in an increase of the friction coefficient. However, the plasticizing effect of a coalescing agent (Texanol™) was much stronger and therefore much more increased the friction coefficient, which only slowly decreased upon film aging. The film containing 10% of Texanol was used as the reference system for the remainder of the study. To this reference, different classical formulation ingredients were successively added (reference plus ethanol, reference plus thickener, plus crosslinker, plus pigment). Curiously, the presence of ethanol in the aqueous phase, often added in order to speed up evaporation, had an increasing effect, although slight, on the friction coefficient of the dry film. The opposite was observed with the thickener, result interpreted in terms of a marked lubrication phenomenon. Crosslinking or pigment addition significantly decreased the friction coefficient. A positive correlation between friction and film softness could be observed (increase of μ when film softness increased and inversely), except when lubrication was dominating. These results should be helpful to formulators of waterborne films in several application areas.
Volume 113, December 2017, Pages 189-199