Abstract
There is a trend in the coatings industry to replace organic solvent-based systems with equivalently-performing, environmentally benign products. Waterborne colloidal polymers represent a promising alternative. The development of mechanical strength and hardness is often enhanced by chemical crosslinking that creates a three dimensional network. If crosslinking occurs prior to particle coalescence, however, the network will not be continuous throughout the film and a weaker product will result. Therefore, an understanding of the relative rates of polymer interdiffusion and crosslinking is imperative. The competition between these two processes in an acrylic latex containing acrylamide functionality utilising the “keto-hydrazide” crosslinking reaction has been studied. The mechanism and factors influencing the rate of the crosslinking reaction were investigated in a model system, using spectroscopic techniques. The crosslinking reaction was favoured by the loss of water during drying and the simultaneous decrease in pH arising from the evaporation of ammonia. The fundamental understanding achieved from the model system was used to explore the phenomenon within a drying latex film. Measurement of the latex pH relative to polymer mass fraction during film formation clarified the expected point of onset for crosslinking in relation to particle packing. Gel fraction and swelling measurements were used to probe the temporal evolution of the cross-link density. Crosslinking is expected when the latex particles are close packed, and there is a rapid increase in gel content in a short period. Atomic force microscopy was used to follow surface levelling relative to the competing influence of crosslinking with film ageing. The flattening of latex particles and interdiffusion lead to the blurring of particle/particle interfaces and both were inhibited by crosslinking. The amount of particle deformability was identified as a key factor in the amount of surfactant exudation. Factors that increase the amount of particle deformation and coalescence (e. g. higher film formation temperature, lower latex Tg, and no crosslinking) promoted surfactant exudation. Surfactant exudation was inhibited when particles were less deformable and slower to coalesce. A more hydrophilic atmosphere (high humidity) encouraged surfactant segregation at the interface with the film. Exudation to a “clean surface” was triggered by raising the temperature or by raising the humidity.