Hypothesis: Drying of latex dispersions often results in particle gradients at the latex-air interface. We expect that, by increasing the carboxylic acid content of latex particles, inter-particle interactions at the interface change. With dilatational rheology one could detect particle-particle interactions in an early stage of the drying process and elucidate the nature of these interactions.
Experiments: Acrylic latex dispersions were prepared with different amounts of methacrylic acid (MAA), ranging from 2 to 10 wt% on dry mass. Dilatational rheology studies during drying at different relative humidities RH were performed using profile analysis tensiometry. Visco-elastic properties of latex surfaces were used to identify inter-particle interactions at the surfaces depending on the drying rate and particle composition.
Findings: Drying at 85% RH did not show significant changes of the mechanical properties of the latex surfaces. Drying at 65 and 53% RH resulted in a change of the mechanical properties, ultimately showing non-linear visco-elastic behavior. This indicates that capillary and/or Van der Waals forces were operating between particles at the surface. With increasing MAA content the viscous contribution decreased, possibly due to the formation of more gel-like structures at the particle surface due to higher solubility of polymer segments near to the surface.
Keywords: Dilatational rheology; Drying; Interface; Latex; Methacrylic acid.
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