Abstract
During the past two decades, an improved understanding of the operative particle deformation mechanisms during latex film formation has been gained. For a particular colloidal dispersion with a known particle size, the Routh-Russel deformation maps predict the dominant mechanism for particle deformation for a particular set of conditions (evaporation rate, temperature, and initial film thickness). Although qualitative tests of the Routh-Russel model have been reported previously, a systematic study of the relationship between the film formation conditions and the resulting water concentration profiles is lacking. Here, the water distribution during the film formation of a series of acrylic copolymer latexes with varying glass transition temperature, Tg (values of 22, 11, 4 and 19 ºC) has been obtained using GARField nuclear magnetic resonance profiling. A significant reduction in the rate of water loss from the latex with the lowest Tg was found, which is explained by its relatively low polymer viscosity enabling the growth of a coalesced skin layer. The transition between slowed drying and unimpeded drying occurs at the boundary between the capillary deformation and wet sintering regimes, in full agreement with the Routh-Russel model. An inverse correlation between the model’s dimensionless control parameter and the dimensionless drying time is discovered, which is useful for the design of fast-drying waterborne films. The effect of particle size and bimodality on the film formation of latexes was also investigated. The differences in the drying rates at the early stages of drying due to different particle sizes were discovered. The latexes with unimodal smaller particle sizes were found to have longer characteristic drying times compared to those with unimodal larger particle sizes. This was postulated to be due to the Kelvin-Laplace theory of the reduction of vapour pressure of water due to the radius of the water menisci between polymer particles at the early stages of the drying process. Acrylic acid functionalised latexes were then studied. The effect of neutralising the latexes to a higher pH of 9 on the drying of latexes was investigated. It was found that the latexes with a higher pH had faster characteristic drying times compared to those with a pH of 2 which could be useful when designing stable waterborne coatings.