Understanding the mechanism by which films fail during drying is the first step in controlling this natural process. Previous studies have examined the spacing between cracks with predictions made by assuming a balance between elastic energy released with a surface energy consumed. We introduce a new scaling for the spacing between cracks in drying dispersions. The scaling relates to the distance that solvent can flow, to relieve capillary stresses, as a film fails. The scaling collapses data for a range of evaporation rates, film thicknesses, particle sizes, and materials. This work identifies capillary pressures, induced by packed particle fronts travelling horizontally across films, as responsible for the failure in dried films.
The change of minimum film formation temperature (MFFT) with time was studied utilizing a temperature gradient bar. Fitting the data to a theory which assumed that particles deform due to the action of the polymer-air surface tension, the glass transition temperature, T g , of the latex was predicted. This T g was compared to the value obtained by differential scanning calorimetry (DSC) and good agreement was observed between the two measurements.
Latex dispersions with different glass-transition temperatures have been dried in Petri dishes over a range of temperatures. The crack spacing is reported and is observed to increase as the temperature approaches the glass-transition temperature of the latex. The data for crack spacing over a range of temperatures is collapsed to a single curve by adapting a scaling for the crack spacing of hard particles. The increase in temperature requires an account to be made for particle deformation and, hence, an increase in particle volume fraction, as well as the temperature dependence of the evaporation rate. The importance of measuring the stress at which films crack is highlighted.
Uneven distribution of surfactant in dried latex films can affect the final film properties such as its water-resistance, gloss, and adhesiveness. Therefore, it is important to understand the driving force for surfactant transport during drying. In this paper, the accumulation of surfactant on the surface of poly(styrene-co-butyl acrylate) latex is studied using Rutherford Backscattering (RBS) and compared with results from a model that is based on the diffusive transport of particles and surfactant. Experimentally, a 30-50 nm thick surface layer, rich in surfactant, is seen and the concentration in the bulk of the film, obtained from RBS, agrees, at least qualitatively, with the model predictions for two of the surfactants tested.
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