Fast Drying and Film Formation of Latex Dispersions Studied with FTIR Spectroscopic Imaging

Abstract

Drying of thin latex films (∼20 μm) at high drying speeds (of the order of seconds) has been studied by fast chemical imaging. ATR-FTIR spectroscopic imaging combined with a fast "kinetic" mode was used to acquire spectral images without coaddition, enabling the amount of water and homogeneity of the drying film to be studied over time. Drying profiles, constructed from analyzing the water content in each image, show two stages of drying, a fast and a slow region. The formulation of latex dispersions affects the onset of slow drying and the volume fraction of water remaining at the onset of slow drying. In this work, the effect of physical properties, film thickness and glass transition temperature (Tg), were investigated, as well as the effect of monomer composition where two monomoers, 2-ethylhexyl acrylate and n-butyl acrylate, and the amount of hydrophilic comonomer, methyl methacrylate (MMA), were varied. It was found that thicker films produced slower overall drying and that the formulation with a Tg above the minimum film formation temperature did not dry evenly, exhibiting cracking. However, the drying kinetics of high and low Tg films were similar, highlighting the advantage of using a spatially-resolved spectroscopic approach. Formulations containing more MMA dried faster than those with less. This was due to the hydrophilicity of MMA and the increase in Tg of the dispersion from the addition of MMA. Overall, FTIR spectroscopic imaging was shown to be a suitable approach in measuring film drying at high speeds as both chemical changes and chemical distribution could be analyzed over time.

A critical and quantitative review of the stratification of particles during the drying of colloidal films

“…Ekanayake et al 62 confirmed in NMR depth profiling experiments that the particle concentration gradient is steeper when Pe is increased, as was predicted by Routh and Zimmerman. 46 Kimber et al 63 studied latex films dried at elevated temperatures for which the evaporation rate was very high. With the relatively low thickness of their films, on the order of H = 20 m, Pe was approximately 5.…”

Interdiffusion during film formation of ionically cross‐linked acrylics investigated with Förster resonance energy transfer (FRET)

“… Here, it is assumed that a fraction of SDS molecules is still located at the boundaries between deformed particles. Results of film formation studies based on different techniques have indicated that particles of low‐ T g polymers can deform before drying has completed because of their softness . Since the films dried at 92% are turbid, it is expected that they still contain a small amount of water.…”

Molecular Aspects of Film Formation of Partially Cross-Linked Water-Borne Secondary Dispersions that Show Skin Formation upon Drying

“…For rapid drying of latex dispersion, the accumulation of latex particles at the surface forms a skin layer, which prevents the particles from redistributing or diffusing in time and restricts the evaporation of water. Water evaporation can only occur via passage through capillaries in the film or diffusion through the polymer itself. , Following Adams and co-authors, we separate the skin formation and progress of polymer diffusion into three separate stages that we depict in Figure . In stage I, the rapid evaporation of surface water causes the polymer NPs to migrate to the surface where they are captured when they form part of the skin.…”

“…These authors concluded that the drying times of these films of similar thickness, under the same atmospheric conditions, were inversely related to T g . This view is supported by Kazarian et al, who studied the drying of thin latex films by Fourier-transform infrared spectroscopy imaging. They monitored the water content in each image, showing two stages of drying.…”