Water whitening: Comparison between water- and solvent-borne films
Water whitening of polymer films derived from solution, bulk and emulsion polymerisation processes was studied by researchers at University of New Hampshire.
In addition to quantifying the wavelength dependent light scattering of the films over time, the different physical forms of water present in blushed films were quantified by differential scanning calorimetry (DSC). Scanning electron microscopy (SEM) was used to observe sections of the films and characterise the scattering domains responsible for the whitening phenomenon.
Comparing the blushing of both films
The scientists studied the same polymers with and without the surfactants and salts used in emulsion polymerisation, and compared the blushing of water-borne and solvent-borne films. They have found that all of the wide variety of (co)polymers used water whiten under the right conditions of time and temperature. Residual surfactants and salts in latex derived films make the blushing process more rapid and extensive than for the same polymer without them, but they are not the principal cause for water whitening. Neither is the particulate nature of the starting point for latex films, as the same whitening process occurs in solvent-borne films of the same polymer.
Water domain size growth within the matrix
Both absorbance measurements and SEM images show that there is water domain size growth within the polymeric matrix over time. The size and number of the water domains are responsible for the water whitening effect and both can be restricted by the stiffness of the polymeric matrix. Mechanistic modeling of the time dependence of whitening has led to the prediction that the extent of whitening of non-latex based polymer films is directly proportional to the inherent water solubility in the polymer as well as the diffusivity of water within the polymer at the temperature of testing.
The study is published in: Progress in Organic Coatings, Volume 105, April 2017, Pages 56–66
