United States. Researchers at the University of New Hampshire studied the water bleaching of polymer films derived from polymerization processes in solution, mass and emulsion.
In addition to quantifying the wavelength-dependent light scattering of the films over time, the different physical forms of water present in the reddened films were quantified using differential scanning calorimetry (DSC). Scanning electron microscopy (SEM) was used to observe sections of the films and characterize the dispersion domains responsible for the bleaching phenomenon.
The scientists studied the same polymers with and without the surfactants and salts used in emulsion polymerization, and compared the flush of water- and solvent-transmitted films. They found that the entire wide variety of (co)polymers used bleach water under the right weather and temperature conditions.
Surfactants and residual salts in latex-derived sheets make the flushing process faster and more extensive than for the same polymer without them, but they are not the main cause of water bleaching. Nor is the particle nature of the starting point for latex films, as the same bleaching process occurs in solvent-based films of the same polymer.
Both absorbance and SEM measurements show that there is a growth in the size of the water domain within the polymer matrix over time. The size and number of water domains are responsible for the bleaching effect of water and both may be restricted by the rigidity of the polymer matrix.
Mechanical modeling of the temporal dependence of bleaching has led to the prediction that the degree of bleaching of non-latex-based polymer films is directly proportional to the water solubility inherent in the polymer, as well as to the diffusivity of water within the polymer at the test temperature.
Source: Progress in Organic Coatings.
