“Did you know…….?.” series for March - April 2022

Did you know…. that nearly all polymers absorb water to some extent, and some of them actually whiten (blush) upon extensive exposure? This is a subject of great importance to the emulsion polymer community as the polymer films and coatings are derived from water dispersions. How do we determine the amount of water a film absorbs? We have to consider that in the first step of the filming/coating process, one is driving off the water by drying it. This is followed by subjecting the dried film/coating to either water vapor (perhaps at different relative humidities) or liquid for various times (and maybe temperatures) to measure the uptake of water into the polymer. It should be recognized that the polymer in latex form is saturated with water since it was formed in that environment and this saturation level can be quantified by measuring the “wet Tg” via DSC. The same test can be used to measure the amount of water absorbed in a dried film after exposure, if desired, but gravimetry is also used. But the DSC signal can distinguish between the water absorbed along the polymer chains from that contributing to whitening. Water whitening is a rather startling phenomenon when first seen and is often described as blushing. The mechanism responsible for this result is made up of several, sequential steps: 1. molecular diffusion of water into the polymer and hydrogen bonding to the more polar constituents of the repeating monomer units in the chain until equilibrium is reached; 2. additional water penetration, driven by water attaching to those molecules hydrogen bonded to the polar groups on the polymer chain (as described in step 1); 3. the formation of nano-sized domains of “free” water adjacent to the earlier water molecules; and finally 4. the growth of the nano-domains into larger, micron-sized domains. Light is scattered more intensely as progression from steps 3-4 takes place. Interestingly, the domain size growth ceases when the pressure inside the growing domains is effectively resisted by the tensile strength of the polymer. Below we show an SEM photo of a section of polymer that has whitened to its fullest extent. Here one can see that the domains are of micron size. Further details for this interesting phenomenon can see viewed in the following publication: Progress in Organic Coatings, 105, 56- 66 (2017). Since the above descriptions of the absorption process are based on molecular diffusion of water into the coating, there is inherently a “rate process” to consider. It is well known that water-borne films/coatings absorb water faster (and perhaps to a somewhat greater extent) than the otherwise equivalent solvent-borne films. This is clearly related to the fact that there are surfactant and other ionic species in the latex than do not evaporate as the film dries. There are added mechanisms for water penetration at play when these non-polymer species exist somewhere within the dried film, especially for surfactants. This we leave as a topic for future discussion. As always, we invite your questions and comments via our website, epced.com