Liquid absorption and evaporation in porous coating structures studied by near-infrared spectroscopy

The dynamic wetting of, and absorption into, model porous coatings in the form of compressed particulate pigment tablets by mono-, dual- and multi-component liquid droplets, has been studied by observation of apparent contact angle and near infra-red spectroscopy to identify the liquid water/moisture content. The absorption of the liquids was studied in a corresponding vapor-saturated environment. Liquid evaporation was determined for the tablets at both equilibrium starting pore saturation and under limited volume-filling conditions as evaporation proceeds. The changes in water and moisture content within the coatings as a function of time were also determined gravimetrically to relate the water uptake and evaporation being observed to changes in the near infra-red spectral data. Model and commercial offset printing fountain solutions were compared in respect to both absorption and evaporation. For the solutions containing isopropyl-alcohol in water, a non-linear behavior in the water response in the near infra-red spectra during absorption is observed as a function of time, which can be related to the fast evaporation of the alcohol. The non-linear region was followed by a decline in water and moisture content as the penetration/evaporation of the water phase proceeded. Comparing the NIR water volume dependency in the upper layers of the structure with weight loss during evaporation showed that the mechanism of liquid transport to the surface-air interface reflected the logarithmic volume distribution of pore sizes, as might be expected from capillarity considerations and pore condensation hysteresis.