The dynamic wetting of, and absorption into, model porous coatings in the form of compressed particulate pigment tablets by monocomponent, dual-component, and multicomponent liquid droplets has been studied by observation of apparent contact angle and near-infrared 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-infrared spectral data. Model and commercial offset printing fountain solutions were compared with respect to both absorption and evaporation. For the solutions containing isopropyl alcohol in water, a nonlinear behavior in the water response in the near-infrared spectra during absorption is observed as a function of time, which can be related to the fast evaporation of the alcohol. The nonlinear region was followed by a decline in water and moisture content as the penetration/evaporation of the water phase proceeded. Comparing the near-infrared 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.
To obtain more knowledge of the properties affecting print quality and adhesion characteristics in the printing process, attention has been directed to the nature of surface energy. The aim was to compare different surfaceenergy calculation models and to investigate the influence of surface roughness on wetting of coated offset papers. The wetting process was studied by static contact angle measurements using a series of reference liquids. Topographical characterization was carried out using atomic force microscopy. Surface energy components were determined using different calculation models. The determination can be considerably simplified using a mono-monopolar model, which has been proven in previous studies. The surface energy components were derived from both apparent and topography-corrected contact angles. The surface topography had a significant effect on wetting of the samples studied.
The aim of this study was to investigate the spreading of sessile drops of polar probe liquids (water and ethylene glycol) on pigment coated offset papers. Furthermore, existing theoretical models for spreading were applied to evaluate the experimental results. The hydrodynamic model gave a better fit to the results at lower spreading rates, while the molecular-kinetic theory gave a good fit over a larger velocity range. Factors introduced to correct for the exponential dependency on time of the drop base radius and its contact angle can be interpreted as coupled processes. Differences in the spreading between the papers were found to correlate with the acid and base components of the surface energy, rather than with differences in surface roughness.
Novel models are developed and previously published simple models characterizing the spreading of probe liquids and inks on solid surfaces are evaluated with water and ethylene glycol. The aim is to determine the most appropriate models which would be suitable to describe liquid and ink spreading on both chemically and structurally heterogeneous paper surfaces. The surface energy and its components (Lifshitzvan der Waals, acid and base) for the paper samples have previously been characterized at semi-equilibrium conditions. The equilibrium work of adhesion and work of spreading is found to be linearly related to the maximum print tack force of model inks. The dynamic spreading are found to be determined by coupled spreading mechanisms, being related both to the surface structure and surface chemistry.
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