Printing of dye and pigment based aqueous inks onto porous substrates
Résumé
In this article, the dynamics of droplet impingement and absorption into microporous materials for pigment based aqueous inks and dye based aqueous inks are compared. For dye based inks it was shown earlier that three main phases could be resolved: inertial spreading, absorption, and evaporation of the liquid, leading to the final equilibrium condition on which the typical customer is evaluating the image quality. For the inertial spreading phase it could be shown that the spreading behavior is largely determined by the hydrodynamic properties, and is easily amenable to dimensionless analysis. The absorption phase could be well described by a capillary wicking process according to imbibition models. Evaporation is the slowest process only being finalized after many seconds. These results are now compared with droplet impingement and absorption of pigment based inks on microporous receivers. It is shown that these inks behave totally differently from dye based inks. Immediately after impingement and initial spreading the pigment particles start to coagulate on the surface of the microporous layer, creating a filter cake limiting the passage of carrier liquid. As a result much longer absorption times are observed and the equilibrium dot stays on top of the microporous layer. Most polymer stabilizers in the pigment based inks create a colored polymer layer having polymeric blend characteristics limiting considerably the penetration of water compared to the capillary wicking process. The capillary imbibition models are not valid any more because now the build-up of the filter cake changes not only the receding contact angle but also introduces a diffusion process changing as a function of time during the drying of the wet ink.
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