Absorption Properties of Coatings: A Selected Overview of Absorption Criteria Derived from Recent Pore Network Modelling

Abstract

Absorption of fluids into porous media controls the printability characteristics of coated papers in respect to ink setting rate and phenomena such as wicking and bleeding. The dynamic of absorption is related to the runnability on printing presses. Pore structure, size distribution, and surface chemistry are the parameters which can be used to design structures for optimal absorption. To achieve a working understanding it has been necessary to develop the theory of absorption to encompass the short timescale fast absorption occurring during wetting of the fluid ink onto the surface of the paper. The phenomena of fluid spread and absorption are studied and shown to depend on the observed pore size distribution rather than porosity alone. This finding calls into question the traditional acceptance of the Lucas–Washburn relationship for such porous networks as the hydraulic absorption radius determined from experimental absorption rate measurements fails to follow the actual measured pore size trend. It is necessary to invoke the concept of a preferred pathway of absorption in which only selected groups within the finest connecting pores or throats of the network in combination with intermediate pore reservoirs contribute maximally to the transmission of fluid through the coating structure as porosity increases. By defining the pore size range involved in this mechanism using an inertial wetting term, found in the short time solution to the Bosanquet equation, it is possible for the first time to identify the connective structures responsible for the important time dependent absorption properties of coatings and their relationship to the ink tackification and/or setting rates for a range of printing ink solvents and diluents, including offset, rotogravure, flexographic, and ink jet inks. Additionally, the differential chromatographic effect of coatings and associated binder systems results in further time dependent offset ink setting phenomena in which miscible ink oils of different natures separate within the coating pores and between the pores and synthetic latex binders. In situ determination of the viscosity and solids content of an offset ink via static ink tack measurements allows the dynamic of ink setting to be studied on real coating systems. Combining these criteria, the necessary steps can be followed to design the coating pigment structures required for optimising existing and future fluid‐based printing technologies.