Computer modelling of fluid permeation in porous coatings and paper - an overview

The behaviour of fluids in paper and paper coatings is of prime importance, both with respect to the fabrication of papers and paper coatings, and sub sequent printing on them. The behaviour depends primarily on the nature of the fluids themselves, the geometry of the void structures within the paper or coating, and the forces acting at the interface between the fluid and t he voids. Various methods to monitor these properties are in widespread use within the industry. The methods usually mimic some property directly rele vant to the manufacturing or printing process, but involve averaging and mi xing of the fundamental fluid processes occurring. Recent advances in computing technology have allowed the development of a r ange of computer models to describe the behaviour of fluids in a more funda mental and precise manner. Such models range from a continuum approximation , through highly simplified structures, to an explicit three-dimensional vo id structure. Properties of the fluids or fluid systems are incorporated in to these models, and the modelled behaviour of the fluid compared to experi mental measurements. The process of obtaining agreement between the simulat ed and actual behaviour leads to an increased fundamental understanding of the processes that occur, and a predictive capability applicable to other s ystems. This overview will give examples of models of each of the three types. Bind er migration is modelled using a continuum approximation. Wicking and absor ption methods involve the use of a simple one-dimensional hydraulic stream tube, or modifications of this approximation. The behaviour of fluids in ex plicitly three-dimensional structures has been studied in various ways. Scr iven and co-workers have used a network with sites on a cubic lattice, with percolation controlled by constrictions in the throats joining the larger pores. Closer inspection of the shapes of voids has been undertaken by Knac kstedt, Niskanen, Bousfield, Toivakka and their co-workers. Knackstedt and Niskanen have used lattice gas methods, whereas Bousfield and Toivakka digi tise the void shapes. My own research group has developed the 'Pore-Cor' mo del comprising a cubic lattice of sites with simplified geometry, and has a ttempted to correlate properties using the same network for each. These various approaches are discussed in terms of the complexity of the vo id space, and the applicability and reality of the model. Using these crite ria, it is possible to look ahead to ways of improving both reality and app licability in the future. The ultimate goal is to achieve a computer model that is so close to realit y that it has a powerful role in explaining liquid permeation in current sy stems, and predicting fluid permeation characteristics in specially enginee red systems. Not only are such systems of direct commercial value in manufa cturing papers and coating with ever-improved properties, but also there ar e closely related environmental issues. These include the relative environm ental impacts of using water- or oil-based inks, and the relative energy an d environmental impact of using mined china clay or the various forms of ca lcium carbonate.