Heat transfer through porous multiphase systems measurement, modelling and applications in printing of coated papers

摘要

This work examines the thermal transfer through porous media by means of measurement of the effective thermal conductivity and modelling of the structural parameters. While most of the work is of a general nature, the porous media in focus are pigment plus latex binder systems as used for paper coatings. As such, the application field of thermal effects in printing is discussed. At first the use of a dynamic measurement method for the thermal conductivity of porous coating pigment binder systems in the form of tablets is evaluated. In addition, a Lumped Parameter Model is developed to analyse the measured thermal conductivities in terms of the material volume fractions and their structural configuration. The model therefore uses two independent structural parameters instead of porosity alone as the governing parameter. This enables a discussion following modelling approaches adopting parameter modelling and structural modelling. A combination of both is used to discuss the solid-solid connectivity of pigment binder systems. It is shown by the model parameter of pigment connectivity that a disruption of the pigment packing already at low concentrations of binder occurs related to surface and colloid chemistry factors which cause the binder to accumulate first at pigment nodal points. The analysis of different pigment binder systems shows that, due to the presence of two interacting solid phases, the effective thermal conductivity cannot be modelled by using porosity and pore structure alone, but also requires the additional parameter of dual solid phase connectivity. It is proposed, therefore, that a combination of pore structure analysis and thermal conductivity measurement can be used to identify and parameterise subtleties in interactive particulate systems. For the application of toner fusing in electrophotography it is shown that a thermally insulating coating layer leads to a concentration of the fusing energy on the surface, thus aiding the fusing/adhesion development of the toner while shielding the base paper from undesired heating. In the heatset web-offset drying process, an insulating precoating in combination with liquid barrier properties can lead to a similar shielding of the base paper and thus helps to maintain an even moisture profile effectively reducing the waving/fluting tendency.