Effective Thermal Conductivity of Liquid-Saturated Coatings and Their Liquid Vaporisation Behaviour

摘要

The effective thermal conductivity is an important element in understanding the thermal response to heating of a paper coating, e.g. during drying in heatset web-offset, and thus it not only affects the drying efficiency but also affects print quality detriments like web fluting. This study examines both the effective thermal conductivity of liquid-saturated ground calcium carbonate coating structures as well as the vaporisation behaviour from these structures. The liquids used for saturation were mineral oil and water in order to resemble ink and fountain solution, respectively, both of which are present in the traditional heatset web-offset process. The effective thermal conductivities of liquid-saturated coating structures are discussed in regard to the corresponding unsaturated systems by using a Lumped Parameter Model. It is shown that the liquid saturation has a dominant effect in determining the effective thermal conductivity. Since this effect is not fully captured by the model, other mechanisms like an apparent pigment-pigment connectivity increase by liquid bridging and the role of liquid in changing the contact resistance during the measurement of thermal conductivity are discussed. In addition, the transformation of three-dimensional structures to an equivalent two-dimensional modelling is evaluated. The vaporisation behaviour of mineral oil and water is studied by a thermogravimetric analysis. By following the changes in maximum evaporation temperatures and evaporation rates, the addition of binder is seen to lead to a reduction in the vaporisation rate of both liquids. Since there is little to no interaction between the liquids and the binder, the confinement caused by the geometry change induced by binder addition is identified as the mechanism resulting in elevated vapour pressure within the structure.