A Structural Analysis of the Thermal Conductivity of Paper Coatings: Application of Particle Deposition Simulation to a Lumped Parameter Model

Abstract

Thermal conductivity of paper coating structures can be regarded as an important property for many processes involving the application of thermal energy on coated papers. This work analyses the thermal conductivity of coatings in terms of their structure. A Monte Carlo simulation-based particle deposition was used to create idealised two-dimensional coating structures. They acted as a master template for the superimposed parameters of a Lumped Parameter Model for the calculation of thermal conductivity, in which pigment and binder are treated as separate solid phases within a fluid (air). Binder alone was initially assumed to provide the necessary thermal connectivity. Comparison of the numerically calculated conductivities with corresponding experimental results, obtained from ground calcium carbonate pigment structures, showed generally lower calculated conductivities and clear differences in the change of conductivity when increasing latex binder content. Two different mechanisms are suggested as the cause of this lack of correlation. Firstly, it is shown that both the simulation and the current Lumped Parameter Model do not account sufficiently for pigment connectivity. This is the reason for the underestimation, especially evident when no binder is present. The nature of pigment connectivity is related to polymer dispersant on the pigment surface and the surface crystallite planar structures, if present, mostly related to larger particles. Secondly, it is confirmed that surface and colloid chemistry factors cause binder to accumulate first at pigment nodal points, which causes a disruption of the pigment packing already at 6 w/w% binder. This creates in homogeneity in the real coating structure which is not accounted for by the homogeneous assumption of the model. It could be shown that an introduced parameter of pigment connectivity becomes lower for the binder concentrations for which pigment disruption occurs. It is shown that the method is sensitive enough in respect to refinement of both pigment and latex connectivity factors to allow identification and parameterisation of the subtleties occurring in real colloidally interactive particulate systems that are reflected in the thermal conductivity response of the dried coating structure.