Measurement and modelling of heat transfer in paper coating structures
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Thermal conductivity of paper coatings is increasingly important in the performance of many printing processes, including traditional heatset web offset and thermal papers, as well as digital processes, such as electrophotography. This work studies the extension of a Modified Lumped Parameter Model, previously used successfully to describe talc coatings, to model the thermal conductivity–coating structure relationships of calcium carbonate (gcc: 60 wt% < 2 μm) coatings. A series of compact tablets were used to provide experimental values of thermal diffusivity and conductivity. The samples studied covered a range of latex binder addition levels, namely 0–25 parts, based on 100 parts pigment, of 0.2 μm styrene acrylate latex. Combining the observed thermal properties with knowledge of the pore structure changes induced by the latex addition, it is possible to establish initial correlation with the model, in which the connectivity of the structure is increased at low latex dose illustrating the initial increase in effective thermal conductivity. The practically hard sphere properties of the latex used, combined with the broad size distribution of the gcc, produce a disruptive packing effect as the dose level increases, such that the conductivity reflects a competition between the increasing connectivity provided by the latex versus the increasing relative pore size in the network structure. It is recognized that the fixed pigment volume in the model unit cell diverts from a true representation of the residual porosity. At the highest latex dose levels, the intrinsically less conducting properties of the latex begin to dominate.
Grateful thanks are given to Omya Development AG for its continuing financial support of this work. A particular thanks also to Dr. Cathy J. Ridgway, Senior Scientist at Omya’s R&D Laboratories, Oftringen, Switzerland, for her expertise and support in assisting in the sample construction and determination of coating pore structure.
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