COATED PAPER QUALITY: AN INNOVATIVE APPROACH TO UNDERSTANDING AND OPTIMIZING DRYING STRATEGY

摘要

Back trap print mottle and drying strategy have been shown to be intimately linked. From researchers to papermakers, it is widely recognized that drying strategy can have a serious impact on paper printability. The objective of this study is to present an innovative approach allowing for a better understanding and quantification of the heat and mass transfer mechanisms involved in coating consolidation to predict print mottle. This paper draws upon a well-established heat and mass transfer numerical model based on the finite volume method for drying simulation. Using a mixed theoretical and experimental approach, we have developed a new module for water drainage from coating into the base stock and have coupled it with the existing drying simulation model. Consistent with extant research, the model is built around two levels of coating dryness: the first and second critical concentrations. However, we redefine these concentration levels as follows: 1. First critical concentration is defined as the situation where the compacted (immobilized) bed of particles is surmounted by a water (and soluble binder) film having a thickness of approximately the coating roughness level. 2. Second critical concentration simply corresponds to the pigment particles compaction. In the first drying phase, the model allows for water migration quantification to find a correlation between water migration and mottle in correspondence. The results are systematically compared to experimental results extracted from the existing literature. In the second drying phase, the model is used to compare the influence of drying load on the computed binder concentration at the surface. It is suggested, and demonstrated, that mottle can be the consequence of over-drying, leading to non-uniform binder deposit at the coating surface during the second phase of drying. As a consequence, coated paper quality is the result of a compromise: a high evaporation rate in the first phase to avoid water and binder migration, but halted at the right time to avoid the risk of binder skinning. Evidence is provided that the methodology used explains most of the results from the existing literature and supports real-life industrial cases. We intend to develop the new approach further using trials in future work.