Compressional rheology of model paper coatings

• The surface treatment of paper is commonly undertaken in order to improve a set of key end-use properties, including optical response and printability. These properties can be influenced, to a significant extent, by the sub-surface structure of the coating layer. The nature of coating dewatering also has strong implications for machine runnability. Thus, there is a clear need to understand in suitable detail the nature of the coating consolidation process. In this study, we have applied a novel approach to characterising the equilibrium consolidation state of calcium carbonate sediments, both with and without polymeric thickener. The aim is to provide a quantitative link between the structure of consolidated layers and their network strength, though the compressive yield stress. A suspension, prepared at a given volume fraction of solid, is centrifuged to produce a consolidated particle sediment (or gel). The solidity variation of that sediment as a function of depth is then measured using one-dimensional magnetic resonance imaging (MRI), and the compressive yield stress calculated directly from the volume fraction profile. The results obtained are discussed in the light of particle network structure, the effect of polymer on particle consolidation, and the relation to viscoelastic properties of the suspensions. The link to the dewatering of coating suspensions, and structure formation in coating layers, is also considered.

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