Effect of Water Retention and Rheology Modifier (WRRM) Chemistry on Paper Coating Structure

摘要

Synthetic water retention and rheology modifiers (WRRM's) are gaining increasing acceptance in the paper coating industry. Their intended function is to provide an optimum rheological behavior in a range of shear rates and water retention of the coating colour. Optimal thickener dose and chemistry will thus provide certain flow characteristics and water retention properties required for coater runnability and coating structure. In the latter case, it is crucial to understand the influence of the WRRM's on the interactions in the dispersion that cause relevant physical and chemical properties of the coated sheet. Recently it was demonstrated that the synergistic effects induced by WRRM chemistry could influence the coating structure of the coating colour, particularly the distribution of the coating ingredients along the Z-direction. Optimal interaction prior to immobilization is a critical phenomenon necessary to avoid the strong particle co-aggregation or depletion, which can occur on both the lateral (X-Y) plane as well as along the depth (Z-direction). These findings enable opportunities to further tailor coating structures, via the control of shear-dependent interactions between the various coating ingredients. In our continuing efforts to improve the fundamental understanding of mechanisms important in dewatering of coating colours and the subsequent formation of the coating structure, particularly the surface morphology of coated sheets, the coating formulations evaluated earlier were applied on a pilot coater (CTC, Raisio, Finland), on a woodfree base stock. These coated sheets were subsequently printed using sheetfed offset inks on a commercial printer (FPC, Raisio, Finland). Latex distribution both on the X-Y plane and in Z-direction showed local concentration variations, which can be partially ascribed to the particle-thickener interaction state prior to the consolidation and immobilization of the coating layer. Latex concentration on the surface, determined semi-quantitatively from SEM images, indicated small differences between the samples, which seemed to affect the physical-chemical properties of the coated sheets. A method to estimate the ink film thickness based on image analysis is presented. Based on the results, we demonstrate that the ink film thickness can be controlled indirectly via appropriate choice of WRRM chemistry.