Improving the combinations of critical properties and process parameters of printing and writing papers and paperboards by new paper-filling methods

The present study focused on improving the property combinations of printing and writing papers that become critical when lowering grammage. The target was to clarify whether improved combinations can be achieved through structuring precipitated pigments, pulps, and pulp fines in a new way. A kinetic model for diffusion of ions within the fiber wall was developed and used to interpret data from diffusion experiments in order to clarify the limitations of fiber loading, which was also briefly studied. Fiber loading, however, failed to provide paper with adequate properties. A new composite filler produced by precipitating calcium carbonate on pulp fines (SuperFill) was introduced and a mechanism for the paper filler suggested. Based on the diffusion coefficients obtained and a general knowledge of crystallization, it can be concluded that the tendency of calcium carbonate to precipitate into the lumen or deeper within the fiber wall during fiber loading is minor, when precipitation is performed for birch fibers using the reaction between calcium hydroxide and carbon dioxide. Consequently, fiber loading did not offer significant benefits compared with fillers added normally to the stock. SuperFill provided paper with a significantly improved combination of light scattering coefficient and strength compared with fillers added normally to the stock. The proposed mechanism for SuperFill consists of three elements. First, SuperFill provides paper with a more uniform spatial filler distribution with reduced agglomeration, which may contribute to paper light scattering coefficient through the combined effect of the pigment itself, interruption of the fiber bonded area, and reduced fines collapse. Second, the fines component of SuperFill most probably contributes to increased activation through enhanced bonding, resulting in increased paper strength. Third, it was shown that SuperFill influences paper in such a way that a more optimum pore structure with respect to paper light scattering is achieved. The combination of formation and retention was also improved with SuperFill. The economic value of SuperFill technology was estimated to be considerable.reviewe