Pigment impacts on strength in a paper coating & movement of binder during coating colour consolidation

The reduction of binder concentration in coatings, within the constraints of acceptable final paper quality, is a constant focus for cost effective production of paper and board coatings. As binder levels are reduced, issues of strength and printability become more and more critical, due to the increasing probability of creating a heterogeneous binder distribution within the coating structure. Obtaining the maximum strength for a given amount of binder added is a goal for the cost-conscious paper maker. In this presentation, the complexity of strength measurement of a coating is outlined as well as the factors which impact the IGT dry pick test. The dry pick test is shown to have a good correlation with a commercial press trial; where coating pick on the printing blanket was assessed after a controlled print run; and is more relevant than a laboratory wet pick test using pre-damping water. Pigment properties such as particle size distribution, surface area and shape factor of the minerals in the coating layer have a profound impact on the resultant strength and these are discussed. The impact of how the latex is added to the slurry is also determined. The broad particle size distribution GCC was separated into two fractions - fine and coarse. The effects of adding the latex binder to either the fine or the coarse fraction separately, or in sequence, was explored, and compared to a situation where the latex is added to the combined blend. It was found that mixing the latex first with the finer pigment fraction and then blending in the coarse fraction does not significantly influence the strength of the coatings or indeed the other properties. It appears that mixing ensured uniform binder distribution herein. The process of applying a coating colour onto a porous paper substrate results in some loss of the aqueous phase into the substrate. Along with the water, some binder, latex or starch may be lost, depleting the coating layer and reducing strength. This might be at least partly offset by enrichment of binder at the coating-base interface. In this study, we have also attempted to relate the effects of binder depletion on the type and location of failure observed in the dry pick test. We present evidence that there is a depletion of latex binder from the coating surface during the coating application stage, and a subsequent enrichment of the binder at the coating-basepaper interface. This effect is greatest with an absorbent basepaper and a coating pigment which has poor water retention characteristics. A total formulation approach, incorporating thickeners and water retention AIDS is required to obtain an optimised coating solution.