Artigo Técnico – Pigment Impacts

Authors: Janet Preston1, Martti Toivakka3, Andrew Findlay1, Rui Sergio Tsukuda2, Evandro Oliveira2, Edenil Costa21 Imerys Minerals Ltd., Par Moor Centre, Par Moor Rd, Par, Cornwall, UK PL253EH2 Imerys do Brasil – Av. Valentina M.F. Borestein, 545 – 08735-270 – Mogi das Cruzes/SP3 Åbo Akademi, Laboratory of Paper Coating and Converting, FinlandABSTRACTThe reduction of binder concentration in coatings, withinthe constraints of acceptable final paper quality, is a constantfocus for cost effective production of paper and board coatings.As binder levels are reduced, issues of strength and printabilitybecome more and more critical, due to the increasing probabilityof creating a heterogeneous binder distribution within the coatingstructure. Obtaining the maximum strength for a given amount ofbinder added is a goal for the cost-conscious paper maker.In this presentation, the complexity of strength measurement ofa coating is outlined as well as the factors which impact the IGTdry pick test. The dry pick test is shown to have a good correlationwith a commercial press trial; where coating pick on the printingblanket was assessed after a controlled print run; and is morerelevant than a laboratory wet pick test using pre-damping water.Pigment properties such as particle size distribution, surfacearea and shape factor of the minerals in the coating layer havea profound impact on the resultant strength and these arediscussed. The impact of how the latex is added to the slurry isalso determined. The broad particle size distribution GCC wasseparated into two fractions – fine and coarse. The effects of addingthe latex binder to either the fine or the coarse fraction separately,or in sequence, was explored, and compared to a situation wherethe latex is added to the combined blend. It was found that mixingthe latex first with the finer pigment fraction and then blending inthe coarse fraction does not significantly influence the strength ofthe coatings or indeed the other properties. It appears that mixingensured uniform binder distribution herein.The process of applying a coating colour onto a porous papersubstrate results in some loss of the aqueous phase into thesubstrate. Along with the water, some binder, latex or starch maybe lost, depleting the coating layer and reducing strength. Thismight be at least partly offset by enrichment of binder at thecoating-base interface. In this study, we have also attempted torelate the effects of binder depletion on the type and location offailure observed in the dry pick test. We present evidence thatthere is a depletion of latex binder from the coating surface duringthe coating application stage, and a subsequent enrichment of thebinder at the coating-basepaper interface. This effect is greatestwith an absorbent basepaper and a coating pigment which haspoor water retention characteristics.A total formulation approach, incorporating thickeners and waterretention aids is required to obtain an optimised coating solution.Keywords: Coating strength, coating color dewatering,consolidation, IGT dry pick testCorresponding author: Janet Preston, Imerys Minerals Par Moor Rd, Par, Cornwall, PL24 2SQ UK Phone: +44-7768427259e-mail [email protected]