The Effect of Pigment Particle Size, Chemically Structured Kaolin, and Coating Pore Structure on Rotogravure Print Quality, 2006 Coating & Graphic Arts Conference

Light Weight Coating (LWC) studies were conducted using a cylindrical laboratory coater (CLC) to determine the effects of pigment particle size, chemical structuring of kaolin, and coating pore structure on coated sheet properties and rotogravure printability. These studies examined pigments spanning a wide range of particle size, while binder content was held constant. The results showed that, as expected, the larger the particle size, the larger the coating pore size. Pore volume remained relatively constant for the standard kaolin samples while it decreased for talc. Rotogravure printability as measured by the Heliotest Missing Dot Method improved with increasing particle size while Parker Print Surf (PPS) did not show any trend with particle size. Chemically structured kaolin increased pore size, pore volume, PPS smoothness, brightness, opacity, and most importantly rotogravure printability, while it tended to decrease sheet gloss compared to an unstructured counterpart.

Analysis of all of the available data indicated that, of the variables evaluated in this study, large pore size and smooth surface were the two most important parameters for improving rotogravure printability. Deficiency of one parameter may be compensated for by the other as long as minimum threshold smoothness is achieved. The Compressibility Factor measured from PPS values at two different pressures, and often used as an indicator of compressibility, did not correlate well with printability. Now that the significant factors affecting rotogravure printability have been identified, coating formulations can be designed to increase pore size and smoothness by appropriately selecting coating materials for improved rotogravure printability. From the pigment selection standpoint, a coarser delaminated clay or talc would increase pore size and smoothness. Chemically structured kaolin would also provide larger pore size and a smoother surface.