Paper coatings and their influence on offset paper and ink interaction

In multicolor offset printing, ink film splitting and tack development are, to a certain extent, dependent on surface properties of the paper. These processes and coated paper properties are effectively altered by the choice of coating raw materials and the coating application method. In the present work, we quantified ink tack buildup rates and the pick strength of coated papers, using a laboratory technique which correlates well with the performance of papers in multicolor sheetfed and web offset commercial presses. The study focuses on polymeric properties of styrene butadiene latex and coating drying conditions. Results indicate that the rate of tack buildup decreases with: (a) increasing total binder level and the presence of a soluble cobinder, such as starch, in the coating formulation; (b) a higher degree of latex polymer crosslinking and a smaller latex particle size, and (c) coating drying with infrared and forced hot air when compared to drying with infrared only. The tack rate increases correspondingly with: (a) a raised level of butadiene in the latex (within the range 35-45% which is of practical interest for paper coating latices), and (b) incorporation of polystyrene into the coating, latex particles with a relatively high Tg. Coating drying plays an important role on tack rate of the final paper, irrespective of the binder system. This is attributable to the intensity of drying; higher paper surface temperatures and air impingement in drying with hot air lead binder to migrate to the coating surface and tack rate reduces. Binder film positioning and its solubility play a significant role on ink tack rates, primarily through the affinity of the ink solvent to the binder. Building on this work, coating formulation strategies can be developed that result in better paper and offset ink interactions, ultimately resulting in papers with excellent multicolor offset printability.