A Re-Evaluation of Factors Controlling Print Rub on Matt and Silk Coated Papers, 2006 Coating & Graphic Arts Conference

Print rub, or ink scuff, remains one of the greatest challenges for the successful development of matt and silk matt offset coated papers. Using a recently proposed method for evaluating print rub, involving a non-slip pressure and friction-induced stress contact between a printed and unprinted sheet, this work seeks to test a newly-formulated hypothesis that the ability to improve print rub resistance is a function of coating surface capillarity (absorption) and adsorption properties, in combination with the roughness and compressibility/conformability of the basepaper-coating composite on the fibre-floc scale. A series of coated papers have been tested, ranging from woodfree single to multicoated grades, including the evaluation of compressibility and stretchability under pressure and shear both individually (pre-rubbing), to simulate external real-up, calendering and converting processes, and in combination with the aged and conditioned printed surface. The findings are used to illustrate the corner points of an hypothesis, from which is deduced that the most resistant surface to print-rub is a combination of high capillarity and high surface area of the pigmented coating, aiding absorption and adsorption, together with a uniformly compressible basepaper-coating composite structure, such that contact shear forces are minimised. Under conditions of compromise, such as a coating surface exhibiting low capillarity, an incompressible/non conforming sheet can help reduce print rub due to minimised contact area. The evidence tends to suggest that a too rigid coating-basepaper combination, such as developed by many modern paper machines together with multi-coating, is exacerbating performance difficulties in this paper sector.