A Structural Approach to Paper Surface Compressibility – Relationship with Printing Characteristics

Abstract

Three-dimensional topographical maps of paper surfaces under load have been quantified using the confocal laser scanning microscopy. Distributions of the paper surface pores of the same area under different loads were evaluated by the Equivalent Surface Pore (ESP). The ESP roughness of the uncompressed and compressed surfaces of TMP and bleached kraft papers, calendered to the same Print-Surf roughness with different calendering processes, were used to evaluate the local static compressibility of these paper surfaces. Assuming an exponential decay of roughness with pressure, the local static compressibility is defined as the slope of the roughness as a function of the logarithm of the applied pressure. Upon calendering, the local compressibility of the paper surface decreases. The compressibility after calendering depends both on the calendering process and on the furnish. The stiffer TMP fibres present more residual compressibility than the kraft fibres, already pre-collapsed in the uncalendered sheet. The surface compressibility increases with the internal pore volume. The calendered papers were gravure printed at different printing pressures and the number of missing dots counted. A theory is developed which links roughness to ink coverage. It is proposed that roughness is linearly related to the logarithm of the number of missing dots where the slope represents the surface compressibility. Theoretical derivations have been experimentally verified. It was also found that the static roughness is linearly related to the dynamic roughness.