Predicting Sheet Structure Effect on Ink Oil Contribution to Print Through of Newsprint

With the current trend towards lower basis weights for newsprint, pressroom performance issues such as print through are becoming more serious. In this report, we introduce a systematic method of quantifying the effect of non-drying coldset ink vehicle oil on the opacity of paper. This new approach allows us to predict (with high accuracy) paper's contribution to the negative effect of ink oil on print through. We demonstrate that the contribution of the oil to print through is inversely proportional to the pore volume of the sheet, which is defined as the product of the paper's porosity and caliper. Since ink oil contributes up to 70% to the measured print through, it is essential to control both porosity and caliper. Evaluation of 12 different newsprints with basis weights of 45 and 48 g/m2 showed that the use of deinked pulp as well as the filler can reduce the porosity of paper, thus leading to an increased propensity to opacity loss. Calendering the paper to a lower caliper helps smoothness, but reduces the pore volume, thus aggravating the print through problem.