Application of imaging reflectometry to studies of print mottle on commercially printed coated papers

It has been well documented that homogeneous coating structures are necessary for obtaining good, mottle-free printed papers. Methods such as mercury intrusion porosimetry are widely used in the paper industry, but will deliver a bulk porosity measure, which contains contributions from both the surface layers and the bulk of the coating. One way of assessing the surface porosity is to measure its effective refractive index (RI) because the RI measured is a combination of the material RI and air. In this work three papers coated with PCC, kaolin, and GCC were RI mapped using an imaging reflectometer. The data were then treated with a mathematical transform in order to be able to quantify the degree of variation that was taking place at each length scale. These results were then compared to the print mottle, which was evident after both lab scale printing and full scale sheetfed printing of the pilot coated papers. The commercial mottle was quantified both visually and using image analysis and there was a reasonable correlation between these two methods. In the halftone printed area, the propensity for mottle correlated with the degree of porosity variation of the coating. However, the rankings for the full tone print areas better correlated with the short time water absorbency of the coatings. FFT band-pass analysis confirmed that the same spatial distribution of unevenness occurred in the basepaper, coating layer and printed areas.