Analysis of Porosity Distribution in Coating Layers, 1997 Advanced Coating Fundamentals Symposium Proceedings

A method is developed which permits the porosity distribution of the coating layer on calendered paper to be calculated. The method is based on a burn-out test, which is performed on both uncalendered and calendered paper, and measurement of the reflectivity distribution of the burned coating surfaces using image analysis. For uncalendered material, the reflectivity distribution is a measure of the coating mass distribu-tion, and for calendered paper, it is a measure of the sum of the distributions of mass and porosity. The porosity distribution is obtained as the difference between the reflectivity distributions for calendered and uncalendered paper. Mercury porosimetry was used to transform the reflectivity into porosity in percentage units.

The method was applied to a blade-coated and a MSP (Metering Size Press)-coated material produced and superca-lendered at three moisture contents in the 3-8 % range. The paper was printed in a sheet-fed offset press and the mottle in the print was evaluated. A non-uniform porosity distribution was found in the coating layer of the MSP-coated material which led to a non-uniform ink transfer and to mottle in the print.