ANALYSIS OF POROSITY DISTRIBUTION IN COATING LAYERS

A method has been developed that permits the porosity distribution of the coating layer on calendered paper to be calculated. The method is based on a burnout test, which is performed on both uncalendered and c alendered paper, and measurement of the reflectance distribution of th e burned coating surfaces using image analysis. For uncalendered mater ial, the reflectance distribution is a measure of coating mass distrib ution, and for calendered paper, it is a measure of the sum of the dis tributions of mass and porosity. The porosity distribution is obtained as the difference between the reflectance distributions for calendere d and uncalendered paper. Mercury porosimetry was used to transform th e reflectance into porosity in percentage units. The method was applie d to a blade-coated and a metering size press-coated (MSP-coated) mate rial produced and supercalendered at three moisture contents within th e 3-8% range. The paper was printed in a sheet-fed offset press, and t he mottle in the print was evaluated. A nonuniform porosity distributi on was found in the coating layer of the MSP-coated material; this led to a nonuniform ink transfer and to mottle in the print.