The Porous Structure of Paper and Ink Penetration

Abstract

Ink penetration is of great importance in printing processes. Especialy, an initial setting of printing ink is greatly influenced by the ink penetration which depends on the porosity of a printing paper. In order to investigate the ink penetration accurately, we developed a measuring system of the ink penetration depth using a CCD video camera and an image processing board (Photron FDM 98-4). The ink penetration depth was determined from the reflection density of the reverse side of the printing paper. Suspensions of carbon black in linseed oil at various concentrations were used as sample inks and the penetration behavior of these suspensions into wood free paper and coated paper was observed. The phenomenon of ink penetration can be predicted by Lucas-Washburn's equation; the penetration depth increases in proportion to square root of elapsed time and mean radius of pores in a paper stock. The ink penetration speed in the coated paper is not so fast and not influenced by a consistency of ink which depends on the volume fraction of pigment. The reason for this phenomenon is explained as follows. The pore structure in the coating layer is different from that in the body stock and pore size of the former is extremely small. The pigment particles and/or flocs formed by them lerger than the pore size of the coating layer may be filtered. The measurement of the time dependence of the penetration depth is an effective method to investigate the interconnected fiber structure in paper stocks. We can also evaluate the ink-setting characteristics from the dependence of the penetration depth on time.