Characterization of Pigment Coating Layer Structure Using SEM and Digital Image Analysis Techniques, 2000 Coating Conference Proceedings

G. Chinga, T. Helle, P. O. Johnsen--Digital image analysis and electron microscopy techniques have proved to be reliable techniques for the study and characterization of paper structure details. SEM technique and resolution, and imaging speed and analysis that have been especially useful for the study of coating layer structures. Backscatter (BEI) imaging of ground cross sections of epoxy embedded coated paper yields digital SEM micrographs excellently suited for direct image analysis, without the sample distortion experienced when using cut samples. Detailed analyses of the distribution, local composition and structure of the coating layer were thus gained in the present study. Obviously, details like pigment particle sizes and inter-particle space volume fraction, as well as particle orientation will affect the coating layers’ interaction with the printing ink. In the present study, these factors were assessed and related to some coating color compositions, coating layer thickness variations and calendering of the paper. Several lab coated paper grades were studied to characterize the coating layer in the micron and sub-micron scale. Relationships between coating thickness variation, orientation of pigment particles, pores (inter-particle volumes) and calendering were analyzed. The detailed thickness in the coating layer varies depending on the base paper’s surface profile. For thin coating layer details, that is, over “hills” in the base paper, the inter-particle void volume was lower than in thicker areas. Also, micro roughness was lower. Probably, this is caused by higher blade pressure on the coating layer during the coating. Calendering of the paper enhances these differences. The clay particle orientation was quantified by assessing the angle between the particles’ largest axis and the paper surface plane. The particles are clearly oriented already off the coater, and they become more oriented by calendering. Even here, the effect is more pronounced in thin coat area than in thicker ones.