Structure characterisation of pigment coating layer on paper by scanning electron microscopy and image analysis

The ever rising need for detailed knowledge of the structure of paper coating urges the improvement and development of new techniques for gaining new and valuable information. The assessment, study and characterisation of factors that are assumed to affect the interaction between printing inks and coated paper surface may be accomplished by microscopy and image analysis techniques. The scanning electron microscope (SEM) is a most suitable tool for morphometrical studies of fibres and paper. Digital images acquired in a SEM, by the secondary electron imaging (SEI) or backscattered electron imaging (BEI) mode have suitable quality and resolution for further image processing and analysis.The present study exemplifies how microscopy and image analysis may be applied for structural studies of the coating layer. Image processing and analysis routines for acquiring information on the coating layer surface roughness and porosity details like the pore shape, size and orientation are described. A routine, based on the Sobel operators, for measuring the pigment particle orientation, is also described. The developed semi-automatic image analysis routines are useful methods for morphometric analysis of paper structure details, thus facilitating numerical data acquisition of the pores and the pigment particles’ geometry. Parts of the obtained information is assumed to be unattainable by other methods at this point.To discern the binder in the coating layer from the embedding epoxy and thus observe individual pores in the coating, the binder has to be stained before embedding. In the study, osmium tetroxide (OsO4) was used to stain the latex binder in the coating layer. Morphometric analyses of paper cross-sections using SEM, BEI-mode images and stereo images were used to quantify the paper coating structure details. The smallest details to be quantified are approaching the limitations given by the imaging methods.It is shown that the staining procedures allow a quite accurate determination of pore fraction in the coating, and individual readings of the pores like pore area, aspect ratio and orientation. Two compared staining methods differed somewhat in their effect on the paper. OsO4 dissolved in water gave good staining throughout the base paper/coating sandwich, allowing good visualisation and assessment of the coating structure. However, moisture tends to make the base paper fibre expand, raising the paper thickness and thus the macro-roughness. When staining by dry OsO4 crystals, the base paper expansion is avoided, however, even the staining of the fibres is reduced. Critical coating parameters like mean pore area and micro-roughness of the coating layer do not differ between the two methods.In the study, the structure of commercial LWC paper coating layers were assessed and quantified by SEM, BEI-mode and digital image analysis of cross sections. It is quantified how clay particles cause a more densely packed coating structure compared to coatings containing mainly CaCO3. The pores of clay coatings have a larger aspect ratio and are oriented more parallel to the paper surface. Pore orientation is affected by the pigment particle orientation. Besides the size of the pores, their orientation is proved to be an essential parameter affecting the achieved print gloss level for a given paper grade. The pore’s orientation/ diameter ratio seems to be a suitable parameter for describing its geometry and influence on print gloss. For a given pore diameter, the more horizontally oriented, the higher the delta gloss (print gloss-paper gloss).