Three-dimensional observation of coated paper by confocal laser scanning microscope

A new method to image the coating layer uses a confocal laser scanning microscope (CLSM). A fluorescent dye, Rhodamine B, stains coated paper and is rinsed off. Rhodamine B preferentially stains latex binder in the coating, only slightly staining kaolin clay, calcium carbonate pigments, and cellulose in the base paper. A laser beam scans a specimen, and a photodetector collects the flourescence light emitted. Information about the coating layer can be obtained nondestructively over a wide area. With this new technique, the three-dimensional structure of a paper coating layer was observed. The results were used to characterize the depth of the coating layer penetration into the base paper, the occurrence, of bubbles, and the possible distribution of latex and optical brightener. The structure of double-coated papers was also discernible. The technique could be used to locate other coated paper components if they can be tagged with fluorescent dye.