Application of Confocal Laser Scanning Microscopy (CLSM) for Observing Adhesives in Paper

Abstract

Many kinds of adhesives are used to produce paper and prints. However, it is difficult to observe the distribution of these adhesives in situ because most are transparent and of low content. We have developed a fluorescence staining technique, which observes adhesives in paper with a confocal laser scanning microscope (CLSM) after staining with specific fluorescent dyes. In this review, we present our observations using this technique on a wet strength resin in paper, the vehicle of offset printing ink on uncoated paper, and the coating binder in coated paper. The polyamide epichlorohydrin (PAE) resin, a wet strength resin, was selectively stained with Sulforhodamine 101 acid chloride, while the pulp fibers were stained with Acridine Orange. The distribution of PAE could be observed in a CLSM fluorescence image at the excitation wavelength of 543 nm, and the pulp fibers could be observed in a fluorescence image at the excitation wavelength of 473 nm. From this it was found that PAE tends to concentrate at fiber crossings. In order to quantify the penetration of the ink vehicle into the base paper sheet, cyan offset inks stained with Rhodamine B were printed on various paper samples. The location of the vehicle was determined with CLSM. The penetration of the ink vehicle was observed in three-dimensional images. Coated papers were soaked in a solution of Rhodamine B in ethanol, and then they were rinsed with pure ethanol. The styrene–butadiene latex of the coating binder was selectively stained with Rhodamine B. The distribution and position of latex in the coating layer was characterized with CLSM.