Confocal Raman microscopy in chemical and physical characterization of coated and printed papers

Abstract

In offset printing on coated papers an uneven print, mottling, is a serious problem. Uneven styrene-butadiene (SB) latex distribution may be the reason for the mottled print. In this work, confocal Raman microscopy methods were developed for chemical and physical characterization of printed and coated papers. The main emphasis was laid on analysis of the SB-latex distribution in x-y-z direction.

A depth profiling method was developed. An immersion method was found to be essential in the depth profiling of light scattering coated papers. Model samples were employed to obtain a better understanding of the depth profiling method. The method was applied in the analysis of SB-latex migration and measurement of the thickness of paper coating layers and ink films.

Lateral mapping of paper coatings was developed for bulk and surface analysis. In mapping of coating bulk, the whole coating layer was measured and the analysis could be done through print. Lateral mapping of the bulk layer provided information on SB-latex distribution, and variations in coat weight and ink density could be mapped simultaneously. Bulk mapping was combined with light microscopy images acquired from the same area.

A higher magnification objective gave the means to measure the outermost 1-2 µm of the coating layer. Analysis of the coating surface is important because of the ink–coating interaction taking place on the coating surface. In the surface analysis, SB-latex content and its variation in different scales were of interest. The total areas analysed contained many areas of 1 mm × 1 mm.