Characterization of Coating Layer Structural and Chemical Uniformity for Samples with Backtrap Mottle
Publication Name: Nordic Pulp and Paper Research Journal
Backtrap mottle is a common printing defect that is difficult to correct. A good understanding of the coating structure that leads to backtrap mottle is still lacking in the literature. A confocal laser scanning microscopy (CLSM) and other techniques are used to evaluate samples that had different tendencies towards backtrap mottle. The coating structure is observed by staining the coating with a fluorescent dye. The final location of the ink varnish was observed by staining the alkyd varnish with a fluorescent dye. Final prints that had backtrap mottle could be directly observed after printing from the fluorescence of the magenta ink pigment. The distribution of magenta ink tends to be obscure on prints that have backtrap mottle. Samples with uneven coated layer distributions correlate to the occurrence of backtrap mottle even though the average coat weight is constant between samples. In addition, the fluorescence intensity was strong at portions of the thin coated layer; this may be caused by a decrease in the coating pore volume in these regions. As a result of the observation of varnish penetration, samples with uneven ink varnish distribution tend to have backtrap mottle. Moreover, the varnish penetrates little in regions with thin coated layers.