Fast evaluation of spatial coating layer formation using ultraviolet scanner imaging

Print quality demands are steadily growing. Several print quality problems are known to be closely related to coating layer uniformity (e.g., print or gloss mottle). This study evaluates the potential for using ultraviolet (UV) scanner imaging as a fast method for characterization of the spatial coating layer formation. The method measures the visual light response of the excited fluorescent whitening agent (FWA) in the coating color. UV scanner imaging was compared to two reference methods for determining the coating layer uniformity, the burnout test, and β-radiography. Within the study, a commercially produced lightweight coated paper, a laboratory coated commercial woodfree base paper, and laboratory coated handsheets were examined. Image analysis showed that UV scanner imaging indeed reveals the coating layer formation, especially for lower coat weights. Furthermore, the influence of varying amounts of FWA in the coating color and in the base paper on the measurement results was investigated. The results indicate that UV scanner imaging of coating layer uniformity works best for coat weights below 20 g/m² and moderate FWA concentrations in the coating color.