Reduction of Back-Trap Mottle Through Optimization of the Drying Process for Paper Coatings, Part I, 1997 Coating Conference Proceedings

The drying of paper coatings can have a significant effect on the back-trap mottle of coated paper. The primary objectives of this study were to identify the optimum drying strategy for a mill and the best latex binder to reduce back-trap mottle. Samples were prepared on a pilot coater under different drying conditions and were printed on a commercial six-color press. These data were statistically analyzed using a Thurstone all pairs comparison ranking to generate a numerical scale for mottle, which was used for regression and factor analysis. Computer modeling was used to better understand the critical drying process parameters and to translate the pilot coater results to the drying process of the mill. In addition, scanning electron microscopy (SEM), cryo-SEM, and x-ray photoelectron spectroscopy (XPS) data were obtained to help understand the mechanism(s) for mottle. Key findings included the following: 1) The variables having a significant effect on back-trap mottle were: (a) the evaporation rate within a critical coating solids zone of 72 to 78%, (b) the sheet temperature profile between the second and third pilot coater air flotation dryers, and (c) the type of S/B latex binder. 2) The calculated critical drying zones were narrower than the region bounded by the first critical concentration (FCC) and second critical concentration (SCC). In fact, the degree of back-trap mottle seems to be highly sensitive to drying conditions near the FCC, which in turn, seems to be located closer to the start of the falling rate of evaporation phase than previously thought. 3) XPS data showed there were no significant differences in the distribution of surface components between samples with good and poor mottle, except when a mild-early followed by a harsh-late drying profile was used. Also, drying conditions, over the range covered in this study, did not significantly affect end-use coating properties other than back-trap mottle.