Cause of Backtrap Mottle: Chamical or Physical?, 2000 Coating Conference Proceedings

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Y. Xiang, D. W. Bousfield, P. Coleman, A. Osgood

Backtrap mottle (BTM) is a serious concern for coated papers in multi-color offset printing. In the first part of our study, the point-to-point variation in the tack buildup on coated paper was found to correlate well with the backtrap mottle. Therefore, we concluded that the local variation in the physico-chemical properties of coated paper affecting ink setting is the key to backtrap mottle. In this part, the same series of eight samples with different degrees of backtrap mottle were examined physically and chemically using mercury porosimetry, environmental scanning electron microscope (ESEM), laser-induced plasma spectroscopy (LIPS), and energy dispersive spectroscopy (EDS). These samples had identical coating composition and the same coat weight, but were dried under different conditions. All large scale tests, such as gloss, brightness and mercury porosimetry results, do not show any significant differences between good and poor printing samples. The results obtained indicate that the sample with heavy backtrap mottle has more regions of "closed" regions on the surface compared with the paper sample with little backtrap mottle. No significant differences in the point-to-point variation of chemical composition were found between these samples with EDS or LIPS. The non-uniform distribution in surface pore structure seems to be the most important factor related to backtrap mottle with coated paper in multi-color offset printing. Two mechanisms are proposed to explain the variation in surface pore distribution within a sample.