Details:
Summary:
In multi-color offset printing, backtrap mottle (BTM) has been a serious concern for many coated papers. 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, the local variation in the physico-chemical properties of coated paper affecting ink setting might be the key to backtrap mottle. In this part, two coated papers with much different degrees of backtrap mottle were examined physically and chemically using mercury porosimetry, stylus profilometer, environmental scanning electron microscope (ESEM), energy dispersive spectroscopy (EDS) and contact angle measurements. These two samples had identical coating composition and the same coat weight, but were dried under difference conditions. The results obtained indicate that the sample with heavy backtrap mottle has larger regions of closed surface on the surface compared with the samples with little backtrap mottle. There is little difference in distribution of binder, wettability and roughness between two samples. It is concluded that the presence of the closed areas is the most important factor for backtrap mottle with coated paper in multi-color offset printing.