Back-trap mottle is a common and serious print quality problem in lithographic offset printing of coated papers. It is caused by nonuniform ink retransfer from an already printed surface when it passes through a subsequent printing nip with the print in contact with the rubber blanket in that nip. A nonuniform surface porosity gives rise to mottle. A key parameter in mottling contexts is the coating mass distribution, which must be uniform. Good relationships between mottle and mass distribution have also been reported; the mottle pattern coincides with that of the coating mass distribution. High blade pressures, compressible base papers, and high water pick-up between application and metering, which plasticizes the paper, yield uniform mass distributions, but these parameters might have a detrimental effect on the runnability in blade coating in terms of web breaks. The general opinion has been that nonuniform surface porosity is caused by binder migration and enrichment of binder in the coating surface, more in the high coat weight areas and less in the low coat weight areas. Recent research has suggested that a more probable mechanism is depletion of binder in the coating surface. Nonuniform shrinkage of the pigment matrix (filter cake) formed during the consolidation between the first critical concentration (FCC) and the second critical concentration (SCC) is another possible mechanism. Relevant relaxation times for latex and the time scales for consolidation show that the high coat weight areas shrink more than the low coat weight areas in the coating layer. A recent pilot-scale experiment has shown that the drying strategy did not affect the differences in shrinkage between high and low coat weight areas. The drying strategy has a pronounced impact on mottle. A high evaporation rate at the beginning of the evaporation results in less mottle than a low evaporation rate. The least mottle is obtained if the drying is performed with a gap in the course of evaporation between the FCC and the SCC.

A pilot coating trial was performed to study 1) whether the drying strategy introduces porosity variations in the coating layer and 2) whether porosity variations caused by the subsequent supercalendering are linked to back-trap mottle in offset prints. The porosity variations and the mean porosity were indirectly measured using a burn-out test. Coating colours were compared with three different binder systems 1) carboxy methyl cellulose (CMC)/SB-latex, 2) dextrin/SB-latex and 3) oxidized starch/SB-latex. The results showed that neither the mean porosity nor the porosity variations were affected by the drying strategies studied. During calendering the coating with the CMC/SB-latex binder system was compressed the most, but the porosity variations were nevertheless small. Both the compression and the porosity variations were influenced by the drying strategy, which shows that the strategy had an impact on the mechanical properties of the dry coating layer. With CMC/SB-latex and dextrin/SB-latex, the mottle increased when the porosity variations introduced by the calendering increased. In the case of the oxidized starch/SB-latex, the opposite was true. It was concluded that with CMC/SB-latex and dextrin/SB-latex, print mottle is caused by calendering and with oxidized starch/SB-latex by non-uniformities introduced during dewatering and drying, possibly binder migration.

A pilot coating trial has been carried out where the synthetic binder in the coating colour was partially replaced with dextrin starch. The coating thickness and the coating thickness distribution were investigated by means of burnout tests and mercury porosity measurements. The coating thickness distribution is governed by the surface profile of the base paper underneath the blade; this surface profile is controlled by the compressibility of the base paper and the blade pressure. Dewatering of the coating colours plasticises the paper and increases its compressibility; a linear relationship between dewatering and the standard deviation in the coating thickness was found for a given blade pressure. The characterization of the coating layers showed a small but measurable increase in the thickness variation for increasing starch content, which was caused by the higher water retention of the starch colours. The surface profile of the uncoated and coated paper was investigated with an optical profilometer. The data from those measurements and the coating thickness distribution investigation were used analytically to calculate the roughness change of the base paper during coating. The roughness change was found to be inversely proportional to the original base paper roughness, indicating that the rougher base paper was more compressible.

Polyethylene-extrusion-coated greaseproof paper exhibited significantly lower oxygen permeability than predicted from the oxygen permeability of polyethylene and greaseproof paper itself. Two mechanisms proposed for the superior oxygen permeability of polyethylene-extrusion-coated greaseproof paper were compared.