Processes that convert paperboard into finished products include, for example, printing, where the paperboard is subjected to rapid Z-directional (ZD) compression in the print nip. However, measuring and evaluating the relevant properties in the thickness direction of paperboard are not necessarily straightforward or easy. Measuring at relevant, millisecond deformation rates further complicates the problem. The aim of the present work is to elucidate some of the influences on the compressive stiffness. Both the initial material response and the overall compressibility of the paperboard is studied. In this project, the effect on the material response from the surface structure and the millisecond timescale recovery is explored. The method utilized is a machine called the Rapid ZD-tester. The device drops a probe in freefall on the substrate and records the probe position, thus acquiring the deformation of the substrate. The probe is also allowed to bounce several times on the surface for consecutive impacts before being lifted for the next drop. To investigate the time dependent stiffness behavior, the probe is dropped several times at the same XY position on the paperboard from different heights, thus achieving different impact velocities. The material response from drops and bounces combined allows study of the short-term recovery of the material. The material in the study is commercial paperboard. The paperboard samples are compared to material where the surface has been smoothed by grinding it. Our study shows that there is a non-permanent reduction in thickness and a stiffening per bounce of the probe, indicating a compaction that has not recovered in the millisecond timescale. Additionally, a higher impact velocity has an initial stiffening effect on the paperboard, and this is reduced by smoothing the surface.

Industry processes, such as printing, subjects paperboard to rapid, Z-directional compression. However, measuring and evaluating the relevant properties in the thickness direction are not necessarily straight forward or easy. Measuring at relevant, millisecond, deformation rates complicate the problem further. The aim of the present work is to elucidate on some of the influences on the compressive stiffness. Both the initial material response and the overall compressibility of the paperboard is studied. In this project the effect on the material response from the surface structure and the millisecond time-scale recovery is explored. The method utilized is a machine called the Rapid ZD-tester. The device drops a probe in free fall on the substrate and records the probe-position, thus acquiring the deformation of the substrate. To investigate the time dependent stiffness behavior the probe is dropped several times at the same xy-position on the paperboard from different heights, thus achieving different impact velocities. The probe is also allowed to bounce several times on the surface before lifted for consecutive drops. The drop-bounce cycle allows study of the short-term recovery of the material. The material in the study is commercial paperboard. The paperboard samples are compared to material where the surface has been smoothed by grinding it. Our study shows that there is a non-permanent reduction in thickness and a stiffening per bounce of the probe, indicating a compaction that has not recovered in the millisecond timescale. Additionally, a higher impact velocity has an initial stiffening effect on the paperboards, and that this is reduced by smoothing the surface.

Print mottle is a serious and yet common print defect in offset printing. An imbalance between the feed of fountain solution and the ability of the paper substrate to absorb and transport this water away from the surface can cause moisture/water interference problems. In the study presented here, we have investigated the uniformity of aqueous absorption and coating structure of pilot-coated papers with different types and dosages of dispersants and linked this to print mottle and uncovered areas (UCA). In earlier studies, the print quality of these papers indicated that a moderate addition of excess dispersant caused ink refusal, ink-lift-off (ink-surface adhesion failure) and water-interference mottle when printing at elevated fountain feed. In the present study, we have shown that a majority of the samples with uneven water/moisture absorption and an uneven burn-out reflectance tended to have more severe printing problems related to surface-moisture/water.An aqueous staining technique was used to characterise the absorption non-uniformities. This method has been developed previously with focus on absorption of flexographic water-based inks but can clearly give relevant information also for offset printing, when it comes to moisture/water interference mottle. .

The interaction between a liquid and a paper surface is important for a number of paper treatment processes, where absorption is of special significance during printing. Many absorption measurement techniques use a large available volume of liquid to characterise absorption, when compared to the volume of the coating. The uniformity of the absorption is also seldom characterised. We have developed a new technique, which is presented in this article, to study the uniformity of absorption of a small amount of liquid. This technique is based on the short-time absorption (tenth of a second) of a coloured liquid, the blotting of excess liquid and a characterisation of the pattern of the stain. This method made it possible to detect differences among coating layers with different compositions. In many cases, the absorption non-uniformity could be linked to variations in the coating thickness and/or wettability. The thinner and thicker areas of the coating layers were interpreted as having different pore structures. Neither the coating thickness nor the wettability could provide a full explanation, which showed the need to develop a method to characterise absorption uniformity instead of only relying on measuring the total absorption potential.