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.

The out-of-plane properties of paperboard are important in several converting applications such as printing, sealing, creasing, and calendering. A juxtaposed tensile and compression curve in the z direction (ZD) will, however, appear to have a kink or discontinuity at 0 stress. The purpose of the present work is to capture the continuous transition between tension and compression and to increase the understanding of the complex ZD properties of paperboard by cyclic testing. In this attempt to unify the ZD tensile and compressive behavior of paperboard, samples were laminated to the testing platens using heat seal laminate film. The method for adhering the samples was compared to samples that were laminated and glued to the testing platens. The edge effects of the cutting method were evaluated in compression testing with samples not attached to the testing platens. The flat slope seen in the initial part of the pure compression curve disappeared when the samples were laminated to the testing platens. The flat slope was instead replaced by a continuous response in the transition across 0 N. The stiffness in the transition region resembled the response in tensile testing. When the testing is cycled, the material exhibits a history dependence. Starting the cycle in either compression or tensile will show an effect on the stiffness at the transition, as well as the compressive stiffness. However, the ultimate tensile strength is unaffected. © TAPPI Press 2024.

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.

The ZD-tensile strength is tested by attaching the top and bottom sides of a paperboard to rigid blocks that are pulled apart. In a production laboratory the strength is recorded using a tape as an adhesive. In specialized laboratories a more thorough method is available that also measures the force-displacement curve of the sample. The advanced method involves laminating and gluing the paperboard sample to metal blocks which are mounted in a universal testing machine. 

In this study the advanced ZD-tensile method was refined by removing the glue step and laminating the paperboard directly to the blocks. The new method was validated against the regular method with adequate results. The limits of the refined method were explored with regards to ZD-strength and paper/paperboard thickness. 

In an attempt to unify the ZD-tensile and -compressive behaviour of paperboard, samples were laminated and tested in combined compression and tension testing. The compressive properties were compared to non-laminated samples. The laminated samples showed a different behaviour than the non-laminated samples. The flat slope seen in the initial part of the pure compression curve disappeared, replaced by a continuous response passing 0 N. The stiffness in this region resembled the response in tensile testing.