MRS Advances (2022) 7:789–798 https://doi.org/10.1557/s43580-022-00282-7
ORIGINAL PAPER
Relating papermaking process parameters to properties of paperboard with special attention to through‑thickness design
Mikael Nygårds 1,2
Received: 23 March 2022 / Accepted: 26 April 2022 / Published online: 6 May 2022 © The Author(s) 2022
Abstract A biaxial stress state has been proposed to formulate a failure criterion for paperboard during bending. About 100 paper- boards have been splitted, such that top, middle, and bottom plies have been free-laid and tested in the machine direction, cross-machine direction as well as in out-of-plane direction (ZD). The purpose was to determine the failure stresses and its dependency of papermaking parameters: density, degree of orientation, and fiber length for each layer. A linear model to predict the geometrical strength of a plies was suggested. Analytically simulations of different paperboard structures behavior during bending were performed. The density of the middle ply affected the location of the failure position in ZD, as well as the maximum bending moment. The impact of orientation and degree of anisotropy was simulated, which can be used to optimize the ZD property gradient by tweaking the properties, and hence optimize paperboard performance.
Introduction In the papermaking process, fibers with different fiber length are mixed into a pulp. In the process, it is possible to steer a paperboard machine to control the density by refining and by chemical additives in different plies in a paperboard. It is also possible to steer the orientation of the fiber by control- ling the headbox outlet speed in relation to the moving web. Hence, by utilizing papermaking parameters paperboards can be engineered to have different properties in the plies of a multiply paperboard. The product paperboard is characterized with respect to its bending stiffness, thickness, or grammage. To opti- mize these properties and the paperboard functionality the through-thickness (ZD) profile can be engineered. The straightforward path to optimize the bending stiffness would be to make a paperboard with and I-beam structure. How- ever, when a paperboard is folded an I-beam structure might not be optimal to comply with the stress state that arise. One complicating factor is that the in-plane tensile and compres- sion behavior is different for paperboard [1]. Depending on
how the ZD profile is engineered different failure mecha- nisms can be activated when a paperboard is folded. Before a paperboard becomes a package, it needs to be converted; creasing is used to score the paperboard such that it will fold along pre-defined lines [2]. For optimal convert- ing behavior it is important to control where the intentional damage created during creasing develops. This put addi- tional requirements on the paperboard design that can be considered for optimal folding performance. The key element to design paperboard properties in the ZD is to utilize multiply board structures, such that at least three plies can have different properties. To evaluate how the effect of the papermaking process in each ply, it is necessary to free- lay the plies and perform testing, where the intention is to isolate the constituent plies, and test each of them separately. While multiply paperboard has been available for a long time, effective and rigorous methods to free-lay (or isolate) the plies have not been available until recently. Initially surface grinding was used [3], and the technique has also successfully been used to determine properties of top, middle, and bottom plies [4–7]. The technique works to determine properties, but is dependent on good calibration of the machinery, and is very time con- suming. A more time efficient machine has been developed by Fortuna Gmbh [8], where a rotating knife is placed after a nip. With this technique top, middle, and bottom plies can be free-laid in a very time efficient procedure. The machine has also been used to characterize properties of plies [9–11].
* Mikael Nygårds
Mikael.nygards@billerudkorsnas.com
1
BillerudKorsnäs, 801 81 Gävle, Sweden
2 Solid Mechanics, Department of Engineering Mechanics, KTH, 100 44 Stockholm, Sweden
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