Design of multilayer paperboard for optimisedcompression strength

Abstract: This master thesis was issued by BillerudKorsnäs Gruvön. The focus of this thesis was theinvestigation of the effects different fibre types has on the compression strength of the wholepaper product. This stems from wanting to better understand how to produce a better paperboardconstruction with respect to stacking, storage, and transport. The goal, thus, was to learn howto better design multiply board - utilising laboratory test and a literature study. An alternative way of forming a multiply paperboard, stratified forming, has been studied -with insight in to the past and the present. A test plan was developed, the decision was to test bleached softwood kraft pulp (BSKP),unbleached softwood kraft pulp (UBSKP), neutral sulfite semi chemical Pulp (NSSC), andbleached hardwood kraft pulp (BHKP). The grammage was set to 140 g /m2 for all sheets. Thecombinations tested for the multilayered sheets were NSSC/BHKP, NSSC/BSKP, BSKP/BHKP,and BHKP/UBSKP with the configurations of 40/100, 70/70, and 100/40 g /m2 for all respective combinations. The method to produce a sheet for use in laboratory test was the dynamic sheet former(DSF). The sheets were then subsequently pressed and dried, and finally conditioned to 23◦Cand 50% RH. Measurements of the grammage and thickness, and analysis of the fractures, wereconducted at Karlstad University, and the SCT were conducted at Gruvön in standard condition. The SCT results showed that for single sheets, BSKP was the strongest fibre type in its SCTindex. The difference in strength between BSKP and UBSKP affirms the theory that bleachingimproves the fibre-fibre bonding potential, which increases the strength. For two layered sheets, the highest SCT index was combinations BSKP/BHKP with the70/70 configuration. When the difference was big between two different fibre types there alsoseemed to be a relation for the SCT index; if one fibre type is dominant, the SCT index showsa relation. For similar SCT indices, there was no relation, and the 70/70 was strongest. The fractures visible complied with the theory of how the fracture in SCT occur; with eithera shear zone buildup, bending or a bulging. Bulging occured in many two layered sheets, thisdue to the weaker interface between the 2 different pulps. This complied with previous studiesstating the mechanisms to be shearing and delamination according to the shear strength profile. The ratio of anisotropy was different for all sheets. This was remedied by taking the geometricaverage for the SCT index. Further testing with Rapid-Köthen (RK) could verify at the veryleast the single layer geometrically averaged results since RK yields isotropic papers. Otherfuture works could be to find a mixing rule that could be applied to multiply sheets.