On the relation between paperboard properties and packaging performance
Time: Mon 2021-01-18 13.15
Subject area: Solid Mechanics
Doctoral student: Gustav Marin , Hållfasthetslära
Opponent: Docent Christer Korin, Örebro Universitet
Supervisor: Professor Sören Östlund, Hållfasthetslära; Docent Mikael Nygårds, BillerudKorsnäs AB
Paper-based materials, such as paperboard, are commonly used as packaging materials. Inaddition to the advantage that wood as a raw material is renewable, there are also many otherbenefits of paperboard. From a mechanical point of view, paperboard has a high bendingstiffness compared to its relatively low weight and has a high foldability, which both areproperties of significance in the design of packages. However, a distinct drawback withpaperboard is its significant sensitivity to moisture. The moisture reduces the mechanicalproperties of the paperboard and consequently reduces the performance of the package. Thisthesis is starting with an investigation of the relation between moisture and differentmechanical properties on a continuum material level, and then these relations are applied onthe packaging design level through experimental testing and simulations.
In Paper A, a material characterization was performed on a series of five paperboards withdifferent grammages from the same producer. Five types of mechanical tests to characterizethe paperboards’ material properties were performed:
• In-plane tensile test,
• Out-of-plane tensile test,
• Short-span Compression Test (SCT),
• Bending stiffness test,
• Double-notch shear test.
All tests were performed at several levels of relative humidity (RH). Linear relations betweenthe mechanical properties normalized with their respective value at 50 % RH and moistureratio were found.
Paper B examined whether the linear relationships discovered in Paper A are true also forother paperboard series as well. Therefore, 15 paperboards from four producers wereinvestigated in this study, at the same levels of RH as before. Here, the in-plane stiffnessesand strengths and SCT-values were evaluated as a function of moisture. When also themoisture ratios in the investigated paperboards were normalized, it turned out that allpaperboards followed the same linear relationship between normalized mechanical propertyand normalized moisture ratio. Additionally, a bilinear elastic-plastic in-plane model wasdeveloped, that can predict the stress-strain relation of an arbitrary paperboard at an arbitrarymoisture level, and without requiring any mechanical testing except at standard condition(50% RH, 23 °C).
In Paper C, this relation was used to estimate input material parameters for simulating a BoxCompression Test (BCT) at different moisture levels. The result showed that it was possibleto accurately predict the load-compression curve of a BCT when moisture was accountedfor.