Abstract
This study examines the effect of pulp type on the formability and elongation of paperboard, which are of key importance when producing 3D packages. Material performance was studied with a press-forming machine using laboratory handsheets as substrates. The handsheets were prepared from bleached softwood and hardwood kraft pulps, chemi-thermomechanical pulp, recycled newsprint, and mixtures of birch kraft and other pulps. The effect of microfibrillated cellulose (MFC) on substrate properties and material formability was also investigated. The 3D elongation of MFC-free handsheets varied between 1.2 and 5.5%. Depending on the pulp type and sheet composition, three essential sheet properties were recognized. These properties were bulk, elastic modulus and bending stiffness, the first two of which affect material bending stiffness. Sheets made from softwood fibers were superior to other samples, but their low bending stiffness resulted in distortion of formed trays. A partial replacement of birch kraft with recycled newsprint did not lead to a loss of sheet extensibility, which suggests that the interactions between the converting tools and the substrate have a great effect on material convertibility. Scanning electron micrographs showed that the pulp type affects the mechanism behind fracture initiation. The ability of MFC to increase extensibility was limited to mechanical pulps, indicating the important role of the main pulp component in defining tensile properties.Graphical abstract
Highlights
Wood-fiber-based packaging materials have several significant benefits such as better sustainability and good printability compared to most common thermoplastics
The required properties vary by the planned end use, and no single pulp composition can be claimed to be optimal for every purpose, our findings suggest that pulp mixtures can potentially be used to tailor paperboards for processes requiring 3D elongation
In terms of 3D elongation, the kraft pulps were found to be superior to chemi-thermomechanical pulp (CTMP), RCF and the commercial reference board
Summary
Wood-fiber-based packaging materials have several significant benefits such as better sustainability and good printability compared to most common thermoplastics. These properties explain the growing interest in paperboard packages, but complex shapes cannot be produced because of factors such as moderate elongation, differences in physical properties between the machine-direction and cross-direction, and local fluctuation of tensile properties due to material heterogeneity and the nature of sheet formation. Mechanical fibers are stiffer than chemical fibers, especially in wet conditions Both mechanical pulp and chemi-thermomechanical pulp (CTMP) provide bulk, which is an essential material property for many converting processes. Interest in using microfibrillated cellulose (MFC) in paperboard grades has grown in recent years, since MFC makes it possible to decrease grammage without loss of strength (Kajanto and Kosonen 2012)
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