Reinforcement effect of pulp fines and microfibrillated cellulose in highly densified binderless paperboards

This study investigates the use of papermaking fines as a reinforcement fibre for fully bio-based highly densified paperboards. Paperboards were prepared in a wet process followed by subsequent densification, aiming to exploit the self-bonding capabilities of cellulosic fibres without addition of synthetic binders. The cohesion of paper-based materials can be mainly attributed to the presence of strong hydrogen bonds which effectively interlink individual pulp fibres especially in highly densified paperboard. However, at a given board density, cohesion of pulp fibres may be further enhanced by adding micro- or nanoscale cellulosic fibres which provide a high surface area and thus enable extensive hydrogen bonding. In the present study, primary papermaking pulp and microfibrillated cellulose (MFC) were used to mechanically reinforce highly densified paperboards prepared from unrefined kraft pulp. All fibre materials used were characterised with regard to morphology, size distribution and dewatering behaviour (Schopper-Riegler freeness). The prepared paperboards were characterised in uniaxial tensile tests and fracture surfaces were investigated by scanning electron microscopy. As revealed by tensile tests, both reinforcement materials were highly effective, resulting in an ultimate strength increase of 50% for primary fines and 60% for MFC. Even at an addition level of 1 wt%, both reinforcement materials increased board strength by around 25%. In terms of mechanical properties, the paperboards produced in this study were found to be competitive to densified paper-based materials described in literature and similar boards which are already commercially available.