A toolbox for improving the processability of composites with a high content of thermomechanical pulp

Licentiatavhandling, 2021

The use of oil-based products causes large volumes of non-biodegradable waste and contributes to climate changes. Focus has consequently shifted towards the use of polymers derived from wood. The processability of such polymers, however, presents a challenge but this can be overcome through the use of composite materials, i.e., a blend of wood-derived material and fossil fuel-based polymers. Wood-derived material requires either additives or modification in order to achieve high loading of the renewable polymer section. Two possible routes for modifying thermomechanical pulp (TMP) are outlined here, with the aim of achieving composites with a high content of TMP that not only have good mechanical properties but are also processable. The addition of physical modifiers, such as magnesium stearate (MgSt) and molybdenum disulfide (MoS2), and their effects on the matrix of polyolefin copolymer poly (ethylene-co-acrylic acid (EAA)) in the TMP loading were the first route studied. The composites were prepared by compression moulding and had dry TMP contents of 30, 50 and 70 wt%, respectively, and 5 wt% additive relative to the weight of TMP. The second route was chemical modification of TMP fibres using three different alkyl ketene dimers (AKD), and was tested within the matrix of polypropylene (PP). The AKDs had carbon chains that were 6, 12 and 18 carbons in length. The AKD-modified TMP was extruded with PP at 50 wt% TMP. The addition of TMP increased the mechanical strength of the composites, independent of matrix (EAA or PP). The addition of MgSt or MoS2 increased the strength of the TMP-EAA composites further. Also, the presence of MoS2 improved the interface compatibility between EAA and TMP, as revealed by the use of rheology. In the case of AKD-modified TMP, surface energy measurements showed that modification reduced the surface energy of the fibres and increased hydrophobicity. The colour, shape of the extruded filament and reduced complex viscosity all indicate the easier processability of the TMP modified with the longest AKD carbon chain. It may be concluded that both the physical and chemical modifications made showed improvements in the processability of these composites.