Oona Korhonen’s research while affiliated with Aalto University and other places

All-cellulose composites were prepared by dispersing kraft fibers in a matrix made from pulp dissolved in NaOH-water. To hydrophobize the composite surface while maintaining the material fully bio-based and biodegradable, layer-by-layer deposition of cationic starch and carnauba wax was performed. Various options of surface coating, drying, and curing were tested, including partial and complete melting of the wax. The composite surface was characterized by water contact angle, roughness and scanning electron microscopy, and material properties by adsorption and absorption of water (in vapor and liquid form) and tensile testing. The highest water contact angle was obtained when the layer-by-layer deposition was performed by dipping the dry composite into cationic starch solution, then in wax dispersion, and partially melting the wax after coating. However, it was demonstrated that the dipping approach was detrimental to material tensile properties, due to heterogeneous swelling of cellulose during the treatment and multiple drying sequences. Process optimization via spraying resulted in composites with Young’s modulus of 6 GPa and hydrophobic surface with water contact angle 122 °C.

Porous all-cellulose composites were prepared from pulps dissolved in 8 wt-% NaOH-water and dispersed softwood kraft fibers. Freeze-drying and drying with supercritical CO2 were used to vary the morphology of the composites. As cellulose dissolution in NaOH-water is known to be limited by pulp degree of polymerization, the latter was varied to obtain different amounts of non-dissolved fractions in solution. The goal of the work was to understand the influence of fibers originating from the incomplete dissolution and/or from added kraft fibers on the morphology and properties of porous all-cellulose composites. Density, porosity, pore volume, specific surface area and mechanical properties under compression were determined and correlated with the composite formulations and morphology.

It is well known that when cellulose is dissolved in aqueous NaOH-based solvent, solutions are gelling with increasing time and temperature. The goal of this work was to understand if the presence of non-dissolved cellulose fibers influences gelation behavior of the whole system. One of the motivations is to control gelation when making all-cellulose composites with short fibers dispersed in cellulose-NaOH-water solutions. Gelation kinetics of cellulose(dissolving pulp)-NaOH-water solutions with added softwood kraft fibers were investigated using dynamic rheology. Fiber concentration, dissolving pulp degree of polymerization and solution temperature were varied. In all cases the addition of kraft fibers accelerates gelation and increases modulus at gel point while the presence of “inert” carbon fibers does not influence solution gelation kinetics. It was suggested that acceleration of gelation and reinforcement of cellulose gels is due to the interactions between dissolved and non-dissolved cellulose.

All-cellulose composites were prepared by dispersing short softwood kraft fibers in dissolving pulp-8 wt% NaOH–water. The degree of polymerization of the dissolving pulp used for the matrix and the concentration of reinforcing fibers were varied. Morphology, density, crystallinity, cellulose I content and mechanical properties of the composites were investigated. A special attention was paid on the presence of non-dissolved fibers originating from incomplete dissolution of pulp in 8 wt% NaOH–water thus decreasing the actual concentration of dissolved cellulose in matrix solution. This “lack of matter” induced the formation of pores, which strongly influenced the morphology of composites. Density was shown to be the main parameter contributing to the mechanical properties of the prepared all-cellulose composites. The results demonstrate the complexity of the system and the need in taking into account the dissolution power of the solvent. Graphical abstract Morphology of all-cellulose composites: matrix is from low-DP dissolving pulp (a) and from high-DP pulp (b). Open image in new window