Browsing by Author "Budtova, Tatiana, Prof., Aalto University, Department of Bioproducts and Biosystems, Finland"

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Short-Fiber Reinforced All-Cellulose Composites Using Aqueous NaOH Solution
(2020) Korhonen, Oona
School of Chemical Technology | Doctoral dissertation (article-based)
The demand for producing materials utilizing renewable resources instead of petroleum has increased with environmental concerns. This dissertation focuses on all-cellulose composites produced via short-fiber dispersion approach using commercial pulps as raw material and NaOH (aq) as cellulose solvent. The goal was to study the complete process of all-cellulose composite preparation and to understand how the pulp characteristics, special features of NaOH (aq) solvent and the processing parameters affect the material's properties. The influence of incomplete dissolution on the gelation properties of cellulose-NaOH systems was determined, different drying methods for all-cellulose composite production were studied and the effect of layer-by-layer hydrophobization treatment on the properties of these materials was examined. NaOH was found to be a suitable solvent for the production of all-cellulose composites if a sufficient concentration of dissolved cellulose in the matrix is achieved. The presence of non-dissolved cellulose in the cellulose-NaOH solution was found to reinforce the formed gels, but to accelerate gelation of the system, which should be taken into account for successful manufacturing. Density was found to be the main contributor to the mechanical performance of all-cellulose composites. Drying under compression resulted in materials with maximum Young's modulus of 8 GPa and tensile strength 53 MPa. In the case of porous all-cellulose composites prepared via supercritical drying, the addition of fibers significantly decreased the specific surface area. The best performance was found for the system containing only non-dissolved fiber fragments, which provided the reinforcement (compressive modulus 8 MPa), without sacrificing the high specific surface area (335 m2/g). Freeze-drying resulted in stronger materials, but surface area was significantly lower due to larger pores in the matrix. Additionally, the origin of the reinforcement (non-dissolved fragments or added fibers) was shown to have less influence on the composite's properties compared to fiber concentration. Layer by layer deposition of cationic starch and carnauba wax was shown to result in surface hydrophobized all-cellulose composites. Process optimisation was performed to obtain high water contact angle (122 °) while maintaining the composite mechanical properties. Overall, the results obtained in this work show that the properties of short-fiber reinforced all-cellulose composites can be altered and controlled, therefore allowing to design these materials to fulfil the requirements for various applications.