Dried nanofibrillated cellulose and its bionanocomposites

During the past decade there has been a growing interest in the reinforcement of synthetic polymers with cellulose nanowhiskers and nanofibrillated cellulose (NFC) obtained from plants or bacteria. Their beneficial mechanical properties like high stiffness and strength, in combination with their low mass allowed successful reinforcement of water based polymer dispersions (latexes) for the production of solution cast composite films. However, the production of fully degradable or biocompatible nanocomposites containing NFC with high aspect ratio and diameters below 100 nm is still a challenging task. One of the main issues to overcome is irreversible agglomeration (hornification) of NFC. Hornification can occur during drying of aqueous NFC suspensions or during compounding of NFC with hydrophobic polymers and it can be explained with the formation of a large number of hydrogen bonds between the hydroxyl groups of adjacent nanofibrils. This process is accompanied by a considerable decrease of the NFC aspect ratio and consequently results in the complete loss of its beneficial properties. Therefore, the objective of this PhD work was to chemically functionalize NFC in order to prevent hornification during drying and to develop novel bionanocomposites with well dispersed NFC, displaying improved properties compared to the neat polymers. Successful preparation of such bio-based composites could open up ways to new applications in e.g. medicine, bio-packaging or horticulture. In this study, a method for the preparation of water-redispersible NFC in powder form was developed, comprising carboxymethylation and mechanical disintegration of refined, bleached beech pulp (RBP). The powders formed stable gels when dispersed in water and SEM images confirmed that carboxymethylation had successfully prevented hornification of NFC during drying. Dynamic mechanical analysis (DMA) of poly(vinyl acetate) latex composites showed that carboxymethylation did not negatively influence the reinforcing potential of NFC. Consistently, the reinforcing potential of c-NFC was not altered by the drying procedure, as was shown by DMA experiments and tensile tests of hydroxypropyl cellulose composites containing dried and never-dried c-NFC. In a subsequent study, bionanocomposites were developed by UV-photopolymerization of N-vinyl-2-pyrrolidone in presence of a trimethacrylate crosslinker and water-redispersed c-NFC powder to yield a biocompatible hydrogel for the replacement of degenerated human Nucleus Pulposus (NP) in intervertebral discs. The native structure and function of the NP was mimicked by the randomly oriented c-NFC fibrils in the hydrogel matrix. The biocomposite hydrogels showed similar values for swelling ratio and modulus of elasticity in compression, compared to native NP. A final study focused on the feasibility of an industrial up-scaling of poly(lactic acid) composites containing compatibilized c-NFC using extrusion.Godkänd; 2011; 20110204 (ysko); DISPUTATION Opponent: Professor Olli Ikkala, Dep of Applied Physics and Molecular Materials, Aalto University, Finland Ordförande: Professor Kristiina Oksman, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Fredag den 8 april 2011, kl 10.30 Plats: E246, Luleå tekniska universite