Preparation and properties of dried nanofibrillated cellulose and its nanocomposites (2010)

During the past decade there has been a growing interest in the reinforcement of synthetic polymers by cellulose fibres or fibre fragments. Some work has already been carried out on cellulose nanowhiskers and nanofibrils obtained from plants or animals. These whiskers or fibrils have been used as reinforcement components in synthetic polymers for the production of films and lacquer. Depending on the proportion of cellulose, the composites have shown improved mechanical properties.&lt;br /&gt;\nThe production of fully degradable nanocomposites with biopolymers as matrix and cellulose nanofibrils with high aspect ratios as reinforcement is still a challenging task. Also, due to the large amount of hydroxyl groups on the surface of these nanofibrils, they tend to irreversibly agglomerate during drying. This process, known as hornification, decreases the aspect ratio of the nanofibrils. Consequently, their reinforcing potential in nanocomposites is lowered. Thus, the objective of this PhD project is to produce novel biopolymer composites that are reinforced by functionalised cellulose nanofibrils in powder form. A successful preparation of such bio-based composites could open up ways to new applications in e.g. medicine, bio-packaging or horticulture. In order to induce an optimal compounding of the fibrils with different biopolymers, good fibril/matrix embedding is required. Therefore, the cellulose nanofibrils have to be modified appropriately to match the hydrophilic or hydrophobic nature of the polymer matrix.&lt;br /&gt;\nIn the first study, water-redispersible, nanofibrillated cellulose (NFC) in powder form was prepared from refined, bleached beech pulp (RBP) by carboxymethylation and mechanical disintegration. The sequence of the treatments influenced the stability of the final products in water. When carboxymethylation was applied first, enhanced disintegration of RBP into its sub-structural elements was observed. The prepared powder of this route formed a stable gel in water without sedimentation after 20 h. SEM images affirmed a significant reduction of cellulose nanofibrils agglomeration compared to unmodified NFC. The results suggest that NFC in dry form could be used as an alternative to conventional NFC in aqueous suspensions used as starting material for derivatization and compounding with biopolymers.&lt;br /&gt;\nThe second study focused on the characterization of composites containing poly(vinyl acetate) and the above mentioned chemically modified cellulose nanofibrils by dynamic mechanical analysis (DMA). Also, the suitability of using the nanofibrils to formulate PVAc adhesives for wood bonded assemblies was studied. The results showed that the presence of nanofibrillated cellulose had a strong influence on the viscoelastic properties of PVAc latex films. For all nanocomposites, increasing amounts of NFC (treated or untreated) led to increasing reinforcing effects in the glassy state. This reinforcement primarily resulted from interactions between the cellulose fibrils network and the hydrophilic PVOH matrix that led to the complete disappearance of the PVOH glass transition (tan delta peak) for some fibril types and contents. At any given concentration in the PVOH transition, the nanofibrils which were prepared by chemical modification followed by the mechanical disintegration provided the highest reinforcement. Finally, the use of the chemically modified nanofibrils to prepare adhesives for wood bonding was promising; even though they generally performed worse in dry and wet conditions the boards showed superior heat resistance (EN 14257) and passed the test for durability class D1.&lt;br /&gt;\nIn the third study nanocomposites of hydroxypropyl cellulose (HPC) and nanofibrillated cellulose (NFC) were prepared by solution casting. The various NFC were in form of powders and were prepared from refined, bleached beech pulp (RBP) by mechanical disintegration, optionally combined with a pre- or post mechanical carboxymethylation. Dynamic mechanical analysis (DMA) and tensile tests were performed to compare the reinforcing effects of the NFC to those of their never-dried analogues. Carboxymethylated NFC showed the same mechanical properties in HPC, regardless of being dried or not before casting. This suggested that the effect of irreversible agglomeration of the fibrils during drying (hornification) was prevented by the carboxylate groups on the surface of the fibrils. SEM characterization confirmed a homogeneous dispersion of dried, carboxymethylated NFC within the HPC matrix. These results clearly demonstrate that drying of carboxymethylated NFC to a powder does not decrease its reinforcing potential in the (bio)nanocomposites.&lt;br /&gt;\nAll these strategies have in common that the matrix and the dried nanofibrils form a noncovalently bound composite. Using different reactants with various polarities, modified cellulose fibrils compatible with different biopolymer matrices will be presented. A thorough characterization of morphological, physical-chemical and thermal-mechanical properties of the composites completes the research program.