Preparation and characterization of nanocellulose films and coatings from industrial bio-residues

Sammanfattning: The aim of this work has been to study cellulose nanocrystals (CNC) from bio-residues and their use as green coating materials. In the first part of this work, physical and chemical properties of different types of cellulose nanocrystals, from different raw material sources and different treatments were compared. It was found that all CNC suspensions had similar flow birefringence behavior, and similar diameters of the crystals. The mechanically isolated CNC from bio-ethanol residue had lower crystallinity but higher thermal stability compared chemically isolated CNC. Also the surface charge of these nanocrystals was different, being lower for the mechanically isolated CNC.In the second part, nanocrystals with lower and higher surface charges, were used to spin-coat films. The films were prepared with two different configurations: one with alternating layering of an anchoring polymer and CNC and the other with a single polymer layer coated with multilayers of CNC. The CNC with higher surface charge resulted in a smoother surface and the surface charge impacted the films’ hydrophobicity, being highest for CNC with lower surface charge. The gas permeability coefficient was dependent primarily on the surface charge of the CNC and secondly on the film configuration. The films with lower surface charge CNC were gas barriers for nitrogen and oxygen, and showed gas selectivity with some gas combinations while the films with higher surface charge CNC did not show gas selectivity.In the third part, coatings of CNC were prepared using spin- and dip-coating methods, on porous cellulose substrates. Microscopy studies showed that spin coating technique was suitable for the substrate with smaller pore size, while dip coating was suitable for the substrate with the larger pore size. The coating thickness ranged from some hundreds of nanometers for the spin-coated samples, to some micrometers for the dip-coated ones. The contact angle increased with the coating thickness and roughness. Additionally, the samples showed low oxygen permeability at 23% of relative humidity (RH). It was also seen that eight months storing reduced the barrier properties of the coatings when compared with fresh materials.In the last study, a new coating design as well as sorbitol plasticizer and citric acid crosslinker were used to improve the barrier and mechanical properties. This resulted in a dramatic improvement of the barrier properties and decreased water vapor permeability of more than 60%. Oxygen permeability value as low as 0.7 mL'μm/(m2'day'kPa) at 49% RH was reached when sorbitol was used. Overall, it was seen that the addition of sorbitol increased the thermal stability, barrier properties, and maximum strength of the CNC coatings.This work has demonstrated that nanocellulose films and coating from bio-residues can potentially be used to produce functional barrier coatings, thereby increasing the value of the forest resources.