This work is driven by the effort towards increased environmental sustainability and aims to develop a new food packaging material based on renewable materials, and more specifically cellulose as the most abundant biomass resource on Earth. Focus was on microfibrillated cellulose (MFC) known for its exceptional diffusion barrier performance at low relative humidity, which however rapidly degrades with increasing humidity, and on strategies to reduce this sensitivity to moisture.
The first objective was to investigate alternative MFC from algae and bacteria. A commercially available wood-MFC was selected as a benchmark. The algae-MFC was extracted from Boergesenia forbesii algae. The bacterial cellulose (BC) was produced via fermentation of Komagataeibacter xylinus bacteria in two variations: a standard BC-Control and BC-Cremodan, cultivated in a foamed medium with cremodan and xanthan. Aqueous suspensions of MFCs were shear-thinning and exhibited a yield stress with increasing concentration. The BC-Cremodan and algae-MFC suspensions with high nanosized fractions and fewer physically entangled agglomerates were less stiff than the BC-Control and wood-MFC suspensions with rougher and longer macro-fibers. The analysis of self-standing nanopapers produced with these MFC revealed the complex interplay between microstructural factors such as crystallinity, nanofraction and residual water content and macroscopic properties, as the Young's modulus and the water vapor permeability. The oxygen transmission rate (OTR) of the MFC films at high humidity was comparable to the values of conventional packaging materials such as PET and their water vapor transmission rate (WVTR) was an order of magnitude higher than that of PET, but an order of magnitude lower than values for previous MFC films.
The second objective was to suppress the considerable sensitivity of MFC to moisture, and more specifically to maintain its very high barrier against the diffusion of oxygen at high relative humidity. The approach was to encapsulate the MFC films with hydrophobic polymers in the form of multilayer coatings on a paper substrate. Two different types of polymers, namely photo-curable organic-inorganic epoxy-based formulations, and an acid-modified polyolefin suspension (PO) were selected. The epoxy hybrid approach improved the OTR and WVTR of the MFC by an order of magnitude, and the addition of a sol-gel inorganic network into the epoxy network was instrumental to achieve a high mechanical integrity, with no delamination at the polymer-MFC interface. The PO approach was tested with the wood-MFC and algae-MFC and enabled to largely decrease the WVTR of the MFC, both at 50%RH and 85%RH, with a marginal effect on the OTR. The algae-MFC with an open porosity led to a high adhesion with cohesive failure within the MFC layer owing to a mechanical interlocking effect with the PO layers.
The phenomenon of water transport within the MFC/epoxy hybrid and MFC/PO multilayers was further explored using transient and steady-state permeation approaches. This analysis confirmed the absence of a strong synergy within the multilayer architectures with a limited encapsulation effect. The assumption of a Fickian diffusion moreover led to artefacts in the derivation of the diffusion and solubility coefficients for water. This was considered to reflect a diffusion-reaction type transport, where water molecules absorb on active sites at the interfaces and within the MFC network.