Photocurable Hybrid-Nanocellulose Multilayer Coatings on Biobased Substrates for Food Packaging Applications

With waste management concerns, researchers are now focusing on developing recyclable or biodegradable materials in an innovative way. To comply with the food industry requirements, current studies aim to develop materials with high barrier properties to oxygen and water vapor while considering their entire lifecycle. Microfibrillated cellulose (MFC) produced in the form of thin flexible films have attracted an increasing interest in packaging applications due to their excellent grease and oxygen barrier properties. However, the permeability of such materials degrades exponentially with increasing relative humidity (RH) due to disruption of the Hydrogen bonds and swelling of the MFC. The moisture sensitivity of MFC can be reduced through thermomechanical treatments, or by encapsulation with a hydrophobic matrix.<br/>This work aims to develop high diffusion-barrier materials in the form of multilayer coatings composed of MFC and UV-curable organic-inorganic hybrids coated on paper and biobased polymer substrate. Two end of life strategies are considered. The first is based on easy recyclable materials in an already existing paper mill. The second focuses on the development of biodegradable composites within relevant degradable environment.<br/>In the case of a recyclable packaging, MFC is coated as a diluted aqueous suspension and dried to form a dense film. The hybrid interlayers combine a cycloaliphatic epoxy oligomer, 3-glycidoxypropyl trimethylenesilane (GPTS) as a coupling agent and tetraethylorthosilicate (TEOS) to favor interfacial interactions with the hydroxyl-terminated MFC layers. Particular attention is paid to the photo-induced conversion of the hybrid network, to the MFC-hybrid interface and resulting oxygen and water vapor permeability. Multilayers based on pure epoxy and MFC layers on paper led to oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) of 25 cm3/m2/day/bar (23°C, 85%RH) and 14 g/m2/day (23°C, 85%RH), respectively. The achieved water vapor barrier performance is mainly coming from the epoxy. Moreover, epoxy did not properly wet the MFC surface and the interfacial adhesion was low. Adding TEOS and GPTS improved both the wettability and adhesion properties of the resin onto the MFC surface. Indeed, the addition of those two compounds led to the formation of covalent bonds through condensation reactions between the hydrolyzed TEOS and MFC surface. The photopolymerization rate and gel-time of UV-curable resins increased with increasing TEOS and GPTS concentration. A systematic analysis of barrier and adhesion properties is performed to assess the effect of the number of MFC interlayers and the composition of the UV-curable hybrid. The assessment of the recyclability of these multilayer films within a standard paper recycling scheme highlights their capability to be recyclable into Category II grade.<br/>Focus is also given to a fully biodegradable, MFC-based multilayer film alternative using a biobased epoxidized soya bean oil (ESBO) as an organic matrix and a polyhydroxyalkanoate/polylactic acid blend (PHA/PLA) as substrate. Preliminary biodegradability tests confirm the ability of a consortium of bacteria to use each material as a carbon source. The polarity contrast between the precursor (MFC suspension) and the PHA/PLA substrate leads to important delamination issues during film formation. To face this problem, a treatment of the polymer with an organosilane 3-aminopropyl)triethoxysilane (APTES) was performed showing great improvement. Permeability tests are ongoing to further investigate the performances of this new biodegradable MFC multilayer films.