Multiscale characterisation of cellulose nanofibril networks using three 3D imaging methods

Cellulose materials are suitable to replace plastic in food packaging. They are hydrophilic and may have poor barrier properties that affect the shelf life of the food due to the migration of contaminants. The wet lamination of microfibrillated cellulose films on cellulose materials appears as a promising way to improve their barrier properties by forming a bilayer material. These barrier properties depend on the microstructural properties (porosity, pores connectivity, specific surface area or contact surface area between the two layers) of both layers, notably the film, which are poorly known. Therefore, a multiscale approach is proposed to estimate such microstructural parameters by combining three 3D imaging methods: synchrotron X-ray micro-/nanotomography and FIB-SEM tomography. The 3D microstructure of two different bilayer materials obtained with two Microfibrillated Cellulose (MFC) grades is investigated. For the first time, a full 3D representation of such material is presented. Regardless of the scale under consideration, our results showed that both films present a dense structure with very low porosity and no pore connectivity along the thickness. The MFC film produced with the smallest MFC fibrils led to a more homogeneous and less porous layer with a larger contact surface to the paper.