New simulation approach to mechanical properties of nanocellulose aerogels

Highly porous, mechanically stable aerogels were prepared by freeze drying of nano-fibrillar-cellulose (NFC) suspensions. The NFC concentration was varied to alter the morphological structure and mechanical properties of the samples. Three dimensional samples were prepared and their compressibility was tested. The increase of NFC content and thus density clearly increased the mechanical strength. The pore morphology of the structures was investigated with SEM and X-ray tomography. The main feature in the structures was the formation of micron-sized nanocellulose sheets. From the 3D tomography image, we obtained 2D cross-sections that allowed the recording of microscopic geometric features. This structure was used to simulate the stresses and strains during compression with a rigid surface. The model considered a microscopic 2D structure formed from thin beams representing the NFC sheet-like layers. The microscopic model was implemented in a commercial FE-code. We also simulated similar artificial structures obtained by placing beams randomly in a rectangular cell. The resulting stress-strain curves were consistent with experimental findings. The sheet-like layers improve effectively the stiffness of the structures.