Effect of foaming on mechanical properties of microfibrillated cellulose-based porous solids

All-cellulose porous solids have been prepared by foaming watery suspensions of microfibrillated cellulose (MFC) and the foaming agent methyl cellulose (MC) followed by freeze-drying. Mechanical and structural characterization of foamed and unfoamed porous solids with and without MC was performed to evaluate the effect of the foaming agent and of the foaming process. In unfoamed systems, partial replacement of MFC by MC led to decreased mechanical stability and a stronger dependency of mechanical properties on density. The foaming process allowed to reach gas volume fractions of up to 52% through interfacial stabilization by MC and thus reduce specific volume of water before drying by half. The incorporated gas bubbles withstood the freezing and drying process as shown by scanning electron microscopy of freeze-dried samples. Final density and porosity were tailored by adjusting solid content in the suspension, foaming time or concentration of foaming agent. In uniaxial compression, foamed porous solids showed similar or even higher Young’s modulus and yield stress compared to unfoamed systems at same composition and density. In foamed porous solids with increased mechanical stability, X-ray μ-computed tomography revealed the occurrence of aligned tubes, which act as reinforcing substructure. Thus, foaming can be applied prior to freeze-drying to drastically reduce water content, while the mechanical performance is unaffected or even improved through restructuring.