Microfibrillated cellulose foams obtained by a straightforward freeze–thawing–drying procedure

Microfibrillated cellulose (MFC) is continuously gaining attention due to its outstanding mechanical properties, in particular high strength-to-weight ratio. Recently, more and more studies target the production of porous materials, such as foams, out of this natural resource. Commonly, an energy-consuming freeze–drying method is utilized for producing pure MFC porous structures from water-based suspensions, which renders these products particularly unattractive for industry. Although alternatives for foam production have been proposed, using either modified MFC or with various additives, the freeze–drying step is still one of the most critical bottle-neck of MFC foam production upscaling. A novel straightforward freeze–thawing–drying procedure assisted by the common additive urea was herein proposed. Such method allows the production of mechanically stable, lightweight MFC structures under low-cost ambient conditions drying. The influence of the cellulose fibril characteristics, the suspension formulation and the process parameters on the final foam properties have been studied in terms of porosity, density and mechanical properties.