Nanocellulose/Clay Thin Films and Foams: Biobased Nanocomposites with Superior Flame Retardant Properties

Nanocellulose has attracted great interest as a natural building block for nanocomposites characterized by an easy and green approach to nanostructuration. Different forms of nanocellulose exist and can be roughly divided into three main groups: cellulose nanocrystals (CNC), nanofibrillated cellulose (CNF) and bacterial nanocellulose (BNC). CNF is particularly interesting as its production can potentially be scaled up at industrial levels. In the present contribution CNF was successfully employed with lamellar shape nanoparticles (sodium montmorillonite) for the production of nanostructured materials with outstanding flame retardant properties. Water-based nanocellulose/clay colloids were either filtered in paper-like processing to form nanostructured films or freeze dried in order to produce light and mechanically strong foams. The structure and morphology of the achieved materials have been deeply characterized proving the formation of brick and mortar structures in which oriented lamellar nanoparticles are intercalated by nanocellulose that is employed as continuous fibril matrix. The flame retardant properties have been evaluated by means of flammability and cone calorimetry. During flammability tests both the thin films and the foams showed self-extinguishing behavior. When exposed to the cone calorimetry heat flux (35 kW/m2), nanocellulose/clay films did not ignite while foams burned with very low heat release rate peaks (below 75 kW/m2) and negligible production of smoke. The mechanism for such impressive flame retardant properties is identified in the achieved nanostructuration: the oriented clay with numerous nanocellulose/clay interfaces provides barrier properties and low transverse thermal conductivity, whereas CNF provides toughness and favorable charring. Finally, a proof of concept is presented for the application of nanocellulose/clay as thin protective surface layers for composites or wood.