Flame retardancy of polymers

We show that the pH-dependent molecular pathways for cross-linking boric acid and nanocellulose have a strong influence on the flame retardancy and ignition resistance of nanocellulose-based foams. Solid-state NMR revealed that cross-linking at alkaline conditions results in the formation of thermally stable borate ester bonds. Boric acid cross-linking also modifies the thermal degradation pathway of nanocellulose; the cellulose structure is transformed into stable aromatic char during thermal annealing and the evolution of flammable volatile levoglucosan is small. We show that composite foam panels of the nanocellulose-borate hybrids and sepiolite nanoclay prepared at alkaline conditions did not ignite on radiant heat exposure in cone calorimetry testing and maintained their structural integrity after prolonged heat exposure.