Layer-by-Layer-Coated Cellulose Fibers Enable the Production of Porous, Flame-Retardant, and Lightweight Materials

: New sustainable materials produced by green processing routes are required in order to meet the concepts of circular economy. The replacement of insulating materials comprising flammable synthetic polymers by bio-based materials represents a potential opportunity to achieve this task. In this paper, low-density and flame-retardant (FR) porous fiber networks are prepared by assembling Layer-by-Layer (LbL)-functionalized cellulose fibers by means of freeze-drying. The LbL coating, encompassing chitosan and sodium hexametaphosphate, enables the formation of a self-sustained porous structure by enhancing fiber-fiber interactions during the freeze-drying process. Fiber networks prepared from 3 Bi-Layer (BL)-coated fibers contain 80% wt of cellulose and can easily self-extinguish the flame during flammability tests in vertical configuration while displaying extremely low combustion rates in forced combustion tests. Smoke release is 1 order of magnitude lower than that of commercially available polyurethane foams. Such high FR efficiency is ascribed to the homogeneity of the deposited assembly, which produces a protective exoskeleton at the air/cellulose interface. The results reported in this paper represent an excellent opportunity for the development of fire-safe materials, encompassing natural components where sustainability and performance are maximized.