Phosphorylated Cellulose Nanofibrils : A Nano-Tool for Preparing Cellulose-Based Flame-Retardant Materials
Abstract: The growing awareness of the need for a circular society and a circular chemistry has spurred the interest in using wood-based cellulose as a raw material for the preparation of new macroscopic devices and construction materials. The interest has been particularly focused on cellulose nanofibrils (CNF), which has led to the development of new material concepts through a nanoscale bottom-up engineering using renewable CNF. In order to be industrially applicable, the CNF must however possess a set of properties among which good flame-retardation is crucial. This thesis presents a) a way to chemically modify delignified wood fibers by phosphorylation to produce phosphorylated CNF, b) the fabrication and characterization of flame-retardant thin films, coatings and nanocomposite foams from the phosphorylated fibrils and c) the flame-retardant mechanisms of the phosphorylated CNF-based substances.Chemically delignified fibers have been phosphorylated by (NH4)2HPO4 in the presence of urea, and the resulting material has been used to prepare phosphorylated CNF (P-CNF). The flame-retardant properties of the phosphorylated fibrils were significantly improved by the phosphorus functionalization of the cellulose chain, converting the fibrils to an inherently flame-retardant material. The P-CNF was applied to make thin films/coatings using the Layer-by-Layer (LbL) technique. All-cellulose free-standing films were prepared through LbL self-assembly of the P-CNF and fibrils prepared from aminated cellulose-rich fibers (cationic CNF). The LbL-assembled film showed a high thermal stability, excellent flame resistance and superior mechanical performance. P-CNF/chitosan (CH) assemblies were also prepared as a fire protection for polyurethane (PU) foams. The five bilayer CH/P-CNF coating yielded a nano-exoskeleton on the surface of PU foam, shown to be capable of increasing the modulus of the foam by a factor of three and entirely preventing its melt dripping during the flammability testing.P-CNF/montmorillonite (MMT), sepiolite (Sep) clay or sodium hexametaphosphate (SHMP) films were also fabricated by vacuum filtration/solvent casting of the composite suspensions, and the structural and compositional features of these different films were used to study the mechanisms behind their flame-retardant properties. Only the P-CNF/MMT films were able to completely prevent ignition during cone calorimetry, when used as coatings for highly flammable polyethylene (PE) films and this was mainly ascribed to the excellent barrier properties of these films. The results also showed that the excellent strength and stiffness of the P-CNF/MMT samples, compared to those of the P-CNF/Sep and P-CNF/SHMP films, were essential for maintaining the barrier effect during combustion. Finally, nanostructured foams were prepared by freeze-casting of the P-CNF/Sep suspensions. The foams showed extensive flame-resistance, maintaining a temperature drop of more than 600 °C across the thickness during the flame penetration test. This performance was related mainly to the charring capability of the phosphorylated fibrils combined with the significant thermal insulation of Sep clay.
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