2D-nanostructures as flame retardant additives: Recent progress in hybrid polymeric coatings

The use of polymers in day-to-day life is undeniable; nevertheless, applicability of these polymers in the fire risk sector poses serious limitations as most of the polymeric materials/coatings employed are prone to fire. Hence to improve the fire retardant (FR) properties of polymers, researchers recommend the use of FR materials/additives, either physically blended or incorporated chemically via suitable modifications. However, to achieve sufficient FR property, usually higher amounts of traditional FRs are required which understandably deteriorates the mechanical and other important properties of the polymers. Moreover, use of halogenated FRs are under immense scrutiny due to the possible release of carcinogenic and organic pollutants. As a result development of halogen-free FRs is an emerging field of research. In this context, incorporation of nanostructured two-dimensional (2-D) materials to form polymer composites that can not only reinforce the mechanical, thermal and other important properties but also improve flame retardancy has opened up new prospects. The 2-D nanostructured materials, particularly, layered double hydroxide (LDH), MXenes, Graphene and its derivatives, Boron Nitride (BN) and molybdenum disulphide (MoS₂) have demonstrated capabilities to enhance the FR properties as a green and environmentally benign material. Incorporation of these 2-D materials into polymers to form nanohybrids can be achieved either as conventional filler or as surface modified systems chemically bonded to the parent matrix. In the present review, the recent development strategies of surface modifications employed on 2-D nanostructured materials (LDH, MXenes, GO, BN and/or MoS₂) to form polymeric nanocomposites and the FR properties achieved are discussed. The significant outcomes reported by various research groups, the key insights gained and viewpoints are deliberated. A plausible underlying mechanism for flame retardancy offered by 2-D nanostructured materials (LDH, MXenes, GO, BN and/or MoS₂) based polymer nanocomposites as extended by several research groups is discussed.