Layer by Layer ammonium polyphosphate-based coatings for flame retardancy of polyester–cotton blends

Ammonium polyphosphate (APP)-based coatings have been prepared through Layer by Layer deposition, in order to enhance the thermal stability in air and the flame retardancy properties of polyester–cotton blends. To this aim, two different counterparts, i.e. chitosan molecules and silica nanoparticles, have been coupled with APP. These species have been selected in order to prepare two novel flame retardant systems based on a different action mechanism. Indeed, the chitosan–APP pair represents an intumescent-like system, in which chitosan can act as both carbon source and foaming agent, whereas APP produces in situ phosphoric acid at high temperatures, favouring the char formation. On the other hand, the silica–APP pair exploits the joint effect between the phosphoric acid generated by APP that induces the carbonization of the polymer, and the thermal insulator behaviour of a ceramer such as silica. The two systems under study turned out to be responsible of an overall enhancement of the flame retardancy. Indeed, both the coatings were able to suppress the afterglow phenomenon and to leave a remarkable residue after the flammability test. In the case of chitosan-based assemblies, the residue appeared more coherent than that left by silica. Furthermore, silica/APP system showed a significant increase of the time to ignition and a strong decrease of the total heat release during cone calorimeter tests.