Polyallylamine-montmorillonite as super flame retardant coating assemblies by layer-by layer deposition on polyamide

In this paper, a polyamide-6 (PA6) substrate has been coated with a flame retardant film, made from polyallylamine (PAH) (polycation) and montmorillonite (MMT) (polyanion), elaborated by Layer-by-Layer (LbL) technique. The (PAH-MMT)n assembly (with n the number of bilayers deposited) exhibits an exponential growth regime. At n = 20 bilayers deposition of PAH and MMT, the film reaches a considerable thickness of ∼5 μm with the alignment of MMT in the direction parallel to the substrate. Scanning Electron Microscopy (SEM) analysis of the cross-section and Atomic Force Microscopy (AFM) analysis display a regular and continuous morphology of the obtained films. Thermogravimetric analysis shows that the presence of (PAH-MMT)n films at 10 and 20 bilayers enhances the thermal stability of the polyamide substrate. Cone calorimetry evidences excellent reaction to fire of the material since peak of heat release rate (HRR) is decreased by more than 60% in the presence of 20 bilayers of PAH-MMT film in comparison with uncoated PA6. Continuous charred layer was observed during the combustion and the thickness of the coating at the end of the combustion test is twice higher than that of the initial thickness. The presence of this expanded charred layer at the surface acts as protective limiting heat and mass transfer.