UV-cured hybrid organic-inorganic Layer by Layer assemblies : effect on the flame retardancy of polycarbonate films

In the present work, UV-curable layers have been exploited for assembling hybrid organic-inorganic Layer by Layer (LbL) coatings, able to enhance the thermo-oxidative stability and flame retardancy of polycarbonate (PC) films (500 μm thick). More specifically, 5 or 10 quad-layers (QL), each consisting of a branched polyethylene imine, negative silica nanoparticles (average diameter: 30 nm), positive alumina-coated silica nanoparticles (average diameter: 10 nm) and a UV-curable acrylic aliphatic polyurethane resin have been assembled on the PC films. The obtained architectures have been eventually exposed to UV radiation in order to crosslink them and to study their durability to washing treatments and to a concentrated ammonia solution. Scanning electron microscopy has shown that the UV-curing process allows forming a continuous and homogeneous coating, as compared to the un-cured counterpart. In addition, UV-cured 5 and 10QL have turned out to be capable to protect PC films from the application of a methane flame (3s) suppressing the dripping, the smoke release and the particulate formation. These coatings have proven to be still efficient also when the samples have been washed at 50 °C for 1 h in deionized water. Indeed, the same performances have been observed on such samples when horizontal flame spread tests have been repeated after washing, thus demonstrating durability features of these LbL architectures. The best results have been achieved when at least 5QL have been deposited on the substrate and subjected to UV-curing. Thus, the combination of LbL assembly and UV-curing may represent an effective way for obtaining durable flame retardant coatings.