Materials engineering for surface-confined flame retardancy

Polymer combustion is fuelled by pyrolysis products escaping from its surface due to the heat transferred from the flame to the polymer surface and also radiated from the flame itself. The oxygen required for sustaining the flaming combustion diffuses in and from the surrounding air [1, 2]. The most significant polymer degradation reactions usually occur in the condensed phase, as they take place mainly within 1mm of the interphase between the flame and polymer. These reactions involve the polymer and any additives (in particular flame retardants) included in the formulations or applied as surface treatments. The volatile species formed during combustion escape into the flame zone, whilst heavier species undergo further reactions and may eventually turn into char: this multi-lamellar carbonaceous structure acting as a thermal insulator protects the surrounded polymer. Therefore, the polymer surface can be considered the critical zone in the polymer combustion scenario because, being the interface between gas and condensed phase, it controls mass and heat transfers which are the processes responsible for flame fuelling. Indeed, the heat reaching the polymer surface is transmitted to the polymer bulk, from which volatile products of thermal degradation diffuse towards the surface and the gas phase, feeding the flame. Thus, the polymer surface plays a key role in polymer ignition and combustion [2]. Here, it is shown how the combination of advancements in polymer surface engineering and development of nanotechnologies supply an innovative environmentally friendly approach to fire retardance, based on providing polymer material products with a surface barrier, which either reradiates heat and/or slows down heat transmission and volatiles diffusion, without affecting the bulk properties. To this aim, Layer by Layer assembly has proven to be one of the most effective approaches and here it will be thoroughly described. Numerous examples applied to films, fabrics and foams will be presented and discussed. References 1. Alongi J, Carosio F, Horrocks AR, Malucelli G, Eds. Update on Flame Retardant textiles: State of the art, Environmental Issues and Innovative Solutions. Shawbury, Shrewsbury, Shropshire (UK): Smithers RAPRA Publishing, 2013. 2. Malucelli G, Carosio F, Alongi J, Fina A, Frache A, Camino G. Materials engineering for surface-confined flame retardancy. Materials Science & Engineering R Reports, 2014;84(October):1-20.