Layer by Layer Assembly: A Current Emerging Technique for Conferring Flame Retardancy

Layer by Layer (LbL) assembly consists in a step-by-step film build-up based on electrostatic interactions; it was introduced in 1991 for polyanion/polycation couples in order to obtain the so-called polyelectrolyte multilayers [1], and subsequently extended to inorganic nanoparticles [2] exploiting different interactions (e.g. covalent bonds, hydrogen bonds, etc.) beside the electrostatic one. The LbL assembly through electrostatic interactions simply requires the alternate immersion of the substrate into an oppositely charged polyelectrolyte (usually water-based) solution (or dispersion). Thus, an assembly of positively and negatively charged layers piled up on the substrate surface is obtained, exploiting a total surface charge reversal after each immersion step. LbL was first described in 1966 [3] and has been rediscovered and optimized decades later [4, 5]. Very recently, such approach proved to be extremely advantageous when exploited for the flame retardancy of foams [6, 7], thin and thick films [8-10], fibres and fabrics [11-13]. More specifically, different types of architectures have been deposited on fabrics: namely, i) inorganic LbL coatings; ii) hybrid organic-inorganic or intumescent LbL coatings; and iii) char-former/enhancer LbL coatings. The main results collected in the literature will be described in the present work, with particular attention to the possible industrial exploitations. Indeed, although dipping has been widely investigated as deposition technique, surprising results have been obtained employing spray, as well. A deep overview of all these results will be presented. References 1. Decher G, Hong JD. Buildup of ultrathin multilayer films by a self-assembly process, 1 consecutive adsorption of anionic and cationic bipolar amphiphiles on charged surfaces. Makromol Chem, Macromol Symp 1991;46(1):321-327. 2. Tang Z, Kotov NA, Magonov S, Ozturk B. Nanostructured artificial nacre. Nature Mater 2003;2(6):413-418. 3. Decher G. In: Decher G, Schlenoff JB, editors. Multilayer thin films, sequential assembly of nanocomposite materials. Weinheim (Germany): Wiley VCH, 2003. p. 1. 4. Decher G, Schlenoff J, editors. Multilayer thin films, sequential assembly of nanocomposite materials. Weinheim (Germany): Wiley VCH, 2002. 5. Decher G. In: Decher G, Schlenoff JB, editors. Multilayer thin films, sequential assembly of nanocomposite materials. Weinheim (Germany): Wiley VCH, 2003. p. 1. 6. Laufer G, Kirklan C, Cain AA, Grunlan JC. Clay−chitosan nanobrick walls: completely renewable gas barrier and flame-retardant nanocoatings. ACS Appl Mater Interfaces 2012;4(3):1643-1649. 7. Kim YS, Davis R, Cain AA, Grunlan JC. Development of layer-by-layer assembled carbon nanofiber-filled coatings to reduce polyurethane foam flammability. Polymer 2011;52(13):2847-2855. 8. Apaydin K, Laachachi A, Ball V, Jimenez M, Bourbigot S, Toniazzo, Ruch D. Polyallylamine–montmorillonite as super flame retardant coating assemblies by layer-by layer deposition on polyamide. Polym Degrad Stab 2013;98(2):627-634. 9. Laachachi A, Ball V, Apaydin K, Toniazzo V, Ruch D. Diffusion of polyphosphates into (poly(allylamine)-montmorillonite) multilayer films: flame retardant-intumescent films with improved oxygen barrier. Langmuir 2011;27(22):13879-13887. 10. Carosio F, Di Blasio A, Alongi J, Malucelli G. Layer by Layer nanoarchitectures for the surface protection of polycarbonate. Eur Polym J 2013;49(2):397-404. 11. Carosio F, Alongi J, Frache A, Malucelli G, Camino G. In: Morgan AB, Nelson GL, Wilkie CA, editor. Fire and polymers VI: new advances in flame retardant chemistry and science. Washington DC (USA): ACS Symposium Series 1118, 2012. Chapter 22. 12. Alongi J, Carosio F, Malucelli G. Current emerging techniques to impart flame retardancy to fabrics, Polymer Degradation and Stability, In press. 13. Alongi J, Carosio F, Horrocks AR, Malucelli G, editors. Update on Flame Retardant textiles: State of the art, Environmental Issues and Innovative Solutions. Shawbury, Shrewsbury, Shropshire (UK): Smithers RAPRA Publishing, 2013, ISBN: 978-1-90903-017-6.