Layer by Layer Assembly of fire proofing coatings for textiles, films and foams

This paper presents the recent achievements by our research group on fire retardancy based on surface modification of textiles, films and foams achieved through layer by layer assembly. Such approach will be discussed through selected case studies involving the deposition of coatings with different fire proofing mechanisms (i.e. inorganic thermal shielding barriers or micro-intumescent coatings) [1-3]. Layer by Layer (LbL) assembly is a step by step deposition technique that allows the molecularly-controlled fabrication of surface-confined nanostructured materials. The layer by layer has been discovered by Iler in 1966 and reinvented by the group of Decher in early 90s [4]. It simply consists in an alternate adsorption of chemical species on the selected substrate due to an interaction (e.g. the electrostatic interaction that occurs in between positively and negatively charged species); because of unlimited possibilities of materials choice and deposition parameters it is possible to target a wide range of application fields including flame retardancy. The first papers that showed the potentialities of LbL were focused on the LbL assembly of totally inorganic or hybrid organic-inorganic coatings made, as an example, of oppositely charged silica nanoparticles or polyelectrolytes coupled to zirconium phosphate nanoplatelets [5,6]. The same inorganic architectures can be also assembled by employing spray-assisted LbL depositions; in a direct comparison with dipping, only the horizontal spray allows the assembly of a very homogeneous coating that subsequently leads to the best fire retardant properties [7,8]. Pursuing this research, the technique was extended to other substrates, such as rigid plastics [9, 10], and, mostly important, the coating composition and fire proofing action were directed toward the intumescence field [11-14]. Seeking for an intumescent-like coating capable of protecting both synthetic and natural fabrics, the Layer by Layer has been exploited for building coatings with enhanced char forming ability based on polyacrylic acid and ammonium polyphosphate [12,13]. The deposited coatings were able to protect both cotton and PET fabrics, preventing flame spread in flammability tests and reducing the heat release rate and total heat release when tested by cone calorimetry under different irradiative heat fluxes (namely, 25, 35 and 50 kW/m2). Our research group has recently proposed the use of deoxyribonucleic acid (DNA) coupled with chitosan in order to assembly novel and environmentally sustainable LbL coatings.[15] In particular, the DNA macromolecule has been demonstrated to represent an all in one intumescent system that, when applied to cotton, is capable of extinguishing the flame during horizontal flammability tests, increase cotton LOI values as well as to strongly reduce the combustion rate in cone calorimetry tests. DNA has been also deposited in a 100 μm coating on ethylene vinyl acetate (EVA) copolymers and compared with its bulk addition via melt blending. The collected results have shown that the DNA coating can greatly delay the ignition of the copolymer when tested by cone calorimeter (35 kW/m2 heat flux), increasing the time to ignition by 228s (+380%), while the bulk addition led to an anticipation of combustion. A similar effect has been observed under a heat flux of 50 kW/m2 with an increase of 102s (+625% with respect to pure EVA). [16] As far as foamed materials are concerned, a LbL architecture containing poly(acrylic acid), chitosan, and poly(phosphoric acid) has been recently assembled; the deposited coating was able to adapt to flame or heat exposure and to evolve into thermally stable carbon-based structures capable of a 55% reduction in heat release rate during cone calorimetry tests under different irradiative heat fluxes (from 35 up to 75kW/m2). In addition, when subjected to a flame torch penetration (Tflame≈1300°C), the LbL-coated foam was capable of maintaining its three-dimensional structure, thus successfully insulating the unexposed side. [17] Up to now, most of the LbL coatings for fire protection have been deposited on substrates characterized by high surface to bulk ratios, such as textiles and open cell foams. The use of substrates, such as bulk plastic films (100 to 1000μm thick), represents a challenge; as an example, inorganic coatings made of silica nanoparticles have been successfully assembled for the surface protection of polycarbonate films of different thickness. Thinner substrates (200 μm) achieved the best flame retardant properties with the suppression of incandescent melt dripping during flammability tests, unlike thicker samples (1000μm), for which limited improvements have been observed.[9]. Very recently, first we have demonstrated that LbL can be very performing also when applied to closed cell polyethylene terephthalate foams (sample thickness: 10 mm) [18]. Two coating compositions have been selected in order to achieve an intumescent behavior during combustion: more specifically, the flame retardant features of ammonium polyphosphate have been compared with those of deoxyribonucleic acid. The coating growth characterization proved that both the selected architectures can bring to the formation of continuous coatings, characterized by different sub-micronic thicknesses, just confined on the surface of PET foams, as assessed by electron microscopy. Flammability and cone calorimetry tests have clearly shown the superior performances of the LbL coatings containing ammonium polyphosphate (APP), as compared to the DNA-based counterparts. Indeed, only APP-based architectures were able to suppress the melt dripping behavior typical of PET and to reduce the heat release rate peak by 25%: more specifically, these findings were ascribed to the development, during combustion, of intumescent structures that act as thermal shield and protect the underlying PET foam. References [1] Alongi J, Carosio F, Malucelli G. Current emerging techniques to impart flame retardancy to fabrics. Polymer Degradation and Stability, 2014,106(August 2014):138-149. [2] 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. [3] 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. [4] G. Decher and J. D. Hong. Buildup of ultrathin multilayer films by a self-assembly process, 1 consecutive adsorption of anionic and cationic bipolar amphiphiles on charged surfaces. Makromol. Chemistry, Macromolecular Symposia, 1991, 46(June):321-327. [5] Carosio F, Laufer G, Alongi J, Camino G, Grunlan JC. Layer by layer assembly of silica-based flame retardant thin film on PET fabric. Polymer Degradation and Stability 2011,96(5):745-750. [6] Carosio F, Alongi J, Malucelli G. α-zirconium phosphate-based nanoarchitectures on PET fabrics through Layer-by-Layer assembly: morphology, thermal stability and flame retardancy. Journal of Materials Chemistry 2011; 21(28):10370-10376. [7] Alongi J, Carosio F, Frache A, Malucelli G. Layer by Layer coatings assembled through dipping, vertical or horizontal spray for cotton flame retardancy. Carbohydrate Polymers 2013;92(1):114-119. [8] Carosio F, Di Blasio A, Cuttica F, Alongi J, Frache A, Malucelli G. Flame retardancy of polyester fabrics treated by spray-assisted Layer by Layer silica architectures. Industrial & Engineering Chemistry Research 2013;52(28):9544-9550. [9] Carosio F, Di Blasio A, Alongi J, Malucelli G. Layer by Layer nanoarchitectures for the surface protection of polycarbonate. European Polymer Journal 2013;49(2):397-404. [10] Alongi J, Di Blasio A, Carosio F, Malucelli G. UV-cured hybrid organic-inorganic Layer by Layer assemblies: effect on the flame retardancy of polycarbonate films. Polymer Degradation and Stability, 2014;107(September):74-81. [11] Carosio F, Alongi J, Malucelli G. Layer by Layer ammonium polyphosphate-based coatings for flame retardancy of polyester-cotton blends. Carbohydrate Polymers 2012;88(4):1460-1469. [12] Alongi J, Carosio F, Malucelli G. Layer by Layer complex architectures based on ammonium polyphosphate, chitosan and silica on polyester-cotton blends: flammability and combustion behavior. Cellulose 2012;19(3):1041-1050. [13] Alongi J, Carosio F, Malucelli G. Influence of ammonium polyphosphate-/poly(acrylic acid)-based Layer by Layer architectures on the char formation in cotton, polyester and their blends. Polymer Degradation and Stability 2012;97(9):1644-1653. [14] Carosio F, Alongi J, Malucelli G. Flammability and combustion properties of ammonium polyphosphate-/poly(acrylic acid)- based Layer by Layer architectures deposited on cotton, polyester and their blends. Polymer Degradation and Stability 2013;98(9):1626-1637. [15] Carosio F, Di Blasio A, Alongi J, Malucelli G. Green DNA-based flame retardant coatings assembled through Layer by Layer. Polymer 2013;54(19):5148-5153. [16] Alongi J, Di Blasio A, Cuttica F, Carosio F, Malucelli G. Bulk or surface treatments of ethylene vinyl acetate copolymers with DNA: investigation on the flame retardant properties. European Polymer Journal 2014;51(1):112-119. [17] Carosio F, Di Blasio A, Cuttica F, Alongi J, Malucelli G. Self-assembled hybrid nanoarchitectures deposited on poly(urethane) foams capable of chemically adapting to extreme heat. RSC Advances, 2014;4(32):16674-16680. [18] Carosio F, Cuttica F, Di Blasio A, Alongi J, Malucelli G. Layer by layer assembly of flame retardant thin films on closed cell PET foams: efficiency of ammonium polyphosphate versus DNA. Polymer Degradation and Stability, In press. Acknowledgements The European COST Action MP 1105 “Sustainable flame retardancy for textiles and related materials based on nanoparticles substituting conventional chemicals” – FLARETEX – is gratefully acknowledged.