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Layer by Layer Assembly of fire proofing coatings for textiles, films and foams
No full textThis 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
Materials engineering for surface-confined flame retardancy
No full textPolymer 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
Layer by Layer Assembly: A Current Emerging Technique for Conferring Flame Retardancy
No full textLayer 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
Fire protection of films, fabrics and foams achieved through surface nano-structuring
No full textGenerally speaking, 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 environment. Solid particles escape from the flame as smoke, which is accompanied by gaseous species, some of which can be toxic1, 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, where the temperature raise is high enough. These reactions involve the polymer and any additives (in particular flame retardants) included in the formulations or applied as surface treatments. Experimental studies of this region have been published by Price and co-workers3 and by Marosi and coworkers4. 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 because its chemical and physical characteristics affect the combustible volatiles flux towards the gas phase5.
One of the most valuable fire retardant strategy pursued by bulk addition, proved to be the production or accumulation of a thermally stable surface layer able to act as a barrier to mass and/or heat exchange. Such a layer is built during the early stage of combustion as a consequence of polymer surface layer decomposition, in the presence of different kinds of fire retardants, including inorganic nanoparticles. However, the time required for build-up of the surface barrier is straightforwardly connected to the development of the fire in the early stage, consequently adversely affecting the effectiveness of the protective barrier.
Results and discussion
Here it is shown how the combination of advancements in polymer surface engineering and development of nanotechnologies, supplies 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 purpose, nanoparticle adsorption6, sol-gel and dual-cure processes7, 8, Layer by Layer assembly8, will be thoroughly described. By building the fire protection onto the original polymer surface, its effectiveness will be larger than in the case of protection created during combustion as usually happens with traditional bulk addition. Numerous examples of the above mentioned approaches applied to films, fabrics and foams will be presented. A glimpse on the use of biomacromolecule-based coatings will be proposed, as well9, 10.
Conclusion
Engineering the polymer surface is shown to provide a potential promising, environmentally-friendly and effective approach to polymer fire retardance, particularly when combined with nanostructurating technologies. Feasibility is demonstrated for textiles, films and foams while present efforts are directed towards composites with possible future extension to thick polymer materials. A major interest in this approach to surface polymer properties is the possibility to simultaneously confer multifunctional features that, besides fire retardance, may involve gas barrier, hydrophobicity, biocide activity, surface electrical conductivity, etc.
Keywords: surface; coatings; Layer by Layer; sol-gel; combustion;
Acknowledgments
The European COST Action “Sustainable flame retardancy for textiles and related materials based on nanoparticles substituting conventional chemicals“, FLARETEX (MP1105) is gratefully acknowledged.
References
1. J. Alongi, F. Carosio, A.R. Horrocks, G. Malucelli G, Update on Flame Retardant textiles: State of the art, Environmental Issues and Innovative Solutions, Shawbury, Shrewsbury, Shropshire (UK): Smithers RAPRA Publishing, 2013.
2. T.R. Hull, “Challenges in fire testing: reaction to fire tests and assessment of fire toxicity” in Advances in Fire Retardant Materials, edited by D. Price and A.R. Horrocks, Cambridge (UK): Woodhead Publishing, 2008, pp. 255-290.
3. D. Price, F. Gao, G.J. Milnes, B. Eling, C.I. Lindsay, T.P. McGrail, Polym. Degrad. Stab. 64, 403-410 (1999).
4. G. Marosi, “Use of Organosilicone Composites as Flame Retardant Additives and Coatings for Polypropylene” in Fire Retardancy of Polymers: New Strategies and Mechanisms, edited by T.R. Hull and B.K. Kandola, Cambridge (UK): The Royal Society of Chemistry, 2009, pp. 49-58.
5. G. Malucelli, F. Carosio, J. Alongi, A. Fina, A., Frache, G. Camino, Mater. Sci. Eng. R 84, 1-20(2014).
6. J. Alongi, J. Tata, F. Carosio, G. Rosace, A. Frache, G. Camino, Polymers 7, 47-68(2015).
7. J. Alongi, F. Carosio, G. Malucelli, Polym. Degrad. Stab. 106, 138-149(2014).
8. J. Alongi, G. Malucelli, J. Mater. Chem. A 22, 21805-21809(2012).
9. G. Malucelli, F. Bosco, J. Alongi, F. Carosio, A. Di Blasio, C. Mollea, F. Cuttica, A Casale, RSC Adv. 4, 46024-46039(2014).
10. J. Alongi, F. Bosco, F. Carosio, A. Di Blasio, G. Malucelli, Mater. Today 17, 152-153(2014)
Innovative Solutions To Enhance The Flame Retardancy Properties Of Natural And Synthetic Fabrics: Sol-Gel, Dual-Cure Processes And Layer By Layer Assembly
No full textNowadays, nanotechnology has a real and great potential in the textile industry, considering that conventional methods used to give different properties to fabric textiles often do not lead to permanent effects against wearing or laundering; in such scenario, the use of nanotechnology has been proved to give high durability for fabrics, improve the performance of fibres and create unprecedented functions [1]. The idea is to develop a new method based on the introduction of nanoparticles during the finishing step. Multilayered thin films made of nano size layers, with total thickness ranging from submicron to a few millimetres, can be prepared with a new technique called layer-by-layer (LbL). Now, the technique can be tailored to multimaterial assembly of several compounds without special chemical modifications, thus enabling the production of multilayer films. Furthermore, the technique is very easy and applicable to different field applications, such as textile sector. It consists in an alternate immersion or more simply in alternate spraying of the substrate with an oppositely charged polyelectrolyte solutions, thus creating a structure of positively and negatively charged layers piled up on the substrate surface. On the other hand, the nano-sized particles have been synthesized by sol-gel process, which involves inorganic precursors (organo-silicates, -titanates-aluminates, etc.) that undergo several reactions resulting in the formation of a three-dimensional molecular network. The sol-gel process is conducted at a low temperature which also enables the incorporation of organic compounds into the inorganic structure without decomposition.
[1] J. Alongi, F. Carosio, R.A. Horrocks, G. Malucelli. Flame Retardant Textiles: State of the Art, Environmental Issues and Innovative Solutions, Rapra, In press
Layer by Layer ammonium polyphosphate-based coatings for flame retardancy of polyester-cotton blends
No full textAmmonium polyphosphate (APP)-based coatings have been prepared through Layer by Layer deposition, in order to enhance the thermal stability in air and the flame retardancy properties of polyester-cotton blends. To this aim, two different counterparts, i.e. chitosan molecules and silica nanoparticles, have been coupled with APP. These species have been selected in order to prepare two novel flame retardant systems based on a different action mechanism. Indeed, the chitosan-APP pair represents an intumescent-like system, in which chitosan can act as both carbon source and foaming agent, whereas APP produces in situ phosphoric acid at high temperatures, favouring the char formation. On the other hand, the silica-APP pair exploits the joint effect between the phosphoric acid generated by APP that induces the carbonization of the polymer, and the thermal insulator behaviour of a ceramer such as silica. The two systems under study turned out to be responsible of an overall enhancement of the flame retardancy. Indeed, both the coatings were able to suppress the afterglow phenomenon and to leave a remarkable residue after the flammability test. In the case of chitosan-based assemblies, the residue appeared more coherent than that left by silica. Furthermore, silica/APP system showed a significant increase of the time to ignition and a strong decrease of the total heat release during cone calorimeter test
Three organic/inorganic nanolayers on flexible foam allow retaining superior flame retardancy performance upon mechanical compression cycles
No full textInfluence of layer by layer coatings containing octapropylammonium polyhedral oligomeric silsesquioxane and ammonium polyphosphate on the thermal stability and flammability of acrylic fabrics
No full textThe use of Layer by Layer (LbL) assembly for modifying the thermal stability and flammability of acrylicfabrics is proposed. Many efforts have been devoted to improve the flame retardancy of acrylic materialsthat vigorously burn, releasing dense smoke when exposed to a flame or a heat flux. In spite of this, noone has still attempted LbL as new nanotechnological solution up to now.Here, nanostructured LbL coatings based on ammonium polyphosphate (APP) and octapropyl ammo-nium polyhedral oligomeric silsesquioxane (POSS) layer have been deposited on acrylic fabrics exploitingthe LbL assembly. More specifically, the coatings consisting of 4 or 6 bi-layers have been found to homo-geneously cover each fibre, creating an intimate interconnection between those adjacent. By this way, theacrylic fibres turned out to be efficiently protected the exposure to a 20 mm methane flame or 35 kW/m2heat flux. As a result, the melt dripping phenomenon has been completely suppressed and the com-bustion rate significantly reduced. The success of the proposed treatments has been ascribed to (i) thechar-former character of APP, (ii) the ceramic thermal insulating barrier created by POSS and (iii) theintimate contact between these two species, only occurring in a LbL assembly. These three aspects haveproven to be responsible of a significant modification in the thermal degradation mechanism of acrylicfibres that discussed as key role in their combustion
Few durable layers suppress cotton combustion due to the joint combination of layer by layer assembly and UV-curing
No full textIn the last five years, Layer by Layer (LbL) assembly has proven to be one of the most innovative solutions for conferring flame retardancy to fabrics. In spite of this, two main issues for the breakthrough of this approach at an industrial scale are still unsolved: namely, the use of few efficient layers characterized by washing durability. In this context, the present study shows that both these limitations can be overcome by coupling LbL with UV-curing processes in a joint action. In detail, 3 bi-layers (BL) consisting of an anionic UV-curable aliphatic acrylic polyurethane latex doped with a phosphorus-based flame retardant (namely, ammonium polyphosphate, APP) and chitosan have been initially deposited on cotton fabrics by dipping. Subsequently, this assembly has been exposed to UV radiation, thus resulting in a thin coating in which APP is in intimate contact with chitosan within a UV-cured network. This system has proven to be an efficient flame retardant system with exceptional durability features. Indeed, cotton self-extinguishment in horizontal flame spread tests has been achieved, even after washing in water at 65 °C for 1 h. Furthermore, this coating managed to withstand the attack of a 1 M solution of acetic acid or ammonia for 1 h, without losing its original structure. Indeed, no visible signs of coating hydrolysis or slight degradation phenomena have been observed by scanning electron microscopy
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