Self-assembled hybrid nanoarchitectures deposited on poly(urethane) foams capable of chemically adapting to extreme heat

Abstract

In the present paper the layer by layer (LbL) technique has been adopted for the construction of hybrid organic–inorganic nanoarchitectures capable of adapting to extreme heat or flame exposure and chemically evolving into thermally-stable carbon based structures. More specifically, the LbL technique has been applied to an open cell poly(urethane) (PU) foam in order to increase its thermal and flame stability. Scanning electron microscopy showed that the LbL assembly covered each surface of the PU complex three-dimensional structure without altering its open cell morphology. When exposed to a direct flame, the treated PU foam was capable of stopping combustion within a few seconds after ignition, unlike the untreated foam that burned completely. Under different irradiative heat fluxes (from 35 up to 75 kW m²), the coating demonstrated exceptional performances by reducing the rate of heat release up to 60% with respect to the untreated counterpart. Finally, when subjected to a flame torch penetration (T_flame ≈ 1300 °C), the LbL-coated PU foam was capable of maintaining its three-dimensional structure, thus successfully insulating the unexposed side (T below 100 °C after two flame torch applications) with temperature drops of 800 °C achieved with a specimen thickness of only 10 mm.

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Introduction

In the last few decades, the layer by layer (LbL) deposition technique has become a worldwide standard for the construction of functional coatings capable of enhancing the properties of several substrates.

Despite the first discovery by Iler in 1966, the potential of this simple self-assembly technique has been explored heavily only after the reinvention by the group of Decher in the early 90s.^1,2 Among the nearly infinite variants of the deposition, the most common is certainly the one that relies on electrostatic interactions occurring during the alternate adsorption of positively and negatively charged species on the selected substrate.^3,4 Nowadays, this technique is employed for the production of surface-confined nanostructured materials, the complex functionality of which goes beyond the simple definition of coating. Indeed, the LbL technique presents a wide spectrum of applications ranging from nanoscale reactors to smart functional coatings.^5–8

During the last years, the LbL technique has been applied to build-up protective coatings directed toward the fire safety and fire protection fields, clearly proving that this technique can be successfully adopted as a versatile tool to confer flame retardancy (FR) properties.^9–13

The present work focuses on the build-up of adaptive hybrid organic–inorganic nanoarchitectures capable of withstanding the exposure to extreme heat, by exploiting a chemical reaction, which leads to the formation of thermally-stable carbon based structures (char). The chosen LbL architecture consists of a four layer repetitive unit made of poly(acrylic acid) (−), chitosan (+), poly(phosphoric acid) (−) and chitosan (+). In the coating composition, chitosan and poly(acrylic acid) exhibit natural char forming characteristics that can be further enhanced by the dehydration abilities of poly(phosphoric acid).^14–16 Although these three components have been already employed in different LbL coatings with FR characteristics, this is the first time they are employed together in a single LbL architecture.

Poly(urethane) foams have been selected as a suitable substrate to protect against heat and fire by these adaptive coatings; indeed, nowadays their fire hazard is still considered a severe treat to human safety.¹⁷ More specifically, PU foams, that can be found in almost every home (e.g. sofas, mattresses), represent one of the first item to be ignited in fires as they can catch fire and burn very quickly, releasing toxic gases and leading to flashover events that subsequently result in loss of rooms, properties and, most important, life. Unfortunately, the FR chemistry that is currently protecting PU foams is under regulatory scrutiny due to perceived environmental issues;^18,19 as a consequence, most of the high performing FRs have been either banned or limited as far as their application is concerned, thus leaving PU foams practically unprotected.^20–22 As the PU FR field is seeking for new, valuable and high performing solutions, the LbL technique has proven to be an extremely versatile tool able to penetrate inside the foam and homogeneously coat each available surface with fire-proof nanostructured materials.^23–25 The first published results demonstrated that LbL coatings made of carbon nanofibers,²³ montmorillonite²⁴ or simply polyelectrolytes²⁵ can successfully reduce the fire hazard of treated PU foams when exposed to irradiative heat fluxes corresponding to developing fires (i.e. 35 kW m⁻²). However, the LbL treated PU foams have never been characterized under higher heat fluxes or heat insulation conditions, thus leaving the potential of the LbL technique undisclosed.

To this aim, the PU foams coated by the adaptive coatings presented in this work have been characterized under both standard and extreme heat conditions. First, the coating growth and morphology have been assessed by infrared spectroscopy (IR) and scanning electron microscopy (SEM), respectively. Then, the thermal shielding performances and the subsequent FR effect on PU foam flammability have been tested by assessing the reaction to a direct flame application (i.e. through flammability tests) and to an irradiative heat flux (by using cone calorimetry tests). Flammability tests allow to evaluate the coating reaction to a direct methane flame application, while cone calorimetry measurements evaluate the coating behaviour under irradiative heat fluxes that can be found at different stages of a fire development.²⁶

Finally, the LbL treated PU foams have been tested as an insulating barrier by assessing their resistance to a propane–butane flame torch penetration (T ≈ 1300 °C), while monitoring the temperature on the unexposed side of the foam.

The collected results clearly demonstrate the extreme performances of the proposed adaptive layer by layer coatings making this approach a safe and high performing alternative for the protection of materials.

Results and discussion

Coating growth and morphology on PU foams

First of all, each layer constituent has been evaluated by FT-IR spectroscopy (see Fig. S1†).

Poly(acrylic acid) shows the characteristic strong band of carbonyl group located at 1720 cm⁻¹ along with weaker peaks located at 1450 and 1250 cm⁻¹ and ascribed to CH₂ and C–O groups, respectively.²⁷ For what concerns chitosan, the characteristic signals related to NH₃⁺ asymmetric and symmetric stretching vibrations can be found at 1640 and 1550 cm⁻¹, respectively, whereas at lower wavenumbers the peaks of the CH₂ groups (1410 cm⁻¹) and of the glycoside linkage (1080 cm⁻¹) can be detected.²⁸ Poly(phosphoric acid) (PPA) shows characteristic peaks at 1260, 1020 and 885 cm⁻¹ attributable to the stretching of P=O, PO²⁻ and P–O⁻ groups, respectively; a broad band, due to OH deformation vibration, can be detected around 1700 cm⁻¹.²⁹

Fig. 1 plots the acquired IR spectra for the first three layers adsorbed during the deposition and the evolution of the coating signals as a function of the increasing QL number.

Fig. 1 FT-IR spectra of the LbL architecture: (a) first layer adsorbed (PAA), (b) second layer adsorbed (chitosan), (c) third layer adsorbed (PPA) and (d) evolution of the IR signals as a function of the QL number.

After the adsorption of the first layer of PAA, it is possible to observe the formation of a signal located at 1710 cm⁻¹ and related to the C=O stretching vibration of the PAA carboxylic group: this signal is the strongest one for PAA (see Fig. S2†), while weaker bands can be also located in between 1300–1100 cm⁻¹ due to CH₂ groups. The subsequent adsorption of chitosan leads to the formation of new and characteristic peaks strictly related to this macromolecule; indeed, as reported in Fig. 1, new strong signals appear in the region 1200–1000 cm⁻¹. The strongest one is found at 1075 cm⁻¹ and can be ascribed to the C–O–C signals of the glycoside linkage; in addition, two bands related to the NH₃⁺ asymmetric and symmetric stretching vibrations appear at 1640 and 1550 cm⁻¹. The adsorption of the negatively-charged PPA brings two new strong peaks to the IR spectrum of the assembled layers: the first, located at 1260 cm⁻¹, refers to the stretching vibration of the P=O group, while the second at 880 cm⁻¹ can be ascribed to the P–O⁻ stretching. This latter signal appears shifted if compared with that reported in Fig. S1 for PPA;† such a shift is attributed to the formation of P–O^{− +}H₃N– electrostatic bonds during the LbL assembly. When the number of deposited layers further increases, the above signals grow in intensity as clearly observable from the IR spectra of 1, 2, 3, 4 and 5 QL reported in Fig. 1b.

The coating can be easily transferred to the PU foam as assessed by SEM microscopy: some typical micrographs are collected in Fig. 2.

Fig. 2 SEM micrographs of untreated PU foam (a) and PU foam treated by 1 (b), 2 (c), 3 (d), 4 (e) and 5 (f) QL.

Untreated PU shows a structure typical of open cell foams: the surface of each open cell appears to be smooth as imaged at higher magnifications (see inlet of Fig. 2a). When the PU foams are treated with 1 QL, a change in the surface morphology is clearly observable because of the deposition of a thin and homogeneous coating on each available surface. By increasing the number of deposited QL, the coating thickness increases and its nanotexture becomes clearly visible. When 4 and 5 QL are assembled, the LbL architecture is capable of conformally coating the PU 3D structure following its complex geometry (see inlets of Fig. 2e and f) without altering the open cell characteristic of the substrate. Elemental analyses performed on LbL coated PU revealed the presence of phosphorous (Fig. S2†), thus confirming the outcome of the LbL deposition. The mass gain, evaluated by weighting the samples before and after the LbL treatment, reaches 10, 13, 23, 27 and 48% for 1, 2, 3, 4 and 5 QL, respectively.

Flammability behaviour

Being among one of the first ignited items during a fire developing in a room, it is important to study the foam reaction to a direct flame application. For this purpose, untreated and LbL-treated foams have been subjected to flammability tests; the collected results are reported in Table 1.

Table 1 Flammability data of untreated and LbL treated PU foams

Sample	Burning time [s]	Dripping	Cotton ignition	Residue [%]
PU	65	Yes	Yes	0
1 QL	67	No	No	8
2 QL	96	No	No	17
3 QL	110	No	No	45
4 QL	53	No	No	88
5 QL	33	No	No	94

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Upon application of a methane flame, the untreated PU immediately ignites and burns completely in less than 75 seconds, leaving no residue at the end of the test. During combustion, the formation of molten polymer droplets can be observed; such droplets fall beneath the sample holder igniting the dry cotton positioned underneath before starting the test. This phenomenon, known as melt dripping, is extremely dangerous as it can easily spread the fire to other ignitable materials (e.g. a carpet) and dramatically increase the speed of the developing fire.

The deposition of 1 QL completely changes the behaviour of the PU foam during the test: indeed, no melt dripping occurs during combustion and, while 1 QL sample still burns for its entire length, it is possible to collect a compact and self-standing residue that is averagely the 8% of the initial weight. By increasing the number of deposited QL to 2 and 3, the PU behaviour during combustion can be further ameliorated: indeed, the residues after combustion increase (17 and 45%, for 2 and 3 QL, respectively); in addition, although both 2 and 3 QL samples burn completely, the burning rate is significantly reduced, thus indicating an inefficient combustion.

4 and 5 QL deposited on the PU foam are capable of stopping the flame propagation by reaching the self-extinguishment during the test: indeed, because of the presence of the coating, the advancing flame is forced to involve a progressively smaller portion of the foam until the heat produced by the flame cannot sustain the combustion anymore, hence resulting in self-extinguishment. Samples treated by 5 QL reach the best performances achieving the self-extinguishing phenomenon within the first 2.5 cm and in a shorter time; furthermore, any subsequent flame application is not capable of igniting the foam again. It is worth mentioning that, during the test of 5 QL, the flame dimensions decrease dramatically below 1 cm (for untreated PU the flame is more than 5 cm long). The final residue is greatly enhanced reaching 88% for 4 QL and even 94% for 5 QL; the snapshot of the residues after the flammability tests are reported in Fig. S3.†

Limit oxygen index (LOI) values (i.e. the minimum concentration of oxygen, expressed as a percentage, that will support combustion of the sample) have been also evaluated. The LOI values of the PU foam increase from 17.3 (untreated PU) to 18.5, 19.8, 20.8, 21.5 and 22.9% for 1, 2, 3, 4 and 5 QL, respectively; these values, which increase by increasing the QL number, further support the significant FR properties observed in the flammability tests.

From the flammability tests it is clear that the deposited nanoarchitectures are capable of protecting the PU foam from a direct flame application with an efficiency that increases by increasing the QL number; indeed, 1 QL is enough to suppress the melt dripping phenomenon, while 4 and 5 QL reach the self-extinguishing of the flame. More in details, upon flame application, the coating constituents start reacting together (adapting to the new temperature conditions) in order to create a protective coating. Indeed, both chitosan and poly(acrylic acid) are able to produce a thermally stable char; in addition, these char forming reactions can be further enhanced by the dehydration abilities of the poly(phosphoric acid). This coating is able to protect the underlying PU from heat and oxygen and also to prevent it from collapsing, hence giving rise to the formation of a pool of molten polymer. By increasing the number of QL, a more stable and insulating protection can be achieved during combustion, hence resulting in better performances.

Combustion tests

Cone calorimetry operates in a well-defined flaming condition, forced by external radiation, typical for a developing fire scenario (the stage of which is defined by the adopted heat flux). It gathers data regarding the burning behaviour of a sample from ignition (as a consequence of the heat flux exposure) through the subsequent flaming combustion (rate of heat release) to the end of combustion. Treated and untreated PU foams have been tested under irradiative heat fluxes of 35, 50 and 75 kW m⁻² that can be referred to the average heat fluxes found in early stage, developing and fully developed fires, respectively.²⁶ Table 2 reports the collected data.

Table 2 Cone calorimetry data of untreated and LbL-treated PU

Sample	TTI [s]	pkHRR [kW m^−2]	FIGRA [kW sm^−2]	THR [MJ m^−2]	Residue [%]
Heat flux: 35 kW m^−2
PU	1 ± 1	395 ± 15	26	7.6 ± 0.5	4
1 QL	2 ± 1	395 ± 18	28	7.5 ± 0.1	8
2 QL	2 ± 1	249 ± 27	12	6.8 ± 0.3	9
3 QL	3 ± 1	214 ± 26	12	7.3 ± 0.1	10
4 QL	4 ± 1	189 ± 26	8	6.9 ± 0.3	12
5 QL	4 ± 1	177 ± 5	7	6.2 ± 0.1	13
Heat flux: 50 kW m^−2
PU	1 ± 1	536 ± 24	50	7.4 ± 0.4	4
4 QL	2 ± 1	240 ± 4	17	6.4 ± 0.5	8
5 QL	3 ± 1	246 ± 3	13	7.7 ± 0.2	10
Heat flux: 75 kW m^−2
PU	1 ± 1	623 ± 57	85	7.5 ± 0.2	4
4 QL	1 ± 1	336 ± 3	37	6.9 ± 0.2	7
5 QL	1 ± 1	259 ± 8	18	6.3 ± 0.2	10

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Upon exposure to the cone calorimeter heat flux, the untreated PU foam instantly ignites and collapses creating a pool of molten polymer that burns completely in few seconds, leaving no residue and releasing a total heat of 7.6 MJ m⁻² with a heat release rate peak (pkHRR) of 395 kW m⁻².

The deposition of 1 QL does not change the burning behaviour of the PU foam (in other words, the sample still ignites very quickly and collapses during combustion); however, 9% residue can be found at the end of the test. With the deposition of 2 QL, the burning behaviour of the treated PU foams completely changes; indeed, the foam does not collapse anymore during combustion, thus preventing the formation of the molten polymer pool. As a consequence, the pkHRR of 2 QL is significantly reduced by 37% with respect of the untreated foam. The increase of the QL number leads to a further pkHRR reduction that reaches the maximum decrease when 5 QL are assembled (−55%).

The performance of the PU foams treated by 4 and 5 QL have been evaluated under higher heat fluxes, namely 50 and 75 kW m⁻², that can mimic the scenarios of developing and developed fires, respectively. As reported in Table 2, both the treated samples are capable of remarkably reducing the pkHRR, regardless of the adopted heat flux; in details, 5 QL can reach a 55% reduction when tested at 50 kW m⁻²; a further decrease up to 60% was found upon exposure to a higher heat flux (i.e. 75 kW m⁻²). These findings are of great importance as one of the widely recognized flaw of currently employed FR relies in their bad efficiency when tested at high heat fluxes.¹⁷

Fire growth rate (FIGRA) values of untreated PU and PU treated by 4 and 5 QL have been also calculated (Table 2). FIGRA represents a fire safety engineering parameter and can be related to the rate of fire growth for a material once exposed to heat. Higher FIGRA values suggest faster flame spread and an increase chance of ignition for nearby objects. As shown in Table 2, FIGRA values are strongly reduced by the presence of the coatings, regardless of the adopted heat flux and hence they highlight the excellent performance of these LbL nanoarchitectures. Once again, as observed for flammability, such extraordinary performance can be ascribed to the adaptive nature of the assembly that is capable of creating a carbonaceous shield that both protects the PU slowing down heat and oxygen transfer from the flame and prevents the PU foam from collapsing, as demonstrated by the residues collected at the end of the test (Fig. S4†). In detail, both 4 and 5 QL samples left a coherent residue bearing a structure similar to that of the original foam but with decreasing dimensions with increasing the adopted heat flux. Again, 5 QL assemblies are capable of yielding the highest and most coherent residues.

SEM microscopy has been exploited for imaging these residues in order to investigate their structure and morphology; Fig. 3 presents the collected micrographs of 4 and 5 QL residues tested under each adopted heat flux at different magnifications.

Fig. 3 SEM micrographs performed on the residues after cone calorimetry tests: 35 kW m⁻² ((a) 4 QL and (b) 5 QL), 50 kW m⁻² ((c) 4 QL and (d) 5 QL) and 75 kW m⁻² ((e) 4 QL and (f) 5 QL).

As clearly observable from Fig. 3a and b, the residues left under 35 kW m⁻² still possess the original open cell structure of the PU foam (compare Fig. 4a and b with Fig. S5†). By increasing the magnification, the real structure of the residue is revealed; surprisingly, the coating deposited is now the only component of the three-dimensional structure of the residue. Indeed, as clearly depicted in Fig. 3a and b inlets, the walls of the cells are hollow and consist of the solely LbL assembly. It seems that during combustion the coated PU foam has served as 3D template for the structure of the final residue. Indeed, upon exposure to a heat flux, the architecture constituents start reacting together, hence changing the assembly into a thermally stable shield that remarkably slows down the PU foam combustion evolution, as demonstrated by cone calorimetry tests. Although well protected by the coating, the PU foam eventually burns completely, thus leaving a 3D structure resembling the original foam but made of the coating only. The residues found at 50 kW m⁻² show the same 3D structure as depicted in Fig. 3c and d. However, at 75 kW m⁻², the 4 QL structure appears to be severely damaged (see Fig. 3e inlet); on the contrary, the residue resulting from the 5 QL assembly is capable of maintaining the 3D structure even at such high heat flux (see Fig. 3f).

Fig. 4 Temperatures measured on the unexposed side of the foam during flame penetration test.

Resistance to flame penetration

The thermal barrier of the coated foam has been tested by flame penetration tests. The layout adopted for these tests is usually employed to evaluate the flame penetration resistance of composites for aerospace applications (i.e. to very high performing materials); during the tests, the sample is placed in between a thermocouple and a flame torch. According to the adopted experimental conditions, the flame torch has been turned on three times for 5 second each; meanwhile, the temperature on the back side (unexposed one) of the sample has been monitored. Fig. 4 reports the maximum measured temperatures after each flame torch application for the untreated PU and for PU treated by 4 and 5 QL.

It is noteworthy that the untreated PU foam is completely penetrated by the torch upon the first application; as a consequence, the temperature measured during the first application is around 685 °C, while the subsequent flame applications yield the foam to behave as an unshielded system (i.e. the measured temperatures are similar to those detected when no sample is placed in between the torch and the thermocouple). The snapshots of untreated PU and 5 QL-treated PU during the flame penetration tests and the final residue of 5 QL samples are depicted in Fig. 5.

Fig. 5 (a) snapshots of untreated PU and 5 QL treated PU during flame penetration test (inlets: unexposed side of the sample), (b) residue of 5 QL sample after the test (left) and its cross section (right).

Conversely, when 4 and 5 QL samples are tested, the treated LbL foam is capable of withstanding the flame torch penetration keeping its structure and thermally insulating the unexposed side; indeed, as reported in Fig. 5a for 5 QL, only one side of the sample is damaged by the torch, and, at the end of the test, the specimen still shows undamaged areas as reported in Fig. 5b. As a consequence, the temperature readings, for each torch application, were found to be remarkably below those corresponding to the untreated PU. Once again, as observable from temperature bars in Fig. 4, the best results are achieved with the deposition of 5 QL; indeed, the maximum temperatures reached during the first and second torch application are below 100 °C (68 and 80 °C, respectively) and raise to an average of 247 °C only during the third flame application. It is worthy to notice that these temperatures, as compared with the unshielded system, correspond to temperature drops of 806, 794 and 627 °C (for each subsequent torch application) that are reached in only 10 mm (i.e. within the sample thickness). This insulating capability is impressive, especially taking into account that it is achieved with the simple deposition of a thin LbL assembly that not only protects the PU foam but also shields other possible underlying materials from heat, further increasing its potentialities.

Experimental

Materials and methods

Materials. Open cell polyurethane foams (Fig. S5†) with a density of 18 g dm⁻³ and a thickness of 10 mm have been purchased from the local warehouse. Chitosan (75–85% deacetylated), poly(acrylic acid) and poly(phosphoric acid) (115%) were purchased from Sigma Aldrich (Milwaukee, WI) and used as received for preparing 1 wt% water solutions, using 18.2 MΩ deionized water supplied by a Q20 Millipore system (Milano, Italy). The pH of the chitosan solution was adjusted to 3.5 using acetic acid (Sigma-Aldrich). In order to limit the effect of poly(phosphoric acid) hydrolysis, the solution was prepared 1 hour prior to the LbL deposition.

Layer by layer deposition. Si wafers or PU foams were alternately dipped into the negatively poly(acrylic acid) (PAA) and poly(phosphoric acid) (PPA) and positively (chitosan) charged solution in order to deposit a LbL coating consisting of a quad-layer (QL) repetitive unit with the following composition: PAA/chitosan/PPA/chitosan. The dipping time of the first 4 layers was set to 5 minutes, the subsequent layers were achieved after 1 minute. The foams were washed with ultrapure water in between each adsorption step by immersion and vigorous squeezing. The building up of the LbL architecture started with the deposition of negative poly(acrylic acid) in order to improve the coating adhesion and homogeneity; indeed, it has been reported in the literature that the PAA can be easily adsorbed on PU foam substrates due to the formation of hydrogen bonding between the urethane groups and the undissociated carboxylic groups (the degree of dissociation of PAA in 1 wt% is averagely below 5%).²⁵ The process was repeated till 1, 2, 3, 4 and 5 QL were deposited. Fig. S6† schematize the LbL process adopted in this paper.

Infrared spectroscopy. The growth of the LbL assembly was monitored using a FT-IR spectrophotometer (32 scans and 4 cm⁻¹ resolution, Perkin Elmer SPECTRUM GX).

Scanning electron microscopy. The morphology of untreated and treated PU foams has been studied using a LEO-1450VP scanning electron microscope (imaging beam voltage: 5 kV); elemental analysis has been performed with an X-ray probe (INCA Energy Oxford, Cu-Kα X-ray source, k = 1.540562 Å, beam voltage: 20 kV). PU pieces (5 × 5 mm²) were cut using a doctor blade and fixed to conductive adhesive tapes and gold-metallized.

Flammability test. The flammability of prepared samples has been tested in horizontal configuration following the ASTM D 4986 standard; the sample (50 × 150 × 10 mm³) was positioned on a metallic grid and ignited from its short side by a 20 mm methane flame (flame application time: 5 s). Dry cotton wad was placed 30 cm beneath the metallic grid in order to assess the incandescent melt dripping phenomenon. The test was repeated 4 times for each formulation in order to ensure reproducibility; during the test, parameters such as burning time, final residue and formation of incandescent droplets of molten polymer were registered.

Limiting oxygen index (LOI). LOI tests were carried out using a FIRE oxygen index apparatus following the ASTM D2863 standard.

Cone calorimetry test. Cone calorimetry (Fire Testing Technology, FTT) was employed to investigate the combustion behaviour of square samples (50 × 50 × 10 mm³) under different irradiative heat fluxes (35, 50 and 75 kW m⁻²) in horizontal configuration, following the ISO 5660 standard. Time To Ignition (TTI), Heat Release Rate and corresponding peak (pkHRR), Total Heat Release (THR), and final residue were evaluated. FIre GRowth Rate (FIGRA) values have been calculated as the ratio between the pkHRR and the time at which the peak occurs; FIGRA represents a fire safety engineering parameter that can be related to the rate of fire growth for a material once exposed to heat (high FIGRA values suggest faster flame spread and an increase chance of ignition for nearby objects). For each sample, the experiments were repeated four times in order to ensure reproducible and significant data. The experimental error was evaluated as standard deviation (σ).

Flame penetration test. The tests were carried out placing the specimen (50 × 50 × 10 mm³) in a ceramic frame, held in vertical configuration, and applying a butane–propane flame (65 : 35 v/v) toward the specimen centre. The flame, generated from a torch positioned at 100 mm distance from the surface of the specimen, was applied for 5 s for three times. The temperature on the back side of the specimen (i.e. the surface not exposed to the flame) was measured by using a thermocouple (stainless steel sheathed K-type; 0.5 mm diameter). The test was duplicated.

Conclusions

In this work, LbL architectures capable of adapting to extreme heat have been successfully designed and employed for the fire protection of open cell polyurethane foams. The coating, made of poly(acrylic acid), chitosan and poly(phosphoric acid), was found to cover each surface of the PU complex three-dimensional structure homogeneously and without altering its open cell morphology, as assessed by SEM measurements.

When exposed to a methane flame, the LbL-treated PU foam achieved the self-extinguishment within few seconds after ignition; on the contrary, the untreated PU foam burned completely with a vigorous flame, producing, at the same time, incandescent melt droplets capable of spreading the fire to other ignitable materials. Under different irradiative heat fluxes (from 35 to 75 kW m⁻²) the coating demonstrated exceptional performances in reducing the rate of heat release up to 60%, with respect to the untreated foam, regardless to the adopted heat flux and hence demonstrating a wide spectrum efficiency. The achievement of a self-extinguishing behaviour in flammability tests and the significant reductions of the rate of heat release assessed during cone calorimetry tests represent two significant key findings that would likely slow fire growth in real world fire scenarios, giving people more time to escape or extinguish the foam, thus preventing dangerous flashover events.

Finally, when subjected to a flame torch penetration (T ≈ 1300 °C), the LbL-treated foams showed astonishing insulating capabilities as: (i) they kept their three-dimensional structure and (ii) they achieved, within a sample thickness of only 10 mm, temperature drops of 800 °C on the unexposed side of the specimens. Therefore, the adaptive all-polymer nanoassemblies here discussed can provide a safe and high performing alternative for the protection of flammable materials.

Acknowledgements

Prof. Giovanni Camino is acknowledged for the fruitful discussions.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: IR spectra of coating constituents, EDS analysis on LbL treated PU foam, residues after flammability tests, residues after combustion tests, low magnification micrographs of untreated PU foam and schematization of the adopted LbL process. See DOI: 10.1039/c4ra01343c

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