Sandwichlike Coating Consisting of Alternating Montmorillonite and Î²

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Sandwich-like coating consisting of alternating montmorillonite and #-FeOOH for reducing the fire hazard of flexible polyurethane foam Wei Wang, Haifeng Pan, Yongqian Shi, Bin Yu, Ying Pan, Lei Song, Kim Meow Liew, and Yuan Hu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b00805 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 26, 2015

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ACS Sustainable Chemistry & Engineering

Sandwich-like coating consisting of alternating montmorillonite and β-FeOOH for reducing the fire hazard of flexible polyurethane foam Wei Wang a, b, Haifeng Pan a, b, Yongqian Shi a, b, Bin Yu a, b, Ying Pan a, Kim Meow Liew b, c, Lei Song *, a, Yuan Hu *, a, b

a

State Key Laboratory of Fire Science, University of Science and Technology of

China, 96 Jinzhai Road, Hefei, Anhui 230026, People's Republic of China. b

Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute of University of

Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, People's Republic of China. c

Department of Architecture and Civil Engineering, City University of Hong Kong,

Tat Chee Avenue Kowloon, Hong Kong.

*

Corresponding author. Fax/Tel: +86-551-63601664.

E-mail address: [email protected] (Yuan Hu); [email protected] (Lei Song).

ACS Paragon Plus Environment


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ABSTRACT: A novel nanocoating, composed of montmorillonite (MMT) nanosheets and β-FeOOH nanorods, was deposited on the surface of flexible polyurethane (FPU) foams through a layer-by-layer assembly technique to reduce its fire hazard. The coating growth was performed by alternately immersing FPU foams into polyethylenimine (PEI) solution, MMT-alginate suspension and β-FeOOH dispersion. Structural and morphological characterization indicated that MMT nanosheets and β-FeOOH nanorods were distributed homogeneously on the surface of the matrix and formed a sandwich-like topology. The coated FPU foams showed the lower peak heat release rate (pHRR), compared to the counterparts fabricated by introduction of MMT nanosheets or β-FeOOH nanorods alone. Furthermore, the concentration of volatile products released from the coated FPU foams was reduced remarkably, indicating that the binary components had tremendous advantages in reducing the fire hazards of the material. These significant improvements in flame retardancy and toxic gas suppression could be caused by the reason: the MMT-based layers and the network formed by β-FeOOH nanorods exhibited “labyrinth effect” to retard the permeation of heat, oxygen and mass between the flame and underlying FPU matrix. Keywords: layer-by-layer assembly; montmorillonite; β-FeOOH nanorods; fire hazard; mechanism.


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Introduction Flexible polyurethane (FPU) foams are widely applied in various fields of furniture, carpet underlay automobiles, etc properties

1-2

because of its excellent cushioning and physical

3-5

. The demand for FPUF is increasing with the rapid development of

automobile manufacturing industry 6. Unfortunately, FPU foam, as the highly cellular materials, is flammable, and releases a great amount of smoke and toxic gaseous products during combustion

7-9

. Due to increasingly strict standards being developed

in traffic safety regulation, considerable attention has focused on improving flame-retardant properties of FPU foam 10-12. It is well known that flame retardant additives containing halogen, phosphorus, nitrogen, boron, or silicon can improve the flame retardancy of FPU foams on the basis of condense-phase and/or gas-phase mechanism

8, 13

. Nevertheless, the

application is usually limited due to their relatively low efficiency or toxic gases production 14. It is of great significance to develop advanced strategies to solve these issues. To date, various nano-additives have been widely used as flame retardants, such as polyhedral oligomeric silsesquioxane 15, carbon nanotubes 16, graphene 17, and molybdenum disulfide

18

, etc, to endow polymers with excellent flame retardancy.

These sheet-like or tubular flame retardants show the physical barrier effect during burning process, improving the flame retardancy and reducing the evolution and spread of toxic gases 19. Recently，layer-by-layer (LbL) assembly technique has been applied to build efficient fire hazard suppression systems

20-26

. The LBL assembly technique can



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integrate various polymers and molecules into a thin film through electrostatic interaction. It is easy to obtain good dispersion and controlled properties of the thin film on the surface of the substrate. One-Dimensional (1D) material has been utilized to construct the nanocoating by LBL assembly. Kim et al. prepared carbon nanofiber (CNF) based coating on the surface of FPU to improve its flame retardancy. This bilayer (BL) coating consisted of two opposite charged components, including a cationic layer prepared by a solution blend of CNFs and branched polyethylenimine and an anionic layer (poly(acrylic acid))

20

. Most recently, Pan et al. deposited

titanium dioxide nanotubes on the surface of PUF via a LBL assembly technique. The obtained results exhibited a remarkable reduction in peak heat release rate (PHRR) and smoke production. The improvements in toxic gases and smoke suppression were attributed to the physical barrier effect and oxygen absorption capacity of the nanocoating

25

. Similarly, clay-based coating was deposited on the surface of FPUF

for improving its flame retardancy. Li et al. prepared montmorillonite (MMT)-based multilayered nanocoating on the surface of PUFs, and a 30% decrease in PHRR was achieved at 4.8 wt.% loadings 27. The fascination of layer by layer assembly technique derives from the ability which can realize the functional diversity for substrates. It is worth mentioning that LBL assembly technique has been developed by the use of trilayer approach instead of the bilayer 20. Generally, inorganic material-based coatings have been demonstrated as physical insulation barriers for fire safety improvement. However, LbL coatings to improve fire safety of substrates based on the synergistic mechanisms are seldom


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investigated. Therefore, it is expected that the combination of the physical barrier effect from MMT layers (2D) and β-FeOOH nanorods (1D) via LBL assembly technique is better to properties improvement of the hosts than the single one. In the present work, a hybrid coating based on β-FeOOH nanorods and MMT nanosheets was deposited on the surface of FPUFs by a trilayer approach. Cone calorimeter was employed to investigate the flame retardancy of coated FPUFs. The pyrolysis toxic gaseous products of the control and coated flexible polyurethane foams were measured by thermal gravimetric infrared analysis (TG-IR). The dispersion of nanocoating on the surface of FPUFs was evaluated by scanning electron microscopy (SEM).

Experimental Raw Materials Flexible polyurethane foam (DW30) was obtained from Jiangsu Lvyuan New Material Co., Ltd. Branched polyethylenimine (PEI, branched, Mw=10000 g/mol) and sodium alginate (SA) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Polyacrylic acid (PAA; MW ~ 100000) was purchased from Sigma-Aldrich. Na-montmorillonite (Na-MMT) was provided by Zhejiang FengHong new material Co., Ltd. (Zhejiang, China). Sodium hydroxide, acetic acid, and concentrated hydrochloric acid (HCl; 36.5%-38%) were received from Changzheng Chemical Reagent Corp. Preparation of β-FeOOH nanorods β-FeOOH nanorods were synthesized by a simple hydrothermal method according to



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the literature 28. In a typical procedure, 40 mL of 0.5 M FeCl3 solution was transferred into a 50 mL Teﬂon-lined autoclave. The autoclave was heated to 120 °C, and kept for 12 h. After cooling to room temperature, the resulting precipitates were washed several times with deionized water and absolute ethanol, and then dried at 80 °C for 6 h. Preparation of solution and dispersion β-FeOOH nanorods suspension was firstly prepared. The synthetic β-FeOOH nanorods (5 mg.mL−1) were dispersed in deionized water; the pH was adjusted to 6 with 1 M HCl solution, and then strongly stirred for 24 h to produce a stable suspension. Analogous to that of β-FeOOH nanorods, the preparation of sodium alginate (SA)/MMT suspension was performed as follows: MMT (5 mg.mL−1) and SA (3 mg.mL−1) were dispersed in water, and then stirred for 24h to obtain a stable dispersion. PEI solution (5mg.mL−1) was prepared by adding PEI to DI water and thereafter the pH was tuned to 9 with 1 M HCl or NaOH solution, then stirred for 24 h. SA (3 mg.mL−1) solution was prepared using similar process to that for preparation of PEI solution. Layer by layer deposition process As stated in our previous work, to create a positively charged surface on its surface, FPU foam was presoaked in a 0.1 M HNO3 solution for 5 min before deposition and excess acidic solution was squeezed out. Then, FPU foams were soaked in 1.0% PAA solution for 5min, which induced a negatively charge on the foam surface. Then the FPU foams were soaked by a dipping sequence of PEI solution, MMT/SA dispersion


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and β-FeOOH suspension. Each dip was followed by rinsing with DI water for 2 min and wringing out to expel liquid from among the FPU foams. This fabrication process was denoted as the first trilayer. After the desired number of trilayers was deposited, the coated FPU foams were dried at 60 °C overnight before testing. For comparison, the samples deposited by only MMT or β-FeOOH based coating are also prepared by the identical process: the pretreated FPU foams were soaked by the dipping sequence of PEI solution, MMT/SA dispersion and the dipping sequence of PEI solution, SA solution and β-FeOOH suspension. The samples were marked, as shown in Table 1. The preparation process is illustrated in Scheme 1. Characterization X-ray diffraction (XRD) measurements of β-FeOOH nanorods was performed with a Japan Rigaku D=Max-Ra rotating anode X-ray diffractometer equipped with a Cu-Ka tube and Ni filter (λ= 0.1542 nm). Transmission electron microscopy (TEM) images of β-FeOOH nanorods and MMT nanosheets were obtained on a Jeol JEM-100SX transmission electron microscope with an acceleration voltage of 100 kV. UV-vis

absorption

measurements

were

taken

using

a

UV-visible

spectrophotometer (Cary 100 Bio, Varian, USA). Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra, in the frequency region of 4000-400 cm-1 at a 4 cm-1 resolution, were recorded by a Nicolet 6700 spectrometer (Thermo-Nicolet) using 32 scans. The morphologies of control and treated flexible PU foams, along with the char



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residues after cone calorimetry, coated with a gold layer in advance were observed using scanning electron microscopy (SEM; AMRAY1000B, Beijing R&D Center of the Chinese Academy of sciences, China). The thermogravimetric analysis (TGA) of samples was examined on a TGA-Q5000 apparatus (TA Co., USA) from 50 to 700 °C at a heating rate of 20 °C.min-1. The weight of all samples was kept within 3-5 mg in an open platinum pan. TG/FTIR of the control and coated flexible PU foams was performed using a TGA Q5000IR thermal gravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer. Approximately 5.0 mg of the sample was put in an platinum crucible and heated from 30 to 600 °C at a heating rate of 20 °C.min-1 (Helium atmosphere, flow rate of 45 mL· min-1). The combustion test was carried out on cone calorimeter (Fire Testing Technology, U.K.) according to ISO 5660 standard procedures, with 100 × 100 × 25 mm3 specimens. Each specimen was exposed horizontally to 35 kW.m2 external heat flux.

Results and discussion Characterization of β-FeOOH and MMT Figure 1 presents the TEM images of β-FeOOH (a), MMT (b) and XRD pattern of β-FeOOH nanorods (c). It is evident that MMT has a diameter of ca. 3 µm and stacked structure, as shown in Figure 1(b). As can be observed from Figure 1(a), the as-synthesized

β-FeOOH

nanorods

show

the

rods-like


morphology

with

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homogeneous size distribution. The diameter and the length of the rods are around 0.2 and 1.4 µm, respectively. In addition, all the diffraction peaks in Figure 1(c) are indexed to the β-FeOOH nanorods, which is in good consistence with JCPDS card (No. 75-1594). Characterization of the control and coated FPU foams ATR-FTIR spectroscopy was conducted to qualitatively investigate the chemical structures of the FPU foam surface. Figure 2 is the ATR-FTIR spectra of the control and coated FPU foams. The absorption peaks at 1098 and 1222 cm-1 are the characteristic peaks of the pure FPU foam, attributing to the non-symmetric stretching vibration of C-O-C and the stretching vibration of aromatic C-O, respectively, while the peak located at 1536 cm-1 is assigned to deformation and stretching vibration of N-H

29-30

. In the case of FPU3, FPU4, FPU5 and FPU6, due to the overlap of

MMT-based coating, the new characteristic peak at 1043 cm-1 appears, which is corresponding to the stretching vibration of Si-O-Si from MMT.26 Moreover, the absorbance increases with increasing of the number of layers, demonstrating the successful LBL deposition. In addition, for FPU1, FPU2, FPU5, and FPU6, another new peak at 3488 cm-1 is visible, which is ascribed to the characteristic absorption of H-O for β-FeOOH

31

. Also, the absorbance increases with the increment of layer

number. The successful deposition can be further demonstrated by the SEM measurement, as depicted in Figure 3. SEM was employed to observe the surface morphology of the control and coated FPU foams. The smooth and clean surface feature of the control FPU foam is



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observed in Figure 3a and b. After coating, the surface of FPU foams becomes rough, indicating the successful deposition of the inorganic filled coating. Through the observation from Figure 3(d), it is clearly seen that β-FeOOH nanorods are well introduced onto the surface of FPU foam. Several nanorods form cross-linking network, providing the physical blocking effect 25. However, the β-FeOOH nanorods exhibit the poor dispersion; the meshes in the network are relatively large, which maybe weaken the physical barrier effect. Figure 3f is the high magnification image of FPU4, indicating a uniform dispersion. The dispersion of MMT is benefit to the improvement of flame retardancy due to its physical barrier effect and high thermal stability. As shown in Figure 3g and h, the excellent dispersion of both β-FeOOH nanorods and MMT nanosheets are observed in FPU6. Furthermore, striking network is visible, and the meshes disappear, due to the overlap of the MMT. UV-visible absorption spectrometry was used to monitor the coating growth on the quartz slides substrate. Figure 4 plots the representative UV-visible absorption spectra of (PEI/SA/β-FeOOH), (PEI/MMT-SA) and (PEI/MMT-SA/β-FeOOH) multilayer of 3, 6, 9 cycles prepared on quartz slides. As can be seen from Figure 4a, the absorption peak at 490 nm is assigned to the β-FeOOH 32. The intensity increases as the cycle number rises. In Figure 4b, a strong absorption band occurs at 200 nm, caused by the ultrathin films 33. In addition, there is a linear relationship between the absorbance and the number of deposited cycles, demonstrating that the deposition process is reproductive from unit to unit. Figure 4c shows the UV-visible absorption spectra of PEI/MMT-SA/β-FeOOH multilayer film. The characteristic peak of


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β-FeOOH at 490 nm along with the strong absorption band around 200 nm is observed obviously. The linear relationship is also existent between absorption intensity and the number of deposited cycles. These results further demonstrate that the LBL assembly was successfully performed by alternating deposition of MMT and β-FeOOH. Thermal stability Figure 5 depicts the measured TGA curves of all the samples under He and air conditions. Two-stage thermal degradation is observed for all the samples. The first stage in the range of 220-270 °C is initiated by the liberation of diisocyanate compound, which is caused by depolymerization of the urethane and the bi-substituted urea groups. The second stage is attributed to the pyrolysis of the remaining polyether chain

25, 34

. For FPU0, there is almost no char residue left under

He and air conditions. It is clearly found that the thermal stability of the samples with 3 deposited cycles is better than that of those with 1deposited cycle. In special, FPU6, FPU4 and FPU2 have the char residue of 5.8%, 3.7% and 1.5% at 700 °C under air condition, respectively. Although the mass gains of FPU2 (7.4%) and FPU4 (7.5%) are more than FPU6 (6.8%), as is shown in Table 1. The results reveal the presence of the synergistic effect between MMT nanosheets and β-FeOOH nanorods. Fire hazards of the control and coated FPU foams Cone calorimeter is useful to simulate a developing fire scenario on a fixed sized specimen and evaluate the forced burning fire performance of polymeric materials. Herein, some parameters, including the PHRR, total heat release (THR) were obtained



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for the control and coated FPU foams. The HRR and THR curves of control and coated FPU foams are shown in Figure 6 and Figure 7, respectively. The corresponding data are recorded in Table 2. It can be clearly observed from Figure 6 that the combustion of the control FPU foam consists of two peaks, caused by the pyrolysis of isocyanate and polyol

4, 35

. For FPU1 and

FPU2, as the deposited cycle increases, the PHRR values decrease, attributing to the physical barrier effect of network formed by β-FeOOH nanorods. As the loading increases, the network becomes compact, leading to the enhanced insulating blocking effect. For FPU3 and FPU4, the PHRR values are reduced significantly, indicating that MMT layers act as good physical insulating barriers to inhibit the combustion. Furthermore, the reduction in flammability is positively aligned with the coating mass. As shown in Table 2, the higher deposited cycle, the greater reduce in PHRR values for all the coated samples. Combination of MMT layers and β-FeOOH nanorods further reduces the PHRR value for FPU6 and FPU5. Moreover, the second peak is completely eliminated when the number of assembly is increased to 3. These improvements can be explained as physical barrier effect of MMT layers and the network formed by β-FeOOH nanorods. The FPU6 exhibits the most outstanding reduction in THR among all the resulting samples, because as the mass gains, the synergistic effect performs better. The analysis of char residues after cone calorimeter test In general, the char residue with the characteristics of less crack, denser and thicker structure and faster formation, can illustrate the greatly improved fire safety for


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polymeric materials. The digital photos of char residue of coated samples are shown in Figure 8. From Figure 8a and b, it can be seen that the FPU foams coated by β-FeOOH nanorods cannot keep the original shape and have little char residue left， indicating that the network could not support the whole structure of FPU foam. Figure 8c and d suggest that MMT-based coatings facilitate the formation of char residue. However, the collapse and contraction lead to unfavorable formation of physical barriers. As can be seen from Figure 8e and f, FPU5 and FPU6 keep a good square shape, especially FPU6. With the increase of the deposition cycle, the char residue of FPU6 has little change. Figure9 shows the SEM images of the char residue of FPU2 (a, b), FPU4 (c, d) and FPU6 (e, f) under low and high magnification. As can be seen in Figure 9a and b, the char residues collapse significantly. The crosslinking structure can be seen obviously, indicating that β-FeOOH nanorods can promote forming network structure and delay the heat release rate. In Figure 9c and d, the char residues also collapse, the surface of the residue seems relative smooth, due to the overlap of MMT layers. It can be seen in Figure 9e and f that the structure of the char residue retains well, no collapse and fractures. In high magnification, the surface of the residue keep smooth and the structure of β-FeOOH nanorods can be easily seen, demonstrating that the combination of the MMT layers and β-FeOOH nanorods can effectively protect the foam matrix and improve the flame retardancy more efficiently. Smoke hazards of Control and Coated FPU Foams estimated by TG-IR Cone calorimeter could objectively evaluate the fire hazards of polymer materials in a real fire instead of pyrolysis gaseous products during their thermal decomposition



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under inert atmosphere. TG-IR measurement was widely used to analyze the volatilized products after the thermal decomposition of samples, which contributes to investigating the thermal degradation process and the pyrolysis gaseous products. Figure 10 shows the FTIR spectra of volatilized pyrolysis products emitted from FPU0, FPU2, FPU4 and FPU6 at maximum evolution rate. It can be observed that the characteristic absorption peaks of the pyrolysis products of FPU2, FPU4 and FPU6 are identical to that of FPU0, indicating that the deposition of MMT and β-FeOOH based coating does not significantly alter the thermal decomposition process. Some of the volatilized pyrolysis products of FPU foam can be identified by FTIR signal: the bands at 3450-3600 cm-1 are attributed to the vibration absorption of hydroxide groups, indicative of water vapor; the bands at 2876-2982 cm-1 are ascribed to the aliphatic C-H bonding arising from various alkanes; the peak at 2358 cm-1 is due to the stretching vibration of CO2 ; the sharp characteristic peak at 1750 cm-1 is assigned to the stretching vibration of C=O group; the strong absorption band at 1112 cm-1 is due to the stretching vibration of C-O-C bond from ethers 25, 36. To further highlight the influence of β-FeOOH nanorods, MMT nanosheets and the hybrid system on amount variation of released gases, the absorbance of gaseous volatiles for the control and coated FPU foams versus time is presented in Figure 11. It can be clearly seen that the maximum absorbance of the pyrolysis products for the coated FPU foams are all lower than that for pure PUF remarkably. To study the influence of LBL coating on volatilized pyrolysis products, some typical compounds, such as ethers (1112 cm-1), aromatic compounds (1608 cm-1),


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carbonyl compounds (1750m-1), CO (2180 cm-1), -NCO (2275 cm-1), CO2 (2358 cm-1) and hydrocarbons (2980 cm-1), were selected. As can be seen from Figure 11, the flammable gases are reduced obviously, meaning less “fuel” to be fed back to the flame, and thereby the reduced HRR and THR are observed in cone calorimeter tests. Importantly, the reduction of aromatic compounds and carbonyl compounds results in the toxic decrement of pyrolysis products, which decreased the fire hazards during combustion. Moreover, the reduction of organic volatiles, such as aromatic and carbonyl compounds, leads to the inhibition of smoke particles, thus benefiting the visibility and breathing safety when a fire occurs. In fire scene, -NCO containing compounds released by FPU can do great harm to human health by only few quantity. Therefore, it is meaningful to decrease its generation during combustion. The generated trends of –NCO containing compounds are shown in Figure 11(e). The absorbance of –NCO for FPU6 is much lower than pure FPU, FPU2 and FPU4, attributing to the synergistic physical barrier effect, which can effectively decrease the fire hazards of FPU. Carbon monoxide (CO) and carbon dioxide (CO2) are more dangerous than carbonyl compounds in a real fire scene, thus it is vital to decrease its production. As depicted in Figure 12, the absorbance of CO and CO2 reduced significantly, compared to the samples coated by single component. The reduction in CO and CO2 is tremendously helpful for fire rescue when accidents occur. Flame retardancy mechanism In summary, the possible mechanism of the reduced fire hazards for coated FPU foams is proposed and shown in Scheme 2. Physical insulating barrier effect from



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MMT layer and β-FeOOH nanorods are the main cause for the reduced fire hazards. The sandwich-like structure formed by alternatively deposited MMT layers and β-FeOOH nanorods network, delays the pyrolysis of matrix and reduces evolution of mass into the gas-phase. Simultaneously, the further formed char layer acts as an effective protective shield to delay the transfer of heat, oxygen and volatile gases between the flame and underlying matrix. This work would provide an efficient way to extend the application of 1D and 2D material in reducing fire hazards of polymeric material.

Conclusion Layer-by-layer assembly technique could realize the purposeful deposition on the surface of FPU foams by alternating MMT nanosheets (1D) and β-FeOOH nanorods (2D) to decrease its fire hazards. Characterization by means of ATR-FTIR spectra and SEM images indicated that MMT nanosheets and β-FeOOH nanorods were homogeneously distributed on the surface of the matrix and formed a random entanglement network covered by MMT nanosheets. This coating constructed by MMT-based layers and β-FeOOH nanorods networks significantly improved the fire safety of FPU foams, e.g. the improved flame retardancy and smoke suppression properties. The MMT-based layers and the network formed by β-FeOOH nanorods could behave as a physical insulation barrier to retard the permeation of heat, oxygen and mass between the flame and underlying FPU matrix Acknowledgements The work was financially supported by the National Basic Research Program of China


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(973 Program) (2012CB719701), the National Key Technology R&D Program (2013BAJ01B05) and the Research Grants Council of the Hong Kong Special Administrative Region, China (Grant No. 9042221, CityU 11300215).

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resisting by melamine and expandable graphite: Industrial approach. J. Cell. Plast. 2011, 47, 549-565. (8) Weil, E. D.; Levchik, S. V. Commercial flame retardancy of polyurethanes. J. Fire Sci. 2004, 22, 183-210. (9) König, A.; Kroke, E. Methyl‐DOPO—a new flame retardant for flexible polyurethane foam. Polym. Advan. Technol. 2011, 22, 5-13. (10) Wang, C. Q.; Ge, F. Y.; Sun, J.; Cai, Z. S. Effects of expandable graphite and dimethyl methylphosphonate on mechanical, thermal, and flame‐retardant properties of flexible polyurethane foams. J. Appl. Polym. Sci. 2013, 130, 916-926. (11) Denecker, C.; Liggat, J.; Snape, C. Relationship between the thermal degradation chemistry and flammability of commercial flexible polyurethane foams. J. Appl. Polym. Sci. 2006, 100, 3024-3033. (12) Wang, W.; Pan, H.; Yu, B.; Pan, Y.; Song, L.; Liew, K. M.; Hu, Y. Fabrication of carbon black coated flexible polyurethane foam for significantly improved fire safety. RSC Adv. 2015, 5, 55870-55878. (13) Modesti, M.; Lorenzetti, A.; Besco, S.; Hrelja, D.; Semenzato, S.; Bertani, R.; Michelin, R. Synergism between flame retardant and modified layered silicate on thermal stability and fire behaviour of polyurethane nanocomposite foams. Polym. Degrad. Stab. 2008, 93, 2166-2171. (14) Andersson, A.; Magnusson, A.; Troedsson, S.; Lundmark, S.; Maurer, F. H. Intumescent foams—A novel flame retardant system for flexible polyurethane foams. J. Appl. Polym. Sci. 2008, 109, 2269-2274.


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(15) Dasari, A.; Yu, Z.-Z.; Mai, Y.-W.; Cai, G.; Song, H. Roles of graphite oxide, clay and POSS during the combustion of polyamide 6. Polymer 2009, 50, 1577-1587. (16) Huang, G.; Wang, S.; Song, P. a.; Wu, C.; Chen, S.; Wang, X. Combination effect of carbon nanotubes with graphene on intumescent flame-retardant polypropylene nanocomposites. Compos. Part A-Appl. S. 2014, 59, 18-25. (17) Wang, X.; Song, L.; Yang, H.; Lu, H.; Hu, Y. Synergistic effect of graphene on antidripping and fire resistance of intumescent flame retardant poly (butylene succinate) composites. Ind. Eng. Chem. Res. 2011, 50, 5376-5383. (18) Matusinovic, Z.; Shukla, R.; Manias, E.; Hogshead, C. G.; Wilkie, C. A. Polystyrene/molybdenum disulfide and poly (methyl methacrylate)/molybdenum disulfide nanocomposites with enhanced thermal stability. Polym. Degrad. Stab. 2012, 97, 2481-2486. (19) Wang, B.; Zhou, K.; Wang, B.; Gui, Z.; Hu, Y. Synthesis and Characterization of CuMoO4/Zn–Al Layered Double Hydroxide Hybrids and Their Application as a Reinforcement in Polypropylene. Ind. Eng. Chem. Res. 2014, 53, 12355-12362. (20) Kim, Y. S.; Davis, R.; Cain, A. A.; Grunlan, J. C. Development of layer-by-layer assembled carbon nanofiber-filled coatings to reduce polyurethane foam flammability. Polymer 2011, 52, 2847-2855. (21) Kim, Y. S.; Harris, R.; Davis, R. Innovative approach to rapid growth of highly clay-filled coatings on porous polyurethane foam. ACS Macro Lett. 2012, 1, 820-824. (22) Kim, Y. S.; Davis, R. Multi-walled carbon nanotube layer-by-layer coatings with a trilayer structure to reduce foam flammability. Thin Solid Films 2014, 550, 184-189.



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(23) Carosio, F.; Di Blasio, A.; Alongi, J.; Malucelli, G. Green DNA-based flame retardant coatings assembled through layer by layer. Polymer 2013, 54, 5148-5153. (24) 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. Polym. Degrad. Stab. 2014, 113, 189-196. (25) Pan, H.; Wang, W.; Pan, Y.; Song, L.; Hu, Y.; Liew, K. M. Formation of Layer-by-Layer assembled titanate nanotubes-filled coating on flexible polyurethane foam with improved Flame retardant and Smoke suppression properties. ACS Appl. Mater. interfaces 2014, 7, 101-111. (26) Pan, H.; Pan, Y.; Wang, W.; Song, L.; Hu, Y.; Liew, K. M. Synergistic Effect of Layer-by-Layer Assembled Thin Films Based on Clay and Carbon Nanotubes To Reduce the Flammability of Flexible Polyurethane Foam. Ind. Eng. Chem. Res. 2014, 53, 14315-14321. (27) Li, Y.-C.; Kim, Y. S.; Shields, J.; Davis, R. Controlling polyurethane foam flammability and mechanical behaviour by tailoring the composition of clay-based multilayer nanocoatings. J. Mater. Chem. A 2013, 1, 12987-12997. (28) Zhu, C.; Chen, Y.; Wang, R.; Wang, L.; Cao, M.; Shi, X. Synthesis and enhanced ethanol sensing properties of α-Fe 2 O 3/ZnO heteronanostructures. Sensor. Actuat. B-Chem. 2009, 140, 185-189. (29) Wang, X.; Hu, Y.; Song, L.; Yang, H.; Xing, W.; Lu, H. In situ polymerization of graphene nanosheets and polyurethane with enhanced mechanical and thermal properties. J. Mater. Chem. 2011, 21, 4222-4227.


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(30) Liu, Y.; Ma, J.; Wu, T.; Wang, X.; Huang, G.; Liu, Y.; Qiu, H.; Li, Y.; Wang, W.; Gao, J. Cost-Effective Reduced Graphene Oxide-Coated Polyurethane Sponge As a Highly Efficient and Reusable Oil-Absorbent. ACS Appl. Mater. Interfaces 2013, 5, 10018-10026. (31) Yang, L.; Hu, C.; Nie, Y.; Qu, J. Surface acidity and reactivity of β-FeOOH/Al2O3 for pharmaceuticals degradation with ozone: In situ ATR-FTIR studies. Appl. Catal. B-Environ. 2010, 97, 340-346. (32) Xiong, Y.; Xie, Y.; Chen, S.; Li, Z. Fabrication of Self‐Supported Patterns of Aligned β ‐ FeOOH Nanowires by a Low ‐ Temperature Solution Reaction. Chem-Eur. J. 2003, 9, 4991-4996. (33) Huang, S.; Cen, X.; Peng, H.; Guo, S.; Wang, W.; Liu, T. Heterogeneous ultrathin films of poly (vinyl alcohol)/layered double hydroxide and montmorillonite nanosheets via layer-by-layer assembly. J. Phys. Chem. B 2009, 113, 15225-15230. (34) Zammarano, M.; Krämer, R. H.; Harris, R.; Ohlemiller, T. J.; Shields, J. R.; Rahatekar, S. S.; Lacerda, S.; Gilman, J. W. Flammability reduction of flexible polyurethane foams via carbon nanofiber network formation. Polym. Advan. Technol. 2008, 19, 588-595. (35) Krämer, R.; Zammarano, M.; Linteris, G.; Gedde, U.; Gilman, J. Heat release and structural collapse of flexible polyurethane foam. Polym. Degrad. Stab. 2010, 95, 1115-1122. (36) Wang, X.; Pan, Y.-T.; Wan, J.-T.; Wang, D.-Y. An eco-friendly way to fire retardant flexible polyurethane foam: layer-by-layer assembly of fully bio-based



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substances. RSC Adv. 2014, 4, 46164-46169.

Figure captions Scheme 1 The schematic of the layer-by-layer assembled process: Scheme 2 Schematic illustration of the possible mechanism of the reduced fire hazards of FPU6. Figure 1 TEM images of β-FeOOH (a) and MMT (b); XRD pattern of β-FeOOH nanorods (c). Figure 2 ATR-FTIR spectra of the control and coated samples. Figure 3 SEM images of FPU0 (a,b), FPU2 (c,d), FPU4 (e,f) and FPU6 (g,h) under low magnification and high magnification. The scale bars are 20 µm and 1 µm, respectively. Figure 4 UV-visible absorption spectra of PEI/SA/β-FeOOH, PEI/MMT-SA and PEI/MMT-SA/β-FeOOH films on quartz slides. Figure 5 TGA curves of control and coated FPU foams under He and Air Figure 6 HRR curves of control and coated FPU foams during cone test. Figure 7 THR curves of control and coated FPU foams during cone test. Figure 8 Digital photos of char residue of FPU1 (a), FPU2 (b), FPU3 (c), FPU4 (d), FPU5 (e) and FPU6 (f) after the cone test. Figure 9 SEM images of char residue of FPU2 (a, b), FPU4 (c, d) and FPU6 (e, f) under low and high magnification. The scale bars are 200µm and 10µm, respectively. Figure 10 FTIR spectra of volatilized pyrolysis products emitted from FPU0, FPU2, FPU4 and FPU6 at maximum evolution rate.


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Figure 11 Absorbance of decomposition products versus time for the control and coated FPU foams Figure 12 Absorbance of CO (a) and CO2 (b) versus time for the control and coated FPU foams Table captions Table 1 The corresponding concentration of the solution, layer numbers and the mass gain of the coated samples. Table 2 Cone data of control and coated FPU Foams.



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Scheme 1


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Scheme 2



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Figure 1


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Figure 2



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Figure 3


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Figure 4



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Figure 5


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Figure 6



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Figure 7


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Figure 8



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Figure 9


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Figure 10



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Figure 11


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Figure 12



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Table 1

Layer

β-FeOOH

MMT-SA

SA

PEI

numbers (n)

(mg·mL-1)

(mg·mL-1)

(mg·mL-1)

(mg·mL-1)

Mass Gain (%wt.)

0 1 3 1 3 1 3

0 5 5 0 0 5 5

0 0 0 5/3 5/3 5/3 5/3

0 3 3 0 0 0 0

0 5 5 5 5 5 5

0 2.8 7.4 3.1 7.5 2.4 6.8

Samples

FPU0 FPU1 FPU2 FPU3 FPU4 FPU5 FPU6


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Table 2

FPU0 FPU1 FPU2 FPU3 FPU4 FPU5 FPU6

PHRR (kW/m2)

THR (MJ/m2)

781±46 748±32 671±41 592±30 433±29 540±33 417±25

21.7±0.9 20.8±0.7 21.5±0.8 20.4±0.6 22.1±0.5 21.4±0.7 19.5±0.4



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For Table of Contents Use Only

Sandwich-like coating consisting of alternating montmorillonite and β-FeOOH for reducing the fire hazard of flexible polyurethane foam Wei Wang, Haifeng Pan, Yongqian Shi, Bin Yu, Ying Pan, Kim Meow Liew, Lei Song *, Yuan Hu *

Sandwich-like nanocoating, composed of montmorillonite and β-FeOOH, was deposited on FPUF through the self-assembly technique to reduce its fire hazard.


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Sandwichlike Coating Consisting of Alternating Montmorillonite and Î²

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