Layered double hydroxide-decorated flexible polyurethane foam: significantly improved toxic effluent elimination

Abstract

A layered double hydroxide-based fire-blocking coating was deposited on the surface of a flexible polyurethane foam using a layer-by-layer method to improve its thermal stability, flame retardancy and smoke suppression properties. The assembly process was carried out by alternately submerging a flexible polyurethane foam into chitosan solution, layered double-hydroxide suspension and alginate solution. Analysis of the cone calorimeter data indicated that all the coated flexible polyurethane foam showed a lower peak heat release rate (pHRR), total heat release (THR), peak smoke production rate (pSPR), total smoke release (TSR), peak carbon monoxide (CO) level and peak carbon dioxide (CO₂) level compared with the control flexible polyurethane foam. Such a significant improvement in flame retardancy and smoke suppression and a decrease in toxic gas release was ascribed to the physical barrier effect of the layer by layer structure. At the same number of layers, a nickel-aluminum layered double-hydroxide-containing coating performs better than a magnesium-aluminum layered double-hydroxide-filled coating with respect to elimination of toxic effluents from FPUFs.

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1. Introduction

Layered double hydroxides (LDHs), a class of anion-exchangeable lamellar compounds, are made up of positively charged brucite-like layers with an interlayer region containing charge-compensating anions and salvation molecules.^1,2 Owing to their layered structure and high anion exchange capacity, LDHs have been applied in numerous fields, such as pharmaceuticals as precursors for preparing CO₂ adsorbents, catalysis, UV absorbents, fire retardant additives, drug delivery hosts and cement additives.^3–5 In recent years, LDHs have attracted increasing attention as novel flame retardant additives.^6–8 Due to their unique chemical composition and layered structure, LDHs can offer outstanding flame retardancy and smoke suppression properties. As is well known, three conditions are required for a fire to occur: a source of heat, oxygen and combustible material. During combustion, LDHs lose their interlayer water, intercalated anions and dehydroxylate to form mixed metal oxides. The process of decomposition can absorb large amounts of heat and dilute the O₂ in the air. Combustion eventually stops when there is not enough fuel to propagate the reaction. Nowadays, the use of halogen-containing flame retardants has been gradually reduced or even banned in some jurisdictions.^9,10 Simultaneously, people have been looking for a novel, environmentally friendly flame retardant as a substitute for halogen-containing flame retardants. LDHs have been regarded as a promising new type of environmentally friendly and highly efficient flame retardant for polymer applications.

In fact, LDHs can be easily positively charged and have strong interlayer electrostatic interactions, which means that they have electrostatic adsorption capacity. As such, multilayer films based on LDHs have been constructed using Langmuir–Blodgett (LB) techniques and Layer-by-Layer (LbL) assembly.^11–13

Layer-by-layer (LbL) assembly is a technique for constructing multilayered films by the alternate deposition of oppositely charged polyelectrolytes or particles on a substrate through electrostatic attraction. Each positive and negative pair deposited is referred to as a bilayer (BL). Through this technique, various properties can be realized, including controlled drug release, chemical sensing, water repellency and oxygen blocking. Recently, the LbL technique has attracted increasing attention with respect to the flame retardant treatment of various polymeric materials such as flexible polyurethane foam (FPUF) and cotton fabrics.^14–16 Owing to excellent cushioning and physical properties, flexible polyurethane foams are broadly applied in various fields, for instance, cushioning materials in furniture, carpet underlays and automobiles.^17,18 Unfortunately, FPUFs, as highly cellular polymers, are easy to ignite and burn rapidly with a high rate of heat release and evolution of smoke without any residual char. Due to stricter standards being developed in traffic safety regulation, more attention has been paid to improving the flame-retardant properties of FPUF. Kim et al. originally improved the flame retardancy of FPUF by using carbon nanofibers (CNFs) as the main component of coatings.¹⁸ Their bilayer coating contains an anionic layer (polyacrylic acid, PAA) and a cationic layer synthesized by a solution blend of CNFs and branched polyethylenimine (BPEI). It should be noted that there was less than 1% mass gain with a 100 nm thick coating consisting of 20 bilayers, thus this coating failed to adhere to FPUF. At the same time, a trilayer (TL) approach, which can result in fast film growth and mass gain, was used to improve flame retardancy, as well as the mechanical and physical properties of FPUF.¹⁶ A trilayer approach was found to be an ideal method to effectively endow FPUF with flame retardancy.

In this study, in order to probe the differences in the performance of different types of layered double hydroxides, we selected NiAl-LDH and MgAl-LDH as flame retardants for comparison. The coatings of layered double hydroxide, chitosan and alginate were alternatively deposited on the surface of FPUF using a trilayer approach. The flame retardancy and smoke suppression properties of coated FPUFs were mainly investigated by cone calorimetry and thermogravimetric infrared analysis (TG-IR). It can be anticipated that a layered double hydroxide-filled coating can reduce the elimination of toxic effluents by FPUFs and reduce their fire hazards.

2. Experimental section

2.1 Raw materials

Flexible polyurethane foam (polyether type, DW30) was obtained from Jiangsu Lvyuan New Material Co., Ltd. Chitosan (viscosity 50–800 mPa s, degree of deacetylation 80–95%) and alginate were purchased from Sinopharm Chemical Reagent Co. Ltd. Magnesium nitrate (Mg (NO₃)₂), aluminium nitrate (Al(NO₃)₃), nickel nitrate (NiNO₃), sodium hydroxide (NaOH) and hydrochloric acid (HCl 36–38%) were purchased from Sinopharm Chemical Reagent Co. Ltd and were used without further purification. Polyacrylic acid (PAA, M_w – 100 000) was purchased from Sigma-Aldrich. Deionized water with a resistance of 18.2 MΩ was used for all experiments.

2.2 Preparation of solution

Layered double hydroxides were synthesized through a hydrothermal method according to previous reports.¹⁹ A layered double-hydroxide suspension was prepared by a similar method according to a previous report.²⁰ Layered double hydroxides (4 mg mL⁻¹) were dispersed in formamide solution and stirred for 24 h at room temperature to yield a translucent solution.²¹ Similarly, chitosan solution and alginate solution were prepared as per our previous study. Chitosan solution (5 mg mL⁻¹) was prepared by adding chitosan to deionized water and stirring constantly for 24 h. The final pH was adjusted to 5 using 1 mol L⁻¹ HCl solution. The preparation process of alginate (3 mg mL⁻¹) solution was the same as for chitosan solution.

2.3 Layer by layer deposition process

The deposition process was similar to that in our previous study.²² Prior to deposition, the pure FPU foam was pre-soaked in a 0.1 mol L⁻¹ HNO₃ solution for 5 min. After squeezing, FPUF was washed with plenty of deionized water. Afterwards, the FPUF was immersed in 0.1% PAA solution for 5 min to form a primer layer to improve its surface adhesion. After pretreatment, these FPUFs were alternately dipped into chitosan solution, alginate solution and layered double-hydroxide suspension. Each process of soaking was sustained for 2 min and followed by rinsing with deionized water for 2 min, then wringing out. After the desired numbers of trilayers were obtained, these coated FPUFs were placed in an oven at 60 °C overnight.

Table 1 shows detailed data on the concentrations of suspensions and solutions and information on the weight gains of the coatings. According to the number of trilayers, the control and coated FPUFs are marked as pure FPU, FPU/NiAl-LDH3, FPU/NiAl-LDH6, FPU/NiAl-LDH12, FPU/MgAl-LDH3, FPU/MgAl-LDH6 and FPU/MgAl-LDH12.

Table 1 The formulae of the coated FPUFs and weight gain information

Sample	Number of trilayers (n)	NiAl-LDH (mg mL^−1)	MgAl-LDH (mg mL^−1)	Chitosan (mg mL^−1)	Alginate (mg mL^−1)	Weight gain (%wt)
Pure FPU	0	0	0	0	0	0
FPU/NiAl-LDH3	3	4	0	5	3	1.7 ± 0.3
FPU/NiAl-LDH6	6	4	0	5	3	2.3 ± 0.1
FPU/NiAl-LDH12	12	4	0	5	3	4.1 ± 0.2
FPU/MgAl-LDH3	3	0	4	5	3	1.8 ± 0.1
FPU/MgAl-LDH6	6	0	4	5	3	2.5 ± 0.1
FPU/MgAl-LDH12	12	0	4	5	3	4.0 ± 0.2

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Prior to deposition, quartz slides (10 mm × 20 mm) were pretreated with boiling piranha solution (H₂O₂–H₂SO₄ 1 : 3 v/v) at 90 °C for 50 min; this was freshly prepared. (Dangerous! The dripping rate of the H₂O₂ solution must be slow.) Furthermore, these quartz slides were washed thoroughly with deionized water for further use. The deposition process on treated quartz slides was the same as that on FPUF.

2.4 Measurements

X-ray diffraction (XRD) measurements were employed to characterize layered double hydroxides with a Japan Rigaku D = Max-Ra rotating anode X-ray diffractometer equipped with a Cu-Kα tube and Ni filter (λ = 0.1542 nm). The scanning rate was 4° min⁻¹ and the range was 5–65°.

UV-vis absorption measurements were taken using a UV-visible spectrophotometer (Cary 100 Bio, Varian, America).

Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra, in the frequency region of 4000–400 cm⁻¹ at 4 cm⁻¹ resolution, were obtained with a Nicolet 6700 spectrometer (Thermo-Nicolet) using 32 scans.

Thermogravimetric analysis (TGA) of samples in air and nitrogen atmospheres were examined on a TGA-Q5000 apparatus (TA Company, 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 alumina pan.

The morphologies of control and FPUFs 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).

A combustion test was performed using a cone calorimeter (Fire Testing Technology, UK) according to ISO 5660 standard procedures, with 100 × 100 × 25 mm³ specimens. Each specimen was exposed horizontally to 35 kW m⁻² external heat flux.

Thermogravimetric analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR) was performed using a TGA Q5000 IR thermogravimetric analyzer, which was coupled with a Nicolet 6700 FT-IR spectrophotometer via a stainless steel transfer pipe.

3. Results and discussion

3.1 Characterization of the control and coated samples

ATR-FTIR can detect changes in an infrared beam with internal reflection as it comes into contact with the surface of samples. Hence, this IR sampling technique can be used to qualitatively investigate the surface chemical structures of the control and coated FPU foams. ATR-FTIR spectra of the control and coated FPU foams are shown in Fig. 1. The absorptions at 1536, 1222 and 1098 cm⁻¹ are ascribed to the stretching vibration and deformation of N–H, the stretching vibration of aromatic C–O and the non-symmetric stretching vibrations of C–O–C in FPUF.²³ It can be seen that the absorbances of all three bands decreased after coating, which can be attributed to the coverage of layered double hydroxide-based coating. For the FPU/NiAl-LDH12 and FPU/MgAl-LDH12, a new weak peak at 1038 cm⁻¹ appears after the coating process, which corresponds to C–O–C stretching for an alginate structural unit.²² Hence, the ATR-FTIR results verify the presence of the coatings on the surface of FPU foam after the deposition process.

Fig. 1 ATR-FTIR spectra of the control and coated samples.

Fig. 2 shows the control and coated samples with 12 trilayers at low and high magnifications. The image of pure FPUF in Fig. 2a reveals the complex, irregular architecture, which has a reported 50–70% open-celled structure and a smooth and clean surface can be seen at high magnification. A rough surface can be observed for the two coated FPUFs owing to the presence of NiAl-LDH and MgAl-LDH-filled coatings, as shown in Fig. 2b and c, respectively. The two coated samples have a uniform nanotexture throughout the entire foam thickness, revealing coverage of every pore wall that does not alter the macroscale porosity. Furthermore, as shown in the magnified SEM images b and c, the outline of layered double hydroxides can be obviously observed in image b but vaguely in image c, suggesting that the positively charged NiAl-LDH can absorb the negative polyelectrolyte more efficiently than MgAl-LDH during the self-assembly deposition process.

Fig. 2 SEM images of the control FPU (a), FPU/NiAl-LDH12 (b) and FPU/MgAl-LDH12 (c).

3.2 Characterization of the multilayer coating

An LDH/alginate/CS nanocoating was synthesized by alternate adsorption of positive polyelectrolyte amino-chitosan, LDH and negatively charged alginate on the surface of FPUF. Film growth was monitored by UV-vis absorption spectrometry to determine if the LbL assembly process was uniform. Fig. 3 shows the UV-vis absorption spectra of NiAl-LDH (a) and MgAl-LDH (b) based multilayers of 3, 6, 12 and 24 bilayers and 5, 10, 15, 20 and 30 bilayers, respectively. As can be observed in Fig. 3a, the absorbance of the coating increases with the number of deposition cycles. Absorbance at 220 nm is plotted against the number of layers (insert plot in Fig. 3a). A linear relationship between the absorbance and the number of layers can be observed, indicating that the fabrication procedure is reproducible from unit to unit in all cases.²⁴ In addition, an almost linear increase is observed for the absorption intensity at 260 nm relative to the number of trilayers, as shown in the insert plot, indicating that almost the same amount of MgAl-LDH is deposited at each assembly cycle. The results obtained above clearly demonstrate that the layer-by-layer assembly process can be carried out successfully.

Fig. 3 UV-visible spectra of NiAl-LDH (a) and MgAl-LDH (b) based coatings on the surface of a silica wafer.

3.3 Thermal stability of the control and coated FPU foams

Thermogravimetric analysis (TGA) was employed to explore the thermal degradation behaviors of uncoated and coated FPUFs. TGA profiles in air and nitrogen for uncoated and coated FPU foams as a function of temperature at a heating rate of 20 °C min⁻¹ are shown in Fig. 4. The whole thermal degradation process for FPU foams consists of two stages in both air and nitrogen. In the first stage, FPU foams decompose at a significant rate over a temperature range of 250–290 °C, which is related to decomposition of the urethane and urea bonds in FPU foams and regeneration of the liquid precursors, such as polyol and isocyanate but only minimal volatile gas production. The second stage is ascribed to pyrolysis of the remaining polyether chain.²³ The inset plots in Fig. 4a and b show that the thermal stability of the coated FPUFs is significantly improved as the number of deposition cycles increases both in air and nitrogen. Moreover, the thermal stability of the NiAl-LDH-based FPUFs is better than that of MgAl-LDH-based FPUFs with the same number of deposition cycles, especially in nitrogen with 12 deposition cycles.

Fig. 4 TGA curves of control and coated FPU foams in air (a) and nitrogen (b).

3.4 Flammability

Cone calorimeter test. In an effort to better understand the effect of NiAl-LDH and MgAl-LDH-filled coatings on the flammability of FPUF, cone calorimetry was performed on these coated samples. Cone calorimetry is a routine bench-scale fire test that simulates a developing fire scenario on a small specimen and quantitatively measures the inherent flammability of polymeric materials. Many important combustion parameters, such as peak heat release rate, total heat release, peak smoke production rate, total smoke release, carbon monoxide production and carbon dioxide production, can be used to directly indicate the potential fire threat of polymeric materials.

Fig. 5 shows heat release rate (HRR) and total heat release (THR) curves for control and coated foam samples. The corresponding data are shown in Table 2. A typical curve with two stages of heat release is observed for the control foam. The first step is associated with the initial decomposition stage of foam combustion, attributed to the pyrolysis of isocyanate.²² The pyrolysis of polyol and subsequent formation of a quickly vaporizing melt pool leads to the second, larger pHRR. All the coated FPUFs show at least 44% reduction in pHRR, contributing to physical blocking effect of the LDH layers. It can be seen that the HRR curves of the coated FPUFs do not show the traditional two significant peaks during the combustion process. Especially for the coated FPUF with an NiAl-LDH filled coating, the FPU/NiAl-LDH12 sample is able to completely eliminate the second peak and effectively extend the time for the whole combustion, which can efficiently reduce the pHRR value and allow more time for rescue. Furthermore, the reduction in pHRR for NiAl-LDH-coated FPUFs is more than that for the MgAl-LDH-coated FPUFs for the same number of deposition cycles. It can be clearly seen that the pHRR value for FPU/NiAl-LDH12 (197 kW m⁻²) is lower than that for FPU/MgAl-LDH12 (276 kW m⁻²). In addition, the THR values for the coated samples are also remarkably reduced compared to the uncoated sample. For the same number of deposition cycles, NiAl-LDH-coated FPUFs show a lower THR value than MgAl-LDH-coated PFUFs.

Fig. 5 HRR and THR curves of the control and coated samples during cone test.

Table 2 Cone calorimetry data of the pure and coated samples

Sample	Time to ignition (s)	Peak HRR (kW m^−2)	THR (MJ m^−2)	Peak SPR (m^2 s^−1)	TSP (m^2 m^−2)	Peak CO production (g s^−1)	FPI (m^2s kW^−1)
Pure FPU	2	801 ± 55	22.1 ± 1.5	0.0837	6.241	0.0215	0.00250
FPU/NiAl-LDH3	2	333 ± 36	16.7 ± 1.1	0.0677	1.947	0.0118	0.00600
FPU/NiAl-LDH6	3	274 ± 37	15.8 ± 1.1	0.0472	1.803	0.0104	0.01095
FPU/NiAl-LDH12	4	197 ± 35	16.2 ± 1.2	0.0329	1.024	0.0072	0.02030
FPU/MgAl-LDH3	3	451 ± 48	17.3 ± 1.3	0.0722	4.122	0.0167	0.00665
FPU/MgAl-LDH6	3	381 ± 26	17.3 ± 0.9	0.0308	2.831	0.0142	0.00787
FPU/MgAl-LDH12	3	276 ± 31	16.7 ± 1.0	0.0454	1.625	0.0106	0.01087

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Fig. 6 shows the CO and CO₂ production curves of the samples in order to investigate the toxicity of the generated gases. The related parameters are listed in Table 2. After the fabrication process, the coated FPUFs show a significant reduction in CO (Fig. 6a) and CO₂ (Fig. 6b) production compared with the uncoated sample. As the number of deposition cycles increases, the reductions in CO and CO₂ production are more obvious. Moreover, it can be clearly seen that the samples coated by NiAl-LDH obtain a lower CO and CO₂ production rate compared with MgAl-LDH-based samples.

Fig. 6 CO and CO₂ production curves of the pure and coated samples during cone test.

In a fire, the emission of smoke is extremely harmful for humans; the smoke particles can make the person suffocate and hamper the rescue. Therefore, it is necessary to reduce smoke production. Smoke production rate (SPR) and total smoke production (TSP) curves are shown in Fig. 7. The related data are listed in Table 2. The coated FPUFs have lower SPR and TSP values compared with pure FPUF. It can be seen that FPU/NiAl-LDH12 and FPU/MgAl-LDH12 show a remarkable reduction in SPR and TSP. In particular, the sample of FPU/NiAl-LDH12 has the lowest SPR value (0.0329 m² s⁻¹) compared to other samples, showing a 61% reduction compared with uncoated samples (0.0837 m² s⁻¹). In addition, the TSP values of FPU/NiAl-LDH12 and FPU/MgAl-LDH12 are 1.024 and 1.625 m² m⁻², which correspond to 84% and 74% reduction compared with the control FPUF (6.241 m² m⁻²). These results indicate that an LDH-based coating on the surface of FPUFs can efficiently reduce smoke release during the combustion process.

Fig. 7 SPR and TSP curves of the control and coated samples during cone test.

Fig. 8 shows the digital images of the char residues of the coated samples after a cone calorimeter test. It can be clearly observed that the char residue becomes thicker and denser as the number of deposition cycles increases. Moreover, the char residues of FPU/NiAl-LDH perform better than those of FPU/MgAl-LDH and have no shrinkage. These images demonstrate that NiAl-LDH can be a more efficient flame retardant than MgAl-LDH to endow FPUF with outstanding fire safety. Excellent char formation means less “fuel” for the combustion and less heat and smoke particles emitted into the environment, which is indicated by the HRR, THR, SPR and TSP values.

Fig. 8 Digital images of char residues from FPUF/NiAl-LDH3 (a), FPUF/NiAl-LDH6 (b), FPUF/NiAl-LDH12 (c), FPUF/MgAl-LDH3 (d), FPUF/MgAl-LDH6 (e) and FPUF/MgAl-LDH12 (f) after cone test.

3.5 Smoke hazards of control and coated FPUFs estimated by TG-IR

TG-IR measurement is usually employed to investigate the pyrolysis products after thermal decomposition of the samples. Fig. 9 shows the FTIR spectra of the volatilized products released from the control and coated samples. The characteristic absorption peaks of pyrolysis gases emitted from the coated samples are the same as for those emitted from uncoated FPUF, which indicates that the fabrication of LDH on the surface of FPUFs cannot alter the thermal decomposition process. The bands at 3450–3600 cm⁻¹ are ascribed to the vibration absorption of hydroxide groups, indicative of water vapor; the characteristic bands at 2880–2980 cm⁻¹ are assigned to aliphatic C–H bonding arising from various alkanes; the sharp peak at 1750 cm⁻¹ is attributed to the stretching vibration of the C=O group and the peak at 1112 cm⁻¹ is due to the stretching vibration of the C–O–C bond from ethers.²⁵

Fig. 9 FTIR spectra of volatilized pyrolysis products emitted from pure FPU, FPU/NiAl-LDH12 and FPU/MgAl-LDH12 at maximum evolution rate.

To further investigate the influence of NiAl-LDH and MgAl-LDH on the released gases, the absorbance of pyrolysis gases for the control and coated FPUF versus time is shown in Fig. 10. The maximum absorbance of the volatilized gases for the coated samples is lower than that for the pure FPUF. Furthermore, the absorbance of FPU/NiAl-LDH12 is lower than that of FPU/MgAl-LDH12. Some typical compounds are selected to study the influence of LBL coating on volatilized pyrolysis products. As can be observed in Fig. 10, the volatiles emitted from the coated FPUFs are reduced remarkably. The reduction of organic volatiles, such as aromatic and carbonyl compounds, leads to the inhibition of smoke particles, which corresponds to the cone calorimeter data. In addition, only a small quantity of –NCO containing compounds released from FPU can be very harmful to human health. The absorbance of –NCO (Fig. 10e) for FPU/NiAl-LDH12 is much lower than for uncoated FPUF and FPU/MgAl-LDH; toxic effluents are thus effectively eliminated in FPU/NiAl-LDH12.

Fig. 10 Absorbance of decomposition products versus time for the control and coated FPU foams.

3.6 Probable flame retardancy mechanism

A plausible mechanism was proposed for the reduced toxicity of the coated FPUFs. The LDH-filled coating on the surface of FPUFs can act as a physical barrier to delay the permeation of heat, oxygen and volatiles, which can effectively reduce the fire hazards of the FPUFs. As a significant functional metal element, Ni species have outstanding catalytic effects, because of their distinctive physical and chemical properties. Research conducted by Tang et al. demonstrated that a nickel catalyst can promote the catalyzed carbonization of polypropylene, improving its fire retardancy. Hu et al. found that the incorporation of NiAl-LDH can effectively enhance the flame retardancy of poly(methyl methacrylate), which is attributed to the catalytic carbonization of NiAl-LDH. Therefore, from the combination of the digital graphs provided in Fig. 10, it can be reasonably believed that the catalytic effect of NiAl-LDH plays a role in the elimination of toxic gases, since this is more effective than MgAl-LDH in enhancing flame retardancy.

4. Conclusion

NiAl-LDH and MgAl-LDH were successfully fabricated on the surface of FPUFs using a layer-by-layer method for the purpose eliminating toxic effluents. Cone calorimetry and TG-IR results indicated that the deposition of LDH can significantly improve the thermal stability, flame retardancy and smoke suppression properties of FPUFs. Moreover, NiAl-LDH-containing coatings perform better than MgAl-LDH-filled coatings on the elimination of toxic effluents by FPUFs.

Acknowledgements

The study was financially supported by the Natural Science Foundation of China (U1332134), the National Basic Research Program of China (973 Program) (2012CB719701) and the National Natural Science Foundation of China (21374111).

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