Fabrication of binary hybrid-filled layer-by-layer coatings on flexible polyurethane foams and studies on their flame-retardant and thermal properties

Abstract

A binary hybrid-filled flame-retardant coating, consisting of graphene oxide (GO) and amino-terminated silica nanospheres (KH-550-SiO₂), was fabricated onto a flexible polyurethane (FPU) foam using the layer-by-layer assembly method. The coexistence morphology between GO nanosheets with KH-550-SiO₂, as a percolated network structure, can be observed on the FPU foam surface as shown by the scanning electron microscopy images. The comparative cone test study revealed that the binary hybrid-filled coating had a larger reduction in peak heat release rate (PHRR) (50.9% reduction) and could eliminate the second PHRR for FPU foams compared with the single (GO or KH-550-SiO₂)-filled coating. Such an obvious improvement in flame-retardancy for FPU foam could be ascribed to the barrier effect of the binary hybrid network structure formed, which can reduce the amounts of organic volatiles available for burning, and the heat and oxygen transfers between the flame and underlying foam.

---

1. Introduction

Owing to the excellent cushioning and physical properties, flexible polyurethane (FPU) foam has extensive applications in modern homes decorated with soft furniture (mattresses, and cushions). However, this material usually has high flammability and often burns rapidly with a high heat release rate and large amounts of dense smoke and toxic gaseous products, easily leading to many fire fatalities. Therefore, FPU foam must be filled with flame retardants or protected with a fire resistant barrier to reduce its flammability. Halogen-containing compounds have proven to be effective flame retardants for FPU foam,^1,2 but they usually produce black smoke and corrosive gases during burning, restricting their further application by legislation.^3–5 With increasing concern for the environment, health and safety, a trend for environmentally friendly flame retardant technology is developing.

The Layer-by-Layer (LbL) assembly technique is usually a water-based technique and it can provide the possibility of using a large range of materials in coatings construction, including polyelectrolytes, dendrimers, carbon materials and layered inorganic nanoparticles.^6–10 These LbL coatings have attracted much attention owing to their water-based chemistry (green solution) and ease of fabrication. Recently, this technique was used to develop as a novel flame retardant technology for polymeric materials.^11–14 Inorganic nanoparticles were widely studied to fabricate LbL coatings onto FPU foams, including 2-dimensional layered nanoparticles (montmorillonite and layered double hydroxide), and 1-dimensional nanoparticles (carbon nanotubes).^13,15–18 In general, these LbL coatings can form a continuous network-like structure to protect underlying polymeric materials, resulting in improved flame-retardancy for polymeric materials.

Graphene oxide (GO), which generally comes from chemical exfoliation, is a 2-dimensional graphene-like carbon nano-material with graphene domains, defects and residual oxygen-containing groups on its surface. Recently, as a flame retardant additive, GO has been incorporated into polymeric materials to improve their fire safety.^19–21 Similar to traditional layered nanoparticles, GO can act as a physical barrier to delay the diffusion of thermal decomposition products, gases and heat. However, owing to the high thermal conductivity, GO shows worse flame retardation efficiency with earlier ignition times and higher heat release rates in specific polymers, and a high loading level is needed to achieve a good flame retardation effect.^22,23 As is well known, silica particles are also common flame-retardant fillers in polymer composites. With the incorporation of silica particles into polymers, silica particles can accumulate near the surface to act as the ‘‘insulating blanket” to reduce the amount of volatiles available for burning in the gas phase and the amount of heat that feeds back to the polymer surface.^24,25

Recently, with increasing research in flame retardant applications of different nano-fillers, the combination of different nano-fillers with varied geometries (spherical, rod, and layered) have been widely studied to prepare multicomponent polymer composites. Among these combinations, binary hybrids based on 2-dimensional layered nanoparticles (clay or GO) with other nanoparticles is one of the common types. Fang's group has revealed that the incorporation of clay or graphene with carbon nanotubes (CNT) can form a more effective confined space and enhance the network structure, which obviously slowed down the whole combustion process.^26,27 In addition, some graphene-based hybrids, such as graphene/Ni–Fe layered double hydroxide hybrids, graphene/POSS, grapheme/Co₃O₄ hybrids, were prepared and then incorporated into polymeric materials to enhance fire safety. This fire safety effect is usually caused by the barrier and the adsorption effects or catalytic effects of graphene-based hybrids.^28–30 Accordingly, the binary combination of inorganic nano-fillers usually exhibits extraordinary synergies to enhance the flame retardation property of polymers.

Suggested by the above reports, LbL coating with binary-hybrids may be a good choice to improve flame retardation efficiency. In our present work, the binary hybrid-filled coating made from GO and amino-terminated silica nanospheres was fabricated onto the FPU foam by the LbL assembly method. Here, GO could be considered as a negatively charged nanosheet due to the presence of oxygen-groups, and silica nanospheres with amino groups show a positively charged surface. The coating morphologies on the FPU foam surface were characterized by scanning electron microscopy (SEM) and the thermal degradation behaviors of the foams were evaluated using thermogravimetric analysis (TGA). The flame retardation property of FPU foams was mainly investigated by cone calorimetry. This comparative study was carried out to probe the differences in the performance of the coatings with different structures (hybrids or single component). It can be expected that the binary hybrid-filled LbL coating can improve the flame retardancy of FPU foams.

2. Experiment

2.1 Materials

Flexible polyurethane foam (polyether type, DW30) was obtained from Jiangsu Lvyuan New Material Co., Ltd. Sodium alginate (SA) and branched polyethylenimine (PEI, branched, M_w = 10 000 g mol⁻¹) were purchased from Sinopharm Chemical Reagent Co. Ltd. Polyacrylic acid (PAA, M_w ∼ 100 000) was purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 36–38%) was received from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water with a resistance of 18.2 MΩ was used for all experiments.

2.2 Preparation of graphene oxide (GO), SiO₂ nanospheres and amino-terminated silica nanospheres (KH-550-SiO₂)

GO was synthesized from expandable graphite by a modified Hummers' method.³¹ The detailed description can be seen in the ESI.† The SiO₂ nanospheres were prepared according to the Stöber method,³² which is described in the ESI.† The preparation of KH-550-SiO₂ can also be seen in the ESI.† Furthermore, the characterization of GO, SiO₂ nanospheres and KH-550-SiO₂ are also provided and discussed in the ESI.†

2.3 Preparation of solutions

All solutions or suspensions were prepared with 18.2 MΩ deionized water. PEI solution (5 mg mL⁻¹) was prepared by adding PEI into deionized water, and the pH was adjusted to 9.0 with 1 M hydrochloric acid. SA solution (3 mg mL⁻¹, pH = 7) was prepared by introducing alginate into deionized water and overnight stirring. 1 mg mL⁻¹ GO with 3 mg mL⁻¹ SA solution (pH = 7.0) can be obtained by mixing SA powders with GO into a common solution, and the suspension ultrasonicated for 30 min, then stirred for 24 h. For KH-550-SiO₂ suspension (3 mg mL⁻¹), KH-550-SiO₂ powders were dispersed in aqueous solution, and the pH was adjusted to 3 with 1 M HCl solution, and then stirred for 24 h.

2.4 LbL deposition process

(GO/KH-550-SiO₂)-coated FPU foams could be obtained according to the description as follows. Firstly, FPU foams were soaked in 0.1 M HNO₃ solution for 5 min to create a positively charged surface.³³ After that, the FPU foam was immersed into 1.0% PAA solution (pH = 4) for 5 min as a primer layer to improve the adhesion. FPU foam was soaked by the dipping sequence of PEI solution (cationic), then GO/SA solution (anionic) and then KH-550-SiO₂ suspension (cationic). Each dip was followed by rinsing with water for 2 min and was wrung out to expel liquid from within the FPU foam. This method completes the fabrication of the first trilayer. After the desired number was deposited, FPU foam was dried at 35 °C overnight before testing. The preparation process is shown in Scheme 1(a).

Scheme 1 Preparation of coated FPU foams by the LbL assembly method: (GO/KH-550-SiO₂)-coated FPU foams (a), (KH-550-SiO₂)-coated FPU foams (b) and (GO)-coated FPU foams (c).

(KH-550-SiO₂)-coated FPU foams were prepared by alternately dipping the FPU foams into PEI solution (cationic), SA solution (anionic) and KH-550-SiO₂ suspension (cationic). The remaining operational process was the same to that of (GO/KH-550-SiO₂)-coated FPU foams. The preparation process is shown in Scheme 1(b).

(GO)-coated FPU foams were prepared as follows. The FPU foam was alternately immersed into PEI solution (cationic) and SA/GO suspension (anionic). The remaining operational process was the same to that of (GO/KH-550-SiO₂)-coated FPU foams. The preparation process is shown in Scheme 1(c).

All samples were marked as shown in Table 1.

Table 1 Coating mass gains of coated FPU foams

Sample	Number (n)	PEI (mg mL^−1)	GO/SA (mg mL^−1)	KH-550-SiO2 (mg mL^−1)	SA (mg mL^−1)	Coating mass gains (wt%)	The density (g cm^−3)
FPU0	—	—	—	—	—	—	0.0280
(KH-550-SiO2)-2	2	5	—	3	3	1.0	0.0282
(KH-550-SiO2)-5	5	5	—	3	3	2.6	0.0287
(GO)-2	2	5	3/1	—	—	4.1	0.0291
(GO)-5	5	5	3/1	—	—	6.6	0.0299
(GO/KH-550-SiO2)-2	2	5	3/1	3	—	3.05	0.0288
(GO/KH-550-SiO2)-5	5	5	3/1	3	—	13.0	0.0321

---

2.5 Characterization

The density (ρ) of the foam was calculated using the equation: ρ = m/V, where m is the weight of the foam and V is the volume of the foam.

Morphologies of FPU foams and residual char were coated with a gold layer and were observed using scanning electron microscopy (SEM, AMRAY1000B, Beijing R&D Center of the Chinese Academy of sciences, China).

Combustion tests were performed on the cone calorimeter (Fire Testing Technology, Stanton Redcroft) 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/infrared spectrometry (TGA-IR) of the samples was performed using a TGA Q5000 IR thermogravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer through a Thermo-Nicolet TGA special connector.

Tensile measurements were performed with a CMT6104 universal testing machine (Shenzhen SANS Material Detection Co., Ltd. China) according to ISO 1798: 2008 at a crosshead speed at 500 mm min⁻¹. Sample sheets were cut to a dumbbell shape with the size of 40 × 10 × 10 mm³.

Compression set was determined according to the ISO1856-2000 standard. The dimensions of the samples were 50 × 50 × 20 mm³. The thickness of the foams was measured, and the foams were then compressed between two metallic plates to 50% of their original thickness and left at 70 ± 1 °C for 22 h. The residual thickness was measured after 30 min when the pressure was released. Compression set is defined as the percentage of thickness loss.

3. Results and discussion

3.1 Characterization of coated FPU foams

Table 1 shows the coating mass gains and density of coated FPU foams as a function of number of deposition cycles and the assembled component concentration. When the number of deposition cycles is the same, (GO/KH-550-SiO₂)-coated FPU foams show a higher coating mass gain than that of single GO or KH-550-SiO₂-coated FPU foams. A possible reason is that the binary-hybrid structure has a greater number of absorption sites between the (GO/SA) layer and (KH-550-SiO₂) layer during the deposition process. The density of the FPU0 is 0.028 g cm⁻³, and the density of coated FPU foams increased with increases in coating mass gain. The (GO/KH-550-SiO₂)-5 has a maximum ρ value of 0.0321 g cm⁻³, which has a 12.7% increase compared with FPU0. As a result, the LbL coating does not significantly change the density of the FPU foam.

Surface morphologies of coated FPU foams were imaged using SEM for direct comparison. Fig. 1 shows the typical top views of the prepared samples at low and high magnifications. First, the images at low magnification (Fig. 1a–h) are discussed. The SEM image of FPU0 reveals a complex and irregular architecture with open-celled structures, and shows a smooth and clean surface. For the three coated FPU foams, the macroscale porosity and nano-texture structure of the foams were not altered, indicating the conformal nature of LbL deposition. However, their surfaces become rough because of the deposition of coatings on the wall. The same case also appeared in the center of coated FPU foams (Fig. S5†). Second, the images at high magnification (Fig. 1i–k) are discussed. The images of the three coated FPU foams have some differences. For (KH-550-SiO₂)-5, the KH-550-SiO₂ nanospheres can be clearly seen, but are thinly distributed on the FPU foam surface and densely arranged morphology cannot be observed. This case is ascribed to the low loading level of KH-550-SiO₂-filled LbL coatings. For (GO)-5, its surface has some wrinkles, which is believed to be caused by the wrinkling effect of GO nanosheets.³⁴ For (GO/KH-550-SiO₂)-5, the coexisting phenomenon between GO nanosheets with KH-550-SiO₂ was found to cover the FPU foam surface and the wrinkled GO sheets were still observed. In addition, some KH-550-SiO₂ nanospheres were found to disperse on the surface, and the others were found to inlay the LbL coating. Here, KH-550-SiO₂ nanospheres can overlap the crack between the wrinkled GO sheets and were even deposited onto the surface of GO nanosheets. By contrast, GO nanosheets filled the bore and gaps among KH-550-SiO₂ nanospheres. Accordingly, the binary GO/KH-550-SiO₂ hybrids implemented a complementary effect between GO sheets and KH-550-SiO₂ nanospheres.

Fig. 1 SEM images of FPU0 (a and e), (GO)-5 (b, f and i), (KH-550-SiO₂)-5 (c, g and j) and (GO/KH-550-SiO₂)-5 (d, h and k).

3.2 Thermal stability

Fig. 2 shows the TGA curves of FPU0 and coated FPU foams under nitrogen atmosphere, and the representative parameters are summarized in Table 2. The decomposition of FPU0 was carried out almost without any char residue. All coated FPU foams showed lower initial thermal decomposition temperatures (T_−5%) than that of FPU0, which was mostly caused by desorption of the physically adsorbed water (LbL coating is usually hydrophilic). However, the solid residues of the regions from 400 °C to 700 °C in the coated FPU foams were obviously higher than that of FPU0, indicative of the enhancement of thermal stability in higher temperature ranges. The (GO/KH-550-SiO₂)-5 has 6.9% char residue at 700 °C, which is the highest value among all samples. The GO/KH-550-SiO₂ network structure formed could improve the thermal stability of FPU foam owing to its barrier effect, which could delay the evolution of organic volatiles from the pyrolysis of underlying materials.

Fig. 2 TGA curves of FPU0 and coated FPU foams under nitrogen atmosphere.

Table 2 TGA data of control and coated FPU foams

Sample	T−5% (°C)	T−50% (°C)	Char residue at 700 °C (%)
FPU0	256	373	0.63
(KH-550-SiO2)-2	254	370	2.3
(KH-550-SiO2)-5	255	376	2.5
(GO)-2	245	377	3.4
(GO)-5	253	374	4.6
(GO/KH-550-SiO2)-2	251	370	3.5
(GO/KH-550-SiO2)-2	244	378	6.9

---

3.3 Cone calorimetry of coated FPU foams

Coating effects on the FPU flammability was assessed by cone calorimetry. The cone calorimeter is a standard bench scale tool for measuring material flammability. Cone tests were conducted in the presence of an external heat flux, the results are often used to predict full-scale flammability characteristics. The most common parameters of interest are time to ignition (TTI) and heat release rate (HRR). In particular for the peak heat release rate (PHRR), it represents the point in a fire where heat is likely to propagate further or ignite adjacent objects, so the reduction of this parameter is important for fire safety. Fig. 3 shows the HRR curves of all samples during cone tests, and the corresponding data are shown in Table 3. The combustion of FPU0 was accompanied by a significant change in volume due to the collapse of the foam to a low viscosity liquid.³⁵ Its HRR curve has two PHRR, which represent the pyrolysis of isocyanate (first peak) and polyol (second peak), respectively.³⁵ All coated FPU foams had a higher first PHRR than that of FPU0, which was caused by the larger exposed area of coated FPU foams and lower heat flux for FPU0 during cone testing. During the first peak, FPU foam completely collapses and forms a pool of combustible compounds.³⁵ The distance between the top FPU foam surface and electric heat increases as the FPU0 collapses, resulting in the lower heat flux. However, coated FPU foams can maintain its shape, owing to the protective effect of the LbL coating, resulting in a larger exposed area than that of FPU0. FPU foams covered with one single KH-550-SiO₂ have higher PHRR than that of FPU0, indicating that this coating has a negative effect on the enhancement in flame retardancy. For GO-coated FPU foams, they have a notable reduction in PHRR, but the two PHRR can still be clearly observed. (GO/KH-550-SiO₂)-coated FPU foams obtained the highest flame retardation efficiency. (GO/KH-550-SiO₂)-5 had a 50.9% reduction in PHRR, and the elimination of the second PHRR can be observed. In general, the accumulation of SiO₂ nanospheres was not completely dense; bores and gaps can be found in the LbL coating, and the similar case also appeared for GO-filled coatings. More specifically, GO nanosheets still had high thermal conductivity, which would deteriorate the barrier effect during combustion tests.¹⁹ The combining of GO nanosheets with KH-550-SiO₂ nanospheres by the self-assembled style could improve the barrier effect of GO nanosheets, owing to the protective effect of silica nanospheres, similar with Bao's work.¹⁹ Moreover, the combination of the binary hybrids could implement a complementary effect and enhance the barrier effect of their network structure to show a synergistic effect that improves the flame retardancy of FPU foams. Nevertheless, (GO/KH-550-SiO₂)-5 only had a 50.9% reduction in PHRR, and it seemingly did not obtain a large improvement in flame retardancy. In fact, for the flame retardant enhancement of FPU foams, the elimination of the second PHRR has more significance. The second PHRR is associated with the pyrolysis of polyol, which is the main mass fraction in the foam component. Therefore, the elimination of the second PHRR means that the binary hybrids can effectively delay the release of pyrolysis gaseous products from the polyol, which fuels the combustion process. In addition, this binary hybrid-filled coating realizes the target that a moderate loading level of the coating (approximately 10%) along with a low number of deposition cycles (∼5) can effectively improve the flame retardancy of FPU foams.

Fig. 3 HRR curves of control and coated FPU foams during a cone test.

Table 3 Cone data of control and coated FPU foams

Sample	TTI (s)	Peak HRR (kW m^−2)	FPI (s m^2 kW^−1)
FPU0	2	738	0.00271
(KH-550-SiO2)-2	2	748	0.00267
(KH-550-SiO2)-5	2	790	0.00253
GO-2	2	425	0.00470
GO-5	2	392	0.00510
(GO/KH-550-SiO2)-2	2	395	0.00506
(GO/KH-550-SiO2)-5	2	362	0.00552

---

3.4 Pyrolysis products analyzed by TGA-IR

TGA-IR was used to analyze the gaseous products during thermal degradation. 3D TGA-IR spectra of the gas phase in the thermal degradation of FPU0 and (GO/KH-550-SiO₂)-5 are shown in Fig. 4. From these spectra, the gaseous products of (GO/KH-550-SiO₂)-5 were the same to those of FPU0. This similarity implies that the LbL coating cannot significantly change the thermal decomposition process of pristine FPU foam. The absorption characteristic peaks mainly appear in the regions of 3400–3700, 2800–3100, 2250–2400, 1650–1800 and 1000–1250 cm⁻¹. The bands at 3450–3600 cm⁻¹ are ascribed to the vibrations of hydroxide groups, indicating the release of water vapor;³⁶ the bands at 2876–2982 cm⁻¹ are assigned to aliphatic C–H bonds arising from various alkanes. The sharp band at 1740 cm⁻¹ is caused by the stretching vibration of the C=O group; the strong absorption band at 1108 cm⁻¹ is caused by the stretching vibration of the C–O–C bond from ethers.

Fig. 4 3D TGA-IR spectra of the gas phase in the thermal degradation of FPU0 and (GO/KH-550-SiO₂)-5.

To further understand the mechanism of the improved flame retardancy for coated foam by the LbL coatings, and the absorbance of total and selected gaseous products of FPU0 and (GO/KH-550-SiO₂)-5 versus time is revealed in Fig. 5. The absorption intensity of some gaseous products for (GO/KH-550-SiO₂)-5 was lower than that of FPU0, especially for hydrocarbons and CH₃OH. The reduced amount of gaseous products means less “fuel” to be fed back to the flame, and thereby reduces the heat release rate observed in cone calorimeter test. The barrier effect of the (GO/KH-550-SiO₂)-hybrid network formed was considered to be the reason. In addition, the absorption intensity of H₂O for (GO/KH-550-SiO₂)-5 was higher than that of FPU0. Generally, GO, KH-550-SiO₂ and SA in LbL coatings are all hydroxyl-containing materials. Therefore, the extra water vapor should come from the dehydration reaction of GO, KH-550-SiO₂ and SA during thermal decomposition. During combustion, water vapor can reduce the heat of its surroundings and result in an inert gas to dilute the flammable volatiles, so it can help to improve the flame retardancy of FPU foams.³⁷

Fig. 5 Absorption intensity of some gaseous products of FPU0 and (GO/KH-550-SiO₂)-5.

3.5 Char residue analysis

Fig. 6 shows the residual char photographs of FPU0, (KH-550-SiO₂)-5, (GO)-5 and (GO/KH-550-SiO₂)-5 after the cone test. FPU0 has no char residue. Few char is left for (KH-550-SiO₂)-5, and the char residue shows a crack morphology. However, the other two samples left high amounts of char residue, but show some differences. Apparently, the thickness of the char residue for (GO/KH-550-SiO₂)-5 is higher than that of (GO)-5. Therefore, (GO/KH-550-SiO₂)-5 showed the best flame retardancy among all samples.

Fig. 6 Residual char photographs of FPU0, (KH-550-SiO₂)-5, (GO)-5 and (GO/KH-550-SiO₂)-5 after the cone test.

To further testify the effect of the char layer during combustion, the morphologies of the chars of all samples after the cone test were measured by SEM. From Fig. 7, the cell structure of FPU foams have been broken for the char of (KH-550-SiO₂)-5 and (GO)-5, but the char residue of (GO/KH-550-SiO₂)-5 still retains the FPU cell structure. Thus, the (GO/KH-550-SiO₂) hybrid network structure formed can act as an excellent barrier to protect the underlying materials during combustion.

Fig. 7 The SEM images of char residues of (KH-550-SiO₂)-5, (GO)-5 and (GO/KH-550-SiO₂)-5.

3.6 Mechanical properties of the control and coated FPU foams

It is interesting to note that the flame retardation property of coated FPU foams is not the only aspect of the foams influenced. The mechanical properties are also of concern in the application to various industries. The tensile strength, elongation at break and compression set of the control and coated FPU foams are shown in Fig. 8a–c. The data is shown in Table 4. The tensile strength, elongation at break and compression set of FPU0 were measured to be 0.122 MPa, 81% and 5% respectively. The increased loading of the LbL coatings enhanced the tensile strength, but decrease the elongation at break and increased the compression set. The maximum tensile strength of 0.151 MPa was achieved for (GO/KH-550-SiO₂)-5, which had the coating mass gain of 13.0%. Compared with the FPU0, the tensile strength of (GO/KH-550-SiO₂)-5 increased by 19%. This can be ascribed to the interfacial interactions from between the individual layers of the rigid LbL coatings that cover the substrate.³⁸ Furthermore, the elongation at break (54%) of (GO/KH-550-SiO₂)-5 decreased by 33% and the compression set increased from 5% to 22% compared with FPU0. These changes can be ascribed to the chain rigidity of polyelectrolyte and the addition of the inorganic portions.

Fig. 8 Photo of tensile strength (a), elongation at break (b) and compression set (c) of control and coated FPU foams.

Table 4 The data of tensile strength, elongation at break and compression set of the control and coated FPU foams

The number	Sample	Tensile strength (MPa)	Elongation at break (%)	Compression set (%)
1	FPU0	0.122 ± 0.002	81 ± 2	5 ± 1
2	(KH-550-SiO2)-2	0.125 ± 0.005	78 ± 3	7 ± 2
3	(KH-550-SiO2)-5	0.130 ± 0.003	75 ± 2	10 ± 1
4	(GO)-2	0.138 ± 0.004	68 ± 3	12 ± 2
5	(GO)-5	0.143 ± 0.006	62 ± 4	15 ± 1
6	(GO/KH-550-SiO2)-2	0.130 ± 0.005	65 ± 3	10 ± 2
7	(GO/KH-550-SiO2)-5	0.150 ± 0.005	54 ± 3	22 ± 3

---

4. Conclusion

Binary-(GO/KH-550-SiO₂)-hybrid filled coatings were successfully constructed by the LbL assembly method onto FPU foam to improve the flame-retardation efficiency. The comparative study by coating mass gains showed that the binary-(GO/KH-550-SiO₂)-hybrid filled coating obtained higher mass gains compared with single-coated samples. The characterization by SEM images presented that the combination of GO nanosheets with KH-550-SiO₂ nanospheres successfully coexisted in the LbL coating. The comparative study in cone testing has confirmed that the binary-(GO/KH-550-SiO₂)-hybrid filled coating has a larger enhancement in flame retardation properties for FPU foams compared with the single-coated (GO or KH-550-SiO₂) samples. The performance was mainly shown by the larger reduction in PHRR and elimination of the second PHRR. The combination of the binary hybrids can implement complementary effects, enhance the barrier effect of their network structure and show a synergistic effect on improving the flame retardation properties of FPU foams.

Acknowledgements

The work was financially supported by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG160608), National Basic Research Program of China (973 Program) (2012CB719701), National Natural Science Foundation of China (51276054) and National Natural Science Foundation of China (51303165).

References

1. H. M. Stapleton, S. Klosterhaus, S. Eagle, J. Fuh, J. D. Meeker, A. Blum and T. F. Webster, Environ. Sci. Technol., 2009, 43, 7490–7495.
2. V. Babrauskas, A. Blum, R. Daley and L. Birnbaum, Fire Safety Science, 2011, 10, 265–278.
3. R. Renner, Environ. Sci. Technol., 2004, 38, 14–15.
4. M. A. Uddin, T. Bhaskar, J. Kaneko, A. Muto, Y. Sakata and T. Matsui, Fuel, 2002, 81, 1819–1825.
5. H. M. Stapleton, S. Klosterhaus, A. Keller, P. L. Ferguson, S. van Bergen, E. Cooper and A. Blum, Environ. Sci. Technol., 2011, 45, 5323–5331.
6. S. L. Clark, M. F. Montague and P. T. Hammond, Macromolecules, 1997, 30, 7237–7244.
7. H. Y. Zhang, Y. Fu, D. Wang, L. Y. Wang, Z. Q. Wang and X. Zhang, Langmuir, 2003, 19, 8497–8502.
8. D. Yan, J. Lu, M. Wei, J. Han, J. Ma, F. Li and X. Duan, Angew. Chem., 2009, 121, 3119–3122.
9. M. Zhang, Y. Yan, K. Gong, L. Mao, Z. Guo and Y. Chen, Langmuir, 2004, 20, 8781–8785.
10. P. Podsiadlo, Z. Liu, D. Paterson, P. B. Messersmith and N. A. Kotov, Adv. Mater., 2007, 19, 949–955.
11. Y. C. Li, S. Mannen, A. B. Morgan, S. Chang, Y. H. Yang, B. Condon and J. C. Grunlan, Adv. Mater., 2011, 23, 3926–3931.
12. T. Zhang, H. Yan, M. Peng, L. Wang, H. Ding and Z. Fang, Nanoscale, 2013, 5, 3013–3021.
13. D. Patra, P. Vangal, A. A. Cain, C. Cho, O. Regev and J. C. Grunlan, ACS Appl. Mater. Interfaces, 2014, 6, 16903–16908.
14. J. Alongi, A. Di Blasio, F. Carosio and G. Malucelli, Polym. Degrad. Stab., 2014, 107, 74–81.
15. Y. S. Kim, Y. C. Li, W. M. Pitts, M. Werrel and R. D. Davis, ACS Appl. Mater. Interfaces, 2014, 6, 2146–2152.
16. Y. C. Li, Y. H. Yang, J. R. Shields and R. D. Davis, Polymer, 2015, 56, 284–292.
17. L. Liu, W. Wang and Y. Hu, RSC Adv., 2015, 5, 97458–97466.
18. Y. S. Kim and R. Davis, Thin Solid Films, 2014, 550, 184–189.
19. C. Bao, Y. Guo, L. Song, Y. Kan, X. Qian and Y. Hu, J. Mater. Chem., 2011, 21, 13290–13298.
20. C. Bao, L. Song, C. A. Wilkie, B. Yuan, Y. Guo, Y. Hu and X. Gong, J. Mater. Chem., 2012, 22, 16399–16406.
21. W. Hu, B. Yu, S. D. Jiang, L. Song, Y. Hu and B. Wang, J. Hazard. Mater., 2015, 300, 58–66.
22. A. Dasari, Z. Z. Yu, Y. W. Mai, G. Cai and H. Song, Polymer, 2009, 50, 1577–1587.
23. X. Wu, L. Wang, C. Wu, J. Yu, L. Xie, G. Wang and P. Jiang, Polym. Degrad. Stab., 2012, 97, 54–63.
24. F. Y. Hshieh, Fire Mater., 1998, 22, 69–76.
25. T. Kashiwagi, A. B. Morgan, J. M. Antonucci, M. R. VanLandingham, R. H. Harris, W. H. Awad and J. R. Shields, J. Appl. Polym. Sci., 2003, 89, 2072–2078.
26. P. Song, L. Liu, S. Fu, Y. Yu, C. Jin, Q. Wu, Y. Zhang and Q. Li, Nanotechnology, 2013, 24, 125704.
27. H. Ma, L. Tong, Z. Xu and Z. Fang, Nanotechnology, 2007, 18, 375602.
28. X. Wang, L. Song, H. Yang, W. Xing, B. Kandola and Y. Hu, J. Mater. Chem., 2012, 22, 22037–22043.
29. X. Wang, L. Song, H. Yang, W. Xing, H. Lu and Y. Hu, J. Mater. Chem., 2012, 22, 3426–3431.
30. D. Wang, Q. Zhang, K. Zhou, W. Yang, Y. Hu and X. Gong, J. Hazard. Mater., 2014, 278, 391–400.
31. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339.
32. W. Stöber, A. Fink and E. Bohn, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci., 1968, 26, 62–69.
33. J. W. Gilman, T. Kashiwagi and J. D. Lichtenhan, SAMPE J., 1997, 33, 40–46.
34. D. D. Nguyen, N. H. Tai, S. B. Lee and W. Kuo, Energy Environ. Sci., 2012, 5, 7908–7912.
35. J. Lefebvre, B. Bastin, M. L. Bras, S. Duquesne, C. Ritter, R. Paleja and F. Poutch, Polym. Test., 2004, 23, 281–290.
36. K. Wu, Y. Hu, L. Song, H. Lu and Z. Wang, Ind. Eng. Chem. Res., 2009, 48, 3150–3157.
37. Y. Dong, Z. Gui, Y. Hu, Y. Wu and S. Jiang, J. Hazard. Mater., 2012, 209, 34–39.
38. J. H. Park, B. S. Kim, Y. C. Yoo, M. S. Khil and H. Y. Kim, J. Appl. Polym. Sci., 2008, 107, 2211–2216.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03760g

---