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Improving the flame retardancy of polymeric materials is an imperative yet challenging task without deteriorating ... zirconium phosphate,15. LDH,12,16.
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Research Article http://doi.org/10.22180/na221

Volume 3, Issue 2, 2018

CO2 Induced Synthesis of Zn-Al Layered Double Hydroxide Nanostructures towards Efficiently Reducing Fire Hazards of Polymeric Materials Xin Wang,a* Ehsan Naderi Kalali,b Weiyi Xing,a and De-Yi Wang b* a

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei 230026, Anhui Province, P. R. China b

IMDEA Materials Institute, C/Eric Kandel, 2, 28906 Getafe, Madrid, Spain

*Corresponding author. E-mail: [email protected] (X. Wang); [email protected] (D. Y. Wang) Received December 9, 2017; Revised February 4, 2018

Citation: X. Wang, E. N. Kalali, W. Xing, and D. Y. Wang, Nano Adv., 2018, 3, 12-17. Improving the flame retardancy of polymeric materials is an imperative yet challenging task without deteriorating their intrinsic properties. Herein, we reported a facile approach to grow zinc-aluminum layered double hydroxide (Zn-Al LDH) nanostructures on the surfaces of polymer substrates in order to overcome their high fire hazards. CO2 induced growth of Zn-Al LDH allowed slow and homogeneous nucleation, leading to the as-synthesized Zn-Al LDH with ultra-large size (several microns) and perfect hexagonal shape. The Zn-Al LDH uniformly covered the surfaces of substrates and provided excellent flame-retardant effect. Cone calorimeter measurements revealed that the peak heat release rate of Zn−Al LDH-coated wood and rigid polyurethane foam (RPUF) were significantly decreased by 55% and 51%, respectively, compared to those of the pristine wood and RPUF; also, Zn−Al LDH-coated wood and RPUF showed notable reduction in total smoke production up to 47% and 28%, respectively. These findings demonstrate that CO2 induced synthesis of Zn-Al LDH nanostructures is a feasible and effective solution to polymeric materials with desirable flame-retardant and smoke-suppression properties. KEYWORDS: CO2 induced synthesis; Layered double hydroxides; Flame retardancy; Wood; Polyurethane foam

1. Introduction

charged metal hydroxide sheets with anions and water molecules within the layers.4 The general chemical formula of LDHs can be represented as [M2+1−xM3+x(OH)2]x+·[(An−)x/n·yH2O]x-, where M2+, M3+, and An−denote divalent and trivalent metal cations, and interlayer anions, respectively.5 Due to the highly tunable of M2+, M3+, and An− as well as the “x” value, various composition of LDHs have been prepared. LDHs have been widely investigated as photocatalysts,6 dye absorbents,7 drug carriers8 and flame retardants.9, 10 As one kind of well-known flame retardant, LDH has been intensively studied as additives to blend with polymeric materials. Under this situation, the flame retardant improvement depends strongly upon the dispersion of LDHs, and the addition of LDHs might alter the intrinsic properties of polymer matrix as well. As an alternative solution, in recent years, construction of coatings on the polymeric materials using nano-materials has attracted considerable interests because the presence of these nanocomposite coatings could greatly improve the flame resistance of polymer substrates without destructing the intrinsic

Polymeric materials have found a wide variety of applications and brought substantial convenience to modern society in everyday life. Despite of numerous advantages, polymeric materials, including natural polymers and synthetic polymers, possess a major problem because most of them are composed of organics and thus flammable. The flammability restricts their application in the fields of electrical and electronic devices, furniture, and building and transportation materials. Flame retardant additives are most commonly used to reduce the fire hazards of polymers. However, these additives also encounter a dilemma, such as negative impact on health and environments during the combustion,1,2 and deterioration in mechanical and/or thermal properties.3 Consequently, there is increasing demand for developing novel flame retardant technology for polymeric materials. Layered double hydroxide (LDH), also known as anionic clay, is an important host-guest material that is composed of positively

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Scheme 1. CO2 induced synthesis of Zn-Al layered double hydroxide nanostructures on polymeric materials.

properties. Over the last decade, multi-layered coatings consisted of various nano-materials, including montmorillonite,11–13 graphene,14 zirconium phosphate,15 LDH,12,16 polyhedral oligomeric silsesquioxanes,17 have been deposited onto polymer substrates through the layer-by-layer (L-b-L) assembly technique. These nanocomposite coatings built by the L-b-L method have been demonstrated to be effective in suppressing flame spread and/or reducing heat release rate of cotton fabrics,17,18 polyester fabrics,15 polyurethane foams,12,14 cellulose aerogels.13 However, the L-b-L technique requires multiple immersion-washing-drying cycles, which consumes a long time. Very recently, a two-step hydrothermal process was reported to fabricate Mg−Al LDH coating on a wood substrate.19 Significant reduction in peak heat release rate (-49%) and total smoke production (-58%) were observed in the Mg−Al LDH-coated wood in comparison to the untreated wood, but the hydrothermal process limits its scalable production. Up to now, LDHs are mainly synthesized through two routes: co-precipitation6,7 and hydrothermal method.20,21 However, the LDHs prepared by these two methods usually show small lateral size, low specific surface area and crystalline defect.6,7,21 In this work, we developed a new and simple approach to synthesize the Zn-Al LDH through exposing the mixed alkali solution of divalent and trivalent metal ions to air atmosphere. With the transformation of CO2 into CO32− as the interlayer anions, the Zn-Al LDH with ultra-large size, high specific surface area and beautiful crystalline structure was coated on the polymeric materials, such as wood and rigid polyurethane foam (RPUF), to confer flame-retardant properties.

China fir wood was purchased from local market and cut into specialized size for measurements. Rigid polyurethane foams (RPUFs) were made in our laboratory according to the receipt (Table S1). 2.2 Carbon dioxide induced synthesis of layered double hydroxide ZnAl-LDH was synthesized induced by carbon dioxide, as shown in Scheme 1. Briefly, zinc nitrate hexahydrate (0.02 mol) was dissolved in deionised water (20 ml) and subsequently 2 M sodium hydroxide aqueous solution was added into it dropwise until the resultant white precipitate re-dissolved to obtain a transparent solution (A). Meanwhile, aluminium nitrate nonahydrate (0.01 mol) was also dissolved in deionised water (20 ml) and subsequently treated by 2 M sodium hydroxide aqueous solution dropwise until the resultant white precipitate re-dissolved to obtain a transparent solution (B). The solution A was mixed with the solution B and then diluted to 500 ml by the deionised water. The wood board or rigid polyurethane foam was placed at the bottom of the mixture, followed by exposing to air statically for 12 h. During this period, the transparent mixture gradually became muddy, and the resultant white precipitate was deposited onto the wood board or rigid polyurethane foam, and then dried in the oven at 60 °C for 48 h. 2.3 Characterization The powder X-ray diffraction (XRD) patterns were carried out on a Philips X’Pert PRO diffractometer (Netherlands) equipped with Cu Kα radiation (λ = 0.15405 nm), operating at 45 kV voltage and 40 mA current. Fourier transformed infrared (FTIR) spectra were recorded on a NICOLET iS50FTIR spectrometer (USA) using KBr disc method. Thermogravimetric analysis (TGA) of the samples was measured on a Q50 thermal analyzer (TA Instruments, USA) in air atmosphere from 30 to 800 °C at a heating rate of 10 °C/min. Transmission electron microscopy (TEM) images were observed using a Talos F200X microscopy (FEI, Netherlands) combining outstanding high-resolution TEM imaging. The morphology was investigated using an EVO MA15

2. Experimental 2.1 Materials Zinc nitrate hexahydrate, aluminum nitrate nonahydrate and sodium hydroxide were purchased from Sigma-Aldrich Reagents Company and used as received. Deionised water is used for all experiments unless otherwise stated. A heartwood section of

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Nano Advances and 671 cm−1 are ascribed to the anti-symmetric stretching vibration and the angular bending mode of carbonate, respectively.24 The TG and differential TG curves (Figure 1c) of the Zn-Al LDH reveals two major stages of mass loss. These two stages locate at around 155 and 245 °C, respectively, as clearly observed in the differential TG curve. The first one corresponds to the loss of adsorbed moisture and interlayer water molecules, while the second one is owing to the release of water loosely coordinated to the interlayer carbonate.7 The TEM image (Figure 1d) reveals a regular hexagonal platelet of the Zn-Al LDH. The selected area electron diffraction (SAED) pattern (inset in Figure 1d) shows hexagonally arranged spots, indicating a typical single crystalline of the material, in good agreement with the XRD results. The formation mechanism of Zn-Al LDH is proposed as follows: an aqueous precursor solution containing Na2ZnO2 and NaAlO2 is exposed to air via carbon dioxide induced co-precipitation. The Na2ZnO2 is obtained from solubilising Zn(OH)2 by excessive NaOH solution (Equation 1). Similarly, the NaAlO2 is obtained from solubilising Al(OH)3 by excessive NaOH solution (Equation 2). Simultaneously, CO2 is turned into CO32− and intercalated into the interlayer gallery as anion. Due to the excessive existence of NaOH and the low solubility of CO2 in solution, this aqueous precursor solution provides an alkaline medium favourable for heterogeneous nucleation of Zn-Al LDH (Equation 3).

Figure 1. (a) Wide angle X-ray scattering profile; (b) FTIR spectrum; (c) Thermogravimetric analysis curve and (d) TEM images (Inset: the corresponding SAED pattern) of the Zn-Al LDH induced by CO2.

scanning electron microscopy (SEM, Zeiss, Germany). The samples were sputtered with a conductive gold layer before SEM observation. To determine the thickness of the Zn-Al LDH coatings, a clean glass plate was put into the solution instead of polymeric materials. After 12 h, the glass plate was washed, dried and then observed using SEM. The combustion behaviors of the samples were assessed by a cone calorimeter (Fire Testing Technology, UK) according to the standard ISO 5660-1. Squared specimens (100 mm × 100 mm × 10 mm) were horizontally exposed to a heat flux of 50 kW·m–2.

( (

)

( ) (2)

[

( ) ] ( ) (3) SEM micrographs were taken to provide morphological information of the Zn-Al LDH nanostructures on wood substrates. Figure 2 gives the SEM images of the untreated wood, Zn-Al LDH coated wood and the corresponding EDX elemental mapping analysis. As can be observed, untreated wood shows a rough surface consisted of many fibre bundles (Figure 2a). In contrast, the treated wood exhibits a Zn−Al LDH nanostructure coating contained numerous hexagonal platelets with the average size of several microns (Figure 2b). High-magnification observations of the Zn−Al LDH coating indicates an ultra-large size (> 5 μm) and beautiful hexagonal shape of Zn-Al LDH obtained through the CO2 induced growth method (Figure 2c). Table S2 compares the as-prepared Zn-Al LDH with their counterparts in the previous reports.6,7,21,24,25 Most of the Zn-Al LDH reported up to now exhibits smaller size and irregular shape. This is attributed to that the CO2 induced growth of Zn-Al LDH allows slow and homogeneous nucleation in comparison to the conventional methods (co-precipitation or hydrothermal method). The elemental composition measured by EDX analysis (Figure 2d) reveals that high percentages of Zn and Al elements are clearly detected on the surface of the coated wood samples besides C and O elements corresponding to the wood

3. Results and discussion The structure of the Zn-Al LDH induced by CO2 was characterized by wide angle X-ray diffraction (Figure 1a). As can be observed, the Zn-Al LDH displays the typical features of LDH materials with sharp intense peaks at low theta values, while they become weaker and less defined at higher angular values. The characteristics peaks at 2theta = 11.7o, 23.4o and 34.8o are attributed to the (003), (006) and (009) diffraction peaks, respectively.22 As is well known, the d(003)-spacing is equal to the basal spacing of LDH materials.23 From the basal reflection (003) at 2theta = 11.7o, the basal spacing of Zn-Al LDH is estimated to be 0.76 nm according to the Bragg equation. This value is in good coincidence with the value for carbonate-intercalated LDH materials.23 Furthermore, all the diffraction peaks appear to be of very sharp shape, implying the large-size crystalline obtained under free growth condition. The FTIR spectrum of the Zn-Al LDH (Figure 1b) clarifies the characteristic peaks of hydrotalcite-like compounds. The wide and strong peak centered at 3454 cm−1 is assigned to the stretching of the hydroxyl groups and water molecules. The weak peak at 1620 cm−1 can be attributed to the bending vibration of the interlayer water. The strong peaks around 1380

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Figure 3. Zn-Al LDH coating thickness as a function of the Zn and Al ions amount.

polymer substrates. The fire protection effect of the Zn−Al LDH coating on the wood and RPUF was investigated using cone calorimeter. As one of the most important parameter obtained in cone calorimeter, the heat release rate (HRR) is usually used to assess the intensity of flame. The peak HRR values of the Zn−Al LDH-coated wood and RPUF are significantly decreased by 55% and 51%, respectively, compared to those of the pristine wood

Figure 2. SEM images of (a) untreated wood and (b, c) Zn-Al LDH coated wood under different magnifications. (d) EDX elemental mapping analysis of (e) Zn, and (f) Al of the Zn−Al LDH-coated wood.

components. Additionally, the EDX mapping images (Figure 2e and 2f) confirm that the Zn and Al elements are distributed uniformly on the wood surfaces. Similar phenomenon is observed in the RPUF and Zn-Al LDH coated RPUF, as shown in Figure S1 in the supporting information. In short, the SEM and EDX analyses demonstrate that the Zn−Al LDH has been prepared successfully and uniformly deposited onto the wood and RPUF surfaces. The influence of the loadings of the deposition mixtures on the growth of the Zn-Al LDH coatings was also investigated. Four different amounts of Zn and Al ions are used to prepare the Zn-Al LDH coatings whose growth is shown in Figure 3. All four formulations grew linearly as a function of amounts of Zn and Al ions deposited. The coating thicknesses ranged from several to hundred micrometers. So, it is very convenient to control the coating thicknesses by adjusting the loading of Zn and Al ions. The XPS survey spectra of the wood, Zn-Al LDH coated wood, RPUF and Zn-Al LDH coated RPUF are shown in Figure 4. The relative intensity of C1s and O1s of Zn-Al LDH coated wood is lower than that in the untreated wood, suggesting that the Zn-Al LDH is covered on the wood surface. In addition, peaks at 1044.5 and 74.3 eV in the Zn−Al LDH-coated wood are assigned to the Zn2p and Al2p binding energies in Zn−Al LDH, respectively. Similar phenomenon is observed in the RPUF and Zn-Al LDH coated RPUF. These findings further confirm that Zn−Al LDH has been successfully synthesized on the surfaces of

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Figure 4. XPS survey of untreated wood and the Zn−Al LDH-coated wood, and untreated RPUF and the Zn−Al LDH-coated RPUF. XPS high-resolution spectra of Al2p and Zn2p of the Zn−Al LDH-coated wood and the Zn−Al LDH-coated RPUF.

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Table 1. Cone calorimeter data of wood, RPUF before and after LDH treatment

Sample

TTI (s)

PHRR (kW/m2)

THR (MJ/m2)

Av-EHC (MJ/kg)

Time to PHRR (s) FIGRA (kW/(m2·s))

Wood

11

340

18.45

11.05

110

3.09

LDH-Wood

10

152

14.99

8.10

108

1.41

RPUF

2

325

13.97

20.11

10

32.5

LDH-RPUF

3

159

11.59

15.51

12

13.3

and RPUF (Figure 5a and 5e). Accompanying with the significantly reduced peak HRR values of the Zn−Al LDH-coated samples, their time to ignition (TTI) and time to peak HRR values slightly change (Table 1). The fire growth rate index (FIGRA), calculated by the ratio of PHRR and time to PHRR,26 is usually used to evaluate the flame propagation rate of materials. The FIGRA values of the Zn−Al LDH-coated wood and RPUF are dramatically reduced in comparison to the untreated ones (Table 1), indicating the suppressed fire hazard quality of the materials. With the presence of the Zn−Al LDH coating, the total heat release (THR) values of the treated wood and RPUF are decreased by 19% and 17%, respectively, compared to those of the untreated wood and RPUF (Figure 5b and 5f). A comparison of the performance of Zn-Al LDH coating with other flame retardant coatings is summarized in Table S3. The current Zn-Al LDH coating exhibits better flame retardant effect than other flame retardant coatings made by layer-by-layer assembly such as chitosan and poly(phosphoric acid),27 chitosan and poly(vinyl sulfonic acid sodium salt),28 poly(diallydimethylammonium chloride), poly(acrylic acid) and ammonium polyphosphate,29 brucite, 3-aminopropyltriethoxysilane and alginate.30 Flame retardant effect is similar to that of Mg−Al LDH coating,19 but the current coating is easier to achieve scalable production. Furthermore, both the Zn−Al LDH-coated wood and RPUF show notable reduction in total smoke production (TSP) up to 47% and 28%, respectively, compared to that of pristine samples (Figure 5c and 5g). The decreased TSP can be explained by that the presence of Zn−Al LDH coating served as physical barrier to restrain organic macromolecules from converting into the organic volatiles that are the major source of smoke particles.31 This phenomenon is also supported by the decreased av-EHC values (Table 1), suggesting that lower amount of volatile gases (fuels) goes into gaseous-phase. CO production, regarding the toxicity of gas released from combustion of samples, is the major reason for casualties in fire incidents.32 The CO production versus time curve (Figure 5d and 5h) shows that the CO production of the Zn−Al LDH-coated samples is significantly decreased by 41% and 29%, respectively, compared to that of

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pristine samples. These dramatic reductions in smoke and toxic CO production are beneficial to evacuation and rescue when an incident happens. These improved fire retardant properties induced by Zn-Al LDH can be attributed to the heat absorption and char formation. During the degradation process, LDHs lose the interlayer water molecule, which is an endothermic reaction.33 LDHs can absorb a large amount of heat and meanwhile the water vapor dilute the

Figure 5. (a, e) HRR, (b, f) THR, (c, g) SEA and (d, h) CO production versus time curves of wood, Zn-Al LDH-coated wood, RPUF and Zn-Al LDH-coated RPUF, respectively.

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flammable volatiles, and thus, the heat release rate is suppressed. From the digital photograph of the char residue, it can be observed that a white inorganic layer is formed on the surface of LDH-wood compared to pristine sample (Figure S2a and S2b). For the RPUF, almost no residue is left after the cone calorimeter and the aluminium foil is burned out (Figure S2c). In contrast, the LDH-RPUF shows much more residue (Figure S2d). The increased char yield is also evidenced by the mass loss profiles as a function of burning time (Figure S3). The formation of such a char layer functions as a physical barrier to inhibit the mass and heat transfer between polymeric substrates and flame.

7. 8.

9. 10. 11.

4. Conclusions

12.

In summary, we developed a facile CO2 induced synthesis method to deposit Zn-Al LDH coatings onto the surfaces of polymeric materials including wood and rigid polyurethane foam (RPUF). The morphology and chemical compositions characterized by SEM and EDX spectroscopy confirmed that the Zn-Al LDHs were uniformly covered on the surfaces of substrates. The presence of the Zn-Al LDH coating resulted in significantly improved fire retardancy of polymeric materials. Cone calorimeter results showed that the peak heat release rate of the Zn−Al LDH-coated wood and RPUF was reduced by 55% and 51%, respectively, compared to those of the pristine wood and RPUF; also, notable reduction in total heat release, total smoke production, and CO production were also observed. This flame retardant treatment reported herein provides an innovative and efficient strategy to reduce fire hazards of polymeric materials.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Acknowledgements 23.

We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21604081), and Anhui Provincial Natural Science Foundation (1608085QE99).

24. 25. 26.

Notes and references

27.

Supporting information for this article is available: Figures S1-S3 and Table S1-S3 mentioned in the text. See doi: 10.22180/na221. 1. 2. 3. 4.

5. 6.

28. 29.

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How to cite this article: X. Wang, E. N. Kalali, W. Xing, and D. Y. Wang, Nano Adv., 2018, 3, 12−17. doi: 10.22180/na221.

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