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Research Article

Nano Adv., 2018, 3, 12−17.

2016, 1, X−X. Nano Advances

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: wxcmx@ustc.edu.cn (X. Wang); deyi.wang@imdea.org (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

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

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−xM

3+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

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Research Article Nano Advances

Nano Adv., 2018, 3, 12−17.016, 1, X−X.

doi: 10.22180/na221

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.

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

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

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

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Research Article Nano Advances

Nano Adv., 2018, 3, 12−17.016, 1, X−X.

doi: 10.22180/na221

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.

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

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).

( ) ( )

( ) (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

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.

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Research Article Nano Advances

Nano Adv., 2018, 3, 12−17.016, 1, X−X.

doi: 10.22180/na221

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

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.

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

amount.

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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Nano Adv., 2018, 3, 12−17.016, 1, X−X.

doi: 10.22180/na221

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

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.

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

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Research Article Nano Advances

Nano Adv., 2018, 3, 12−17.016, 1, X−X.

doi: 10.22180/na221

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.

4. Conclusions

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.

Acknowledgements

We gratefully acknowledge financial support from the National

Natural Science Foundation of China (Grant No. 21604081), and

Anhui Provincial Natural Science Foundation (1608085QE99).

Notes and references

Supporting information for this article is available: Figures

S1-S3 and Table S1-S3 mentioned in the text. See doi:

10.22180/na221.

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How to cite this article: X. Wang, E. N. Kalali, W. Xing, and D.

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