Fabrication of flame retardant coating on cotton fabric by alternate assembly of exfoliated layered double hydroxides and alginate

Abstract

In our current work, a layer-by-layer flame retardant coating, assembled from MgAl layered double hydroxides (MgAl-LDH) and alginate, was firstly fabricated onto the surface of cotton fabric for the purpose of reducing its flammability. First, the MgAl–NO₃ LDH was prepared by the hydrothermal method, then it was exfoliated into the positively charged nanosheet in the presence of formamide. Second, the coating deposition on cotton fabrics was carried out by alternately immersing the fabrics into MgAl-LDH suspension and alginate solution. In the vertical flame test, the cotton fabric coated with 20 bilayers could preserve almost 70% of the weave structure after the burning, while pure cotton fabric was completely burned out. The test by microscale combustion calorimetry revealed that the cotton fabrics only with 3.52 wt% coating could obtain 34.6% reduction in peak heat release rate and 25.6% reduction in total heat release compared with those of the pure one. In addition, as evidenced by thermogravimetric analysis-Fourier transform infrared spectroscopy results, the decreased amount of volatilized pyrolysis products and toxic volatiles (CO) during the degradation of coated cotton fabrics was another important factor to improve the flame retardancy. The notable improved flame retardancy of coated cotton fabrics is attributed to the fact that the MgAl-LDH filled coating can form an inorganic protective layer, which can act as a barrier to retard the transfer of heat, oxygen, and the diffusion of volatilized pyrolysis products between the combustion zone and underlying cotton fibers.

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1. Introduction

Cotton fabric, made up of biodegradable cellulose fibers, is one of the most commonly used natural textiles in the world. Owing to its superior properties including high intensity, fast moisture absorption, and good thermal conductivity, cotton fabric has been widely used in producing apparel production, home furnishing, medical textiles and other industrial products. However, the high flammability and ignitability of cotton fabric bring potential fire hazards during its applications.¹ Lots of fire statistics have illustrated that the ignition of household textiles is one of the most common fire reasons of residential home fires, which led to the considerable fire-related fatalities and amount of property damage. Therefore, a great effort should be made to reduce its potential fire hazards.

Various strategies have been developed to get flame retardant fabrics: surface photo-induced grafting treatment,^2,3 fire-retardant additives or co-monomers in synthetic fabrics,^4,5 fiber blending,⁶ etc. Recently, layer-by-layer (LbL) assembly has been applied as a surface treatment technique to endow fabrics with flame retardancy.⁷ LbL assembly is a simple and versatile method to create a coating onto the substrate surface by submersing the substrate into positively and negatively charged polyelectrolyte and/or nanoparticle solutions. The positive–negative pairing nanolayers in the coating can be held together mostly through electrostatic attraction.⁸ Since the simplicity and controlled properties, LbL assembly technique has been the focus of significant research interest and studied for various applications such as oxygen barrier, drug delivery, electrochemical film and sensing application.⁸ Recently, LbL assembly has been developed as a novel flame retardant technology for polymer matrixes. Since Grunlan and co-workers firstly used the LbL assembled coating comprised of branched polyethylenimine and LAPONITE^® clay platelets to improve flame retardancy of cotton fabric,⁹ various nanoparticles including sodium montmorillonite,¹⁰ a-zirconium phosphate,¹¹ carbon nanotubes,¹² silica particles,¹³ titanate nanotubes,¹⁴ etc., have been successively fabricated onto fabrics to endow them flame retardancy.

Layered double hydroxides (LDH) are a class of two-dimensional inorganic layered matrices. The general formula of LDH can be expressed as [M_1−x²⁺M_x³⁺(OH)₂]X⁺(Aⁿ⁻)_x/n·mH₂O, where M²⁺ and M³⁺ represent divalent and trivalent metal cations in the octahedral positions of brucite-like layers. These brucite-like layers always yield excessive positive charge, and Aⁿ⁻ represents an n-valent anion which balances the positive charges on the layers.^15,16 The kinds of metal cations and their proportions can be changed under certain conditions without altering the structure of the material. Their lamellar structure with anion exchange properties makes them good candidates for many applications, such as ion-exchangers, adsorbents, pharmaceutical stabilizers. With the growing concern about the environmental problems, LDH, as a non-toxic material, has been developed as a flame retardant nanofiller to incorporate into polymer matrixes.¹⁷ Owing to its notable physical barrier effect, LDH could work as heat stabilizer and flame retardants or efficient smoke suppressants to reduce fire hazard of polymeric materials. For example, as an effective thermal stabilizer, ZnAlLa–CO₃-LDHs could improve the long-term thermal stability of PVC;¹⁸ Costache et al. have reported that the incorporation of LDH could lead to the significant reduction in peak heat release rate for various polymer matrixes, including 55% for polyethylene, 39% for ethylene vinyl acetate, and 35% for polystyrene;¹⁹ Research on epoxy resin/LDH nanocomposites indicated that the smoke production rate and maximum smoke density and smoke shading index decreased significantly with the loading of LDH.²⁰

Suggested by the non-toxicity feature and excellent fire resistance of LDH, we decided to choose LDH as a filler of the coating to improve the flame retardancy of cotton fabric. In our current work, the flame retardant coating consisting of MgAl-LDH and alginate was fabricated onto fabric surface via layer-by-layer assembly technique. MgAl based LDH is large size nanoplates, and its color is white, which do not significantly change the whiteness of cotton fabric. These excellent properties inspired us to choose MgAl-LDH as the representative to give a study. As far as we know, it is the first report about LDH filled flame retardant coating to be fabricated onto cotton fabric by layer-by-layer assembled method. Here, the thermal degradation behaviors of the cotton fabric were evaluated using thermogravimetric analyzer (TGA). The flame retardancy and combustion behavior were investigated by micro combustion calorimetry and vertical flame test. The incorporation of 3.52 wt% coating could result in 34.6% reduction in peak heat release rate and 25.6% reduction in total heat release for cotton fabric. Furthermore, the cotton fabric with 3.52 wt% coating could preserve almost 70% of weave structure during combustion, while pure cotton fabric was completely consumed. As a result, the good flame retardant effect could be obtained for the cotton fabric after covering with MgAl-LDH filled coating.

2. Experiment

2.1 Materials

Cotton fabrics were provided by Jiangxi JingZhu RamieTextile Co. Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, urea, H₂O₂ (30%), H₂SO₄ (98%), HNO₃ (68%), NaNO₃, alginate and formamide were all purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water with a resistance of 18.2 M was used for all experiments.

2.2 The synthesis and exfoliation of MgAl–NO₃ LDH

The processes of synthesis and exfoliation of MgAl–NO₃ LDH were similar to the reports by the literatures.^21,22 Typically, Mg(NO₃)₂·6H₂O (0.002 mol), Al(NO₃)₃·9H₂O (0.001 mol) and urea (0.012 mol) were dissolved in aqueous solution (70 ml), then the mixture of which was sealed in a Teflon-lined stainless steel autoclave and heated at 110 °C for 24 h. After that, the product from the autoclave was washed with water, and dried in air at 70 °C. This sample (0.3 g) was further treated with 400 ml of an aqueous solution containing NaNO₃ (0.50 mol) and HNO₃ (0.0015 mol) whilst purging with nitrogen gas, and was shaken for 24 h at ambient temperature. The resulting LDH was washed with hot distilled water, and then dried in a vacuum at 70 °C. 2 g of this sample was shaken in 998 g of formamide solution for 24 h to produce a stable colloidal suspension of exfoliated MgAl-LDH nanosheets.

2.3 The coating deposition

Alginate (0.3 wt%) solution was prepared by introducing alginate into deionized water and stirring the mixture for 24 h. Cotton fabric was washed with distilled water, and then dried with filtered air at room temperature for 24 h before coating deposition. As is shown in Scheme 1, a typical coating deposition process on cotton fabric is as follows: the treated cotton fabric was alternately dipped into MgAl-LDH suspension (positive) and alginate solution (negative). The first dip in the MgAl-LDH suspension were 5 min. Subsequent dip was carried out in the alginate solution for 1 min. Each dip was followed by rinsing with deionized water and wringing out by hand to expel liquid among cotton fabric. This procedure would complete the fabrication of the first bilayers. After the designed bilayer number (5, 10, and 20) was achieved, the coated cotton fabrics were dried under vacuum at 70 °C for 12 h before testing. This preparation process can be shown in Scheme 1.

Scheme 1 The scheme of coating deposition on the cotton fabrics using layer-by-layer assembly technique.

In order to monitor the coating growth, LbL coatings were also deposited onto quartz slides substrates (10 × 20 mm) for observation. Before deposition, the quartz slides were cleaned using boiling piranha solution (H₂O₂–H₂SO₄ 1 : 3 v/v) at 85 °C for 40 min. After this procedure, the quartz slides were rinsed and washed thoroughly with deionized water. Then, the layer-by-layer deposition process on treated quartz slides was similar to that of cotton fabric. At last, coated quartz slides were dried with N₂.

2.4 Characterization

X-ray diffraction (XRD) measurements of MgAl-LDH was performed 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).

FTIR spectra were recorded on a Nicolet MAGNA-IR750 FTIR spectrometer. The transition mode was used, and the wavelength range was set from 4000 to 500 cm⁻¹.

The morphologies of MgAl–NO₃ LDH, pure and coated cotton fabrics including the char residues after vertical flame tests were observed using scanning electron microscopy (SEM, AMRAY1000B, Beijing R&D Center of the Chinese Academy of sciences, China).

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 from 4000 to 700 cm⁻¹ at a 4 cm⁻¹ resolution, were recorded by a Nicolet 6700 spectrometer (Thermo-Nicolet) using 32 scans.

The thermogravimetric analysis (TGA) of samples was undertaken using TGA-Q5000 apparatus (TA Co., USA) from 50 °C to 700 °C under nitrogen atmosphere at a heating rate of 20 °C min⁻¹.

Heat release rate (HRR) and total heat release (THR) were measured in a microscale combustion calorimeter (GOVMARK MCC-2). About 5 mg samples were heated at a heating rate of 1 K s⁻¹ in a nitrogen stream flowing at 80 cm³ min⁻¹. The degradation products of the sample in nitrogen atmosphere were mixed with a 20 cm³ min⁻¹ stream of pure oxygen prior to entering a 900 °C combustion furnace.

Vertical flame test was performed according to ASTM D6413-11, using a vertical burning tester (CZF-3, Nanjing Jiangning Analytical Instrument Factory, China). The samples (300 × 76 mm), held 19 mm over the Bunsen burner, were first exposed to the flame for a period of 12 s and then removed rapidly.

Thermogravimetric analysis/infrared spectrometry (TGA-FTIR) of pure and coated cotton fabrics were performed using the TGA Q5000IR thermogravimetric analyzer that was interfaced to the Nicolet 6700 FTIR spectrophotometer. About 5.0 mg of the sample was put in an alumina crucible and heated from room temperature to 700 °C at a heating rate of 20 °C min⁻¹ (nitrogen atmosphere, flow rate of 45 ml min⁻¹).

The mechanical property of all cotton fabrics were measured with a universal testing machine (Instron1185) at the temperature of 25 ± 2 °C according to the modified GB/T 3923.1-1997. The 250 × 30 mm² specimens were tested with a speed of 100 mm min⁻¹. The breaking strength was recorded.

3. Results and discussion

3.1 Characterization of MgAl–NO₃ LDH and its LbL coating

Owing to the positive layer charge, the exfoliation of LDH is thought to be a useful and versatile method to obtain unilamellar nanosheets.²¹ The best known route is the exfoliation in formamide,¹⁶ which has been demonstrated to be the most effective reagent to exfoliate LDH through a direct reaction without any heating or refluxing treatment. In addition, due to high affinity of the carbonate to the LDH layer, the interlayer CO₃²⁻ must be converted to other ions such as NO₃¹⁻ or bipolar ion to enable the exfoliation of LDH layers.^23,24 This is why we must synthesized MgAl–NO₃ LDH with uniform, highly crystalline structure. Fig. 1a presents the XRD patterns of the product that was prepared by the Mg(NO₃)₂–Al(NO₃)₃–urea system. The sharp and symmetric features of the diffraction peaks strongly suggest that the product is the highly crystallized MgAl–NO₃ LDH with a three-dimensional order, indicating the high purity of the product. The d₀₀₃ represents the interlayer distance (8.95 Å) in the layered double hydroxide structure, which confirms the formation of the nitrate form of the LDH.²⁵ Furthermore, intercalation of NO₃¹⁻ anions can be further confirmed by FTIR spectra. As shown in Fig. 1b, a sharp and strong absorption peak at 1384 cm⁻¹ can be observed, which is attributed to the N–O stretching vibration of NO₃⁻¹ anions.²⁵ Therefore, the NO₃⁻¹ intercalated double hydroxide structure has been successfully prepared. The morphology of the MgAl–NO₃ LDH is visually characterized using SEM. As shown in Fig. 1c, the prepared MgAl–NO₃ LDH reveals a hexagonal nanoplate crystal with the diameters of several micrometers. As is well known to all, the nanoplate-like organic nanoparticles could perform well in flame retardant application owing to their good physical barrier effect.

Fig. 1 XRD patterns of MgAl–NO₃ LDH (a), FTIR spectra of MgAl–NO₃ LDH (b), and Scanning Electron Microscopy (SEM) image of MgAl–NO₃ LDH (c).

In order to make clearly whether the LbL assembly process was uniform, the coating growth on the quartz slides was measured using UV-visible absorption spectrometry. Fig. 2 plots the representative UV-visible absorption spectra of 5, 10, 15, and 20 bilayers that prepared on quartz slides. It can be found that the absorption in the spectral range of 240–350 nm increase as respect to the bilayer number. The absorption peak at 266 nm is attributed to the absorption of alginate. As shown in the inset plot, its absorption intensity increases as the bilayer numbers rises. Furthermore, there is an almost linear relationship between the absorbance and the bilayer number, demonstrating that the deposition process of coating is reproductive from unit to unit.

Fig. 2 UV-visible absorption spectra of MgAl-LDH/alginate based LbL coating on quartz slides. The inset plot shows the absorption intensity at 266 nm as a function of bilayer number.

3.2 Characterization of coated cotton fabrics

Table 1 gives the weight gain of LbL coatings on cotton fabrics as a function of the bilayer number. The pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers are marked as CF0, CF5, CF10, CF20, and the corresponding weight gain is 0%, 0.80%, 1.62%, and 3.52%, respectively. Obviously, the weight gain shows an almost linear growth trend relative to the bilayer number, suggesting that the coating loading that prepared on cotton fabrics can be adjusted by changing the bilayer number.

Table 1 The concentration of the solution, bilayer number, and the coating weight gain of cotton fabrics with different bilayers

Sample	Number (n)	Alginate (wt%)	MgAl–NO3 LDH (wt%)	Coating weight gain (wt%)
CF0	—	—	—	—
CF5	5	0.3	0.2	0.80
CF10	10	0.3	0.2	1.62
CF20	20	0.3	0.2	3.52

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Here, in order to monitor the coating growth on cotton fabrics, the ATR-FTIR spectroscopy was used to qualitatively detect the surface chemical structure of pure and coated cotton fabrics. ATR-FTIR is an IR sampling technique and can measure the changes in an infrared beam with totally internal reflection when it comes into contact with the surface of the samples. As can be seen from Fig. 3, compared with CF0, the spectra of all coated cotton fabrics appear a new peak at 1627 cm⁻¹, which is believed to be ascribed to the stretching vibration of carboxyl group from alginate.²⁶ Furthermore, the intensity of the peak absorbance increase obviously as the bilayer number increase, indicating the regular coating growth on fabric surface.

Fig. 3 ATR-FTIR spectra of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers (a), and the FTIR spectra of alginate (b).

In order to provide direct visual information, the surface morphologies of pure cotton fabric and cotton fabrics coated with different bilayers were imaged using SEM for direct comparison. The SEM images of all samples are shown in Fig. 4. For CF0, it has a smooth and clean surface. For CF5, CF10, and CF20, the LbL coatings obviously distribute onto the surface of cotton fibers, resulting in the rough surface. Furthermore, with the increasing of bilayer number, the surface coverage by LbL coatings substantially increase on cotton fabric surface, indicating the effectiveness of the use of MgAl-LDH and alginate as cationic and anionic polyelectrolyte, respectively. In detail, when the bilayer number is 5 (CF5), the individual cotton fibers are easily discerned. As the bilayer number is increased to 20 (CF20), the gap between the cotton fibers get smaller, even small part of interstices between the fibers are filled with LbL coating. It is likely that coating draped between fibers provided additional surface area for deposition during coating growth. As a result, this LbL coating with good distribution would perform well as protective layer on the surface of cotton fibers.

Fig. 4 SEM images of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers.

3.3 Thermal stability

TGA is an efficient method to assess the thermal behavior of materials. It can provide direct information about the thermal property through measuring the weight loss of the sample as a function of temperature. Fig. 5 depicts the TGA curves of CF0, CF5, CF10, and CF20 under nitrogen atmosphere, and the representative parameters are summarized in Table 2. The initial degradation temperature (T_−5%) (the weight loss is 5 wt%) of samples, the maximum weight loss temperature (T_max) and the solid residue left at 500 and 700 °C can be obtained from TGA curves. As it can be observed, for all samples, the main weight loss is happened in the temperature around 360 °C, which is attributed to the dehydration and decarboxylation reactions which produce combustible gasses like aldehydes, ketones, ethers, etc.^27–29 Compared with CF0, CF5, CF10, and CF20 all show lower initial degradation temperature. This is thought to be mainly caused by the earlier degradation of MgAl-LDH. However, the solid residues of the regions from 400 to 700 °C in the coated cotton fabrics are obviously higher than that of CF0, indicative of the enhancement of thermal stability in high temperature range. Furthermore, this enhancement is dependent on the bilayer number and LbL coating loading. For example, CF20 has the highest char residue (11.3%) at 700 °C. Hence, the TGA results highlight the importance of the LbL coating loading that added to the cotton fabric. And such an improvement in thermal stability for coated cotton fabrics is ascribed to the protective effect from the MgAl-LDH.

Fig. 5 TGA curves of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers under nitrogen atmosphere.

Table 2 Thermal properties of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers under nitrogen atmosphere

Sample	T−5% (°C)	Tmax (°C)	Solid residue at 500 °C (wt%)	Char residue at 700 °C (wt%)
CF0	321	369	9.3	6.0
CF5	294	357	9.8	6.7
CF10	300	361	10.8	7.7
CF20	292	363	15.3	11.3

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3.4 Flammability

The microscale combustion calorimeter (MCC) is an effective tool for obtaining information regarding the combustibility and fire hazard of materials. It can quickly and easily measure the key fire parameters from just a few milligrams of specimen in minutes, including the temperature at maximum heat release rate (T_max), peak heat release rate (PHRR) and total heat release (THR). The representative HRR and THR curves of pure cotton fabric and cotton fabrics with different bilayers as function of temperature are presented in Fig. 6, and the detailed data are summarized in Table 3. Compared to the pure cotton fabric, the PHRR and THR values of all coated samples are shown to decrease, indicating the enhancement in flame retardancy. This can be attributed to the protective effect by MgAl-LDH, which can form an inorganic protective layer to retard the evolution of flammable volatile products and shield the underlying material from further pyrolysis.^30,31 Furthermore, with increasing of bilayer number, the PHRR and THR values of cotton fabric decrease. The higher bilayer number, the greater reduction in PHRR and THR. The results demonstrate that the flame retardancy of cotton fabric can be adjusted through changing the bilayer number. Especially for CF20, its PHRR and THR values are 134 W g⁻¹ and 6.1 kJ g⁻¹, with the reductions of 34.6% and 25.6%, respectively, compared to those of CF0. Another obvious observation is that the T_max values of all coated fabrics shift to lower temperatures compared to that of CF0, which is consistent with the results from TGA test.

Fig. 6 Heat release rate and total heat release curves of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers.

Table 3 Microscale Combustion Calorimeter (MCC) data of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers

Sample	PHRR (W g^−1)	THR (kJ g^−1)	Tmax (°C)
CF0	205 ± 5	8.2 ± 0.2	387 ± 7
CF5	187 ± 3	6.7 ± 0.2	370 ± 6
CF10	166 ± 4	6.4 ± 0.1	381 ± 6
CF20	134 ± 3	6.1 ± 0.1	371 ± 5

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Further assessment of the flammability properties for all cotton fabrics was provided by vertical flame test. The photographs of the collected char residues after the vertical flame test are shown in Fig. 7A. As it can be seen, CF0 was almost completely consumed after the test, while the other samples left some residues. Furthermore, the char residue become more intact and structurally continuous with the increasing of bilayer number. Especially for CF20, its char residue is more continuous and relatively intact with just some slit in the left and top edge and little shrinkage on the bottom side, and almost 70% of weave structure can be preserved in the end. This result indicates that this MgAl-LDH filled coating performs good protective effect to the underlying cotton fabrics during combustion.

Fig. 7 Photographs of the collected residues of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers after the vertical flame test (A), and SEM images of char residues of cotton fabrics coated with 5, 10, and 20 bilayers after the vertical flame test (B).

The char residues after vertical flame test were analyzed using SEM to further understand the combustion behavior of cotton fabrics. As is shown in Fig. 7B, the char residues of CF5, CF10, and CF20 retain the weave structure, but each of them shows some differences. For CF5 and CF10, the fibers shrink and gaps between yarns are relative larger due to the breakage of cellulose backbone and the following release of organic fragments during combustion. This is a somewhat expected result owing to their lower coating loadings. While for CF20, the density of weave structure of residues become compact, and the gaps between fibers obviously decrease, suggesting that the higher coating loading is contributed to better protect underlying cotton fibers from pyrolysis during combustion. Therefore, the best flame retardant property is obtained for CF20, which has been proven by the MCC results.

3.5 Volatilized pyrolysis products analysis of pure cotton fabric and cotton fabric coated with 20 bilayers

TGA-IR spectroscopy was conducted to identify the volatilized pyrolysis products during thermal decomposition for better understanding of the influence of the LBL coating on the thermal degradation of the cotton fabric.³² Fig. 8 presents the main absorbance of volatilized pyrolysis products from CF0 and CF20. Total and some specific volatilized pyrolysis products, including H₂O (3548 cm⁻¹), hydrocarbons (2920 cm⁻¹), CO₂ (2358 cm⁻¹), CO (2180 cm⁻¹), carbonyl compounds (1741 cm⁻¹) and CH₃OH (1084 cm⁻¹), are chosen to be studied. As shown in Fig. 8, it can be found that the absorption peaks of the volatilized pyrolysis products of CF20 are almost identical to that of pure CF0, indicating that the MgAl-LDH filled coating did not alter the main thermal degradation process significantly. Obviously, compared with CF0, the intensity of all of the adsorption peaks in CF20 is greatly reduced, especially for CH₃OH, hydrocarbons, and CO. These reduced amount of the organic volatiles means less “fuel” to be fed back to the flame, so the reduced peak heat release rate and total heat release are both obtained for coated cotton fabrics in MCC test. In a fire scene, the released CO by fabrics is usually the real killer that can do great harm to human health. Here, the reduced amount of toxic volatiles (CO) means the inhibition of toxic gas by MgAl-LDH filled coating. Therefore, this LbL coating can also effectively decrease the fire toxicity of cotton fabric as the fire occurs.

Fig. 8 Intensity of characteristic peaks of volatilized pyrolysis products of CF0 and CF20.

3.6 Mechanical property of all cotton fabrics

Table 4 shows the mechanical property of all cotton fabrics. It can be found that the increased loading of the MgAl-LDH filled coating substantially enhances the tensile strength in both warp and weft directions. However, there is just a little enhancement, even for CF20. Therefore, low coating loading does not change obviously the strength of cotton fabrics. In addition, data of tensile strength also suggest that the alternate assembly between MgAl-LDH and alginate mainly makes a coating onto the cotton fiber instead of forming the crosslinking structure among fibers (which would reduce the mobility of cellulose chains),^33,34 thereby leading to the increased tensile strength.

Table 4 The tensile strength of pure cotton fabric and cotton fabrics coated with 5, 10, and 20 bilayers

Sample	Tensile strength (N)
	Warp	Weft
CF0	111.4	105.4
CF5	110.2	105.8
CF10	112.5	106.4
CF20	113.8	107.5

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3.7 The possible flame retardant mechanism

It is believed that the possible flame retardant mechanism can be explained by the protective effect of MgAl-LDH filled coating. During the pyrolysis of coated cotton fabric, the MgAl-LDH can quickly form an inorganic protective layer on the surface of cotton fibers. This protective layer can act as a physical barrier to retard the transfer of heat, oxygen, and the diffusion of volatilized pyrolysis products between the combustion zone and underlying cotton fibers. Therefore, the reduction in PHRR and THR can be observed for coated cotton fabrics in MCC test. In addition, as a condensed-phase flame retardant coating, the physical layer can effectively hinder the release of toxic gases (CO) generated from cotton fabric, so the reduced amount of the toxic volatiles (CO) can also be observed (shown in Fig. 8). The possible mechanism of the improved flame retardant property for coated cotton fabric is shown in Fig. 9.

Fig. 9 The scheme of the possible flame retardant mechanism.

4. Conclusion

The layer-by-layer assembled coatings consisting of MgAl-LDH and alginate were successfully built on cotton fabrics to endow them with flame retardancy. By altering the bilayer number, the LbL coating loading was changed. The analysis by TGA suggested that the incorporation of MgAl-LDH filled coating could enhance the thermal stability of cotton fabric at high temperature range (400 to 700 °C). The MCC results indicated that all coated cotton fabrics show lower PHRR and THR relative to pure one, especially for the sample with 3.52 wt% coating, with 34.6% and 25.6% reduction in PHRR and THR, respectively. Furthermore, in the vertical flame test, the cotton fabric with 3.52 wt% coating could preserve almost 70% weave structure. Such an obvious improvement in flame retardancy could be ascribed to the protective effect from MgAl-LDH filled coating, which could form an inorganic physical barrier between the combustion zone and underlying cotton fibers.

Acknowledgements

The work was financially supported by the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG160608).

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