Flame Retardancy and Smoke Suppression of MgAl Layered Double

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The #ame retardancy and smoke suppression of MgAl LDHs containing P and Si in polyurethane elastomer Wenzong Xu, Baoling Xu, Aijiao Li, Xiaoling Wang, and Guisong Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b02708 • Publication Date (Web): 10 Oct 2016 Downloaded from http://pubs.acs.org on October 13, 2016

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Industrial & Engineering Chemistry Research

The flame retardancy and smoke suppression of MgAl LDHs containing P and Si in polyurethane elastomer Wenzong Xu, *,†,‡ Baoling Xu, † Aijiao Li, † Xiaoling Wang, † Guisong Wang † †

School of Materials Science and Chemical Engineering, Anhui Jianzhu University,

292 Ziyun Road, Hefei, Anhui 230601, People’s Republic of China ‡

State Key Lab of Fire Science, University of Science and Technology of China, Hefei,

Anhui 230026, People’s Republic of China Correspondence to: Wenzong Xu (*Tel./Fax:+86-0551-63828157.

Email: [email protected])

ABSTRACT: MgAl LDHs containing tripolyphosphate (P3O105-) (P-LDHs) was prepared by the anion exchange method; subsequently, aminopropyltriethoxysilane (APTS) was grafted onto the surface of P-LDHs to prepare S-MgAl LDHs (S-LDHs) through induced hydrolysis silylation. The dispersion, flame retardancy and smoke suppression of N-LDHs (MgAl LDHs with nitrate in its interlayer), P-LDHs and S-LDHs in polyurethane elastomer (PUE) were investigated. The X-ray diffraction (XRD) and Transmission electron microscopy (TEM) showed that S-LDHs could disperse uniformly in PUE. The cone calorimetry and smoke density test indicated that S-LDHs performed best in flame retardancy and smoke suppression. By analyzing the char layer with Laser Raman spectroscopy (LRS), Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), it could be concluded that, besides the catalytic charring effect of LDHs, the stable structure of -P(=O)-O-Cand -P(=O)-O-Si- could improve the thermal oxidative resistance and stabilize the char layer, resulting in better flame retardancy and smoke suppression. KEYWORDS:

MgAl

LDHs;

Tripolyphosphate;

Aminopropyltriethoxysilane;

Polyurethane elastomer; Flame retardancy; Smoke suppression

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1. INTRODUCTION Metal hydroxides as environmentally-friendly flame retardants have received much attention, and aluminium hydroxide and magnesium hydroxide are the most widely used inorganic flame retardants. Metal hydroxides act as flame retardants mainly by releasing water vapour through endothermic decomposition and leaving a thermal stable inorganic residue. However, typically at least 60% by mass of aluminium hydroxide or magnesium hydroxide is generally required to provide good flame retardancy in polymers, and aluminium hydroxide or magnesium hydroxide is extremely difficult to form intercalated or exfoliated nanocomposites due to its grain size and morphology.1,2 Therefore, traditional metal hydroxides as flame retardants are usually associated with low efficiency.3,4 Fortunately, a new green halogen-free flame retardant, layered double hydroxides (LDHs), has been found. LDHs that consist of positively charged layers of mixed metal hydroxides require the presence of interlayer anions to maintain overall charge neutrality. Generally, the typical formula of LDHs can be represented by [MII+1−xMIII+x(OH)2]x+[(An-)x/n•mH2O]x- where MII+ and MIII+ are divalent and trivalent metal ions, respectively, and An- denotes a valent anion.5-7 Compared with traditional metal hydroxides, the vast interlayer anion and its layered structure enable LDHs to exhibit greater flame retardancy in polymers.8,9 Therefore, LDHs has attracted much attention in recent years. Jiang et al.10 added MgAl LDHs into ethylene vinyl acetate (EVA) and the cone calorimeter test results showed that the heat release rate (HRR) of EVA composites decreased obviously, the MgAl LDHs significantly enhanced the flame retardancy of EVA. Wang et al.11 reported that 6 wt% MgAl LDHs decreased the peak heat release rate (pHRR) and total smoke production (TSP) values of epoxy resin (EP) composites by 62% and 45%, respectively, compared with those of neat EP. It is because LDHs nanofillers can improve the quality of char residue, which could effectively enhance the flame retardancy and smoke suppression of EP composites. In order to further improve the flame retardancy, various modified methods for LDHs have been studied recently. One of the most effectively modified methods is 2/41


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through intercalating flame retardant elements into the LDHs interlayer. Zhang et al.12 prepared dihydrogen phosphate anion-intercalated LDHs (M-LDHs) with an anion exchange procedure and incorporated M-LDHs into polypropylene (PP). The anti-flammability of PP composites was improved significantly after the addition of M-LDHs. It was suggested that a more compact char layer can be obtained during the process of combustion when dihydrogen phosphate was intercalated into LDHs interlayer. The compact char layer effectively prevented the heat transfer, and thus the flame retardancy of PP composites was improved significantly. Edenharter et al.13 synthesized a modified MgAl LDHs (PP-LDHs) through intercalating phenyl phosphate into MgAl LDHs interlayer. Then unmodified MgAl LDHs (UM-LDHs) and PP-LDHs were incorporated into polystyrene (PS) to form UM-LDHs/PS and PP-LDHs/PS composites, respectively. The flame retardancy of PS composites was tested by cone calorimetry, and the results showed that the pHRR of UM-LDHs/PS and PP-LDHs/PS were decreased by 22% and 47%, respectively, compared with pure PS. Obviously, the LDHs modified with phosphate performed higher flame retardancy. However, the satisfactory dispersion of LDHs in the polymer matrix is always difficult to achieve. A large amount of -OH on the surface of LDHs gives the surface of LDHs a hydrophilic characteristic, making it difficult for LDHs to disperse in polymer.14,15 In order to make LDHs more compatible with polymers, an appropriate interlamellar and surface modification of LDHs is necessary. Mallakpour et al.16 prepared L-aspartic acid containing dicarboxylic acid modified MgAl LDHs and the enhanced dispersion in polymer was confirmed. Manzi-Nshuti1 et al.17 modified LDHs with oleate and incorporated them into polymethyl methacrylate (PMMA); their results showed that the dispersion of LDHs in polymer was improved significantly and flame retardancy was enhanced greatly. Hu et al.18 prepared aminopropyltriethoxysilane grafted ZnAl LDHs and added it into polyaniline (PANI). Their study showed that grafted LDHs was exfoliated partly in the PANI matrix which was due to the interfacial interaction between PANI and grafted LDHs, and the 3/41



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dispersion was improved. In this study, tripolyphosphate was intercalated into MgAl LDHs by an anion exchange procedure to obtain P-MgAl LDHs, and subsequently APTS grafted LDHs (S-LDHs) was obtained through the induced hydrolysis silylation method. Furthermore, the dispersion, thermal stability, flame retardancy and smoke suppression of modified LDHs in PUE were investigated. Scheme 1 presents the modification process of LDHs.

2.

EXPERIMENTAL

2.1. Materials. Mg(NO3)2•6H2O was obtained from Shanghai Qiangshun Chemical Corporation. Al(NO3)3•9H2O was obtained from Tianjin Zhiyuan Chemical Corperation. Sodium tripolyphosphate (STPP), aminopropyltriethoxysilane were obtained from Aladdin Chemical Reagent Corperation. NaOH was obtained from Sinopharm Chemical Corperation. Toluene was kindly supplied by Jiangsu Qiangsheng Chemical Reagent Corperation. Toluene diisocyanate was obtained from Mitsui Chemical Corperation. Polyester polyol was kindly supplied by Shandong Dexin Chemical Corperation. 3,3'-Dichloro-4,4'-diaminodiphenylmethane (MOCA) was obtained from Jinan Haiwu Chemical Corperation.

2.2. Modification of LDHs. N–LDHs were synthesised by a traditional hydrothermal method.19 P3O105- intercalated LDHs were synthesised by anion exchange. Firstly, 1g N-LDHs was dispersed into 200 ml distilled water and stirred for 30 min at room temperature to obtain a homogeneous dispersion solution. Then 1.2g STPP was dispersed in 100 ml distilled water, and added to the above homogeneous dispersion solution, and the mixed solution was stirred for 4 h at room temperature. Finally, the suspension was centrifuged and washed with deionized water several times, and then dried in an oven at 60 °C for 10 h. Modified LDHs was thus obtained (P-LDHs). APTS was grafted onto P-LDHs through induced hydrolysis silylation. 1g P-LDHs was dispersed in 30 ml toluene solution and ultrasonicated for 15 min. Then the 4/41


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solution was poured into a three-necked flask with 100 ml toluene solution, and 3g APTS was added to the mixed solution at the same time. The mixed solution was heated to 60 °C and stirred for 6 h. Subsequently, the mixture was centrifuged and washed with toluene solution several times before being dried in an oven at 60 °C overnight. The product was referred to as S-LDHs.

2.3. Preparation of LDHs/PUE composites. LDHs/PUE composites were prepared by a two-step method.20 Firstly, the prepolymer with NCO group sealing side was prepared through the dehydration reaction between polyester diol and toluene diisocyanate. Then the prepolymer, MOCA and LDHs were mixed and stirred vigorously. The mixture was poured into a teflon mold, and heated at 80 °C for 6 h and 120 °C for 4 h in the oven to obtain LDHs/PUE composites. The formulation of LDHs/PUE composites is shown in Table 1.

2.4. Characterization. X-ray diffraction (XRD) patterns were obtained by using a Shimadzu XRD-6000 (Japan) diffractometer with monochromatic Cu Kα radiation (λ=0.15406 nm). Fourier transform infrared (FTIR) spectra were obtained from a Nicolet 6700 FTIR spectrophotometer (U.S.) by using a conventional KBr pellet method. X-ray photoelectron spectroscopy (XPS) was performed with a XSAM80 (Kratos, U.K.) using Al Ka excitation radiation (hν=1486.6 eV). Scanning electron microscopy (SEM) images were taken with a German Zeisse Ultra 55 electron microscope. The samples were placed on a copper plate and then coated with a conductive layer for observation. Thermogravimetric analysis (TGA) was operated on a German Netzsch 209 F1 thermal analyzer instrument under an air flow of 20 °C/min. The samples were heated from room temperature to 700 °C. Transmission electron microscopy (TEM) tests were performed with JEM 200CX (Japan) at an accelerating voltage of 100 KV. All samples were cut with an 5/41



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ultramicrotome. Laser Raman spectroscopy (LRS) of char was obtained by a SPEX-1403 laser Raman spectrometer (U.S.). Cone calorimeter combustion tests were carried out using a JCZ-2 cone calorimeter (China) in accordance with ISO5660. All samples (100×100×3 mm3) were wrapped in tinfoil and irradiated at a heat flux of 50 kW/m2. Smoke density tests were carried out using a JSC-2 smoke density test instrument (China) in accordance with ISO5659-2. All samples (75×75×2.5 mm3) were wrapped in tinfoil and exposed horizontally under an external heat flux of 25 kW/m2.

3.

RESULTS AND DISCUSSION

3.1. Characterization of LDHs 3.1.1. XRD characterization. Figure 1 shows the XRD patterns of N-LDHs, P-LDHs and S-LDHs. The peaks of (003), (006), (110) and (113) belong to diffraction peaks of typically nitrate LDHs.21 For N-LDHs, the basal diffraction peaks (003) at 2θ=9.9˚ and (006) at 2θ=19.8˚ are sharp and intense, and the diffraction peaks of (110) and (113) can be clearly seen. It suggests that N-LDHs possess a good crystalline phase and structural integrity. However, some impurity peaks can be found in XRD patterns which are marked by asterisk. It indicates that a small amount of impurities exist in the sample, and the impurity is γ-AlO(OH) (JCPDS File No. 21-1307) which forms at the early stage of the hydrothermal reaction.22,23 The intensity of (006) reflection in as synthesized MgAl LDHs is less than the (003) reflection while (006) reflection shows higher intensity after the modification. The reason for this is that the electron density of atoms in the LDHs interlayer increases after P3O105- is intercalated into the LDHs interlayer, and the strong electron density of atoms result in large X-ray scattering.24 According to the Bragg equation 2dsinθ=λ, the basal d-spacing of (003) is 8.9 Å. As for P-LDHs, the basal diffraction peaks of (003) and (006) shift to a lower diffraction angle. It can be calculated that the d-spacing of P-LDHs is 10.5 Å. The thickness of the N-LDHs layer board is 4.8 Å,25,26 and thus the interlayer spacing of N-LDHs and 6/41


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P-LDHs are 4.1 and 5.7 Å, respectively. It means that the interlayer spacing of P-LDHs is increased 1.6 Å after modification, which is increased 39% in comparsion with that of N-LDHs. The size of P3O105- is simulated by ChemBio 3D Ultra, and its maximum size is 8.4 Å. From the above data, it could be concluded that P3O105- anion is oriented at 43° angle with respect to the plane of the LDHs layer board (arcsinα=5.7 Å/8.4 Å, α=43°). From the above analysis, it could be concluded that P3O105- anions are intercalated into the interlayer of N-LDHs by anion exchange. Through grafting modification, the d-spacing of S-LDHs is unchanged, which indicates that APTS has been grafted onto the surface of S-LDHs.

3.1.2. FTIR characterization. Figure 2 reveals the FTIR spectra of STPP, APTS, N-LDHs, P-LDHs and S-LDHs. In the N-LDHs spectrum, the strong and broad absorption peak at about 3450 cm-1 corresponds to the O-H stretching vibration of LDHs and interlayer adsorbed water. The peak at 1627 cm-1 is attributed to the flexible vibration of O-H. The intense adsorption peak around 1382 cm-1 is attrbuted to the symmetric stretching mode of NO3-. In the spectrum of STPP, the absorption peak at 1219 cm-1 can be associated with the stretching vibration of P=O and the absorption peak at 1151cm-1 belongs to the stretching vibration of P-O. Similar to N-LDHs, the absorption peak at 3450 and 1627 cm-1 can be observed obviously in P-LDHs, but the absorption peak at 1382 cm-1 almost disappears. Meanwhile, new absorption peaks appear at 1219 and 1151 cm-1, indicating that NO3- is almost all replaced by P3O105-. And according to the XRD pattern, the diffraction peak of (003) moves to a low angle which means the interlayer of LDHs becomes larger. It could further confirm that P3O105- is intercalated into the interlayer of LDHs. The combination of XRD patterns and FTIR spectra could fully prove that P3O105- is intercalated into the interlayer of LDHs.27 Compared with P-LDHs, new absorption peaks appear around 2990-2860 cm-1 in S-LDHs that belong to the C-H stretching band of CH2 and CH3 groups. The CH2 and CH3 groups of APTS are introduced into S-LDHs, attributed to APTS grafted onto S-LDHs.28 All the analysis could confirm 7/41



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that P3O105- is intercalated into the interlayer of LDHs and APTS is grafted onto the surface of S-LDHs.

3.1.3. XPS characterization. Figure 3 shows the XPS spectra of N-LDHs, P-LDHs and S-LDHs. As can been seen in Figure 3(a), the peak values of binding energy at 75.0 eV and 1305.1 eV are Al2p and Mg1s, respectively. The N1s binding energy at 406.0 eV belongs to NO3-. In the spectrum of P-LDHs, the binding energy of N1s disappears and P2p (Figure 3(b, c)) appears at 133.5 eV, indicating that NO3- is substituted by P3O105-.29 The emerged binding energy of Si2p at 103.6 eV (Figure 3(d)) in S-LDHs implies the successful graft of ATPS onto the surface of S-LDHs. The results are consistent with the analysis of XRD and FTIR.

3.1.4. TG characterization. The TGA curves for N-LDHs, P-LDHs and S-LDHs under air condition are shown in Figure 4. For N-LDHs, the weight loss below 200 °C is mainly due to the loss of water, including adsorbed water and interlayer crystalline water in N-LDHs. The weight loss of N-LDHs above 200 °C is attributed to dehydroxylation and deionization of NO3-.30 Meanwhile the weight loss of P-LDHs and S-LDHs is higher than that of N-LDHs below 390 °C. It may be due to the fact that phosphate derivatives decomposed by P3O105- during heating possess strong dehydration performance.12,31 Compared with P-LDHs, the weight loss of S-LDHs is less below 350 °C. This is because that the water is consumed in the process of APTS grafting onto the S-LDHs surface and -OH groups are consumed when -Si-OH condensates with the S-LDHs surface to form -Si-O-Mg or -Si-O-Al.32 It is noted that the weight loss of S-LDHs is higher than that of P-LDHs above 350 °C, which is attributed to the decomposition of APTS. At 700 °C, the char yield of S-LDHs is 63.1%, and it is lower than that of P-LDHs, 65.2%, which could further confirm that APTS is grafted onto the S-LDHs surface.

3.1.5. SEM characterization. Figure 5(a), (b) and (c) present the SEM images of 8/41


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N-LDHs, P-LDHs and S-LDHs. As can be seen from the images, all LDHs exhibit uniform and hexagonal platelets, and their lateral size ranges from 100 to 200 nm. P-LDHs keep a similar crystallite shape and size to N-LDHs. Compared with N-LDHs and P-LDHs, the crystallite shape of S-LDHs has been destroyed to some extent, which may be due to the APTS grafting onto the surface of S-LDHs.

3.2. Characterization of LDHs/PUE composites 3.2.1. Morphology of LDHs/PUE composites. The XRD patterns of PUE composites with different LDHs additions can be observed in Figure 6. It is evident that all the PUE composites have a broad peak at 2θ=20.5°. This could be ascribed to the amorphous conformation of PUE.33 In the XRD patterns of PUE2 and PUE3, the basal diffraction peaks at 2θ=9.9˚ (003) and 2θ=19.8˚ (006) still remain, indicating that N-LDHs are not exfoliated in the composites and have poor compatibility with PUE.34 In regard to PUE5 and PUE6, the basal diffraction peak (003) at 2θ=8.3˚ means that P-LDHs remain unexfoliated in PUE. However, the basal diffraction peaks of (003) and (006) are absent in PUE8 and PUE9, and it can be concluded that S-LDHs are entirely exfoliated in PUE after the APTS grafting onto the surface of S-LDHs, and it is benefical to the dispersion of S-LDHs in the PUE matrix. TEM can intuitively display the disperse state of inorganic nanoparticles in polymer. The TEM images of PUE5 and PUE8 are shown in Figure 7. The obvious aggregation of P-LDHs can be observed from Figure 7(a) and their size ranges from 1 to 3 µm, indicating that the dispersion of P-LDHs in PUE is not homogenous. Compared with PUE5, the S-LDHs in PUE8 have no obvious aggregation and disperse well in PUE, and their size ranges from 200 to 500 nm. It is indicated that aggregation of LDHs can be decreased by the grafting of APTS which could improve the dispersion of LDHs in the polymer matrix. The results are consistent with the XRD pattern.

3.2.2. Thermal stability of LDHs/PUE composites. Thermogravimetric analysis is commonly utilized for the evaluation of the thermal decomposition property of 9/41



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polymers. The TG curves for PUE0, PUE3, PUE6 and PUE9 under air conditions are shown in Figure 8. Clearly, the LDHs/PUE composites show lower decomposition temperature in comparsion with pure PUE. It is attributed to the low decomposition temperature of LDHs and the catalytic effect of LDHs on PUE composites, resulting in low thermal stability of PUE composites at an early stage.35 However, it can be observed that the decomposition temperatures of PUE3, PUE6 and PUE9 are 337, 342 and 349 °C at T50 (T50 defined as the temperature at 50 wt% mass loss), respectively. Apparently, the T50 of PUE9 is the highest among PUE3, PUE6 and PUE9. Furthermore, the char residue yields of PUE3, PUE6 and PUE9 at 700 °C are 2.76, 5.05 and 6.93%, respectively, increased by 2.74, 5.03 and 6.91%, compared with that of PUE0 at 0.02%. Meanwhile, the calculated char yield of PUE3, PUE6 and PUE9 can be obtained through adding the corresponding char yield of 95wt% pure PUE and 5wt% N-LDHs, P-LDHs and S-LDHs under air conditions at 700 °C, and they are 2.61, 3.44, and 3.17%, respectively. The char yield of PUE3 is higher than the char yield of pure PUE. It is due mainly to the formation of mixed metallic oxides during the decomposition of LDHs and the barrier effect of the LDHs layer which could promote the char of PUE composites. In addition, the char yield of PUE6 is higher than that of PUE3, which is attributed to the P elememt that could further promote the formation of char of PUE composites. Obviously, the char yield of PUE9 is the highest. This may be because S-LDHs is modified by APTS so that it is well dispersed in PUE and the effective barrier can prevent the transmission of heat.36 Furthermore, the functions of P and Si could stabilize the char layer. It can also be seen obviously that the experimental char yield of PUE3, PUE6 and PUE9 are higher than the calculated char yield, respectively. It is due to the catalytic char effect of LDHs, and further the catalytic char effect of phosphorus could form condensed char layers during decomposition.37,38 Moreover, the functions of P and Si in S-LDHs contribute to the formation of a more compact and stable char layer.

3.2.3. Flame retardancy of LDHs/PUE composites. Figure 9(a), (b) and (c) show 10/41


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the heat release rate curves of pure PUE and LDHs/PUE composites tested by cone calorimetry. It can be seen that the pHRR of PUE1, PUE4 and PUE7 with 1wt% LDHs are 556, 523 and 461 kW/m2, respectively, decreased by 39%, 43% and 49% in comparison with PUE0, 913 kW/m2. The pHRR gradually decreases with the increased content of LDHs. When the loading of LDHs is increased to 5wt%, the pHRR of PUE3, PUE6 and PUE9 drop to 401, 389 and 267 kW/m2, respectively, decreased by 56%, 58% and 70%, compared with PUE0. The pHRR of LDHs/PUE is decreased remarkably with the addition of LDHs. This can be analyzed in the following three aspects: (1) The decomposition of LDHs is an endothermic process which can absorb heat and decrease the temperature in the process of combustion. (2) Combustible gas can be diluted by the H2O and non-flammable gas produced by the decomposition of LDHs. (3) Stable char residue can be formed on the surface of LDHs/PUE composites due to the catalytic charring effect of LDHs, which can prevent the diffusion of the flammable gas and the transmission of heat during combustion.38,39 To further study the effect of modified LDHs on flame retardancy of PUE, one more aminopropyltriethoxysilane alone grafted N-LDHs (G-LDHs) sample was prepared, and 3wt% G-LDHs was added into PUE to obtain G-LDHs/PUE composite. The HRR curves of PUE0, PUE2, PUE5, PUE8 and G-LDHs/PUE are shown in Figure 9(d). As seen from the HRR curves, the pHRR of PUE2 is 481 kW/m2, decreased by 47% in comparsion with PUE0, 913 kW/m2. Compared with PUE0, the pHRR of PUE5 and G-LDHs/PUE are 441 and 456 kW/m2, decreased by 52% and 50%, respectively. The P-LDHs and G-LDHs can both improve the flame retardancy of PUE, and the flame retardancy of P-LDHs is more apparent. It is noted that the pHRR of PUE8 is 375 kW/m2, decreased by 58% in comparsion with PUE0, indicating that the flame retardancy of S-LDHs is the best among N-LDHs, P-LDHs, G-LDHs and S-LDHs. It may be because S-LDHs possess better dispersion in the PUE matrix and the functions of P and Si in S-LDHs could faciliate the stability of char residue and increase the barrier effect.

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3.2.4. Smoke suppression of LDHs/ PUE composites. The smoke density curves of pure PUE and LDHs/PUE composites are shown in Figure 10, and the specific data are listed in Table 2. The Ds,10min and Ds,max are defined as smoke density at 10min and the maximum smoke density, respectively. It can be seen from Figure 10, when LDHs is added into PUE, the Ds,max and Ds,10min of LDHs/PUE composites are all decreased. When 1wt% LDHs is added to PUE, the Ds,max and Ds,10min of PUE1, PUE4 and PUE7 are decreased by 9, 12%; 12, 18% and 20, 29%, respectively, compared with PUE0. The Ds,max and Ds,10min are decreased gradually with more LDHs added into PUE. When the addition of LDHs is up to 5wt%, the Ds,max and Ds,10min of PUE3, PUE6 and PUE9 are decreased by 21, 28%; 26, 35% and 36, 52% in comparison with PUE0, respectively. It could be explained as follows: (1) The MgO and Al2O3 decomposed from LDHs can contribute to the production of a compact char layer during combustion, so the spread of smoke can be limited. (2) Smoke can be absorbed quickly by the metal oxide mixture due to its relatively large surface area.40 The effect of smoke suppression of P-LDHs is better than that of N-LDHs. It is because the P element possesses a catalyic charring effect and the good char layer could restrict the release of smoke. As for S-LDHs, the effect of smoke suppression is better than that of P-LDHs. It is attributed to the good dispersion of S-LDHs in PUE, which could provide a better barrier effect. Moreover, the function of P and Si in S-LDHs could contribute to the formation of a more compact and stable char layer.

3.3. Char residue analysis 3.3.1. LRS analysis of char residue. The Raman spectra of char residue of PUE0, PUE2, PUE5, PUE8 are shown in Figure 11. The first band (D band, 1360cm-1) is associated with the vibrations of carbon atoms with dangling bonds in the plane terminations of disordered graphite or glassy carbons. The latter peak (G band, 1580cm-1) corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline. The ratio of the intensity of the G and D bands (ID/IG) is generally used to assess the graphitization degree of char residue. The 12/41


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lower the value of ID/IG, the higher graphitization degree of char residue, which means the better flame retardant performance.41 The ID/IG ratio follows the order of PUE8 (3.31)< PUE5 (3.74)< PUE2 (4.23)< PUE0 (4.34). Obviously, the value of PUE8 is lowest. It is implied that PUE8 has the best flame retardancy, which is consistent with the results of the cone calorimeter test.

3.3.2. FTIR analysis of char residue. The FTIR spectra of the char residue are shown in Figure 12. Compared with the spectra of PUE0 and PUE2, a new absorption peak appears obviously around 1040 cm-1 in the spectrum of PUE5. It is attributed to the vibration absorption of PO2-. As for PUE8, the band located at 945cm-1 is assigned to the vibration absorption of -P(=O)-O-Si, the band at 1006 cm-1 to the vibration absorption of polyphosphate or SiO2, and the band at 1052 cm-1 to the vibration absorption of Si-O.42 The above illustrate that P and Si are involved in the formation process of char residue to stabilize the char layer, so as to improve the flame retardancy of S-LDHs/PUE composites.

3.3.3. XPS analysis of char residue. The char residue of PUE0, PUE2, PUE5 and PUE8 after the cone calorimeter test is investigated through XPS, as shown in Figure 13. In the C1s spectra, three characteristic peaks are observed at 284.7, 285.8 and 287.5 eV, ascribed to C-H and C-C in aliphatic and aromatic species, C-O in ether and/or hydroxyl groups and C=O (carboxylic groups), respectively. Generally, the thermal oxidative resistance of char is studied by Cox/Ca (Cox: oxidized carbons, Ca: aliphatic and aromatic carbons) values, and the thermal oxidative resistance is better when the Cox/Ca value is lower.43 As can be seen in Table 3, the Cox/Ca values of the PUE0, PUE2, PUE5 and PUE8 are 0.97, 0.93, 0.77 and 0.64, respectively. It shows that with the addition of LDHs, the thermal oxidative resistance of PUE composites is improved. Moreover, the thermal oxidative resistance of PUE5 and PUE8 is significantly improved, especially for PUE8, indicating that Si and P elements enhance the stability of char residue to provide better flame retardancy. N1s is also 13/41



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analyzed, and the peaks at 399, 400.3 and 402 eV are assigned to nitrogen functionalities in pyridinic (N-6) and inpyrrolic (N-5) groups, to quaternary nitrogen (N-Q), respectively. N-Q/(N-5+N-6) can be used to measure the condensation degree of the polyaromatic network: the higher the N-Q/(N-5+N-6) value, the more stable the char layer.44 The data listed in Table 3 show that the char residue of PUE8 contains more quaternary nitrogen in the graphene layers, suggesting that PUE8 forms a more stable char layer which has previously been proved in the LRS analysis. As for the O1s spectra, four bands located at 533.3, 531.7, 530.9 and 529.9 eV are attributed to C-O-C, Si-O-Si, M-O-M and P=O groups, respectively. Evidently, there are P=O and Si-O-Si groups in the char layer of PUE8, compared with those of other PUE composites, which is consistent with the FTIR analysis results. Furthermore, the results of the P2p spectra show that the -P(=O)-O-Si- structure is formed in the char layer of PUE8 in addition to the -P(=O)-O-C- structure, and the formation of -P(=O)-O-Si- linkages could increase the quantity of char.45 From all of the above analysis, it can be concluded that P and Si could stabilize the char layer and improve the flame retardancy.

4.

CONCLUSIONS

In this work, N-LDHs, P-LDHs and S-LDHs were synthesized by the hydrothermal, anion exchange and induced hydrolysis silylation method, respectively, and subsequently they were characterized by XRD, FTIR, XPS, TG and SEM. The results showed that tripolyphosphate was successfully intercalated into the MgAl LDHs interlayer and APTS was grafted onto the surface of MgAl LDHs. PUE composites with different loadings of N-LDHs, P-LDHs or S-LDHs were prepared by two step methods. The dispersion, thermal stability, flame retardancy and smoke suppression of different LDHs/PUE composites were investigated. Compared with N-LDHs and P-LDHs, S-LDHs showed a better dispersion in the PUE matrix. With the same loadings, P-LDHs and S-LDHs were proved to be excellent in flame retardancy and smoke suppression. The pHRR of PUE6 and PUE9 were decreased by 58% and 70%, 14/41


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compared with that of neat PUE. Ds,max and Ds,10min for PUE6 and PUE9 were decreased by 26%, 35% and 36%, 52%, respectively. It indicates that PUE9 performs best in flame retardancy due to the better dispersion of S-LDHs in the PUE matrix. Furthermore, the char residues were studied through LRS, FTIR and XPS to investigate the mechanism of flame retardancy and smoke suppression. The results showed that LDHs could facilitate the transformation of the char residue from disordered graphite to graphite crystalline. Meanwhile, the thermal oxidative resistance and the condensation degree of the polyaromatic network of the char layer were improved. As for S-LDHs, the -P(=O)-O-C- and -P(=O)-O-Si- structure formed in the combustion process could stabilize the char layer significantly, which could prevent the further burning and the release of smoke of the LDHs/PUE composites by cutting off oxygen and heat transmission.

ACKNOWLEDGEMENTS

The authors are grateful to the Research Fund for the Doctoral Program of Anhui Jianzhu University (2014) and National Key Technology R&D Program （2013BAJ01B05）for their financial support. REFERENCES (1) Hollingbery, L. A.; Hull, T. R. The fire retardant behaviour of huntite and hydromagnesite-A review. Polym. Degrad. Stab. 2010, 95, 2213. (2) Yang, J.; Liang, J. Z.; Tang, C. Y. Studies on melt flow properties during capillary extrusion of PP/Al(OH)3/Mg(OH)2 flame retardant composites. Polym. Test. 2009, 28, 907. (3) Wang, M.; Han, X. W.; Liu, L.; Zeng, X. F.; Zou, H. K.; Cheng, J. F. Transparent Aqueous Mg(OH)2 Nanodispersion for Transparent and Flexible Polymer Film with Enhanced Flame-Retardant Property. Ind. Eng. Chem. Res. 2015, 54, 12805. (4) Hewitt, F; Rhebat, D. E.; Witkowski, A.; Hull, T. R. An experimental and numerical model for the release of acetone from decomposing EVA containing aluminium, magnesium or calcium hydroxide fire retardants. Polym. Degrad. Stab. 15/41



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Scheme, Figures and Tables captions Scheme 1. Modification process of LDHs. Figure 1. XRD patterns of LDHs. Figure 2. FTIR spectra of APTS, STPP and LDHs. Figure 3. XPS spectra of LDHs (a), P2p (b, c) and Si2p (d). Figure 4. TG curves of N-LDHs, P-LDHs and S-LDHs. Figure 5. SEM of N-LDHs [(a)×100000], P-LDHs [(b)×100000] and S-LDHs [(c)×100000]. Figure 6. XRD patterns of neat PUE and LDHs/PUE composites. Figure 7. TEM images of PUE5 (a) and PUE8 (b) Figure 8. TG curves of neat PUE and LDHs/PUE composites. Figure 9. HRR (a,b,c,d) curves of neat PUE and LDHs/PUE composites. Figure 10. Smoke density (a,b,c) curves of neat PUE and LDHs/PUE composites. Figure 11. Laser Raman spectra of the char residue of neat PUE and LDHs/PUE composites. Figure 12. FTIR of the char residue for neat PUE and LDHs/PUE composites. Figure 13. XPS spectra (C1s, N1s, O1s and P2p) of the char residue of LDHs/PUE composites. Table 1. Formulation of neat PUE and LDHs/PUE composites. Table 2. Table 2 Data of smoke density of neat PUE and LDHs/PUE composites. Table 3. Results of C1s and N1s XPS of char residue of LDHs/PUE composites.

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Scheme 1. Modification process of LDHs.

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Figure 1. XRD patterns of LDHs.

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Figure 2. FTIR spectra of APTS, STPP and LDHs.

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Figure 3. XPS spectra of LDHs (a), P2p (b, c) and Si2p (d).

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Figure 4. TG curves of N-LDHs, P-LDHs and S-LDHs.

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Figure 5. SEM of N-LDHs [(a)×100000], P-LDHs [(b)×100000] and S-LDHs [(c)×100000].

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Figure 6. XRD patterns of neat PUE and PUE/LDHs composites.

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Figure 7. TEM images of PUE5 (a) and PUE8 (b)

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Figure 8. TG curves of neat PUE and PUE/LDHs composites.

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Figure 9. HRR (a,b,c,d) curves of neat PUE and PUE/LDHs composites.

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Figure 10. Smoke density (a,b,c) curves of neat PUE and PUE/LDHs composites.

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Figure 11. Laser Raman spectra of the char residue of neat PUE and PUE/LDHs composites.

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Figure 12. FTIR of the char residue of neat PUE and PUE/LDHs composites.

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Figure 13. XPS spectra (C1s, N1s, O1s and P2p) of the char residue of PUE/LDHs composites.

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Table 1. Formulation of neat PUE and PUE/LDHs composites LDHs Samples

Prepolymer

MOCA

(g)

(g)

PUE0

45.31

PUE1

N-LDHs

contents

P-LDHs

S-LDHs

(g)

(g)

(g)

(wt%)

4.69

0

0

0

0

44.86

4.64

0.5

0

0

1

PUE2

43.96

4.54

1.5

0

0

3

PUE3

43.05

4.45

2.5

0

0

5

PUE4

44.86

4.64

0

0.5

0

1

PUE5

43.96

4.54

0

1.5

0

3

PUE6

43.05

4.45

0

2.5

0

5

PUE7

44.86

4.64

0

0

0.5

1

PUE8

43.96

4.54

0

0

1.5

3

PUE9

43.05

4.45

0

0

2.5

5

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Table 2. Data of smoke density of neat PUE and PUE/LDHs composites Sample

Ds,max

D10min

Sample

Ds,max

D10min

PUE0

495

455

/

/

/

PUE1

449

401

/

/

/

PUE2

430

386

/

/

/

PUE3

390

328

/

/

/

PUE4

435

372

PUE7

396

322

PUE5

380

319

PUE8

357

304

PUE6

365

296

PUE9

314

217

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Table 3. Results of C1s and N1s XPS of char residue of PUE/LDHs composites Element

PUE0

PUE2

PUE5

PUE8

C-C（284.7ev）

0.51

0.52

0.57

0.61

C-O（285.8ev）

0.33

0.28

0.28

0.29

C=O (287.5ev)

0.16

0.20

0.15

0.10

0.97

0.93

0.77

0.64

N-6（399.0ev）

0.36

0.37

0.44

0.35

N-5（400.3ev）

0.47

0.44

0.35

0.42

N-Q（402.0ev）

0.17

0.19

0.21

0.23

N-Q/（N-5+N-6）

0.21

0.23

0.27

0.30

Cox/Ca

40/41


Page 41 of 41

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Abstract graphic:

41/41


Flame Retardancy and Smoke Suppression of MgAl Layered Double

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