Layer-by-Layer Assembly of Multifunctional Flame ... - ACS Publications

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Layer-by-Layer Assembly of Multifunctional Flame Retardant Based on Brucite, 3‑Aminopropyltriethoxysilane, and Alginate and Its Applications in Ethylene-Vinyl Acetate Resin Yiliang Wang, Xiaomei Yang, Hui Peng, Fang Wang, Xiu Liu, Yunguo Yang, and Jianwei Hao* National Engineering Technology Research Center of Flame Retardant Materials, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China S Supporting Information *

ABSTRACT: An efficient and multifunctional brucite/ 3-aminopropyltriethoxysilane (APTES)/nickel alginate/APTES (B/A/Nia/A) hybrid flame retardant was fabricated via the layer-by-layer assembly technique with brucite, silane coupling agents, nickel chloride, and sodium alginate. The morphology, chemical composition, and structure of the hybrid flame retardant were characterized. The results confirmed the multilayer structure and indicated that the assembled driving forces were electrostatic interactions, dehydration condensation, hydrogen bonds, and coordination bonds. When used in ethylene-vinyl acetate (EVA) resin, the multifunctional flame retardant had better performance than brucite in improving the flame retardancy, smoke suppression, and mechanical properties. With 130 phr loading, the multifunctional flame retardant achieved a limiting oxygen index value of 32.3% and a UL 94 V-0 rating, whereas the brucite achieved only 31.1% and a V-2 rating, respectively. The peak heat release rate and total heat released decreased by 41.5% and 8.9%, respectively. The multifunctional flame retardant had an excellent performance in reducing the smoke, CO, and CO2 production rates. These improvements could be attributed to the catalyzing carbonization of nickel compounds and the formation of more protective char layers. Moreover, the elongation at break increased by 97.5%, which benefited from the improved compatibility and the sacrificial bonds in the nickel alginate. The mechanism of flame retardant, smoke suppression, and toughening is proposed. KEYWORDS: brucite, nickel alginate, flame retardant, smoke suppression, layer-by-layer assembly

1. INTRODUCTION Recently, halogen-free flame retardant (HFFR) polymeric materials with low emission of poisonous gases have become an important trend in modern industries. Mineral flame retardants comprise the most important segment in the market of HFFR because they are environmentally friendly, nontoxic, and inexpensive and because they exhibit good smoke suppression and no corrosion.1 Natural brucite is a unique mineral that contains more than 94 wt % of magnesium hydroxide (MH), which can be used as an HFFR directly. It is known that brucite can decompose endothermically and release water to cool the combustion zone.2 The vaporous water can dilute the concentration of combustible gases, the active MgO layers provide heat shielding and a smoke suppression effect, inhibiting the liberation of flammable volatile organic compounds into the gas phase. However, the drawbacks of brucite are its irregular shape, the broad particle-size distribution, and the high loading (more than 60 wt %) required to achieve the desired flame retardant efficiency. These lead to poor dispersion of the particles, negatively affecting the interfacial interaction between the flame retardant and substrates. Generally, surface modification with low-molecular weight coupling agents could enhance the interfacial interaction,3 but © XXXX American Chemical Society

the coupling agents have a tendency to migrate out from the interfaces. Using other types of flame retardants with brucite can improve its efficiency,4−6 but the dispersion and mechanical properties remain poor. How to integrate advantages and avoid disadvantages has confused materials scientists for a long time. Recently, a simple and environmentally friendly layer-by-layer (LbL) assembly technique has supplied a viable solution. The LbL assembly technique consists of a multistep deposition process driven by some interactions of different polyelectrolytes and/or nanoparticles,7 such as electrostatic interaction,8 hydrogen bonds,9 and covalent bonds.10 Using this technique, several flame retardant thin films can be formed on the polymer material surface acting as a barrier, which either reradiates heat and/or slows down heat transmission and volatiles diffusion without affecting the product bulk properties.11 However, up to now, most applications of the LbL assembly technique have been related to depositing special flame retardants on substrates with high surface-to-bulk ratios, such Received: January 25, 2016 Accepted: March 22, 2016

A

DOI: 10.1021/acsami.6b00998 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces as fabric,12,13 foam,14,15 and thin films,16 in the field of flameretardant polymers. Very few studies have focused on the fabrication of layer-by-layer-assembled flame retardants. In the present work, the LbL assembly technique was employed for the first time to deposit 3-aminopropyltriethoxysilane (APTES) and alginate onto brucite, thus enhancing the efficiency and interfacial interaction of brucite with polymers. Sodium alginate (Sa), an anionic polysaccharide extracted from marine algae, is one of the most popular materials used in LbL-based microcapsules.17−19 It is composed of linear chains of a-L-guluronic acid (G) and b-D-mannuronic acid (M). Divalent and trivalent metal ions such as calcium, barium, and nickel can bind preferentially to the G-blocks in a highly cooperative manner, forming strong and rigidly ordered structures, the hydrogel.20 It is commonly acknowledged that a Ni catalyst (originating from nickel compounds) can catalyze the dehydrogenation and aromatization of intermediate compounds (produced by the degradation of polymer) into carbonaceous protective layers.21,22 APTES is a hydrolyzable organosilane compound containing a NH2 group. The hydrolyzed APTES can react with the hydroxy of brucite and alginate through dehydration to form a protective silicon layer.4 Natural brucite is found primarily in China, the United States, Canada, and Russia, and according to statistics, the world’s proven reserves are more than 5.29 × 107 t.23 Natural brucite can be used as an HFFR, replacing synthesized magnesium hydroxide. Many varieties of brucite flame retardants, such as Hydrofy and Magseeds, have already been developed. Nevertheless, most of them are just modified with different coupling agents or surfactants, which cannot overcome the above disadvantages.24,25 In this work, an efficient and multifunctional brucite (B/A/Nia/A) flame retardant was fabricated via the LbL assembly method with brucite, APTES, sodium alginate, and nickel chloride. The assembled driving forces and interfacial interactions were studied. The flame retardant and smoke suppression properties, mechanical properties, and compatibility of the EVA composites were measured. The results affirmed this novel strategy in preparing an efficient, multifunctional, structurally controllable, and environmentally friendly flame retardant.

Scheme 1. Schematic Illustration of the Synthesis Process of B/A/Nia/A

reaction, the system was kept vigorous stirring (300 rpm) at 30 °C for other 1 h. Afterward, 9.5 g nickel chloride aqueous solution (0.05 mol/L) was added dropwise into the system, kept the reaction conditions same for another 1 h. The product (Brucite/APTES/Nickel alginate, B/A/Nia) was immediately filtered and washed with deionized water several times until no Cl− was detected by 0.5 mol/L AgNO3 aqueous solution, ensuring the product was pure. After drying and grinding, the product was modified by APTES as described in section 2.2. Finally, the obtained powder was dried under 100 °C for 10 h in a drying oven, yielding a new brucite flame retardant (brucite/APTES/nickel alginate/APTES, B/A/Nia/A) modified by APTES and nickel alginate (Nia). 2.4. Preparation of EVA+Flame Retardant Composites. The B/A and B/A/Nia/A were used as flame retardants (FR) for poly(ethylene-vinyl acetate) copolymers (EVA). The EVA+FR composites were prepared by twin-roll mixer at 120 °C for 10 min. The sheets with different thickness were prepared by compression molding under a pressure of 10 MPa at 120 °C for 10 min. 2.5. Characterization. The morphology of the FR particles was characterized by field emission scanning electron microscopy (FESEM, FEI Quanta x50) and transmission electron microscopy (TEM, FEI Tecnai G2 F30). The cryofractured sections, the etching sections, and the char residue of EVA+FR composites were characterized by FESEM. The etching sections were prepared by etching the cryofractured samples with hydrochloric acid for 2 h to remove the exposed inorganic particles. The char residue was obtained from the cone calorimeter tests. The elements in the samples were examined by energy dispersive X-ray spectroscopy (EDS) measurements in SEM and X fluorescence spectrometer (XRF, Shimadzu XRF-1800). The interactions between these elements were characterized by X-ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM) at 25 W under a vacuum lower than 10−6Pa. The crystal structures of the FR were analyzed by X-ray diffraction (XRD), which was performed with a rotating anode X-ray diffractometer (Japan Rigaku D/Max-Ra) equipped with a Cu Kα (λ = 0.1542 nm) radiation at 2θ values ranging from 10° to 80°. Thermogravimetric analysis (TGA) was performed with a thermal analyzer (Netzsch 209 F1) at a heating rate of 20 °C/min from 40 to 800 °C. Fourier transform infrared (FTIR) spectroscopy was performed with an ATR IR spectrometer (Nicolet 6700) in detection mode and the spectra were collected at 32 scans with a spectral resolution of 4 cm−1. Limiting oxygen index (LOI) was measured by a FTA-II instrument (Rheometric Scientific Ltd.) with specimen dimensions of 130 × 6.5 × 3 mm3, according to ASTM D 2863-08. The UL 94 vertical burning test was carried out using a CZF-3 instrument (Jiangning Analysis Instrument Factory) with specimen dimensions of 125 × 12.5 × 3.2 mm3, according to GB/T 2408−2008. The cone calorimeter measurement was performed with a fire testing technology apparatus (FTT) under 50 kW/m2 external radiant heat flux conforming to ISO 5660 protocol. The specimen dimension is 100 × 100 × 3 mm3. The tensile properties were conducted at room temperature on a universal testing machine (CMT-410 4, MTS Systems (China) Co., Ltd.) with a cross-head speed of 50 mm/min following the ASTM-D412 standard.

2. EXPERIMENTAL SECTION 2.1. Materials. The brucite powder (2500 mesh) was a commercial product from Yinkou Deer Technology New Material Co., Ltd. (China). The sodium alginate (Sa, (C6H7NaO6)n, chemical grade) was purchased from Tianjin Guangfu Fine Chemical Research Institute and its viscosity range from 1.05 Pa·s to 1.15 Pa·s. The coupling agent (3-aminopropyltriethoxysilane, APTES) was supplied by Diamondchem Co., Ltd. The nickel chloride hexahydrate (NiCl2·6H2O, analytical grade) was purchased from Shantou Xilong Chemical Factory Guangdong. Poly(ethylene-vinyl acetate) copolymers (Elvax, 28 wt % of vinyl acetate) was supplied as pellets by DuPont company. 2.2. Preparation of Brucite/APTES. First, 100 g of brucite powder and 300 mL of absolute ethyl alcohol were mixed by vigorous stirring (200 rpm) at 30 °C for 5 min. The brucite absorbs water easily. Then, 3 g of APTES was dropped into the mixture, and vigorous stirring (200 rpm) was applied during this procedure for 10 min. The obtained product was dried under 100 °C for 10 h in a drying oven. The obtained white powder was the surface-modified brucite with APTES (brucite/ APTES, B/A). 2.3. Preparation of Brucite/APTES/Nickel Alginate/APTES. In the typical synthesis process (Scheme 1), 100 g of B/A powder was suspended in 300 mL of deionized water by vigorous stirring (300 rpm). Meanwhile, the slurry was heated in a water bath at 30 °C for 1 h. Then, a 1.4 L sodium alginate aqueous solution (5 g/L) was added to the

3. RESULTS AND DISCUSSION 3.1. Morphology of the Flame Retardant. TEM images of brucite and B/A/Nia/A particles are shown in Figure 1 and Figure S1. The brucite particle-size distribution was broad and B


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with Si−O−H, forming Si−O−Si, or it was free,28 making it possible to form hydrogen bonds with Sa. 3.2.2. Structure of Sa/APTES. Figure 3 shows the Fourier transform infrared (FTIR) spectra of Sa, Sa/APTES, and

Figure 1. TEM images of the brucite particles (a,b) and the B/A/Nia/A particles (c,d).

the diameter was lower than 20 μm (Figure S2a). Most brucite particles were irregular in shape owing to the physical grinding. It is obvious that the B/A/Nia/A particles have a different morphology from the brucite particles. Figure 1a,b show the regular lamellar structures of brucite, whereas Figure 1c shows that the lamellar structures are coated by some floccules as a result of the high and variable molecular weight of alginate. Also, the nickel alginate hydrogel agglutinated with some small, irregular brucite particles, as shown in Figure 1d. The floccules and agglutination indicated the formation of amorphous nickel alginate. 3.2. Chemical Compositions and Structural Characterization of Flame Retardants. 3.2.1. Structure of Brucite/ APTES. The reactions between brucite and APTES were characterized by X-ray photoelectron spectroscopy (XPS). The XPS pattern of the O 1s spectra is displayed in Figure 2. In Figure 2a, the peak at 531.9 eV indicates −OH on the pristine brucite surfaces, which was caused by incomplete dehydration and broken bonds or crystalline imperfections. After modification by APTES, the intensity of the −OH peak decreased and the binding states of O changed, as shown in Figure 2b. The three new peaks at 531.5, 532.3, and 532.9 eV are attributed to the binding energies of Mg−O−Si, Si−O−Si, and Si−O−H, which proved that layered APTES was coated on the surface of brucite.26,27 Si−O−H arose from the hydrolysis of APTES, which could form Mg−O−Si with the −OH of the brucite. The other Si−O−H either had a dehydration condensation reaction

Figure 3. FTIR spectra of Sa (a), Sa/APTES (b), and APTES (c).

APTES. Sa/APTES was made from Sa, APTES, and deionized water after vigorous stirring at 30 °C. More detailed absorption band assignments for Sa (Figure 3a) and APTES (Figure 3c) are shown in Table S1.29−32 In Sa/APTES (Figure 3b), the N−H and COO− vibrations changed into three new peaks at approximately 1564, 1479, and 1299 cm−1 (amide I, II, and III, respectively), indicating that the carboxylate groups of Sa were attracted to the oppositely charged −NH3+ groups of hydrolyzed APTES. The −CH3 peak (2975 cm−1) and Si−OC2H5 peak (1104 cm−1) disappeared because of the hydrolyzation of APTES.28,33−36 Also, the stretching vibration peak of −OH (at approximately 3260 cm−1 in Sa) weakened and a broader band appeared at 3300−2000 cm−1, which was caused by the dehydration condensation reactions between the Si−OH of hydrolyzed APTES and the −OH groups of alginate. The most prominent peaks for the Sa/APTES spectrum appeared at 950 and 1250 cm−1, where Si−O−Si and Si−O−C modes have been reported.37 The changes of several peaks (C−C, C−O) at approximately 1000 cm−1 could reflect the conformational changes of the alginate chains caused by electrostatic interactions and hydrogen bonding.38 The spectral changes evidently demonstrate that Sa could react with APTES.

Figure 2. O 1s XPS spectra of brucite surfaces before (a) and after (b) modified with APTES. C


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ACS Applied Materials & Interfaces 3.2.3. Structure of Nia. The reaction between Ni2+ and sodium alginate, which resulted in the formation of a nickel alginate hydrogel, was analyzed by FTIR. The spectra of alginic acid (AD), Sa, and Nia are displayed in Figure 4. In Figure 4c,

approximately 856 and 645 eV in B/A/Nia and B/A/Nia/A, respectively, can be attributed to Ni 2p3/2 and Ni LMM, affirming that nickel alginate was well attached on the surfaces of the brucite.43 Besides, the bulk content of Ni was characterized by XRF, as shown in Table 1. After analysis and calculation, the contents of nickel alginate in B/A/Nia and B/A/Nia/A were 8.63 and 8.43 wt %, respectively.44 3.2.5. Mechanism of the Synthesis. On the basis of the above conclusions, the assembled driving forces are thought to be dehydration condensation, electrostatic interactions, hydrogen bonds, and coordination bonds. Through these driving forces, it is possible to control the surface microstructure when flame retardants are synthesized. The synthesis mechanism of the LbL-assembled B/A/Nia/A is illustrated in Scheme 2. 3.3. Thermal Stability of the Flame Retardants. The thermal degradation of B/A/Nia/A and brucite was analyzed by thermogravimetric analysis (TGA). As revealed in Figure 6a, the thermal decomposition of B/A/Nia/A mainly took place in two steps, which was also the case with brucite. In the temperature range 100−350 °C, B/A/Nia/A showed greater weight loss, which was mainly caused by the destruction of glycosidic bonds in the nickel alginate. In the range 350 °C−410 °C, the glycosidic bonds were further destroyed and most Mg(OH)2 decomposed into MgO.45,46 The second step (410 °C−700 °C) could be ascribed to the decomposition of dolomite [CaMg(CO3)2], which is the primary impurity in brucite. It is clear that nickel alginate decreased the thermal stability of B/A/Nia/A. However, the derivative thermogravimetric (DTG) curves in Figure 6b show that the derivative weight peak of B/A/Nia/A (395 °C) was higher than that of brucite (390 °C), even though the weight of the residue was lower. 3.4. Morphology of EVA+FR Composites. The morphology of EVA+FR composites is shown in Figure 7. From Figure 7a,b, it was found that B/A/Nia/A particles with much less exposed surfaces blended in EVA better than the B/A particles. In the cryofractured sections etched by hydrochloric acid (Figure 7c,d), it is obvious that the EVA+B/A/Nia/A composites had more partly opened or unopened rough holes, which indicated better interfacial interactions or compatibility between the B/A/Nia/A particles and the matrix. The evenly distributed holes were the most compelling evidence for the good dispersion of B/A/Nia/A particles in EVA. Overall, the strategy improved the dispersibility of brucite particles and had a good effect on the interfacial interactions of the EVA+FR composites. 3.5. Mechanical Properties of EVA+FR Composites. The mechanical properties of the EVA+FR composites are listed in Table 2. When the FR content exceeded 110 phr, the tensile strength (TS) of the EVA+B/A composites was higher than EVA+B/A/Nia/A composites. Also, the TS of the EVA+B/A composites increased considerably with the addition of brucite particles. These phenomena were caused by the unique lamellar nanostructure, which was integrated into the EVA matrix and increased the contact area.47 As for the fracture toughness, the EVA+B/A/Nia/A composites did better. For example, the elongation at break (EB) values of A-3 and B-3 were 121% and 252%, respectively. The mechanical properties are usually related to the compatibility and interfacial interactions. At the interface between B/A/Nia/A and EVA, stiff and brittle nickel alginate chains could form an interpenetrating polymer network (IPN) structure with ductile EVA chains. The ductile polymer network could sustain large deformation, whereas the brittle network could break into small clusters to efficiently disperse the stress

Figure 4. FTIR spectra of AD (a), Sa (b), and Nia (c).

some peaks (1000−1150 cm−1) shift to lower frequencies, which indicated weakening bonds. The COO− peaks at 1589 and 1409 cm−1 (Sa) broaden and shift to 1592 and 1403 cm−1 (Nia), respectively.29,39 These peaks changed not only in terms of their frequencies but also their shapes, which were caused by the coordination bonds between Ni2+, −OH, and −COO−. Actually, the COO− stretching vibration peak was specific to ionic and the peaks of the nickel alginate hydrogel were 1592 and 1403 cm−1.40 These results are in agreement with the transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images. 3.2.4. Chemical Compositions of Flame Retardants. The chemical compositions of brucite, B/A, B/A/Nia, and B/A/ Nia/A were carefully examined by XPS. The results are shown in Figure 5 and Table 1. The main elements of brucite and modified

Figure 5. XPS spectra of B/A/Nia/A, B/A, B/A/Nia, and brucite.

brucite were C, O, Mg, Si, and Ca. The changes of surface atomic concentration in the XPS test results had reflected the LBL assembly progress of the different layers. The binding energy of these elements are shown in Figure 5.41,42 The new peaks at D


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ACS Applied Materials & Interfaces Table 1. Chemical Components of Brucite, B/A, B/A/Nia, and B/A/Nia/A XPS (%)

XRF (wt %)

sample

C

N

O

Mg

Si

Ca

Ni

Ni

Nia

brucite B/A B/A/Nia B/A/Nia/A

28.71 29.39 47.96 52.49

0 1.12 0.96 1.61

45.31 43.29 34.20 31.19

18.79 18.54 10.92 9.01

0.59 1.58 0.76 1.68

6.60 6.08 3.67 2.82

0 0 1.53 1.20

0 0 1.24 1.21

0 0 8.63 8.43

Scheme 2. Schematic Illustration of the Synthesis Mechanism of B/A/Nia/A

Figure 6. TGA (a) and DTG (b) curves of brucite, B/A/Nia/A in the nitrogen atmosphere.

the water vapor arising from the decomposition could dilute combustible gases. The catalysis played a crucial role in promoting more stable and compact char layers. In order to further evaluate the flame retardancy of the EVA+FR composites, cone calorimeter testing was carried out. The cone calorimeter is frequently considered to be the best characterization method for a full-scale flame by evaluating parameters such as heat release rate (HRR), peak heat release rate (PHRR), total heat released (THR), smoke production rate (SPR), and time to ignition (TTI). HRR and THR results for EVA+FR composites are displayed in Figure 8. EVA burned out rapidly within 300 s after ignition, and all the flame retardants reduced the heat release of the EVA composites. A-1, A-2, and A-3 had two obvious HRR peaks, where the second peak was higher and sharper, which indicated a violent combustion over a short period of time. Indeed, the phenomenon could be ascribed to the thin and fragile MgO layers with many cracks.47 The HRR curves of B-1 and B-2 had two lower gentle peaks, which prolonged the combustion times to 360 and 430 s, respectively. Only one low peak was found for

around the crack tip into the surrounding damage zone. The polymer network could also supply many sacrificial bonds that could break and dissipate energy before the material fails.48−50 The toughening mechanism is shown in Scheme 3. 3.6. Flame Retardancy and Smoke Suppression of the EVA+FR Composites. 3.6.1. Flame Retardancy of the EVA +FR Composites. LOI and UL 94 were used to determine the flame retardancy of EVA+FR composites and the results are listed in Table 2. It was found that brucite and the modified products were efficient for improving the flame retardancy of EVA. The LOI for the EVA+FR composites improved with the increasing amounts of FR and the EVA+B/A/Nia/A composites did even better. B-3 with 130 phr of B/A/Nia/A had the highest LOI value (32.3%), which was 10.7% more than that of EVA. The composites with low FR loading failed to pass the UL 94 V-2 rating for melt dripping. When the addition of B/A/Nia/A increased to 130 phr, the EVA+B/A/Nia/A composite passed the V-0 rating. These improvements might be attributed to the good dispersion of B/A/Nia/A, the endothermic decomposition of Mg(OH)2, and the catalysis of nickel alginate. In addition, E


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Scheme 3. Schematic Illustration of the Sacrificial BondBreaking Mechanism in EVA+B/A/Nia/A Composites

of significant importance to reduce the smoke toxicity of polymers. According to past research, most visible smoke is mainly composed of poisonous intermediate aromatic compounds and benzene derivatives, which could be catalyzed into more stable carbon materials or polyaromatic species by a Ni catalyst. The catalyzing carbonization is confirmed by a decrease of the smoke produce rate (SPR). As shown in Figure 9a, the peak SPR value of B-3 (0.013 m2/s) decreased significantly as compared to that of A-3 (0.039 m2/s). The dynamic curves of the CO production rate (COP) and CO2 production rate (CO2P) were similar, which is thought to be due to the catalytic properties of the nickel compounds and active MgO.47 More detailed data for smoke suppression is found in Table 4. In Figure 9b and Table 4, the char residue weight of B-3 (48.4 wt %) was higher than that of all other samples. Additionally, the residual mass of B/A/Nia/A with nickel alginate was lower than that of B/A, as shown in Figure 6. Therefore, the nickel alginate must have catalyzed the carbonization of the burning EVA. To further evaluate the catalytic characteristics of the nickel alginate, analysis of the chemical composition and morphological analysis of the char residue was necessary. 3.7. Chemical Compositions and Morphology of the Char. 3.7.1. Chemical Composition of the Char. Figure 10 shows the XRD patterns of the residual char of nickel alginate (Nia, Figure 10a) and the EVA+B/A/Nia/A composition (B-3, Figure 10b). The main components of brucite were Mg(OH)2 and CaMg(CO3)2. After the cone calorimeter test, Mg(OH)2 decomposed into MgO and some dolomite [CaMg(CO3)2] remained that was decomposed at a higher temperature,53 as shown in Figure 10b. No XRD patterns of nickel compounds were observed because of the strong diffraction of MgO and the similarity between MgO and nickel oxide. Therefore, the XRD patterns of the nickel alginate char subjected to the same combustion environment was tested. As shown in Figure 10a, the main characteristic peaks belonged to NiO (JCPDS 44-1159) and NiC (JCPDS 14-0020), which indicated that nickel alginate was converted into nickel oxide and nickel carbide.

Figure 7. SEM images of cryofractured sections: EVA+B/A, A-3 (a); EVA+B/A/Nia/A, B-3 (b); A-3 etched by hydrochloric acid (c); and B-3 etched by hydrochloric acid (d).

B-3. In addition, with equivalent FR loading, the PHRR values of the EVA+B/A/Nia/A composites were much lower than those of the EVA+B/A composites. For example, the PHRR of B-3 (at 100 s) was 152.5 kW/m2 less than the 260.6 kW/m2 of A-3 (the second peak, at 315 s). The THR curves had a similar regularity, as shown in Figure 8b. These results suggest that more effective and protective char layers formed on the burning surfaces of the EVA+B/A/Nia/A composites. The char layers could act as an insulating barrier, preventing oxygen diffusion and feedback of heat from reaching the underlying material.51 Meanwhile, the barrier could inhibit the liberation of flammable volatile organic compounds into the gas phase. The condensed phase effect of the additives was mainly responsible for the reduction of HRR and THR. Table 3 shows some additional detailed cone calorimeter data for the compositions. According to the TG test, B/A/Nia/A had a lower thermal stability than B/A, which was caused by the low thermal degradation temperature of alginate.45,46,52 However, the TTI values of the EVA+B/A/Nia/A composites were higher than those of the EVA+B/A composites, which indicated that the poor thermal stability of nickel alginate did not deteriorate the TTI. We can conclude that the good dispersion and catalysis of B/A/Nia/A were beneficial for extending TTI. 3.6.2. Smoke Suppression of the EVA+FR Composites. The vast majority of fire fatalities are attributable to the lethal atmosphere resulting from the combustion of polymers. Therefore, it is

Table 2. Formulation, Combustion, and Mechanical Properties Test Results of the EVA+FR Composites composition (phr)a

a

sample

EVA

B/A

B/A/Nia/A

LOI (%)

EVA A-1 A-2 A-3 B-1 B-2 B-3

100 100 100 100 100 100 100

0 90 110 130 0 0 0

0 0 0 0 90 110 130

21.6 ± 0.2 25.7 ± 0.2 28.4 ± 0.1 31.1 ± 0.1 28.1 ± 0.1 29.2 ± 0.1 32.3 ± 0.1

ΔLOI (%)

UL 94 3.2 (mm)

TS (MPa)

EB (%)

4.1 6.8 9.5 6.5 7.6 10.7

NR NR NR V-2 NR. V-2 V-0

18.1 ± 1.2 7.2 ± 0.2 8.3 ± 0.3 9.1 ± 0.3 8.6 ± 0.2 8.3 ± 0.4 8.6 ± 0.4

2085 ± 57 379 ± 33 154 ± 17 121 ± 24 522 ± 37 266 ± 13 252 ± 27

phr = parts per hundreds of resin. F


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Figure 8. HRR (a) and THR (b) curves for EVA, A-1, A-2, A-3, B-1, B-2, and B-3.

Table 3. Cone Calorimeter Data for EVA+FR Compositions composites

EVA

A-1

A-2

A-3

B-1

B-2

B-3

TTI (s) PHRR (kW/m2) THR (MJ/m2)

39 1087.6 170.4

50 355.3 81.8

54 274.5 70.6

53 260.6 63.9

52 265.0 77.8

55 197.4 66.4

59 152.5 58.2

Figure 9. Smoke production rate (a), residue weight (b), CO (c) and CO2 (d) production rate.

Table 4. Smoke Suppression and Residue Weight (700s) of EVA+FR Compositions composites 2

SPR (m /s) COP (g/s) CO2P (g/s) residue weight (wt %)

EVA

A-1

A-2

A-3

B-1

B-2

B-3

0.122 0.0095 0.638 1.4

0.059 0.0029 0.205 33.8

0.044 0.0022 0.170 37.7

0.039 0.0022 0.156 42.6

0.055 0.0024 0.162 35.5

0.037 0.0016 0.126 38.6

0.013 0.0009 0.105 48.4

As reported in a previous study, the nickel compounds catalyzed the carbonization of the burning polymers.54 Furthermore, the Ni 2p XPS spectrum of the B-3 residual char was examined

(Figure 11), and two groups of obvious peaks belonged to Ni 2p1/2 and Ni 2p3/2, respectively. The Ni 2p1/2 peaks (873.3 and 879.8 eV) and Ni 2p3/2 peaks (854.9 and 861.4 eV) were assigned G


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18.4 eV, which indicated the well-defined symmetry of Ni2+ in oxide form.56 The peak at 856.3 eV and its satellite at 864.2 eV were attributed to Ni3+ in Ni2O3, which was produced by the further oxidation of NiO at 400 °C.57 There were no Ni 2p3/2 peaks at approximately 852.9 eV due to the low levels of NiC as well as to the interference of high MgO loading in the residual char of B-3.58 3.7.2. Morphology of the Char. The morphology of the char residue was analyzed using a digital camera and SEM, and Figure 12 shows some interesting visual information on the char obtained by cone calorimetry. The thin brittle char on the surface of A-3 could not protect the underlying material, and all the composites had burnt to a residue that was composed of MgO and ashes, as shown in Figure 12a. On the other hand, the char of B-3 was unbroken and firm, and thus it acted as a barrier for both heat flow and gas transport, resulting in the remarkable reduction of HRR and THR (Figure 8a,b).59 The SEM images of the residual char are shown in Figure 13. It is easy to discern the differences between the exterior char structure of A-3 (Figure 13a) and that of B-3 (Figure 13c); the former was fragile and cracked, whereas the latter was porous and firm. Also, unlike A-3, the morphology of the interior char of B-3 was different from that of the exterior. The exterior looked like a fluffy sponge with some undecomposed polymer, which indicated the better barrier effect of the protective exterior char. This phenomenon could be attributed to the release of gases from the decomposition of alginate and the catalyzing carbonization of nickel compounds. 3.8. Mechanism of Flame Retardant and Smoke Suppression. It is clear that during the combustion process, the polymer materials were first degraded into lower molecular weight fractions (gases and liquid intermediate compounds). These degradation products were transformed into carbon materials or burnt up.60 The combustion of the EVA+B/A/Nia/ A composites is described below. First, when the temperature increased to above 130 °C, EVA began to soften. The composites swelled by the gases (noncombustible gas, combustible gas, and vapor) released from the decomposition of alginate and Mg(OH)2 (Figure 6). Subsequently, with the continued increase of temperature (350 °C−410 °C), most Mg(OH)2 decomposed endothermically into active MgO and released free water, thus cooling the combustion zone. The vaporous water swelled the composites and diluted the concentration of combustible gases. EVA began to decompose at approximately 320 °C, which produced more gases and liquid intermediate compounds.61 As a result, the exterior surfaces of composites appeared sponge-like with many bubbles. The bursting of the bubbles pushed the FR particles away from bubbles, which promoted the accumulation of FR particles on the exterior surfaces. At the same time, Ni (originating from the reduction of nickel compounds) catalyzed the degradation and aromatization of the intermediate compounds. Some kinds of carbon materials were formed on the surfaces of the Ni catalyst, followed by the growth of materials

Figure 10. XRD patterns of residual char obtained by cone calorimetry Nia (a) and B-3 (b).

Figure 11. Ni 2p XPS spectrum of B-3 residual char obtained by cone calorimetry.

Figure 12. Photographs of chars from A-3 (a) and B-3 (b) after cone calorimeter tests.

to Ni2+ in the NiO produced by the thermal decomposition of Nia,55 which is consistent with the XRD results. Meanwhile, the energy difference between Ni 2p3/2 and Ni 2p1/2 splitting was

Figure 13. SEM images of residual char obtained by cone calorimetry: exterior (a) and interior (b) of A-3 and exterior (c) and interior (d) of B-3. H


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Scheme 4. Schematic Illustration of the Flame Retardant and Smoke Suppression Mechanism of EVA+B/A/Nia/A Composites

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that formed a stable char residue with active MgO/SiO2.62 As a result, more protective char layers were formed. Finally, with progression of the combustion process, the exterior char of the EVA+B/A/Nia/A composites became firm and many tiny pores appeared, but the interior char retained its bubbles (Figure 13d). On the basis of the above analysis, the flame retardant and smoke suppression mechanisms of the EVA+B/A/Nia/A composites are summarized in Scheme 4.

4. CONCLUSION In this work, an efficient and multifunctional brucite flame retardant was fabricated via the layer-by-layer (LbL) assembly technique with brucite, APTES, sodium alginate, and nickel chloride. The driving forces of the fabrication were electrostatic interactions, dehydration condensations, hydrogen bonds, and coordination bonds. The new B/A/Nia/A flame retardant effectively improved the flame retardant and smoke suppression properties of the EVA+FR composites. The improvements were attributed to the formation of more protective layers and the catalyzing carbonization of nickel compounds. The new B/A/ Nia/A particles were well dispersed in EVA well and had a toughening effect on the composites due to the sacrificial bonds supplied by nickel alginate. The mechanism of flame retardance, smoke suppression, and toughening for the EVA+B/A/Nia/A composites were proposed. In short, we explored some simple and easy methods to produce an inexpensive, efficient, multifunctional, and structurally controllable green flame retardant.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00998. SEM and TEM images of the brucite particles, B/A/Nia/A particles; detailed assignment of absorption band for Sa and APTES (PDF)

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REFERENCES

(1) Polymer Green Flame Retardants; Elsevier: Athens, Greece, 2014; Vol. 1, pp 27−30.10.1016/B978-0-444-53808-6.00001-9 (2) Liu, J. H.; Yu, Z. L.; Chang, H. B.; Zhang, Y. B.; Shi, Y. Z.; Luo, J.; Pan, B. L.; Lu, C. Thermal Degradation Behavior and Fire Performance of Halogen-free Flame-retardant High Impact Polystyrene Containing Magnesium Hydroxide and Microencapsulated Red Phosphorus. Polym. Degrad. Stab. 2014, 103, 83−95. (3) Ma, Z. L.; Wang, J. H.; Zhang, X. Y. Effect of Silane KH-550 to Polypropylene/Brucite Composite. J. Appl. Polym. Sci. 2008, 107, 1000− 1005. (4) Yu, Z. L.; Liu, J. C.; Zhang, Y. B.; Luo, J.; Lu, C.; Pan, B. L. Thermooxidative Degradation Behavior and Fire Performance of High Impact Polystyrene/Magnesium Hydroxide/Microencapsulated Red Phosphorus Composite with an Alternating Layered Structure. Polym. Degrad. Stab. 2015, 115, 54−62. (5) Wang, X. S.; Pang, H. C.; Chen, W. D.; Lin, Y.; Ning, G. L. Nanoengineering Core/Shell Structured Brucite@Polyphosphate@ Amine Hybrid System for Enhanced Flame Retardant Properties. Polym. Degrad. Stab. 2013, 98, 2609−2616. (6) Laoutid, F.; Bonnaud, L.; Alexandre, M.; Lopez-Cuesta, J.-M.; Dubois, P. New Prospects in Flame Retardant Polymer Materials: From Fundamentals to Nanocomposites. Mater. Sci. Eng., R 2009, 63, 100− 125. (7) Köklükaya, O.; Carosio, F.; Grunlan, J. C.; Wågberg, L. FlameRetardant Paper from Wood Fibers Functionalized via Layer-by-Layer Assembly. ACS Appl. Mater. Interfaces 2015, 7, 23750−23759. (8) Fang, F.; Xiao, D. Z.; Zhang, X.; Meng, Y. D.; Cheng, C.; Bao, C.; Ding, X.; Cao, H.; Tian, X. Y. Construction of Intumescent Flame Retardant and Antimicrobial Coating on Cotton Fabric via Layer-bylayer Assembly Technology. Surf. Coat. Technol. 2015, 276, 726−734. (9) Li, Q. L.; Sun, Y. F.; Sun, Y. L.; Wen, J. J.; Zhou, Y.; Bing, Q. M.; Isaacs, L. D.; Jin, Y. H.; Gao, H.; Yang, Y. W. Mesoporous Silica Nanoparticles Coated by Layer-by-Layer Self-assembly Using Cucurbit[7]uril for in Vitro and in Vivo Anticancer Drug Release. Chem. Mater. 2014, 26, 6418−6431. (10) Sun, J.; Wu, T.; Liu, F.; Wang, Z.; Zhang, X.; Shen, J. Covalently Attached Multilayer Assemblies by Sequential Adsorption of Polycationic Diazo-Resins and Polyanionic Poly(acrylic acid). Langmuir 2000, 16, 4620−4624. (11) Malucelli, G.; Carosio, F.; Alongi, J.; Fina, A.; Frache, A.; Camino, G. Materials Engineering for Surface-confined Flame Retardancy. Mater. Sci. Eng., R 2014, 84, 1−20. (12) Cerkez, I.; Kocer, H. B.; Worley, S. D.; Broughton, R. M.; Huang, T. S. N-Halamine Biocidal Coatings via a Layer-by-Layer Assembly Technique. Langmuir 2011, 27, 4091−4097. (13) Carosio, F.; Fontaine, G.; Alongi, J.; Bourbigot, S. Starch-Based Layer by Layer Assembly: Efficient and Sustainable Approach to Cotton Fire Protection. ACS Appl. Mater. Interfaces 2015, 7, 12158−12167. (14) Pan, H. F.; Pan, Y.; Wang, W.; Song, L.; Hu, Y.; Liew, K. M. Synergistic Effect of Layer-by-Layer Assembled Thin Films Based on Clay and Carbon Nanotubes To Reduce the Flammability of Flexible Polyurethane Foam. Ind. Eng. Chem. Res. 2014, 53, 14315−14321.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone/fax: +86 10 6891 3075. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the kind gift of brucite from Yinkou Deer Technology New Material Co., Ltd. Furthermore, the authors gratefully acknowledge National Engineering Technology Research Center of Flame Retardant Materials for supplying test equipment. I


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Retardant and Smoke Suppression Properties. ACS Appl. Mater. Interfaces 2015, 7, 101−111. (35) Kurayama, F.; Suzuki, S.; Oyamada, T.; Furusawa, T.; Sato, M.; Suzuki, N. Facile Method for Preparing Organic/Inorganic Hybrid Capsules Using Amino-functional Silane Coupling Agent in Aqueous Media. J. Colloid Interface Sci. 2010, 349, 70−76. (36) Zhang, W. B.; He, S.; Liu, Y.; Geng, Q. Q.; Ding, G. L.; Guo, M. C.; Deng, Y. F.; Zhu, J. L.; Li, J. Q.; Cao, Y. S. Preparation and Characterization of Novel Functionalized Prochloraz Microcapsules Using Silica−Alginate−Elements as Controlled Release Carrier Materials. ACS Appl. Mater. Interfaces 2014, 6, 11783−11790. (37) Pasternack, R. M.; Rivillon Amy, S.; Chabal, Y. J. Attachment of 3(Aminopropyl)triethoxysilane on Silicon Oxide Surfaces: Dependence on Solution Temperature. Langmuir 2008, 24, 12963−12971. (38) Staroszczyk, H.; Sztuka, K.; Wolska, J.; Wojtasz-Pajak, A.; Kołodziejska, I. Interactions of Fish Gelatin and Chitosan in Uncrosslinked and Crosslinked with EDC Films: FT-IR Study. Spectrochim. Acta, Part A 2014, 117, 707−712. (39) Fourest, E.; Volesky, B. Contribution of Sulfonate Groups and Alginate to Heavy Metal Biosorption by the Dry Biomass of Sargassum fluitans. Environ. Sci. Technol. 1996, 30, 277−282. (40) Papageorgiou, S. K.; Kouvelos, E. P.; Favvas, E. P.; Sapalidis, A. A.; Romanos, G. E.; Katsaros, F. K. Metal Carboxylate Interactions in Metal−alginate Complexes Studied with FTIR Spectroscopy. Carbohydr. Res. 2010, 345, 469−473. (41) Santamaria, M.; Di Quarto, F.; Zanna, S.; Marcus, P. Initial Surface Film on Magnesium Metal: A Characterization by X-ray Photoelectron Spectroscopy (XPS) and Photocurrent Spectroscopy (PCS). Electrochim. Acta 2007, 53, 1314−1324. (42) Liu, J. H.; Feng, N.; Chang, S. Q.; Kang, H. L. Preparation and Characterization of Poly(glycidyl methacrylate) Grafted from Magnesium Hydroxide Particles via SI-ATRP. Appl. Surf. Sci. 2012, 258, 6127− 6135. (43) Grosvenor, A. P.; Biesinger, M. C.; Smart, R. S. C.; McIntyre, N. S. New Interpretations of XPS Spectra of Nickel Metal and Oxides. Surf. Sci. 2006, 600, 1771−1779. (44) Hassan, R. M.; Wahdan, M. H.; Hassan, A. Kinetics and Mechanism of Sol-gel Transformation on Polyelectrolytes of Nickel Alginate Ionotropic Membranes. Eur. Polym. J. 1988, 24, 281−283. (45) Zhang, J. J.; Ji, Q.; Shen, X. H.; Xia, Y. Z.; Tan, L. W.; Kong, Q. S. Pyrolysis Products and Thermal Degradation Mechanism of Intrinsically Flame-retardant Calcium Alginate Fibre. Polym. Degrad. Stab. 2011, 96, 936−942. (46) Liu, Y.; Zhao, J. C.; Zhang, C. J.; Ji, H.; Zhu, P. The Flame Retardancy, Thermal Properties, and Degradation Mechanism of Zinc Alginate Films. J. Macromol. Sci., Part B: Phys. 2014, 53, 1074−1089. (47) Wang, X. S.; Pang, H. C.; Chen, W. D.; Lin, Y.; Zong, L. S.; Ning, G. L. Controllable Fabrication of Zinc Borate Hierarchical Nanostructure on Brucite Surface for Enhanced Mechanical Properties and Flame Retardant Behaviors. ACS Appl. Mater. Interfaces 2014, 6, 7223−7235. (48) Haque, M. A.; Kurokawa, T.; Gong, J. P. Super Tough Double Network Hydrogels and Their Application as Biomaterials. Polymer 2012, 53, 1805−1822. (49) Ducrot, E.; Chen, Y. L.; Bulters, M.; Sijbesma, R. P.; Creton, C. Toughening Elastomers with Sacrificial Bonds and Watching Them Break. Science (Washington, DC, U. S.) 2014, 344, 186−189. (50) Gong, J. P.; Katsuyama, Y.; Kurokawa, T.; Osada, Y. DoubleNetwork Hydrogels with Extremely High Mechanical Strength. Adv. Mater. 2003, 15, 1155−1158. (51) Zhou, Y.; Feng, J.; Peng, H.; Qu, H. Q.; Hao, J. W. Catalytic Pyrolysis and Flame Retardancy of Epoxy Resins with Solid Acid Boron Phosphate. Polym. Degrad. Stab. 2014, 110, 395−404. (52) Zhang, J. J.; Ji, Q.; Wang, F. J.; Tan, L. W.; Xia, Y. Z. Effects of Divalent Metal Ions on the Flame Retardancy and Pyrolysis Products of Alginate Fibres. Polym. Degrad. Stab. 2012, 97, 1034−1040. (53) Correia, L. M.; de Sousa Campelo, N.; Novaes, D. S.; Cavalcante, C. L.; Cecilia, J. A.; Rodríguez-Castellón, E.; Vieira, R. S. Characterization and Application of Dolomite as Catalytic Precursor for Canola

(15) Carosio, F.; Cuttica, F.; Di Blasio, A.; Alongi, J.; Malucelli, G. Layer by Layer Assembly of Flame Retardant Thin Films on Closed Cell PET Foams: Efficiency of Ammonium Polyphosphate versus DNA. Polym. Degrad. Stab. 2015, 113, 189−196. (16) Huang, G. B.; Yang, J. G.; Gao, J. R.; Wang, X. Thin Films of Intumescent Flame Retardant-Polyacrylamide and Exfoliated Graphene Oxide Fabricated via Layer-by-Layer Assembly for Improving Flame Retardant Properties of Cotton Fabric. Ind. Eng. Chem. Res. 2012, 51, 12355−12366. (17) Sun, C.; Deng, Y. F.; Wan, L. N.; Qin, X. S.; Chen, G. H. Graphene Oxide-Immobilized NH2− Terminated Silicon Nanoparticles by CrossLinked Interactions for Highly Stable Silicon Negative Electrodes. ACS Appl. Mater. Interfaces 2014, 6, 11277−11285. (18) Mu, B.; Zhong, W.; Dong, Y.; Du, P.; Liu, P. Encapsulation of Drug Microparticles with Self-assembled Fe3O4/alginate Hybrid Multilayers for Targeted Controlled Release Alginate Hybrid Multilayers for Targeted Controlled Release. J. Biomed. Mater. Res., Part B 2012, 100B, 825−831. (19) Liu, H.; Gu, X. Y.; Hu, M.; Hu, Y.; Wang, C. Y. Facile Fabrication of Nanocomposite Microcapsules by Combining Layer-by-layer Selfassembly and Pickering Emulsion Templating. RSC Adv. 2014, 4, 16751−16758. (20) Grant, G. T.; Morris, E. R.; Rees, D. A.; Smith, P. J. C.; Thom, D. Biological Interactions between Polysaccharides and Divalent Cations: The Egg-box Model. FEBS Lett. 1973, 32, 195−198. (21) Gong, J.; Tian, N. N.; Liu, J.; Yao, K.; Jiang, Z. W.; Chen, X. C.; Wen, X.; Mijowska, E.; Tang, T. Synergistic Effect of Activated Carbon and Ni2O3 in Promoting the Thermal Stability and Flame Retardancy of Polypropylene. Polym. Degrad. Stab. 2014, 99, 18−26. (22) Wen, X.; Gong, J.; Yu, H. O.; Liu, Z.; Wan, D.; Liu, J.; Jiang, Z. W.; Tang, T. Catalyzing Carbonization of Poly(L-lactide) by Nanosized Carbon Black Combined with Ni2O3 for Improving Flame Retardancy. J. Mater. Chem. 2012, 22, 19974−19980. (23) Wilson, I. Brucite bonanza. Ind. Miner. (London, U. K.) 2007, 481, 74−79. (24) Matsui, S. (Konoshima Chemical Co., Ltd.), Japanese Patent JP 2003-003167, January 8, 2003. (25) Matsui, S. (Konoshima Chemical Co., Ltd.), Japanese Patent JP 2003-096463, April 3, 2003. (26) Rasalingam, S.; Kibombo, H. S.; Wu, C.-M.; Budhi, S.; Peng, R.; Baltrusaitis, J.; Koodali, R. T. Influence of Ti−O−Si Hetero-linkages in the Photocatalytic Degradation of Rhodamine B. Catal. Commun. 2013, 31, 66−70. (27) Amouzou, D.; Fourdrinier, L.; Maseri, F.; Sporken, R. Formation of Me−O−Si Covalent Bonds at the Interface between Polysilazane and Stainless Steel. Appl. Surf. Sci. 2014, 320, 519−523. (28) Li, H. Y.; Wang, R. G.; Hu, H. L.; Liu, W. B. Surface Modification of Self-healing Poly(urea-formaldehyde) Microcapsules Using Silanecoupling Agent. Appl. Surf. Sci. 2008, 255, 1894−1900. (29) Sartori, C.; Finch, D. S.; Ralph, B.; Gilding, K. Determination of the Cation Content of Alginate Thin Films by FTi.r. spectroscopy. Polymer 1997, 38, 43−51. (30) Wei, B. G.; Chang, Q.; Bao, C. X.; Dai, L.; Zhang, G. Z.; Wu, F. P. Surface Modification of Filter Medium Particles with Silane Coupling. Colloids Surf., A 2013, 434, 276−280. (31) Su, D.; Huang, C. Y.; Hu, Y.; Jiang, Q. W.; Zhang, L.; Zhu, Y. F. Preparation of Superhydrophobic Surface with a Novel Sol−gel System. Appl. Surf. Sci. 2011, 258, 928−934. (32) Aissaoui, N.; Bergaoui, L.; Landoulsi, J.; Lambert, J.-F.; Boujday, S. Silane Layers on Silicon Surfaces: Mechanism of Interaction, Stability, and Influence on Protein Adsorption. Langmuir 2012, 28, 656−665. (33) Liu, X.; Chen, X.; Li, Y. F.; Wang, X. Y.; Peng, X. M.; Zhu, W. W. Preparation of Superparamagnetic Fe3O4@Alginate/Chitosan Nanospheres for Candida rugosa lipase Immobilization and Utilization of Layer-by-Layer Assembly to Enhance the Stability of Immobilized Lipase. ACS Appl. Mater. Interfaces 2012, 4, 5169−5178. (34) Pan, H. F.; Wang, W.; Pan, Y.; Song, L.; Hu, Y.; Liew, K. M. Formation of Layer-by-Layer Assembled Titanate Nanotubes Filled Coating on Flexible Polyurethane Foam with Improved Flame J


Research Article

ACS Applied Materials & Interfaces and Sunflower Oils for Biodiesel Production. Chem. Eng. J. 2015, 269, 35−43. (54) Yu, H. O.; Liu, J.; Wang, Z.; Jiang, Z. W.; Tang, T. Combination of Carbon Nanotubes with Ni2O3 for Simultaneously Improving the Flame Retardancy and Mechanical Properties of Polyethylene. J. Phys. Chem. C 2009, 113, 13092−13097. (55) Wu, H. J.; Wu, G. L.; Wu, Q. F.; Wang, L. D. Facile Synthesis and Microwave Absorbability of C@Ni−NiO Core−shell Hybrid Solid Sphere and Multi-shelled NiO Hollow Sphere. Mater. Charact. 2014, 97, 18−26. (56) Wang, Y. F.; Qu, F. D.; Liu, J.; Wang, Y.; Zhou, J. R.; Ruan, S. P. Enhanced H2S Sensing Characteristics of CuO-NiO Core-shell Microspheres Sensors. Sens. Actuators, B 2015, 209, 515−523. (57) Carley, A. F.; Jackson, S. D.; O’Shea, J. N.; Roberts, M. W. The Formation and Characterisation of Ni3+  an X-ray Photoelectron Spectroscopic Investigation of Potassium-doped Ni(110)−O. Surf. Sci. 1999, 440, L868−L874. (58) Mahata, N.; Cunha, A. F.; Orfao, J. J. M.; Figueiredo, J. L. Simultaneous Elimination of Pyrophoricity and Enhancement of Activity of Raney Nickel by Carbon Deposition: Synthesis of Highly Active NiC Catalysts. ChemCatChem 2010, 2, 330−335. (59) Feng, J.; Hao, J. W.; Du, J. X.; Yang, R. J. Flame Retardancy and Thermal Properties of Solid Bisphenol A Bis(diphenyl phosphate) Combined with Montmorillonite in Polycarbonate. Polym. Degrad. Stab. 2010, 95, 2041−2048. (60) Laoutid, F.; Bonnaud, L.; Alexandre, M.; Lopez-Cuesta, J. M.; Dubois, P. New Prospects in Flame Retardant Polymer Materials: From Fundamentals to Nanocomposites. Mater. Sci. Eng., R 2009, 63, 100− 125. (61) Carpentier, F.; Bourbigot, S.; Le Bras, M.; Delobel, R.; Foulon, M. Charring of Fire Retarded Ethylene Vinyl Acetate Copolymer -magnesium Hydroxide/Zinc Borate Formulations. Polym. Degrad. Stab. 2000, 69, 83−92. (62) Gong, J.; Liu, J.; Wan, D.; Chen, X. C.; Wen, X.; Mijowska, E.; Jiang, Z. W.; Wang, Y. H.; Tang, T. Catalytic Carbonization of Polypropylene by the Combined Catalysis of Activated Carbon with Ni2O3 into Carbon Nanotubes and Its Mechanism. Appl. Catal., A 2012, 449, 112−120.

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