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Spray drying assisted Layer-by-Layer assembly of alginate, 3aminopropyltriethoxysilane, and magnesium hydroxide flame retardant and its catalytic graphitization in ethylene-vinyl acetate resin Yiliang Wang, ZhiPeng Li, Yuanyuan Li, Jingyu Wang, Xiu Liu, TianYou Song, Xiaomei Yang, and Jianwei Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01556 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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ACS Applied Materials & Interfaces

Spray drying assisted Layer-by-Layer assembly of alginate, 3-aminopropyltriethoxysilane, and magnesium hydroxide

flame

retardant

and

its

catalytic

graphitization in ethylene-vinyl acetate resin

Yiliang Wang,

†,§

†

†

†

†

†

Zhipeng Li, Yuanyuan Li, Jingyu Wang, Xiu Liu, Tianyou Song, Xiaomei 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 §

Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education,

Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China. ABSTRACT: Alginates (nickel alginate: NiA, copper alginate: CuA or zinc alginate: ZnA) and 3aminopropyltriethoxysilane (APTES) were alternately deposited on magnesium hydroxide (MH) surface by spray drying assisted layer-by-layer assembly technique, fabricating some efficient and environmentally benign flame retardants (M-FR, including Ni-FR, Cu-FR and Zn-FR). The morphology, chemical compositions and structures of M-FR were investigated. With 50 wt% loading, compared with EVA28+MH, the peak heat release rate, smoke production rate and CO production rate of EVA28+Ni-FR decreased by 50.78%, 61.76% and 66.67%, respectively. The metals or metal oxide nanoparticles arising from alginates could catalyse the pyrolysis intermediates of EVA into graphene and amorphous carbon, which could bind the inorganic compounds (the decomposition products of MH and APTES) together and form some more protective barriers. For each M-FR, the flame retardant and

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smoke suppression efficiency were different, which were caused by the diverse carbonization and graphitization behaviors of three alginates. ZnA generated some ZnO aggregations couldn’t catalyse the graphitization of intermediates. For CuA, the catalytic graphitization was limited by the tightly binding graphene layer. As for NiA, the configuration of Ni atom could not provide strong binding of Ni substrate and carbon. The liquid like Ni nanoparticles could restructure and get out from firm graphene shells, so the catalytic graphitization of NiA was efficient and sustainable. This work displayed the catalytic graphitization mechanism of alginates, while exploring a simple and novel strategy for fabricating efficient green flame retardants. KEYWORDS: spray drying, alginate, catalytic graphitization, flame retardant, layer-by-layer

1. INTRODUCTION Nowadays, the fire protection of polymer productions is facing critical condition as the safety and efficiency of the flame retardants (FR) normally adopted in the field have been questioned.1 According to the studies, the most effective and commonly used flame retardants are halogenated additives, bromine and chlorine can readily be released and take part in the combustion process, especially with the previously discussed free-radical mechanism occurring in the gas phase.2 But they are bioaccumulative, persistent in the environment, and hazard for animals and humans.3-5 Many regulations and policy have been established to limit the use of hazardous flame retardants, for instance, the restriction of hazardous substances (RoHS) directive and REGULATION concerning the registration, evaluation, authorization and restriction of chemicals (REACH).6 In such a worldwide scenario, efficient and environmentally benign flame retardants are in urgent need. Magnesium hydroxide (MH) comprises an important segment in the market of halogen-free flame retardants (HFFR), it can decompose endothermically, release water to cool the combustion and form inorganic protective barrier. However, MH particles has a bad compatibility with polymers, negatively affecting the interfacial interaction. The traditional solution is surface modification with coupling agents, which are easy to migrate out from the interfaces.7 Layer-by-layer (LbL) assembly technique has supplied a viable solution, which could deposit polyelectrolytes and/or nanoparticles on different substrates with high surface-to-bulk ratios, such as fabric, foam, and thin films.8 There are three main conventional LbL deposition methods, immersive LbL, spin-assisted LbL and spray-assisted LbL.9 In our last work, immersive LbL assembly technique had been used to fabricate flame retardant (FR) particles for the first time, alginate and 3-aminopropyltriethoxysilane (APTES) had been deposited onto brucite (a unique mineral containing more than 94 wt % of MH) particles, enhancing the interfacial interaction with polymers.10 But it couldn’t forbid the aggregation of FR particles effectively in the LbL assembly process. Spray drying (Sd) is an energy intensive, continuous and scalable drying process, which can generate nano to micron size particles and prevent the aggregation of nanoparticles in a very short time-frame.11 In this work, spray drying-assisted LbL is used as an efficient method to prepare multilayer-coated FR for the first time. As a flame retardant, MH is inherent inefficient; the formed char layer is thin, brittle and full of cracks, which deteriorate the protection effect. Fabricating protective barrier by catalyzing the polymer matrix or charring agent into carbonized residues has become one of the most efficient strategy for fire protection.12 The graphite has better thermal stability than amorphous carbon; improving the graphitization degree of residual char is beneficial to build more protective barrier.13 Transition metals are the most common catalysts for synthesizing carbon nanotubes and graphene14-16, which are also efficient for the catalytic conversion of polymeric materials to graphite materials.17 Transition metals ions could form stable hydrogels with sodium alginate (Sa) in water, which caused by the chelation of


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poly-a-L-guluronic acid (G), poly-b-Dmannuronic acid (M) and metal ions (Scheme 1).18 Although alginates have been used to produce porous carbon and carbon/metal (oxide) hybrids with fine nanostructure.19-21 However, there are no researches about the catalytic graphitization of alginates in polymers, which contributes to forming more protective barrier with high graphitization degree during the combustion of polymers. In the present paper, we proposed a new strategy to prepare flame retardant rapidly via spray drying assisted layer-by-layer assembly technique, which could reduce the aggregation of FR to some extent. The spray drying process was carried out after the deposition of alginates and APTES on MH particle. The morphology and compositions of new flame retardants (M-FR) had been characterized. After using in EVA resin, the flame retardant and smoke suppression properties of the composites were investigated. The catalytic graphitization and carbonization mechanism of alginates for EVA were revealed for the first time.

2. EXPERIMENTAL SECTION 2.1 Materials. The MH powder (d90, 4.895 µm) was obtained from HeFei ZhongKe Flame Retardant New Material Co., LTD. The sodium alginate (SA, (C6H7NaO6)n, 200 mPa·s, chemical grade) was purchased from Aladdin Industrial Corporation. The silane coupling agent (APTES) was supplied by Diamondchem Co., LTD. The nickel chloride hexahydrate (NiCl2·6H2O, analytical grade) was purchased from Shantou Xilong Chemical Factory Guangdong. The copper chloride trihydrate (CuCl2·3H2O, analytical grade) and zinc chloride hexahydrate (ZnCl2·6H2O, analytical grade) were purchased from Tianjin Guangfu Fine Chemical Research Institute. Poly (ethylene-vinyl acetate) copolymers (EVA28) were supplied as pellets by DuPont company (Elvax, 28 wt% of vinyl acetate). 2.2. Preparation of M-FR The preparation of M-FR by spray drying assisted layer-by-layer (LbL) assembly process was illustrated in Scheme 1. The driving forces of the fabrication were electrostatic interactions, dehydration condensations, hydrogen bonds, and coordination bonds.10 The preparation of MH-A. 1000 g of MH and 2 L of deionized water were mixed by vigorous stirring (200 rpm) at 30 °C for 5 min. Then, 30 g of APTES was dropped into the mixture, and vigorous stirring (200 rpm) was applied during this procedure for 10 min to obtain FR slurry, which was spray dried in a small spray dryer, as displayed in Figure S1i and Scheme 1. The FR slurry was injected into the dry tower with injection rate of 20 ml/min. The pressure of compressed air was 1.5 MPa. The temperature of hot air in the inlet and outlet were maintained at 140 oC and 80 oC, respectively. The obtained powder was the surface-modified MH with APTES (MH-A). Scheme 1. Schematic illustration of spray drying assisted layer-by-layer assembly process of M-FR and the principle of spray dryer.


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The preparation of MH-A-SA. 1000 g of MH-A powder was suspended in 1.5 L of deionized water by vigorous stirring (300rpm) for 10 min. Then, a 2.5 L sodium alginate aqueous solution (60 g/L) was added to the reaction, the system was kept vigorous stirring (300 rpm) at room temperature for 1h. After centrifuging and washing (CW) with deionized water several times, the product FR slurry (20 % solid content) was spray dried as described above and MH-A-SA was prepared. The preparation of MH-A-MA. 1000 g of MH-A-SA powder was suspended in 1.5 L of deionized water by vigorous stirring (300rpm) for 10 min. 1 L metal chloride (NiCl2, CuCl2 or ZnCl2) aqueous solution (0.4 mol/L) was added dropwise into the system, kept the reaction conditions same for another 1h. The product was immediately centrifugated and washed with deionized water several times until no Cl− was detected by 0.5 mol/L AgNO3 aqueous solution. Then the FR slurry was spray dried to prepare MH-A-MA. The MH-A-MA was modified by APTES as described above. The obtained powder was dried under 100 °C for 10 h in a drying oven, yielding the new flame retardant (M-FR). 2.3 Preparation of EVA28+FR Composites and EVA28+Alginate Composites. The EVA28+flame retardant (EVA28+FR) composites containing 50 wt% EVA28 and 50 wt% flame retardants were obtained in a twin-roll mixer at 120 oC for 10 min with a speed of 60 rpm. After the mixing, the composites were transferred to a steel mold which were preheated at 120 oC for 5 min and then pressed at 10.0 MPa for 10 min. The EVA28+Alginate composites containing 90 wt% EVA28 and 10 wt% alginates were mixed in tetrahydrofuran (THF) solution. EVA28+Nickel alginate (EVA28+NiA), EVA28+Copper alginate (EVA28+CuA) and EVA28+Zinc alginate (EVA28+ZnA) were prepared with this method. 2.4 Measurements and Characterization. Transmission electron microscopy (TEM) images of FR particles were obtained on FEI Tecnai G2 F30 with an acceleration voltage of 300 kV. Field emission scanning electron microscopy (FESEM) images of FR particles were acquired with FEI Quanta x50. A copper coating was deposited on all samples prior to SEM test to prevent charging. The size distributions of the FR particles were analyzed by a laser diffraction apparatus (Mastersizer 2000, Malvern Instruments Ltd.), the particles were mixed with Ethanol by a sample handling unit (Hydro 2000MU, Malvern Instruments Ltd.) under constant pump speed (1700 rpm). During sample addition, laser obscuration was maintained between 7% and 10%, and readings were obtained using refractive indices of 1.52. The elements in FR were examined by X-ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM). The binding energy of elements in char residues and partial decomposed samples were characterized by XPS at 15kV×25 W under a vacuum lower than 10-6Pa. The partial decomposed EVA28+FR samples were handled in tube furnace for 3min at different temperatures. Raman spectra of EVA+Alginates residual char were measured at room temperature using a 532 nm laser (Horiba Jobin Yvon HR Evolution). Fourier transform infrared (FTIR) spectroscopy was examined on 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. To investigate the non-volatile pyrolysis products of the EVA28+FR composites, FTIR (NICOLET 6700) was used to examine the partial decomposed samples. As for the volatile pyrolysis products, simultaneous thermal analyzer/IR spectrometer (Perkinelmer STA8000/Frontier) was performed under nitrogen atmosphere from 50 oC to 800 oC at a heating rate of 20 oC/min. The cone calorimeter measurement (Cone) was performed on a fire testing technology apparatus (FTT) conforming to ISO 5660 protocol. Each specimen with dimensions of 100 × 100 × 3 mm3 was exposed horizontally to 50 kW/m2 external heat flux.


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3. RESULTS AND DISCUSSION 3.1 Morphology of the Flame Retardants The morphology of MH and M-FR particles were characterized by TEM and SEM, as shown in Figure 1 and Figure S1, respectively. Figures 1a shows the smooth surfaces of MH, whereas Figure 1bd show that the surfaces have been coated by some floccules with different nanometer thickness. These floccules were mainly alginates hydrogel and the morphology was decided by the high molecular weight and variable molecular structures of sodium alginate.22 These alginates hydrogel may agglutinate with some small MH particles, as shown in Figure S1. The morphology of alginates in MFR were different from each other, which were caused by the different chelate ability of alginate molecular chains for Cu2+, Ni2+ and Zn2+.23

Figure 1. TEM images of the flame retardants: (a) MH particles; (b) Ni-FR particles; (c) Cu-FR particles; (d) Zn-FR particles. 3.2 Size Distribution and Chemical Compositions of the Flame Retardants The size distribution of MH and M-FR are displayed in Figure 2a. The results were consistent with TEM and SEM images, the distribution were broad and the diameter arranged from about 160 nm to 30 µm. The diameters of MH and M-FR particles could be divided into three sections: section A (160 nm600 nm), section B (600 nm-6 µm) and section C (6 µm-30 µm). In section A, the volume of small MH particles was higher than M-FR, however, it was obvious that the volume of M-FR particles was higher than MH in section B. It could be concluded that small particles in section A were slightly shifted to larger size section in B after the LbL assembly process. In section C, the volume of MH particles was almost same with Cu-FR and Zn-FR, indicating the spry drying process could reduce the aggregation of flame retardants. As for Ni-FR, there were more bigger particles (3µm-30 µm), which was attribute to the strong chelate ability of alginate molecular chains for Ni2+.23


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The chemical compositions of MH and M-FR were carefully examined by XPS, as shown In Figure 2b. In MH spectrum, the main elements of MH were C (C adsorbed on MH surface), O and Mg. In MFR spectra, the new peaks around 400 eV and 155 eV belonged to N 1s and Si 2p, indicating that APTES had been attached on the surfaces of MH. In Ni-FR, the new peaks at approximately 856 eV and 645 eV can be attributed to Ni 2p and Ni LMM, respectively. The Cu-FR and Zn-FR curves had similar results that some new peaks belonging to Cu 2p, Cu LMM and Zn 2p, Zn LMM appeared. These new peaks affirmed that alginates had been assembled on MH surfaces.24-26 The chemical components of MH and M-FR are listed in Table 1, the atomic concentrations of Ni, Cu and Zn in three M-FR were 0.84%, 0.81% and 0.93%, respectively.

Figure 2. (a) size distribution of MH and M-FR; (b) XPS spectra of MH and M-FR. Table 1. Chemical components of MH, Ni-FR, Cu-FR and Zn-FR XPS (Atomic Concentration, %) Sample C Mg O N Si Ma MH 32.77 21.72 45.51 0 0 0 Ni-FR 36.53 16.75 39.15 3.65 3.08 0.84 Cu-FR 36.35 16.73 40.22 2.98 2.91 0.81 Zn-FR 36.67 16.09 40.18 3.28 2.85 0.93 a

M: Ni, Cu or Zn.

3.3 Flame Retardant and Smoke Suppression Properties of EVA28 and EVA28+FR Composites The cone calorimeter is frequently considered to be the most effective method for a full-scale evaluation of the flame retardancy of materials.27 It can provide many important parameters such as heat release rate (HRR), peak heat release rate (PHRR), total heat release (THR), smoke production rate (SPR), total smoke production (TSP), time to ignition (TTI), CO produce rate (COP) and CO2 produce rate (CO2P). Cone calorimeter results for EVA28 and EVA28+FR compositions are displayed in Figure 3 and Table 2. The pure EVA28 ignited after 37 s in the fast burning, reached a PHRR value of 1026.1 kW/m2 and burned out rapidly within 350 s. All flame retardants reduced the HRR of the EVA28+FR composites. EVA28+MH had one broad and high HRR peaks, indicating a bad barrier effect for heat flow, which should be ascribed to the thin and fragile char layers with many cracks.28 The HRR peaks of EVA28+Cu-FR and EVA28+Zn-FR were lower and milder, which prolonged the combustion times to 400 and 550 s, respectively. And the second peaks were higher indicated the protective char layers had lost the protective effect in the end of combustion. As for EVA28+Ni-FR, the HRR curve had a


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long plateau and the PHRR (at 90 s) was 201.1 kW/m2 much less than 408.6 kW/m2 of EVA28+MH (135 s). And EVA28+Ni-FR had the lowest THR value (70.4 MJ/m2).These results suggest that some more protective and effective char layers could form on the burning surface of EVA28+Ni-FR composites, the insulating barrier could prevent oxygen diffusion and feedback of heat from reaching the underlying material.29 And the photographs of residual chars obtained from cone tests supplied some visual information about the barrier; as shown in Figure 4, the char of EVA28+MH were thin, brittle and full of multiple cracks, while the unbroken char of EVA28+Ni-FR was firm and had fewer defects. In Figure 4f and Table 2, compared with EVA28+MH char, the residue weight (RW) of EVA28+Ni-FR char (38.92 wt%) increased by 7.45%, indicating more combustion intermediate product were kept in condensed phase. According to the past study, alginates were easy to decompose at low temperature.30 However, the TTI values of EVA28+M-FR were not less than EVA28+MH, which indicated that the poor thermal stability of alginates did not deteriorate the TTI. In Table 2, the TTI of EVA28+Cu-FR is shorter than EVA28+Ni-FR and EVA28+Zn-FR, which should be attribute to the worse thermal stability of CuA.30

Figure 3. Cone calorimeter curves of EVA28 and EVA28+FR compositions: (a) HRR curves; (b) SPR curves; (c) THR curves; (d) TSP curves; (e) COP curves (f) CO2P curves. Table 2. Cone calorimeter data for EVA28 and EVA28+FR compositions TTI PHRR THR SPR TSP COP CO2P RW Sample 2 2 2 2 (s) (kW/m ) (MJ/m ) (m /s) (m /kg) (g/s) (g/s) (wt %) EVA28 37 1026.1 106.4 0.129 13.57 0.0093 0.569 1.87 EVA28+MH 51 408.6 82.1 0.068 10.22 0.0039 0.212 36.06 EVA28+Ni-FR 55 201.1 70.4 0.026 6.68 0.0013 0.124 38.92 EVA28+Cu-FR 52 220.6 74.9 0.048 9.16 0.0023 0.128 37.81 EVA28+Zn-FR 61 327.0 76.2 0.062 8.12 0.0034 0.188 37.06 It is well-known that most fire fatalities are attributable to the lethal atmosphere resulting from the combustion of polymers. Therefore, reduction of smoke and toxic gases (especially carbon monoxide) will be beneficial to fire rescue when a fire accident occurs.31 According to past research, most visible smoke is mainly composed of poisonous aromatic intermediate compounds and benzene derivatives, which could be catalyzed into more stable carbon materials and polyaromatic species by metal catalysts.17 The catalytic carbonization had been confirmed by the changes of SPR and TSP curves in Figure 3. The peak SPR value of EVA28 (0.129 m2/s) decreased significantly after adding 50 wt% MH


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(0.068 m2/s), which is due to the catalytic carbonization of active MgO. Our LbL assembled flame retardants could further improve the smoke suppression properties. EVA28+Ni-FR (0.026 m2/s) behave better than EVA28+Cu-FR (0.048 m2/s) and EVA28+Zn-FR (0.062 m2/s); TSP curves in Figure 3d shows the remarkable advantage, compared with EVA28+MH, TSP of EVA28+Ni-FR were only 6.68 m2/kg, decreased by 34.63%. In addition, the dynamic curves of COP and CO2P had similar rules, as shown in Figure 3e and f. Compared with EVA28+MH, the peak COP and peak CO2P of EVA28+NiFR decreased by 66.67% and 41.51%, respectively. Indeed, these phenomena should be ascribed to the different catalytic graphitization and carbonization ability of metal compounds that introduced by alginates. And alginates also could prevent the emigration of MH and SiO2 (arising from APTES), which contribute to form more protective and effective char layers.32 The morphology and chemical structures of the residual char supplied in Figure 4 had provided the strongest evidence.

Figure 4. Photographs and weight curves of residual char obtained from cone calorimeter tests: (a) EVA28; (b) EVA28+MH; (c) EVA28+Ni-FR; (d) EVA28+Cu-FR; (e) EVA28+Zn-FR and (f) residue weight curves. Besides, a traditional layer-by-layer assembly process without spray drying had been used to fabricate nickel alginate and APTES on brucite, preparing a new efficient brucite FR (B-Ni-FR). When used in EVA18, the Cone results also confirmed the obvious improvement for flame retardant and smoke suppression properties, as shown in Figure S2 and Table S1. 3.4 TG-IR Analysis of EVA28+Alginates Composites The different flame retardant, smoke suppression properties and the diverse morphology of EVA28+M-FR composites were mainly caused by the different alginates. After removing the interference from MH and APTES, TG-IR technique was chosen to monitor the evolution of volatilized products formed during thermal degradation of EVA28 and EVA28+Alginates. According to the past study, there are two steps in the EVA28 decomposition. The first degradation step, in the range of 345450 oC, can be assigned to the pyrolysis of vinyl acetate groups, whereas the second step at 455-510 oC is due to the degradation of polyethylene chains. In Figure 5a, compared with EVA28, the CO2 peaks at


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2360 cm-1 and the hydrocarbons around 2920 cm-1 weakened obviously in the 3D diagrams of EVA28+NiA and EVA28+CuA. From Grame-Schmidt (GS) curves in Figure 5b, we can find the volatilized products of EVA28+NiA and EVA28+CuA decreased obviously during decomposition. And FTIR spectra obtained at the maximum evolution rate for the first (Figure 5c) and second (Figure 5d) decomposition step have shown more detail information, the intensity of H2O, CH3COOH, aromatic gases and CO2 decreased, which indicated less visual smokes were produced and more volatilized intermediate compounds had been catalyzed into solid char. For EVA28+ZnA, the intensity of hydrocarbons decreased and more CO2 produced, which means the EVA28 burnt more fully. Besides, early beginning of the first and second decomposition steps have been observed in the Grame-Schmidt (GS) curves of EVA28+alginates, which caused by the worse thermal stability of alginates.30

Figure 5. TG-IR test results of EVA28 and EVA28+Alginates obtained under N2 atmosphere: (a) 3D diagram; (b) TGA and GS curves; FTIR spectra obtained at the maximum evolution rate for the (c) first and (d) second decomposition step. 3.5 XPS Analysis and Raman Spectroscopy of EVA28+Alginates Residual Char X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy were employed to analyze the compositional changes of the EVA28+Alginates residual char. In the C1s XPS spectra (Figure 6), five types of carbon with different chemical states had been observed, which appear at 284.6-284.9 eV (C– H, C–C/graphite), 286.2-286.5 eV(C–O), 288.7-288.8 eV (O–C=O), and 283.8-283.9 eV(Metal-C), respectively.33 At the room temperature (RT), only four types of carbons(C–H, C–C, C–O, O–C=O)


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could been found, which represents the major of carbon in the EVA28 and alginates.34 When the temperature arrived at 385 oC, the first degradation step of EVA28 happened and most of vinyl acetate groups had removed from the EVA28 surface. As the results, only C–H, C–C and few C–O could be observed, which was consistent with the results obtained from Figure 5.

Figure 6. C1s XPS spectra of EVA28+Alginates residual char obtained from tube furnace under N2 atmosphere at different temperatures, the heating rate is 20 oC/min and the high temperature preservation is 3 minutes: (a) EVA28+NiA; (b) EVA28+CuA; (c) EVA28+ZnA. Table 3. Atomic concentration data for EVA28 and EVA28+Alginates residual char Atomic Concentration (%) Sample Elements RTb 385 oC 585 oC 735 oC C 89.02 98.22 88.23 91.31 EVA28+NiA O 10.94 1.71 11.47 8.11 Ni 0.04 0.07 0.31 0.57 C 87.68 99.58 86.13 86.34 EVA28+CuA O 12.30 0.37 13.54 12.96 Cu 0.02 0.05 0.33 0.70 C 85.88 97.68 83.62 84.56 EVA28+ZnA O 14.10 2.28 15.43 14.14 Zn 0.03 0.04 0.95 1.30 b

RT: room temperature.

According to past study about the decomposition of alginates, metal oxides generate when the temperature is over 350 oC.35 The oxides in EVA28+NiA and EVA28+CuA could be reduced by aromatics, carbon and various intermediate hydrocarbon products (main components of the smoke) arising from the decomposition of alginate as well as EVA28.36,37 Then some hydrocarbon molecules could be adsorbed on the metallic Ni or Cu particles surface, dissociated, and C atoms dissolved into the bulk metallic Ni or Cu.38 So at 585 oC, the new peaks (283.8 eV) in the Figure 6a and b could be attributed to Ni–C and Cu–C bonds, respectively. It is impossible for reducing ZnO into zinc, a new peak at 283.8 eV appeared in Figure 6c, indicating carbon substitution for oxygen and formation of ZnC bonds in the carbon-doped ZnO.39 When the temperature rises to 735 oC, the C1s spectra of EVA28+NiA were different from the others, only C–C, Ni–C and few O–C=O can be found; there were more Ni–C than C–C. This phenomenon could be due to the higher carbon solubility of Ni than Cu and ZnO.40 Besides, in Figure 6a, the disappearance of C–O and the weakening of O–C=O relative intensity indicated that there was less oxygen in EVA28+NiA residual char. As we know, less oxygen content in char barrier means better thermal oxidation resistance as well as better thermal insulation properties,


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which was responsible for the decrease of HRR. The measuring depth of XPS for organic materials is about 10 nm, so almost no metal can be detected at low temperature due to the cover of polymer chains. Although the alginate contents were same for three EVA28+Alginates composites, the Ni concentration in EVA28+NiA was less than Cu and Zn. It seems like there were more graphene layers on the Ni surface, which interfered with the XPS test. Raman spectra of the EVA28+Alginates residual char are shown in Figure 7. The G-band (1600 cm−1) was related with the in-plane tangential vibration of graphite carbon and the D-band at 1338 cm−1 was associated with amorphous carbon. The intensity ratio of D to G bands (ID/IG) could been used to characterized the defects in the graphite planes or the extent of graphitization in carbonized polymers.41 The ID/IG of EVA28+NiA (0.89) was less than the ID/IG of EVA28+CuA and EVA28+ZnA (0.97), indicating more graphite carbon formed in EVA28+NiA residual char and this was in accordance with the XPS results.

Figure 7. Raman spectra of EVA28+Alginates residual char obtained from tube furnace under N2 atmosphere at 735 oC, the high temperature preservation is 3 minutes. 3.6 The Catalytic Graphitization Mechanisms of Alginates in EVA28 Resin The XRD patterns of EVA28+Alginates residual char listed in Figure 8 shows more useful information. In XRD patterns of EVA28+NiA, three sharp diffraction peaks can be indexed to metallic Ni, in accordance with the reported data (JCPDS No. 04-0805). The broad diffraction peaks around 26。 was belong to graphite (JCPDS No. 41-1487).42 Similarly, the XRD patterns of EVA28+CuA should be attributed to the Cu (JCPDS No. 04-0836), amorphous carbon and graphite.43 And the curves of EVA28+ZnA showed the XRD pattern of C-doped ZnO, the major diffraction peaks can be indexed to ZnO (JCPDS 75-0576);44 the broad peaks arising from 22。 were attributed to the amorphous carbon.43 These results proved that metallic Ni, metallic Cu and ZnO could be formed during the decomposition of EVA28+Alginates; three alginates could catalyse the carbonization of EVA28, but only NiA and CuA had a catalytic graphitization for EVA28 resin. And this result was consistent with the Raman and XPS results.


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Figure 8. XRD patterns of EVA28+Alginates residual char obtained from tube furnace under N2 atmosphere at 735 oC, the high temperature preservation is 3 minutes. The TEM images showed the morphology and typical structures of EVA28+Alginates residual char. In Figure 9a and Figure S3, some irregularly shaped Ni nanoparticles were distributed in the amorphous carbon and the diameter did not exceed 50 nm, which were indeed coated by several layers of graphene. In EVA28+CuA residual char (Figure 9b and Figure S3b), the Cu nanoparticles were regularly circular and the diameter ranged from 35 to 40 nm. In EVA28+ZnA residual char (Figure 9c and Figure S3c), only some aggregates of ZnO and amorphous carbon could be found. In Figure 9a and b, there were some shape changes in the formation process of Ni and Cu nanoparticles, which are usually explained by surface diffusion and free-energy minimization.45 The melting point of finite-sized metal nanoparticles decreased and made the particles like liquid. In addition, the liquid like Ni and Cu particles could catalyse the hydrocarbons and aromatics into graphene layers, which leaded to strong wall stress, making the shape of nanoparticles change.46 However, the mechanisms taking place during graphene growth process with regard to metal catalysts properties were completely different.47 The Cu with close ground state valence shell configuration (3d104s1) had a very low carbon solubility (0.1 atomic %). In addition, the configuration of Ni atom was isotropic and could not provide strong binding of Ni substrate and carbon, which drove carbon atoms from the bulk to the surface of Ni nanoparticles forming layer-by-layer graphene (Figure 9a, Figure 10a and c).48,49 With increase of graphene layers, liquid like nickel particles would restructure and get out from firm layers, so many graphene shells of different sizes left, as shown in Figure 10c.50, 51 And this is the most visible evidence for the efficient and sustainable catalytic graphitization of NiA. The different catalytic graphitization mechanisms of CuA and NiA in EVA28 resin were summarized in Scheme 2.


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Figure 9. Low-magnification TEM images of EVA28+Alginates residual char obtained from tube furnace under N2 atmosphere at 735 oC, the high temperature preservation is 3 minutes: (a) EVA28+NiA; (b) EVA28+CuA; (c) EVA28+ZnA. The black arrows in (a) mark the graphene layers on the Ni nanoparticles surfaces. The white arrows in (b) show the diameter of Cu nanoparticles.

Figure 10. High-magnification TEM images of EVA28+Alginates residual char obtained from tube furnace at 735 oC: (a) EVA28+NiA; (b) EVA28+CuA; (c) detail of EVA28+NiA. The white arrows in (c) show the thickness of graphene layers. Scheme 2. Schematic illustration of the catalytic graphitization of CuA and NiA.


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3.7 The Flame Retardancy Mechanisms of M-FR in EVA28+FR Composites In the pyrolysis process of M-FR, the Mg(OH)2 and APTES would decompose into MgO and SiO2, which were the most common catalyst supports for the catalytic synthesis of graphene, carbon nanotube and carbon fibre.52-54 The EVA28 and alginates resulted into metal or metal oxide, graphene and amorphous carbon, which could bind the inorganic compounds (MgO and SiO2) together and form some more protective char barriers. The alginate layers could inhibit the emigration and aggregation of inorganic compounds during combustion, as a result, some fluffier char formed.31 This point could be supported by the macro-appearance of residual char displayed in Figure 4. The EVA28+ Ni-FR char (Figure 4b) looked like fluffy sponges composed of high temperature resistant inorganic compounds and metal/graphene/amorphous carbon mixture, which act a better barrier slowing down heat transmission and volatiles diffusion. The fluffy char had a bigger contact area and few cracks, which indicated more gas products had been kept in condensed phase for the catalytic graphitization of metal Ni. However, the EVA28+Cu-FR, EVA28+Zn-FR char (Figure 4c and Figure 4d) were full of cracks, which meant more gases had released in the end of combustion. The mechanism agreed with the cone calorimeter results and the catalytic graphitization mechanisms of alginates in EVA28 resin. The SEM images (Figure S4) and TEM images (Figure S5) of EVA28+B-A and EVA28+B-Ni-FR residual char had supplied more evidence. 4. CONCLUSION In summary, we proposed a new strategy to prepare flame retardant rapidly, alginates and APTES were deposited on MH via spray drying assisted layer-by-layer (LbL) assembly technique. The morphology, chemical compositions structures of the flame retardants were characterized. The new flame retardants effectively improved the flame retardant and smoke suppression properties of the EVA+FR composites. The influence of alginates on the EVA resin pyrolysis were studied. These improvements were attributed to the catalytic graphitization and carbonization of alginates as well as the formation of more protective layers. The catalytic graphitization of alginates in polymer was revealed for the first time.

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Supporting Information SEM images of the flame retardants; The cone results of EVA18+FR; photographs and TEM images of EVA28+FR residual char obtained from cone calorimeter tests; TEM images of EVA28+alginates residual char. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]；Tel./fax:+86 10 6891 3075. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21474008) and National Key Research and Development of China (2016YFB0302104). REFERENCES (1) 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. (2) 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. (3) Salamova, A.; Hites, R. A. Brominated and Chlorinated Flame Retardants in Tree Bark from Around the Globe. Environ. Sci. Technol. 2013, 47, 349−354. (4) Zhou, S. N.; Buchar, A.; Siddique, S.; Takser, L.; Abdelouahab, N.; Zhu, J. P. Measurements of Selected Brominated Flame Retardants in Nursing Women: Implications for Human Exposure. Environ. Sci. Technol. 2014, 48, 8873−8880. (5) Hermanson, M. H.; Isaksson, E.; Forsstorm, S.; Teixeira, C.; Muir, D. C. G.; Pohjola, V. A.; Van de wal, R. S. V. Deposition History of Brominated Flame Retardant Compounds in an Ice Core from Holtedahlfonna, Svalbard, Norway. Environ. Sci. Technol. 2010, 44, 7405–7410. (6) Kemmlein, S.; Herzke, D.; Law, R. J. Brominated flame retardants in the European chemicals policy of REACH—Regulation and determination in materials. Environ. Sci. Technol. 2010, 44, 7405–7410. (7) 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. (8) Köklükaya, O.; Carosio, F.; Grunlan, J. C.; Wågberg, L. Flame Retardant Paper from Wood Fibers Functionalized via Layer-byLayer Assembly. ACS Appl. Mater. Interfaces 2015, 7, 23750−23759. (9) Richardson, J. J.; Cui, J. W.; Björnmalm, M.; Braunger, J. A.; Ejima, Hirotaka.; Caruso, F. Innovation in Layer-by-Layer Assembly. Chem. Rev. 2016, 116, 14828−14867. (10) Wang, Y. L.; Yang, X. M.; Peng, H.; Wang, F.; Liu, X.; Yang, Y. G.; Hao, J. W. Layer-by-Layer Assembly of Multifunctional Flame Retardant Based on Brucite, 3 Aminopropyltriethoxysilane, and Alginate and Its Applications in Ethylene-Vinyl Acetate Resin. ACS Appl. Mater. Interfaces 2016, 8, 9925−9935. (11) Singh, A.; Mooter, G. D. Spray drying formulation of amorphous solid dispersions. Advanced Drug Delivery Reviews 2016, 100, 27–50. (12) Shao, Z. B.; Deng, C.; Tan, Y.; Yu, L.; Chen, M. J.; Chen, L.; Wang, Y. Z. Ammonium polyphosphate chemically-modified with ethanolamine as an efficient intumescent flame retardant for polypropylene. J. Mater. Chem. A 2014, 2, 13955–13965. (13) Nine, M. J.; Cole, M. A.; Tran, D. N. H.; Losic, D. Graphene: a multipurpose material for protective coatings. J. Mater. Chem. A 2015, 3, 12580–12602. (14) Sarac, M. F.; Anderson, B. D.; Pearce R. C.; Railsback, J. G.; Oni, A. A.; White R. M.; Hensley, D. K.; LeBeau, J. M.; Melechko, A. V.; Tracy, J. B. Airbrushed Nickel Nanoparticles for Large-Area Growth of Vertically Aligned Carbon Nanofibers on Metal (Al, Cu, Ti) Surfaces. ACS Appl. Mater. Interfaces 2013, 5, 8955−8960.


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