Transparent Aqueous Mg(OH)2 Nanodispersion for Transparent and

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Transparent aqueous Mg(OH)2 nanodispersion for transparent and flexible polymer film with enhanced flame-retardant property Miao Wang, Xing-Wei Han, Long Liu, Xiaofei Zeng, Haikui Zou, Jie-Xin Wang, and Jianfeng Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03172 • Publication Date (Web): 04 Dec 2015 Downloaded from http://pubs.acs.org on December 8, 2015

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Industrial & Engineering Chemistry Research

Transparent aqueous Mg(OH)2 nanodispersion for transparent and flexible polymer film with enhanced flame-retardant property Miao Wanga, Xing-Wei Hana, Long Liua, Xiao-Fei Zeng*a, Hai-Kui, Zoub, Jie-Xin Wang*b and Jian-Feng Chenab a

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

b

Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing, 100029, China *

Corresponding author. Tel.: +86 10 64447274; Fax: +86 10 64423474

E-mail: [email protected]; [email protected];

ACS Paragon Plus Environment

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Abstract A transparent aqueous nanodispersion of magnesium hydroxide (MH) is firstly synthesized by using a rotating packed bed (RPB) reactor. Based on this nanodispersion, we further fabricate a transparent poly(vinyl alcohol) (PVA)/MH nanocomposite with enhanced flame-retardant property by a solution-mixing method. The PVA/MH nanocomposite can keep good transparency even at high MH contents. The limiting oxygen index (LOI) of PVA/MH nanocomposite with 50 phr MH is improved by 45% compared to pure PVA. The peak of heat release rate (pHRR) in the microscale combustion calorimetry (MCC) significantly reduces from 453 W/g for pure PVA to 332 W/g. Importantly, a smooth and compact residues layer for the PVA/MH nanocomposite is formed during combustion, which yields a barrier for the transfer of heat and oxygen. These results indicate that the PVA/MH is an excellent transparent flameretardant material, which opens a door to manufacture transparent and fire-resistant polymerbased nanocomposites with environmental-friendly process. Keywords: Magnesium hydroxide; Transparent aqueous nanodispersion; Nanocomposite; Flame-retardant


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1. Introduction Organic-inorganic hybrid materials have recently attracted considerable interest due to their enhanced properties, compared to the corresponding inorganic or organic component only.1-3 In particular, a great deal of study has focused on the preparation of transparent nanocomposites via incorporating inorganic nanoparticles into optical polymer matrices by a chemical or physical method, owing to their widely applications in the fields of liquid crystal displays, touch panels, light emitting diodes, smart windows and solar cells.4-10 However, how to disperse the inorganic nanoparticles mono-uniformly into organic matrices without aggregation and maintain the transparency of the polymer is still a great challenge. For traditional powder technology, the aggregation of nanoparticles formed during drying process is almost inevitable.11 To overcome this problem, an efficient way is to prepare transparent dispersion of inorganic nanoparticles in a liquid medium containing the monomer or polymer and then the polymer-inorganic hybrid materials are fabricated via directly drying, melt blending or chemical polymerization method. Such transparent dispersion of inorganic nanoparticles can be regarded as a “transparent filler” to maintain the transparency of the resulting nanocomposites.12-16 Actually, polymer materials are very sensitive to flame, which would restrict further application of transparent nanocomposites to a certain extent. Therefore, it is necessary to improve their fire resistance by adding flame retardants.17,18 Halogen-containing chemicals are the most effective and widely used flame retardants, but they usually release toxic gases and corrosive smoke during combustion. For environmental requirement, an efficient halogen-free flame retardant for transparent nanocomposites is desirable. Nano-sized magnesium hydroxide (MH) as a toxic- and smoking-free additive has been acceptable for halogen-free flame-retardant polymeric composites in the past decade.19-25 However, with the decrease in particle size, the surface area will be increased and may induce the aggregation of nanoparticles more easily, causing degradation in the mechanical and physical properties of the composites. Therefore, it is essential to modify the surface of MH nanoparticles to improve the dispersion and the compatibility with polymer matrix. Chen et al.26 reported superfine MH with surface treatment was added into polypropylene (PP) as flame retardant. Compared with pure PP, the LOI value of PP composites containing untreated MH (100/100) was increased from 18.0 to 28.2, whereas the corresponding value of PP composites containing modified MH was improved further to 29.2 because of the enhanced dispersion of the modified


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MH particles in the polymer matrix. Numerous investigations on the modification of MH have been reported.27-29 However, the size or dispersity of the MH particles is hard to meet the requirement on the transparent nanocomposites. Transparent nanodispersion of MH and its application in transparent polymer seem not available so far. Rotating packed bed (RPB), as known as Higee apparatus, can generate a high-gravity environment with the centrifugal force produced by the rotation.30 Since the RPB can significantly intensify the mass transfer and micromixing,31,32 it has been regarded as a promising reactor for the production of inorganic nanoparticles. Compared with the conventional reactors, RPB has several advantages in preparing nanoparticles, such as small particle size with narrow size distribution, short reaction time, easy industrialization, etc. Our group has previously reported a facile route to synthesize transparent ethylene glycol dispersion of MH nanocrystals with high MH content via a RPB reactor.33 The as-prepared MH nanocrystals were well-dispersed in ethylene glycol and could be stable for more than three months without any sedimentation. In this work, to sustain the growing environmental-friendly demand far away from the pollution caused by organic solvents, transparent aqueous phase dispersion of MH nanocrystals was successfully prepared via the precipitation method in RPB with subsequently surface treatment. Moreover, a transparent nanocomposite was fabricated by a simple solution blending of the obtained MH nanodispersion and poly(vinyl alcohol) (PVA) aqueous solution. The transparency of the composite was maintained, and simultaneously its flame retardancy was enhanced. 2. Experimental section 2.1 Materials and setup Magnesium chloride hexahydrate (MgCl2•6H2O), sodium hydroxide (NaOH), sodium hexametaphosphate (SHMP) and ethanol were obtained from Beijing Chemical Reagent Co., Ltd. (China). Poly(vinyl alcohol) (PVA) with the polymerization degree of 1750 ± 50 was provided by Sinopharm Chemical Reagent Co., Ltd. (China). Silane coupling agent γmethacryloxypropyltrimethoxysilane (KH-570) was purchased from Nanjing Capatue Chemical Co., Ltd. (China). All the chemicals were used without further treatment. The key experimental setup for preparing nanoparticles is a rotating packed bed (RPB) reactor, which consists of a rotator with stainless packing, a casing, two liquid inlets, an outlet and a


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motor. The schematic diagram of a rotating packed bed is shown in Figure 1, and more details regarding the RPB can be found in our previous work.31 2.2 Preparation of transparent Mg(OH)2 aqueous nanodispersion . A 0.2 mol/L MgCl2·6H2O ethanol solution and a 0.4 mol/L NaOH ethanol solution were simultaneously pumped into RPB with the same flow rate of 400 ml/min. The operating condition of RPB was a rotating speed of 2000 rpm, and the temperature was kept at room temperature during the experimental run. The generated MH nanoparticles suspension was then collected into a modification tank. Afterwards, a certain amount of silane coupling agent KH570 (KH570/MH=8 wt %) was added into the tank. The mixture was kept by mechanical stirring at 70 °C for 3h. The products were filtered, and washed sequentially with ethanol and deionized water for three times, respectively. Finally, the MH filter cake was redispersed in deionized water by ultrasonication with an addition of sodium hexametaphosphate (SHMP, SHMP/MH = 10 wt %), which eventually formed a transparent aqueous dispersion of MH nanocrystals (NMH). As comparison, a blank unmodified MH sample (UMH) was prepared without KH570 and SHMP under the same processing conditions. 2.3 Preparation of PVA/MH nanocomposites A certain amount of PVA was dissolved in hot water at 95 °C with vigorous stirring for 3 h to form a transparent 10 wt% PVA aqueous solution. Then, the as-synthesized MH nanodispersion was added to the PVA solution with mechanical stirring for about 1 h. After stirring, the transparent solution was poured onto a glass plate to obtain a film with thickness of ~0.5 mm, which was placed at ambient temperature for 3 days. Finally, the PVA/MH nanocomposites films were dried at 60 °C in vacuum for another 24 h. Instead of the MH aqueous nanodispersion, the unmodified MH suspended in water was also used for the preparation of nanocomposites. The PVA/MH nanocomposites filled with modified MH and unmodified MH nanoparticles were denoted as PVA/NMH and PVA/UMH, respectively. The obtained nanocomposites films were further cut into pieces for LOI and UL-94 tests. 2.4 Characterization The crystal structure of the synthesized MH nanoparticles was characterized by X-ray diffraction (XRD) with CuKα radiation (λ=0.15406 nm) on a SHIMADZU 6000 X-ray diffractometer. The laser particle size analysis and zeta potential measurement were performed by MALVERN Zetasizer Nano ZS90. The morphology of the MH nanodispersion and PVA/MH


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nanocomposites were measured by a JEOL JEM-2010F transmission electron microscope (TEM). FT-IR spectrum was carried out with a Bruker Vector 70 spectrometer employing a KBr pellet method. The thermal behavior was examined on a Netsch STA449C thermal gravimetric analyzer at a heating rate of 10°min-1 under air or nitrogen. The limiting oxygen index (LOI) test was detected by a Jiangning JF-3 oxygen index meter with dimensions of 140 × 52 × 0.5 mm3 according to GB/T 2406-93. The UL-94 vertical burning test was measured on a Jiangning CZF2 vertical burning tester with dimensions of 130 × 13 × 0.5 mm3 according to ASTM D3801. The microscale combustion calorimetry (MCC) was performed on a microscale combustion calorimeter (FAA-PCFC, Fire Testing Technology Ltd, UK). The microstructures of the residues after LOI test were observed using a JEOL JSM-6701 scanning electron microscope. The transparency of PVA/MH nanocomposites was characterized by a SHIMADZU UV-2501 UVVis spectrometer in the range of 400-800 nm. 3. Results and discussion 3.1 Characterizations of the MH aqueous nanodispersion The transmission electron microscope (TEM) image of the MH nanodispersion is shown in Figure 2a. As can be seen, the MH nanoparticles in the nanodispersion exhibit regular lamella morphology, and are well-dispersed in water nearly monodispersed. The size of the MH nanoparticles ranges from 20 nm to 60 nm with an average value of ~ 40 nm. By contrast, the unmodified MH sample presents obvious agglomerations as shown in Figure 2b, and the particle size is larger than that of the MH particles in the nanodispersion. Furthermore, the size distribution of the unmodified MH and modified MH nanoparticles were measured by dynamic light scattering (DLS). The DLS curves of the MH nanoparticles are shown in Figure 2c. It is found that the particle size distribution of the modified MH is narrower, which indicates the MH nanoparticles are well-dispersed in the nanodispersion. The peak of the curve of the unmodified MH moves to larger scale due to the cohesion of the aggregates. Meanwhile, the zeta potentials were also detected. The zeta potential of the modified MH in the nanodispersion decreases from 5.42 to -30.3 mV compared to the unmodified MH, which is attributed to the adsorption of the SHMP.34 The increased negative charges on the MH surface can inhibit the aggregates or agglomerations by the electrostatic repulsion. Figure 2d shows the photographs of the MH aqueous nanodispersion and unmodified MH suspension in water with the MH solid content of 1 wt%. It is clearly that the MH nanodispersion has better transparency compared to the


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unmodified MH suspension. Agglomeration of particles leads to harmful effects on the transparency of the unmodified MH suspension. After storage for three days, obvious sedimentations were found in the unmodified MH sample. However, the MH aqueous nanodispersion kept stable without sedimentation, which was very important for further applications. Therefore, it is believed that the combined effect of KH570 and SHMP is beneficial for the stable and transparent dispersion of MH in water. Figure 3a shows the XRD spectra of the unmodified MH and modified MH, respectively. Both diffraction peaks are indexed very well as hexagonal phase MH according to the standard data (JCPDS 7-239). No additional characteristic peaks are observed in the XRD spectra. Moreover, it is noted that the peaks of the modified-MH are significantly broadened and weakened, especially in the (001), (101) and (102) lattice plans, which proves that the modified MH nanoparticles have small grain size. The FT-IR spectra of the unmodified MH and modified MH are shown in Figure 3b. In the unmodified MH sample, remarkable strong peaks at 3699 cm-1 and 440 cm-1 are assigned to the stretching vibrations of the O-H bond and the Mg-O bond in the MH nanocrystals, respectively.35 The absorption at 1437 cm-1 is the bending vibration of water. The peaks at 3406 cm−1 and 1649 cm−1 can be assigned to the stretching and bending vibration of water.36 In the modified MH sample, it is found that there are some weak peaks at 2980-2800 cm−1, which is due to the C-H stretching vibration in KH570. And the peaks at 1109 cm-1 and 1066 cm-1 for the stretching vibration of the Si-O bond also can be observed. These results prove that the KH570 was grafted on the surface of MH successfully. Meanwhile, the characteristic peak of P-O bond at 881 cm-1 could be detected, indicating the SHMP was adsorbed onto the surface of MH. Figure 3c shows the TGA curves of the unmodified MH and modified MH under air atmosphere. Compared with the unmodified MH, the lower decomposition temperature and more weight loss in the temperature range from 100 to 340 °C of the modified MH are mainly caused by the decomposition of KH570 grafted on the MH, and the enhanced residue above 340 °C is assigned to the present of SHMP. It can be further calculated that the amount of modifier on the MH nanoparticles is about 13 wt%. 3.2 Optical properties of the PVA/MH nanocomposites It is well-known that PVA has been widely used as coating and packing materials,37 and high transparence is necessary and important in these applications. Figure 4a and 4b show the UV-


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Visible transmittance spectra in the visible region (400-800 nm) of pure PVA, PVA/NMH and PVA/UMH films with various MH contents. With the increase of MH loading, the PVA/NMH films display a slight decline in the transmittance at 400-800 nm, but they remain comparatively high transmittance in the visible region. In contrast, the PVA/UMH films show lower transmittance value even at low MH contents. For instance, the transmittance at 500 nm of the PVA/NMH film containing 50 phr of MH is also over 90%, while that of the PVA/UMH film with 10 phr MH decreases to 54% from 98% for the pure PVA film. The dramatic difference of the optical transparency is further shown by the photographs of the PVA/NMH and PVA/UMH films. The PVA/NMH films are quite transparent that close to the pure PVA film. Moreover, the PVA/NMH film with 10 phr MH content exhibits the desired flexibility as shown in the inset of Figure 4c. The transparency of the PVA/NMH films slightly decreases with the increasing MH contents. The PVA/UMH films, by contrast, are translucent and even opaque at high MH contents. Figure 4e and 4f show the TEM images of PVA/NMH and PVA/UMH nanocomposites. It can be seen in the TEM image of PVA/NMH that the modified MH nanoparticles remain their original size of ~40 nm and disperse well in the PVA matrix at a nanoscale level. But for the PVA/UMH sample, the unmodified MH nanoparticles exhibit serious aggregation in the PVA matrix, with the large aggregated size up to microscale, which causes the significant decrease in the transparency of the PVA/UMH nanocomposites as above discussion. 3.3 Thermal behavior of the PVA/MH nanocomposites The thermal behavior of pure PVA and PVA/MH nanocomposites were investigated by the thermal gravimetric analysis (TGA) under a nitrogen atmosphere. Figure 5 shows the TGA and differential thermogravimetric (DTG) curves of pure PVA, PVA/NMH, and PVA/UMH with the same MH loading of 50 phr, and the corresponding data are summarized in Table 1. The degradation of pure PVA in nitrogen atmosphere mainly contains two stages, corresponding to the degradation of side groups (release of water and acetic acid) and the main chain degradation (chain-scission and cyclization).38 In comparison with pure PVA, both the PVA/UMH and PVA/NMH nanocomposites show better thermal stability with enhanced amount of residues above 280 °C and the delayed stage of the main chain degradation (increased T2). This is caused by the decomposition of MH to magnesium oxide along with elimination of water and absorption of heat, which benefits for the improvement of flame retardancy. Compared with PVA/UMH, although there is no remarkably difference of the residue yield, the PVA/NMH


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exhibits higher initial decomposition temperature. That is to say, the thermal stability of PVA/NMH is better than that of the PVA/UMH. In addition, both of the Tinitial and T1 of PVA/NMH and PVA/UMH shift to lower temperature, owing to the catalytic effect on the degradation of side groups by MH nanoparticles.39 3.4 Flame retardancy of PVA/MH nanocomposites The limiting oxygen index (LOI) and UL-94 vertical burning test are widely employed as indicator to evaluate the flame retardancy of polymeric composites.40 Figure 6 shows the LOI of PVA/UMH and PVA/NMH with different MH contents, and the UL-94 results are summarized in Table 2. It is obvious that the LOI of pure PVA is 19.5, and it fails to reach any of the UL-94 ratings. With increasing MH content, the LOI of PVA/NMH and PVA/UMH increase gradually, and higher UL-94 ratings can be obtained when more MH is incorporated. For instance, the LOI value of PVA/NMH composites with the MH content of 50 phr can reach to 28.3 and the UL-94 can achieve V-0 rating, while the corresponding LOI value of PVA/UMH is only 25.6 and the UL-94 only reaches V-1 rating. Obviously, the LOI of PVA/NMH is higher than that of the PVA/UMH with the same MH content. In other word, a smaller amount of modified MH is needed to meet the same LOI value. As a result, the modified MH enhances the flame retardancy of PVA more effectively than the unmodified MH. The microscale combustion calorimetry (MCC) is also a easy way to characterize the flammability properties of polymeric materials.41 Figure 7 shows the heat release rate (HRR) curves of pure PVA, PVA/NMH and PVA/UMH with the same MH content of 50 phr, and the primary parameters obtained from the HRR curves are presented in Table 3. Generally speaking, the lower the pHRR, THR, and HRC value, the less the fire hazard. The flame-retardant mechanism of MH mainly results from the endothermic reaction of its decomposition, along with releasing water and forming a magnesium oxide layer to hold back heat transfer and flame gases. Obviously, the incorporation of MH nanoparticles remarkably reduced all the data as shown in Table 3. Compared with PVA/UMH, the pHRR value of PVA/NMH reduces from 380 to 332 W/g. Meanwhile, the THR and HRC values also decrease apparently. In addition, with the addition of MH nanoparticles, the Tp of PVA/NMH and PVA/UMH shift to low temperature, which are attributed to the acceleration effect of MH on the removal of water and residual acetate groups, corresponding to the TGA results.


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The scanning electron microscope (SEM) images in Figure 8 present the microstructures of the residues of the PVA/UMH and PVA/NMH after the LOI test. As shown in Figure 8a, there are many cracks and holes on the outer surface of residue of the PVA/UMH sample; however, the incontinuous residue layer is not a suitable barrier to endow the underlying materials with high flame retardancy. By contrast, the PVA/NMH sample shows compact and continuous residue layer without pores. This kind of residue layer is significant to reduce the heat transfer and cut off oxygen from the degraded volatiles effectively, and consequently can prevent the materials from further combustion. On the basis of the nanoscale dispersion of MH nanoparticles in PVA matrix, the compact char layer of PVA/NMH could be achieved so that it has higher LOI value and better fire hazard evaluation in the MCC test at the same MH content. 4. Conclusions In summary, we have successfully prepared the transparent MH nanodispersion in aqueous medium via a RPB reactor with subsequently surface treatment through synergistic effect of KH570 and SHMP. Based on the MH aqueous nanodispersion, a transparent PVA/MH nanocomposite has been fabricated by solution blending of the obtained MH nanodispersion and aqueous PVA solution. The lamellar MH nanoparticles with the mean size of 40 nm can be welldispersed in water without agglomeration. Due to the perfect dispersion of the MH nanoparticles, the PVA/MH nanocomposites can keep good transparency even at high MH content up to 50 phr. Compared with pure PVA, the nanocomposites exhibited excellent flame retardancy with the increased LOI value from 19.5 to 28.3, UL-94 V-0 rating, and decreased pHRR in MCC from 453 to 332 W/g. Moreover, a compact and continuous magnesium oxide layer was formed during combustion, which was beneficial for holding back the transfer of heat and flame gases. As a green fire retardant material, the transparent MH aqueous nanodispersion would be potential applications in optical flexible materials and devices with high transparency and excellent flame retardancy. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 21476024 and No. 21576022), Beijing Municipal Science and Technology Project (Z151100003315005), National ‘863’ Program of China (NO. 2013AA032003).


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(19) Li, Z. Z.; Qu, B. J. Flammability Characterization and Synergistic Effects of Expandable Graphite with Magnesium Hydroxide in Halogen-Free Flame-Retardant EVA Blends. Polym. Degrad. Stab. 2003, 81, 401. (20) Balakrishnan, H.; Hassan, A.; Isitman, N. A.; Kaynak, C. On the Use of Magnesium Hydroxide Towards Halogen-free Flame-Retarded Polyamide-6/Polypropylene Blends. Polym. Degrad. Stab. 2012, 97, 1447. (21) Liu, J. C.; Yu, Z. L.; Shi, Y. Z.; Chang, H. B.; Zhang, Y. B.; Luo, J.; Lu, C. A Preliminary Study on the Thermal Degradation Behavior and Flame Retardancy of High Impact Polystyrene/Magnesium Hydroxide/Microencapsulated Red Phosphorus Composite with a Gradient Structure. Polym. Degrad. Stab. 2014, 105, 21. (22) Kim S. Flame Retardancy and Smoke Suppression of Magnesium Hydroxide Filled Polyethylene. J. Polym. Sci. Part B: Polym. Phys. 2003, 41, 936. (23) Sener, A. A.; Demirhan, E. The Investigation of Using Magnesium Hydroxide as a Flame Retardant in the Cable Insulation Material by Cross-Linked Polyethylene. Mater. Des. 2008, 29, 1376. (24) Gholamian, F.; Salavati-Niasari, M.; Ghanbari, D.; Sabet, M. The Effect of Flower-Like Magnesium Hydroxide Nanostructure on the Thermal Stability of Cellulose Acetate and Acrylonitrile–Butadiene–Styrene. J. Clust. Sci. 2013, 24, 73. (25) Ghanbari, D.; Salavati-Niasari, M.; Sabet, M. Preparation of Flower-Like Magnesium Hydroxide Nanostructure and Its Influence on the Thermal Stability of Poly Vinyl Acetate and Poly Vinyl Alcohol. Composites: Part B 2013, 45, 550. (26) Chen, X. L.; Yu, J.; Guo, S. Y. Structure and Properties of Polypropylene Composites Filled with Magnesium Hydroxide. J. Appl. Polym. Sci. 2006, 102, 4943. (27) Cabrera-Álvarez, E. N.; Ramos-deValle, L. F.; Sánchez-Valdes, S.; Candia-García, A.; Soriano-Corral, F.; Ramíre-Vargas, E.; Ibarra-Alonso, M. C.; Patiño-Soto, P. Study of the Silane Modification of Magnesium Hydroxide and Their Effects on the Flame Retardant and Tensile Properties of High Density Polyethylene Nanocomposites. Polym. Compos. 2014, 35, 1060.


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(28) Yang, Z.; Cai, J.; Zhou, C. G.; Zhou, D.; Chen, B. F.; Yang, H.; Cheng, R. S. Effects of the Content of Silane Coupling Agent KH-560 on the Properties of LLDPE/Magnesium Hydroxide Composites. J. Appl. Polym. Sci. 2010, 118, 2634. (29) Yan, H.; Zhang, X. H.; Wei, L. Q.; Liu, X. G.; Xu, B. S. Hydrophobic Magnesium Hydroxide Nanoparticles via Oleic Acid and Poly(methyl methacrylate)-Grafting Surface Modification. Powder Technol. 2009, 193, 125. (30) Chen, J. F.; Wang, Y. H.; Guo, F.; Wang, X. M.; Zheng, C. Synthesis of Nanoparticles with Novel Technology: High-Gravity Reactive Precipitation. Ind. Eng. Chem. Res. 2000, 39, 948. (31) Yang, H. J.; Chu, G. W.; Xiang, Y.; Chen, J. F. Characterization of Micromixing Efficiency in Rotating Packed Beds by Chemical Methods. Chem. Eng. J. 2006, 121, 147. (32) Yang, Q.; Wang, J. X.; Guo, F.; Chen, J. F. Preparation of Hydroxyaptite Nanoparticles by Using High-Gravity Reactive Precipitation Combined with Hydrothermal Method. Ind. Eng. Chem. Res. 2010, 49, 9857. (33) Sun Q.; Chen B.; Wu X.; Wang M.; Zhang C.; Zeng X. F.; Wang J. X.; Chen J. F. Preparation of Transparent Suspension of Lamellar Magnesium Hydroxide Nanocrystals Using a High-Gravity Reactive Precipitation Combined with Surface Modification. Ind. Eng. Chem. Res. 2015, 54, 666. (34) Ji, Y.; Wang, J.; Xiang, L. (NaPO3)6-Assisted Wet Formation of Dispersive Mg(OH)2 Nanoplates. Mater. Res. Innov. 2015, 19, S2-147. (35) Wu, J. M.; Yan, H.; Zhang, X. H.; Wei, L. Q.; Liu, X. G.; Xu, B. S. Magnesium Hydroxide Nanoparticles Synthesized in Water-in-Oil Microemulsions. J. Colloid Interface Sci. 2008, 324, 167. (36) Jiang, W. J.; Hua, X.; Han, Q. F.; Yang, X. J.; Lu, L. D.; Wang, X. Preparation of Lamellar Magnesium Hydroxide Nanoparticles via Precipitation Method. Powder Technol. 2009, 191, 227.


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(37) Lin, J. S.; Chen, L.; Liu, Y; Wang, Y. Z. Flame-Retardant and Physical Properties of Poly(vinyl alcohol) Chemically Modified by Diethyl Chlorophosphate. J. Appl. Polym. Sci. 2012, 125, 3517. (38) Guo, D.; Wang, Q.; Bai, S. B. Poly(vinyl alcohol)/Melamine Phosphate Composites Prepared through Thermal Processing: Thermal Stability and Flame Retardancy. Polym. Adv. Technol. 2013, 24, 339. (39) Parveen, M. F.; Umapathy, S.; Dhanalakshmi, V.; Anbarasan, R. Synthesis and Characterization of Nanosized Mg(OH)2 and Its Nanocomposite with Poly(Vinyl Alcohol). Nano 2009, 4, 147. (40) Bahattab, M. A.; Mosnáček, J.; Basfer, A. A.; Shukri, T. M. Cross-Linked Poly(ethylene vinyl acetate) (EVA)/Low Density Polyethylene (LDPE)/Metal Hydroxides Composites for Wire and Cable Applications. Polym. Bull. 2010, 64, 569. (41) Lu, H. D.; Wilkie, C. A.; Ding, M.; Song, L. Flammability Performance of Poly(vinyl alcohol) Nanocomposites with Zirconium Phosphate and Layered Silicates. Polym. Degrad. Stab. 2011, 96, 1219.


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Figure 1. Schematic diagram of a rotating packed bed (RPB)


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Figure 2. TEM images of (a) the MH aqueous nanodispersion and (b) the unmodified MH suspension in water; (c) DLS-based size distribution of the modified MH and unmodified MH nanoparticles; (d) The photographs of the MH nanodispersion and unmodified MH suspension in water (MH content: 1 wt%).


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Figure 3. (a) XRD spectra of the unmodified MH (1) and modified MH (2); (b) FT-IR spectra of the unmodified MH (1) and modified MH (2); (c) TGA curves of the unmodified MH (1) and modified MH (2) under air atmosphere.


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Figure 4. The UV-Vis transmittance spectra of (a) PVA/NMH and (b) PVA/UMH films; The photographs of (c) PVA/NMH and (d) PVA/UMH films with various MH contents; The TEM images of (e) PVA/NMH and (f) PVA/UMH nanocomposites.


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Figure 5. (a) TGA and (b) DTG curves of PVA and PVA/MH nanocomposites with MH content of 50 phr under nitrogen atmosphere.


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Figure 6. The limiting oxygen index (LOI) of PVA/NMH and PVA/UMH nanocomposites


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Figure 7. Heat release rate (HRR) curves of PVA, PVA/NMH and PVA/UMH in the microscale combustion calorimetry.


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Figure 8. SEM images of the residues of (a) PVA/UMH and (b) PVA/NMH after the LOI test.


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Table 1. TGA data of PVA, PVA/NMH and PVA/UMH Sample

Tinitiala (°C)

T1b (°C)

TMHc (°C)

T2b (°C)

Residues yield at 800 °C (%)

PVA

232

268

-

435

9.5

PVA/NMH

220

247

377

445

33.8

PVA/UMH

217

237

355

443

34.2

a

Tinitial, temperature at 5% weight loss; bT1 and T2, the temperature at maximum rate of the weight

loss; cTMH, the temperature at maximum rate of the weight loss of MH.


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Table 2. The formulations and UL-94 results of the PVA/MH nanocomposites Sample

Component

UL-94

PVA

NMH (phra)

UMH (phr)

PVA

100

-

-

No rating

PVA/NMH-30

100

30

-

No rating

PVA/NMH-40

100

40

-

V-1

PVA/NMH-50

100

50

-

V-0

PVA/UMH-30

100

-

30

No rating

PVA/UMH-40

100

-

40

No rating

PVA/UMH-50

100

-

50

V-1

a

phr, parts per hundred PVA resin by weight.


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Table 3. MCC data of PVA, PVA/NMH and PVA/UMH Sample

pHRR (W/g)

THR (kJ/g)

HRC (J/g·K)

Tp (°C)

PVA

453

19.4

273

274

PVA/NMH

332

7.3

198

264

PVA/UMH

380

14.9

228

260

pHRR, peak heat release rate; THR, total heat released; HRC, heat release capacity; Tp, temperature at pHRR.


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For Table of Contents Only:


27

Transparent Aqueous Mg(OH)2 Nanodispersion for Transparent and

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