Preparation of Ultrafine Nanoparticles under Supergravity Field and

Nov 6, 2014 - optimized using response surface methodology (RSM). The .... 2000 laser particle size analyzer from Malvern (Malvern, U.K.) was used. 2...
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Preparation of Ultrafine Nanoparticles under Supergravity Field and Their Flame-Retardant Properties Hong-Yan Zeng,* Pei-Han Zhu, Sheng Xu, Meng-Chen Liao, Zhi-Qing Zhang, Xiao-Jun Liu, and Jin-Ze Du School of Chemical Engineering, Xiangtan University, Xiangtan 411105, Hunan China ABSTRACT: Ultrafine Mg/Al hydrotalcite (MHT) nanoparticles were prepared using a new method in a rotating packed bed (RPB). The MHT nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), Fourier transform infrared (FT-IR) spectroscopy, and laser particle size analysis. By varying preparation parameters such as precipitation temperature, precipitation time, aging time, and [CO32−]/[NO3−] molar ratio in reaction solution, we investigated the effects of the parameters on the precipitation yield of the MHT nanoparticles. Response surface methodology (RSM) coupled with Box−Behnken design was used to optimize the preparation parameters, and an 85.5% yield of MHT nanoparticles with a Mg/Al molar ratio of 3 (MHT-3) was produced under the optimum conditions. The MHT-3 nanoparticles were melted and blended with polypropylene (PP) at different loadings of MHT-3. The PP/MHT-3 composite (PP/MHT-3) with a 10 wt % loading achieved an LOI value close to 21% and a UL 94 grade of V-2, and the mechanical properties of the PP/MHT-3 composite remained mostly unchanged compared to those of PP. Thermogravimetric analysis showed that the addition of 10 wt % MHT-3 resulted in an increase in both the onset and decomposition temperatures of PP polymer, indicating that the PP/ MHT-3 composite exhibited good thermal properties. The results suggest that ultrafine hydrotalcite nanoparticles can be obtained in an RPB, providing a new approach for the industrial preparation of ultrafine hydrotalcite nanoparticles with good antiflaming performances.

1. INTRODUCTION Layered nanostructured materials have attracted much attention as fire retardants for polymers. Layered silicate nanomaterials with good dispersion have been found to exhibit outstanding mechanical and physical properties and to improve the flame-retarding effect of neat polymers.1,2 Layered double hydroxides (LDHs) are a class of nanolayer materials that have positively charged brucite-like sheets, between which are located intercalated anions and some water molecules.3 Recently, many studies have focused on polymer−LDH composites.4,5 However, the flame-retardant efficiencies of the resulting composites remain low, because high loadings result in steep drops in the mechanical properties of the flameretardant materials. Metal hydroxides are typically added in the amount of 40−65 wt % to obtain flame-retardant polymers.6 Many efforts have been made to improve the flame-retardant effectiveness of hydrotalcites. A loading of Mg/Al−CO3 hydrotalcites of at least 50 wt % was required to reach an appreciable flame-retardant behavior of ethylene−vinyl acetate (EVA) polymer with a loss on ignition (LOI) of 29.5% and an HB flame-retardant rating.7 When dodecylsulfate-intercalated LDH particles were blended with polypropylene (PP) at a 10 wt % loading, the LOI value (19.3%) was improved, and the UL 94 V testing time to start dripping was 21 s.8 Ultrafine nanoparticles are believed to be very effective in improving the flame retardance and mechanical properties of polymeric materials. To enhance the flame-retardant efficiency of polymers, the loading of LDHs must be decreased. The use of ultrafine nanoparticles is an essential method for reducing the loading. To obtain nanoparticles with smaller sizes and © 2014 American Chemical Society

narrower size distributions by the coprecipitation method, supergravity fields have been widely applied in the low-cost and high-volume production of ultrafine nanoparticles. Because supergravity can greatly intensify micromixing (mixing on the molecular scale) and gas−liquid or liquid−liquid mass transfer, it has been successfully applied to the synthesis of well-defined nanoinorganic powders.9 It was reported that nanoparticles with small sizes and narrow size distributions can be successfully produced in supergravity rotating packed bed (RPB) reactors.10−12 For example, hydroxyapatite nanoparticles with a mean size of 55−110 nm and a narrow size distribution were successfully prepared in a supergravity field.9 Chen et al.12 employed an RPB reactor to prepare CaCO3 nanoparticles with mean sizes of 17−36 nm and SrCO3 nanoparticles with a mean size of 40 nm. The RPB reactor has advantages over conventional reactors in the production of nanoparticles, such as short reaction time, small particle size, narrow size distribution, and easy application in industry. The present work demonstrates a new method for the preparation of ultrafine Mg/Al hydrotalcite (MHT) nanoparticles by high-gravity reactive precipitation in an RPB reactor and includes an analysis of key preparation parameters influencing reactive precipitation. The key preparation parameters (precipitation temperature, aging time, and [CO32−]/[NO3−] molar ratio in the reaction solution) were Received: Revised: Accepted: Published: 18380

July 6, 2014 November 3, 2014 November 6, 2014 November 6, 2014 dx.doi.org/10.1021/ie502689e | Ind. Eng. Chem. Res. 2014, 53, 18380−18389

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MHT nanoparticles with Mg/Al molar ratios of 2.0, 3.0, and 4.0 are designated as MHT-2, MHT-3, and MHT-4, respectively. The precipitation yield (%) of MHT nanoparticles was determined as the ratio of the experimental mass of precipitate to the theoretical value. Response surface methodology (RSM) is a powerful technique for testing multiple process variables with few experimental trials and can be used evaluate the effects of several factors and the interactions among them. After approximation of the optimal conditions by the “one-factorat-a-time” method, RSM was used to test the effects of precipitation temperature, precipitation time, aging time, and [CO32−]/[NO3−] molar ratio in the reaction solution on the precipitation yield of sample MHT-3. Box−Behnken design (BBD) was used to design the experiments and optimize one response variable, namely, precipitation yield (Y) from the precipitation process of MHT-3. Each independent variable was coded at three levels between −1 and +1, where the ranges of the variables precipitation temperature (T), precipitation time (P), aging time (A), and [CO32−]/[NO3−] molar ratio (M) are listed in Table 1. Twenty-nine experiments were augmented

optimized using response surface methodology (RSM). The effects of the supergravity field on the microstructure and morphology of the MHT nanoparticles were investigated by Xray diffraction (XRD), scanning electron microscopy/energydispersive spectroscopy (SEM/EDS), transmission electron microscopy/selected-area electron diffraction (TEM/SAED), and Fourier transform infrared (FT-IR) analyses. The nanoparticles were incorporated into polypropylene (PP) to produce PP/LDH composites with improved flame-retardant properties of the PP material.

2. EXPERIMENTAL SECTION 2.1. Materials. Polypropylene particles (K8303, melt flow rate of 2.6 g·10 min−1 at 230 °C and 2.16 kg) with a particle size about 1 mm were purchased from Yanshan Petrochemical I Co., Ltd. (Beijing, China). All chemicals were of analytical grade and were purchased from Sinopharm Chemical Reagent Co. Ltd. (Beijing, China). All other reagents used in the experiments were of analytical grade, and all solutions were made with deionized water. 2.2. Preparation of MHT Nanoparticles. The supergravity rotating packed bed (RPB) reactor was from the School of Chemical Engineering, Xiangtan University, and is shown in Figure 1. Mg/Al hydrotalcite (MHT) nanoparticles with Mg/Al

Table 1. Experimental Ranges and Levels of the Independent Variables code levels independent variable

symbol

units

−1

0

1

precipitation temperature precipitation time aging time [CO32−]/[NO3−] molar ratio

T P A M

°C min h −

45 15 2 1.0

60 30 4 1.5

75 45 6 2.0

with three replications at the design center to evaluate the pure error and were carried out in randomized order as required in many design procedures. After completion of the reaction under the specified conditions, the response Y was measured. The statistical software package Design Expert (version 8.0.6) was used for regression analysis of the experimental data and for plotting the response surface. The model generated during RSM implementation was validated by conducting experiments on the given optimal setting. The following second-order polynomial model was applied to predict the response variable (Y)

Figure 1. Experimental setup for producing Mg/Al hydrotalcite (MHT) nanoparticles in a rotating packed bed (RPB) reactor.

molar ratios of 2.0, 3.0, and 4.0 were prepared in the RPB reactor according to the method described in the literature.11 A mixed salt solution containing Mg(NO3)2·6H2O and Al(NO3)3·9H2O was prepared. (NH4)2CO3 was used as the precipitant, and solutions of (NH4)2CO3 were prepared with varying concentrations based on [CO32−]/[NO3−] molar ratios (assuming an equal volume of mixed salt solution) of 0.5−3.0. Then, the mixed salt solution and the (NH4)2CO3 solution (each 3.0 L) were poured into separate tanks (see Figure 1) at 25 °C. First, the mixed salt solution was pumped by its distributor into the reaction bed. When the flow rate of the mixed salt solution was stable at about 500 L·h−1, the (NH4)2CO3 solution was pumped at a flow rate of 250 L·h−1 into the reaction system. The reaction system was rotated at 5000 rpm. Its temperature was kept at 25−80 °C by a circulating water from a thermostat to the jacket of the RPB reactor, and the pH in the reaction system was in the range of 8.5−10. After 40 min, the preparation process was ended. All samples were obtained after the following steps: aging at 80 °C for a desired time (0−12 h), filtration, washing with deionized water until neutral, and drying at 100 °C. For convenience,

4

Y = β0 +

4

∑ βi Xi + ∑ j=1

i=j=1

2

βijXi 2 +

4

∑ ∑ i=1 j=i+1

βijXiXj (1)

where Y is the response value (precipitation yield) and β0, βi, βii, and βij are the regression coefficients for the intercept, linear, quadratic, and interaction terms, respectively. Xi and Xj represent the independent variables. 2.3. Preparation of PP/MHT-3 Composites. Polypropylene/MHT-3 (PP/MHT-3) composites were prepared by melting the PP and then mixing various loadings of MHT-3 into the PP matrix in a GH-10A high-speed mixer (Beijing Plastic Machinery Factory) at a rotor speed of 250 rpm at 230 °C for 15 min. The mass loadings of MHT-3 added (corresponding to the mass of pure PP) were 3, 5, 10, 20, 30, 40, and 50 wt %. The admixtures were molded into bars (120 × 10 × 4 mm3) using a JK-WZM-I micro injection molding machine with a twin-screw extruder (SHJ-30A) (Beijing Heng Odd Instrument Co., Ltd.) for the testing. 18381

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2.4. Characterization of the MHT Nanoparticles. X-ray diffraction (XRD) patterns were collected on a Rigaku D/max2550PC instrument with Cu Kα radiation (λ = 1.5406 Å). The scan step was 0.0671°/s, with a filament intensity of 30 mA and a voltage of 40 kV. Scanning electron microscopy (SEM) images were obtained with a JEOL JSM-6700F instrument, and energy-dispersive spectrometry (EDS) analysis was performed with a Noran SystemSix instrument. EDS was used to determine the contents of C, O, Mg, and Al elements in the samples. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained using a JEM-2010 electron microscope (JEOL, Tokyo, Japan) at 200 kV, where the samples were dispersed by an ultrasonic cleaner. Fourier transform infrared (FT-IR) spectroscopy was performed on a Perkin-Elmer Spectrum One B instrument using the KBr pellet technique. A Mastersizer 2000 laser particle size analyzer from Malvern (Malvern, U.K.) was used. 2.5. Characterization of the PP/MHT-3 Composites. Thermogravimetric analysis (TGA) was performed on a PerkinElmer Pyris-1 instrument. Ten milligrams of sample (PP/ MHT-3 composite) was loaded in an open ceramic crucible and heated in an air atmosphere at a heating rate of 10 K·min−1. The phase morphology of the PP/MHT-3 composite with 10 wt % MHT-3 was observed by SEM. The specimen for SEM observations was prepared by cryogenic fracturing in liquid nitrogen. The impact strength was measured with an XJJ-50 plastic izod impact testing machine (Laryee Technology Co., Ltd., Beijing, China) at room temperature based on standard GB/ T1043-1993 with a 45° V-shaped notch and a notch-tip radius of 0.2 mm. Five specimens were repeated, and the average value was used to obtain reproducible results. Other mechanical properties were measured using an RGD-5 electronic tensile test machine (Shenzhen Reger Instrument Co., Ltd., Shenzhen, China) with a crosshead speed of 30 mm·min−1. Tensile strength, fracture elongation, and elongation at break were measured based on standards GB/T1042-1992, GB/T10421992, and GB/T9341-2000, respectively. At least three specimens were repeated to determine the average value to obtain reproducible results. The limiting oxygen index (LOI) was measured using a JF-3 instrument (Shanghai Precision Instrument Co., Ltd., Shanghai, China) on a bar with dimensions of 120 × 10 × 4 mm3 according to the standard oxygen index test (GB/T 2406.22009). The vertical burning UL 94 test was carried out with 120 × 10 × 4 mm3 specimens based on standard ANSI/UL 941985 and averaged over five measurements for each composition.

Figure 2. XRD patterns of samples MHT-2, MHT-3, and MHT-4.

CO3 LDH (JCPDS 70-2151) with sharp and intense (003), (006), (009), (110), and (113) reflections and broadened (015) and (018) reflections, and the interlayer distance d003 was in the range 0.760−0.780 nm for the CO32−.3 The XRD pattern of MHT-4 showed the presence of Mg(OH)2CO3 as a major impure phase, due to the increase of magnesium content. For MHT-2 and MHT-3, no other crystalline phases were observed in the XRD patterns, indicating that these two samples were pure crystalline hydroxide structures. The crystallographic features showed that both the sharpness and intensity of the XRD patterns were closely related to the well-developed lattice planes, as clarified by the chemical bonding theory of singlecrystal growth.14,15 Compared to MHT-2, MHT-3 exhibited sharper and higher diffraction peaks with a more stable baseline, indicating a better-crystallized layered structure and implying that MHT-3 had the highest crystallinity. To investigate the morphology of the MHT nanoparticles, the samples were observed by SEM and TEM/SAED. The TEM diffraction patterns of the three samples were consistent with a perfect single-crystal layered structure (Figure 3). The SAED patterns of MHT-2 (Figure 3A-3) and MHT-3 (Figure 3B-3) were consistent with a well-developed hexagonal crystal lattice, but that of MHT-4 suggested an irregular crystal lattice (Figure 3C-3), possibly due to the presence of the Mg(OH)2CO3 impurity phase. The TEM images of MHT-2 (Figure 3A-2) and MHT-4 (Figure 3C-2) revealed the presence of crystal agglomeration in various degrees, whereas that of MHT-3 (Figure 3B-2) demonstrated a monodisperse crystal. According to the SEM and TEM images, the MHT-3 particles were expected to have a well-developed layered hexagonal structure with monodisperse and edge-rounded platelets. MHT-2, with some hexagonal particles, formed thin flat crystals with various edges, indicating a layered structure with some tendency for platelets to aggregate in a clumpy manner. In contrast to MHT-2 and MHT-3, MHT-4 consisted mainly of aggregates in blocks, possibly due to the formation of the Mg(OH)2CO3 impurity phase. Semiquantitative EDS analysis was used to detect the elements of C, O, Mg, and Al in the three samples. The EDS analyses revealed that the Mg/Al molar ratios in MHT-2, MHT-3, and MHT-4 were about 1.82, 2.73, and 3.75, respectively, and that the three samples contained C, implicating CO32− in the interlayers. These results were in agreement with the above-reported XRD analyses (Figure 2). FT-IR spectra of MHT-2, MHT-3, and MHT-4 in the range of 400−4000 cm−1 are displayed in Figure 4 and represent typical FT-IR spectra of Mg/Al−CO3 hydrotalcites. Peaks at around 3500 cm−1 (structural −OH-group stretching vibra-

3. RESULTS AND DISCUSSION 3.1. Determination of Preparation Parameters. 3.1.1. Mg/Al Molar Ratio. Mg/Al−CO3 LDHs with different Mg/Al molar ratios exhibited some changes in crystal structure.13 The variation in crystal structure of MHT nanoparticles with different Mg/Al molar ratios was studied at a precipitation temperature of 60 °C, a precipitation time of 30 min, a [CO32−]/[NO3−] molar ratio of 1.5, and an aging time of 4 h. Samples MHT-2, MHT-3, and MHT-4 were characterized by XRD, SEM/EDS, and FT-IR spectroscopy, and the results are shown in Figures 2−4, respectively. As can be seen in Figure 2, samples MHT-2, MHT-3, and MHT-4 had the typical layered double hydroxide structure of the Mg/Al− 18382

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Figure 3. (1) SEM images of (A-1) MHT-2, (B-1) MHT-3, and (C-1) MHT-4; (2) TEM images of (A-2) MHT-2, (B-2) MHT-3, and (C-2) MHT4; and (3) SAED patterns of (A-3) MHT-2, (B-3) MHT-3, and (C-3) MHT-4.

Figure 4. FT-IR spectra of samples MHT-2, MHT-3, and MHT-4.

tions) and around 1640 cm−1 (water bending vibrations) were observed in the three samples.16,17 The main absorption peak of CO32− was observed at about 1400 cm−1, and the peak at 680 cm−1 was interpreted as a Mg−OH translation mode.3,18 Especially in the MHT-4 spectrum, a new peak at about 1508 cm−1 was probably due to Mg2(OH)2CO3,19 implying that Mg2(OH)2CO3 impurity phase was present in the MHT-4 crystal. The results further confirmed the XRD and EDS analyses, and on the basis of the combined XRD, SEM/EDS, TEM/SAED, and FT-IR analyses, only sample MHT-3 was chosen for further tests. 3.1.2. Precipitation Temperature. The effects of precipitation temperature on the preparation yield of MHT-3 was investigated under the conditions of a precipitation time of 30 min, a [CO32−]/[NO3−] molar ratio of 1.5, and an aging time of 4 h (Figure 5). The results showed that the optimum

Figure 5. Effects of precipitation temperature on the preparation of MHT-3.

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precipitation temperature was about 60 °C, for which the preparation yield was 80.3%. The decrease in the yield above 60 °C was probably due to a decrease in the pH in the reaction solution, where high temperature sharply promotes NH3 volatilization, leading to a decrease in pH. As can be seen in Figure 5, the samples had the typical structure of Mg/Al−CO3 LDHs (JCPDS 70-2151), where MHT-3 had a higher crystallinity at 45 and 60 °C than at 30 °C. 3.1.3. Precipitation Time. Precipitation times ranging from 5 to 60 min were also investigated under the conditions of a precipitation temperature of 60 °C, a [CO32−]/[NO3−] molar ratio of 1.5, and an aging time of 4 h (Figure 6). The precipitation yield increased steeply with precipitation time and then reached a plateau representative of near-equilibrium after 30 min, where the yield was about 81.0%.

Figure 7. Effects of the [CO32−]/[NO3−] molar ratio on the preparation of MHT-3. Figure 6. Effects of precipitation time on the preparation of MHT-3.

3.1.4. [CO32−]/[NO3−] Molar Ratio. The effects of [CO32−]/ [NO3−] molar ratio on the crystallinity and precipitation yield of MHT-3 were investigated under the conditions of a precipitation temperature of 60 °C, a precipitation time of 30 min, and an aging time of 4 h (Figure 7). As can be seen in Figure 7, the samples also had a typical Mg/Al−CO3 LDH structure (JCPDS 70-2151). The highest yield (79.5%) was obtained using a [CO32−]/[NO3−] molar ratio of 1.5. At normal pressure, NH3 reached a vapor−liquid equilibrium in the reaction system when the [CO32−]/[NO3−] molar ratio was 1.5. At a higher (NH4)2CO3 amount (above 1.5), superfluous NH3 volatilization seemed to result, consequently keeping the amount of NH3 in the reaction solution stable and leading to a stable pH. In addition, MHT-3 at [CO32−]/[NO3−] molar ratios of 1.5 and 2.0 had almost the same crystallinity; as a result, a [CO32−]/[NO3−] molar ratio of about 1.5 was selected for further optimization. 3.1.5. Aging Time. To visualize the effects of aging time on the crystallinity and precipitation yield, the XRD patterns and yield were obtained for MHT-3 aged for various times (Figure 8). As can be seen in Figure 8, MHT-3 also had a typical Mg/ Al−CO3 LDH structure (JCPDS 70-2151). The yield increased with time and then reached equilibrium at about 4 h. From the sharpness and intensity of the peaks, it was concluded that there was no significant change in crystallinity as the aging time was prolonged. To gain a high preparation yield, an aging time of about 4 h was selected. 3.2. Optimization of MHT-3 Preparation by RSM. 3.2.1. Regression Model and Statistical Analysis. Based on Box−Behnken design and the results of our experiments (Table 2), a statistical analysis of variance (ANOVA) was performed

Figure 8. Effects of aging time on the preparation of MHT-3.

with Design Expert software to investigate the fitness and significance of the model and the effects of the individual variables and interaction effects on the response. According to the ANOVA results (Table 3), the model was highly significant with a p value of less than 0.0001 in predicting the response values. In terms of the significant coefficients, all of the variables, namely, precipitation temperature T, precipitation 18384

dx.doi.org/10.1021/ie502689e | Ind. Eng. Chem. Res. 2014, 53, 18380−18389

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Table 2. Experimental Box−Behnken Design Matrix and Its Experimental and Predicted Responses

Table 3. ANOVA for the Response Surface Quadratic Model (α = 0.05)

response Y (mL·g−1)

experimental variables run

T (°C)

P (min)

A (h)

M

expt

predicted

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

60 45 75 60 60 60 60 75 60 60 45 60 60 45 75 60 60 45 75 75 75 60 60 60 45 60 45 60 60

30 30 30 30 45 30 30 30 15 30 15 45 30 30 30 45 30 30 30 15 45 30 15 30 30 15 45 45 15

6 2 2 6 6 4 2 6 4 4 4 4 4 6 4 2 4 4 4 4 4 2 6 4 4 2 4 4 4

1 1.5 1.5 2 1.5 1.5 1 1.5 1 1.5 1.5 2 1.5 1.5 2 1.5 1.5 1 1 1.5 1.5 2 1.5 1.5 2 1.5 1.5 1 2

63.5 67.4 57.4 80.2 81.8 81.3 47.2 71.4 45.3 78.7 67.8 79.6 82.1 75.8 72.4 73.3 78.5 59.6 48.1 53.8 76.4 74.8 70.2 81.3 79.6 57.3 78.5 65.8 74.3

63.4 68.5 57.5 79.9 82.0 80.4 47.1 71.2 44.7 80.4 68.8 81.1 80.4 76.6 72.6 73.3 80.4 58.9 48.6 54.6 75.0 74.4 69.8 80.4 78.7 56.6 77.3 66.8 74.2

source model T P A M TP TA TM PA PM AM T2 P2 A2 M2 residual lack of fit pure error cor total R2 adjusted R2 predicted R2 adequate precision CV

3530.7158 201.7200 626.4075 357.5208 1438.8300 35.4025 7.8400 4.6225 4.8400 57.7600 29.7025 293.5736 145.7358 176.4079 519.8741 23.8222 12.8142 11.0080 3554.5379 0.9933 0.9866 0.9744 39.7546

DF 14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 14 10 4 28

mean square 252.1940 201.7200 626.4075 357.5208 1438.8300 35.4025 7.8400 4.6225 4.8400 57.7600 29.7025 293.5736 145.7358 176.4079 519.8741 1.7016 1.2814 2.7520

F

p value

148.2114 118.5484 368.1321 210.1107 845.5830 20.8056 4.6075 2.7166 2.8444 33.9449 17.4558 172.5296 85.6472 103.6728 305.5237