An Effective Flame Retardant and Smoke Suppression Oligomer for

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An Effective Flame Retardant and Smoke Suppression Oligomer for Epoxy Resin Qiang Lv, Jian-Qian Huang,* Ming-Jun Chen, Jing Zhao, Yi Tan, Li Chen, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan University, Chengdu 610064, China ABSTRACT: An effective flame retardant and smoke suppression oligomer, poly(melamine-ethoxyphosphinyl-diisocyanate) (PMPC), was successfully synthesized. The chemical structure was characterized by Fourier transform infrared (FTIR), 1H nuclear magnetic resonance (1H NMR), and 31P nuclear magnetic resonance (31P NMR) spectroscopies. PMPC was used alone as the flame retardant for epoxy resins (EP). The results showed that PMPC endowed EP with good flame retardancy. The limiting oxygen index (LOI) value of a EP/PMPC system containing 20 wt % PMPC increased to 28.0%, and can achieve a UL94 V-0 rating. The cone calorimeter data showed that the heat release rate (HRR) and total heat release (THR) were considerably reduced with the addition of PMPC, and the smoke production rate (SPR), total smoke production (TSP), and carbon monoxide production (COP) were also reduced. Moreover, the thermal degradation behavior of PMPC and the flameretardant mode of the EP systems were also investigated. emissions of smoke and poisonous gases during a fire. In fact, the smoke and toxic gases come from partially burned material, which could be inhibited by the formation of compact char residue. Nevertheless, the small-molecule phosphorus- and nitrogencontaining flame retardants display many drawbacks such as leaching and poor compatibility with the polymer matrix. Combining P and N into one molecule to synthesize a polymeric flame retardant, which not only shows better flame retardancy and smoke suppression but also displays better compatibility compared with the typical flame retardant in EP. In the present work, a novel P- and N-containing polymeric flame retardant, poly(melamine-ethoxyphosphinyl-diisocyanate) (PMPC)), shown in Scheme 1, was synthesized

1. INTRODUCTION Epoxy resin (EP) is a very important thermosetting material owing to its superior electrical and mechanical properties, good adhesion to many substrates, low shrinkage on curing, and excellent resistance to moisture, solvents, and chemical agents. It is used as a high performance material in many fields, such as adhesives, coatings, laminating capsulations, electronic/electrical insulation, and composite applications.1−5 However, one of the main drawbacks of epoxy resin is its inherent flammability, which restricts its application in many fields with the requirement of fire safety. To overcome this problem, a wide variety of flame retardants have been explored and added to the epoxy resin in the past decades, including boron-,6,7 phosphorus-,8−14 and silicon-containing15−17 compounds. Recently, P- and N-containing compounds have attracted extensive attention for their broad range of applications and good efficiency in the fire protection of polymers. It has been found that flame retardant compounds containing both phosphorus and nitrogen elements exhibit enhanced flameretardant efficiency because of a cooperative effect between phosphorus and nitrogen.18−22 While burning, the P-containing groups can form foamed char layers,23 and the N-containing groups can release inert gases to dilute oxygen and facilitate the expansion of the layers.24 These expanded carbonized layers act as insulating barriers, which can reduce heat transfer between the heat source and the polymer surface and dilute the oxygen concentration of the burning zone.25,26 More importantly, they greatly suppress the generation of the smoke during combustion.27,28 Compared with fire and heat, smoke and toxic gas can do great harm to people’s lives during a disastrous fire.29,30 Toxic gas can choke people, and smoke can decrease visibility, which makes it difficult for people to escape from the fire and for firemen trying to rescue them. According to statistics for disastrous fires, 85% of deaths in a fire are due to inhaling toxic smoke.31 Therefore, it is important to reduce the © 2013 American Chemical Society

Scheme 1. Structure of PMPC

successfully and applied to the enhancement of the flame resistance of epoxy resin. The mode of the degradation and flame retardance of PMPC, which may be helpful to the design of the molecular structure of additives, was also investigated. Knowledge of standard requirements can be helpful in the design of compounds with enhanced flame-retardant efficiency and may improve the performance of thermosets into which they are incorporated. Received: Revised: Accepted: Published: 9397

March 21, 2013 May 30, 2013 June 5, 2013 June 5, 2013 dx.doi.org/10.1021/ie400911r | Ind. Eng. Chem. Res. 2013, 52, 9397−9404

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× 100 mm × 3 mm) were exposed to an external heat flux of 35 kW/m2. Thermogravimetric analysis (TGA) of the samples was performed using a TG 209F1 (NETZSCH, Germany) thermogravimetric analyzer. About 3.0 mg of samples were placed in Al2O3 pans and heated from 40 to 700 °C at a heating rate of 10 °C/min (N2 or air, 60 mL/min). TG-FTIR of the samples was performed using a TG 209 F1 thermogravimetric analyzer that was interfaced to a Nicolet 6700 FTIR spectrophotometer. About 10.0 mg of the sample was put in an Al2O3 pan and heated from 40 to 700 °C at a heating rate of 10 °C/min (N2, 50 mL/min).

2. EXPERIMENTAL SECTION 2.1. Material. Melamine (C 3 H 6 N 6 ), urethane (H2NCOOC2H5), ethanol (CH3CH2OH), pyridine, tetrahydrofuran (THF), and N-methylpyrrolidone (NMP) were of reagent grade and were purchased from Chengdu Kelong Chemical Reagent Factory, China. Phosphorus oxychloride (POCl3) was also of reagent grade and was supplied by Chengdu Jinshan Chemical Reagent Co., China. Epoxy resin (DGEBA, commercial name: E-44) was of industrial grade and was provided by Lanzhou Lanxing Resin Co., China. Polyamide (650) was of industrial grade and was obtained from Yueyang Zhongzhan Technology Co., China. 2.2. Synthesis of PMPC. POCl3 (61.3 g, 0.4 mol), pyridine (64 mL, as acid-scavenging agent), and THF (200 mL) were introduced into a 1000-mL, three-neck, round-bottom glass flask equipped with a reflux condenser and a mechanical stirrer. A solution of H2NCOOC2H5 (71.3 g, 0.8 mol) in THF (200 mL) was added dropwise into the stirred solution of POCl3. The resulting mixture was stirred and maintained at 75 °C for 8 h. After the mixture was cooled to 50 °C, pyridine (32 mL, as acid-scavenging agent) and CH3CH2OH (18.4 g, 0.4 mol) were added and stirred for about 1 h. The solvent was then removed by distillation. A dispersion of C3H6N6 (50.5 g, 0.4 mol) in NMP (400 mL) was added to the residue. The mixture was stirred at 190 °C for 8 h and then cooled to room temperature. The product was collected by filtration and washed several times with DI water and ethanol, respectively. The brown product was dried to constant weight at reduced pressure at 80 °C. Yield: 61%. 2.3. Preparation of Epoxy Thermosets. DGEBA, PMPC, and the curing agent polyamide (650) were blended and stirred at 60 °C. The mixture was poured into preheated molds, and the mixture was cured at 100 °C for 2 h. The formulations of EP/PMPC composites are listed in Table 1.

3. RESULTS AND DISCUSSION 3.1. Characterization of PMPC. The FTIR spectrum of PMPC is shown in Figure 1. It contains broad absorption at

Figure 1. FTIR spectrum of PMPC.

3404 cm−1 and weak absorption at 3228 cm−1, corresponding to the stretching vibration of −NH− and −NH2, respectively. Absorptions at 2800−3000 cm−1 may be attributed to −CH2− and −CH3, and the characteristic absorption for CO is observed at 1662 cm−1. Meanwhile, the skeleton vibration of triazine appears at 1502 cm−1, the characteristic absorption of PO is observed at 1266 cm−1, and the absorption of P−O−C is observed at 1095 cm−1. The 1H NMR spectrum of PMPC is shown in Figure 2. The signals occurring at 2.50 ppm (−CH3), 5.00 ppm (−OCH2−), 8.30−8.50 ppm (−NH−), 8.80 ppm (the terminal −NH2 groups), and 9.10 ppm (−NH2 at the chains) are present. The structure of PMPC is also confirmed by the 31P NMR spectrum shown in Figure 3. The signal occurring at −25.8 ppm corresponds to phosphorus linkage with amine (P−NH−). The degree of polymerization (Dp) of PMPC is about 9, and the number-average molecular weight (Mn) is about 2844, which may be calculated using the following two equations: I Dp = n = 4 9.10 + 1 I8.80 (1)

Table 1. Flame-Retardant Properties for Pure EP and EP/ PMPC Composites sample

DGEBA (wt %)

polyamide (wt %)

PMPC (wt %)

UL-94

LOI (%)

EP EP1 EP2 EP3 EP4

66.7 60 56.7 53.3 50

33.3 30 28.3 26.7 25

0 10 15 20 25

N.R.a N.R. V-1 V-0 V-0

20.5 26.0 27.5 28.0 30.0

a

N.R.: no rating in UL-94 test.

2.4. Characterization. FTIR spectra of the samples were recorded using a Nicolet FTIR 170SX spectrometer and KBr pellets over the range from 500 to 4000 cm−1. 1H NMR and 31P NMR spectra were performed using an FT-80A NMR (400 MHz) and d-TFA as a solvent with TMS as an internal standard. The LOI value was determined using an HC-2C oxygen index instrument (Jiangning, China) according to ASTM D2863-97. The dimensions of the samples were 130 mm × 6.5 mm × 3.2 mm. The UL-94 vertical burning test was performed using a CZF-2 instrument (Jiangning, China) according to ASTM D3801. The dimensions of the samples were 130 mm × 13 mm × 3.2 mm. The flammability of the samples was examined using a cone calorimeter (Fire Testing Technology). The samples (100 mm

M n = 302 × Dp + 126

(2)

3.2. The Flammability of EP/PMPC System. The results of LOI and UL-94 tests for both EP and EP/PMPC composites are shown in Table 1. The data indicate that pure EP is a highly flammable thermosetting resin with a low LOI value (20.5%) and no rating in the UL-94 test. However, the LOI value 9398

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Figure 2. 1H NMR spectrum of PMPC.

Figure 3. 31P NMR spectrum of PMPC.

Table 2. Cone Calorimeter Data for EP and EP/PMPC Composites at 35 kW/m2 sample

PHRR (kW/m2)

THR (MJ/m2)

PCOP (g/s)

PSPR (m2/s)

TSP (m2)

EP EP1 EP2 EP3

781 390 292 235

76 33 30 27

0.018 0.015 0.012 0.010

0.19 0.13 0.10 0.08

21.5 8.1 7.8 5.9

increases gradually from 20.5% to 30.0% (for EP containing 25 wt % PMPC) with increasing PMPC. Meanwhile, the EP/ PMPC composite containing 20 wt % PMPC achieves a UL-94 V-0 rating. These results indicate that PMPC imparts good flame retardancy to epoxy resins. Cone calorimetry is one of the most effective bench-scale techniques for the evaluation of the flammability performance of polymeric materials. The combustion environment of the samples in the test is similar to that of a real fire so that the cone calorimeter results largely reflect the fire performance of the samples.32,33 These results can provide a wealth of information about the combustion behavior of the material and some reflection on its expected behavior in a fire. Table 2 and Figures 4 and 5 contain data and plots from cone calorimeter tests for EP and EP/PMPC at an incident heat flux of 35 kW/m2. The heat release rate (HRR) is a very important parameter and can be used to evaluate the intensity of a fire. An effectively flame-retarded polymeric matrix normally displays a low HRR value. It can be clearly seen that the pure EP burns very fast after ignition with a peak heat

Figure 4. HRR curves for EP and EP/PMPC composites.

release rate (PHRR) of 781 kW/m2. When PMPC increased to 10 wt %, the PHRR decreased to 390 kW/m2, which is much lower than that of pure EP. When the content of PMPC increased to 20 wt %, the PHRR decreased to 235 kW/m2. The 9399

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Figure 7. TSP curves for EP and EP/PMPC composites.

Figure 5. THR curves for EP and EP/PMPC composites.

total heat release (THR) is commonly used to evaluate the fire safety of the materials in a real fire. Figure 5 shows that the THR of pure EP is 76 MJ/m2, and the THR values of EP1−3 are 33, 30, and 27 MJ/m2, respectively. It can be seen that the THR value dramatically decreased as the content of PMPC increased, which is attributed to the earlier decomposition of the flame retardant, to form a char barrier on the surface of the EP/PMPC composites. Smoke in a real fire means more risk of suffocation, even more fatal than heat release. It is clear that PMPC can slow the SPR and hence reduce the peak value of SPR (PSPR), from 0.19 m2/s of EP to 0.08 m2/s when the content of PMPC is increased to 20 wt % (Figure 6). The data in Figure 7 show that Figure 8. COP curves for EP and EP/PMPC composites.

The TG and DTG curves for PMPC in a nitrogen atmosphere are shown in Figure 9. The data from TG and

Figure 6. SPR curves for EP and EP/PMPC composites.

Figure 9. TG and DTG curves for PMPC under N2 atmosphere.

the TSP of the composite is greatly reduced with addition of PMPC. All of the results suggest that PMPC can suppress the formation of smoke very well, which is advantageous in reducing fire risk. Moreover, the data presented in Figure 8 are CO release during sample burning, which can be greatly inhibited with the addition of PMPC. The peak of carbon monoxide production (PCOP) decreases from 0.018 g/s for neat EP to 0.010 g/s for EP/PMPC (20 wt %). 3.3. Thermal Degradation and Mode of Flame Retardance. 3.3.1. Thermal Degradation of PMPC. TGA is one of the most widely used techniques for evaluating thermal stability of different materials. It can provide some indication of the decomposition behavior of polymers at various temperatures.

DTG are listed in Table 3. As can be seen from Figure 9, the onset decomposition temperature (Tonset) of PMPC is about 291 °C. There are two main decomposition stages in the temperature ranges of 280−400 °C and 400−600 °C, with two corresponding differential thermogravimetric (DTG) peaks. The first stage reflects the main decomposition process. The temperature of maximum weight loss (Tmax1) is about 358 °C. At this stage, many chain scissions are probably occurring rapidly with the release of small molecules (NH3, CO2). It is likely that complex chemical reactions, such as the dissociation of the triazine structure, occur at the second stage. The temperature of maximum weight loss (Tmax2) for this 9400

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Table 3. Data for TG and DTG of EP and EP/PMPC Composites under N2 Atmosphere sample PMPC EP EP1 EP2 EP3

Tonset (°C)b Tmax (°C)b 291 333 292 288 282

358, 563 424 366 359 357

rate of Tmax (%/min)

char residues (%)

4.6, 2.9 9.8 9.8 9.0 6.9

29.8 3.9 10.2 11.4 13.5

b Tonset: 5% weight loss temperature. Tmax: maximum weight loss temperature.

decomposition is about 563 °C. The char residue remaining at 700 °C is about 29.8 wt %. The TG and DTG curves for PMPC in air atmosphere are shown in Figure 10. The data are listed in Table 4. As can be

Figure 11. FTIR spectra of volatile products at representative temperatures during thermal degradation of the PMPC.

FTIR spectroscopy was also employed to investigate the residue of PMPC after thermal degradation at different temperatures. The FTIR spectra of the samples at 25, 290, 360, 560, and 700 °C are displayed in Figure 12, respectively.

Figure 10. TG and DTG curves for PMPC under air atmosphere.

Table 4. Data for TG and DTG of EP and EP/PMPC Composites under Air Atmosphere sample

Tonset (°C)

Tmax (°C)

rate of Tmax (%/min)

char residues (%)

PMPC EP EP1 EP2 EP3

289 312 287 285 276

364, 583 367,399,538 342,364,384,549 353,379,543 349,388,547

4.3, 2.4 6.6,7.7,4.0 7.5,7.3,7.6,2.6 7.9,7.0,1.8 7.6,6.0,1.6

27.4 0 4.8 9.4 11.3

Figure 12. FTIR spectra for condensed products from the decomposition of PMPC at different temperatures.

At 25 °C, the main absorptions of PMPC are observed at 3404 cm−1 (−NH−), 2850, 2822 cm−1 (−CH2−, −CH3), 1662 cm−1 (CO), 1502 cm−1 (triazine), 1266 cm−1 (PO), and 1095 cm−1 (P−O−C). When the temperature is increased to 290 °C, the absorption at 1095 cm−1 (P−O−C) nearly disappears, and the absorption at 2850 cm−1 (−CH2−, −CH3) becomes weaker, which reflects the disappearance of −OCH2CH3 group. As the temperature increases to 360 °C, the absorption at 3404 cm−1 becomes weaker and weaker but the absorption at 3228 cm−1 becomes stronger and stronger, which is reflective of the breaking of an amido bond and formation of the free melamine structure. When the temperature increases to 560 °C, the absorption at 3228 cm−1 disappears completely, which suggests that melamine undergoes progressive condensation and formation of polymeric products ‘melon’ or the decomposition of the melamine structure. The decomposition of structure containing the CO group is also indicated from gradual disappearance of the absorption at 1662 cm−1. With the increase of temperature, two other changes can be observed. The first change is that the absorption at 1266 cm−1 gradually disappears, which reflects the disappearance of the PO

seen from Figure 10, the decomposition process of PMPC in air is similar to this in N2 atmosphere. The onset degradation temperature (Tonset) of PMPC is about 289 °C. There are two different stages in the temperature ranges of 280−400 °C and 400−600 °C and there are accordingly two differential thermogravimetric (DTG) peaks. The char residue remaining at 700 °C is about 27.4 wt %. The FTIR spectra of volatile products at representative temperatures during thermal degradation of the PMPC, such as 290 °C (Tonset), 360 °C (Tmax1), 560 °C (Tmax2), and 700 °C, are presented in Figure 11. At 290 °C, the peaks of CO2 (2284, 669 cm−1) and CO (2250 cm−1) can be detected, and may be attributed to the loss of the −OCH2CH3 group, which becomes stronger and stronger with the temperature increasing to 560 °C, indicating the decomposition of the CO and triazine structures. Once the temperature increases to 360 °C, a large amount of nitrogen-containing products (3509, 964, 928 cm−1) are detected, probably due to the dissociation of the triazine and P−NH− structures. 9401

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Scheme 2. Possible Mode of Thermal Degradation PMPC

with the addition of PMPC, the Tonset decreases, and the Tonset of EP1−3 are 292, 288, and 282 °C, respectively, which are much lower than the onset degradation temperature (330 °C) for neat EP. The char yield at 700 °C increases notably from 3.9 wt % for neat EP to 13.5 wt % of EP/PMPC (20%). These results suggest that PMPC has poor thermal stability. Degradation products may catalyze char formation for the decomposing composites. The incorporation of PMPC can improve the stability of the EP/PMPC hybrids at higher temperature and promote the formation of char residue. The TG and DTG curves of EP and EP/PMPC hybrids obtained in air atmosphere are shown in Figure 15a and b. The corresponding data are summarized in Table 4. It can be seen that the degradation EP and EP/PMPC hybrids in air atmosphere are much more complex than those in N2 atmosphere. As for pure EP, there are three stages in the thermal degradation process. The onset degradation temperature (Tonset) is about 312 °C, the temperatures of maximum rate of weight loss (Tmax) are about 367 °C, 399 °C, 538 °C. As can be seen, with the addition of PMPC, there are three or four stages in the thermal degradation process. Figures 16 and 17 show the FTIR spectra of volatile products at representative temperatures during thermal degradation of EP/PMPC system. From the TG-DTG curves of the PMPC and EP, several representative temperatures (330 °C [Tonset of EP], 360 °C [Tmax1 of PMPC], 420 °C [Tmax of EP] and 500 °C) were selected to study the volatile components released during the thermal degradation process. As shown in Figure 16, the main products of the thermal decomposition of EP are compounds containing aromatic nucleus (1604, 1509 cm−1), phenol (747 cm−1), hydrocarbon compounds (2933 cm−1, 1176 cm−1), and water (3650 cm−1). As shown in Figure 17, the evolved gases for EP3 contain, in addition to the decomposition products of neat EP, other new nonflammable gas products such as NH3 (964, 928 cm−1) and CO2 (2250− 2400 cm−1). The release of new compounds from decomposition EP3 at 330 °C may suggest that the flame retardant can influence the thermal decomposition of EP. Furthermore, the peak intensity of the pyrolysis products from EP3 is much lower than that from EP, especially for hydrocarbons and

group. The second change is that a new small absorption at 1117 cm−1 is observed. The absorption at 1117 cm−1 is a characteristic absorption for the ‘−P−O−P−O−’ structure.4 Therefore, it is likely that some structures containing the PO group are converted into ‘−P−O−P−O−’ group containing structures via elimination reactions. From what we have discussed above, a possible mode of thermal degradation of PMPC may be suggested as shown in Scheme 2. 3.3.2. Flame Retardant Mode of the EP/PMPC System. Digital images of residues after cone calorimeter combustion of composite formulations are shown in Figure12. It can be seen that there is almost no char left for pure EP after the cone calorimeter test (Figure 13a), while for EP containing 10 wt %

Figure 13. Digital photos of residues after cone calorimeter tests. (a) EP. (b) EP1. (c) EP3.

PMPC (Figure 13b), a homogeneous, tight and swollen char formed. When PMPC content increased to 20 wt % (Figure 13c), the intumescence phenomenon is much more obvious, which corresponds to better flame retardancy. From Figure 13, it also can be seen that there is some unburned composite under the char residue which means that the material is protected well by the char residue. The TG and DTG curves for EP and EP/PMPC hybrids obtained in a nitrogen atmosphere are shown in Figure 14a and b. The corresponding data are summarized in Table 3. As for pure EP, there is one stage in the thermal degradation process. The onset degradation temperature (Tonset) is about 330 °C, the temperature of maximum weight loss (Tmax) is about 420 °C, and the residue at 700 °C is about 3.9 wt %. As can be seen, 9402

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Figure 14. TG and DTG curves for EP and EP/PMPC composites under N2 atmosphere.

Figure 15. TG (a) and DTG (b) curves for EP and EP/PMPC composites under air atmosphere.

Figure 16. FTIR spectra of volatile products at representative temperature during thermal degradation of pure EP.

Figure 17. FTIR spectra of volatile products formed at representative temperature during thermal degradation of EP3.

aromatic compounds. Consequently, the addition of intumescent flame retardant obviously reduces the release of combustible gases and the loss of weight. From the TG and TG-FTIR studies, it could be concluded that the PMPC can catalyze the formation of a protective char layer and the release of nonflammable gas leading to expansion of the layer. This expanded char layer could inhibit heat transmission and heat diffusion effectively.

V-0 rating in the UL-94 test. Cone calorimeter data shows that the HRR and THR significantly decreased upon the addition of PMPC. Especially, SPR, TSP, and COP strongly decreased. The main volatile products formed in thermal degradation of PMPC are CO2, CO, and NH3, and the residues contain ‘melon’ and ‘−P−O−P−O−’ groups. PMPC may influence the thermal decomposition of EP at low temperature to form a compact, continual char layer and release nonflammable gas to promote expansion of the char layer. The expanded char layer can hinder heat and substance transmission effectively, thus improving the fire resistance of the composite.

4. CONCLUSION A new oligomeric intumescent flame retardant (PMPC) has been synthesized successfully. It displays good flame retardancy and intumescent effects in EP/PMPC system. The EP/PMPC (20 wt %) blend displays a LOI value of 28.0% and passes the 9403

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (J.-Q.H.) and [email protected] (Y.-Z.W.). Tel: +86-28-85410259. Fax: +86-28-85410755. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support from the Natural Science Foundation of China (Grant Nos. 50933005 and 51121001) and the Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026).

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dx.doi.org/10.1021/ie400911r | Ind. Eng. Chem. Res. 2013, 52, 9397−9404