Flame Retardancy and Thermal Decomposition of Phosphorus

Flame retardancy and thermal degradation mechanism of a novel post-chain extension flame retardant waterborne polyurethane. Gang Wu , Jinqing Li , Yun...
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Flame Retardancy and Thermal Decomposition of PhosphorusContaining Waterborne Polyurethanes Modified by Halogen-Free Flame Retardants Limin Gu†,‡ and Yunjun luo*,† †

School of Materials Science and Engineering, Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District, Beijing, 100081, China ‡ School of Chemical and Pharmaceutical Engineering, Hebei University of Science and Technology, 26 Yuxiang Street, Shi Jiazhuang, Hebei Province 050018, China ABSTRACT: A series of halogen-free phosphorus-containing flame-retardant waterborne polyurethanes (DPOWPUs) were synthesized by comodification with Exolit OP550 and ODDP (a phosphorus−nitrogen flame retardant with biphosphonyl structure), and the structure of DPOWPU was characterized. In addition, the thermal decomposition mechanism of DPOWPU was obtained. With the incorporation of OP550 acting as soft segment flame retardant, the decomposition temperatures of each degradation stage decreased. When the OP550 content reached 13 wt %, the limiting oxygen index increased to 31.4%. The cone calorimetery results of materials were greatly improved. Furthermore, the carbon yields of the DPOWPUs increased distinctly, and the firm char layer could much more efficiently protect the unburned materials and prevent the transmission of mass and heat during thermal decomposition. In addition, the phosphorus content of carbon residues grew progressively, which showed that Exolit OP550 displayed condensed phase flame retardation.

1. INTRODUCTION In recent years, many experts have dedicated efforts to studying waterborne polyurethane (WPU) because of its economic viability, environmental friendliness1−3 (low VOC emissions during manufacture), and good applicability to coatings compared to the conventional solvent polyurethane (SPU).4,5 Efforts toward the WPU development have focused on improving the flame retardancy of polymers by adding flame retardants, which is recognized as the most effective method of achieving that goal.6 In order to improve the flame retardancy of polymer materials, experts commonly use two approaches: intrinsic flame-retarded method (the fire retardant reacts with polymer molecular chains) and blending method (blending the polymer with fire retardant). The blending method has several drawbacks, such as the requirement of a load of retardants which are likely to move to the surface of the material after application and cause slowdowns in the performance of the polymer.7 By contrast, the reactive-type flame retardants, such as halogenated compounds (especially bromo substituted organic compounds),8 organophosphorus compounds, and phosphorus−nitrogen (P−N) containing flame retardants,9,10 will not affect the polymer film properties but will maintain its good performance. Because of this, the intrinsic flameretarded method can improve the miscibility between the fire retardant and the materials and increase the efficiency of fire retardancy.11,12 Halogenated compounds, because of their low price and high efficiency, have been used in polymer modification for many years.13 However, many halogenated compounds have been proven to be harmful to human health. They can bioaccumulate in the human body and have potential toxicity to people. Currently, some halogenated compounds have been banned for use as flame retardants.14 © XXXX American Chemical Society

Recently, phosphorus compounds, because of high efficiency and low production of toxic gases, have been widely used in the flame retardation modification of polymers. Cheema et al.15 prepared two bifunctional phosphorus−nitrogen fire retardant monomers: ethyl di(acryloyloxyethyl) phosphorodiamidate (EDAEP) and N,N-dimethyl di(acryloyloxyethyl) phosphoramide (DMDAEP). Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and vertical flame test were chosen to illustrate the properties of the monomers. They found DMDAEP had better performance than EDAEP, and the presence of one more nitrogen atom was the main reason for the excellent properties of DMDAEP, which enhanced the P−N synergistic function, generating more thermostable and rugged carbon residue; Liao et al.16 synthesized a phosphorus− nitrogen−silicon polymeric flame retardant (PNSFR) via polycondensation. The limiting oxygen index (LOI) of the polylactic acid (PLA) composites with 20 wt % PNSFR was approximately 25.0% compared to 20.0% for pure PLA, and a V-0 rating of UL-94 test was attained; Luo and co-workers17 synthesized a new biphosphonyl halogen-free phosphorus−nitrogen synergistic flame retardant with cyclic structure: octahydro-2,7di(N,N-dimethylamino)-1,6,3,8,2,7-di-oxadiazadiphosphecine (ODDP), and introduced it into WPU as a hard segment flameretardant to prepare flame-retardant WPU (DPWPU). The UL-94 received a V-0 classification, and the LOI value of DPWPU was up to 30.6% when the ODDP content increased to 15 wt %, which showed the good flame retardancy of the material. Received: November 24, 2014 Revised: January 28, 2015 Accepted: February 20, 2015

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DOI: 10.1021/ie5045692 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Composition and Fire Retardancy of Different DPOWPU Samples sample

TDI (g)

PPG1000 (g)

DMPA (g)

OP550 (g)

ODDP (g)

TEA (g)

H2O (g)

LOI (%)

UL-94

DPWPU9 OPWPU9 DPOWPU95 DPOWPU97 DPOWPU99 DPOWPU911 DPOWPU913

16.6 13.2 15.2 15.3 15.3 15.4 15.4

25.4 28.8 24.8 23.7 22.7 21.6 20.6

3.0 3.0 3.0 3.0 3.0 3.0 3.0

0 4.5 2.5 3.5 4.5 5.5 6.5

4.5 0 4.5 4.5 4.5 4.5 4.5

2.0 2.0 2.0 2.0 2.0 2.0 2.0

150.0 150.0 150.0 150.0 150.0 150.0 150.0

30.1 30.3 30.5 30.9 31.2 31.3 31.4

V-0 V-1 V-0 V-0 V-0 V-0 V-0

FT-IR 170SX spectrometer over the wavenumber range from 500 to 4000 cm−1 under ambient temperature. 2.3.2. In Situ FTIR. The Nicolet MAGNA-IR 750 spectrometer over the wavenumber range of 500−4000 cm−1 was used to obtain in situ FTIR spectra. The residue samples were ground with KBr and then tested in the temperature range of room temperature to 500 °C. 2.3.3. Elementary Analysis. The actual phosphorus content was tested by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the sample was dissolved in nitric acid before testing. 2.3.4. Characterization of Char Residues. The morphologies of different residue samples after cone testing were obtained by an INSPECTF SEM instrument under a 15 kV accelerating voltage. The char residues were treated by spraygold before testing. Energy-dispersive X-ray (EDX) spectroscopy was used to perform the determination of phosphorus content in the residue samples. 2.3.5. Limiting Oxygen Index. A HC-2C oxygen index instrument based on ISO Standaard 4589-1984 was used to test the LOI of different DPOWPU samples. The form (length × width × thickness) of the sample was 150 × 50 × 3 mm3. 2.3.6. UL-94 Vertical Burning Test. A CZF-II horizontal and vertical burning tester was used to test the value of UL-94. The size of samples was 127 × 12.7 × 3 mm3 according to the ASTM Standard D 3801-1996. 2.3.7. Thermogravimetric Analysis. A METTLER TGA/ DSC1 instrument was used to study the thermogravimetric behavior of DPOWPUs. The heating rate was 10 °C/min over the temperature range from 30 to 600 °C, under N2 atmosphere. 2.3.8. TG-FTIR Analysis. A thermal analyzer system attached to a FTIR instrument was used to perform the TGA-FTIR tests. The heat rate of TG testing was 10 °C/min, and the temperature ranged from 30 to 600 °C, under N2 atmosphere. The gas products were measured by the FTIR coupled with the TGA instrument. 2.3.9. Fire Test. The combustion behavior was measured by a cone calorimeter instrument at a heat flux of 25 kW/m2, and the size (length × width × thickness) of samples was 100 × 100 × 3 mm3.

In this research, a novel waterborne polyurethane (DPOWPU) comodified by both soft segment and hard segment was synthesized by phosphorus containing polyol (Exolit OP550) and ODDP. Exolit OP550 reacted with the prepolymer as a soft segment because of its high phosphorus content, thus imparting good flame resistance; similarly, ODDP was chosen as hard segment chain extender because of its high P−N content. The research was aimed at increasing flame resistance of polyurethane and studying the effects of phosphor-containing flame retardants on the performance and decomposition behavior of DPOWPU.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Triethylamine (TEA) and butanone (MEK) were purchased from Yili Fine Chemicals Co., Ltd. of Beijing (China). 2,2-Bis(hydroxymethyl)propionic acid (DMPA), toluene diisocyanate (TDI 80/20), and polyether polyols (PPG1000; molecular weight, 1000; hydroxyl number, 99.1 mg KOH/g) were purchased from Beijing Linshi Chemical Industrial Co., Ltd. (China), Kelun Chemicals Co., Ltd. (China), and Shanghai Polyurethane Products Factory (China), respectively. Organophosphonate oligomer (Exolit OP550; hydroxyl number, 170 mg KOH/g) was purchased from Clariant chemical limited company. Before being used, the PPG1000, DMPA, and Exolit OP550 were placed under vacuum for heating and drying for 4 h. ODDP was synthesized in Dr. Yunjun Luo’s laboratory, Beijing Institute of Technology (China). 2.2. Preparation of DPOWPU. Taking DPOWPU95 for example, 15.2 g of toluene diisocyanate (TDI), 24.8 g of PPG1000, and 3 g of DMPA were put into a four-necked flask, which was equipped with a thermometer, a mechanical stirrer, a condenser, and a nitrogen inlet, and reacted at 85 °C for 1 h. Then, 2.5 g of Exolit OP550 was added to this system and reacted with the others until the content of NCO reached the theoretical value. After that, adding 30 mL of MEK into the reactor reduced the viscosity of the system. Next, ODDP was added to the system at 85 °C. Four hours later, the compound was cooled to 40 °C, and 30 mL of MEK was added to the flask to reduce the viscosity. Then, 2.0 g of TEA was added into the system as neutralizer. Ten minutes later, the compound was poured into a beaker with 150.0 g of water, and the mixture wase dispersed by a high-shear dispersion homogenizer for 5 min. The waterborne polyurethanes comodified by both OP550 and ODDP (DPOWPUs) were obtained, and the solid content of polymer emulsion was 25 wt %. The compositions of the resutling DPOWPUs are shown in Table 1, and Scheme 1 shows the reaction mechanism for the preparation of DPOWPU. 2.3. Characterization. 2.3.1. Fourier Transform Infrared Spectroscopy. The FTIR spectrogram of the OP550, ODDP, and all the DPOWPU samples were captured from a Nicolet

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. Characterization of DPOWPU. The FTIR spectra of ODDP, Exolit OP550, and DPOWPUs are shown in Figure 1. The characteristic peaks appearing at 1283 cm−1 (PO), 1028, and 986 cm−1 (P−O−C) in the FTIR spectra of OP550 were due to the groups containing phosphorus, and characteristic peaks like these were obtained in DPOWPU97, too. The peaks appearing at 3270.5 cm−1 (N−H), 1727.1 cm−1 (CO), 1677.9 cm−1(the carbonyl group in amides), 1103 cm−1 (C−O−C), 1311.45 cm−1, 1004.6 cm−1, and 742.1 cm−1 (P−N−(CH3)2) in the FTIR spectra of ODDP were similar to the IR absorption peaks of DPOWPU97. B

DOI: 10.1021/ie5045692 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Reaction Mechanism for the Preparation of DPOWPU

3.2. Thermogravimetric Analysis. The TG and DTG curves of DPOWPU are shown in Figures 3 and 4, respectively. All the DPOWPUs present three-step degradation processes at 185−252, 252−302, and 302−400 °C. The decomposition of DPOWPU at 185−252 °C is due to the degradation of fire retardant (ODDP and OP550). The first decomposition peak of DPOWPU95−DPWPU913 became wider and wider (as shown in Figure 4) with the increase of OP550 weight percent. The second decomposition peak is attributed to the depolymerization of the hard segments of polyurethane to form polyols, isocyanates, amines, carbon dioxide, and olefins.18 The third decomposition peak at 302−400 °C, which is the main degradation step of DPOWPUs, is due to the degradation of the soft segments of DPOWPU. The detailed thermal degradation temperatures of DPOWPUs are shown in Table 3. The table shows that there is a decrease of T5% and Tmax I, Tmax II, and Tmax III (the first, second, and third maximum weight loss temperatures, respectively) when the OP550 content is increased. However, compared with those of DPWPU9, T5% and Tmax I increased and Tmax II and Tmax III decreased. This is maybe caused by the addition of OP550 as soft segments flameretardant. On the one hand, OP550 is a phosphorus-containing polyol, and it has thermal stability higher than that of ODDP,17 thus resulting in the increase of T5% and Tmax I. On the other hand, phosphoric acid is one of the thermo-decomposition products of OP550, and with the increase of OP550 content in samples, the phosphoric acid production increases. Phosphoric acid can catalyze the breakdown of polyurethane; therefore, Tmax II and Tmax III of DPOWPUs decreased. We can see clearly that with increased OP550 content, the char residue at 480 °C increased (Table 3). On the one hand,

Figure 1. FTIR spectra for ODDP, OP550, and DPOWPUs.

3.1.2. Elementary Analysis. ICP-AES was used to test the actual phosphorus content of different samples to further confirm the structure of the DPOWPU, and the testing results are shown in Table 2. The actual phosphorus content was similar to the theoretical value, and the relative error was less than 0.3%. When these results are combined with the elemental analysis and FTIR results, it can be concluded that the Exolit OP550 and ODDP were successfully segmented into the waterborne polyurethane macromolecule. Figure 2 shows the possible DPOWPU chain structure formed between TDI, N-210, OP550, and ODDP. C

DOI: 10.1021/ie5045692 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 2. Results of Phosphorus Content Testing sample

DPOWPU95

DPOWPU97

DPOWPU99

DPOWPU911

DPOWPU913

theoretical content found content

2.66% 2.59%

2.98% 2.88%

3.30% 3.24%

3.60% 3.52%

3.94% 3.88%

Figure 2. Possible DPOWPU chain structure formed between TDI, N-210, OP550, and ODDP.

Table 3. Data of TGA sample

T5%(°C)

Tmax I (°C)

Tmax II (°C)

Tmax III (°C)

char yield

DPWPU9 DPOWPU95 DPOWPU97 DPOWPU99 DPOWPU911 DPOWPU913

214.1 224.0 223.6 223.1 222.0 220.3

230.1 239.1 239.1 238.7 238.6 237.7

301.6 296.6 295.4 295.2 294.5 292.8

350.1 348.1 347.8 345.7 345.1 343.9

8.2 8.6 11.9 12.0 13.6 16.1

Figure 3. TG curves for DPOWPUs.

Figure 5. HRR curves of different DPOWPU samples.

3.3. Flame Retardancy. The flame retardancy of samples was characterized by LOI and UL-94 tests, and the results are listed in Table 1. The results show that the LOI value of DPOWPUs improved when the OP550 content increased, and the LOI value increased to 31.4% when the OP550 content reached 13 wt %. The vertical flammability tests indicate that all samples achieved a V-0 classification. The results of flame retardancy testing indicate that the structural modification method in which polymer chains are comodified by the soft and hard segment fire retardants will improve flame retardation of waterborne polyurethane materials. Cone calorimeter measurements are being used more and more widely to assess the combustion behavior of materials. The cone calorimeter can provide many parameters to elaborate flame retardant of the materials, including the time to ignition (TTI), the heat release rate (HRR), the maximum average rate of heat emission (MARHE), the average effective heat of combustion of volatiles (Av-EHC) and the total heat release (THR). The cone calorimetry testing data are listed in Table 4, and the curves of the heat release rate and the total heat release

Figure 4. DTG curves for DPOWPUs.

P−N synergistic can promote char formation. On the other hand, the soft and hard segment fire retardants comodified on polyurethane molecular chains, which increase the density of phosphorus in polyurethane molecular chains, and the phosphorus can product phosphoric acid, polyphosphoric acid, or other derivatives at higher degradation temperature, and these phosphorus-containing compounds promote the dehydration of polyols to form olefin.19 D

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Industrial & Engineering Chemistry Research

Figure 5, we can see the OPWPU9 had only one HRR peak, and the HRR peak appeared around 150 s. The DPWPU9 and DPOWPUs samples had two HRR peaks, and all the first HRR peaks appeared around 50s. The second peak of DPOWPU95−97 appeared around 110s, but the second peak of DPOWPU911 appeared around 150 s. In the initial thermal decomposition stage of DPOWPUs, all the DPOWPU samples degraded and produced phosphoric acid, isocyanate, polyol, and CO2, and the production of carbon residue was less; therefore, the first PHRR was unchanged. However, in the main decomposition stage of DPOWPUs, with the increase of OP550 content, more residual char was formed (as shown in Table 4), which leads to the reduction of the second HRR and the delay of time to second HRR. Furthermore, the MARHE and THR gradually decreased with the increase of OP550 wt %, which also indicates that OP550 can promote the carbonization process. From the Av-EHC values we know that many gases are degraded from the materials. Compared with that of DPOWPU97, DPOWPU99, DPOWPU911, and DPOWPU913, the Av-EHC value of DPOWPU95 is higher. This shows that the addition of soft segment fire retardant can effectively reduce the gases decomposed from the materials, which is further evidence illustrating that OP550 took an active role in char formation. 3.4. Char Residues and Evolved Gases Analysis. To further study the flame retardancy of DPOWPUs, EDX spectroscopy and SEM were used to investigate the elemental composition and morphology of the residues. Figure 7 shows the char mophology of various samples obtained from cone calorimeter

Figure 6. THR curves of different DPOWPU samples.

are shown in Figures 5 and 6, respectively. As seen from Figure 5 and Table 4, the PHRR decreased with the addition of OP550; at the same time, the time to PHRR increased. When the content of OP550 increased to 13 wt %, the PHRR decreased to 264 kW/m2. In addition, the time to PHRR reached 110 s, which was prolonged by 60 s. There are two possible reasons for the decrease of the PHRR: (1) the char formation promoted by OP550 and (2) the nonflammable gas produced because of P−N synergistic effect. Moreover, from Table 4. Results of Cone Calorimeter Testing samples

TTI (s)

time to PHRR (s)

PHRR (kw/m2)

MARHEα (kW/m2)

THR (MJ/m2)

Av-EHCβ (MJ/kg)

char yield (%)

DPWPU9 OPWPU9 DPOWPU95 DPOWPU97 DPOWPU99 DPOWPU911 DPOWPU913

25 26 24 24 23 24 22

165 150 110 110 110 150 50

531.7 513.5 496.6 400.0 281.0 269.4 264.0

250.1 232.6 206.7 189.7 184.3 156.1 147.2

35.8 37.6 34.3 34.0 30.8 28.4 23.6

23.7 24.5 22.8 22.5 22.3 22.1 19.2

6.8% 8.2% 10.0% 11.4% 11.8% 17.0% 27.8%

Figure 7. SEM morphology of residues: (a) DPWPU9, (b) OPWPU9, (c) DPWOPU95, (d) DPOWPU97, (e) DPOWPU99, (f) DPOWPU911, and (g) DPOWPU913. E

DOI: 10.1021/ie5045692 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 8. EDX spectra of char residues: (a) DPWOPU95, (b) DPOWPU97, (c) DPOWPU99, (d) DPOWPU911, and (e) DPOWPU913.

Figure 9. 3D TGA-FTIR spectrogram of DPOWPU97.

Figure 11. FTIR spectrogram of residues of DPOWPU97.

The elemental composition and content analyses of the DPOWPU residues were performed by EDX spectroscopy. Figure 8 shows the results of EDX spectroscopy testing. The phosphorus contents of char residues reach 15.61, 20.41, 20.52, 22.94, and 23.32 wt %, and the percentage of phosphorus in DPOWPU913 residue is much higher than that of the DPOWPU95. This shows that the phosphorus remaining in the residues probably comes from OP550. P/O = Figure 10. FTIR spectrogram of volatile products of DPOWPU97.

Pwt% /31 Owt% /16

(1)

Equation 1 is a method to calculate the ratio of phosphorus to oxygen.20 The average content of phosphorus and oxygen in the char residue are marked as P wt % and O wt %, respectively. Taking DPOWPU95 for example, the ratio of P to O is approximately 1:4 according to eq 1, which suggests that phosphoric acid, polyphosphoric acid, or other phosphorus-containing compounds stayed in the char residue.

testing, and one can clearly see that with the increase of OP550 content, the surface of the chars becomes more and more smooth and compact and presents a continual and rugged structure. This compact char can efficiently inhibit oxygen transmission and heat diffusion during combustion. F

DOI: 10.1021/ie5045692 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 2. Possible Thermal Degradation Mechanism of DPOWPU

To further illustrate the thermal decomposition products of DPOWPU97, the residues of DPOWPU97 at various degradation temperatures were studied by in situ FTIR, as shown in Figure 11. At room temperature, the main absorptions of DPOWPU97 were observed at 1282 cm−1 (PO); 1430 cm−1 (−C−C−); 1085 and 981 cm−1 (−P−O−C−); 3364.5 cm−1 (−N−H); 1727.3 cm−1 (CO); 1675.5 cm−1(the carbonyl group in amides); 1311.5, 1004.6, and 742.1 cm−1 (P−N− (CH3)2); and 1103 cm−1 (C−O−C). At 250 °C, the intensity of some characteristic peaks was dimmed a little. The spectra of residues at 300 °C showed the characteristic peaks at 1310.4, 1004.6, and 742.1 cm−1 due to the P−N-(CH3)2 groups, but the peak at 1085 cm−1 almost disappeared. The characteristic peak at 2851−2988 cm−1 became weaker, which reflected the disappearance of P−O−CH2, P−N−(CH3)2, and −OCH2CH3 groups. In other words, the fire retardant (OP550 and ODDP) in the soft and hard segment degraded first. At 350 °C, the

Figure 9 is the 3D TGA-FTIR spectrogram of DPOWPU97 during degradation, and Figure 10 is the IR spectra of volatile products obtained from Figure 9. Figure 11 is the IR spectra of residues of DPOWPU97 at different temperatures. We can see from Figure 10 that gases containing −NCO groups and HCN (2250−2400 cm−1) release at 200 °C and 300−450 °C and CO2 (2250−2400 cm−1) releases during 350−450 °C. This is attributed to the degradation of the NHCOO− groups at these temperatures.21 In addition, at 350 °C, the peaks appearing at 1020−1245, 1720−1767, and 2755−3000 cm−1 were due to CH2O, C2H4O, or their mixture; peaks at 1210−1310 and 2851−2988 cm−1 were due to hydrocarbons (−CH2− and −CH3); and the absorption peak at 1400− 1500 cm−1 was attributed to the structures containing aromatic rings. In addition, other nonflammable volatile products appeared at 350 °C, for example, NH3 (964 and 928 cm−1), CO2 (2250−2400 cm−1), etc. G

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Industrial & Engineering Chemistry Research characteristic peaks at 1103, 1282, and 1430 cm−1 vanished, indicating the complete degradation of OP550. The characteristic peaks at 3264 cm−1 completely disappeared when the temperature increased to 450 °C. This result shows that the amido bonds have been completely broken down. On the basis of the related articles,22 and in combination with the results of TGA, cone calorimeter test, TG-FTIR, SEM, and EDX spectroscopy, the thermal decomposition mechanism of DPOWPU was obtained. Scheme 2 shows the detailed information on the decomposition mechanism. First, at 160− 252 °C, the polyurethane chains degrade and release gases such as CO2 and HCN. At the same time, ODDP and OP550 degrade and produce phosphoric acid. Second, at 252−302 °C, phosphoric acid catalyzes the breakdown of polyurethane into polyols, isocyanates, olefin, amines, and CO2. Meanwhile, phosphoric acid polymerization generates polyphosphoric acid. Third, at 302−450 °C, with the catalysis of phosphoric acid and polyphosphoric acid, the polyol and isocyanate are further decomposed into dienes and carbodiimides, and NH3, CO2, and H2O are released at the same time.

(6) Song, Y. P.; Wang, D. Y.; Wang, X. L.; Lin, L.; Wang, Y. Z. A method for simultaneously improving the flame retardancy and toughness of PLA. Polym. Adv. Technol. 2011, 22, 2295. (7) Neisius, M.; Liang, S.; Mispreuve, H.; Gaan, S. PhosphoramidateContaining Flame-Retardant Flexible Polyurethane Foams. Ind. Eng. Chem. Res. 2013, 52, 9752. (8) Chattopadhyay, D. K.; Raju, K. V. S. N. Structural engineering of polyurethane coatings for high performance applications. Prog. Polym. Sci. 2007, 32, 352. (9) Konig, A.; Kroke, E. Methyl-DOPOA new flame retardant for flexible polyurethane foam. Polym. Adv. Technol. 2011, 22, 5. (10) Jeng, R. J.; Shau, S. M.; Lin, J. J.; Su, W. C.; Chiu, Y. Flame retardant epoxy polymers based on all phosphorus containing components. Eur. Polym. J. 2002, 38, 683. (11) Ranganathan, T.; Zilberman, J.; Farris, R. J.; Coughlin, E. B.; Emrick, T. Synthesis and Characterization of Halogen-Free Antiflammable Polyphosphonates Containing 4,4′-Bishydroxydeoxybenzoin. Macromolecules. 2006, 39, 5974. (12) Huang, W. K.; Yeh, J. T.; Chen, K. J.; Chen, K. N. Flame retardation improvement of aqueous-based polyurethane with aziridinyl phosphazene curing system. J. Appl. Polym. Sci. 2001, 79, 662. (13) Celebi, F.; Polat, O.; Aras, L.; Gunduz, G.; Akhmedov, I. M. Synthesis and characterization of water-dispersed flame-retardant polyurethane resin using phosphorus-containing chain extender. J. Appl. Polym. Sci. 2004, 91, 1314. (14) Grassie, N.; Mendoza, G. A. P. Thermal degradation of polyether-urethanes: 5, polyether-urethanes prepared from methylene bis(4-phenylisocyanate) and high molecular weight poly(ethylene glycols) and the effect of ammonium polyphosphate. Polym. Degrad. Stab. 1985, 11, 359. (15) Cheema, H. A.; El-Shafei, A.; Hauser, P. J. Conferring flame retardancy on cotton using novel halogen-free flame retardant bifunctional monomers: Synthesis, characterizations and applications. Carbohydr. Polym. 2013, 92, 885. (16) Liao, F.; Zhou, L.; Ju, Y.; Yang, Y.; Wang, X. Synthesis of a Novel Phosphorus−Nitrogen-Silicon Polymeric Flame Retardant and Its Application in Poly(lactic acid). Ind. Eng. Chem. Res. 2014, 53, 10015. (17) Gu, L.; Ge, Z.; Huang, M.; Luo, Y. Halogen-Free FlameRetardant Waterborne Polyurethane with a Novel cyclic structure of Phosphorus−Nitrogen Synergistic Flame Retardant. J. Appl. Polym. Sci. 2015, 132, 765. (18) Ravey, M.; Pearce, E. M. Flexible polyurethane foam. I. Thermal decomposition of a polyether-based, water-blown commercial type of flexible polyurethane foam. J. Appl. Polym. Sci. 1997, 63, 47. (19) Price, D.; Liu, Y.; Milnes, G. J.; Hull, R.; Kandola, B. K.; Horrocks, A. R. An investigation into the mechanism of flame retardancy and smoke suppression by melamine in flexible polyurethane foam. Fire Mater. 2002, 26, 201. (20) Chen, M.-J.; Chen, C.-R.; Tan, Y.; Huang, J.-Q.; Wang, X.-L.; Chen, L.; Wang, Y.-Z. Inherently Flame-Retardant Flexible Polyurethane Foam with Low Content of Phosphorus-Containing CrossLinking Agent. Ind. Eng. Chem. Res. 2014, 53, 1160. (21) Singh, H.; Jain, A. K. Ignition, combustion, toxicity, and fire retardancy of polyurethane foams: A comprehensive review. J. Appl. Polym. Sci. 2009, 111, 1115. (22) Chattopadhyay, D. K.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34, 1068.

4. CONCLUSIONS This work reports the successful synthesis of a group of new halogen-free flame-retardant WPUs by comodification, and the structures of the polymers were characterized by means of FTIR. With the incorporation of OP550 that acted as soft segment fire retardant, the T5% and Tmax III of DPOWPUs were reduced. The flame retardancy of the DPOWPUs was shown to be improved, as the LOI results increased to 31.4% at Exolit OP550 content of 13 wt %. Moreover, with the increase of Exolit OP550 content, the cone calorimeter results were greatly improved, and the carbon residue of the DPOWPUs increased distinctly. Moreover, the structure of the char is continual and rugged, and this compact char layer can effectively inhibit oxygen transmission and heat diffusion during combustion. In addition, the phosphorus content in the residues grew with the increase of OP550 in DPOWPUs, which suggests that OP550 takes an active role in the condensed phase during the decomposition.

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*Tel: +86 10 68913698. Fax: +86 10 68911608-207. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/ie5045692 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX