Novel Flame Retardants Containing 9,10-Dihydro-9-oxa-10

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Novel Flame Retardants Containing 9,10-Dihydro-9-oxa-10phosphaphenanthrene-10-oxide and Unsaturated Bonds: Synthesis, Characterization, and Application in the Flame Retardancy of Epoxy Acrylates Xiaodong Qian,†,‡ Lei Song,*,† Saihua Jiang,†,‡ Gang Tang,† Weiyi Xing,† Bibo Wang,† Yuan Hu,*,†,‡ and Richard K. K. Yuen‡,§ †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡ USTC−CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road Suzhou, Jiangsu 215123, People’s Republic of China § Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong ABSTRACT: A novel flame retardant containing 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and unsaturated bonds (DOPO-HEA) was synthesized and incorporated into epoxy acrylates (EA) in different ratios. The effects of DOPO-HEA on the thermal stability, flame retardant properties, and degradation mechanisms of EA were investigated. The flame retardant properties of the resins showed that the incorporation of DOPO-HEA into EA could improve the limiting oxygen index (LOI) values and greatly reduce the peak heat release rate (pHRR) of EA. The thermal properties of the resins indicated that the incorporation of DOPO-HEA into EA could distinctly improve the char residues. Moreover, the thermal degradation processes of the EA/DOPO-HEA resins were investigated by direct pyrolysis/mass spectrometry (DP−MS). As for the variation in the condensed phase, FTIR and Raman spectroscopies were adopted to investigate the char residues. Due to the catalyzing charring effect and the gas flame retardant effect of phosphorus, the resins exhibited significant improvements in flame retardant properties. rated into polymer materials as additives or reactive flame retardants. Additive flame retardants are normally more economical but tend to leach out, and they have a negative impact on the processability and mechanical properties of the polymer materials.8 Reactive phosphorus-containing flame retardants are usually incorporated into the backbone of the polymer matrix, resulting in high flame retardant efficiency due to the homodispersion of the flame retardants. Among the phosphorus-containing flame retardants, 9,10dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) has attracted extensive attention in recent years due to its excellent flame retardancy to polymer materials and it has already been investigated in detail.9−11 However, as for the epoxy acrylates, DOPO and its derivatives may have great effects on the flame retardant properties. Nevertheless, the flame retardants modified by DOPO reported above are usually solid flame retardants.11,12 The general disadvantages of such solid flame retardants for UV-curing coating is that it is difficult for the resins to be cured by UV light at room temperature because the solid DOPO-containing flame retardants could not be dissolved in the resins at low temperature. As a result, novel DOPO-

1. INTRODUCTION UV curing, the process of photoinitiated conversion of polymeric materials from liquid to solid, is a popular alternative to conventional thermal curing. The UV curing technology has found use in a large variety of applications, in particular to achieve fast drying of varnishes and printing inks and quick setting of adhesives and composite materials.1,2 Among the UVcured resins, epoxy acrylates (EA) exhibit excellent adhesion, good chemical and corrosion resistance, and excellent electrical insulation. Despite having a number of benefits in applications, the known epoxy acrylates, however, are not entirely satisfactory, particularly when they are in fires.3 Thus, some research is required to enhance the fire safety of epoxy acrylates for their comprehensive applications. Compounds containing phosphorus are a growing kind of flame retardants which function differently from other flame retardants such as inorganic materials and halogenated compounds.4,5 Phosphorus-containing flame retardants as additives or reactive monomers are well-known to impart the polymer materials with flame retardant properties through the condensed phase and gas phase mechanisms. Phosphorus could catalyze the char formation in the condensed phase, which could protect the underlying material from heat and act a barrier to reduce the gas release. As for the mechanism in the gas phase, interrupting the exothermic process and suppressing combustion by capturing free radicals are suggested.6,7 Phosphorus-containing flame retardants are usually incorpo© XXXX American Chemical Society

Received: March 18, 2013 Revised: April 24, 2013 Accepted: May 1, 2013

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dx.doi.org/10.1021/ie400872q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

Scheme 1. Synthesis Route of DOPO-HEA

based flame retardants which are liquid and contain reactive unsaturated bonds are required. In this work, a novel monomer (DOPO-HEA) containing DOPO and unsaturated bonds was synthesized and incorporated into the epoxy acrylates through the UV-curing process. It presented a thermal and flame retardant analysis of the epoxy acrylates with different DOPO-HEA (2-hydroxylethyl acrylate) concentrations. The thermal behavior of the cured resins were studied by thermogravimetric analysis (TGA) in both air and nitrogen atmospheres. In addition, Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, and direct pyrolysis/mass spectrometry (DP−MS) were adopted to analyze the flame retardant mechanisms in both condensed and gas phases.

another 12 h at room temperature. After that, precipitated triethylamine hydrochloride was removed by filtering and then the clear filtrate was placed in a rotary evaporator to remove any unchanged reactants and THF. The residues were dissolved in methylene dichloride, and then the product was purified further by washing it with hydrochloric acid (0.01 mol/L, 400 mL), saturated aqueous solutions of sodium bicarbonate (400 mL), and water (500 mL). After drying over sodium sulfate, methylene dichloride was removed by a rotary evaporator, yielding a colorless liquid product, DOPO-HEA. The schematic process of the reaction is presented in Scheme 1. 1 H NMR (CDCl3, ppm): 5.64−6.46 (CHCH2), 3.59−4.43 (−OCH2CH2O−), 6.65−7.93 (aromatic H). 31 P NMR (CDCl3, ppm): 16.71−18.82 (DOPO), −11.9 to −12.32, −13.1 to −13.9 (Ph−O−P−O−CH2−). FTIR (KBr, cm−1): 1263 (−PO), 1190, 979 (−P−O−C), 1726 (−CO), 1637, 1410, 812 (−CC). 2.3. Sample Preparation. The mixtures of EA with DOPO-HEA in different ratios were thoroughly stirred by a magnetic stir bar (60 °C, air atmosphere, absence of light) to get various homogeneous blends according to Table 1. EA,

2. EXPERIMENTAL SECTION 2.1. Materials. The epoxy acrylate (EA), which is a bisphenol A epoxy acrylate with the unsaturation concentration of 3.73 mmol/g and a molar mass of 536 g/mol, was supplied by Tianjin Tianjiao Co. 10-(2,5-Dihydroxyphenyl)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO-HQ) (99.9% purity) was purchased from Shanghai Diwei Co. Ltd. 2-Hydroxylethyl acrylate (HEA), supplied from Dong-fang Chemical Co, Beijing, China, was distilled at reduced pressure and dried over 4 A molecular sieves before use. Tetrahydrofuran (THF) and triethylamine (TEA) were purchased from Shanghai Chemical Reagents Co. of China and were also dried over 4 A molecular sieves before use. Phenyl dichlorophosphate (PDCP), provided by Deheng Chemical Corp. (Shijiazhuang, China), was distilled at reduced pressure before use. 2Hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173), kindly supplied by Ciba Specialty Chemicals, was used as a photoinitiator. Other reagents were used as received without further purification. 2.2. Synthesis of DOPO-HEA. Phenyl dichlorophosphate (PDCP, 21.1 g, 0.1 mol) and THF (200 mL) were added into a three-neck flask equipped with an ice bath, a mechanical stirrer, a flux condenser, an addition funnel, and a nitrogen inlet. After the mixture was saturated with nitrogen atmosphere under vigorous mechanical stirring, a mixture of HEA (11.6 g, 0.1 mol) and TEA (20.2 g, 0.2 mol) was slowly dropped into the above reaction vessel within 2 h at 0−5 °C, and then kept at ambient temperature for 6 h. DOPO-HQ (16 g, 0.05 mol) was then slowly added into the above reactant, and kept stirring for

Table 1. Composition and Limiting Oxygen Index (LOI) Values of EA/DOPO-HEA Resins with Different DOPOHEA Contents sample

epoxy acrylate (g)

DOPO-HEA (g)

DOPO-HEA content (wt %)

LOI

EA0 EA10 EA20 EA30

15.0 13.5 12.0 10.5

0 1.5 3.0 4.5

0 10 20 30

22.0 27.0 28.5 31.5

DOPO-HEA, and their blends were exposed to UV irradiation in the presence of 3 wt % Darocur 1173, and the flame retardant resins were obtained in the ratios of 10 (EA10), 20 (EA20), and 30 wt % (EA30). 2.4. Measurements. FTIR spectra were recorded with a Nicolet 6700 FT-IR spectrophotometer using a thin KBr disk. The transition mode was used, and the wavelength range was set from 4000 to 500 cm−1. 1H NMR and 31P NMR measurements were performed on an AVANCE 400 Bruker spectrometer at room temperature using dimethyl sulfoxide (DMSO) as a solvent. A GOVMARK MCC-2 microscale combustion colorimeter (MCC, according to ASTM D7309) B



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observed in the 31P NMR spectrum, confirming the existence of diastereomers. The FTIR and NMR results indicate that DOPO-HEA has been synthesized. 3.2. Flame Retardant Properties and Thermal Stability of EA and EA/DOPO-HEA Resins. According to ASTM D2863, the LOI is used to reflect the flammability of materials and it is a quantitative method. The LOI just exhibits that some flame retardancy is achieved as the LOI increases (more oxygen required to maintain a candlelike flame). The LOI values of EA and EA/DOPO-HEA resins are given in Table 1. The LOI increases drastically from 22 to 31.5 when the loadings of DOPO-HEA increase from 0 to 30.0 wt %. Those results indicate that DOPO-HEA exhibits good flame retardant effect to epoxy acrylates. The thermal stabilities and thermal degradation of the resins are investigated by thermogravimetric analysis (TGA) under both air and nitrogen atmospheres. The TGA curves mainly involve the stages of the release of decomposition products and the formation of chars. Moreover, Table 2 shows the TGA values of the EA with different ratios of DOPO-HEA. Figure 4 shows the TGA and differential thermogravimetry (DTG) curves of DOPO-HEA, EA, and EA/DOPO-HEA resins under air atmosphere. The cured DOPO-HEA exhibits a 5% weight loss at 248 °C and a 10% weight loss at 329 °C. The degradation process of DOPO-HEA can be divided into two steps: the first region is due to the decomposition of phosphorus and the formation of char in the temperature range 220−550 °C, whereas the second can be assigned to the decomposition of unstable char when the temperature is higher than 550 °C.15 As for the oxidative degradation of neat EA, it begins to lose its weight at about 230 °C and degrades completely below 650 °C. There are two DTG peaks at 420 and 610 °C, corresponding to two decomposition stages. As for the thermal oxidation of the flame retardant resins in air atmosphere, it is clear that the initial decomposition temperature of the flame retardant resins is lowered due to the addition of DOPO-HEA, and it decreases with the increase of DOPO-HEA concentration. Generally, the reduction of the initial degradation temperature is attributed to the lower stability of P−O−C bonds compared with common C−C bonds, which have been discussed in our previous work.16 However, when the temperature is higher than 450 °C, the flame retardant resins exhibit greater thermal stability than that of pure EA. Due to the lower thermal stability of P−O−C bonds, DOPO-HEA degrades and forms phosphoric acid. As a result, the phosphoric acid, which promotes the formation of char at low temperature and acts as the cross-linkers, enhances the thermal stability of the char layer at high temperature. At the same time, the char layers could protect the residues from oxygen and heat, resulting in higher char residue at 650 °C. Those indicate that the incorporation of DOPO-HEA to EA can improve the thermal stability of the EA/DOPO-HEA resins at high temperature. In summary, the TGA and DTG results demonstrate that DOPO-HEA can notably enhance the thermal resistance of the resins at high temperature. Figure 5 shows the TGA and DTG curves of DOPO-HEA, EA, and EA/DOPO-HEA resins in a nitrogen atmosphere. The thermal degradation processes of EA and EA/DOPO-HEA resins have only one stage. From the DTG curves it can be seen that the weight loss rate of the flame retardant resins is lower than that of pure epoxy acrylates. The behavior is in accordance with previous studies of the mechanism of improved flame retardance via phosphorus modification: the phosphorus could

was used to investigate the combustion properties of the UVcured resins. In this system, about 5 mg samples of UV-cured resins were heated to 900 °C at heating rate of 1 K/s and in a stream of nitrogen flowing at 80 cm3/min. The thermogravimetic analysis (TGA) was carried out on a TGA Q5000 IR thermal gravimetric analyzer (TA Instruments). About 5 mg of UV-cured resin was heated from 30 to 800 °C at a heating rate of 10 °C min−1 under nitrogen and air purge, respectively. The DP−MS, which uses the standard direct insertion probe for solid polymer material analysis, was carried out with a Micromass GCT-MS spectrometer at a heating rate of 10 °C min−1, ranging from 30 to 600 °C. The mass data were continuously acquired at a scan rate of 0.1 s. Electron impact (EI) was used for the mass spectra with 70 eV and the mass range of m/z 10−1000. Laser Raman spectroscopy measurements were carried out at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA).

3. RESULTS AND DISCUSSION 3.1. Characterization of DOPO-HEA. The chemical structure of DOPO-HEA was characterized by FTIR, 1H NMR, and 31P NMR. The FTIR spectrum of DOPO-HEA is shown in Figure 1. The strong absorption bands at 1728, 1637,

Figure 1. FTIR spectra of DOPO-HEA.

1411, and 812 cm−1 indicate the existence of acrylate groups.13 Moreover, the specific absorption peak at 580 cm−1 (P−Cl) disappeared and the characteristic absorption bands at 1596 (P−Ph), 1170 (PO), and 937 cm−1 (P−O−Ph) are observed, indicating the reaction between hydroxyl groups and P−Cl groups.7 The 1H NMR spectrum of DOPO-HEA is shown in Figure 2. Since the phosphorus and the adjacent aliphatic carbon in the DOPO structures are both chiral centers, there are diastereomers existing in DOPO-HEA structures.14 As can be seen from Figure 2, the peaks between 3.57 and 4.49 ppm are due to the methylene resonances in the HEA structure. Moreover, the chemical shift of the vinylic group appears between 5.66 and 6.38 ppm and aromatic signals appear in the range 6.62−8.02 ppm. In addition, the specific shifts at 9.13 ppm and 9.51 ppm have disappeared, indicating the reaction between hydroxyl groups in the DOPO-HQ and P−Cl groups. The 31P NMR spectrum of the DOPO-HEA is shown in Figure 3. Three peaks appear at 17.4, 11.9, and −13.5 ppm. The peak at 17.4 ppm belongs to the chemical shift of 31P in the DOPO structure. Moreover, two peaks at −11.9 and −13.5 ppm are C



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Figure 2. 1H NMR spectra of DOPO-HEA and DOPO-HQ.

Figure 3. 31P NMR spectrum of DOPO-HEA.

Table 2. Thermogravimetric Data of the Resins in Air and Nitrogen Atmospheres T10 wt % (°C)

char (650 °C) (wt %)

samples

air

nitrogen

air

nitrogen

EA0 EA10 EA20 EA30

374 341 313 304

371 337 308 287

0.94 7.66 19.16 25.36

13.01 23.10 26.72 29.56

Figure 4. TGA and DTG curves of EA and EA/DOPO-HEA blends in air atmosphere.

approximately 75 wt % of the total weight loss occurs in the temperature range from 300 to 450 °C and the char residues increase from 13.01 to 29.56 wt % at 650 °C. Those indicate that the incorporation of DOPO-HEA could enhance the char

catalyze the thermal degradation of the polymer matrix at the initial stage and enhance the char residues which in turn lowers the mass loss rate and results in lowered heat release because flammable fuel is being released more slowly.17 Moreover, D



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Figure 5. TGA and DTG curves of EA and EA/DOPO-HEA blends in nitrogen atmosphere.

Figure 6. HRR curves of EA0 and EA/DOPO-HEA resins at 1 K/s heating rate, and MCC data of EA and EA/DOPO-HEA resins.

yield and the high char residues are very important for the flame retardancy of the polymer materials. Generally, the flame retardants (DOPO-HEA) could improve the char residues of the resins in both air and nitrogen atmospheres and the char layers are the critical factors which protect the underlying materials from further degradation. 3.3. Microcombustion Calorimeter of EA and EA/ DOPO-HEA Resins. The combustibility of the resins was evaluated using a microcombustion calorimeter (MCC), which is one of the most effective small scale methods to investigate the potential fire hazard of a polymeric material.18 The heat release rates (HRR) of the resins with different DOPO-HEA contents are shown in Figure 6, and the corresponding combustion data are also presented in Figure 6. Evidently, the peak heat release rates (pHRR) of polymers are significantly reduced with the incorporation of DOPO-HEA. When 30 wt % DOPO-HEA was incorporated into the resins, the flame retardant resins, with a reduction as high as 50% in the pHRR, exhibited the lowest pHRR values. As shown in Figure 6, virgin EA has a high value of total heat release (THR; 17.7 kJ/g). After the incorporation of DOPO-HEA into the epoxy acrylate matrix, all the resins containing DOPO-HEA exhibit lower THR values. When the loadings of DOPO-HEA reach 30 wt %, the THR decreases from 17.7 to 11.3 kJ/g. With respect to the ignition temperature (Tign), it can be found that the Tign decreased due to the incorporation of DOPO-HEA into epoxy acrylates. This result is in accordance with the TGA results in nitrogen atmosphere that DOPO-HEA could catalyze the degradation of epoxy acrylates at the initial decomposition stage in nitrogen atmosphere. It is widely known that the

protection mechanism of the phosphate-containing flame retardants is based on the charred layer acting as a physical barrier which slows down heat and mass transfer between the gas and condensed phases. 18,19 Moreover, as for the phosphorus-containing flame retardants, the gas phase mechanism is also suggested. Due to the protection of the phosphorus in the gaseous phase and the condensed phase, the flame retardant resins exhibit reduced pHRR and THR. 3.4. Thermal Degradation Products of the Flame Retardant Resins in Nitrogen Atmosphere. The influence of DOPO-HEA on the thermal degradation behaviors of flame retardant resins at elevated temperature in nitrogen atmosphere is analyzed by DP−MS analysis. The total ion current (TIC) of EA30 by DP−MS is shown in Figure 7a. It can be observed that the TIC of EA30 has two peaks: one obvious peak at 360 °C and one vague peak around 478 °C. Those indicate that the main decomposition happens in the temperature range from 300 to 500 °C, which is in accordance with the TGA results in nitrogen atmosphere. The identification of pyrolysis fragment ions provides insight into the flame retardant mechanisms. Thus, the mass spectra corresponding to the TIC peaks at different times are investigated, as shown in Figure 7b,c. Moreover, the assignments of pyrolysis products are listed in Table 3.20 As for the peak around 360 °C, the peak at m/z 93, which is attributed to the decomposition products of DOPO-HEA, is stronger than others. Those indicate that the P−O−Ph bond is not stable at low temperature. Moreover, the new peaks at m/z 324, m/z 215, m/z 170, and m/z 169 correspond to DOPO-HQ and its E



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Figure 7. (a) Total ion current (TIC) curve of EA30 at different temperatures; EI-MS spectra of the decomposition process at (b) 360 and (c) 478 °C.

M170, M181, M213, and M324 are the main decomposition products of EA and DOPO-HEA. Thus, it can be concluded that the release of DOPO-HQ and its derivatives run through the whole decomposition process. As a result, DOPO-HQ could play the role of flame retardant all through the decomposition process. On the basis of the results of the volatile pyrolysis fragment ions, the following decomposition process can be suggested for DOPO-HEA, as shown in Scheme 2. It can be summarized that

Table 3. Structural Assignments in the DP−MS of EA30

Scheme 2. Oversimplified Decomposition Mechanism of DOPO-HEA

degradation products and the peaks at m/z 27 for C2H3 and m/ z 55 for C3H3O can be assigned to degradation ions of acrylate structures. Those demonstrate that DOPO-HEA decomposes at the initial stages, which is also suggested by the TGA results. The resulting phosphoric acid in the condensed phase and DOPO structures in the gas phase are beneficial to the formation of a char layer. As for decomposition products around 40 min (478 °C), it is noted that M97, M107, M121, F



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Figure 8. (a) FTIR spectra of char residues of EA0 and EA30; (b and c) Raman spectra of char residues of EA0 and EA30.

linkers which link to different aromatic species and reinforce char layers.23 Raman spectroscopy, which is sensitive not only to crystal structures but also to molecular structures, is a powerful tool for analyzing carbonaceous materials. The Raman spectra of the char residues are shown in Figure 8b (the char residue of EA0) and Figure 8c (the char residue of EA30). The spectra usually exhibit two broad and strongly overlapping peaks with intensity maxima at about 1585 and 1360 cm−1. The former band (G band) corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of crystalline graphite, whereas the latter one (the D band) is due to disordered graphite or glassy carbons.24,25 Moreover, the graphitization degree of the char is estimated by a ratio of the intensity of the D and G bands (ID/IG), where ID and IG are the integrated intensities of the D and G bands, respectively.26 Basically, a lower ratio of ID/ IG leads to a higher graphitization degree of the char. According to Figure 8b,c, the ID/IG ratio follows the sequence of EA0 (2.09) < EA30 (2.61), indicating that the highest graphitization degree is EA0. However, the TGA results indicate that the incorporation of DOPO-HEA could improve the char residue significantly. Thus, based on the Raman spectra, it can be concluded that DOPO-HEA creates more glassy carbon as it thermally decomposes. Generally, due to the catalyzing charring effect of phosphorus, the char strengthened effect of phosphorus, and the gas flame retardant effect of phosphorus, the resins exhibited significant improvements in the flame retardancy of epoxy acrylates.

the degradation of EA30 can be divided into two steps. From 250 to 400 °C, the fragment ions are mainly benzene rings and DOPO, which are mainly attributed to the decomposition products of DOPO-HEA. When the temperature is higher than 400 °C, the main decomposition products are bisphenol A, DOPO-HQ, and acrylic acid, which are mainly ascribed to the degradation of epoxy acrylates and DOPO-HEA. Thus, it can be concluded that DOPO-HQ and its derivatives are released at low temperature and high temperature in the process of thermal degradation. 3.5. Residual Chars of EA and EA/DOPO-HEA Resins. It is known that the effective protective of the char layer could improve the flame retardant performance during combustion; thus, it is necessary to evaluate the char layers so as to further investigate the mechanism. Figure 8a plots the FTIR spectra of the char residues of the resins. It is obvious that the spectra are very different from each other. Compared with the char residue of pure EA, the broad peak around 1130 cm−1 (the stretching vibration of PO) appears. Moreover, the broad vibration bands at 1060, 980, and 868 cm−1 are due to the superposition of stretching vibrations of P−O−P bonds.21 Meanwhile, the sharp absorption peak appearing at 1590 cm−1 is assigned to the stretching vibrations of CC in the aromatic compounds.22 Moreover, it is found that the peak at 1750 cm−1 for the CO bands becomes very weak due to the incorporation of DOPO-HEA, implying that DOPO-HEA could promote the carbonization of EA. Compared with pure EA, a new peak at 1610 cm−1 appears, indicating the formation of new aromatic compounds, which are probably due to the formation of P−O− Ph structures. The phosphorus can be regarded as the crossG



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Coating Containing Phosphorus. Polym. Degrad. Stab. 2009, 94, 1176−1182. (8) Artner, J.; Ciesielski, M.; Walter, O.; Döring, M.; Perez, R. M.; Sandler, J. K. W.; Altstädt, V.; Schartel, B. A Novel DOPO-Based Diamine as Hardener and Flame Retardant for Epoxy Resin Systems. Macromol. Mater. Eng. 2008, 293, 503−514. (9) Chen, H. B.; Zhang, Y.; Chen, L.; Shao, Z. B.; Liu, Y.; Wang, Y. Z. Novel Inherently Flame-Retardant Poly(trimethylene Terephthalate) Copolyester with the Phosphorus-Containing Linking Pendent Group. Ind. Eng. Chem. Res. 2010, 49, 7052−7059. (10) Wang, C. S.; Lin, C. H. Flame-Retardant Epoxy Resins from Novel Phosphorus-Containing Novolac. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 3903−3909. (11) Schartel, B.; Braun, U.; Balabanovich, A. I.; Artner, J.; Ciesielski, M.; Doring, M.; Perez, R. M.; Sandler, J. K. W.; Altstadt, V. Pyrolysis and Fire Behaviour of Epoxy Systems Containing a Novel 9,10Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-(DOPO)-Based Diamino Hardener. Eur. Polym. J. 2008, 44, 704−715. (12) Cai, S. X.; Lin, C. H. Flame-Retardant Epoxy Resins with High Glass-Transition Temperatures from a Novel Trifunctional Curing Agent: Dopotriol. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2862− 2873. (13) Chen, X.; Hu, Y.; Jiao, C.; Song, L. Preparation and Thermal Properties of a Novel Flame-Retardant Coating. Polym. Degrad. Stab. 2007, 92, 1141−1150. (14) Lin, C. H.; Lin, H. T.; Chang, S. L.; Wang, H. J.; Hu, Y. M.; Taso, Y. R.; Su, W. C. Benzoxazines with Tolyl, p-Hydroxyphenyl or pCarboxyphenyl Linkage and the Structure−Property Relationship of Resulting Thermosets. Polymer 2009, 50, 2264−2272. (15) Wang, Q.; Shi, W. F. Photopolymerization and Thermal Behaviors of Acrylated Benzenephosphonates/Epoxy Acrylate as Flame Retardant Resins. Eur. Polym. J. 2006, 42, 2261−2269. (16) Qian, X. D.; Song, L.; Hu, Y.; Yuen, R. K. K.; Chen, L. J.; Guo, Y. Q.; Hong, N. N.; Jiang, S. H. Combustion and Thermal Degradation Mechanism of a Novel Intumescent Flame Retardant for Epoxy Acrylate Containing Phosphorus and Nitrogen. Ind. Eng. Chem. Res. 2011, 50, 1881−1892. (17) Braun, U.; Balabanovich, A. I.; Schartel, B.; Knoll, U.; Artner, J.; Ciesielski, M.; Döring, M.; Perez, R.; Sandler, J. K. W.; Altstädt, V.; Hoffmann, T.; Pospiech, D. Influence of the Oxidation State of Phosphorus on the Decomposition and Fire Behaviour of FlameRetarded Epoxy Resin Composites. Polymer 2006, 47, 8495−8508. (18) Braun, U.; Schartel, B.; Fichera, M. A.; Jager, C. Flame Retardancy Mechanisms of Aluminium Phosphinate in Combination with Melamine Polyphosphate and Zinc Borate in Glass-Fibre Reinforced Polyamide 6,6. Polym. Degrad. Stab. 2007, 92, 1528−1545. (19) Liu, Y. L.; Hsiue, G. H.; Chiu, Y. S.; Jeng, R. J.; Ma, C. Synthesis and Flame-Retardant Properties of Phosphorus-Containing Polymers Based on Poly(4-hydroxystyrene). J. Appl. Polym. Sci. 1996, 59, 1619− 1625. (20) Huang, Z. G.; Shi, W. F. Thermal Behavior and Degradation Mechanism of Poly(bisphenyl acryloxyethyl phosphate) as a UV Curable Flame-Retardant Oligomer. Polym. Degrad. Stab. 2006, 91, 1674−1684. (21) Zhu, S. W.; Shi, W. F. Thermal Degradation of a New Flame Retardant Phosphate Methacrylate Polymer. Polym. Degrad. Stab. 2003, 80, 217−222. (22) Ma, H.; Tong, L.; Xu, Z.; Fang, Z.; Jin, Y.; Lu, F. A Novel Intumescent Flame Retardant: Synthesis and Application in ABS Copolymer. Polym. Degrad. Stab. 2007, 92, 720−726. (23) Wang, G. A.; Cheng, W. M.; Tu, Y. L.; Wang, C. C.; Chen, C. Y. Characterizations of a New Flame-Retardant Polymer. Polym. Degrad. Stab. 2006, 91, 3344−3353. (24) Sadezky, A.; Muckenhuber, H.; Grothe, H.; Niessner, R.; Poschl, U. Raman Microspectroscopy of Soot and Related Carbonaceous Materials: Spectral Analysis and Structural Information. Carbon 2005, 43, 1731−1742.

4. CONCLUSION A series of EA/DOPO-HEA resins containing different ratios of DOPO-HEA were prepared. The high phosphorus content of DOPO-HEA contributes an excellent flame retardancy to epoxy acrylates. The MCC results revealed that the resins with DOPO-HEA exhibited lower pHRR and THR compared with those of the pure EA, indicating an improvement in the flame retardancy of EA. Thermogravimetric analysis (TGA) was used to investigate thermal degradation of the resins, demonstrating that DOPO-HEA could notably improve the char residues from 0.94 to 25.36 wt % at 650 °C in air atmosphere. Based on the DP−MS results, the pyrolysis products of EA30 at different stages were fully investigated. From 250 to 400 °C, the fragment ions are mainly benzene rings and DOPO-HQ, which are mainly attributed to the decomposition products of DOPOHEA. When the temperature is higher than 400 °C, the main decomposition products are bisphenol A, DOPO-HQ, and acrylic acid, which are ascribed to the thermal degradation products of epoxy acrylates and DOPO-HEA. Raman spectroscopy showed that DOPO-HEA created more glassy carbon as it thermally decomposed. Due to the catalyzing charring effect, the char strengthened effect, and the gas flame retardant effect of phosphorus, the resins exhibited significant improvements in flame retardant properties.

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AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86-551-3601664. E-mail: [email protected] (L.S.); [email protected] (Y.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the National Basic Research Program of China (973 Program) (2012CB719701), the joint fund of the National Natural Science Foundation of China (NSFC) and the Civil Aviation Administration of China (CAAC) (61079015), the Fundamental Research Funds for the Central Universities (WK2320000007), and the Chinese Academy of Sciences (CAS) and USTC Special Grant for Postgraduate Research, Innovation and Practice.

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REFERENCES

(1) Decker, C. The Use of UV Irradiation in Polymerization. Polym. Int. 1998, 45, 133−141. (2) Decker, C. Kinetic Study and New Applications of UV Radiation Curing. Macromol. Rapid Commun. 2003, 23, 1067−1093. (3) De Espinosa, L. M.; Ronda, J. C.; GaliÀ , M.; Cádiz, V. A Straightforward Strategy for the Efficient Synthesis of Acrylate and Phosphine Oxide-Containing Vegetable Oils and Their Crosslinked Materials. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 4051−4063. (4) Chen, M. J.; Shao, Z. B.; Wang, X. L.; Chen, L.; Wang, Y. Z. Halogen-Free Flame-Retardant Flexible Polyurethane Foam with a Novel Nitrogen−Phosphorus Flame Retardant. Ind. Eng. Chem. Res. 2012, 51, 9769−9776. (5) Levchik, S. V.; Weil, E. D. Review Overview of Recent Developments in the Flame Retardancy of Polycarbonates. Polym. Int. 2005, 54, 981−998. (6) Hergenrother, P. M.; Thompson, C. M.; Smith, J. G., Jr.; Connell, J. W.; Hinkley, J. A.; Lyon, R. E.; Moulton, R. Flame Retardant Aircraft Epoxy Resins Containing Phosphorus. Polymer 2005, 46, 5012−5024. (7) Xing, W. Y.; Hu, Y.; Song, L.; Chen, X. L.; Zhang, P.; Ni, J. X. Thermal Degradation and Combustion of a Novel UV Curable H



Article

(25) Landi, B. J.; Ruf, H. J.; Evans, C. M.; Cress, C. D.; Raffaelle, R. P. Purity Assessment of Single-Wall Carbon Nanotubes, Using Optical Absorption Spectroscopy. J. Phys. Chem. B 2005, 109, 9952−9965. (26) Ko, T.-H. Raman Spectrum of Modified PAN-Based Carbon Fibers during Graphitization. J. Appl. Polym. Sci. 1996, 59, 577−580.

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Novel Flame Retardants Containing 9,10-Dihydro-9-oxa-10

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