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Chinese Chemical Letters 19 (2008) 350–354 www.elsevier.com/locate/cclet
A mechanistic study of flame retardance of novel copolyester phosphorus containing linked pendant groups by TG/XPS/direct Py-MS Nian Hua Huang a,*, Qiang Zhang a, Chou Fan b, Jian Qi Wang c a
b
School of Textile and Materials, Wuhan University of Science and Engineering, Wuhan 430073, China X-ray Photoelectron Spectroscopy Laboratory, South-Central University for Nationalities, Wuhan 430074, China c National Laboratory of Flame Retardant Materials, Beijing Institute of Technology, Beijing 100081, China Received 7 September 2007
Abstract The flame retardant mechanism of the copolyester phosphorus containing linked pendant groups was investigated by thermogravimetric (TG), X-ray photoelectron spectroscopy (XPS) and direct insertion probe pyrolysis mass spectrometry (DPMS) technique. TG results show that the incorporation of phosphorus containing unit linked pendant groups can destabilize the copolyester due to the cleavage of P–CH2 bond, and phosphorus containing units cannot promote the char-formation of the copolyester during the thermal degradation of the copolyester. XPS spectra indicate that with the increase of the temperature, the P– CH2 bonds of the copolyester break down gradually, the concentration of phosphorus in the condensed phase products decrease gradually and the chemical state of phosphorus does not change in the temperature of 250–380 8C. Direct pyrolysis MS suggests that the P–CH2 bonds cleavage occurs at pendant groups and species containing phosphorus can volatilize into the gas phase. A flame retardant mechanism is proposed for the gas phase mode of action of the halogen-free copolyester phosphorus containing linked pendant groups. # 2007 Nian Hua Huang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Phosphorus containing copolyester; Thermal degradation; Flame retardant mechanism; TGA; XPS; Direct Py-MS
In recent years, much attention has been attracted on the necessity for various flame retardant products, and much effort has been made to this purpose. Phosphorus containing flame retardants have been developed to replace the conventional halogen containing flame retardants to meet the requirements of low smoke and low toxicity in view of environmental protection and public security. As the flame retardants are chemically bonded to polymeric main chains or pendant groups, they will not migrate to the surfaces of polymers during processing, such as extrusion, injection moulding or spinning, and thus the flame retardant effect and physical properties are not affected. So, the copolyester phosphorus containing in the main or side chains have been extensively synthesized [1–5]. However, most of researches have focused on the thermal stability, flame retardant performance, rheological and mechanical properties of the copolyesters. In general, the phosphorus containing polymer acts primarily in the gas phase or in the condensed
* Corresponding author. E-mail address: [email protected] (N.H. Huang). 1001-8417/$ – see front matter # 2007 Nian Hua Huang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.11.021
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
351
Scheme 1. Structure of reactive phosphorus containing FR-monomer (a) and reaction route of the copolyesters (b).
phase or both. The flame retardant effect of the copolyester phosphorus containing in the main chains was mainly proposed via a condensed phase mechanism [6]. However, the flame retardant mechanism of the copolyester with phosphorus linkage on the side chain has been little reported. In our previous study [7], a novel copolyester phosphorus containing linked pendant groups (FR-PET) (see Scheme 1) was synthesized. The FR-PET had high LOI value of 31.5% and exhibited excellent flame retardant property and anti-dripping behaviour. In this work, ‘‘pseudo-in-situ’’ XPS (X-ray photoelectron spectroscopy) in combination with TGA and Direct Py-MS is used to investigate the flame retardant mechanism of FR-PET. 1. Experimental Commercial PET was provided by Hubei Kaidi Textile Co., Ltd. The copolyester phosphorus containing linked pendant groups (FR-PET) was obtained from the melt polycondensation of terephthalic acid (TPA), ethylene (EG) and 9,10-dihydro-9-oxa-10[2,3-di(2-hydroxyethoxy)carbonylpropyl]10-phosphaphcnantbrene-10-oxide [7]. The inherent viscosities (I.V.) of PET and FR-PET were determined with an Ubbelohde viscometer at 25 8C in phenol/1,1,2,2-tetrachloroethane (3/2, w/w) solution. Some physical parameters of PET and FR-PET are given in Table 1. Thermogravimetric analysis (TG) was performed on the NETZSCH thermal analyzer (TG 209 F1). Samples weighting about 5.0 mg were heated from room temperature to 700 8C at various values of b (5, 10, 20, 40 8C/min) in a dynamic nitrogen atmosphere. The nitrogen flow rate was 50 ml/min. Brief description of XPS experiment CAE (Constant Analyzer Energy) mode was exercised with Mg Ka (1253.6 eV) at 200 W on Thermo VG Multilab 2000 at pass energy of 70 eV (step length of 0.2 eV and five scans) for better result in quantitative analysis, e.g., for C1s, O1s, and P2p windows. Calibrations of binding energies were referenced on the adventitious carbon (284.6 eV). The XPS experiments were performed at temperatures from ambient up to 600 8C. The pseudo-in-situ technique was utilized. In this technique, the sample is heated outside the XPS chamber under Ar atmosphere and thus, the sample is placed in the ultra high vacuum chamber for XPS analysis [8,9]. The samples were prepared by dissolving 0.5 g PET or FR-PET in 20 ml CHCl3/CF3CH2OH (2:1, v/v) mixture, of which one or two droplets were spread onto an aluminum foil which was well cleaned, by ethyl alcohol and then acetone. The casting film was then prepared using a centrifugal method. Direct pyrolysis MS equipment consists of a direct insertion pyrolysis probe and a control unit, a Quattro II quadruple mass spectrometer and a personal computer for the control of the instrument and data acquisition and processing. Pyrolysis was carried out using the direct insertion probe for solid materials at 400 8C (in order to prevent the polymer main chain from further degradation) under high vacuum (10 6 Pa), using about 5 mg of a polymer sample. Electron impact (EI) mass spectra were obtained at 18 eV. The source temperature was maintained at 260 8C, and the acceleration voltage was 8 kV. Table 1 Some physical parameters of PET and FR-PET Samples
P (%, w/w)
I.V. (dl/g)
Tm (8C)
LOI (%)
Dripping
PET FR-PET
– 0.35
0.64 0.68
253.4 246.5
22.8 31.5
Yes No
352
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
Fig. 1. TG curves of PET and FR-PET in nitrogen at different heating rates: (a) 5 8C/min; (b) 10 8C/min; (c) 20 8C/min and (d) 40 8C/min.
2. Results and discussion Fig. 1 shows the TG curves of the control PET sample and FR-PET at different heating rates (b = 5, 10, 20, 40 8C/ min). The corresponding data can also be seen in Table 2. PET loses weight in the range 387–500 8C at heating rate of 10 8C/min. This process involves a main step with a maximum rate of weight loss at 425 8C. The weight loss of FRPET takes place in the range 373–500 8C with a maximum rate of weight loss at 427 8C at heating rate of 10 8C/min. The onset temperature of FR-PET (Tonset = 373 8C) is lower than that of the control PET sample (Tonset = 387 8C). This reveals that FR-PET has lower thermal stability than PET. This is because of the incorporation of phosphorus linkage as pendant groups. The P–C bonds (ethylene phosphonate unit) cleavage occurs at the pendant groups; therefore, the thermal stability of phosphorus containing copolyester decreases. However, the temperature at the maximum decomposition rate (Tmax) and the maximum decomposition rate (Rmax) of FR-PET are close to those of PET. The residue left by FR-PET at 650 8C is lower than that of PET. These results reveal that: (1) phosphorus unit linkage as pendant groups has little effect on the thermal stability of PET at high temperature (>420 8C); (2) the thermal degradation mechanism of the main degradation stage for FR-PET is probably the same as PET; (3) phosphorus unit linkage as pendant groups does not promote the charring forming of the copolyester, i.e., implying that the charforming mechanism of phosphorus unit linkage as pendant groups is negligible in the condensed phase. Table 3 shows the specific XPS data (C1s, O1s and P2p spectra) of the spectra of PET and FR-PET at different temperatures. A binding energy of 133.3 eV can be assigned to O–P O at room temperature. The concentration of phosphorus in the condensed phase products for FR-PET decreases gradually with increasing temperature. The phosphorus is not detected by XPS at temperatures above 380 8C. Specially, a binding energy of phosphorus element has almost no change in the temperature of 250–380 8C. This reveals that the chemical state of phosphorus does not change. The atomic concentration and its changing trend of carbon element in the condensed phase products for FRPET at different temperatures are very similar to those for PET. These XPS data confirm that phosphorus unit does not promote the charring forming of the copolyester during the thermal degradation of FR-PET, and phosphorus species volatilize into the gas phase. The gas phase flame retardant effect of the FR-PET could be very important. Table 2 Data of TG and DTG of neat PET and flame retardant PET at different heating rates b
5 10 20 40
Neat-PET
FR-PET
Tonset (8C)
T10% (8C)
Tmax (8C)
Rmax (%/8C)
C.R. (%)
Tonset (8C)
T10% (8C)
Tmax (8C)
Rmax (%/8C)
C.R. (%)
374 387 399 413
385 398 410 424
411 425 439 450
1.94 1.92 1.87 1.85
16.54 16.83 16.60 17.48
366 373 392 407
380 390 405 420
411 427 439 450
1.84 1.84 1.78 1.84
15.92 14.04 15.81 14.82
T10% is the temperature for 10% weight loss, respectively; Tmax is the temperature at which the rate of of weight loss reaches a maxium; C.R. is the residue of polymer at 650 8C.
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
353
Table 3 Atomic concentration of PET and FR-PET at different temperatures in Ar Temperature (8C)
Room temp. 250 330 340 350 360 380 440 480 500
Neat-PET
FR-PET
C1s (%)
O1s (%)
C1s (%)
O1s (%)
P2p (%)
74.02 79.23 78.61 77.24 76.47 75.16 73.08 70.41 81.27 89.18
25.98 20.27 21.39 22.76 23.53 24.84 26.92 29.59 18.73 10.82
74.32 77.72 75.70 73.35 72.92 73.79 73.11 73.35 80.62 87.28
25.30 21.91 24.07 26.48 26.98 26.14 26.89 26.65 19.38 12.72
0.38 0.37 0.23 0.17 0.15 0.07 n.d. n.d. n.d. n.d.
n.d.: Not detectable.
Fig. 2. EI mass spectra (18 eV) of the FR-PET at 400 8C. EI mass spectra (18 eV) of the FR-PET at 400 8C (top trace); mass spectra of DOPO is automatically searched in NIST mass spectra database of the Quattro II quadruple mass spectrometer (bottom trace).
Pyrolysis mass spectrometry (MS) technique can be regarded as one of the most powerful analytical methods for thermal analysis of polymers. Not only thermal stability but also degradation products can be analyzed with this technique [10,11]. According to XPS and TG results, the thermal degradation of FR-PET occurred at 400 8C and the concentration of P in the condensed phase is very low. The EI mass spectra of the products evolving from FR-PET at a probe temperature of 400 8C is reported in Fig. 2. There was a strong peak at m/z 216 from 9,10-dihydro-9-oxa-10phospaphenanthrene-10-oxide (DOPO). It is concluded that the P–CH2 bond cleavage occurs at the pendant groups because P–CH2 bond is the weakest during the thermal degradation of FR-PET. Morever, phosphorus species, DOPO, obtained from the P–CH2 bonds cleavage at the pendant groups, should volatilize into the gas phase. In addition to DOPO mass spectra peaks, other mass spectra peaks are also observed, becasue experiment is carried out at the high temperature (400 8C) and under high vacuum condition and the minor degradation of the main chain takes place. 3. Conclusion A lower thermal stability of FR-PET in relation to PET is obvious from the TG data because of the P–CH2 bond cleavage at the pendant groups. Phosphorus species, DOPO, obtained from the P–CH2 bond cleavage, should volatilize into the gas phase as indicated by the DP-MS. The yield of char for the copolyester phosphorus containing linked pendant groups is insignificant relative to neat PET itself, and this suggests that the majority of the crucial flame retardant activity of FR-PET is in the gas phase rather than in the condensed phase. Acknowledgment This work is supported by China National Textile and Apparel Council (CNTAC No. 2006071).
354
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
L.S. Wang, X.L. Wang, G.L. Yan, Polym. Degrad. Stabil. 69 (2000) 127. S.J. Chang, F.C. Chang, Polymer 39 (1998) 3233. S.J. Chang, F.C. Chang, J. Appl. Polym. Sci. 72 (1999) 109. C.S. Wnag, J.Y. Shieh, Y.M. Sun, J. Appl. Polym. Sci. 70 (1998) 1959. J. Asrar, P.A. Berger, J. Hurlbut, J. Polym. Sci. Part A: Polym. Chem. 37 (1999) 3119. S. Moriyuki, H. Kondo, Y. Masaaki, J. Appl. Polym. Sci. 29 (1984) 299. N.H. Huang, Q. Zhang, Z.H. Li., New Chem. Mater. (in Chinese), 2008, in press. H. Tu, J.Q. Wang, Polym. Degrad. Stabil. 54 (1996) 195. J. Wang, J. Du, Polym. Degrad. Stabil. 77 (2002) 249. H. Meuzelaar, W. Winding, A.M. Harper, et al. Science 226 (1984) 268. J.M. John, J. Anal. Appl. Pyrol. 18 (1990) 1.
Chinese Chemical Letters 19 (2008) 350–354 www.elsevier.com/locate/cclet
A mechanistic study of flame retardance of novel copolyester phosphorus containing linked pendant groups by TG/XPS/direct Py-MS Nian Hua Huang a,*, Qiang Zhang a, Chou Fan b, Jian Qi Wang c a
b
School of Textile and Materials, Wuhan University of Science and Engineering, Wuhan 430073, China X-ray Photoelectron Spectroscopy Laboratory, South-Central University for Nationalities, Wuhan 430074, China c National Laboratory of Flame Retardant Materials, Beijing Institute of Technology, Beijing 100081, China Received 7 September 2007
Abstract The flame retardant mechanism of the copolyester phosphorus containing linked pendant groups was investigated by thermogravimetric (TG), X-ray photoelectron spectroscopy (XPS) and direct insertion probe pyrolysis mass spectrometry (DPMS) technique. TG results show that the incorporation of phosphorus containing unit linked pendant groups can destabilize the copolyester due to the cleavage of P–CH2 bond, and phosphorus containing units cannot promote the char-formation of the copolyester during the thermal degradation of the copolyester. XPS spectra indicate that with the increase of the temperature, the P– CH2 bonds of the copolyester break down gradually, the concentration of phosphorus in the condensed phase products decrease gradually and the chemical state of phosphorus does not change in the temperature of 250–380 8C. Direct pyrolysis MS suggests that the P–CH2 bonds cleavage occurs at pendant groups and species containing phosphorus can volatilize into the gas phase. A flame retardant mechanism is proposed for the gas phase mode of action of the halogen-free copolyester phosphorus containing linked pendant groups. # 2007 Nian Hua Huang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Phosphorus containing copolyester; Thermal degradation; Flame retardant mechanism; TGA; XPS; Direct Py-MS
In recent years, much attention has been attracted on the necessity for various flame retardant products, and much effort has been made to this purpose. Phosphorus containing flame retardants have been developed to replace the conventional halogen containing flame retardants to meet the requirements of low smoke and low toxicity in view of environmental protection and public security. As the flame retardants are chemically bonded to polymeric main chains or pendant groups, they will not migrate to the surfaces of polymers during processing, such as extrusion, injection moulding or spinning, and thus the flame retardant effect and physical properties are not affected. So, the copolyester phosphorus containing in the main or side chains have been extensively synthesized [1–5]. However, most of researches have focused on the thermal stability, flame retardant performance, rheological and mechanical properties of the copolyesters. In general, the phosphorus containing polymer acts primarily in the gas phase or in the condensed
* Corresponding author. E-mail address: [email protected] (N.H. Huang). 1001-8417/$ – see front matter # 2007 Nian Hua Huang. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2007.11.021
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
351
Scheme 1. Structure of reactive phosphorus containing FR-monomer (a) and reaction route of the copolyesters (b).
phase or both. The flame retardant effect of the copolyester phosphorus containing in the main chains was mainly proposed via a condensed phase mechanism [6]. However, the flame retardant mechanism of the copolyester with phosphorus linkage on the side chain has been little reported. In our previous study [7], a novel copolyester phosphorus containing linked pendant groups (FR-PET) (see Scheme 1) was synthesized. The FR-PET had high LOI value of 31.5% and exhibited excellent flame retardant property and anti-dripping behaviour. In this work, ‘‘pseudo-in-situ’’ XPS (X-ray photoelectron spectroscopy) in combination with TGA and Direct Py-MS is used to investigate the flame retardant mechanism of FR-PET. 1. Experimental Commercial PET was provided by Hubei Kaidi Textile Co., Ltd. The copolyester phosphorus containing linked pendant groups (FR-PET) was obtained from the melt polycondensation of terephthalic acid (TPA), ethylene (EG) and 9,10-dihydro-9-oxa-10[2,3-di(2-hydroxyethoxy)carbonylpropyl]10-phosphaphcnantbrene-10-oxide [7]. The inherent viscosities (I.V.) of PET and FR-PET were determined with an Ubbelohde viscometer at 25 8C in phenol/1,1,2,2-tetrachloroethane (3/2, w/w) solution. Some physical parameters of PET and FR-PET are given in Table 1. Thermogravimetric analysis (TG) was performed on the NETZSCH thermal analyzer (TG 209 F1). Samples weighting about 5.0 mg were heated from room temperature to 700 8C at various values of b (5, 10, 20, 40 8C/min) in a dynamic nitrogen atmosphere. The nitrogen flow rate was 50 ml/min. Brief description of XPS experiment CAE (Constant Analyzer Energy) mode was exercised with Mg Ka (1253.6 eV) at 200 W on Thermo VG Multilab 2000 at pass energy of 70 eV (step length of 0.2 eV and five scans) for better result in quantitative analysis, e.g., for C1s, O1s, and P2p windows. Calibrations of binding energies were referenced on the adventitious carbon (284.6 eV). The XPS experiments were performed at temperatures from ambient up to 600 8C. The pseudo-in-situ technique was utilized. In this technique, the sample is heated outside the XPS chamber under Ar atmosphere and thus, the sample is placed in the ultra high vacuum chamber for XPS analysis [8,9]. The samples were prepared by dissolving 0.5 g PET or FR-PET in 20 ml CHCl3/CF3CH2OH (2:1, v/v) mixture, of which one or two droplets were spread onto an aluminum foil which was well cleaned, by ethyl alcohol and then acetone. The casting film was then prepared using a centrifugal method. Direct pyrolysis MS equipment consists of a direct insertion pyrolysis probe and a control unit, a Quattro II quadruple mass spectrometer and a personal computer for the control of the instrument and data acquisition and processing. Pyrolysis was carried out using the direct insertion probe for solid materials at 400 8C (in order to prevent the polymer main chain from further degradation) under high vacuum (10 6 Pa), using about 5 mg of a polymer sample. Electron impact (EI) mass spectra were obtained at 18 eV. The source temperature was maintained at 260 8C, and the acceleration voltage was 8 kV. Table 1 Some physical parameters of PET and FR-PET Samples
P (%, w/w)
I.V. (dl/g)
Tm (8C)
LOI (%)
Dripping
PET FR-PET
– 0.35
0.64 0.68
253.4 246.5
22.8 31.5
Yes No
352
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
Fig. 1. TG curves of PET and FR-PET in nitrogen at different heating rates: (a) 5 8C/min; (b) 10 8C/min; (c) 20 8C/min and (d) 40 8C/min.
2. Results and discussion Fig. 1 shows the TG curves of the control PET sample and FR-PET at different heating rates (b = 5, 10, 20, 40 8C/ min). The corresponding data can also be seen in Table 2. PET loses weight in the range 387–500 8C at heating rate of 10 8C/min. This process involves a main step with a maximum rate of weight loss at 425 8C. The weight loss of FRPET takes place in the range 373–500 8C with a maximum rate of weight loss at 427 8C at heating rate of 10 8C/min. The onset temperature of FR-PET (Tonset = 373 8C) is lower than that of the control PET sample (Tonset = 387 8C). This reveals that FR-PET has lower thermal stability than PET. This is because of the incorporation of phosphorus linkage as pendant groups. The P–C bonds (ethylene phosphonate unit) cleavage occurs at the pendant groups; therefore, the thermal stability of phosphorus containing copolyester decreases. However, the temperature at the maximum decomposition rate (Tmax) and the maximum decomposition rate (Rmax) of FR-PET are close to those of PET. The residue left by FR-PET at 650 8C is lower than that of PET. These results reveal that: (1) phosphorus unit linkage as pendant groups has little effect on the thermal stability of PET at high temperature (>420 8C); (2) the thermal degradation mechanism of the main degradation stage for FR-PET is probably the same as PET; (3) phosphorus unit linkage as pendant groups does not promote the charring forming of the copolyester, i.e., implying that the charforming mechanism of phosphorus unit linkage as pendant groups is negligible in the condensed phase. Table 3 shows the specific XPS data (C1s, O1s and P2p spectra) of the spectra of PET and FR-PET at different temperatures. A binding energy of 133.3 eV can be assigned to O–P O at room temperature. The concentration of phosphorus in the condensed phase products for FR-PET decreases gradually with increasing temperature. The phosphorus is not detected by XPS at temperatures above 380 8C. Specially, a binding energy of phosphorus element has almost no change in the temperature of 250–380 8C. This reveals that the chemical state of phosphorus does not change. The atomic concentration and its changing trend of carbon element in the condensed phase products for FRPET at different temperatures are very similar to those for PET. These XPS data confirm that phosphorus unit does not promote the charring forming of the copolyester during the thermal degradation of FR-PET, and phosphorus species volatilize into the gas phase. The gas phase flame retardant effect of the FR-PET could be very important. Table 2 Data of TG and DTG of neat PET and flame retardant PET at different heating rates b
5 10 20 40
Neat-PET
FR-PET
Tonset (8C)
T10% (8C)
Tmax (8C)
Rmax (%/8C)
C.R. (%)
Tonset (8C)
T10% (8C)
Tmax (8C)
Rmax (%/8C)
C.R. (%)
374 387 399 413
385 398 410 424
411 425 439 450
1.94 1.92 1.87 1.85
16.54 16.83 16.60 17.48
366 373 392 407
380 390 405 420
411 427 439 450
1.84 1.84 1.78 1.84
15.92 14.04 15.81 14.82
T10% is the temperature for 10% weight loss, respectively; Tmax is the temperature at which the rate of of weight loss reaches a maxium; C.R. is the residue of polymer at 650 8C.
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
353
Table 3 Atomic concentration of PET and FR-PET at different temperatures in Ar Temperature (8C)
Room temp. 250 330 340 350 360 380 440 480 500
Neat-PET
FR-PET
C1s (%)
O1s (%)
C1s (%)
O1s (%)
P2p (%)
74.02 79.23 78.61 77.24 76.47 75.16 73.08 70.41 81.27 89.18
25.98 20.27 21.39 22.76 23.53 24.84 26.92 29.59 18.73 10.82
74.32 77.72 75.70 73.35 72.92 73.79 73.11 73.35 80.62 87.28
25.30 21.91 24.07 26.48 26.98 26.14 26.89 26.65 19.38 12.72
0.38 0.37 0.23 0.17 0.15 0.07 n.d. n.d. n.d. n.d.
n.d.: Not detectable.
Fig. 2. EI mass spectra (18 eV) of the FR-PET at 400 8C. EI mass spectra (18 eV) of the FR-PET at 400 8C (top trace); mass spectra of DOPO is automatically searched in NIST mass spectra database of the Quattro II quadruple mass spectrometer (bottom trace).
Pyrolysis mass spectrometry (MS) technique can be regarded as one of the most powerful analytical methods for thermal analysis of polymers. Not only thermal stability but also degradation products can be analyzed with this technique [10,11]. According to XPS and TG results, the thermal degradation of FR-PET occurred at 400 8C and the concentration of P in the condensed phase is very low. The EI mass spectra of the products evolving from FR-PET at a probe temperature of 400 8C is reported in Fig. 2. There was a strong peak at m/z 216 from 9,10-dihydro-9-oxa-10phospaphenanthrene-10-oxide (DOPO). It is concluded that the P–CH2 bond cleavage occurs at the pendant groups because P–CH2 bond is the weakest during the thermal degradation of FR-PET. Morever, phosphorus species, DOPO, obtained from the P–CH2 bonds cleavage at the pendant groups, should volatilize into the gas phase. In addition to DOPO mass spectra peaks, other mass spectra peaks are also observed, becasue experiment is carried out at the high temperature (400 8C) and under high vacuum condition and the minor degradation of the main chain takes place. 3. Conclusion A lower thermal stability of FR-PET in relation to PET is obvious from the TG data because of the P–CH2 bond cleavage at the pendant groups. Phosphorus species, DOPO, obtained from the P–CH2 bond cleavage, should volatilize into the gas phase as indicated by the DP-MS. The yield of char for the copolyester phosphorus containing linked pendant groups is insignificant relative to neat PET itself, and this suggests that the majority of the crucial flame retardant activity of FR-PET is in the gas phase rather than in the condensed phase. Acknowledgment This work is supported by China National Textile and Apparel Council (CNTAC No. 2006071).
354
N.H. Huang et al. / Chinese Chemical Letters 19 (2008) 350–354
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
L.S. Wang, X.L. Wang, G.L. Yan, Polym. Degrad. Stabil. 69 (2000) 127. S.J. Chang, F.C. Chang, Polymer 39 (1998) 3233. S.J. Chang, F.C. Chang, J. Appl. Polym. Sci. 72 (1999) 109. C.S. Wnag, J.Y. Shieh, Y.M. Sun, J. Appl. Polym. Sci. 70 (1998) 1959. J. Asrar, P.A. Berger, J. Hurlbut, J. Polym. Sci. Part A: Polym. Chem. 37 (1999) 3119. S. Moriyuki, H. Kondo, Y. Masaaki, J. Appl. Polym. Sci. 29 (1984) 299. N.H. Huang, Q. Zhang, Z.H. Li., New Chem. Mater. (in Chinese), 2008, in press. H. Tu, J.Q. Wang, Polym. Degrad. Stabil. 54 (1996) 195. J. Wang, J. Du, Polym. Degrad. Stabil. 77 (2002) 249. H. Meuzelaar, W. Winding, A.M. Harper, et al. Science 226 (1984) 268. J.M. John, J. Anal. Appl. Pyrol. 18 (1990) 1.