Thermal Transition Behavior, Thermal Stability, and Flame Retardancy

Article pubs.acs.org/IECR

Thermal Transition Behavior, Thermal Stability, and Flame Retardancy of Low-Melting-Temperature Copolyester: Comonomer Effect Xin-Ke Jing, De-Ming Guo, Jun-Bo Zhang, Fei-Yu Zhai, Xiu-Li Wang,* Li Chen, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Phosphorus-containing poly(ethylene terephthalate-co-diethylene terephthalate) (PEDT) with a low melting temperature was synthesized through esterification and copolycondensation of terephthalic acid, ethylene glycol, diethylene glycol (DEG), and 3-(hydroxyphenylphosphinyl)propionic acid (HPPPA). DSC results showed that PEDTs still were crystallizable and that the lowest melting temperature of the PEDTs was reduced to 118.0 °C although the chain regularity was destroyed by the incorporation of comonomers. It was found that the addition of HPPPA can improve the thermal oxidative stability of copolyesters. The results of the limiting oxygen index (LOI) test, the UL-94 vertical test, and the cone calorimeter test indicated that HPPPA endowed PEDT with good flame-retardant properties. The LOI values of the PEDTs were increased to 30.5−34.8, the UL-94 ratings of vertical burning were improved to V-0, and both the peak heat release rate (pHRR) and the total heat release (THR) obviously decreased. 2-carboxyethyl(phenylphosphinic) acid.14 A series of phosphorus-containing copolyesters and their nanocomposites with good flame retardancy have been synthesized by our group.15−19 However, our report is the only one on the flame retardancy of low-melting-temperature polyesters.20 In our previous study, neopentyl glycol, as the third comonomer, was used to lower the melting temperature of LMT polyesters. However, the melting temperature of the LMT polyesters was decreased only to 171.2 °C, which limited its processing technology. In addition, neopentyl glycol is more expensive than ethylene glycol. Therefore, to lower the melting temperature and reduce the cost at the same time, diethylene glycol was chosen as the third comonomer. In this study, a phosphorus-containing low-melting-temperature copolyester, phosphorus-containing poly[(ethylene terephthalate)-co-(diethylene terephthalate)] (PEDT), was synthesized by copolycondensation of terephthalic acid (TPA), ethlene glycol (EG), diethylene glycol (DEG), and 3(hydroxyphenylphosphinyl)propionic acid (HPPPA). The effects of DEG and HPPPA on the thermal behaviors, thermal stability, and flame-retardancy of PEDT were investigated in detail.

1. INTRODUCTION The rapid development of the textile industry has drawn attention to cost-effective processing.1 Nonwoven products based on low-melting-temperature (LMT) polyesters meet the developmental demands of the textile industry for applications in many fields such as agriculture, automotive industry, building trade, civil engineering, and health care.1−4 A third or fourth monomer is usually added during the synthesis of polyester to lower the melting temperature. For example, diethylene glycol, an inexpensive monomer, can be introduced into the polyester structure to lower the melting temperature and cost.5 Because its main chain contains both flexible ether linkages and hard ester segments, this polyester shows good flexibility and dimensional stability.5,6 However, the existing LMT polyesters are inflammable, which limits their applications. Usually, three approaches are used to improve the flame retardancy of polyesters fibers, namely, post-treatment, blending with a flame retardant, and monomer copolymerization with a reactive flame retardant.7−9 Phosphorus-containing reactive flame retardants have been used for the synthesis of flame-retardant polyesters. Phosphine oxides prepared by reacting phosphorylated hydroquinone or 2,6-dihydroxynaphthalene with ethylene glycol were added into the poly(ethylene terephthalate) (PET) main chain by copolymerization.10 Chang and co-workers synthesized sidechain-type copolyesters by copolymerization of 9,10-dihydro10-[2,3-di(hydroxycarbonyl)propyl]-10-phosphaphenanthrene10-oxide, terephthalic acid, and ethylene glycol.11,12 Wang et al. incorporated bis(4-carboxyphenyl)phenyl phosphine oxide into the PET backbone and evaluated its flame retardancy and thermal stability.13 Asrar et al. investigated main-chain phosphorus-containing copolyesters that were synthesized by the copolymerization of terephthalic acid, ethylene glycol, and © 2013 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. TPA was purchased from Jinan Zhenghao Advanced Materials Co. Ltd. (Jinan, China). EG was obtained Received: Revised: Accepted: Published: 4539

October 24, 2012 February 25, 2013 March 5, 2013 March 5, 2013 dx.doi.org/10.1021/ie302919n | Ind. Eng. Chem. Res. 2013, 52, 4539−4546

Industrial & Engineering Chemistry Research

Article

Scheme 1. Reaction Route of PEDTs

Table 1. Basic Characteristics of PEDTs PEDT

DEG (mol %, based on TPA)

HPPPA (mol %, based on TPA)

[η] (dL/ g)

Pta (wt %)

Pab (wt %)

Mn (×104 g/mol)

Mw (×104 g/mol)

PDI

PEDT30 PEDT5-30 PEDT50 PEDT5-50 PEDT60 PEDT5-60 PEDT75 PEDT5-70

30 30 50 50 60 60 75 70

0 5 0 5 0 5 0 5

0.85 0.73 0.82 0.74 0.77 0.80 0.78 0.80

0 0.71 0 0.68 0 0.67 0 0.66

0 0.67 0 0.57 0 0.52 0 0.48

− − 4.03 3.61 4.83 4.62 5.09 3.57

− − 8.40 9.08 8.47 8.30 8.49 7.04

− − 2.08 2.51 1.75 1.80 1.67 1.97

a

Pt, theoretical phosphorus content of the final copolyester. bPa, actual phosphorus content of the final copolyester.

introduced into the autoclave to replace the air. A multistep temperature procedure was applied for esterification. The reaction mixture was heated to 240 °C for 1.5 h, and then the temperature of the reactor was raised to 260 °C, where it was held for another 1 h. The pressure for the esterification was 0.32−0.36 MPa, and water produced by esterification was removed by fractionation. After the theoretical volume of water was achieved, the pressure of the system was dropped down to atmospheric pressure. Afterward, the temperature of the mixture was raised to 280 °C and maintained for 4 h under a pressure of 40−100 Pa. The produced polymer melt was extruded at the nitrogen pressure through an orifice and cooled with water. The synthetic route for PEDTs is shown in Scheme 1, and the basic characteristics of the samples are listed in Table 1. 1 H NMR (CDCl3; δ, ppm): 8.01−8.09 (ArH), 7.76 (Ar H), 7.52 (ArH), 7.43 (ArH), 4.69 (EG units COO CH 2 CH 2 OCO), 4.51 (DEG units COO CH2), 3.89 (DEG units CH2OCH2), 2.63 (

from Xilong Chemical Industries Co. Ltd. (Chengdu, China). 3-(Hydroxyphenylphosphinyl)propionic acid (HPPPA) was provided by Weili Flame Retardant Chemical Industry Co. Ltd. (Chengdu, China). Antimony trioxide (Sb2O3, 99.99%) was purchased from Chengdu Chemical Industries Co. Ltd. (Chengdu, China). DEG, phenol, and 1,1,2,2,-tetrachloroethane were provided by Kelong Chemical Industries Reagent Co. Ltd. (Chengdu, China). 2.2. Preparation of PEDTs. PEDTs were synthesized from PET monomers (TPA and EG), DEG, and HPPPA in a 2-L autoclave by direct polymerization. DEG and HPPPA were the third and the fourth comonomers, respectively. The synthetic procedures for all of the copolyesters were the same. The route for the preparation of PEDT containing 50 mol % DEG and 5 mol % HPPPA is presented here as a representative example. EG (310 mL, 5.56 mol) was added into a 2-L autoclave, and then a mixture of TPA (860 g, 5.18 mol), HPPPA (55.4 g, 0.26 mol), DEG (246 mL, 2.59 mol), and Sb2O3 (0.301 g, as a catalyst) was added under stirring. After that, nitrogen was 4540

dx.doi.org/10.1021/ie302919n | Ind. Eng. Chem. Res. 2013, 52, 4539−4546


Article

Figure 1. 1H and 31P spectra of PEDT5-50.

heating rate was 10 °C min−1, and the scanning range was from 40 to 700 °C. Wide-angle X-ray diffraction (WAXD) patterns of the PEDTs were recorded with an X-ray diffractometer (Philips X’Pert X-ray diffractometer) with Cu Kα radiation. The equipment was operated at room temperature at a scan rate of 2°/min scanning from 10° to 36°. Before testing, PEDTs samples with 0.5-mm thickness were preheated at 90 °C for 24 h. PEDTs with a size of 120 × 6.5 × 3.2 mm3 were prepared on a Ferromatik Milacron K-TEC 40 injection molding machine for LOI testing, which was performed on an oxygen index flammability gauge (HC-2C) according to standard method ASTM D 2863-97. The UL-94 vertical test was performed on a vertical burning test instrument (CZF-2) according to the ASTM D 3801 testing procedure in which the size of the PEDT samples was 125 × 12.7 × 3.2 mm3. PEDTs with a size of 100 × 100 × 6 mm3 were used for cone calorimeters test (FTT cone calorimeter) according to method ISO 5660-1 at a heat flux of 50 kW m−2.

CH2CO), 2.21 (PCH2). 31P NMR (CDCl3; δ, ppm), 43.9−44.5. 2.3. Characterization. NMR spectra (1H, 400 MHz; 31P, 161.9 MHz) were obtained on a Bruker AVANCE AV II-400 NMR instrument at room temperature using CDCl3 as the solvent, with tetramethylsilane (TMS) or phosphoric acid as the internal standard. The samples were first dissolved in chloroform and then precipitated by methanol before NMR spectra were recorded. The intrinsic viscosities of the PEDTs were determined through an Ubbelodhe viscometer with a concentration of 0.5 g dL−1 at 25 °C in 1:1 (v/v.) phenol1,1,2,2,-tetrachloroethane solution. The experimental phosphorus content of the final copolyester was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES; IRIS Advantage, TJA Solutions). Gel permeation chromatography (GPC) was performed to determine the molecular weights and molecular weight distributions of the PEDTs using a Waters chromatograph equipped with a Waters 2414 refractive index detector and three Styragel HT columns. PEDTs were dissolved in chloroform (2.5 mg mL−1), and data were recorded at 30 °C at a flow rate of 1 mL min−1. Monodisperse polystyrene standards were used for the primary calibration. Differential scanning calorimetry (DSC) measurements were performed on a TA Q200 instrument under a nitrogen atmosphere at a flow rate of 50 mL min−1. To eliminate the influence of thermal history and the effect of heat treatment on the crystalline structure of the materials, PEDTs were first heated at 280 °C for 5 min and subsequently cooled to 0 °C (i.e., first cooling). After that, they were reheated to 280 °C (i.e., second heating). The thermal transition behaviors of the annealed samples, obtained by storing them in a vacuum oven at 90 °C for 24 h, were also investigated by DSC (scanned from 0 to 280 °C). The scanning rates for DSC measurements were 10 °C min−1. Thermogravimetric analysis (TGA) was performed on a NETZSCH TG 209 F1 instrument under air and nitrogen atmosphere at a flow rate of 50 mL min−1. The

3. RESULTS AND DISCUSSION 3.1. Structure Characterization of PEDTs. The structure of the PEDTs was confirmed by 1H NMR and 31P NMR spectroscopies. The NMR spectra of PEDT5-50 are shown in Figure 1. The resonances located at 8.01−8.09 ppm (δHe) and 4.69 ppm (δHa) are assigned to PET. The signals at 7.75 ppm (δHf), 7.52 ppm (δHh), and 7.43 ppm (δHi) belong to the aryl protons of HPPPA. The chemical shifts of methylene of HPPPA were found at 2.63 ppm (δHj) and 2.21 ppm (δHk). Furthermore, the signals at 4.51 ppm (δHb) and 3.89 ppm (δHc) are assigned to methylene of DEG. The probable representative structures are shown in Figure 1; however, accurate structural parameters were not determined because their chemical shifts could not be differentiated. Moreover, the 31 P NMR spectrum provides further information for structure 4541



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characterization. The phosphorus signals of PEDT5-50 can be seen around 44.5−43.9 ppm (δP1, δP2, δP3, δP4, and δP5). Thus, PEDTs were successfully synthesized through the polycondensation. GPC was used to determine the molecular weights and molecular weight distributions of the PEDTs except for PEDT30 and PEDT5-30 because of their poor solubility in chloroform. The detailed results are shown in Table 1 and Figure 2. In addition, the actual phosphorus contents of the

Figure 2. GPC diagrams of PEDTs.

PEDTs were determined by ICP-AES (also included in Table 1). The results show that the actual phosphorus contents of the PEDTs were lower than their theoretical phosphorus contents, which was caused by the volatilization of HPPPA during the esterfication and condensation. The GPC traces of the PEDTs exhibit a unimodal peak, indicating that the reaction was completed and no unreacted prepolymer remained.21 From Table 1, one can see that the number-average molecular weight (Mn) was about 4 × 104 g/mol. The molecular weight distributions of all samples were located at about 1.67−2.51, as expected for a traditional polycondensation. Furthermore, we found that the molecular weight distributions of PEDT50 and PEDT60 were narrower than those of PEDT5-50, and PEDT560, respectively, which indicates that the latter had more complicated structures after polycondensation. 3.2. Thermal Transition Behaviors of PEDTs. DSC was used to investigate the melting and crystallization behaviors of the copolyesters. As shown in Figure 3a and Table 2, with an increase in DEG content, the glass transition temperature (Tg) of the copolyesters decreased significantly. Similar results were also found when HPPPA was introduced. Because DEG has a long carbon chain and ether bond and HPPPA acts as an unsymmetrical comonomer with flexible aliphatic units,17 both of these components can increase the chain flexibility of the copolyester. Regarding PEDT30, in addition to a melting peak appearing at 197.3 °C, which is much lower than the value for pure PET, a broad cold crystallization peak (Tc) can also be seen. This indicates that some imperfect crystals derived from the incomplete crystallization of PEDT30 during the previous cooling scan began to crystallize, which is similar to the case for PET. A very weak endothermic peak for PEDT5-30 appears at 188.0 °C, which is lower than the value for PEDT30. This was

Figure 3. DSC thermograms of PEDTs: (a) second heating scans, (b) first heating scan after annealing at 90 °C for 24 h.

Table 2. Thermal Transition Temperatures, Degrees of Crystallinity Xc, and Flame Retardancy Results of PEDTs PEDT PEDT30 PEDT530 PEDT50 PEDT550 PEDT60 PEDT560 PEDT75 PEDT570

Tg (°C)

Tc (°C)

Tm (°C)

Xca (%)

LOI (%)

UL94

cotton ignited

61.1 56.1

153.9 −

197.3b 188.0b

35.0 31.2

25.7 34.8

V-2 V-0

yes no

51.9 49.4

− −

151.2c 143.8c

28.8 19.5

25.3 32.3

V-2 V-0

yes no

47.2 46.3

− −

137.0c 133.6c

24.2 18.4

24.6 32.0

V-2 V-0

yes no

42.8 42.7

− −

121.5c 118.0c

11.0 11.9

23.3 30.5

V-2 V-2

yes yes

a

Determined by WAXD. bObtained from second heating scans. Obtained from the first heating scans after annealing at 90 °C for 24 h.

c

ascribed to the fact that the presence of the pendent phenyl of HPPPA would retard its crystallization. However, the Tm values of the other samples whose DEG contents were higher than that of PEDT30 could not be detected because of their low structural regularity induced by the random copolymerization of TPA, EG, DEG, and HPPPA. To investigate the melting behavior of samples whose DEG contents were higher than that of PEDT30, these samples were 4542



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preheated at 90 °C for 24 h, and then they were scanned from 0 to 280 °C at a heating rate of 10 °C min−1 (shown in Figure 3b). As shown in Figure 3b, all of the samples showed typical multiple melting endothermal peaks. The multiple melting phenomena of semicrystalline polyesters can be explained by the following viewpoints: the melting of crystals with multiple lamellar thicknesses22 and a melting−recrystallization−remelting process.23,24 The lowest endothermic peak (Tm,1) can be ascribed to the melting of crystals produced by the secondary crystallization, or the melting of an intermediate state between amorphous and ordered crystalline states. The intermediate endothermic peak (Tm2) reflects the melting of crystals formed by normal primary crystallization. Thus, Tm,2 is defined as Tm for these samples. The highest endothermic peak (Tm,3) represents the melting of secondary crystallites formed during recrystallization.23,25 The Tm values of PEDTs reported in Table 2 decreased gradually with increasing DEG content. The lowest melting temperature was 118.0 °C, which is much lower than that of pure PET. In addition, it is also lower than that of our previously obtained LMP polyester, in which neopentyl glycol was used as the third comonomer.20 Furthermore, the melting temperature of samples containing HPPPA was further lowered. To obtain the degree of crystallinity (Xc), PEDTs sheet with a thickness of 0.5 mm were preheated at 90 °C for 24 h and then investigated by WAXD. Figure S1 (Supporting Information) displays the diffraction patterns of PEDTs. The Xc of PEDTs was calculated through the deconvolution process for the crystalline and amorphous peaks using Jade 5.0 software.26 The relevant data are listed in Table 2. It is obvious that Xc decreased gradually with increasing DEG content. In addition, the degree of crystallinity of samples containing HPPPA was further lowered compared to that of corresponding samples without HPPPA. This phenomenon can be predicted because the introduction of comonomer would destroy the regularity of the polyester just as it has an effect on the melting temperature. 3.3. Thermal Stability of PEDTs. TGA was used to investigate the thermal stability of PEDTs in nitrogen and air atmospheres. Figure 4 shows the TGA curves of the PEDTs under nitrogen. Table S1 (Supporting Information) lists all relevant thermal decomposition data, including the temperature of 5 wt % weight loss, T5%, defined as the temperature of initial decomposition; the temperature of maximum weight loss rate, Tmax; and the weight of residue at 700 °C, wtR700. It is worth mentioning that both the TGA curves and the degradation temperatures (TD) are very similar, indicating a remarkable stability for PEDTs under nitrogen, to temperatures exceeding 375−390 °C. However, the T5% values of samples with HPPPA are lower than those of the corresponding samples without HPPPA, which suggests that phosphorus monomer accelerates the thermal decomposition of PEDT under nitrogen atmosphere. The typical bond energies of C−C and P−C bonds are 349 and 260 kJ/mol, respectively.27 The acceleration of decomposition is related to P−C bonds, for which chain scission tends to take place more easily than for C−C bonds. It should be noted that the wtR700 values of PEDTs without HPPPA were found to be higher than those of corresponding PEDTs with HPPPA. This difference can be attributed to the flame-retardant mechanism of HPPPA, which enhances melt dripping and leaves no stable intumescent char.28 Figure 5 shows the TGA curves of the PEDTs in air atmosphere. Relevant thermal oxidative decomposition data are

Figure 4. TGA thermograms of PEDTs in N2.

presented in Table S1 (Supporting Information). It can be seen that the PEDTs show three decomposition steps under air atmosphere, including one major (at 425 °C) and two minor (at about 315 and 530 °C) weight-loss stages. This is different from the thermal oxidative decomposition of pure PET, which has two decomposition steps under air atmosphere, including one major weight-loss stage (at about 433 °C) and one minor weight-loss stage (at about 544 °C).29 The first weight-loss stage (at about 315 °C) is mainly the decomposition of DEG chain segments. The weight-loss values (wl) of the first weightloss stage for the PEDTs increased with increasing DEG content. At the same time, the T5% values obviously decreased with increasing DEG content. Some investigations of the thermal oxidative degradation mechanism of DEG-containing polymers have appeared in the literature.6,30,31 The most common viewpoints invoked to explain the phenomenon are that the carbon atom vicinal to the ether oxygen is attacked by oxygen, yielding hydroperoxides, and then the hydroperoxides decompose to alkoxy radicals and hydroxyl radicals (•OH). The alkoxy radicals can generate two new degradation products, which produce compounds with acidic end groups or ester functions after several degradation steps.6,30,31 The low decomposition temperature cannot hinder the application and processing of PEDTs owing to their low melting temperatures. As shown in Table 3, the T5% values of samples containing HPPPA were higher than those of the corresponding PEDTs without HPPPA. This is attributed to fact that, as a type of phosphorus-containing flame retardant, HPPPA can scavenge active radicals and inhibit radical reactions in the gas 4543



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The results of UL-94 vertical tests showed that the PEDTs incorporated with HPPPA reached a V-0 rating, except for PEDT5-70, whereas the PEDTs without HPPPA reached only a V-2 rating. The UL-94 vertical test results also suggest that the incorporation of HPPPA can endow flame retardancy to copolyesters. Cone calorimetry, which can provide a wealth of information through its simulation of real fire, was used to investigate the effects of HPPPA on the combustion behavior of the PEDTs. The time to ignition (TTI), heat release rate (HRR), and total heat release (THR) obtained from cone calorimeter tests (reported in Table 3) are very important parameters for assessing the fire risk of polymers.33,34 As shown in Table 3, the TTIs of PEDT5-50 and PEDT5-60 were delayed by 8 s compared to the TTIs their corresponding samples without HPPPA (PEDT50 and PEDT60, respectively). Some references have reported that the thermal decomposition of materials under heat flux before ignition during cone calorimeter tests is very similar to thermogravimetry under air atmosphere.29,35 From Table S1 (Supporting Information), it is obvious that the T5% values of PEDT5-50 and PEDT5-60 in air were lower than those of their corresponding samples without HPPPA under air. In addition, HPPPA can scavenge active radicals and inhibit radical reactions in the gas phase, which also results in delaying the TTI. Figure 6a and Table 3 show that the peak heat release rates (pHRRs) of copolyesters with HPPPA were obviously reduced. PEDT50 and PEDT60 burned rapidly

Figure 5. TGA thermograms of PEDTs in air.

Table 3. Cone Calorimeter Results for Copolyesters PEDT

TTI (s)

pHRR (kW/m2)

THR (MJ/m2)

PEDT50 PEDT5-50 PEDT60 PEDT5-60

59 67 55 63

726 504 731 565

141 108 140 106

phase,6,28,31,32 by preventing the radicals from attacking the carbon atom vicinal to the ether oxygen. Additionally, the Tmax1, Tmax2, and Tmax3 values of samples containing HPPPA were also higher than those of corresponding PEDTs without HPPPA, which indicates that the addition of HPPPA can improve the thermal oxidative stability of copolyesters. 3.4. Flame Retardancy of PEDTs. The flame retardancy of the PEDTs was assessed by LOI and UL-94 vertical tests. The LOI and UL-94 values of PEDTs are listed in Table 2. It can be clearly seen that the incorporation of HPPPA can significantly improve the flame retardancy of PEDTs. The LOI values of the copolyesters with HPPPA were in the range of 30.5−34.8, whereas the LOI values of the copolyesters without HPPPA were in the range of 23.3−25.7. However, the LOI values decreased progressively with increasing DEG content. This might be due to the fact that the decomposition of copolyester was faster for higher contents of DEG. Generally speaking, materials displaying LOI values above 26 would exhibit selfextinguishing behavior and excellent flame retardancy.7

Figure 6. (a) Heat release rate curves and (b) total heat release curves of copolyesters. 4544



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(6) Botelho, G.; Queirós, A.; Gijsman, P. Thermooxidative studies of poly(ether-esters). 1. Copolymer of poly(butylene terephthalate) and poly(ethlene oxide). Polym. Degrad. Stab. 2000, 67, 13. (7) Wang, Y. Z. Flame-Retardation Design of PET Fibres; Sichuan Science and Technology Press: Chengdu, China, 1994. (8) Wang, C. S.; Shieh, J. Y.; Sun, Y. M. Synthesis and properties of phosphorus containing PET and PEN. J. Appl. Polym. Sci. 1998, 70, 1959. (9) Horrocks, A. R. Developments in flame retardants for heat and fire resistant textilesThe role of char formation and intumescence. Polym. Degrad. Stab. 1996, 54, 143. (10) Ueda, A.; Matsumoto, T.; Imamura, T.; Tsujimoto, K. Flame resistant polyester from diaryl-di(hydroxyalkylene oxy)aryl phosphine oxide. U.S. Patent 5,003,029, 1991. (11) Chang, S. J.; Sheen, Y. C.; Chang, R. S.; Chang, F. C. The thermal degardation of phosphorus-containing copolyesters. Polym. Degrad. Stab. 1996, 54, 365. (12) Chang, S. J.; Chang, F. C. Synthesis and characterization of copolyesters containing the phosphorus linking pendent groups. J. Appl. Polym. Sci. 1999, 72, 109. (13) Wang, L. S.; Wang, X. L.; Yan, G. L. Synthesis, characterisation and flame retardance behaviour of poly(ethylene terephthalate) copolymer containing triaryl phosphine oxide. Polym. Degrad. Stab. 2000, 69, 127. (14) Asrar, J.; Berger, P. A.; Hurlbut, J. Synthesis and characterization of a fire-retardant polyester: Copolymers of ethylene terephthalate and 2-carboxyethyl(phenylphosphinic) acid. J. Polym. Sci. A: Polym. Chem. 1999, 37, 3119. (15) Wu, B.; Wang, Y. Z.; Wang, X. L.; Yang, K. K.; Jin, Y. D.; Zhao, H. Kinetics of thermal oxidative degradation of phosphorus-containing flame retardant copolyesters. Polym. Degrad. Stab. 2002, 76, 401. (16) Zhao, H.; Wang, Y. Z.; Wang, D. Y.; Wu, B.; Chen, D. Q.; Wang, X. L.; Yang, K. K. Kinetics of thermal oxidative degradation of flame retardant copolyesters containing phosphorus linked pendent groups. Polym. Degrad. Stab. 2003, 80, 135. (17) Wang, D. Y.; Ge, X. G.; Wang, Y. Z.; Wang, C.; Qu, M. H.; Zhou, Q. A novel phosphorus-containing poly(ethylene terephthalate) nanocomposite with both flame retardancy and anti-dripping effects. Macromol. Mater. Eng. 2006, 291, 638. (18) Ge, X. G.; Wang, D. Y.; Wang, C.; Qu, M. H.; Wang, J. S.; Zhao, C. S.; Jing, X. K.; Wang, Y. Z. A novel phosphorus-containing copolyester/montmorillonite nanocomposites with improved flame retardancy. Eur. Polym. J. 2007, 43, 2882. (19) Ge, X. G.; Wang, C.; Hu, Z.; Xiang, X.; Wang, J. S.; Wang, D. Y.; Liu, C. P.; Wang, Y. Z. Phosphorus-containing telechelic polyesterbased ionomer: Facile synthesis and antidripping effects. J. Polym. Sci. A: Polym. Chem. 2008, 48, 2994. (20) Jing, X. K.; Ge, X. G.; Xiang, X.; Wang, C.; Sun, Z.; Chen, L.; Wang, Y. Z. A novel phosphorus-containing copolyester with low melting temperature and high flame retardancy. Polym. Int. 2009, 58, 1202. (21) Loh, X. Y.; Tan, Y. X.; Li, Z. Y.; Teo, L. S.; Goh, S. H.; Li, J. Biodegradable thermogelling poly(ester urethane)s consisiting of poly(lactic acid)Thermodynamics of micellization and hydrolytic degradation. Biomaterials 2008, 29, 2164. (22) Gan, Z. H.; Kuwabara, K.; Abe, H.; Iwata, T.; Doi, Y. Metastability and transformation of polymorphic crystals in biodegradable poly(butylene adipate). Biomacromolecules 2004, 5, 371. (23) Ding, C. K.; Cheng, B.; Wu, Q. DSC analysis of isothermally melt-crystallized bacterial poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) films. J. Therm. Anal. Calorim. 2011, 103, 1001. (24) Vasanthan, N.; Ozkaya, S.; Yaman, M. Morphological and conformational changes of poly(trimethylene terephthalate) during isothermal melt crystallization. J. Phys. Chem. B 2010, 114, 13069. (25) Phang, I. Y.; Pramoda, K. P.; Liu, T.; He, C. B. Crystallization and melting behavior of polyester/clay nanocomposites. Polym. Int. 2004, 53, 1282. (26) Zeng, J. B.; Liu, C.; Liu, F. Y.; Li, Y. D.; Wang, Y. Z. Miscibility and crystallization behaviors of poly(butylene succinate) and poly(L-

after ignition, and their HRRs exhibited sharp peaks with pHRRs of 726 and 730 kW/m2, respectively, whereas the pHRRs of PEDT5-50 and PEDT5-60 were 505 and 565 kW/ m2, respectively. Additionally, in Figure 6b and Table 3, it can be seen clearly that the THRs of PEDTs with HPPPA decreased markedly, demonstrating that the fire hazard of the PEDTs was lowered with a low loading of HPPPA. Thus, as a flame retardant, HPPPA can not only act as a comonomer to decrease the melting temperature of copolyester, but also improve the flame retardancy of the copolyester.

4. CONCLUSIONS Phosphorus-containing PEDTs, which showed low melting temperatures and high flame retardancy, were successfully synthesized by esterification followed by copolycondendation. The incorporation of both DEG and HPPPA can decrease the Tm values of copolyester, and the melting temperatures of the PEDTs decreased gradually with increasing comonomer content. The PEDTs exhibited satisfactory thermal stability, but the thermal oxidation stability of the PEDTs decreased upon the introduction of DEG. The lower decomposition temperature does not influence the application and processing of PEDTs because of their low processing temperature. Upon the incorporation of HPPPA, the LOI values of the PEDTs were increased to 30.5−34.8, the UL-94 ratings of vertical burning test were improved from V-2 to V-0, and the peak heat release rates and total heat releases decreased obviously. The resulting novel low-melting-temperature and flame-retardant polyesters will find important applications in nonwoven textiles.

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ASSOCIATED CONTENT

S Supporting Information *

Wide-angle X-ray diffraction patterns, TGA data, heat release rate curves, and total heat release curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.-L. Wang), [email protected]. cn (Y.-Z. Wang). Tel. and Fax: +86-28-85410259. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (50933005, 51121001) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1026).

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REFERENCES

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Thermal Transition Behavior, Thermal Stability, and Flame Retardancy

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