Novel Flame-Retardant and Antidripping Branched Polyesters

Apr 1, 2010 - The samples were molded to a size of 130 × 6.5 × 3.2 mm3 using FERROMATIK MILACRON K-TEC 40 injection molding. The combustion ...
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Ind. Eng. Chem. Res. 2010, 49, 4190–4196

Novel Flame-Retardant and Antidripping Branched Polyesters Prepared via Phosphorus-Containing Ionic Monomer as End-Capping Agent Jun-Sheng Wang, Hai-Bo Zhao, Xin-Guo Ge, Yun Liu, Li Chen, De-Yi Wang, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials, College of Chemistry, State Key Laboratory of Polymer Materials Engineering, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), Sichuan UniVersity, Chengdu 610064, China

A series of novel branched polyester-based ionomers were synthesized with trihydroxy ethyl esters of trimethyl1,3,5-benzentricarboxylate (as branching agent) and sodium salt of 2-hydroxyethyl 3-(phenylphosphinyl)propionate (SHPPP, as end-capping agent) by melt polycondensation. The chemical structures of the branched polyester-based ionomers were confirmed with 1H and 31P NMR spectra. The effects of branching and ionic end groups on the melt behavior and thermal properties were investigated by differential scanning calorimetry and thermogravimetric analysis. The flammability and fire behavior were characterized by limiting oxygen index (LOI) and cone calorimeter. The LOI values of branched poly(ethylene terephthalate)s (PETs) are higher than that of pure PET. The cone calorimeter parameters of the samples such as heat release rate, total heat release, and time to ignition indicate that the fire safety of branched polyester-based ionomers is improved by introducing SHPPP. The effects of SHPPP on accelerating char forming and antidripping could be observed clearly from the pictures of the samples after LOI test. 1. Introduction Poly(ethylene terephthalate) (PET) is a common polymer that has found wide applications in a variety of daily necessities, from fibers to bottles and films.1 However, the melt viscosity and melt strength of PET are relatively low at processing temperature, which limit its applications requiring elongation of the melt.2 Melt strength and other rheological properties have been modified by introduction of branching agents such as timethylolpropane, pentaerythritol,3 trimesic acid,4 and triglycidyl isocyanurate.5 Branched PET (BPET) has been introduced in almost all the applications of PET, such as films, fibers, bottles, foams, and so forth.3,6,7 During preparation of branched PET, monofunctional monomers are often used as end-capping agents to prevent gelation in the presence of branching agents.2,8-10 However, the combustibility and serious melt dripping during combustion of PET greatly limit its applications.11 To the best of our best knowledge, there has been no report about branching chains on flammability of PET. Phosphorus-containing compounds have been recognized as one of the most efficient flame retardants for PET, which have excellent fire retardancy and low toxic gas evolution during combustion compared with halogen-containing ones.11,12 Incorporating a phosphoruscontaining monomer into polymeric backbone is one of the most efficient methods for flame retardation of PET from comparison of blending, finishing, and copolymerization.13-16 A series of phosphorus-containing copolyesters have been synthesized and their flame retardancy and thermal properties were also investigated in our laboratory.17-19 Flame retardation of traditional phosphoruscontaining PET could be mostly achieved through melt dripping by energy removal from the flame zone of the specific specimens.20 Serious melt dripping is very dangerous in fire because flame drips lead to secondary damage and drips with high temperature present a hazard to immediate personnel. Therefore, an antidripping flameretarded PET is in urgent demand. However, from the flame* To whom correspondence should be addressed. Telephone/Fax: +86-28-85410259. E-mail: [email protected].

retarded mechanism of traditional phosphorus-containing PET as mentioned above, the flame retardancy and antidripping of PET during combustion are contradictory. And a few reports have focused on the antidripping behavior of flame-retarded PET except for some patents.21-23 In our previous work, we investigated the preparation, flame retardancy, and antidripping behavior of phosphorus-containing telechelic PET ionomers with monofunctional monomer phosphinate (2-hydroxyethyl 3-(phenylphosphinyl) propionate, SHPPP) systemically.19 In this study, branched polyesters (BPETs) derived from trihydroxy ethyl esters of trimethyl-1,3,5-benzentricarboxylate (THET) as branching agent were synthesized via polycondensation, and SHPPP mentioned above was further introduced as end-capping agent into BPETs to form a series of novel branched polyester-based ionomers (BIPETs). The thermal behaviors of BPETs and BIPETs were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The flammability and fire behavior of the samples were investigated by the limiting oxygen index (LOI) test and cone calorimeter test. 2. Experimental Section 2.1. Materials. Sodium salt 2-hydroxyethyl-3-(phenylphosphinyl)propionate (SHPPP) was synthesized according to our previous work.19 Terephthalic acid (TPA) was provided by Jinan Chemical Fiber Co. Ltd. (Jinan, China). Antimony trioxide (Sb2O3, 99.99%) used as catalyst was purchased from Chengdu Chemical Industries Co. (Chengdu, China). Sodium carbonate anhydrous (Na2CO3), methanol, phenol, and 1,1,2,2-tetrachloroethane were purchased from Kelong Chemical Industries Reagent Co. (Chengdu, China). All reagents were used as received. 2.2. Synthesis of Tris(2-hydroxyethyl)benzene-1,3,5-tricarboxylate. The synthesis route of THET is shown in Scheme 1. A 500-mL round-bottomed flask fitted with a magnetic stirrer and a heating oil bath was charged with 130 g of 1,3,5benzentricarboxylic acid, 240 mL of methanol, and 12 mL of oil of vitriol. The mixture in the flask was heated to reflux from room

10.1021/ie100057n  2010 American Chemical Society Published on Web 04/01/2010

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Scheme 1. Synthesis of Branched Agents (THET)

Scheme 2. Imaginary Chemical Structures of BPET (a) and BIPET (b)

temperature with stirring, maintained there for 6 h, and then cooled to room temperature. A white powder (trimethyl benzene-1,3,5tricarboxylate [TMT], yield: 96.5%) was obtained after filtering and washing with methanol. A 500-mL round-bottomed flask fitted with a magnetic stirrer, condensation tube, and water separator was charged with 50.4 g of TMT, 300 mL of ethylene glycol (EG), and 0.0442 g of zinc acetate. The mixture in the flask was heated to 180 °C with stirring, maintained at 180 °C until 24.6 mL of methanol was obtained, and then cooled to room temperature. Tris(2-hydroxyethyl)benzene-1,3,5-tricarboxylate (THET, yield: 97.2%) was obtained after filtering and washing with EG. IR (KBr, cm-1): 3329.6 (-OH), 2882.2-2997.3 (-CH2-), 1722.7 (-O-CdO). 1H NMR (dimethylsulfoxide-d6 [DMSO-d6], δ, ppm): 8.74 (3H, Ar-H), 5.04 (3H, -OH), 4.38 (6H, -CH2-OH), 3.75 (6H, COO-CH2-). 2.3. Preparation of BPETs and BIPETs. BPETs and BIPETs were synthesized by melt polycondensation in a 2.5-L autoclave equipped with a nitrogen inlet, a condenser, and a mechanical stirrer. The synthesis processes of BPETs and BIPETs were similar. The route for preparation of BIPET containing 2 mol % THET and 5 mol % SHPPP (B2I5PET) is presented here as a representative example. EG (433.8 mL), TPA (860 g), THET (53.2 g), and Sb2O3 (0.301 g) were added to a 2.5-L autoclave. Prior to polymerization, the nitrogen was first introduced into the autoclave to blow out air. The reactor was heated to 240 °C for 2.0 h, and then the temperature of the

reaction mixture was raised to 260 °C in 1.5 h and maintained at 260 °C for 0.5 h. The esterification took place under a pressure of 0.32-0.36 MPa, and water yielded was removed by fractionation. Vacuum was applied and the reaction mixture was cooled to 240 °C slowly after the theoretical volume of water was collected. Thereafter, the solution of SHPPP in EG was added dropwise into the autoclave while keeping the temperature above 220 °C under a steady stream of nitrogen. Afterward, the mixture was heated slowly to 260 °C while vacuum was applied. After this, the pressure of the reactor was reduced to lower than 60 Pa and the temperature was raised to 280 °C in 0.5 h and maintained at 280 °C for 2 h. The resulting polymer melts were extruded via N2 blowing through an orifice and cooled in water. Besides the process of adding SHPPP, BPETs and pure PET were obtained according to a similar process. The possible structure of BIPETs and BPETs is shown in Scheme 2. Basic characteristics of the target polyesters are summarized in Table 1. 1H NMR (CF3COOD/CDCl3 (9/1, v/v), δ, ppm): 8.99 (Ar-H), 8.09-8.12 (Ar-H), 7.44-7.74 (Ar-H), 4.18-4.80 (-CH2-O), 2.76 (-CH2-CdO), 2.48 (-P-CH2-). 31 P NMR (CF3COOD/CDCl3 (9/1, v/v), δ, ppm): 48.6. 2.4. Characterization. NMR spectra (1H, 400 MHz; 31P, 161.9 MHz) were obtained at room temperature by a Bruker AVANCE AV II-400 NMR instrument, with CF3COOD/CDCl3 (9/1, v/v) as the solvent, and tetramethylsilane or phosphoric acid as the internal standard.

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Table 1. Characteristics of Copolyesters samples

content of THET (mol %)

content of SHPPP (mol %)

[η] (dL/g)

LOI (%)

PET B0.5PET B1PET B2PET B1I1PET B1I3PET B1I5PET B1I10PET B0.5I5PET B2I3PET B2I5PET

0 0.5 1 2 1 1 1 1 0.5 2 2

0 0 0 0 1 3 5 10 5 3 5

0.74 0.68 0.71 0.83 0.65 0.70 0.61 0.58 0.64 0.79 0.73

23.5 25.0 27.0 27.0 29.5 27.5 25.9 27.0 25.0 29.0 26.0

The intrinsic viscosities of the polyester were determined with an Ubbelodhe viscometer with a concentration of 0.5 g/dL at 25 °C in 1:1 (v/v) phenol-1,1,2,2-tetrachloroethane solution. TGA was performed on a NETZSCH TG 209 F1 at a heating rate of 10 °C/min. Samples were heated in the temperature range from 40 to 650 °C under a nitrogen and air atmosphere at a flow rate of 50 mL/min, respectively. DSC measurements were performed on a TA Q200 under a nitrogen atmosphere at a flow rate of 50 mL/min. To eliminate the influence of thermal history and the effect of heat treatment on the crystalline structure of the materials, BPETs and BIPETs were first heated at 280 °C for 5 min, then cooled down to 30 °C to record the crystallization temperature, and then reheated to 280 °C, all at a heating rate of 10 °C/min. The LOI values were measured on an HC-2C oxygen index measurement (Jiangning, China) with sheet dimensions of 130 × 6.5 × 3.2 mm3 according to ASTM D2863-97. The samples were molded to a size of 130 × 6.5 × 3.2 mm3 using FERROMATIK MILACRON K-TEC 40 injection molding. The combustion behaviors of BPETs and BIPETs were characterized by a cone calorimeter according to ISO 5660-1. The samples were molded to a size of 100 × 100 × 3 mm3 using FERROMATIK MILACRON K-TEC 40 injection molding. 3. Results and Discussion 3.1. Preparation of Branched Polyester and Branched Polyester-Based Ionomers. Telechelic polyester-based ionomers were synthesized with SHPPP in our previous work.19 In this study, BPETs were synthesized with THET as branched agent and BIPETs were synthesized with THET as branched agent and SHPPP as end-capping agent. Table 1 presents the intrinsic viscosities of BPETs and BIPETs, which shows the effects of branching and end groups on molecular weight. At higher content of THET, the intrinsic viscosities of BPETs and BIPETs can be enhanced; however, they can be depressed with higher SHPPP loading. 1 H NMR and 31P NMR were used to confirm the formation of BPET and BIPET. Figure 1 depicts the NMR spectra for B2PET and B2I5PET and the assignment of the different peaks. In 1H NMR of B2PET, the chemical shift of 8.99 ppm is assigned to the THET, which proves the appearance of THET in B2PET. The sodium phosphinate groups of BIPET converted to phosphinic acid during NMR experiments, for the strong acidity of CF3COOD.19 Besides the signals of BPET, the signals of SHPPP appear in the 1H NMR spectra of B2I5PET, which are assigned by their chemical shift to aryl (7.44-7.74 ppm) and aliphatic hydrogens (2.48-2.76 ppm). Moreover, a single peak at about 48.6 ppm in 31P NMR spectra indicates that the phosphorus atom appears in a polymeric chain which is from SHPPP. Therefore, the BPETs and BIPETs are successfully obtained through a melt polycondensation process.

3.2. Melting Behavior and Crystallization of BPETs and BIPETs. The melting behavior and crystallization properties of BPETs and BIPETs were investigated by DSC analysis. Figure 2 shows the second heating scans of BPETs and BIPETs, for comparison, the corresponding pure PET curve is also included in the figure, and the detailed information is listed in Table 2. Compared with pure PET, Tg temperatures change little; however, those of BIPETs decrease by about 0.6 to 2.9 °C. The branched chains of BPETs are long-chain branching and do not have a large influence on the Tg. However, BIPETs with SHPPP as end-capping agent have short-branched chains that often depress the Tg because of the increase in free volume.10 For pure PET and BPETs, cool crystallization peaks can be observed during heating scans. Comparison with pure PET and BPETs, Tcc of BPETs are shifted to higher temperature while Tc of BPETs and BIPETs during the first cooling process decrease with increasing THET loading. Tm of BPETs decrease from 244.6 to 235.9 °C when THET loading increases from 0.5 to 2 mol %. The results of melting behavior and crystallization of BPETs indicate that branching disrupts the chain symmetry and restricts the segment movements. Compared with BPETs, Tc and Tm of BIPETs are lower than those of BPETs, which indicates the disruption of chain symmetry and restriction of polymeric chains mobility with the presence of ionic aggregation. However, the order for melting enthalpy (∆Hm) of PET, BPETs, and BIPETs are as follows: PET > BPETs > BIPETs, which indicates that the crystallization property of BPETs and BIPETs decreases because of the branching and ionic aggregation. 3.3. Thermal Stability. Thermogravimetric (TG) analysis can serve as the most useful indicator for thermal stability of polymers. Figures 3 and 4 show TG curves of BPETs and BIPETs under nitrogen and air atmospheres, and relevant thermal decomposition data, including the temperature of 5 wt % weight loss, T5, defined as the temperature of initial decomposition, the temperature at maximum weight loss rate, Tmax, and the weight of residue at 650 °C, C650, are summarized in Table 3. In a N2 atmosphere, since the chemical compositions of BPET and BIPET are very similar to that of pure PET, all TG curves of the samples which exhibit only one single maximum decomposition process and are quite similar to each other indicated that the introduction of branching structures and end-capping ionomer hardly affected the mechanism of thermal decomposition of PET. However, compared with those of B1PET and B2PET, T5 of B1I3PET, B1I5PET, and B2I5PET decreased 5.0, 9.6, and 11.5 °C, respectively, which suggests ionic phosphorus-monomer could accelerate the thermal decomposition of the BIPETs under a N2 atmosphere. Typical bond energies of P-C bond and C-C bond are 260 and 349 kJ/ mol,24 respectively. Therefore, the thermal instability of BIPETs could be attributed to the P-C bond, which exhibits greater tendency to chain scission than the C-C bond. Tmax of BIPETs decreased with the increase of SHPPP, which also showed that the ionic phosphorus-monomer accelerated the thermal decomposition of BIPETs under a N2 atmosphere. From the thermal oxidative decomposition of samples in Figure 4, it can be seen that BPETs and BIPETs have two decomposition steps that are the same as those of pure PET under air atmosphere, including a major step (from 430.8 to 433.5 °C) and a minor step (from 500.6 to 544.5 °C). The same behaviors of thermal decomposition under a N2 atmosphere, TG curves of BPETs are very similar to those of pure PET under air atmosphere. Values of T5, Tmax1, and Tmax2 of BPETs all decrease when compared with those of pure PET, which indicate that branching decreases thermal oxidative stability of polyesters

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Figure 1. 1H NMR spectrum of B2PET and 1H and

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31

P NMR spectra of B2I5PET.

under air atmosphere. Branched structure may allow for easier oxidative decomposition than linear structure. However, the trends of T5 and Tmax2 for BIPETs under air atmosphere are very different from those under N2 atmosphere. The T5 of BIPETs are 52.8-60.7 °C higher than those of BPETs; however, the Tmax2 values are 32.5-41.2 °C lower, which indicates that the additive SHPPP is able to improve the thermal oxidative stability in a lower temperature zone while accelerating the thermal oxidative decomposition of copolyesters over 500 °C in air. It is due to the presence of SHPPP

and oxygen. The P-C bond is less stable than the C-C bond; the P-C bond is broken first and then the products from the broken P-C bond are oxidized in air. SHPPP located at the end of BIPETs molecular chain; therefore, the oxidation of the P-C bond does not lead to backbone decomposition of copolyester.19 Furthermore, SHPPP could increase thermal oxidative stability of BIPETs. At high temperature, the dehydration of phosphorus-containing compounds may accelerate decomposition of BIPETs to form compact char, which is the reason for the reduction of Tmax2.

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Figure 3. TG thermograms of BPETs and BIPETs in a nitrogen atmosphere.

Figure 2. DSC thermograms of copolyesters: (a) cooling scans and (b) heating scans. Table 2. DSC Data of Copolyesters samples

Tg (°C)

Tm (°C)

Tc (°C)

Tcc (°C)

∆Hm (°C)

∆Hc (°C)

∆Hcc (°C)

PET B0.5PET B1PET B2PET B0.5I5PET B1I3PET B1I5PET B2I5PET

72.1 73.3 72.2 71.7 69.2 69.8 70.1 71.5

246.2 244.6 241.7 235.9 233.1 231.9 231.8 220.2

190.5 181.3 176.8 163.3 168.0 176.0 171.2 141.4

143.5 148.9 154.6 150.5

30.4 27.7 25.6 25.1 22.2 25.2 24.7 10.9

26.8 25.4 24.4 13.2 21.2 25.8 24.3 5.5

2.4 3.1 1.0 10.9

151.1

Figure 4. TG thermograms of BPETs and BIPETs in an air atmosphere. Table 3. TG Data of Copolyesters nitrogen atmosphere

5.0

The residues of BIPETs increase with the increase of SHPPP and THET at 650 °C under air and nitrogen atmospheres while those of BPETs are all around 0.1%, which indicates SHPPP is the key factor for charring, and branching could also accelerate charring together with SHPPP. The increase in residues of BIPETs suggests that the flame retardancy of BIPET might be better than that of BPET and melt dripping might be improved. From TG results, the effect of the ionic phosphorus-containing end group on thermal and thermal oxidative behaviors is much greater than that of branched chains because the effect of the P-C bond on thermal and thermal oxidative behaviors is much greater than the physical effect of branched chains. 3.4. Combustion Behavior of BPETs and BIPETs. The LOI test is one of the most useful measures to study the flammability of materials. Generally speaking, materials exhibiting LOI values above 26 would show self-extinguishing behavior and would be considered to possess high flame retardancy.11 The LOI values of BPETs and BIPETs are listed in Table 1, which show the effects of branching and ionomer. The LOI values of BPETs increase about 1.5-3.5 units compared with those of pure PET, which may be attributed to branched chains of BPETs. At relatively

air atmosphere

samples

T5 (°C)

Tmax (°C)

C650 (%)

T5 (°C)

Tmax1 (°C)

Tmax2 (°C)

C650 (%)

PET B0.5PET B1PET B2PET B1I3PET B1I5PET B2I5PET

397.7 397.1 396.5 396.1 391.5 386.9 384.6

435.5 436.4 437.5 436.8 434.6 433.7 434.5

12.62 12.48 12.18 11.10 15.52 16.31 17.32

325.2 317.7 320.1 323.0 378.2 380.8 375.8

433.5 432.4 431.9 432.6 430.8 432.1 433.0

544.5 542.9 541.8 533.7 503.7 500.6 501.2

0.09 0.07 0.10 0.03 0.71 0.88 1.25

low shear rates, branched chains can exhibit a viscosity much greater than that of linear polymer with equal molecular weight.10 During LOI test, melt rheological behavior can be considered a polymer melt at low shear rates. Therefore, the melt fluidity of BPETs become more difficult than that of pure PET during the LOI test, allowing BPETs to have enough time to form char to protect the copolyester matrix. Generally, LOI values of phosphorus-containing polymers increase with the increase of phosphorus content. However, LOI values of BIPETs first decrease and then increase, which may be caused by the contravention between melt dripping and char forming. BIPETs with low content of SHPPP such as B1I1PET obtain good flame retardancy by removing energy from the flame zone with melt dripping; and then char formation of BIPETs such as B2I5PET, B1I10PET is obvious with the increase

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Table 4. Cone Calorimeter Parameters of Copolyesters

Figure 5. Pictures of copolyesters after LOI test.

sample

TTI (s)

PHRR (kW/m2)

av-HRR (kW/m2)

THR (MJ/m2)

residual mass (%)

B2PET B1I3PET B1I5PET B2I5PET

46 58 59 56

876 413 438 381

276 149 171 151

78.8 67.6 64.4 63.5

12.7 18.7 19.6 22.2

tively. It should be noted that the PHRR of B2I5PET is only 43.5% of that of B2PET. And the average of HRR (av-HRR) of BIPETs is much lower than that of B2PET. The results suggest that the fire hazard of BIPETs could be improved with low loading of SHPPP. HRR and av-HRR decrease a little with more branched chains when compared to those of B1I5PET and B2I5PET. The detailed data are summarized in Table 4. Figure 7 shows curves of THR of the samples. It can be seen that THR values of BIPETs are always lower than that of B2PET, which suggests that SHPPP could reduce the yield of fuel during combustion. At the end of burning, the total heat release of B2PET is 78.8 MJ/m2, while those of B1I3PET, B1I5PET, and B2I5PET are reduced to 67.6, 64.4, and 63.5 MJ/ m2. THR of B2I5PET is 80.6% of that of B2PET, which indicates part of B2I5PET has not completely combusted for char forming of SHPPP during combustion. The accelerated char-forming effect of SHPPP can be seen clearly in Figure 8, residual mass curves of the samples. Higher char yield corresponds to the lower THR. Thermal decomposition

Figure 6. Heat release rate of copolyesters.

of the SHPPP content, which makes energy removal from the flame zone difficult. It is noteworthy that flame retardancy of BIPETs could be improved when the content of SHPPP is over 3 mol % because of the heat insulation and oxygen isolation from the char formed under flame. Compared with those of BPETs, the LOI values of BIPETs with the same content of SHPPP show consistent trends with the effect of branched chains. The UL-94 test is an important measure to investigate the melt-dripping behavior of materials. However, it is difficult to record the melt-dripping behaviors during the UL-94 test as the melt-dripping behavior appeared for all samples during the UL-94 test, which is improved by SHPPP. Therefore, in this study we took the pictures of the test samples after the LOI test, from which the morphology of the melt-dripping samples could be recorded (see Figure 5). From Figure 5, it can be seen clearly that the yield of char increases with increasing the SHPPP content. The melt dripping was improved with the presence of SHPPP, a compact residue was yielded over the surface of B2I5PET, and no drips can be observed. The cone calorimeter was used to investigate effects of branching and ionic end group on combustion behavior of copolyesters. The cone calorimeter is one of the most useful bench-scale fire test apparatuses and provides a wealth of information from its simulation of real world fire conditions. Fire risk of materials can be investigated by a cone calorimeter such as time to ignition (TTI), heat release rate (HRR), and total heat release (THR).25-27 HRR is one of the most important parameters for predictors of fire hazards. Figure 6 shows the curves of the HRRs of B2PET and BIPETs. It can be observed that B2PET burns rapidly after ignition and the HRR reaches a sharp peak with the peak heat release rate (PHRR) of 876 kW/m2, while PHRRs of B1I3PET, B1I5PET, and B2I5PET are 413, 438, and 381 kW/m2, respec-

Figure 7. Total heat release of copolyesters.

Figure 8. Mass loss of copolyesters.

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of materials under heat flux after ignition with a cone calorimeter can be considered as thermal decomposition under a lack of oxygen atmosphere, which may be very similar to thermal decomposition of materials under N2 atmosphere.27 Therefore, the enhanced char performance of BIPETs from the results shown in Figure 8 agrees with TG experiments under N2 atmosphere. It is shown that BIPETs are more difficult to ignite than B2PET from time to ignition in Table 4, which indicates BIPETs are more safe than B2PET under fire. Thermal decomposition of materials under heat flux before ignition during cone calorimeter test is very similar to that under air atmosphere.28,29 Therefore, the raised T5 temperature of BIPETs in air indicates that it is more difficult to ignite than B2PET. The order of TTI of samples is as follows: B2PET < B2I5PET < B1I3PET < B1I5PET, which highly agrees with the order of T5 temperatures under air atmosphere in TG experiments. 4. Conclusion Branched PETs were synthesized with THET as a branching agent by melt polycondensation. The chemical structure of BPETs was confirmed by 1H NMR spectra. Branched polyesterbased ionomers with THET and SHPPP (as end-capping agent) were synthesized by melt polycondensation and their chemical structure was characterized by 1H and 31P NMR spectra. The crystallization property and Tm temperatures of BPETs and PIPETs decrease because of the branching and ionic aggregation from DSC results. Compared with pure PET, the branched chains have a slight effect on the thermal stability of BPETs. However, the thermal stability of BIPETs with SHPPP could be improved in an air atmosphere and the accelerating char formation of SHPPP can be seen clearly from the residue of BIPETs at 650 °C either in air atmosphere or in nitrogen atmosphere compared with those of BPETs. The LOI values of BPETs are higher than that of pure PET since branched chains make melt flow of BPETs slower than that of pure PET. The flame retardancy of BIPETs could be improved with low content of SHPPP based on LOI results. The inhibition effects on melt dripping of SHPPP can be clearly observed in the pictures of the samples after LOI testing. Cone calorimeter parameters of BIPETs were improved significantly compared with those of B2PET. The results of TTI and mass loss agree with TG results of the samples, indicating the char-forming effect of SHPPP. Acknowledgment This work was supported by the National Science Foundation of China (20804024 and 50933005) and the National Key Technology R&D Program (2007BAE28B01). Literature Cited (1) Wang, Y. Z.; Chen, X. T.; Tang, X. D.; Du, X. H. A new approach to simultaneous improvement of the flame retardancy, tensile strength and melt dripping of polyethylene terephthalate. J Mater. Chem. 2003, 13, 1248. (2) Rosu, R. F.; Shanks, R. A.; Bhattacharya, S. N. Synthesis and characterization of branched poly(ethylene terephthalate). Polym. Int. 1997, 42, 267. (3) Hess, C.; Hirt, P.; Oppermann, W. Influence of branching on the properties of poly(ethylene terephthalate) fibers. J. Appl. Polym. Sci. 1999, 74, 728. (4) Manaresi, P.; Parrini, P.; Semeghini, G. L.; Fornasari, E. Branced poly(ethylene terphthalate) correlations between viscosimetric properties and polymerization parameters. Polymer 1976, 17, 595. (5) Dhavalikar, R.; Yamaguchi, M.; Xanthos, M. Molecular and structural analysis of a triepoxide-modified poly(ethylene terphthalate) from rheological data. J Polym. Sci., Part A: Polym. Chem. 2003, 41, 958.

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ReceiVed for reView January 10, 2010 ReVised manuscript receiVed March 7, 2010 Accepted March 14, 2010 IE100057N