Novel Inherently Flame-Retardant Poly(trimethylene Terephthalate

Jul 2, 2010 - introduced to the poly(trimethylene terephthalate) (PTT) chain. The chemical ... Unfortunately, PTT is a flammable thermoplastic polymer...
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Ind. Eng. Chem. Res. 2010, 49, 7052–7059

Novel Inherently Flame-Retardant Poly(trimethylene Terephthalate) Copolyester with the Phosphorus-Containing Linking Pendent Group Hong-Bing Chen, Yi Zhang, Li Chen,* Zhu-Bao Shao, Ya Liu, 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 novel phosphorus-containing copolyester, poly(trimethylene terephthalate)-co-poly(trimethylene DDP)s (PTTP), was synthesized through esterification and polycondensation of terephthalic acid (TPA), 1,3-propane diol (PDO), and 9,10-dihydro-10-[2,3-di(hydroxycarbonyl)propyl]-10-phospha-phenanthrene-10-oxide (DDP). The analysis of phosphorus content and the test of intrinsic viscosity indicated that DDP was successfully introduced to the poly(trimethylene terephthalate) (PTT) chain. The chemical structure of the resulting copolyesters was further confirmed by 1H NMR and 31P NMR. The thermal behaviors were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), and it was shown that the introduction of DDP lowered the melting temperature and crystallization because of its bulky pendent groups and decreased the initial decomposition temperature in the nitrogen atmosphere due to its weak P-C bond. The flame retardant properties of the resulting copolyesters were characterized by limiting oxygen index (LOI) and cone calorimeter, and it was shown that the copolyesters had good inherent flame retardancy. 1. Introduction Poly(trimethylene terephthalate) (PTT) is a semicrystalline polymer, which was synthesized first by Whinfield and Dickson in 1941.1 PTT has excellent chemical resistance, thermal stability, high strength, and spinnability similar to poly(ethylene terephthalate) (PET), and its crystallization rate and melting point are similar to those of poly(butylene terephthalate) (PBT). Especially, PTT fiber possesses very good durability, softness, and permanent strain resistance, which is in favor of making the special materials with the ability of elastic recovery.2,3 Hereby, since the low-price 1,3-propane diol (PDO) was developed, PTT had been commercialized immediately and was widely investigated as fibers, films, and engineering plastics.1,4-8 Furthermore, 1,3-propane diol can also be derived from biomass such as starch;9,10 therefore, PTT has become a typical partially biobased material, which is meaningful in alleviating the fossil fuel dependency and protecting the environment. Unfortunately, PTT is a flammable thermoplastic polymer, which cannot be used in some fields with the requirement of fire safety. The study on PTT flame retardancy is just at the starting stage globally. To the best of our knowledge, there have not been any reports on the inherent flame retardation of PTT in the journal articles, except for a few patents reported some flame retardant results of PTT. For example, GB1473369 describes a halogen-containing composition of decabromodiphenyl ether, antimony trioxide, and asbestos for flame retarded PTT.11 JP3115195 (B) discloses a process to obtain the halogenfree flame retardant combining N-heterocyclic compounds with a P-based additive. Both the halogen-containing and halogenfree additives could enhance flame retardation of PTT.12 However, the additive flame retardants always exhibit heterogeneous dispersion and poor compatibility in the PTT matrix, which bring a decline in flame-retarding performances and physical properties. Simultaneously, when the polyester is made into fibers, the flame retardant particles may block the spinnerets * To whom correspondence should be addressed. Tel/Fax: 86-2885410755. E-mail: [email protected] (L.C.); [email protected] (Y.-Z.W.).

and destroy the spinnability of PTT, deteriorating the mechanical properties of the target fibers. The copolymerization of polyesters with flame-retardant monomers is another way for their flame retardation.13-15 This method can solve the problems mentioned above and obtain copolyesters with inherent flame retardancy, which is a popular flame retardant technique for some commercial flame retardant polyesters. Several phosphorus-containing monomers have been used to prepare PET-based flame-retardant copolyesters, such as 2-carboxyethyl(phenylphosphinic) acid (CEPP),16 bis 4-carboxyphenyl phosphine oxide (BCPPO), and spirocyclic pentaerythritol di(phosphate acid monochloride)s (SPDPC).17,18 Compared with the synthesis of PET-based copolyesters, high molecular weight copolyesters based on PTT are much more difficult to be synthesized because of lower thermal stability of PTT and a higher boiling point of PDO. In this study, we chose another flame retardant monomer named 9, 10-dihydro-10[2, 3-di(hydroxycarbonyl) propyl]-10-phosphaphenanthrene-10oxide (DDP), which possesses relatively high decomposition temperature (362.9 °C), high reaction activity, and low cost, to prepare a series of flame-retardant poly(trimethylene terephthalate-co-DDP)s (PTTP) through direct melt polycondensation of terephthalic acid (TPA), PDO, and DDP. The chemical structures, thermal properties, thermal stabilities, and burning behaviors of the resulting copolyesters have been studied and presented in this article. 2. Experimental Section 2.1. Materials. TPA (fiber grade) was provided by Jinan Chemical Fiber Co Ltd. (Jinan, China). PDO (fiber grade), tetrabutyl titanate, and triphenyl phosphite (AP) were purchased from Kelong Chemical Reagent Factory (Chengdu, China). Before use, tetrabutyl titanate was dissolved into anhydrous toluene to prepare a 0.2 g/mL solution. DDP was received from Weili Flame Retardant Chemicals Industry, Co. Ltd. (Chengdu, China). Other materials were commercially available and used as received.

10.1021/ie1006917  2010 American Chemical Society Published on Web 07/02/2010

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Scheme 1. Synthesis Process of PTTP

2.2. Preparation of Phosphorus-Containing PTT (PTTP). Phosphorus-containing PTT (PTTP) was synthesized by direct polymerization of TPA, PDO, and DDP, and tetrabutyl titanate (400 ppm) was used as catalyst (Scheme 1). As the procedures for preparing different proportions of PTTPs were almost the same, a representative example, PTTP containing 5% DDP, was given here. TPA (415 g; 2.5 mol), PDO (333 mL; 4.5 mol), and DDP (27.3 g; 0.079 mol) together with catalysts were added into a 1 L autoclave. Prior to the reaction, the apparatus was vacuumed and purged with dry nitrogen three times. The mixture in the apparatus was first heated to 230 °C in 1 h from room temperature, with mechanical stirring at 60 rad/min. Then, the reaction temperature was raised to 255 °C in 2 h. The esterification pressure was between 0.1 and 0.33 MPa. While the water (byproduct at esterification stage) was all removed, the system was vacuumed to 400 Pa so as to undergo a prepolycondensation reaction for 1 h at 230-260 °C. After that, the pressure was further reduced to 20-50 Pa, and the polymerization was carried on at 260 °C for 2 to 3 h. The resulting copolyester was extruded under the pressure of N2 and cooled in cold water. Other samples with different DDP contents were prepared according to the procedures similar to those above. Samples containing 0, 3, 5, 8, and 10 wt % DDP are coded as PTT, PTTP3, PTTP5, PTTP8, and PTTP10, respectively. 2.3. Characterization. Intrinsic viscosites [η] of PTTPs were determined with an Ubbelohde viscometer at 30 °C in 60/40 (w/w) of phenol/1,1,2,2-tetrachloroethane solution. The solution was filtered before testing. NMR spectra (1H, 400 MHz; 31P, 161.9 MHz) were obtained with a Bruker AVANCE AV II-400 NMR instrument. CF3COOD was used as solvent. The actual phosphorus content of the resulting polymer was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; IRIS Advantage, TJA solution). Differential scanning calorimetry (DSC) curves were measured with a TA Q200 DSC apparatus, calibrated with pure indium and zinc standards. Samples (5 ( 0.5 mg) were placed in the aluminum pans to test their thermal behaviors in nitrogen

with a purge flow of 50 mL/min. The test procedures were as follows: at first, those specimens were heated from 0 to 250 °C at a heating rate of 20 °C/min, then kept isothermal for 2 min, and finally cooled to 0 °C at a cooling rate of 20 °C/min. The resulting curves and corresponding data were recorded. Wide-angle X-ray diffraction (WAXD) measurements of PTT, PTTP5, and PTTP10 were performed using an X-ray diffractometer (Philips X’Pert X-ray diffractometer), with Cu KR radiation in a 2θ range from 2° to 50°. The samples (with a size of 120 × 6.5 × 3.2 mm3) for WAXD tests were prepared via injection, then gradually heated to 140 °C and annealed for 5 h to get the maximum crystallinity. Thermogravimetric analysis (TGA) curves were obtained with a NETZSCH TG 209 F1 apparatus. Samples (5.0 ( 0.5) were placed in Al2O3 pans to test their thermal stabilities. The samples were heated from 40 to 600 at 10 °C/min under both nitrogen and air conditions. The initial decomposition temperature, T5%, at which 5 wt % of original weight was lost due to the decomposition, and Td max, at which PTTP possessed the maximum weight loss rate, were used to study the thermal stability along with the char residue. The LOI measurements were performed on the Oxygen Index Flammability Gauge (HC-2C) according to ASTM D 2863-97. The samples had a size of 120 mm × 6.5 mm × 3.2 mm. The cone calorimeter tests were performed according to the procedures described in the ISO 5660 standard with a FTT (UK) cone calorimeter. Specimens with a size of 100 mm × 100 mm × 3.2 mm, a weight of 40 g, were tested under a heat flux of 50 kW/m2, with a total burning time of 225 s. 3. Results and Discussion 3.1. Preparation of Phosphorus-Containing PTT (PTTP). The syntheses of PTT and its copolyesters (especially PTTPs) with high molecular weights are much more difficult than those of PET and its copolyesters although the esterification activity between TPA and PDO is higher than that between TPA and EG.19,20 Some main reasons for those are as follows. PTT has

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Table 1. Characteristic Parameters of PTT and PTTPs sample

DDP content (wt%)

Pta (%)

Ptb (%)

[η] (dL/g)

LOI

PTT PTTP3 PTTP5 PTTP8 PTTP10

0 3 5 8 10

0 0.27 0.45 0.72 0.90

0 0.23 0.40 0.63 0.84

0.93 0.89 0.88 0.89 0.87

21.5 24.4 26.0 26.9 28.0

b

a Theoretical phosphorus content of the resulting Experimental phosphorus content of the resulting polymer.

polymer.

lower thermal stability than PET, which determines its lower polymerization temperature than that of PET,21 and PDO has a higher boiling point than EG, which makes the removal of excessive PDO much more difficult than that of EG. Furthermore, the introduction of phosphorus-containing compounds would further decrease the thermal stability of copolymer due to the weak bond of P-C,22-24 which makes the polycondensation time unable to be long. Therefore, in order to obtain highmolecular-weight PTTP, we took some measures to conveniently remove the excessive PDO during the reaction, such as enhancing the stirring speed, using a large specific surface of reaction apparatus and adding heat stabilizer. High efficient catalysts were also used to accelerate the reaction.25 Our studies found that high-molecular-weight copolyesters (PTTP) could be synthesized in a 1 L autoclave when the titanium-containing catalyst and the triphenyl phosphite heat stabilizer are adopted at a stirring speed of 200 rad/min. The intrinsic viscosities of the resulting copolyesters are summarized in Table 1. From Table 1, we can see that the actual phosphorus content obtained from the ICP-AES test is close to the theoretical results, which confirms that DDP has been introduced successfully to the PTT chains. The data of intrinsic viscosity and phosphorus content indicate that the introduction of DDP does not significantly affect the molecular weight of PTTP, which further demonstrates a high reactivity of DDP with PDO. The chemical structures of the resulting polymers were characterized by 1H NMR and 31P NMR spectroscopy (Figure 1, Schemes 2 and 3). In the 1H NMR spectrum of PTT, the resonance signals occurring at 8.11 (a), 4.56 (b), and 2.38 (c) ppm are remarkably ascribed to aryl, methylene linked to ester group, and the middle methylene of PDO, respectively. The signals at 4.51 (d), 3.92 (e), and 2.20 (f) ppm are assigned to the terminal PDO. In addition to PTT signals, the signals of DDP also appear in a 1H NMR spectrum of PTTP10. The peaks at 7.32-8.01 (g), 2.20-2.33 (i), and 3.10 (h) ppm are reasonably assigned to aryl and methylene of DDP. Particularly, the signals at 4.33-4.60 ppm (j, k) and at 2.4 ppm (t,s) are caused by the methylene groups of PDO with different combinations to TPA and DDP.26 The appearances of these peaks indicate the random sequences of the chain.27 Moreover, the 31P NMR spectrum of the copolyester is also shown in Figure 1. There is only a single peak appearing at 39.20 ppm, indicating that only one type of phosphorus exists, and it belongs to the DOPO segment in the pendent group. Associated with the ICP-AES results listed in Table 1, it can be concluded that the phosphorus-containing PTTPs are synthesized successfully. 3.2. DSC and WAXD Analysis. The thermal transition behaviors of samples were analyzed using DSC. Glass transition temperature (Tg), melting temperature (Tm), fusion enthalpy (∆Hm), crystallization temperatures during heating and cooling (THC, TCC), and the corresponding crystallization enthalpies (∆HHC, ∆HCC) are summarized in Table 2. Figure 2a shows the first heating scans of PTTPs, in which the corresponding curve of neat PTT is also included for comparison. The results indicate that PTT and all PTTPs are semicrystalline. As the

Figure 1. 1H NMR spectrum of (a) PTT; 1H NMR and of (b) PTTP10.

31

P NMR spectra

Scheme 2. Chemical Structure of PTT and PTTP

samples were prepared by quenching in cold water from the melting condition, the crystallization of all the samples are incomplete. Thereby, while heating, some amorphous areas of copolyesters can crystallize again, and an obvious cold crystallization peak occurs. The values of Tm and ∆Hm decrease slightly with increasing DDP content. In addition, the cold crystallization temperatures increase, while the crystallization temperatures from melts decrease, indicating that copolyesters with more DDP content possesses a lower crystallization capacity. This can be explained as follows. On one hand, according to the previous

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Scheme 3. Sequences of PTTP

study,28 lateral substitution resulting in extra molecular twisting due to a steric effect always impairs crystalline properties of the polymers. In order to well-understand the influence of the steric effect on the crystalline behavior of the copolyesters, we used Gaussian03 program to simulate a PTTP molecule image in the crystalline phase (all-gauche conformation).29 The scheme of part of a PTTP molecule was presented in Figure 3. It can be observed that the phosphorus-containing pendent group occupies approximately 0.79 × 0.76 × 0.58 nm3, which takes up over 60% volume of the original unit cell of the PTT crystals.30-32 Therefore, it can be easily speculated that the presence of bulky pendent phosphorus-containing side groups in PTTPs hinders the molecular packing and greatly depresses crystalline properties, giving rise to the increment of cold crystallization temperature of PTTPs. On the other hand, the presence of bulky pendent phosphorus side groups of DDP disrupts the chain regularity and introduces the irregular position along the molecular chain axis due to the asymmetric hydroxyl groups of DDP. Thus, the resulting copolyesters have a lower crystallinity and melting point. Moreover, a faster heating or cooling rate (say, 20 °C/min) will make copolyesters lack sufficient time to crystallize completely. This interpretation is also supported by the peak shape of DSC curves in the cooling process: the crystallization peak of PTT is sharp and narrow, which accords with a higher crystallization rate, but for PTTPs, the crystallization peaks are dull and broad, which means the lower crystallization rates. Observed from Figure 2a, only one single glass transition can be detected during the first heating, proving that the copolyesters are random, which accords with the 1H NMR results: no obvious microphase separation occurs. The glass

Figure 2. DSC thermograms of PTTPs and PTT for the first heating scans (a) and the cooling scans (b) at 20 °C/min.

transition temperature increases with the increase of DDP content, for the pendent side groups might restrict the mobility of the chain. Nevertheless, in the cooling trace, no remarkable glass transition is found due to the better crystallinity of the copolymer after crystallizing in the cooling process. WAXD was used to investigate information of the crystalline of PTT and PTTPs after annealing at 140 °C for a certain time. The crystal structures of PTT, PTTP5, and PTTP10 had been determined by wide-angle X-ray diffraction patterns, as Figure 4 illustrated. The unit cell dimensions of triclinic R-crystal modification is a ) 0.453 nm, b ) 0.62 nm, c ) 1.87 nm, R ) 97.6°, β ) 93.2°, and γ ) 110.1°.33 For all three samples, sharp reflection peaks can be observed, indicating that the PTT crystals formed. The intensities of the peaks neither decrease nor does peak broadening appear. All three patterns are almost the same. The aforementioned phenomenon indicates that the introduction of a small amount of DDP would not affect PTT crystals. 3.3. Thermal Stability. TGA data of PTT and its phosphoruscontaining copolyesters under both a nitrogen and air atmosphere

Table 2. Thermal Behaviors of PTT and PTTPs Obtained from DSC sample

Tg (°C)

TCC (°C)

∆HCC (J/g)

TC (°C)

∆HC (J/g)

Tm (°C)

∆Hm (J/g)

PTT PTTP3 PTTP5 PTTP8 PTTP10

39.15 39.89 41.94 42.23 46.78

65.64 66.11 73.79 77.34 82.13

23.70 20.03 22.24 23.15 23.41

176.80 148.34 143.75 138.34 139.12

46.93 39.47 38.54 31.41 37.88

225.86 223.91 219.74 218.03 215.41

56.36 49.78 47.41 45.68 44.21

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Figure 3. Schematic of part of the PTTP molecule in the crystalline phase (all-gauche conformation).

Figure 5. TGA thermograms of PTTPs and PTT in different ambient atmospheres: (a) nitrogen and (b) air.

Figure 4. WAXD patterns of PTT, PTTP5, and PTTP10.

were obtained and plotted in Figure 5. T5% (°C), Td max (°C), and residue (%) of the samples after the test are summarized in Table 3. Under the N2 atmosphere, all the specimens exhibit a single decomposition process, and the introduction of DDP slightly decreases the stabilities of copolyesters: T5% decreases from 363.9 °C for PTT to 361.2 °C for PTTP10, and Td max decreases from 401.8 °C for PTT to 399.6 °C for PTTP10. Similar to that reported, the decomposition temperature decreases when the chain contains a C-P bond, probably because that C-P bond is less stable than the C-C bond, whose bond energies are 260 and 349 kJ/mol, respectively.34,35 Simultaneously, the phosphoruscontaining PTT remains fewer residues, which was quite different from the similar systems.17,22,36,37 Generally, phosphoruscontaining groups usually form phosphoric acid and then promote carbonization during thermal decomposition.38 Therefore, the char residue of the phosphorus-containing polymer should increase. However, a converse result was obtained in our study. It is reported that phosphorus-containing polymer can also yield P-containing free radical in the gaseous phase when decomposing,39 which may be the main effect of phosphorus in this system. Thus, fewer residues remain for PTTPs. There is little research focusing on the toxicity of such

phosphorus-containing compounds yielded during burning; however, it is very important to fire safety and should be further studied. Under an air atmosphere, the thermo-oxidative degradations of neat PTT and PTTPs both exhibit a two-step process, including a major (at about 400 °C) and a minor (at about 510 °C) weight-loss stage, as shown in Figure 5b. The varying trend of initial decomposition temperature (T5%) is different from that under a nitrogen atmosphere (Table 3) due to the chemical reaction of degrading copolyesters with the oxygen. The copolyesters with higher phosphorus content possess higher T5% value: the T5% value of neat PTT is only 337.1 °C; however, the T5% values of PTTP5 and PTTP10 increase to 345.3 and 354.0 °C, respectively, indicating that a certain amount of DDP can enhance the thermal stability of copolyesters in an air atmosphere. Compared with neat PTT, PTTPs have a slightly decreased Td max1 (the maximum weight-loss rate temperature at the first weight-loss stage). Under the aforementioned testing condition, the comparatively less-stable C-P bond is probably stable at the beginning of the first weight-loss stage, but some other unstable segments start to decompose. It is believed that, when samples are heated to a certain temperature, their decomposition behaviors will be affected by the decomposition of the C-P bond. Td max2, the maximum weight-loss rate temperature of the second weight-loss stage, obviously increases with the increase of phosphorus content. The phenomenon can be explained by the fact that the residues containing phosphorus at the first weight-loss stage are more stable in air than that of neat PTT, which might also be the reason for more residues yielded at the high temperatures.

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Table 3. TGA Data of PTT and PTTPs air atmosphere

N2 atmosphere sample

T5% (°C)

Td max (°C)

residue at 600 °C (%)

T5% (°C)

Td max1 (°C)

Td max2 (°C)

residue at 600 °C (%)

PTT PTTP5 PTTP10

363.9 363.8 361.2

401.8 401.5 399.6

6.66 5.05 3.45

337.1 345.3 354.0

399.6 397.5 396.6

488.2 530.6 531.8

0.37 0.53 1.21

Table 4. Combustion Parameters of PTT and PTTPs Obtained from Cone Calorimeter sample

TTI (s)

Av-HRR (kW/m2)

PHRR (kW/m2)

TTPHR (s)

THR (MJ/m2)

FIGRA

Av-SEA (m2/kg)

TSR

residue (%)

PTT PTTP5 PTTP10

27 32 43

417 349 278

1108 767 648

125 130 155

81 66 56

8.9 5.9 4.2

397 685 823

1781 3185 3474

7.1 4.8 5.9

3.4. Burning Behavior. The limiting oxygen index (LOI) of samples was tested for estimating their flame retardancy. The LOI values of samples with different phosphorus contents are summarized in Table 1. From the table, it can be seen that the LOI values increased from 21.5 for PTT to 28 for PTTP10 as the phosphorus content increases from 0 to 0.84%. When the P content is lower than 0.4% (corresponding to a 5 wt % of DDP content), an obvious increasing tendency of LOI value can be found (from 21.5 to 26.0). However, when the phosphorus content increases to a certain value, LOI value increases scarcely, which is similar to the results of PET-co-poly(ethylene DDP)s.10 Cone calorimeter is a performance-based bench-scale testing method focused on quantitative analysis of flammability to simulate the real-world fire conditions.40 Furthermore, the cone calorimeter offers abundant information on burning behavior of the materials.41,42 It is widely proved to be an effective tool to predict the combustion behavior of material in a real fire. Thus, the relevant data of PTTPs together with neat PTT obtained from a cone calorimeter, such as time to ignition (TTI), heat release rate (HRR), total heat release (THR), specific extinction area (SEA), and total smoke release (TSR), can be used to evaluate their flammability. The detailed data are summarized in Table 4. TTI is used to characterize the flame retardancy, and a longer TTI means better flame retardancy. From Table 4, we can see that TTI becomes longer with the increase of phosphorus content, meaning that, under the irradiation of a heat flux of 50 kW/m2, the copolyesters containing phosphorus can be ignited at a higher temperature than PTT. Figure 6 shows the heat release rate (HRR) of PTT, PTTP5, and PTTP10. All three samples burned quickly after ignition,

with a total burning time of less than 230 s. However, among the three, PTT burned most rapidly and had the biggest HRR peak. The peak heat release rates (PHRR) of PTT, PTTP5, and PTTP10 were 1108, 767, and 648 kW/m2, respectively. It can be known that the PHRR of PTTP10 is only 58.5% of that of PTT. The total heat release (THR) curves of the three samples are shown in Figure 7. The THR value of PTT is 81 MJ/m2; however, the THR values of PTTP5 and PTTP10 are only 66 and 56 MJ/m2. The THR value of PTTP10 decreases to approximately 70% of PTT. The aforementioned data show that the introduction of DDP into PTT can remarkably reduce both the PHRR and THR, endowing PTT with excellent flame retardancy. Another method for evaluating the fire behaviors is to use the slope of fire growth rate (FIGRA),43 which is calculated from the measured data of CCT and defined by the ratio of PHRR to the time at which PHRR has been reached (tP). It has synthesized the effect of time and heat release in a fire, providing an estimation of flame spread in developing fires, the scale of a fire, and the ignitability of the material.40 The FIGRA values are also listed in Table 4; the depressed values of PTTPs imply a slower fire growth rate and better effect of flame retardancy. The FIGRA values of PTTP5 and PTTP10 are 5.9 and 4.2, and both are much lower than PTT as 8.9. Figures 8 and 9 show the curves of specific extinction area (SEA) and total smoke release (TSR), respectively. From the SEA curves and the TSR curves, it can be seen that with the introduction of DDP, more smoke generates. That is an undesired phenomenon in the flame retardant field, for the smoke in a real fire means more risk to suffocate, even more fatal than a scald and burns. The reason why more smoke was formed may be that, when burning, the phosphorus might promote

Figure 6. Heat release rate (HRR) plots of PTT, PTTP5, and PTTP10 as a function of burning time.

Figure 7. Total heat release (THR) plots of PTT, PTTP5, and PTTP10 as a function of burning time.

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Acknowledgment This work was initiated and supported by the National Science Foundation of China (Grant Nos. 50903047 and 50933005). Literature Cited

Figure 8. Specific extinction area (SEA) plots of PTT, PTTP5, and PTTP10 as a function of burning time.

Figure 9. Total smoke release (TSR) plots of PTT, PTTP5, and PTTP10 as a function of burning time.

carbonization. If the compact and firm chars cannot be formed sufficiently, they would run away with the ascending burning gas to form the smoke. In fact, in the process of polymer combustion, the polymer materials will exist in three kinds of different forms: fully burnt parts (transferred into heat and gases), the burnt residue part, and partially burnt parts (the smoke and soot or tiny particles). In our study, the residue mass is nearly the same for the three samples, their total-heat-release order is PTT > PTTP5 > PTTP10, and therefore, the order of total smoke release should be PTT < PTTP5 < PTTP10, which accord with each other. 4. Conclusions In this study, novel flame-retardant poly(trimethylene terephthalate) copolyesters with the phosphorus-containing linking pendent group have been successfully synthesized by direct polycondensation. The introduction of P-containing monomer (DDP) to PTT main chains decreases the melting point and crystallization capacity of the PTT copolyesters but increases the glass transition temperature. P-containing PTT copolyesters (PTTP) have better thermal stabilities in air but are poorer in nitrogen than PTT. The phosphorus chemically incorporated to PTT increases the LOI values of PTT copolyesters and obviously decreases HRR, PHRR, and THR compared to PTT. The FIGRA values also show that PTTPs have better flame retardant effects than PTT. However, PTTPs have higher SEA and TSR than PTT, which is undesired from the viewpoint of fire safety.

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ReceiVed for reView March 20, 2010 ReVised manuscript receiVed May 28, 2010 Accepted June 10, 2010 IE1006917