A novel phosphorus-containing poly(1,4-cyclohexylenedimethylene terephthalate) copolyester: Synthesis, thermal stability, flammability and pyrolysis behavior

Polymer Degradation and Stability 108 (2014) 12e22

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A novel phosphorus-containing poly(1,4-cyclohexylenedimethylene terephthalate) copolyester: Synthesis, thermal stability, flammability and pyrolysis behavior Jun-Bo Zhang, Xiu-Li Wang*, Qiu-Xia He, Hai-Bo Zhao, 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 r t i c l e i n f o

a b s t r a c t

Article history: Received 16 March 2014 Received in revised form 30 May 2014 Accepted 4 June 2014 Available online 12 June 2014

Abstract: Poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) is a commercialized semicrystalline high-temperature thermoplastic polyester, but its flammability restricted its applications in some fields. A third monomer, 2-(6-oxido-6H-dibenz < c,e > <1,2> oxaphosphorin-6-yl)-1,4-hydroxyethoxy phenylene (DOPO-HQ-HE), was used to synthesize an intrinsic flame-retardant copolyester through transesterification and polycondensation. Its chemical structure was confirmed by 1H NMR and ICP-AES. The crystallization behavior of PCTDs was investigated by DSC and WAXD, and found that the introduction of DOPO-HQ-HE slightly reduced the crystallization ability of PCT. TGA results showed that the incorporation of phosphorus-containing monomer improved the thermal stability of copolyesters both in nitrogen and air. FlynneWalleOzawa method was used to analyze the thermal degradation kinetics of copolyesters, and found that the apparent activation energy was enhanced. The microscale combustion calorimetry (MCC) showed that PCTDs had lower heat release rate and total heat release than PCT. The results of the limiting oxygen index (LOI), the UL-94 vertical and the cone calorimeter test indicated that DOPO-HQ-HE endowed PCTDs with flame-retardant properties to some extent. Besides this, the cone calorimeter results show that the introduction of DOPO-HQ-HE remarkably suppressed the smoke release of PCT. The pyrolysis behaviors of PCT and PCTDs were investigated by Py-GC-MS, and found that the decomposition of PCT chains usually happened at ester bond and followed the random chain scission mechanism. The introduction of DOPO-HQ-HE almost had no effect on the thermal degradation mechanism of PCT. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Poly(1,4-cyclohexylenedimethylene terephthalate) 2-(6-Oxido-6Hdibenz < c,e > <1,2 > oxaphosphorin-6-yl)1,4-hydroxyethoxy phenylene Copolymerization Flame retardance Pyrolysis

1. Introduction Poly(1,4-cyclohexylenedimethylene terephthalate) (PCT), as a commercialized semicrystalline high-temperature thermoplastic polyester, was discovered and developed in 1959 by Kibler et al. [1] The melting temperatures of PCT are in the range of 251~318
C depending on the trans/cis isomer ratio of 1,4-cyclohexylenedimethyl (CHDM) units.[2] In addition to possessing the desirable chemical resistance, thermal stability, high strength and fiber spinning properties like poly(ethylene terephthalate), PCT and its copolyesters with excellent heat resistance made them have a great potential in automotive and electronics/electrical applications. [2,3] By adjusting the

* Corresponding authors. Tel./fax: þ86 28 85410755. E-mail addresses: [email protected] (X.-L. Wang), [email protected] (Y.-Z. Wang). http://dx.doi.org/10.1016/j.polymdegradstab.2014.06.003 0141-3910/© 2014 Elsevier Ltd. All rights reserved.

composition, some properties of PCT-based copolyesters can be improved.[48] Usually two methods i.e. alcohol or acid modification are used to prepare PCT copolyesters. When CHDM is partially replaced by other diol, the alcohol modified PCT copolyesters (PCTG) are obtained. If terephthalic unit of PCT is partly replaced by isophthalic ones, another commercial acid modified PCT (PCTA) are gained.[918] However, both of the commercial PCT and its copolyesters are flammable, which restricted their usage in some fields. In general, there are two approaches to make polyester flame retardancy. One is using additive flame retardants, which usually contain flame retardant elements, mixed with polyester. [19,20] The other is introducing reactive flame retardants into polyester's main chain via copolymerization with the monomer of polyester.[21e26] Although endowing polyester with flame retardance by addition flame retardant is convenient, the additive flame retardants always exhibit heterogeneous dispersion and poor compatibility with polyesters, which will migrate from the

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matrix, as well decline the physical properties of polyester. As for as reactive flame retardants are concerned, this method can solve the problems mentioned above and obtain copolyesters with inherent flame retardancy. Several phosphorus-containing reactive monomers, such as bis 4-carboxyphenyl phosphine oxide (BCPPO), 2-carboxyethyl(phenylphosphinic) acid (CEPPA), and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) derivatives, etc, have been used to prepare inherent flameretardant copolyesters.[23,27e30] Unfortunately, the study on the flame retardance of PCT is just at the starting stage up to now. As far as we know, there have been no any reports on the inherent flame retardance of PCT although a few patents deal with flame retardance of PCT using additive flame retardants.[31,32] For example, halogenated organic compounds, antimony compounds and functional olefins were used to improve the flame retardance of PCT's without hurting dimensional stability. [31] It was found halogenated organic compounds containing at least one imide group can also endow PCT with flame retardance.[32] In recent years, some halogen-containing flame retardants have been banned because they will release toxic and corrosive gases, such as dioxins and hydrogen halide during the combustion.[33] In addition, the common defects of adding flame retardants to polymer matrix cannot be ignored. Therefore, it is necessary to prepare halogen-free inherent flame retardant PCT. Phosphous-containing monomer for synthesizing PCT copolyester is difficult to choose because of a higher boiling point of CHDM. Based on our previous work,[34] we chose DOPO derivative named 2-(6-oxido-6H-dibenz < c,e > <1,2 > oxaphosphorin-6-yl)1,4-hydroxyethoxy phenylene (DOPO-HQ-HE), which possesses relatively high decomposition temperature and high reaction activity, to prepare a series of flame-retardant PCT copolyesters. The chemical structures, thermal transition behaviors, thermal degradation kinetics, pyrolysis, and combustion behaviors of the obtained copolyesters have been studied in this paper. 2. Experimental 2.1. Materials Dimethyl-p-phthalate (DMT, CP) was purchased from Sinopharm chemical Reagent Co., Ltd. 1, 4-Cyclohexanedimethanol (99%) was

13

purchased from Aladdin chemistry Co., Ltd. and the ratio of cis to trans is 30:70. Tetrabutyl titanate was purchased from Kelong Chemical Reagent Factory (Chengdu, China). Before using, tetrabutyl titanate was dissolved into anhydrous toluene to prepare a 0.2 g/mL solution. 2-(6-Oxido-6H-dibenz < c,e > <1,2 > oxaphosphorin-6yl)-1,4-hydroquinone (DOPO-HQ) was prepared according our previous work.[35] Other materials were commercially available and used as received. 2.2. Synthesis of DOPO-HQ-HE DOPO-HQ-HE was prepared according to the literature.[35] 1H NMR (d-DMSO; d, ppm): 7.0e8.3 (AreH), 4.0 (eCH2CH2OH), 3.7 (eCH2OH), 3.6 (eCH2CH2OH), 2.9 (eCH2-OH), 4.5 (eOH) and 4.9 (eOH). 2.3. Synthesis of phosphorus-containing PCT (PCTD) Phosphorus-containing PCT (PCTD) was synthesized by direct polymerization of DMT, CHDM and DOPO-HQ-HE, in which tetrabutyl titanate (400 ppm) was used as a catalyst (Scheme 1). Twostep reaction was carried out on a laboratory scale polymerization reactor. The first-step reaction was the transesterification of CHDM and/or DOPO-HQ-HE with DMT at 220~285
C for 5 h under nitrogen atmosphere, in which the ratio of DMT to alcohol was fixed at 1:1.05. The second-step was the polycondensation processed at 290~310
C, in which the pressure was slowly reduced to 40 Pa over 30 min and isothermally held for 2e4 h. Finally, the reactor was cooled to room temperature under vacuum. Samples containing 0, 5, 10 and 20 mol% DOPO-HQ-HE (DOPO-HQ-HE: CHDM) are coded as PCT, PCTP5, PCTP10, and PTTP20, respectively. 2.4. Characterization NMR spectra (1H, 400 MHz) were performed in a Bruker AVANCE AV II-400 NMR instrument. CF3COOD was used as a solvent. About 20 mg of sample was dissolved in 0.5 ml solvent for testing. Intrinsic viscosites [h] of PCTPs were measured at 25
C with an Ubbelohde viscometer in 50/50 (v/v) of phenol/1,1,2,2-

Scheme 1. Synthesis routes of PCTDs.

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tetrachloroethane solution at a polymer concentration of 0.5 g/dL. The solution was filtered before testing. The actual phosphorus content of the coployester was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES; IRIS Advantage, TJA solution). About 10 mg purified copolyester was burned completely in 1 L flask full of oxygen, and the gas was absorbed by 25 mL 0.001 mol/L KOH/KMnO4 solution. At last, the solution was diluted to 100 mL using deionized water. A TA Q200 DSC apparatus, calibrated with pure indium and zinc standards was used to study the crystallization and melting behavior. Samples (0.5 mg) were placed in an aluminum pan to test their thermal behaviors under a nitrogen atmosphere at a flow rate of 50 mL min1. In order to eliminate the influence of thermal history, those specimens were heated to 20
C higher than their respective melting temperature, and kept for 5 min. After that the samples are cooled to 40
C (i.e. first cooling) to record the crystallization temperature, and then reheated to the final temperature (i.e. second heating). All of the scanning rates for DSC test were 10
C min1. The thermal stabilities and thermal degradation kinetics were performed using a NETZSCH TG 209 F1 apparatus. The samples (5.0 ± 0.5 mg) were heated from 40 to 700
C at 10
C/min under both air and nitrogen conditions. The initial decomposition temperature (Ti) (5 wt% of original weight lost) and the maximum weight loss rate temperature (Tdmax) were used to study the thermal stability along with the char residue. The samples were heated from 40 to 700
C at various rates of 5, 10, 20 and 40
C/min. The FlynneWalleOzawa method was used to analyze the thermal degradation kinetics parameters. Limiting oxygen index (LOI) test was performed using an oxygen index flammability gauge (HCe2C) according to ASTM D 2863e97 with sample dimensions of 130 mm  6.5 mm  3 mm (length  width  thickness). UL-94 vertical measurements were performed using a vertical burning instrument (CZF-2) according to ASTM D 3801, in which sample dimensions were 130 mm  12.5 mm  3.2 mm (length  width  thickness). Thermal combustion properties of samples were measured by a microscale combustion calorimetry (MCC-1, FTT). Approximately 7 mg samples were heated to 700
C at a heating rate of 1
C/s in a stream of nitrogen flowing at 80 cm3/min. The volatile, anaerobic thermal degradation products in the nitrogen gas stream are mixed with a 20 cm3/min stream of 20% oxygen and 80% nitrogen prior to entering a 900
C combustion furnace. The heat release rate (HRR) was obtained from the amount of oxygen consumed. The specimens with a size of 100 mm  100 mm  3 mm were used for cone calorimeter tests (FTT cone calorimeter, UK) according to ISO 5660-1at a heat flux of 50 kW m2. Py-GC/MS tests were performed in a pyrolyzer (CDS5200). The pyrolysis chamber was full of He, the relevant samples (300 mg) were heated from ambient to 500
C at a rate of 1000
C/min and kept for 20 s. The pyrolyzer was coupled with DANI MASTER GCTOF-MS Systems, and the carrier gas was He. For the operation, the temperature program of the capillary column (DN-1701 FAST 10 m 0.10 mm 0.10 mm) of GC was as following: 2 min at 45
C, then the temperature increased to 280
C at a rate of 15
C/min and kept at 280
C for 5 min. The injector temperature was 300
C. MS indicator was operated in the electron impact mode at electron energy of 70 eV, and the ion source temperature was kept at 180
C. The detection of mass spectra was carried out using a NIST library. 3. Results and discussion 3.1. Synthesis of phosphorus-containing PCT copolyesters (PCTD)

or PBT. This is ascribed to the higher boiling point of CHDM, which made the removal of excessive CHDM is difficult. By enhancing the stirring speed and the polymerization temperature, a series of PCT copolyesters with intrinsic viscosities in the range of 0.58e0.62 are obtained (Table 1). In their GPC curves (not shown here), only a unimodal peak can be found for all the samples demonstrated the reaction was completed and no un-reacted monomer remained. The phosphorus content of PCTD is determined by ICP-AES and the results are listed 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 value, which indicate that DOPO-HQ-HE has been successfully introduced to the PCT chains. The above results confirm that the introduction of DOPO-HQ-HE does not affect the polymerization of PCT, and copolyesters containing DOPO-HQ-HE can be synthesized successfully via transesterification and polycondensation two steps. The chemical structures of the obtained PCT copolyester are further determined by 1H NMR spectra (Fig. 1). In the 1H NMR spectrum of PCT, all the appeared signals t are assigned to different protons of each repeating unit. The resonance signals occurring at 8.1 (a), 1.2e2.1(d) ppm are remarkably ascribed to aryl and the cyclohexyl of CHDM, respectively. The cyclohexylenedimethylene groups of CT unit have two isomers, i.e. trans and cis, which made the oxymethylene protons divided into two groups, reflected in the chemical shift at 4.3(b) and 4.2(c) in Fig. 1. Compared to PCT, PCTD20 show some different signals belonged to DOPO-HQ-HE. The peaks at 6.9e7.9 ppm (e), 4.0 ppm (f), 3.7 ppm (g), 3.6 ppm (h), 2.9 ppm (i) are reasonably assigned to aryl and methylene of DOPO-HQ-HE. The actual molar ratio of CHDM and DOPO-HQ-HE was determined by the relative peak intensities of methylene (b, c) of CHDM and DOPO-HQ-HE (f, g, h, i), and the detailed data are shown in Table 1. From the Table, it was found that the actual molar ratio of CHDM and DOPO-HQ-HE was almost as same as feed ratio illustrating all DOPO-HQ-HE had been successfully introduced into PCT. 3.2. Thermal transition and crystallization behaviors of PCTDs The thermal transition behaviors of samples are analyzed by DSC (Fig. 2). Glass transition temperature (Tg), melting temperature (Tm), melting enthalpy (DHm), crystallization temperatures (TC), and crystallization enthalpies (DHC) determined from Fig. 2 are summarized in Table 2. From the second heating curves (Fig. 2(a)) of PCT, PCTD5, PCTD10 and PCTD20, a clearly melting peak can be found. This indicated all PCTDs are crystalline polymer even 20 mol % DOPO-HQ-HE is introduced. Since DOPO-HQ-HE is bulky monomer, its introduction will deteriorate chain regularity of PCT and result in lower crystallization ability reflected by the decrease of DHm and melting temperature. Fig. 2(b) shows the cooling curves of PCT and PCTDs. The crystallization peak of PCT is sharp and narrow, which means it has fast crystallization rate. For PCTDs, with increase of DOPO-HQ-HE

Table 1 Characteristic parameters of PCT and PCTDs. Sample

PCT PCTD5 PCTD10 PCTD20 a

The synthesis of PCT and its copolyesters (PCTD) with high molecular weights are more difficult than other polyester like PET

DOPO-HQ-HE: CHDM (mol%) 1

Feed

Calculate by H NMR

0 5 10 20

0 6 9.5 19.5

P ta (%)

Ptb (%)

[h] (dL/g)

LOI

UL-94

0 0.51 0.94 1.6

0 0.48 0.87 1.42

0.65 0.62 0.61 0.58

20.5 23.5 24.5 26.5

NR V-2 V-2 V-2

Theoretical phosphorus content (Pt) of the resulting polymer. Experimental phosphorus content (Pt) determine by ICP-AES of the resulting polymer. b

J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22

15

Fig. 1. 1H NMR spectra of (a) PCT and (b) PCTD20.

content, the crystallization peaks become dull and broad, which means the crystallization rate is low. Besides this, the values of TC and DHC decrease with the increase of DOPO-HQ-HE content also indicating that the introduction of DOPO-HQ-HE decrease the crystallization ability of PCT. On the contrary, the glass transition temperatures (Tg) of PCTDs increase with increase of DOPO-HQ-HE content, which can be ascribed to the rigid chemical structure of DOPO-HQ-HE restrict the mobility of molecular chain. WAXD was used to investigate the crystal structure of PCT and PCTDs (Fig. 3). It has been reported that PCT has a triclinic crystal structure. According to Boye, [36] the unit cell of PCT with above 68% trans-CHDM isomer is nearly the same as that of poly(1,4-transcyclohexylenedimethylene terephthalate) (PtCT). The unit cell dimensions of triclinic crystal is a ¼ 0.646 nm, b ¼ 0.665 nm, c ¼ 1.422 nm, a ¼ 89.45
, b ¼ 47.03
, and g ¼ 114.95
. From Fig. 3, we can see clearly that all three samples show same 2q value, which are found at 15.6
, 16.6
, 23.4
and 25.6
. These reflection peaks are ascribed to 011, 010, 100 and 111 planes of PCT triclinic crystal. And this indicates that the introduction of a small amount of DOPO-HQHE does not affect PCT crystals structure. But their crystalline peak intensity is lower than that of PCT, illustrated the crystallinity of PCTDs is decreased. And this result is also in accorded with DSC results. 3.3. Thermal stability of PCTDs The thermal stability of the PCTDs under both nitrogen and air was investigated with TGA, and the corresponding curves are

Fig. 2. DSC curves of PCTDs and PCT during the second heating scans (a) and the first cooling scans (b) at 10
C/min.

illustrated in Fig 4. Detailed data such as onset decomposition temperature (T5%), maximum decomposition temperature (Tdmax), and char residue are summarized in Table 3. Under N2 atmosphere, it can be noticed that both neat PCT and phosphorus-containing PCT copolyesters decompose in a one-step weight loss process. It has been reported that the introduction of phosphorus-containing compounds would decrease the thermal stability due to the weaker PeC bond in the polymer backbone.[37] In the present study, the thermal stability of samples increases slightly with the addition of phosphorus-containing flame retardant components. We note that the char residue of neat PCT is only 1.4%, which is lower than that similar commercial polyester such as PET, PTT and PBT. It seems that the CHDM-based polyester decompose more completely, and it is difficult to form char residue. Also, the residue of PCTDs at 700
C is almost linear increased with

Table 2 Thermal behaviors of PCT and PCTDs. Sample

Tg (
C)

Tc (
C)

DHc (J/g)

Tm (
C)

DHm (J/g)

PCT PCTD5 PCTD10 PCTD20

96.2 97.2 106.8 111.3

256.2 248.8 231.7 186.6

54.3 32.3 31.7 24.2

286.3 279.6 269.1 250.9

46.7 27.1 23.7 23.0

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J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22 Table 3 TG data of PCT and PCTDs in N2 and Air. sample

PCT PCTD5 PCTD10 PCTD20

N2 atmosphere

Air atmosphere

T5% (
C)

Td max (
C)

Residue at 700
C (%)

T5% (
C)

Tdmax1 (
C)

Tdmax2 (
C)

Residue at 700
C (%)

385.0 385.4 385.8 387.0

415.8 415.7 416.0 417.5

1.4 2.8 4.6 8.6

375.6 382.5 378.8 379.0

405.3 407.6 406.2 407.6

522.8 541.8 554.2 564.2

0.9 0.1 1.0 2.2

more stable, which will postpone the oxygenolysis of copolyester at higher temperature. The char residue at 700
C do not increase obviously as that in N2, demonstrated that char is not stable enough to prevent oxygenolysis occurred. In order to further illustrate the thermal degradation behaviors of neat PCT and PCTDs, FlynneWalleOzawa (Eq. (1)) is used to investigate their thermal degradation kinetics[25,38,39]. Fig. 3. WAXD patterns of PCT, PCTD10, and PCTD20.

log10 b ¼ log10 the increase of DOPO-HQ-HE content. The aryl structure of phosphors-containing monomer which partial replaces the aliphatic di-alcohol CHDM in the main-chain can contribute to the carbonization of decomposition products during heating process. Under air atmosphere, the thermo-oxidative degradations of neat PCT and PCTDs show a two-step process, including a major and a minor weight loss stages. Compared with pure PCT, all the values of T5, Tdmax1, Tdmax2 increase with the addition of phosphoruscontaining monomer indicating that DOPO-HQ-HE can enhance the thermal stability of copolyesters. Especially, Tdmax2 of PCTD20 obviously increases from 522.8
C (PCT) to 564.2
C, which illustrated that the residues of PCTD20 at the first weight-loss stage are

AE E  log10 FðaÞ  2:315  0:4567 R RT

(1)

where A is the pre-exponential factor, E is the activation energy, b is the heating rate, T is the temperature, and F(a) is the integral function of the conversion. Via this method the apparent activation energy of a reaction can be determined without knowing its reaction mechanism. From Eq. (1), E can be calculated from the slope of a plot of logb versus 1/T at a fixed weight loss. Fig. 5 shows thermal degradation curves of PCT, PCTD10 and PCTD20 at different heating rates. Thermal stability of the samples increases with the increase of heating rates, as expected, given the time lag for thermal conduction and volatilization of the decomposition compounds.

Fig. 4. TG and DTG curves of PCT and PCTDs in N2 (a, b) and air (c, d).

J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22

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Fig. 5. TG curves of PCT and PCTD20 at different heating rates in N2.

Fig. 6 shows the plots of logb versus 1/T at 5, 10, 15, 20, 35, 50, 65 and 75% conversion. From the figure, we can see that the fitted lines are nearly parallel and correlation coefficient above 0.99, indicated that this method is valid for this system. The calculated activation energy of the samples with different conversion is listed in Table 4. It was found that at the lower conversion the activation energy of PCTDs is higher than that of PCT, however, this trend became weak when the conversion is beyond 35%. This means that the new ester bond formed by DOPO-HQ-HE and DMT is more stable and needs more energy to break down. And it will break together with the

original ester bond of PCT under the higher decomposition temperature. 3.4. Flame retardancy of PCTDs The flame retardancy of PCTDs is determined by LOI and UL-94 vertical test. The LOI and UL-94 test results are summarized in Table 1. The LOI value of PCT is 20.5% illustrated it is a highly combustible material. The incorporation of DOPO-HQ-HE can improve the flame retardancy of PCTDs, for example, the LOI values of PCTD20 increase from 20.5% to 26.5% as the loading of DOPO-HQHE increases from 0 to 20 mol%. Pure PCT fails in the UL-94 test, however, PCTDs can reach a V-2 rating. During the test, the samples

Table 4 Activation energy(E) and correlation coefficient(r) of PCT and PCTDs using the FlynnWall-Ozawa method. Conversion a (%)

5 10 15 20 35 50 65

Fig. 6. Ozawa plots of PCT and PCTD20 at different fractional weight losses (a).

PCT

PCTD10

PCTD20

E(kJ/mol)

r

E(kJ/mol)

r

E(kJ/mol)

r

181 182 183 184 191 194 196

0.997 0.998 0.999 0.999 0.999 0.998 0.997

192 187 187 187 192 195 198

0.992 0.998 0.999 0.999 0.999 0.999 0.999

186 187 188 188 194 197 198

0.999 0.999 0.999 0.999 0.999 0.999 0.999

Fig. 7. HRR curves of PCT and PCTDs at 1
C/s heating rate in N2.

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Table 5 HRC and THR values of PCT and PCTDs obtained from MCC. Sample

T (
C)

HRC (J/g K)

THR (kJ g1)

PCT PCTD10 PCTD20

422 428 430

821 659 583

29.3 26.1 25.0

Table 6 Data recorded in cone calorimetry measurements.a sample

TTI (s)

PHRR (kW/m2)

TTPHRR (s)

THR (MJ/m2)

TSR (m2m2)

Residue (%)

PCT PCTD10 PCTD20

38 36 30

595 651 580

140 125 115

74 61 49

3738 2739 2598

2.8 7.4 19.8

a TTI: time to ignition; PHRR: peak of heat release rate; THR: total heat release; TSR: Total smoke release; TTPHR: time to peak of heat release rate.

can extinguish in 5 s after fire, but they dripping to ignite the cotton. The heat release properties of PCT and PCTDs are studied by a microscale combustion calorimetry technique (MCC), and the obtained heat release rate (HRR) curves are shown in Fig 7. The heat release parameters included heat release capacity (HRC) and total heat release (THR) are listed in Table 5. The heat release capacity (HRC) is the maximum value of the specific heat release rate divided by the heating rate. The total heat release (THR) is the time integral of the specific heat release rate over the whole testing time. The lower the HRC and THR indicate it maybe has better

flame retardant performance. In this study, a reduction of both HRC and THR is observed for phosphorus-containing PCT. The HRC values decrease from 821 J/g K for neat PCT to 658 J/g K and 583 J/ g K for PCTD10 and PCTD20, respectively. The THR decreases from 29.3 kJ g1 for neat PCT to 25.0 kJ g1 for PCTD20. The above data indicates a reduction in heat release when DOPO-HQ-HE is introduced into PCT chain backbone. The aryl structure of phosphors-containing monomer which replaces aliphatic dihydric alcohol CHDM in the main-chain increases the char residue, resulting in less volatile components produced during the degradation. Cone calorimetry is one of the most useful bench-scale fire tests that attempts to simulate real-world fire conditions and offers abundant information on combustion behavior of materials. The relevant data of PCTDs together with neat PCT obtained from a cone calorimeter, such as time to ignition (TTI), heat release rate (HRR), total heat release (THR), residue mass and total smoke release (TSR), can be used to evaluate their flammability (shown in Table 6). From Table 6, it can be found that the time to ignition is decreased from 38 s of PCT to 30 s of PCTD20. At the same time, the time to peak of heat release rate for PCTDs is also lowered. Fig. 8(a) shows the HRR curves of PCT, PCTD10, and PCTD20. The peak heat release rates (PHRR) of PCT, PCTD10, and PCTD20 are found at 595, 651, and 580 kW/m2, respectively. Unfortunately, the PHRR values are not decreased as we expected when DOPO-HQ-HE is introduced. Compared with other polyester such as PET, PBT or PTT, the PHRR values of PCT is much lower[30,40,41]. This means the thermal radiation of PCT is not as serious as other polyester, which made its PHRR value can not be easily reduced. Total smoke release (TSR) is an important parameter employed to assess the smoke

Fig. 8. Cone calorimetric plots of PCT and PCTDs: (a) heat release rate (HRR), (b) total heat release (THR), (c) mass curves and (d) total smoke release (TSR).

J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22

19

Fig. 9. Digital photographs of the burning residues after cone calorimeter test.

production during burning. As shown in Fig. 8(d) and Table 6, the TSR values of neat PCT, PCTD10 and PCTD20 are 3738 m2 m2, 2739 m2 m2 and 2598 m2 m2, respectively. Especially, the TSR value of PCTD20 decreases to approximately 70% of PCT, which illustrates that the addition of DOPO-HQ-HE has a good smoke suppression effect on PCT. Besides this, the total heat release (THR) also decreases with the increase of the phosphorus-containing ionic monomer. The THR value of neat PCT is 74 MJ/m2, however, the THR values of PCTD10 and PCTD20 decrease to 61, and 49 kW/m2, respectively. This means that the presence of phosphorous-containing monomer promote the carbonization of PCT to make THR decrease. The digital photos of char residue after cone test are shown in Fig. 9. From it, we can see clearly that almost no char remain for PCT, and an intumescent-like char is formed for PCTD10 and PCTD20. For PCTD20, its char residue is as high as 19.8 wt% after cone test, which is much higher than that of PCT (shown in Table 6). These carbonaceous shields can protect PCT from oxygen and heat and made it has flame retardance.

phosphorus-containing compounds does not affect the initiation decomposition of PCT. Combined with the results of Pyrolysis-GC/ MS, we can draw a conclusion that both PCT and PCTDs have same degradation mechanism. The degradation mechanism of PET can be a valuable reference for PCT, because they have similar chemical structure, both of them are the polycondensation products of terephthalic acid and aliphatic diol. The thermal degradation mechanism of PET has been clearly investigated.[43,44] It is well-established that PET chains break down by the random chain scission at ester links, and the principal point of weakness is the b-methylene group.[45] From Table 7, it can be found that less rearrangement reaction occurred in the pyrolysis process of neat PCT, the mainly degradation is similar to that of PET. Based on the results of pyrolysis products of PCT and PCTDs, the possible pyrolysis mechanism of PCT and PCTDs is shown in Scheme 2. Compare with original PCT, the main pyrolysis behavior of PCTD is nearly independent of DOPO-HQ-HE. And this phosphorus-containing compound decomposes alone during the pyrolysis process.

3.5. Pyrolysis behavior of PCTDs

4. Conclusions

To further investigate the composition of pyrolysis products and figure out the decomposition mechanisms of PCT and PCTDs, the Pyrolysis-GC/MS was performed. The pyrogram of the evolved products of PCT and PCTD20 are presented in Fig. 10, and the corresponding areas of peaks as well as their assignments are listed in Table 7. For PCT, massive cyclohexane derivative are found, which is due to the fact that cyclohexane groups are very stable and they will not react with other segments during pyrolysis. The cyclohexylenedimethylene is supposed as a stable gaseous decomposition product during the thermal degradation that can dilute the combustible gases. So many gaseous decomposition products generated during pyrolysis, and this will ultimately results in few char residue. The decomposition products of PCTD20 are almost as same as that of PCT, in which benzoic acid, cyclohexane derivative, cyclohexylenemethylene terephthalate accounts for 80% of all the pyrolysis products. This means that DOPO-HQ-HE don't change the composition of pyrolysis products in essentially. Besides, PCTD20 exhibits more chromatographic peaks than neat PCT during pyrolysis. And some characteristic chemical fragments of DOPO-HQHE (peak16, 17, 18 and 19) such as DOPO ring (M ¼ 216) and dibenzofuran (M ¼ 168) are detected, which is consistent with the previous literature.[42] The above results demonstrated that there are less secondary degradation reaction during the pyrolysis of PCT and PCTD20. As shown by TG, PCT and PCTDs have a comparable onset of decomposition temperature, showing that the introduction of

Novel phosphorus-containing flame-retardant PCTDs have been successfully synthesized. The introduction of DOPO-HQ-HE into PCT main chains decreases its melting point and crystallization capacity. TG data show that the onset weight-loss temperature and

Fig. 10. GC traces for the evolved products of PCT and PCTD20 during pyrolysis.

20

J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22

Table 7 Compounds identified in the pyrograms of PCT and PCTD20. Retention time

Area percent PCT(%)

1

1.48

2,3

M PCTD20(%)

1

1

94

1.98, 2.54

36

31

108

4

2.80

17

18

106

5

3.10

3

2

108

6

4.92

1

1

124

7

5.76

1

1

126

8

7.10

9

13

122

9

11.85

6

4

229

10

12.06

10

7

229

11

12.87

1

1

142

12

13.84

1

1

154

13

14.69

1

2

257

14

19.31

3

5

257

15

6.00

e

<1

150

16

7.75

e

<1

180

Assigned structure

J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22

21

Table 7 (continued ) Retention time

Area percent

M

PCT(%)

PCTD20(%)

17

9.85

e

<1

168

18

10.48

e

<1

178

19

13.523

e

<1

216

Assigned structure

Scheme 2. Pyrolysis process of PCT and PCTDs.

the char residue of PCTDs increased no matter how they are tested in N2 or Air. Besides, the thermal degradation kinetics of PCTDs show that the apparent activation energy increase with the increase of DOPO-HQ-HE content, indicating that DOPO-HQ-HE enhance the thermal stability of PCT. The LOI value of PCTD20 increase to 26.5, and UL-94 rating is V2. Although, the introduction of DOPO-HQ-HE has no contribution on the decrease of PHRR values, it lower the THR and TSR values of PCT. It is noteworthy that DOPOHQ-HE has a good smoke suppression effect and the smoke release is reduced by 30%. The char residue for PCTD20 after cone test is 19.8% indicated the copolyesters show condensed phase flameretardant activity. The Py-GC/MS results show that PCTD has similar pyrolysis mechanisms with PCT, which means that the introduction of DOPO-HQ-HE does not affect the pyrolysis process of PCT.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (50933005, 51121001), the Excellent Youth Foundation of Sichuan (2011JQ0007), and Program for Changjiang Scholars and Innovative Research Team in University (IRT1026). References [1] Kibler CJ, Bell A, Smith JG. Linear polyesters and polyester-amides from 1,4cyclohexanedimethanol. U.S 2,901,466; 1959. [2] Kibler CJ, Bell A, Smith JG. Polyesters 1,4-cyclohexanedimethanol1. J Polym Sci Part Gen Pap 1964;2:2115e25. [3] Goldberg AJ, Burstone CJ. The use of continuous fiber reinforcement in dentistry. Den Mater 1992;8:197e202.

22

J.-B. Zhang et al. / Polymer Degradation and Stability 108 (2014) 12e22

[4] Auerbach AB, Sell JW. Evaluation of poly(1,4-cyclohexylene dimethylene terephthalate) blends for improved processability. Polym Eng Sci 1990;30: 1041e50. [5] Yoo HY, Umemoto S, Kikutani T, Okui N. Co-crystallization behaviour and melting-point depression in poly(ethylene terephthalate-co-1,4cyclohexylene dimethylene terephthalate) random copolyesters. Polymer 1994;35:117e22. [6] Oh TS, Ryou JH, Chun YS, Kim WN. Transesterification reaction of polyarylate and copolyester (PETG) blends. Polym Eng Sci 1997;37:838e44. [7] Lee SW, Huh W, Hong YS, Lee KM. Synthesis and thermal properties of poly(cyclohexylene dimethylene terephthalate-co-butylene terephthalate). Korea Polym J 2000;8:261e7. [8] Sandhya TE, Ramesh C, Sivaram S. Copolyesters based on poly(butylene terephthalate)s containing cyclohexyl groups: synthesis, structure and crystallization. Macromol Symp 2003;199:467e82. [9] Jeong YG, Jo WH, Lee SC. Cocrystallization of poly(1,4cyclohexylenedimethylene terephthalate-co-hexamethylene terephthalate) copolymers. Macromol Res 2004;12:459e65. [10] Sandhya TE, Ramesh C, Sivaram S. Copolyesters based on poly(butylene terephthalate)s containing cyclohexyl and cyclopentyl ring: effect of molecular structure on thermal and crystallization behavior. Macromolecules 2007;40:6906e15. [11] Jeong YG, Jo WH, Lee SC. Crystal structure determination of poly(1,4-transcylcohexylenedimethylene 2,6-naphthalate) by X-ray diffraction and molecular modeling. Macromolecules 2003;36:5201e7. [12] Bang HJ, Kim HY, Jin FL, Park SJ. Fibers spun from 1,4-cyclohexanedimethanolmodified polyethylene terephthalate resin. J Ind Eng Chem 2011;17:805e10. [13] Berti C, Celli A, Marchese P, Marianucci E, Barbiroli G, Di Credico F. Influence of molecular structure and stereochemistry of the 1,4-cyclohexylene ring on thermal and mechanical behavior of poly(butylene 1,4cyclohexanedicarboxylate). Macromol Chem Phys 2008;209:1333e44. [14] Avila-Orta CA, Medellin-Rodriguez FJ, Wang ZG, Navarro-Rodriguez D, Hsiao BS, Yeh FJ. On the nature of multiple melting in poly(ethylene terephthalate) (PET) and its copolymers with cyclohexylene dimethylene terephthalate (PET/CT). Polymer 2003;44:1527e35. [15] Zhang Y, Feng ZG, Feng QL, Cui FZ. Preparation and properties of poly(butylene terephthalate-co-cyclohexanedimethylene terephthalate)-b-poly(ethylene glycol) segmented random copolymers. Polym Degrad Stab 2004;85: 559e70. [16] Kim Y, Heo K, Kim K-W, Kim J, Shin T, Kim J, et al. Time-resolved synchrotron X-ray scattering studies on crystallization behaviors of poly(ethylene terephthalate) copolymers containing 1,4-cyclohexylenedimethylene units. Macromol Res 2014;22:194e202. [17] Yang J, Li WG, Yu AF, Xi P, Huang XA. Li SM sequence distribution, thermal properties, and crystallization studies of poly(trimethylene terephthalate-co1,4-cyclohexylene dimethylene terephthalate) copolyesters. J Appl Polym Sci 2009;111:2751e60. [18] Matsuda H, Nagasaka B, Asakura T. Sequence analysis of poly (ethylene/1,4cyclohexanedimethylene terephthalate) copolymer using H-1 and C-13 NMR. Polymer 2003;44:4681e7. [19] Stackman RW. Phosphorus based additives for flame retardant polyester. 1. Low molecular weight additives. Ind Eng Chem Prod Res Dev 1982;21: 328e31. [20] Stackman RW. Phosphorus based additives for flame retardant polyester. 2. Polymeric phosphorus esters. Ind Eng Chem Prod Res Dev 1982;21:332e6. [21] Chang SJ, Chang FC. Synthesis and characterization of copolyesters containing the phosphorus linking pendent groups. J Appl Polym Sci 1999;72:109e22. [22] Wang CS, Shieh JY, Sun YM. Phosphorus containing PET and PEN by direct esterification. Eur Polym J 1999;35:1465e72. [23] Asrar J, Berger PA, 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;37:3119e28.

€ußler L, Friedel P, et al. Synthesis [24] Fischer O, Pospiech D, Korwitz A, Sahre K, Ha and properties of phosphorus polyesters with systematically altered phosphorus environment. Polym Degrad Stab 2011;96:2198e208. [25] Brehme S, Schartel B, Goebbels J, Fischer O, Pospiech D, Bykov Y, et al. Phosphorus polyester versus aluminium phosphinate in poly (butylene terephthalate) (PBT): flame retardancy performance and mechanisms. Polym Degrad Stab 2011;96:875e84. €ppl T, Schartel B, Fischer O, Altst€ [26] Brehme S, Ko adt V, Pospiech D, et al. Phosphorus polyester-an alternative to low-molecular-weight flame retardants in poly(butylene terephthalate)? Macromol Chem Phys 2012;213:2386e97. [27] Wang LS, Wang XL, Yan GL. Synthesis, characterisation and flame retardance behaviour of poly(ethylene terephthalate) copolymer containing triaryl phosphine oxide. Polym Degrad Stab 2000;69:127e30. [28] Sablong R, Duchateau R, Koning CE, Pospiech D, Korwitz A, Komber H, et al. Incorporation of a flame retardancy enhancing phosphorus-containing diol into poly(butylene terephthalate) via solid state polycondensation: a comparative study. Polym Degrad Stab 2011;96:334e41. [29] Chen HB, Zhang Y, Chen L, Shao ZB, Liu Y, Wang YZ. Novel Inherently flameretardant poly(trimethylene terephthalate) copolyester with the phosphoruscontaining linking pendent group. Ind Eng Chem Res 2010;49:7052e9. [30] Chen HB, Zhang Y, Chen L, Wang W, Zhao B, Wang YZ. A main-chain phosphorus-containing poly(trimethylene terephthalate) copolyester: synthesis, characterization, and flame retardance. Polym Adv Technol 2012;23: 1276e82. [31] Minnick LA. Polyester molding composition having improved flame resistant. U.S.5,021,495; 1991. [32] Keep GT. Process for improving the toughness of PCT formulations by adding rubber impact modifiers. U.S.6,277,905; 2001. [33] Grand AF, Wilkie CA. Fire retardancy of polymeric materials. New York: Marcel Dekker, Inc; 2000. [34] Wang YZ, Chen XT, Tang XD. Synthesis, characterization, and thermal properties of phosphorus-containing, wholly aromatic thermotropic copolyesters. J Appl Polym Sci 2002;86:1278e84. [35] Wang CS, Lin CH, Chen CY. Synthesis properties of phosphorus-containing polyesters derived from 2-(6-oxido-6H-dibenz〈c,e〉〈1,2〉oxaphosphorin-6yl)-1,4-hydroxyethoxy phenylene. J Polym Sci Part A: Polym Chem 1998;36: 3051e61. [36] Boye CA. X-Ray diffraction studies of poly(1,4-cyclohexylenedimethylene terephthalate). J Polym Sci 1961;55:275e84. [37] Ge XG, Wang C, Hu Z, Xiang X, Wang J-S, Wang DY, et al. Phosphorus-containing telechelic polyester-based ionomer: facile synthesis and antidripping effects. J Polym Sci Part A: Polym Chem 2008;46:2994e3006. [38] Ozawa T. A new method of analyzing thermogravimetric Data. Bull Chem Soc Jap 1965;38:1881e6. [39] Wu B, Wang YZ, Wang XL, Yang KK, Jin YD, Zhao H. Kinetics of thermal oxidative degradation of phosphorus-containing flame retardant copolyesters. Polym Degrad Stab 2002;76:401e9. [40] Zhang Y, Chen L, Zhao JJ, Chen HB, He MX, Ni YP, et al. A phosphorus-containing PET ionomer: from ionic aggregates to flame retardance and restricted melt-dripping. Polym Chem 2014;5:1982e91. [41] Gallo E, Braun U, Schartel B, Russo P, Acierno D. Halogen-free flame retarded poly(butylene terephthalate) (PBT) using metal oxides/PBT nanocomposites in combination with aluminium phosphinate. Polym Degrad Stabil 2009;94: 1245e53. €ring M. Pyrolysis [42] Balabanovich AI, Pospiech D, H€ außler L, Harnisch C, Do behavior of phosphorus polyesters. J Anal Appl Pyrol 2009;86:99e107. [43] Buxbaum LH. The degradation of poly(ethylene terephthalate). Angew Chem Int Ed 1968;7:182e90. [44] Zimmerman H, Kim NT. Investigations on thermal and hydrolytic degradation of poly(ethylene terephthalate). Polym Eng Sci 1980;20:80e3. [45] Pohl HA. The thermal degradation of polyesters. J Am Chem Soc 1951;73: 5660e1.