Kinetics of thermal oxidative degradation of phosphorus-containing flame retardant copolyesters

Kinetics of thermal oxidative degradation of phosphorus-containingflame retardant copolyesters

Bo Wu, Yu-Zhong Wang*, Xiu-Li Wang, Ke-Ke Yang, Yong-Dong Jin, Hong Zhao

Center for Degradable and Flame-Retardant Polymeric Materials, School of Chemistry Sichuan University, Chengdu 610064, PR China

Received 12 September 2001; received in revised form 21 January 2002; accepted 27 January 2002

Abstract

The thermal oxidative degradation of phosphorus-containing copolyester and poly(ethylene terephthalate) (PET) in air were

studied by thermogravimetric analysis (TG) in order to obtain the activation energies of pyrolysis. The activation energies weredetermined using the Kissinger method and the Flynn–Wall–Ozawa method, which do not require knowledge of the reactionmechanism (RM). The results show that there is no large difference between the values of activation energy of phosphorus-con-

taining copolyester and PET, which suggests that the flame retardants studied do not greatly influence the mechanisms of pyrolysis.# 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Flame retardant; Phosphorus-containing copolyester; Thermal oxidative degradation

1. Introduction

The pyrolysis and combustion of poly(ethylene tereph-thalate) (PET) is a subject of great concern to a largenumber of people interested in reducing the number offire incidents involving textile materials. Polyester fibers,because of their outstanding characteristics of highstrength, resistance to shrinkage and stretch, good dye-ability, and chemical resistance, are used in carpets,furnishings, and apparel, but their use is limited by theircombustibility. Thus the flame-retardation of polyestersis an urgent problem. As we know, the pyrolysis andcombustion of PET relate to its thermal degradationbehavior, which constrains the use range and techniquesof processing of PET. Thus, it is important to study thethermal oxidative degradation and thermal stability ofpolyesters, in order to ascertain the use and processingrange of PET.Organic polymers undergo thermal degradation when

subjected to sufficient heat flux. In polyesters, the esterlinkage is the point of primary chain scission, leading tothe formation of carboxylic acids and vinyl chain ends.In PET, for example, pyrolysis starts with random chain

scission of C–O bonds aided by the formation of a six-membered ring transition state. For polyesters, phos-phorus-containing compounds appear to be a betteralternative than halogenated compounds because ofenvironmental concerns and the threat of toxic gasemissions during fires. Detailed kinetic analyses ofpolyesters have been performed using thermogravimetricanalysis (TG), to analyze their thermal oxidative behaviorat different heating rates in air. Cooney et al. [1,2] studiedphosphorus-containing PET and observed that phos-phorus species did not greatly influence the degradationkinetics. Granzow [3,4] meanwhile reported that additionof red phosphorus reduced the flammability of thepolymer. There were other reports about degradation offlame retardant polyesters [5–10], but most of themdealt with the additive flame retardant systems ofpolyesters. In this work we synthesized flame retardantcopolyester using a reactive monomer containing phos-phorus, 2-carboxyethyl(phenylphosphinic) acid (CEPP).The thermal oxidative degradation of products wasstudied by thermogravimetric analysis (TG) to analyzetheir thermal oxidative behaviors at different heatingrates in air, and compare with PET.There are many methods of evaluation of the non-

isothermal kinetic parameters of the thermo-oxidativedegradation of polymers [11,12], but the Kissinger andFlynn–Wall–Ozawa methods were chosen here, whichdo not require knowledge of the reaction mechanism.

0141-3910/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(02 )00041-1

Polymer Degradation and Stability 76 (2002) 401–409

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +86-28-5402901; fax: +86-28-

5412907.

E-mail address: [email protected] (Y.-Z. Wang).

2. Kinetic methods

Kinetic information can be extracted from dynamicexperiments by means of various methods. All kineticstudies assume that the isothermal rate of conversion,d�/dt, is a linear function of a temperature-dependentrate constant, k, and a temperature-independent func-tion of the conversion, �, that is:

d�

dt¼ kf �ð Þ ð1Þ

where f(�) depends on the particular decompositionmechanism.According to Arrhenius,

k ¼ Ae�ERT ð2Þ

where A, the pre-exponential factor, is assumed to beindependent of temperature, E is the activation energy,T the absolute temperature, and R the gas constant.Combination of Eqs. (1) and (2) gives

d�

dt¼ Af �ð Þe�

ERT ð3Þ

If the sample temperature is changed at a controlledand constant heating rate, �=dT/dt, the variation in theconversion can be analysed as a function of tempera-ture, this temperature being dependent on the time ofheating. Therefore, the reaction rate may be written asfollows:

d�

dt¼

d�

dT

dT

dt¼ �

d�

dTð4Þ

A combination of Eqs. (3) and (4) leads to

d�

dT¼

A

�e�

ERTf �ð Þ ð5Þ

Integration of this equation from an initial tempera-ture, T0, corresponding to a degree of conversion �0, tothe peak temperature, Tp, where �=�p, gives

ð�p�0

d�

f �ð Þ¼

A

�

ðTP

T0

e�ERTdT ð6Þ

If T0 is low it may be reasonably assumed that �0=0and considering that there is no reaction between 0 andT0:

ð�p0

d�

f �ð Þ¼

A

�

ðTP

0

e�ERTdT ð7Þ

2.1. Kissinger method [13]

The Kissinger method has been used in the literatureto determine the activation energy from plots of thelogarithm of the heating rate versus the inverse of thetemperature at the maximum reaction rate in constantheating rate experiments.The activation energy can be determined by the

Kissinger method without a precise knowledge of thereaction mechanism, using the equation:

ln�

T2max

� �¼ ln

AR

Eþ ln n 1� �maxð Þ

n�1� ��

�E

RTmax

ð8Þ

where � is the heating rate, Tmax is the temperaturecorresponding to the inflection point of the thermaloxidative degradation curves which corresponds to themaximum reaction rate, A is the pre-exponential factor,�max is the extent of conversion at Tmax, and n is thereaction order.From a plot of ln(�/Tmax

2 ) versus 1/Tmax and fitting toa straight line, the activation energy E can be calculatedfrom the slope.

2.2. Flynn–Wall–Ozawa method [14]

This method, which is essentially the same as that ofFlynn and Wall represent a relatively simple method ofdetermining activation energy directly from weight lossversus temperature data obtained at several heating rates.Eq. (7) is integrated using the Doyle approximation. Theresult of the integration after taking logarithms is:

log10F �ð Þ ¼ log10AE

R� log10�� 2:315� 0:4567

E

RTð9Þ

where �, A, E and T have the known meanings, F(�) isthe integral function of conversion.This is one of the integral methods that can determine

the activation energy without a knowledge of reactionorder. It is used to determine the activation energy forgiven values of conversion. The activation energy fordifferent conversion values can be calculated from a ln�versus 1/T plot.

3. Experimental

3.1. Materials

Ethylene glycol (EG) and terephthalic acid (TA) weresupplied by Guang-Kang Chemical Fibers Company(Guanghan, China), 2-carboxyethyl(phenyl)phosphinicacid (CEPP) was offered by Weili Flame-RetardantChemicals Company (Chengdu, China).

402 B. Wu et al. / Polymer Degradation and Stability 76 (2002) 401–409

3.2. Synthesis of phosphorus-containing copolyesters

A reactor was filled with the esterification products ofethylene glycol and terephthalic acid, 2-carboxyethyl(phenyl)phosphinic, and antimony oxide Sb2O3. The reac-tor was fitted to a vacuum system and evacuated threetimes to remove all oxygen. The reactor was then heatedto 270 �C, and after 90 min polymerization was complete.Copolyesters containing 3 and 8% CEPP respectivelywere synthesized, and their limiting oxygen index (LOI)

values are given in Table 1. PET was purchased fromYizheng Chemical Fibers Company (Yizheng, China).

3.3. Measurements

The thermogravimetric analysis was performed with aDupont 2100 instrument. Samples (10�0.1 mg) wereplaced on an open platinum sample pan, and theexperiments conducted in air at a flow rate of 80 ml/min. Nominal heating rates of 5, 10, 20 and 40 �C /minwere used and continuous records of sample temperature,sample weight, and its first derivative were taken.

4. Results and discussion

Figs. 1–3 show thermal oxidative degradation curvesof samples corresponding to dynamic experiments carried

Table 1

Polyesters investigated

Samples %P w/w on samples LOI

PET – 19.6

PET containing CEPP 3% 0.447 30.2

PET containing CEPP 8% 1.225 38.6

Fig. 1. TG curves of PET heated in air at different heating rates.

Fig. 2. TG curves of PET containing 3% CEPP heated in air at different heating rates.

B. Wu et al. / Polymer Degradation and Stability 76 (2002) 401–409 403

Fig. 3. TG curves of PET containing 8% CEPP heated in air at different heating rates.

Fig. 4. DTG curves of PET heated in air at different heating rates.

Fig. 5. DTG curves of PET containing 3% CEPP heated in air at different heating rates.


out at different heating rates. Figs. 4–6 are the corre-sponding DTG curves. Inflection point temperature, Tmand residual mass, Wr, can be determined from thesecurves and are shown in Tables 2–4, respectively.According to previous reports [1], the degradation of

PET can be seen to take place in at least four stages.The activation energies for PET at different stages canbe calculated by the Kissinger method; the activationenergies of other flame retardant PETs were calculatedby the same method. Figs. 7–9 show the activationenergies obtained using the Kissinger method. Forcomparative purpose the results are summarized inTable 5. The activation energy of PET at stage 3 cannotbe obtained by fitting straight line. There are no largedifferences between the activation energy of PET and

that of PET containing phosphorus at stages 1 and 2,which means that the degradation of those samples isfollowing the same reaction mechanism in those stages.However, the activation energy of PET containing 8%CEPP at stage 3 is much higher than that of PET con-taining 3% CEPP, which means the reaction mechanismhas changed.The activation energy can also be determined using

the method of Flynn–Wall–Ozawa, Eq. (9) from a linearfitting of ln� versus 1/T at different conversions. Forthis study, we have used the conversion values 5, 10, 15,20, 35, 50, 65, 75, 80, 85, 90 and 95%. Figs. 10–12 showthat the fitting lines are nearly parallel except above90% conversion, thus indicating the applicability of thismethod to our systems in the conversion range investi-gated, in which the selected conversion values are 5, 20,50, 80 and 90%, and all data were shown in Tables 6–8.The results show that when conversion reaches 90%, wecan not get favorable fitting line, indicating that a com-plicated reaction mechanism acts at this stage, but thatthe conversions range from 5 to 85%, there are nonotable difference among activation energies of eachsample.The use of a multiple heating rate technique was felt

to represent more realistically the variety of heatingrates obtainable during the pyrolysis and combustion

Fig. 6. DTG curves of PET containing 8% CEPP heated in air at different heating rates.

Table 2

Inflection point temperature and residue of PET at 923 K at different

heating rates

Heating rate

(�C/min)

Tinflection point(K)

Residue at 923 K

Wr(%)

5 687 0.23

10 702 0.53

20 716 0.69

40 722 1.28

Table 3

Inflection point temperature and residue of PET containing 3% CEPP

at 923 K at different heating rates

Heating rate

(�C/min)


Residue at 923 K

Wr(%)

5 685 2.2

10 701 0.55

20 713 1.4

40 724 1.4

Table 4

Inflection point temperature and residue of PET containing 8% CEPP

at 923 K at different heating rates

Heating rate

(�C/min)


Residue at 923 K

Wr(%)

5 692 0.13

10 698 0.15

20 714 0.67

40 727 1.08


Fig. 7. The Kissinger plots of PET at different stages.

Fig. 8. The Kissinger plots of PET containing 3% CEPP at different stages.

Fig. 9. The Kissinger plots of PET containing 8% CEPP at different stages.


Table 5

Calcualted activation energies of samples by the Kissinger method

Samples Region Conversion

�Tm (K)

�=10 �C/min

Activation energy E

(kJ/mol)

Correlation coefficient

r

Prestage 0.00–0.09 660 253 0.9999

PET Stage 1 0.09–0.36 688 209 0.9899

Stage 2 0.36–0.92 702 219 0.9756

Stage 3 0.92–1.00 800 – –

Prestage 0.00–0.09 652 292 0.9846

PET containing 3% CEPP Stage 1 0.09–0.36 686 201 0.9882

Stage 2 0.36–0.92 701 213 0.9927

Stage 3 0.92–1.00 811 113 0.9839

Prestage 0.00–0.09 647 192 0.9994

PET containing 8% CEPP Stage 1 0.09–0.36 686 233 0.9966

Stage 2 0.36–0.92 699 223 0.9896

Stage 3 0.92–1.00 814 317 0.9865

Fig. 10. Ozawa plots of PET at different conversions.

Fig. 11. Ozawa plots of PET containing 3% CEPP at different conversions.


reactions occurring when a polymer burns. Ozawa’scalculation is applicable to all points on the TG curves,and Kissinger’s method uses only one point, i.e. thepoint of maximum rate, which is the primary differencebetween these two methods. Comparing values for theactivation energies for the 4 stages of the PET decom-position as calculated via both the Kissinger and Ozawamethods with those given in Ref. [1], we find there areobvious differences between them except at stage 2 inwhich the point of inflection lies. This suggests that it isreasonable to calculate activation energy of PET usingKissinger and Ozawa methods at the primary degrada-tion stage, stage 2. And we think that the actual valuesof activation energy are dependent not only on themathematical treatment used to evaluate the data, butalso upon such factors as sample mass, sample size, flowrate, etc.

Fig. 12. Ozawa plots of PET containing 8% CEPP at different conversions.

Table 6

Activation energies of PET using the Ozawa method

Conversion

�

Activation energy E

(kJ/mol)


r

0.05 183 0.9970

0.10 205 0.9939

0.15 245 0.9926

0.20 285 0.9763

0.35 182 0.9850

0.50 202 0.9844

0.65 208 0.9866

0.75 203 0.9899

0.80 205 0.9914

0.85 179 0.9908

0.90 79.9 0.9629

0.95 83.7 0.9667

Table 7

Activation energies of PET containing 3% CEPP using the Ozawa

method

Conversion

�Activation energy E

(kJ/mol)


r

0.05 235 0.9975

0.10 277 0.9950

0.15 248 0.9999

0.20 196 0.9912

0.35 197 0.9850

0.50 217 0.9844

0.65 217 0.9866

0.75 206 0.9899

0.80 205 0.9914

0.85 192 0.9908

0.90 671 0.9897

0.95 – –

Table 8

Activation energies of PET containing 8% CEPP using the Ozawa

method

Conversion � Activation energy E (kJ/mol) Correlation coefficient r

0.05 187 0.9958

0.10 211 0.9949

0.15 197 0.9989

0.20 268 0.9744

0.35 235 0.9999

0.50 231 0.9999

0.65 230 0.9995

0.75 225 0.9996

0.80 233 0.9997

0.85 234 0.9987

0.90 – –

0.95 285 0.9603


5. Conclusion

From a comparison of the activation energy of PETwith PET containing CEPP at different degradationstages, it can be concluded that there are no largedifferences in the products of pyrolysis. This, ofcourse, suggests that the flame retardants studied inthis investigation do not greatly influence the mechan-isms of pyrolysis, especially at the primary degradationstage.

Acknowledgements

This work was supported by the Doctorial DisciplineProgram and the Key Teacher Training Program of theMinistry of Education of China.

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Kinetics of thermal oxidative degradation of phosphorus-containing flame retardant copolyesters

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