PARTⅠ PYROLYSIS

DETERMINATION OF THE ACTIVATION ENERGY
OF THE DECOMPOSITION OF FLAME RETARDEDHIPS UNDER SPECIAL CONSIDERATION
OF THE COMPENSATION EFFECTGuido Grause*1, Jun Ishibashi1, Tomohito Kameda1, Thallada Bhaskar*2, Toshiaki
Yoshioka1
1Graduate School of Environmental Studies, Tohoku University, Aramaki Aza Aoba 6-6-11,
Aoba-ku Sendai 980-8579, Japan
2
Process, Reaction Engineering, Corrosion, Biomass Conversion Area, , Indian Institute ofPetroleum (IIP), Dehradun 248005, India.
Email-tbhaskar@iip.res.in
E-mail: grause@env.che.tohoku.ac.jpAbstract: The kinetic triplet of high impact polystyrene (HIPS) and fire retarded HIPS wasdetermined using a new method which eliminates the compensation effect after thecalculation of the activation energy and the pre-exponential factor using the Kissinger and theOzawa method. It was found that the change in the pre-exponential factor has a strong impact
on the degradation. The degradation follows nucleation kinetics described by Avrami-Erofeev
kinetics and power-laws.
1. Introduction
In order to improve the flammability of High Impact Polystyrene (HIPS), which isfrequently used for injection molding applications, brominated fire retardants, such as
decabromo diphenyl ether (DPE) and decabromodibenzyl (DDB), are used. The effect ofthese fire retardants can be improved by adding Sb2O3 as a synergist. However, the thermaldecomposition (e.g. during combustion or thermal recycling processes, such as pyrolysis) offire retarded HIPS causes the release of brominated dibenzo dioxins and furans. Since thesecompounds are a threat for health and environment, the behavior of the decomposition of
HIPS is under investigation for a long time[1-4]. One important aspect of the degradation isthe determination of the activation energy EA, the pre-exponential factor k0 and the kineticmodel, since this information allow conclusions about the pathway of the degradation.
However, the results depend strongly on the method used. One reason for this behavior is theso called compensation effect[5], which is visible in a linear relationship between the k0 andEA. The invariant kinetic parameter (IKP) method was found to be sufficient for theelimination of the compensation effect from the method of Coats and Redfern [6-7].
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We will show that the compensation effect observed in the calculation of kinetic
parameters from iso-conventional methods (i.e. Kissinger or Ozawa) can be successfully
eliminated. The results of this procedure are close to those obtained by the IKP method,
however, applicable for a wider decomposition range.
2. Experimental
For this investigation, five samples were used: HIPS, HIPS+DPE (DPE-Sb(0)),
HIPS+DPE+Sb2O3(DPE-Sb(5)),
HIPS+DDB(DDB-Sb(0)),
HIPS+DDB+Sb2O3(DDB-Sb(5)), with contents of 13 wt% fire retardant and 5 wt% Sb2O3. A Seiko
TG/TDA6200 was used in a temperature range between 50 °C and 700 °C with helium gas
flow of 200 ml min-1.
Initially, the EA and k0 values were calculated for 99 weight losses in a weight loss range
between 1 and 99 wt% by two methods, Kissinger[8] and Ozawa[9]. Four of these data pairs
were used to determine the compensation effect parameters α and β using eq. 1, resulting in
24 pairs of α and β:
lg k 0 = a + b * E Aeq. 1

These α and β pairs formed again a straight line, which was used to calculate theinvariant kinetic parameters k*0 and E*A:

a = lg k 0* - E A* * beq. 2

However, it was observed that the degradation process is in general described by morethan one kinetic triplet. The kinetic triplets obtained were used to model the degradation ofthe samples.
3. Results and Discussion
The degradation of HIPS took place between 350 and 450 °C. The thermal stability wasslightly reduced by the addition of a fire retardant and strongly affected in the presence ofSb2O3, visible in an early degradation step prior to the main degradation (Fig.1). However, thedegradation was nearly independent from the fire retardant used.

Fig. 1 TGA and DTG of the degradation of HIPS at 5 K min-1-6-

PARTⅠ PYROLYSISAs it can be seen in Fig. 2a (HIPS) and Fig. 2b (DDB-Sb(5)), the calculation of EA, using
the methods of Kissinger and Ozawa, suggests the change of the EA with the weight loss. Itcan also be seen that there is a linear dependency of k0 from EA (Fig. 2c, Fig. 2d). This
compensation effect is actually an indication that the EA observed is not the real one. As it canbe seen in Fig. 2e and Fig. 2f, the points, obtained from eq. 1 are located close to a commonregression line; however, a closer look reveals that for each sample three degradation regions
can be named.
As it can be expected from Fig. 2a, the changes in the EA for the different regions werequite small (Tab. 1); however, also for the other samples it was found that the variation in EA
was much smaller than it would be expected from the plot of EA over the weight loss. For anyof the fire retarded samples, the EA changed little in a range of about 80 wt%. The degradation,
however, was much more influenced by the change in k0. As it can be seen in Tab. 1, this isoften accompanied by a change in the kinetic model. This might be caused by changes in theproperties of the material.
Tab. 1 Activation energies EA*, pre-exponential factors k0*, and regression coefficients r,
calculated from compensation effect parameters and their weight loss range.
SampleHIPS

DPE-Sb(0)DDB-Sb(0)DPE-Sb(5)DDB-Sb(5)

range
[wt%]
1-96
1-12
13-52
53-96
1-96
1-48
49-80
81-96
1-96
1-45
45-84
85-96
1-96
1-40
41-72
73-96
1-96
1-36
37-84
85-96

Kissinger

Ozawa

*

*

EA
EA
*
2
*
lgk-lg(1-r)-lg(1-r2)
0-1-1 lgk0[kJ mol ][kJ mol ]
202
161
198
200
143
145
144
216
167
166
172
304
147
148
151
211
125
145
150
254

15.1
12.6
14.9
14.4
10.6
11.0
10.4
15.512.3
12.5
12.5
22.0
11.5
12.2
11.2
15.1
9.8
12.0
11.1
18.2

3.29
2.81
3.78
4.83
2.85
3.17
3.54
3.07
2.61
3.90
3.50
3.49
1.99
3.14
2.70
2.10
1.40
2.21
2.54
2.90

202
171
199
201
147
148
148
218
169
168
175
305
149
151
154
211
129
148
154
247

15.1
13.4
15.0
14.6
11.0
11.3
10.8
15.6
12.5
12.7
12.8
22.1
11.7
12.4
11.5
15.1
10.2
12.3
11.5
17.4

3.84
3.81
3.73
4.91
2.99
3.34
3.63
3.20
2.98
3.95
3.38
2.80
1.97
3.12
2.79
2.12
1.46
2.302.54
3.08

Reaction model

A3/2
A3P2A2P2
F1/3
A3/2
P3P4
A2/A3A2
F1/3
A3A2F0-7-The 5th ISFR (October 11-14, 2009, Chengdu, China)

Fig. 2 Degradation of HIPS (left side) and DDB-Sb(5) (right side). a) and b) EA versus
weight loss, c) and d) k0 versus EA, e) and f) compensation effect parameters α and β, g)and h) experimental and calculated TG and DTG plots.
The kinetics of each sample can be described by mechanisms after Avrami-Erofeev(A3/2, A2, A3) or power-laws (P2, P3, P4). This indicates that HIPS and fire retarded HIPS
decomposes by the formation of a small number of centers, where initially radicals are formed.
From these centers the reaction approaches by radical propagation. The results of themodeling are shown in Fig. 2g and 2h. The kinetic triplets used show a good agreement withthe experimental data.

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4. Conclusion
Kinetic parameters of the degradation of fire retarded HIPS were successfully gained bya new method, allowing the elimination of the compensation effect from iso-conventional
methods. The modeling of the TG and DTG plots showed the validity of the obtained data.
These data can be helpful for the investigation of the behavior of HIPS at hightemperature. They can be used for the calculation of the residence time of HIPS in different
reactors during thermal processes; or with a deeper knowledge of the degradation behavior, away of minimizing the evolution of brominated organic compounds might be found.
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[4] E. Jakab, Md. A. Uddin, T. Bhaskar, Y. Sakata J. Anal. Appl. Pyrolysis 2003, 68-69, 83.
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[9] T. Ozawa Bull. Chem. Soc. Jpn. 1965, 38, 1881.

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