Distributed activation energy model (DAEM) of thethermal degradation of flame retarded HIPS
Guido Grause 1 ), * , Jun Ishibashi 1 ) , Tomohito Kameda 1 ) , Thallada Bhaskar 2) ,Toshiaki Yoshioka 1)
1)
Graduate School of Environmental Studies, Tohoku University
2)
Catalytic Conversion Process Division (CCPD), Indian Institute of Petroleum (IIP), India

The distributed activation energy model (DAEM) of the degradation of high impact
polystyrene (HIPS) showed the effect of fire retardants and Sb 2 O 3 . The addition ofthese compounds led to the broadening of the activation energy range. While pure
HIPS degraded to 70% in a range between 200 and 210 kJ mol -1 , there was no favored
activation energy region in the presence of Sb 2 O 3 .
Introduction
Plastics are used in almost all areas of life. Since they consist mainly of carbon andhydrogen, they also show a high flammability. In order to prevent plastics from burning fire
retardants are added. Many common fire retardants contain brominated compounds. These
compounds are very helpful in the prevention of fires, however, cause trouble after use.
Brominated fire retardants have to be removed during the recycling process in order to obtaina valuable product that can be used as a feedstock in the chemical industry.
There are many studies concerning the degradation of fire retardants [1], however, few
investigations were carried out studying the degradation behaviour from the point of view of
the activation energy. The DAEM is a powerful tool for the determination of the kinetic
properties of a material. It allows the identification of different degradation steps and also thedetermination of the activation energies of these steps. It is possible to use the kinetic dataobtained to recalculate the weight loss for any temperature profile. Combined with theknowledge of the evolved products (e.g. brominated products) at any stage allows theimprovement of existing dehalogenation processes.
Experimental
The development of a DAEM is shown in this presentation using HIPS with and without
fire retardant as an example. Two different fire retardants, decabromodiphenylether (DPE)and decabromodibenzyl (DDB), were added to the HIPS. The effect of Sb 2 O 3 was also
investigated.
HIPS was degraded by TGA at four different heating rates (2.5, 5.0, 7.5 and 10.0 K
min -1 ). The activation energy E A and the frequency factor k were determined after the methodof Miura [2] using an interval of 1% weight loss.
Results and Discussion
It was observed for all samples, that logk showed a linear dependence on E A . However,E A was not rising over the whole temperature range. In certain areas of the degradation, E A
was decreasing. The infections can be used to determine the degradation steps of the material.

HIPS degraded in a single degradation step with a peak at 210 kJ mol -1 (Fig. 1).
70 wt% of the material was degraded between 200 and 210 kJ mol -1 . Therefore, E A wassmaller than for polystyrene [3] or butadiene rubber [4].
The addition of DPE or DDB showed similar effects on the degradation of HIPS. The
degradation was slightly delayed in the beginning due to a scavenger effect of the fireFig. 1 DAEM of HIPS

Fig. 2 Experimental and calculated TGA plotof HIPS in the presence of DDB and Sb 2 O 3retardant. When the fire retardant itself started to degrade, HIPS underwent a radical induceddegradation. Since weak links were degraded during the early stage, the degradation wasterminated at a higher temperature than for pure HIPS. About 25 wt% were degraded during asingle peak at 178 and 192 kJ mol -1 for DPE and DDB, respectively.
The addition of Sb 2 O 3 amplified the effect of the fire retardant. The degradation of thefire retardant was accelerated. The end of the HIPS degradation shifted even more to a highertemperature. A main E A as for the Sb 2 O 3 free samples was not observed anymore. The DAEM
showed a broad distribution of E A .
The kinetic data were used for the recalculation of the TGA plot. The discrepancy
between experimental and calculated data was in general below 2.7 K. Just in the presence ofDDB and Sb 2 O 3 , the maximum discrepancy rose to 5.6 K.
Conclusions
The DAEM was a powerful tool in the investigation of the degradation of HIPS with andwithout fire retardant. It was shown that the material did not degrade homogenously, but
accompanied by the change of the E A . However, 70 wt% of HIPS and about 25 wt% in thepresence of DPE or DDB were degraded in a narrow E A range. The addition of Sb 2 O 3 led tothe broadening of E A distribution without significant E A peak.
It is obviously that the change in the reaction mechanism caused also the change in theslope of the E A /lgk relationship. This fact can be used to identify the range of reaction steps
more precise as it is possible by TGA/DTA.
References[1] E. Jakab, Md. A. Uddin, T. Bhaskar, Y. Sakata: Thermal decomposition of flame-retarded
high-impact polystyrene, J. Anal. Appl. Pyrolysis 68-69 (2003) 83.
[2] K. Miura, T. Maki: A Simple Method for Estimating f(E) and k 0 (E) in the Distributed
Activation Energy Model, Energy Fuels 12 (1998) 864.
[3] A. K. Burnham, R. L. Braun: Global Kinetic Analysis of Complex Materials, Energy Fuels
13 (1999) 1.
[4] P. T. Williams, S. Besler: Pyrolysis-thermogravimetric analysis of tyres and tyrecomponents, Fuel 74 (1995) 1277.

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