5-7 OCTOBER

ISFR

2011

67

TOLEDO (SPAIN)TEMPERATURE DEPENDENT PYROLYTIC PRODUCT EVOLUTION PROFILE FOR
POLY (ETHYLENE TEREPHTHALATE)
Sasidhar Gumma, Ujwala Hujuri and Aloke K. Ghoshal
Department of Chemical Engineering, Indian Institute of Technology
Guwahati, Guwahati, INDIA 781039
Email: s.gumma@iitg.ernet.in

Abstract
The structure and purity of the plastics used in the degradation experiments decide the type of products obtained therefrom.
A concurrent study of products distribution and the underlying reaction mechanism of their degradation, helps one tounderstand the pyrolysis behavior of plastics better. In this work, a detailed gas chromatographic study of pyrolysis productsof poly (ethylene terephthalate) (PET) is carried out in the temperature range of 200-600oC. The pyrograms have been
analyzed in terms of mole fractions of different products evolved at various pyrolysis temperatures. At low temperatures(200-300oC), yield of lighter hydrocarbons (C5-C10) is low. It gradually increases until maximum decomposition temperature(446oC) and decreases thereafter. PET degradation can be explained by two possible routes viz., ionic and radicalmechanism. At low temperature it proceeds mainly by ionic mechanism. The following reaction types are considered toexplain its degradation mechanism: (a) heterolytic main chain cleavage to form olefin-end and acid-end structure; (b) intra orintermolecular ester-interchange reactions to yield cyclic oligomers; (c) intramolecular hydrogen transfer to form volatile
products and regeneration of the acid-ends or, olefin-ends; (d) decarboxylation and intramolecular elimination of compounds
like acetaldehyde and acetylene; and (e) hydrogen abstraction and acetylene addition (HACA) mechanism for the formationof polycyclic aromatic hydrocarbons (PAH).

Keywords: gas chromatography (GC); poly (ethylene terephthalate) (PET); product analysis; pyrolysis;
thermogravimetric analysis (TGA)

1. Introduction

2. Materials and Methods

With the increase in consumption of PET in thepackaging sector, there is a parallel increase in thevolume of waste PET. This has triggered the importanceof study of pyrolysis of PET, which can convert theplastic to valuable hydrocarbons. The structure of PETindicates possibility of production of significant amount ofaromatic hydrocarbons as well as PAHs. Literaturereports that decomposition of PET produces about thesame quantitites of gas and liquid with a proportion ofsoilds around 10% [1,2]. The gases contain carbon
oxides (nearly 90%) [1]. In the oils, benzene and toluene
are the major products [2]. Montaudo et al. reported
formation of cyclic oligomers by doing structuralcharacterization of the products obtained by isothermal
degradation of PET in the temperature range of 270370oC [3].

PET used in the study was virgin PET (AS-40 bottlegrade) supplied by South Asian Petrochem Limited,
India. It has a melting point of 248.4oC, heat of fusion of
44.91 J/g and a percentage crystallinity of 39.05%.

In this study, we have carried out degradation of PET in a
TGA and analyzed the product in a GC equipped with
FID detector. We have monitored the product evolution(C5-C44) at six different temperatures from 200oC (wherethere is a negligible product formation) through 600oC(end of degradation). The GC results were used to study
the product distribution and correlate the same with thedecomposition mechanism.

interiores libro.indd 67

Degradation experiments were carried out in a TGA in an
inert environment at a heating rate of 10oC/min. Once thesample reached the desired temperature (200, 300, 400,435 (maximum degradation temperature, Tmax), 500 or
600oC), 1 ml of the evolved sample was collected andinjected into the GC. Calibration of the GC is done using
two reference petroleum standards (ASTM D3710 andASTM D5442) containing 27 known hydrocarbons (C5C44). Mole fractions of the eluent hydrocarbons werecalculated using the response factors obtained from thecalibration procedure.

3. Results and Discussion
A typical derivative theromogravimetric (DTG) curve forrate of degradation dα of PET is shown in Figure 1. It isdTevident from this figure that degradation of PET occurs
over a wide temperature range, and Tmax for PET isaround 435oC.

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6th International Symposium on Feedstock Recycling of Polymeric Materials
OHScheme 1CH O

OCPh CO(CH2)2HOCO

OCPhCOOH

CH2 CHOOCPhCOO

(I)

(II)

CH2(H2C)2OOOOOCC OCCOOO(H2C)2(CH2)2OO CCOO

Scheme 2Fig. 1. DTG curve for PET
GC analysis reveals a change in the evolution pattern ofhydrocarbons with temperature (Figure 2). PET falls
under the category of condensation polymers,
decomposition pathways of which are dominated by thepolarity and reactivity of the functional groups, and at low
temperature it essentially degrade by ionic (heterolytic)
mechanism (scheme 1) forming acid end(I) and olefinend (II) . The repeat unit of PET can be written as:
COPhCOO (CH2)2OOOOOOOOOOOOO
Cyclic Trimer

where, Ph:ab
OCPhCOO CH2CH2 OOCPhCOOHnAs depicted in Figure 2, at low temperature (200-300oC),
C5-C10 produced is insignificant, which could be
attributed to the extra stability of PET due to resonancestabilized structures in the chain backbone. The higherab(I)HOOCPhCOOH

OCPhCOOH(I)
CH2(TPA)
OCPhCOOCH

CHOOCPhCHOOH

CH2CH2(II)(MVT)

CHOOCPhH

HOOCPhH[-C2H2]

Vinyl benzoate

Benzoic acid
Scheme 3

C2H2[-CO2]
Benzene(in four consecutive steps)

HBenzyl radical
Scheme 4

4. Conclusions
Fig.2. Temperature dependency of the evolved products(C5-C44) from PET
hydrocarbons detected might correspond to the linear
and cyclic dimers (C20) and trimers (C30), which could
be present in the PET chain. Moreover, intramolecularester exchange reactions might also - lead to theirformation (scheme 2). As the temperature approaches
Tmax, significant evolution of C5-C10 is observed. Given
the structure of PET, benzene, toluene, terephthalic acid(TPA), monovinyl terephthalate (MVT) would be themajor products, formed by β-CH hydrogen transfer(scheme 3). Beyond 500oC, relative yield of first threefractions is more or less the same, while C32-C44
fraction is almost insignificant. In all the TGA
experiments, a black residue is obtained, which could be
due to the formation of PAHs, which are fused aromaticrings. Their formation is likely at high temperature (at orabove 400oC) by HACA mechanism (scheme 4).

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Products obtained from pyrolysis of PET range fromsimple acetylene molecules to complex PAHs. Its thermal
degradation mainly occurs by ionic mechanism.
However, at high temperature radical reactions are likely
to occur. Temperature plays a crucial role. At Tmax,
maximum evolution of volatiles are observed. Benzene,
MVT, TPA are the major products obtained at T max.

References[1] Williams P.T. and Williams E.A. Recycling of plastic
waste by pyrolysis. J. Inst. Energy 71 (1998) 81-93.
[2] Kaminsky W. and Sinn H. In: Brandrup B.M.; Menges
G. (eds).Recycling and Recovery of Plastics, pp. 434,
Munich: Hansen, 1996.
[3] Samperi F., Puglisi C., Rossana A., Montaudo G.
Thermal degradation of poly(ethylene terephthalate) atthe processing temperature. Polym. Degrad. Stab. 83(2004) 3-10.

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