Temperature Dependent Pyrolytic Product Evolution Profile for Binary Mixtures ofLow density polyethylene and Poly (ethylene terephthalate)
Ujwala Hujuri1, Aloke K. Ghoshal2, and Sasidhar Gumma2*
1Department of Chemical Engineering, Assam Engineering College,Guwahati, INDIA 781013
2Department of Chemical Engineering, Indian Institute of Technology

Guwahati, Guwahati, INDIA 781039
email: s.gumma@iitg.ernet.in

Abstract
The present work aims at evaluating temperature dependency of product evolution from pyrolysis of binary mixtures of lowdensity polyethylene (LDPE), and polyethylene terephthalate (PET). Product distribution studies have been reported for threedifferent mixture compositions (20:80, 50:50, and 80:20) in the temperature range of 300-600oC, for a wide range ofhydrocarbon products (C5-C44) and the same has been explained with mechanistic pathways of decomposition. The
pyrograms have been analyzed in terms of mole fractions of different products evolved at various pyrolysis temperatures.
Enhancement in the production of lighter fractions also support this observation. The following reaction types are considered
to explain the degradation mechanism of the mixtures: (a) heterolytic cleavage of PET to generate acid ends and olefin
ends; (b) homolytic cleavage of LDPE to generate free radicals; (b)attack of these free radicals on PET fragmented chain; (c)intramolecular and intermolecular hydrogen transfer to form volatile products, oligomers and regeneration of radicals, acidends or, olefin-ends.

Keywords: pyrolysis; gas chromatography (GC); poly (ethylene terephthalate) (PET); low density polyethylene (LDPE)

1.

Introduction

Pyrolysis of plastics results in the production of liquid andgaseous hydrocarbons, which have remarkable potential
to be upgraded to fuels or petrochemical feedstock.
Interaction between the constituent plastics is a very
important aspect to be considered to examine theefficacy of any commercial pyrolysis process involving
mixed plastics. This is due to the fact that interaction
between the fragmented chains and radicals of thecomponent plastics at the prevailing high temperature ofpyrolysis might lead to a different product distribution
from that of their single component pyrolysis. In ourearlier study, interaction between the plastics wasobserved in their thermal decomposition experiments [1].
The concept behind the present investigation is toenumerate the interaction between LDPE and PET interms of product evolution from their pyrolysis. In thisstudy, we have carried out degradation of binary mixturesof LDPE and PET in a Thermo Gravimetric Analyzer(TGA) and analyzed the products in a Gas
Chromatograph (GC) equipped with FID detector. We
have monitored the product evolution (C5-C44) at fivedifferent temperatures from 300-600oC. The GC results
were used to study the product distribution and correlate
the same with the decomposition mechanism.

2.

Materials and Methods

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%. LDPE
was obtained from National Chemical Limited, Gujarat,
India. It has a melt flow index of 5.13 and melting point of
126.9oC.
Binary degradation experiments for three different
compositions (20:80, 50:50, 80:20) of the mixtures werecarried out in a TGA in an inert environment at a heating
rate of 10oC/min. Once the sample reached the desired
temperature (300, 400, degradation, Tmax (temperaturecorresponding to the maximum rate of decomposition),
500 or 600oC), 1 ml of the evolved sample was collected
and injected into the GC. Calibration of the GC was doneusing two reference petroleum standards (ASTM D3710
and ASTM D5442) containing 27 known hydrocarbons(C5-C44). Mole fractions of the eluent hydrocarbons
were calculated using the response factors obtained fromthe calibration procedure.

3.

Results and Discussion

The characteristic Tmax for the single plastics and theirbinary mixtures are listed in Table 1. PET is thermallyless stable of the two, which could be due to thepresence of two heterogeneous linkages between O and
C in its backbone [1]. It can be seen that addition of PET

has resulted in an enhancement in the overall
decomposition process.

regenerated in the process. Decarboxylation of TPA
leads to the formation of benzoic acid, which on
subsequent decarboxylation gives benzene (Scheme 3).

Table1. Tmax for the single and the mixed plastics
LDPE (%)
205080100PET (%)
10080
5020-Tmax (oC)435
434
439459470

Figure 1 compares the evolution pattern of hydrocarbons
obtained from the plastics (50:50 composition), astemperature changes from 300oC to Tmax. It is worthwhile
to see that Tmax of the mixtures is a function ofcomposition of the single plastics. The repeat unit of PET
can be written as:
COPhCOO (CH2 )2O

At 300oC, relative yield of higher hydrocarbons (C16 andabove) are more in PET as compared to C5-C10 fraction,
whereas C16-C22 is the major fraction for LDPE.
At Tmax, yield of C5-C10 fraction is more in case of thethe mixture than that from LDPE alone, which could bejustified by the possible interaction between the radicals
and the fragmented chains of the constituent plastics inthe mixture. The major products obtained from themixtures are likely to be isopentane, pentene, benzene,benzoic acid and a host of other aliphatic and aromatichydrocarbons.

where, Ph:

H2CnHOCH2CH O

H2CC
H2LDPE is an aliphatic polymer having the repeat unit
CH2CH2CH2OOCPh C

H2CC
H2OCPhCOOH

CH2 CHOOCPhCOO

(I)

(II)

H2C

H2CC
H2H2C

LDPE

H2C

H2CC
H2C
H2CH2RPnScheme 1H2CHC
H2CH2OCHCH2OOCPhCOOH
(I)

H2C

OCHCH2OOCPhCOOH

C
H2(III)

CH3Scheme 2[-CO2 ]OCHCH2 OOCPhCOOH

HOOCPhCOOH
(TPA)

(III)

[-CO2]

HOOCPhH
Benzoic acid

Scheme 3

4.
Fig.1. Fractional evolution of the products for the plastics
at 300oC andTmax
PET is a condensation polymer which essentiallydecompose by ionic (heterolytic) mechanism formingacid end (I) and olefin end (II); wheras, LDPE is anaddition polymer which decompose by free radical
mechanism resulting into two primary radicals (Rp)
(Scheme 1). Rp formed by random scission of LDPE
might abstract a hydrogen from methylene group of theacid end, transferring the radical center to the PET
fragmented chain (Scheme 2). Thus, PET can undergo
radical reactions at a relatively lower temperature inpresence of LDPE, which is otherwise possible only at ahigher temperature (at or above 400oC). The radical (III),
thus generated can undergo β-CH hydrogen transfer
giving rise to terephthalic acid (TPA). Acid end, II isConclusions

Pyrolysis of the binary mixtures of PET and LDPE is afunction of temperature as well as the mixture
composition. Proper selection of operating temperatureand mixture composition is thus essential for efficientdesign of a pyrolysis procedure of a mixture to havedesired yield of value added products.

References[1] Hujuri U., Ghoshal A.K., Gumma S., Modelingpyrolysis knetics of plastic mixtures. Polym. Degrad.
Stab. 93 (2008) 1832-1837.

Benzene