5-7 OCTOBERISFR2011

37

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

Abstract
Pyrolysis of mixed plastics is a very complex phenomenon, and the feed composition as well as the structure of thecomponent plastics have a pronounced effect on their degradation products . Interaction between the fragmented chains andradicals of the component plastics at the prevailing high temperature of pyrolysis leads to a different product distribution fromthat of their single component pyrolysis. In the present work, detailed gas chromatographic studies on pyrolysis products ofbinary mixtures of poly (ethylene terephthalate) (PET) and polypropylene (PP) are reported in the temperature range of 300600oC. The pyrograms have been analyzed in terms of mole fractions of different products evolved at various pyrolysis
temperatures. Synergistic effects are evident from the fact that addition of PET to PP, enables its degradation at a relatively
lower temperature. 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 acidends and olefin ends; (b) homolytic cleavage of PP 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 andregeneration of radicals, acid-ends or, olefin-ends.

Keywords: mixtures; gas chromatography (GC); poly (ethylene terephthalate) (PET); polypropylene (PP); pyrolysis

1. Introduction
A vast amount of work in plastics pyrolysis has beenreported in the literature and research is continuously
growing in search of innovative ways to narrow down
product distribution to the desired range by appropriate
selection of operating parameters, catalyst etc.
Westerhout et al. observed that polymer mixtures behave
quite similarly to pure polymers and claimed that
additivity rule is the correct approach to the problem [1].
However, interactions between the component plastics
cannot be completely ruled out. This is a very important
aspect to be considered to examine the efficacy of any
commercial pyrolysis process. The concept behind thepresent investigation is to enumerate the interaction
between PET and PP in terms of product evolution fromtheir pyrolysis. In this study, we have carried out
degradation of binary mixtures of PET and PP in a
Thermo Gravimetric Analyzer (TGA) and analyzed theproducts in a GC equipped with FID detector. We havemonitored the product evolution (C5-C44) at five different
temperatures from 300-600oC. The GC results were used
to study the product distribution and correlate the samewith 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 ofinteriores libro.indd 37

44.91 J/g and a percentage crystallinity of 39.05%. PP(Trade Name: Koylene ADL AS030N) was obtained fromIndian Petrochemicals Corporation Limited, Vadodara,
India. It has a melt flow index of 3.0, melting point of
175.6oC, heat of fusion of 62.38 J/g and a percentage
crystallinity of 32.83.
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, maximum degradation
temperature, Tmax, 500 or 600oC), 1 ml of the evolved
sample was collected and injected into the GC.
Calibration of the GC was done using two referencepetroleum standards (ASTM D3710 and ASTM D5442)
containing 27 known hydrocarbons (C5-C44). Mole
fractions of the eluent hydrocarbons were calculatedusing the response factors obtained from the 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 [2]. Difference between their thermal
stability is not much; nonetheless addition of PET has
resulted in an enhancement in the overall degradation
process.

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6th International Symposium on Feedstock Recycling of Polymeric Materials

Table1. Tmax for the single and the mixed plastics
PP (%)
205080100

PET (%)
10080
50
20-Tmax (oC)
435
432
434
440
446

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

It is evident from the figure that evolution of C5-C10hydrocarbons are enhanced in the mixture as comparedto the single plastics. At Tmax, relative yield of C5-C10
fraction is the highest for the mixture, 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.
OHO

CH2(I)
H2C

H2C

where, Ph:

(II)

H2C

CH

CH

CH

CH

CH3CH3CH3CH3H2C

H2C
CH

CH

CH3CH3Rsn

CH
CH3Rp

PPScheme 1

PP is an aliphatic polymer having the repeat unit
CH2(CH3)CH2CH2 CHOOCPhCOO

OCPhCOOHCH O

OCPh Cn
H2C

H2C

H
CH

CH
CH3OCHCH2OOCPhCOOH(I)

CH3H2C

H3C

RpCH

OCHCH2OOCPhCOOH

CH

CH3CH3(III)AlkaneScheme 2[-CO2]OCHCH2 OOCPhCOOH(III)HOOCPhCOOH(TPA)

HOOCPhH
Benzoic acid[-CO2]Benzene

Scheme 3

4. Conclusions

Fig.1. Fractional evolution of the products for the plastics
at 300oC andTmax
PET is a condensation polymer which essentiallydegrade by ionic (heterolytic) mechanism forming acidend(I) and olefin end (II); wheras, PP is an addition
polymer which degrade by free radical mechanism
resulting into a primary radical (Rp) and a secondary
radical (Rs) (Scheme 1). Rp formed by random scission ofPP might abstract a hydrogen from methylene group of
the acid 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 PP, 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) and acid end, II isregenerated in the process. Decarboxylation of TPA
leads to the formation of benzoic acid, which on
subsequent decarboxylation gives benzene (Scheme 3).
At 300oC, relative yield of higher hydrocarbons (C12 andabove) are more in PET as compared to C5-C10
fraction, whereas C12-C15 is the major fraction for PP.

interiores libro.indd 38

The CSBR is suitable for obtaining high yields of liquid
at moderate temperatures. Temperature has animportant effect on product distribution by increasing thearomatization of the volatile fraction and the gas yield.
Moreover, the quality of the solid fraction (adulteratedcarbon black) is improved by operating at hightemperatures. The liquid fraction is of suitable quality for
its use as fuel or it can be an interesting feedstock for arefinery, especially for the hydrocracking process.
Pyrolysis oil contains interesting products in highconcentrations depending on the operating conditions,such as limonene.

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Herriksen. Devolatilization characteristics of large
particles of tyre rubber under combustion conditions.
Fuel 85 (2006)1335–1345.
[2] A.M. Cunliffe and P.T. Williams. Composition of oils
derived from the batch pyrolysis of tyres. J Anal Appl
Pyrolysis 44 (1998) 131–52.
[3] W. K aminsky and C. Mennerich. Pyrolysis ofsynthetic tire rubber in a fluidised-bed reactor to yield
1,3-butadiene styrene and carbon black. J Anal Appl
Pyrolysis 58 (2001) 803–811.

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