ON THE MOLECULAR MASS OF PYROLYSIS OILS FROM POLYOLEFINS
M. Brebu1,*, I. Bunia, M. Silion and K. Murata2
1“Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania
2e-mail: bmihai@icmpp.ro; phone: +40 232 217454; fax: +40 232 211299

Abstract
Pyrolysis of polyethylene (PE), polypropylene (PP) and polystyrene (PS) was performed at 420, 380 and 360 °C
respectively and the obtained oils were analised by GC-MSD and Q-TOF MS. The GC-MSD analysis showed typicalchromatograms and distribution of major compounds in pyrolysis oils, with homologies series of olefins and parafins up ton-C30 for PE and oligomer series for PP (up to P7) and PS (up to S3). However the Q-TOF MS analysis showed that thepyrolysis oils of studied polyolefins also contain much heavier compounds with molecular mass up to around 1 000. Thisrises the question of the origin of high molecular mass compounds and a possible explanation could be theheterogeneous chain-end scissions occuring at liquid – gas interface inside pyrolysis reactor.

Keywords: PE, PP, PS, Q-TOF MS, chain-end scission mechanism

Pyrolysis of polymers have beed extensively studied andthe identification of the thermal degradation mechanism
was among the most important subject of interest
envisaged by researchers. K. Murata proposed amacroscopic mechanism for thermal degradation ofpolyolefins, consisting on two main reactions.[1,2]Random scissions occur in condensed phase (polymermelt) strongly decreasing the length of initialmacromolecular backbone while chain-end scissions
liberate volatile fragments that are collected ascondensable products. It was suggested that the chainend scissions occur at the gas-liquid interface since thesteady state degradation of polyolefins was affected not
only by the liquid phase condition but also by the gas
phase condition inside reactor. This proposed idea wassustained by thermodinamic and kinetic data as well as
by global characterisation of degradation products by gas
chromatography and by gel permeation chromatography.
Here we report our preliminary results on thecharacterisation of pyrolysis oils from polyolefin by GCMSD and by Q-TOF MS, the latter technique opening
new insights on the molecular mass of the producedcompounds

2. Materials and Methods
Pyrolysis of about 0.5g commercial polyethylene (PE),polypropylene (PP) and polystyrene (PS) was performed
at 360, 380 and 420 °C respectively in a semi-batchprocess using a tubular glass reactor of 15 cm length and1 cm internal diameter. The upper 5 cm part of thereactor was not heated, being kept outside the electricfurnace. The volatile degradation products werecondensed and the collected oils were analysed by GCMSD and by Q-TOF MS

3. Results and Discussion
S140P3PEPPPS35

GC-MSD peak area , %1. Introduction

302520S2P415P210S3P550481216202428Carbon numberFig. 1. C-NP gram of pyrolysis oils of PE, PP and PS.
The GC-MSD analysis of pyrolysis oils showed typicalcomposition for studied polyolefins, with homologues
series of paraffins and olefins (C4 – C30) for PE,
propylene oligomers (P2 – P7) for PP and styrene
oligomer structures (S1 – S3) for PS. The GC analysis islimited to mixtures of volatile compounds, the instrument
operating at maximum 300 °C, which corresponds to theboiling point of n-C17 hydrocarbons. On the other handthe pyrolysis temperatures utilised in this study
correspond to the boiling points of n-C22 – n-C25hydrocarbons. However the GC analysis was able toidentiffied compounds up to n-C30. This rise the questionof the nature and origin of the compounds with higher
molecular masss in the pyrolysis oils.
Direct analysis of pyrolysis oils by injecting them in theelectrospray ion source of Q-TOF MS offer double
advantage compared to GC-MSD analysis, namely i)volatilisation is not needed therefore compounds withhigh molecular mass can be analysed and ii) lowionisation energy is involved therefore molecular mass ofcompounds can be determined.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

PE oilboiling points in liquid products that normally should notleave the reactor.

303

counts

183

1001
0.95

0.85
0.8
80

113

0.9

0.75

201

255
271

0.7
0.65
0.6
60
0.55

4. Conclusions

0.5
0.45

40

1075

931

692

0.05

963

804

844

0.1(a)

725

614

431

475

387

519

563

0.2
20
0.15

341

0.3
0.25237

0.35

123
138
152 1670.4(m/z)

00

100

200

300

400

500

600

700

800

900

1000

1100

271

100
counts201

PP oil

183

255

109
123
138
152
167

209
237

80

303

60

1345

700

(b)

1044
1075

600

931
963
996

500

861

697

400

804

580
614

725

411
439
463
481
520

293

323

377
393

20

341

40

0

counts

300

800

900

1000

1100

1200

1300

(m/z)

1400

PS oil
224

100

200

246

100

946

913

870

649
677
693
721
737
766
781
810
825

561

605
633

545

502

517

438
467
473

378
410

260
211

120
135
160

20

190

40

322

285
299

60

348

334

80

(m/z)

001002003004005006007008009001000

Fig. 2. Q-TOF MS spectra of polyolefins pyrolysis oils.
The strongest signals from the Q-TOF MS analysis of thePE oil (Fig. 2.a) were in the range of m/z 183 – 303 (withthe except of the signal at m/z 113 which belongs tooctene) that corresponds to n-C13 – n-C20 hydrocarbons,
which are actually at the tail of the large PE peak of theNP-gram in Fig. 1. However signals at high m/z values
up to 1 075 were found, suggesting that the PE pyrolysis
oil also contains heavier hydrocarbons up to C75.
PP oil gave more Q-TOF MS signals compared to PE oil,
due to its more complex composition, expected from thenumerous tertiary carbon atoms in polypropylene. For PP
pyrolysis oil the highest m/z signal was found at 1 345,
which corresponds to a C96 unsaturated structure while
for PS pyrolysis oil the highest signal was found at m/z
946 which corresponds to a P9C17 nonamer structure (Pstands for phenyl units and C for aliphatic units in thestructure of the degradation product).
Identification of compounds with molecular mass around
1 000 in pyrolysis oils from polyolefins is of great interest
since their volatility is too high for them to pass from thereactor content to the condensed oils through the vapour
phase by volatilisation and condensation process.
A possible explanation for the presence of compoundswith such high molecular mass in pyrolysis oils could be
the heterogeneous chain-end scissions of the primarydegradation products, occurring at the liquid – gasinterface inside the reactor, as proposed by Murata
mechanism.
The chain-end scission at gas-liquid interface could
produce long-chain molecules directly in the gas phase.
However as soon as they are formed, they condense as
liquid droplets (very fine particle like a fog) due to atemperature drop from the liquid to the vapour phase inreactor [2]. Very small droplets are transported out ofreactor by the upward stream of volatile products. Thiseffect explains the presence of compounds with high

The Q-TOF MS analysis of pyrolysis oils obtained at 360,
380 and 420 °C from poystyrene, polypropylene andpolyethylene respectively showed the presence of heavy
compounds with molecular mass up to about 1 000 for
PE and PS and up to 1 350 for PP. These stand for C75
structures in PE oil, C96 structures in PP oil and styrene
nonamer structures (P9C17) in PS pyrolysis oil. The
presence of such heavy compounds in pyrolysis oils
might be explained by the macroscopic degradation
mechanism proposed by Murata, which consider that thechain-end scission responsible with formation of volatilecompounds occur at gas-liquid interface inside reactor.
However more studies are to be performed to confirm thepresence of heavy compounds in pyrolysis oils fromother polymers too.

References[1] K. Murata, Y. Hirano, Y. Sakata and Md. A. Uddin. J.
Anal. Appl. Pyrolysis 65 (2002) 71-90.
[2] K. Murata, K. Sato and Y. Sakata. J. Anal. Appl.
Pyrolysis 71 (2004) 596-589.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013