Solid phase microextraction to concentrate volatile products

from thermal degradation of polymers

Janaina H. Bortoluzzi, Eduardo A. Pinheiro, Eduardo Carasek, Valdir Soldi

*

Chemistry Department, Federal University of Santa Catarina, Campus Universitario, UFSC, 88040-900 Floriano´polis, SC, Brazil

Received 8 October 2004; received in revised form 14 December 2004; accepted 22 December 2004

Available online 17 February 2005

Abstract

Solid phase microextraction (SPME) was used to analyse the products evolved in the thermal degradation of polypropylene (PP)

which was used as a model polymer. The degradation was performed under nitrogen at 470

C using polydimethylsiloxane and

carboxen/polydimethylsiloxane as the fibres for the product pre-concentration. The evolved degradation products were identified by
infrared spectroscopy (FTIR) and gas chromatography/mass spectroscopy (GC/MS). More than 30 evolved products associated
with alkenes (61.0%), alkanes (33.0%) and alkadienes (4.3%), were identified. In agreement with the literature, evolved products
such as 2,4-dimethyl-1-heptene, n-pentane, 2-pentene, propylene, 2-methyl-1-pentene, 2,4,6-trimethyl-1-nonene, etc., were detected
from the polypropylene thermal degradation. The results suggest that the SPME technology offers important factors such as, rapid
analysis and great efficiency in the pre-concentration of evolved products from the thermal degradation of polymers.
Ó 2005 Elsevier Ltd. All rights reserved.

Keywords:

Solid phase microextraction; Volatile products; Thermal degradation; Polymers

1. Introduction

Solid phase microextraction (SPME) has been in-

troduced as a modern alternative to traditional sample
preparation technology. Advantages such as rapid
analysis, total elimination of the use organic solvents
and the possibility to automate the sample preparation
step, have been reported

[1,2]

. Usually, an SPME

technology employs a coated fibre to extract and
concentrate analytes which are then desorbed in the
injection port of a gas chromatograph for analysis

[3,4]

.

SPME has been applied for analyses in various research
fields, such as environmental chemistry, forensic chem-
istry, and in the pharmaceutical and food industries

[5e7]

.

Despite the above applications, SPME technology

has not been extensively used to concentrate volatile
products such as those formed in the thermal degrada-
tion of polymers

[8e11]

. In general, systems such as

thermogravimetry/mass spectrometry (TG/MS)

[12]

, gas

chromatography/mass spectroscopy (GC/MS)

[12e17]

,

size exclusion chromatography/nuclear magnetic reso-
nance (SEC/NMR) and size exclusion chromatography/
matrix assisted laser desorption ionisation (SEC/MAL-
DI), have been used

[9]

. Recently, we analysed the gas

products of thermal degradation of polymers, polysac-
charides and proteins, using a tubular oven connected to
infrared equipment

[18e22]

. The gas products collected

in a cell during the degradation process were analysed
only in terms of the main absorption groups in the
FTIR.

Considering our experience and the impossibility to

use more complex systems such as TG/MS, SEC/NMR
or SEC/MALDI for analysis, in the present work the

* Corresponding author. Tel.: C55 48 331 9219; fax: C55 48 331

9711.

E-mail address:

[e-mail]

(V. Soldi).

0141-3910/$ - see front matter

Ó 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2004.12.021

Polymer Degradation and Stability 89 (2005) 33e37

www.elsevier.com/locate/polydegstab

feasibility of using a single step enrichment procedure
(SPME technology) for thermal degradation products
from polymeric materials under nitrogen atmosphere to
allow their characterization by FTIR and GCeMS was
investigated. Polypropylene (PP) was chosen as a model
polymer because it is known that the thermal degrada-
tion occurs in a single step between 300 and 500

C by

random scission of the main chain

[23]

. At the same

time, PP is a common recycling polymer which has been
studied as a function of its technological applications

[23e27]

. The main idea was to analyse the viability of

using SPME technology to pre-concentrate polymer
evolved products.

2. Experimental

2.1. Materials

Isotactic polypropylene (average M

w

ca. 250 000 g mol

ÿ1

;

average M

n

ca. 67 000 g mol

ÿ1

; density Z 0.900 g cm

3

,

melting temperature 160e165

C) was received from

Aldrich Chemical Company (St. Louis, USA).

The thermogravimetric curves for PP (not shown)

present only one stage of mass loss corresponding to
a maximum degradation rate temperature of 470

C.

2.2. SPME procedure

The SPME device and fibres of polydimethylsiloxane

(PDMS) (100 mm) and PDMS/Carboxen (75 mm) were
obtained from Supelco (Bellefonte, PA, USA). The fibres
were thermally conditioned (300

C) prior to their first

absorption in the hot port of the gas chromatograph
instrument according to the supplier’s instructions.

The thermal degradation of PP was monitored in

a tubular oven (LINDBERG/BLUE) equipped with
a quartz tube for the reaction, connected to a flow
through cell SPME, as shown in

Fig. 1

. The carrier gas

was nitrogen (flow rate of 10 cm

3

min

ÿ1

) and the

temperature was maintained at 470

C during the pre-

concentration step using SPME technology. According
to

Fig. 1

, a flow through cell made of polytetrafluoro-

ethylene (PTFE)

[28]

was connected to the exit of the

oven. The SPME fibre was placed into this cell where the
extraction was performed by simply passing the thermal
degradation products (volatile products) from poly-
propylene around the fibre. After the extraction period
(30 min), the fibre was immediately withdrawn from the
sample and introduced into the GCeMS injector.
Thermal desorption of the degradation products was
carried out in the hot port of the gas chromatograph
instrument for 5 min. After this period no significant
blank values were observed.

2.3. Infrared spectrometry

A FTIR spectrometer (Perkin Elmer, 16-PC) with

a resolution of 4 cm

ÿ1

, was used to characterize the gas

products from the PP thermal degradation. The FTIR
was connected to the tubular oven, in a similar manner
to the SPME device in

Fig. 1

. Polypropylene samples

were submitted to different temperatures (range of 400e
600

C) for thermal degradation. Samples of ca. 150 mg

were heated under nitrogen (50 cm

3

min

ÿ1

) at a heating

rate of 10

C min

ÿ1

.

2.4. Mass spectrometry analysis

The GCeMS investigations were carried out using

a Shimadzu GCeMS QP2000A equipped with a splite
splitless injector. A CPSIL 8B fused silica capillary
column of 50 m ! 0.25 mm ID and with a phase
thickness of 0.25 mm was used for all GC separations.
The temperature program used for the determination of
the degradation products from PP was as follows: the
initial temperature of 30

C was held for 5 min and then

increased to 280

C at 5

C min

ÿ1

. This temperature

was held for 1 min. The run time was 56 min. The
injector and detector temperatures were maintained at
250 and 280

C, respectively. The samples were injected

in the splitless mode and the splitter was opened after
5 min (delay time). The carrier gas used was helium at
1 mL min

ÿ1

. The spectrometer was operated in electron

impact mode (EI) with 70 eV detection volts and scan
range 41e400 m/z.

A

B

C

D

E

F

G

Fig. 1. Scheme of tubular oven/SPME system: (A) carrier gas
(nitrogen); (B) gas flow controller; (C) tubular oven; (D) SPME fibre;
(E) gas outlet; (F) enlargement of the heating region, and (G)
enlargement of the flow through cell containing SPME fibre.

34

J.H. Bortoluzzi et al. / Polymer Degradation and Stability 89 (2005) 33e37

3. Results and discussion

Chan and Balke analysed the thermal degradation of

PP using thermogravimetric data to determine kinetic
parameters such as reaction order, frequency factor and
activation energy

[23]

. Thermogravimetric curves ob-

tained under inert atmosphere (argon) and under air
showed only one stage of mass loss with a maximum
degradation temperature (T

MAX

) in the range of 435e

475

C, depending on the heating rate. In terms of

activation energy, the authors suggested the existence of
two main regions defined as: (i) region I e which
corresponds to a conversion (percentage of mass loss at
low temperatures) up to 10% and (ii) region II e which
corresponds to a conversion in the range of 10e100%
(high temperatures). Activation energy values (average)
of ca. 98 and 328 kJ mol

ÿ1

were determined for regions I

and II, respectively. The low value in region I was
attributed to the existence of weak points in the polymer
chain, whereas, the high value of region II was
associated with high degrees of random scission of the
main chain.

In this study, we analysed the products evolved from

PP thermal degradation at 470

C (the maximum

degradation rate temperature), which may be associated
with the region II described above. The large number of
volatile products evolved from polypropylene degrada-
tion identified in this work, in general, agrees with the
literature

[13,29e31]

and may be associate with a mech-

anism of random scission of the polymeric chain.

Experimentally, the PP degradation was first moni-

tored using a tubular oven connected to infrared
spectrometer. The functional groups of the evolved
products produced from the PP degradation were
qualitatively detected and identified by FTIR. In

Fig. 2

the absorption bands of the volatile products

evolved from PP degradation in the wave number region
of 3500e2500 cm

ÿ1

are shown. The bands observed at

3085, 3016 and 2970 cm

ÿ1

are related to the CeH

stretching. The band at 3085 cm

ÿ1

suggest the formation

of volatile products related to the ]CH

2

group,

characteristic of unsaturated structures.

In

Fig. 3

the absorption bands related to the volatile

products in the region 2000e400 cm

ÿ1

are shown. The

main absorption bands were associated with CH

3

(1460

and 1380 cm

ÿ1

), C]C (1660 cm

ÿ1

), CeC (910 cm

ÿ1

)

and out of the plane CH

2

(670 cm

ÿ1

). In relation to

the absorption bands described above, two important
aspects must be considered: (i) the maximum band
intensities were observed at 470

C, which was the

maximum degradation rate temperature for PP, and (ii)
volatile products associated with saturated and un-
saturated structures were evolved.

The functional groups identified by FTIR, are in

agreement with the formation of alkenes, alkanes and
alkadienes as products from the thermal degradation of

PP discussed in the literature

[13]

. At high temperatures

(O400

C), the products are formed by a mechanism

involving first the random scission of carbonecarbon
bonds, followed by intramolecular hydrogen transfer
processes.

Using SPME technology (for the pre-concentration

step) and GCeMS (for the analysis) we identified more
than 30 evolved products (up to C

15

), in the percentages of

61.0% (alkenes), 33.0% (alkanes) and 4.3% (alkadienes).

3400

3200

3000

2800

2600

500 °C

600 °C

550 °C

470 °C

400 °C

Wavenumber (cm

-1

)

Transmittance (a. u.)

Fig. 2. FTIR spectra for gas products evolved in the PP degradation in
the 3500e2500 cm

ÿ1

region.

2000

1800

1600

1400

1200

1000

800

600

400

400 °C

470 °C

600 °C

550 °C

500 °C

Transmittance (a. u.)

Wavenumber (cm

-1

)

Fig. 3. FTIR spectra for gas products evolved in the PP degradation in
the 2000e400 cm

ÿ1

region.

35

J.H. Bortoluzzi et al. / Polymer Degradation and Stability 89 (2005) 33e37

The same aliphatic compounds have previously been
identified by Bockhorn et al.

[25]

in the percentages of

84.8% (alkenes), 7.6% (alkanes) and 7.6% (alkadienes).
Amorin et al.

[13]

identified various compounds with

M

w

!

240 in the pyrolysis of isotactic PP, in the

percentages of 84.5% (alkenes), 9.4% (alkanes) and
1.9% (alkadienes). The same authors identified com-
pounds in the pyrolysis of atactic PP in the percentages of
77.5% (alkenes), 12.0% (alkanes) and 1.0% (alkadienes).
The differences between our results and those described in
the literature are probably associated with experimental
conditions, such as degradation temperature, atmo-
sphere, sample and material (PP) characteristics.

The main evolved products which have been identi-

fied by different authors

[13,29e31]

include 2,4-di-

methyl-1-heptene,

n

-pentane,

2-pentene,

propylene,

2-methyl-1-pentene, 2,4,6-trimethyl-1-nonene, etc. The
amount (percentage) of products evolved was strongly
dependent on the experimental conditions, mainly the
temperature. For example, the percentage of propylene
obtained by Amorin et al.

[13]

in a temperature range

500e900

C, was ca. 38%. On the other hand, Kiang

et al.

[29]

obtained propylene percentages in the range

15%e28%, when the temperature changed from 388 to
438

C. In this study, despite the differences in the

reaction conditions and methodology, practically the
same above described products (main products) were
identified. In

Fig. 4

, the MS spectra of two identified

compounds: 2,4-dimethyl-1-heptene (

Fig.

4

A) and

5-ethyl-2-methyl heptane (

Fig. 4

B), are shown as

examples. In agreement with the literature, 2,4-dimethyl-
1-heptene was the main product identified in poly-
propylene thermal degradation. Our data indicate an
amount of ca. 45 wt.% for this compound, which is
close to the 42 wt.% determined by Chien and Kiang

[32]

and 40 wt.% determined by Kiang et al.

[29]

. The

5-ethyl-2-methyl heptane accounted for only 3% of the
total identified volatile products. At high temperatures
(O400

C), the appearance of evolved 2,4-dimethyl-1-

heptene must be favoured by the reaction mechanism
associated with the random scission of carbonecarbon
bonds, followed by intramolecular hydrogen transfer
processes.

In summary, the above discussed results suggest the

viability of the use of SPME technology for the pre-
concentration of products evolved from the thermal
degradation of polymers. Rapid analysis and the
efficiency of pre-concentration were the most important
factors associated with the PDMS and PDMS/Carboxen
fibres used in the present study. The products evolved in
the thermal degradation of polypropylene (model poly-
mer) identified using FTIR and GCeMS were in full
agreement with the literature, suggesting that SPME
technology may be extensively used in polymer degra-
dation reactions. Although it is known that PDMS
fibres can be used for a wide range of applications, the
results obtained in this study suggested that the PDMS/
Carboxen fibre was more sensitive for volatile com-
pounds than PDMS.

Acknowledgments

This work was supported by the Conselho Nacional

de Desenvolvimento Cientı´fico e Tecnolo´gico (CNPq)
and Federal University of Santa Catarina (UFSC).

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