Post on 31-Jan-2023
The effect of some fillers on PVC degradation
Tamer Karayildirim a, Jale Yanik a,*, Mithat Yuksel b, Mehmet Saglam b,Cornelia Vasile c, Henning Bockhorn d
aDepartment of Chemistry, Faculty of Science, Ege University, 35100 Izmir, TurkeybDepartment of Chemical Engineering, Faculty of Engineering, Ege University, 35100 Izmir, Turkey
cDepartment of Physical Chemistry of Polymer, ‘‘P. Poni’’ Institute of Macromolecular Chemistry, 700487 ASI, Romaniad Institut fur Chemische Technik, Universitat Karlsruhe (TH), 76131, Germany
Received 5 September 2004; accepted 27 April 2005
Available online 13 June 2005
www.elsevier.com/locate/jaap
J. Anal. Appl. Pyrolysis 75 (2006) 112–119
Abstract
The HCl scavenging effect of particulate fillers such as Red Mud (RM), CaCO3 and dolomite on the thermal degradation of PVC was
investigated by thermogravimetry/mass spectrometry (TG/MS). It was found that, in the presence of carbonates, the peak temperature (Tmax)
of dehydrochlorination was shifted to a higher temperature and the rate of mass loss was decreased, while, in the presence of RM, FeCl3 was
formed and dehydrochlorination of PVC was accelerated, but the second decomposition step of PVC has been retarded. Benzene formation
has been hindered by the acid scavenger additives and retarded by RM, evolution of benzene and other aromatics being shifted to higher
temperatures.
# 2005 Elsevier B.V. All rights reserved.
Keywords: PVC degradation; Hydrogen chlorine absorber; Red Mud; Carbonates
1. Introduction
Tertiary (chemical) recycling including pyrolysis is a
future alternative for the utilization of large amounts of
plastic wastes with ecological and economical benefits [1].
By pyrolysis, the mixed plastic waste is converted into
monomer, fuel or petrochemical feedstock. The main
problem of the pyrolysis of waste plastics is related to
PVC [2–5]. The PVC degradation in waste plastics generates
HCl, which leads to the contamination of all product streams
with chlorinated organics as well as corrosion problems. The
behaviour of PVC in thermal decomposition according to
the decomposition products and to decomposition kinetics
have been widely investigated [3,6–14]. Recently, much
attention has been paid to the dehydrochlorination of
municipal waste plastics (MWP) or PVC containing
polymer mixtures.
* Corresponding author. Fax: +90 232 3888264.
E-mail address: jyanik@sci.ege.edu.tr (J. Yanik).
0165-2370/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jaap.2005.04.012
The dehydrochlorination attempts proposed until now
can be divided into two groups: stepwise and catalytic
pyrolysis. In stepwise pyrolysis, the polymer mixture is
firstly dehydrochlorinated then pyrolysed. As a result of the
relative weak bonding of chlorine in the polymer chain, the
dehydrochlorination of PVC takes place at a lower
temperature (300 8C). After the dehydrochlorination step,
the polymer mixture is pyrolysed above 400 8C to obtain
liquid and gas fuel [15–18]. To investigate the stepwise
pyrolysis procedure, Bockhorn et al. [7,19] studied the
decomposition kinetics of PVC and plastic mixtures by non-
isothermal measurements in dynamic conditions of heating.
They concluded that the stepwise pyrolysis of plastics
mixtures seems to be reasonable way if the various
components decompose at different temperatures.
Recently, the removal of chlorine from the pyrolysis oil
and the fixation of HCl by catalytic dehydrochlorination
received a growing interest. Sakata et al. [20–23] studied the
catalytic dehydrochlorination of PVC containing polymer
mixtures. They reported that iron oxides, iron-, calcium- and
potassium-based carbon composite catalysts were very
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119 113
effective for the removal of HCl. Kaminsky and Kim
employed lime for HCl fixation in the pyrolysis of municipal
waste plastics in a fluidized reactor [24]. As result, the
pyrolysis gases were free from chlorine containing
compounds and the oil had a chloro organic content of
about 15 ppm.
Furthermore, the effect of several metal oxides on the
pyrolysis of PVC has received increased attention, because
of their use as fire retardants and smoke suppressants. It was
found that metal oxides had a suppression effect on
unsubstituted aromatics, much more evident with respect
to alkyl aromatics [25–27], because of metal chlorides
formation. Iida and Goto [25] found that metal chlorides
from acidic oxides of transition elements accelerated the
thermal decomposition of PVC, but that metal chlorides
from basic metal oxides did not.
Another study on the role of metal oxides on the thermal
degradation of PVC [6] showed that the degradation kinetics
of PVC was affected by metal oxides. For example V2O5,
ZrO2, Cr2O3, Fe2O3, MoO3 and CeO2 tended to retard the
dehydrochlorination, while some of metal oxides such as
SnO2, TiO2, Sb2O3, Cu2O, Al2O3 and CuO promoted the
dehydrochlorination. Blazso and Jakab [27] also studied the
effect of some metals, metal oxides and carboxylates on the
thermal decomposition of PVC. They found that HCl
formation was decreased when HCl was able to react with
the metal or oxide forming chlorides.
In our previous studies, the catalytic effect of Red Mud
(RM) on the dehydrochlorination reaction of the PVC
containing polymer mixtures and municipal waste plastics
was studied [28,29]. It has been showed that RM which is a
metal oxide mixture consisting mainly of Fe2O3 was very
effective in the fixation of evolved HCl from the degradation
of PVC. In the pyrolysis of binary blends (PVC/
polypropylene (PP) and PVC/polystyrene (PS)) [28], over
90% of chlorine in PVC recovered in traps as HCl in the
absence of RM. In presence of RM, this amount was 22.1%
for PS and 29.9% for PP. Furthermore, RM affected the
amount and composition of organic chlorine compounds in
liquid pyrolysis product. In the presence of RM, the chlorine
content of the liquid was 318 ppm for PVC/PS and 764 ppm
for PVC/PP whereas in absence of RM, it was 81 and
365 ppm, respectively. In the case of PVC/PP degradation,
organic chlorine compounds of C6–C8 were formed during
thermal degradation, whereas chlorine compounds were
distributed comparatively in the range of C6–C10 in the
presence of RM. In the case of PVC/PS degradation with
RM, a-chloroethylbenzene compound formation was also
observed. In present study, the thermal degradation of PVC
mixed with RM, CaCO3 and dolomite was studied,
emphasizing on qualitatively and quantitatively evaluation
of the degradation products and overall kinetics parameters
were determined. The choice of these fillers was made
taking in consideration the following reasons. (a) The action
of the fillers under study at very high temperatures mainly as
those involved in pyrolysis is less known. (b) RM gave good
results as HCl scavenger and catalyst for binary blends
containing PVC [28] and waste plastic mixtures [29]. (c) It isa by-product from aluminum industry and its volarization is
important from economical and industrial point of view. (d)
Carbonates are already used in PVC-receipts as fillers to
reduce cost. They also improve the aging resistance,
electrical and dielectrical properties, lower shrinkage, lower
plate out, give good storage stability, reduce the stabilizers
content, and increase the tensile strength of PVC material
(up to 5–6%) [30]. (e) All are thermally stable in the studied
temperature range (0–600 8C).
2. Experimental
Experiments were performed with pure poly(vinyl
chloride) (Vinoflex S 5715) BASF (Ludwigshafen, Ger-
many) without stabilizers, fillers and dyestuffs. Red Mud
was supplied by Seydisehir Alumina Plant, Turkey resulted
as a by-products of the electrochemical process of aluminum
production. The components in RM were detected by X-ray
flouresence spectrophotometry. The main analytical data
obtained by this method are: Fe2O3 (37.72 wt.%), Al2O3
(17.27 wt.%), SiO2 (17.10 wt.%), TiO2 (4.81wt.%), Na2O
(7.13 wt.%) and CaO (4.54 wt.%) where silicium and
aluminum in AlSixOy form were detected and iron was
found in Fe2O3 form. CaCO3 (analytical grade, Merck) was
used as received. Analytical composition of dolomite is;
MgCO3 (34 wt.%), CaCO3 (65 wt.%).
PVC and particulate fillers were ground together to a
powder by extensive milling for 15 min to assure a good
homogenization, being known that without melting the
contact of PVC with filler may be not very adequate
influencing the heat transfer. Then the powder has been
screened with a 0.25 mm sieve retaining only the fraction
which passed through this screen. The ratio of polymer to
filler was 5:1 by mass.
The thermogravimetric experiments were carried out
with a thermobalance coupled to a quadrupole mass
spectrometer [7]. The initial mass of each sample was
10 � 0.5 mg in each experiment. Pure helium was used as
purge gas. The flow rate of the purge gas was approximately
100 cm3 min�1. Four heating rates (2, 5, 10, and
15 8C min�1) have been adopted in order to investigate
the degradation kinetics of PVCmixed with inorganic fillers.
All results are presented comparatively with pure PVC. All
values are the average of at least three experiments
effectuated in the same conditions.
Runge–Kutta [5] and isoconversional Flynn–Wall
method [31] methods were applied to assess the modifica-
tion in reaction kinetics in presence of the fillers. Because a
simple kinetic expression has been used in this evaluation
the correlation of the results applying both a differential and
an isoconversional method allows us to obtain reliable
results.
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119114
Fig. 2. TG and DTG curves of PVC with Red Mud decomposition at
different heating rates of 2, 5, 10, 15 8C min�1. The heating rate increases
from left to right (normalized weight was calculated based on the polymer
charge).
3. Results and discussion
Although the degradation kinetics of PVC has been
widely studied, not many investigators researched the PVC
kinetics in the presence of some additives. In this work, the
main interest is to improve the knowledge on the PVC
degradation in the presence of RM, CaCO3 and dolomite,
which have been used as HCl scavengers.
For each TG stage, the following thermal characteristics
were determined onset temperature (Ti) (determined at the
intersection of base line with the first side of DTG curve);
temperature corresponding to the maximum mass loss
(Tmax); temperature corresponding to the end of stage (Tf)
(errors in temperature determination, �2 8C); mass loss
(DW; error, �1%); and overall kinetic parameters such as
activation energy, Ea (error of determination, �10–15 kJ/
mol); pre-exponential factor (A), and reaction order (n).
The TG/DTG data are presented in Figs. 1 and 2 and
Table 1. The TG and DTG curves of PVC recorded in this
work are similar to the curves obtained by other authors
[13,14]. The second step is recorded as a single peak
whereas in the dehydrochlorination step, a large shouldered
peak in the reaction rate is observed. In the first step, the
degradation involves dehydrochlorination to a polyene
structure by the evolution of HCl and small amount of
hydrocarbons especially unsubstituted aromatics. Second
step is the formation of alkyl aromatics with a residual char.
Focusing on the effect of the chlorine fixator (RM
(Fe2O3), CaCO3 and dolomite) in degradation of PVC, the
formed HCl may react with oxides and carbonates, resulting
in chlorides plus water and carbon dioxide, respectively.
Fig. 1. TG and DTG curves of PVC containing additives. b = 10 8C min�1
(normalized weight was calculated based on the polymer charge).
We consider only Fe2O3 for Red Mud as an HCl fixator,
since aluminum and silicium are in the form of an alumina–
silicate compound, they cannot react with HCl [27].
The degradation of PVC was affected by the presence of
additives. The rate of mass loss at peak temperature in the
first step decreased (Fig. 1). In the case of the addition of
CaCO3 and dolomite, the peak temperature (Tmax, tempera-
ture corresponding to maximum mass loss rate) of
dehydrochlorination is shifted to higher temperatures since
the autocatalytic effect of HCl on further dehydrochlorina-
tion was reduced by its adsorption [6,14,32].
It was observed that the fractional residue percent is not
considerably changed by heating rate change (Fig. 2 and
Table 1) for each of the studied samples.
In the case of decomposition in presence of CaCO3 and
dolomite, the mass of residues obtained at both 380 and
600 8C are approximately equal to the sum of the residues of
PVC plus the amount of alkaline-earth metal oxides/
chloride. By considering the formation of chlorides which
lead to a theoretical increase of a maximum of 2% in the
final residue weight, it is clearly seen that the increase of the
residue amount because of the filler is very close to the error
limits. However, in the presence of RM, the percentage of
residue after first degradation step (�51 wt.%) is less than
the calculated sum of the residue of PVC (�49 wt.%) plus
FeCl3 and inert oxides in RM (�12 wt.%). The reasons for
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119 115
Table 1
Thermogravimetric data and overall kinetics parameters of PVC degradation in presence of the particulate fillers
Sample b (8C min�1) First step Second step
Ti(8C)
Tmax
(8C)DWI
(%)
n Ea
(kJ mol�1)
ln k0 Ti(8C)
Tmax
(8C)DWII
(%)
n Ea
(kJ mol�1)
ln k0
PVC 2 184.2 255–296.8 61.5 1.5 190 17.6 356.1 431.9 27.9 1.6 250 18
5 192.4 268.1–311.1 63.4 370.5 442.1 27.9
10 212.9 283.8–338.3 61.6 383.1 456.9 28.2
15 214.3 290.6–339.8 62.6 390.9 466.6 29.4
PVC + dolomite 2 192.4 276.3 57.9 1.6 160 14.3 364.6 435.9 24.4 2.2 328 23.6
5 196.4 290.6 58.7 378.7 448.2 26.0
10 206.7 302.9 59.1 382.7 458.5 26.0
15 215.8 310.5 59.1 392.9 461.4 26.8
PVC + CaCO3 2 198.2 282.5 59.4 1.4 156 13.7 347.4 431.6 23.7 2.4 288 20
5 204.7 300.9 59.1 366.4 446.2 25.6
10 198.5 315.2 60.2 372.5 456.4 26.2
15 202.6 321.3 61.8 390.9 456.4 26.0
PVC + RM 2 202.6 257.9 60.7 2.2 173 14.7 396.1 452.3 18.5 2.9 333 23.2
5 227.2 278.4–304.9 60.3 433.9 472.8 19.7
10 243.6 288.6–321.3 58.1 440.1 487 20.0
15 221.1 278.4–311.1 57.5 409.4 474.9 22.0
Ti = onset temperature; Tmax = temperature corresponding to maximum mass loss rate; DWI and DWII = mass loss corresponding to each thermogravimetric
step; DWr = residue of decomposition.
that might be related the evaporation of certain part of FeCl3(boiling point 375 8C).
As expected, we obtain more final residue with RM since
iron chloride is a catalyst which has the ability of making
crosslinks.
It is noted that in the presence of RM, two Tmax were
observed around 300 8C at the heating rate of 10 8C min�1.
The first is at 288 8C and the second is at 321 8C. At thesetwo temperatures, the rates of mass loss are almost the same.
To explain this unexpected result, the degradation of PVC in
the presence of RM was studied at different heating rates (2,
5, 10 and 15 8C min�1). Peak shapes were changed
depending on the heating rate (Fig. 2). Even at lower
heating rate of 2 8C min�1, the peak shape is different from
that of pure PVC dehydrochlorination, it being asymmetric
with a shoulder at 296 8C. At higher heating rate, this peak
was splitted into two separate peaks. The reason may be due
to the absorber effect of Fe2O3 coupled with catalytic effect
of FeCl3 formed. Iida and Goto suggested that degradation
and dehydrochlorination of PVC should be accelerated
markedly by an increase in the partial polarity of PVC chains
caused by the random introduction of chlorine molecules
resulting from recombination of chlorine atoms [25]. In the
presence of RM, the formation of chlorine molecules from
dissociation of FeCl3 (FeCl3 ! FeCl2 + (1/2)Cl2) besides
from recombination of abstracted chlorine atoms from PVC
may led to formation of second peak in dechlorination stage.
The second degradation step of PVC involves the
condensation and dehydrogenation reactions with deep
dealklylation, isomerization, chain scission, crosslinking
and aromatization [9]. These reactions lead to the volatile
compounds, accompanied by weight loss only if they occur
with low carbon number molecules. Anyway they can
change the characteristic temperatures and kinetic para-
meters of this step. The formation of MgCl2 and CaCl2 did
not effect the decomposition of dehydrochlorinated PVC at
higher temperatures The peak shape and Tmax of the second
step for PVC, PVC/dolomite and PVC/CaCO3 are similar
and are placed at around 456 8C (Fig. 1), whereas it is shifted
to a higher temperature of 487 8C in the case of PVC/RM
mixture. This result is not in accordance with the results
obtained by Blazso and Jakab [27]. They observed that the
second decomposition step of PVC was shifted to a lower
temperature by transition metal oxides. They concluded that
the easier cleavage of the polyenic chain at segments
occurred when they are in contact with transition metal ions.
In our study it seems that iron chloride retarded the evolution
of poly-condensed aromatic intermediates from crosslinked
polyene chain leading to formation of volatiles and char.
Degradation products from PVC are similar to those
found in previous studies [5,27]. Up to around 370 8C, HCl,C4 hydrocarbons and unsubstituted aromatics (e.g. benzene,
naphthalene) were evolved, whereas above 370 8C, C4 and
substituted aromatics (e.g. toluene and methylnaphthalene)
were formed. No organic chlorine compounds are detected.
MS profiles of HCl and benzene are found in the same
temperature interval. Therefore the benzene formation takes
place simultaneously with HCl evolution (Figs. 3 and 4).
The shape of single ion currents (SIC) representing the
formation of HCl from PVC containing additives (Fig. 3)
shows some similarities with DTG curves (Fig. 1). In the
presence of RM,HCl evolution begins at a lower temperature.
Blazso and Jakab [27] observed a similar effect in the
pyrolysis of PVC coated with Fe2O3. They suggested that the
C–Cl bonds areweakened due to the attraction of the chlorine
by themetal and chlorine is driven away from the carbon atom
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119116
Fig. 3. MS ion curves of HCl at m/z 36 from PVC containing additives.
Solid line, PVC; dashed line PVC with RM; dotted line, PVC with dolomite
b = 10 8C min�1.
so it reaches more easily hydrogen atom to form HCl at a
lower temperature. In this study, HCl formation gave a peak at
302 8C, and this peak has a shoulder at 271 8C. This
observation supports the decomposition of FeCl3 during
dehydrochlorination. At the beginning of dehydrochlorina-
tion, some part of HCl is trapped by RM to form FeCl3, then
FeCl3 decomposes to HCl and FeOCl. Therefore, HCl
evolution increases with temperature during dehydrochlor-
ination. In contrast, Blazo obtained a symmetrical HCl peak
around 300 8C. This may be due to the difference in the
contact mode and because the Fe2O3 used was pure, not in a
complexmixture as we used. In our study, RMwas intimately
mixed with polymer while in their studies Fe2O3 and PVC
were in surface contact only. In addition, we have observed
that HCl formation was above the 360 8C even if a small
amount of HCl was detected. This may be due to the reaction
of Fe(III) chloride reduction (reaction (I)) or/and it may arise
from further dehydrochlorination of PVC
FeCl3 þCðHÞ ! FeCl2 þHCl (I)
The presence of FeCl2 in pyrolysis residue was also
qualitatively identified in the following way. The residue
from TGA was extracted with a few drops of water. By
addition of one drop of 1 M NaOH, a green coloured
Fe(OH)2 formation was observed. Then the colour turned to
dark brown colour because of Fe(OH)3 formation. In
Fig. 4. MS ion curves of benzene at m/z 78 from PVC containing additives.
Dashed line PVC, solid line PVC with RM; dotted line PVC with dolomite
b = 10 8C min�1.
conclusion, the absorbed HCl on Fe2O3 partly re-evolved
during the degradation under dynamic conditions (according
to reaction (I)). However, in our previous studies [28,29],
RM showed a positive effect on the fixation of HCl during
pyrolysis of a PVC containing polymer mixture. The reason
for this difference with the previous publications is probably
due to the experimental conditions, such as heating regime
or presence of other polymers.
In the presence of dolomite and CaCO3, the HCl evolution
was decreased due to the reaction of HCl with carbonates. In
contrast to RM, alkaline earth metal carbonates had no
accelerating effect on the dehydrochlorination.
The addition of HCl absorber to PVC resulted also in
suppression of the amount of benzene. The decreasing
intensity can be clearly seen from Fig. 4. This effect is more
important in the case of RM. It is already known that the
evolution of unsubstituted aromatic hydrocarbons during
PVC degradation is suppressed by the presence of some
metal oxides [26,32]. PVC initially undergoes dehydro-
chlorination to generate linear polyene sequences which are
precursors of alkyl benzenes that is formed via a Diels–
Alder reaction in the same time [33]. Subsequently, at higher
temperatures the linear polyenes are predicted to disappear
through two competing processes; intramolecular cycliza-
tion and intermolecular crosslinking. The former produces
only unsubstituted aromatics. The intramolecular cycliza-
tion leads to the formation of crosslinked polyene chains. At
higher temperatures, crosslinked polyene chains of their turn
undergo two competing processes:
(a) F
urther decomposition to produce char.(b) I
ntramolecular cyclization to produce alkyl aromatics.Since transitionmetal chlorides accelerate the crosslinking
reactions, the evolution of unsubstituted aromatics is
suppressed. Although earth metal chlorides do not have a
crosslinking effect, benzene suppression is also observed in
the presence of dolomite. This may be due to the strong
absorption of HCl by carbonates. Chatterjee et al. [32]
reported that HCl might catalyze some reactions in linear
polyene sequences, such as cyclization or interchain cross-
linking of the polyene sequence. In the dehydrochlorination
step, the formation of short polyenes (butadienes and
hexadienes) is observed only in the case of pure PVC
decomposition.
In the last decomposition step, toluene with a small
quantity of methyl naphthalene, short polyenes and
naphthalene are formed. In the RM/PVC mixture, the
evolution of these compounds starts at 450 8C whereas it
starts at 400 8C for PVC/alkaline earth metal carbonates
mixture and PVC alone.
3.1. Determination of kinetic parameters
PVC degradation process is very complex compared with
the degradation of other thermoplastic polymers. Marongiu
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119 117
et al. purposed a mechanism of 40 species and pseudo-
components (molecules and radicals) involved in about 250
reactions [9]. However, the many kinetic models proposed in
the literature refer to a few step mechanism and describe the
pyrolysis process by means of very simplified reaction paths.
A common method to describe a kinetic of polymer
pyrolysis is to adapt a simple kinetic model to the
experimentally obtained conversion [34]. The kinetic model
often used to determine overall kinetic parameters for the
thermal degradation of polymers is based on a single
reaction step, and the reaction rate can be used to describe
Fig. 5. Flynn–Wall plots for PVC, PVC/C
the kinetics of process [34,35]
da=dt ¼ kð1� aÞn (1)
where a is the degree of conversion which is defined as
a = (m0 � m)/(m0 � m1) with m0 being the initial mass; m
the actual mass; m1 the final mass of sample, k the rate
coefficient and n is the reaction order
kðTÞ ¼ k0 e�Ea=RT (2)
where k0 is the pre-exponential factor and Ea is the activation
energy. Introducing the heating rate, b = dT/dt, Eq. (2) is
aCO3, PVC/dolomite and PVC/RM.
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119118
Fig. 6. Activation energy versus conversion in dehydrochlorination of PVC
with and without particulate fillers.
transformed into
da=dT ¼ ðk0=bÞ e�Ea=RTð1� aÞn (3)
For the evaluation of the overall kinetics parameters k0,
Ea and n Eq. (3) is integrated with a fourth order Runge–
Kutta method for given sets of parameters Ea, n and k0. Then
the minimum sum of squares of deviations between the
degree of conversion from the integral rate expression (aI,cal)
and the thermogravimetrically measured ones (aI,exp) is
directly searched by calculating the sum of square in the
parameter space with very high resolution
SSQ ¼ ð1=NÞX
½aI;exp � aI;cat�2 ¼ Min (4)
The advantage of this method compared with some other
methods for deriving the parameters of an overall kinetics
from TG-data as well as the limitations and systematic errors
of the method have been discussed in [5].
It is noted that due to the strong correlation of
temperature and conversion, kinetic parameters derived
from dynamic experiments are very sensitive to divergences
in temperature. Those divergences may be caused by either
physical or chemical factors. Dynamic heating of a polymer
sample generates considerable temperature gradients inside
the material, since polymers are rather poor conductors of
heat. Bockhorn et al [35] investigated the effect of
temperature non-uniformities on kinetic parameters derived
from dynamic experiments. They concluded that the
temperature non-uniformities cause comparably small
deviations of the adapted overall kinetic parameters if
moderate heating rates and sample sizes are applied. In
addition, although degradation of PVC without filler is a
heterogeneous reaction involving gaseous products and a
solid residue, the simplified overall kinetic model was used
to determine kinetic parameters in literature [5,7,34,36–38].
As based on the above explanation we assumed
homogeneity of temperature through the sample containing
additives to simplify the calculations.
The obtained overall kinetic parameters evaluated with
the above-mentioned method are listed in Table 1. It is
clearly seen that additives decreased the overall activation
energy of the dehydrochlorination step. The overall
activation energy does not vary with heating rate in the
case of PVC/dolomite system, a slight decrease from 165
to 148 kJ mol�1 is observed for PVC/CaCO3 mixture,
while the decrease with increasing heating rate is very
important for PVC/RM from 204 kJ mol�1 at 2 8C min�1 to
140 kJ mol�1 at 15 8C min�1. This may be explained with
the change in the dehydrochlorination mechanism. In the
presence of RM, the lower activation energies may be due to
the formation of crosslinked structures rather than a polyene
structure and also due to the catalytic effect of FeCl3 formed
on the degradation. The formation of polycyclic aromatic
hydrocarbons from a crosslinked structure shows a higher
apparent activation energy in the second step.
The change in the reaction mechanism of dehydrochlor-
ination in presence of fillers and with heating rate increases
is much more evident in Flynn–Wall graphs given in Fig. 5.
Up to a conversion degree of about 0.5, straight parallel lines
are found for all samples. Their shape is changed at higher
conversion degree for PVC, PVC/CaCO3 and PVC/
dolomite. No straight lines are obtained for a > 0.5 in the
case of PVC/RM mixture. The activation energy has been
evaluated and its variation with conversion degree was
plotted in Fig. 6.
It can be easily remarked that the activation energy is
almost constant up to a � 0.45 and then increases for the
three systems. It takes slight higher values for the carbonates
containing mixtures in respect with those of PVC. This is in
accordance with the variation of the PVC characteristic
temperatures. The RM incorporation in PVC, decreases the
values of the activation energy increasing conversion degree,
as FeCl3 formed has an catalytic effect.
4. Conclusion
The effect of some chlorine fixators, such as RM, CaCO3
and dolomite, on the thermal degradation of PVC has been
studied. Thermal dehydrochlorination of PVC by metal
carbonates was retarded by the absorption of HCl forming
alkaline metal chlorides. The CaCl2 and MgCl2 had slight
effect on degradation of PVC. In contrast, FeCl3 (from RM)
accelerated the dehydrochlorination but retarded the second
degradation step of PVC. Both alkaline earth metal
carbonates and the iron oxide led to a decrease in the
apparent activation energy of the dehydrochlorination step
and to an increase of the overall activation energy in the
second degradation step of PVC. This may be due to the
change in the dehydrochlorination mechanism, such as the
forming mainly of a crosslinked hydrocarbon-structure
instead of polyene structure in the dehydrochlorination step
and catalytic effect of some components of RM. Benzene
formation was suppressed by the additives whereas no
T. Karayildirim et al. / J. Anal. Appl. Pyrolysis 75 (2006) 112–119 119
considerable suppression was observed in substituted
aromatics. In addition, in the presence of iron oxide, the
evolution of benzene and substituted aromatics was shifted
to higher temperatures.
Acknowledgements
We thank DAAD (German Academic Exchange Service)
for financial support to offer us opportunity to perform this
study in Institut fur Chemische Technik, Universitat
Karlsruhe. It is a pleasure to thank Mr. Hans Weickenmeier
for technical assistance.
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