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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