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J. Blazevska-Gilev, D. Spaseska

251

Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010, 251-254

FORMAL KINETIC ANALYSIS OF PVC THERMAL DEGRADATION


Faculty of Technology and Metallurgy

St.Cyril and Methodious University, P.O. Box 580

MK-1001 Skopje, Republic of Macedonia

E-mail: [email protected]

ABSTRACT

This paper presents an initial attempt at describing poly(vinyl chloride) thermal degradation through kinetic

model. Non isothermal or dynamic thermogravimetry (TGA) has been used for kinetic study of the thermally activated process of poly(vinyl chloride) (PVC), heating by two different rates up to around 723 K. As the easily measured weight

changes of the samples in the defined thermal conditions are a suitable sensor for their structural and chemical changes,

by means of some methods like Gropjanov-Abbakumov ,s one, the useful information for identifying the formal kinetic

parameters of the investigated processes taking place in the course of thermal treatment have been achieved.

The thermal variations of the rate constant as well as the kinetic equations for the examined process depending on the

investigated parameters have been derived.

Keywords: poly(vinyl chloride) , thermal degradation, degradation kinetics.

Received 05 April 2010

Accepted 12 June 2010

INTRODUCTION

Poly(vinyl chloride) (PVC) is commonly used as

a thermoplastic because of its wide variability of proper-

ties and its application in rigid and soft products [1].

Thermal degradation of PVC is a more complex process

than the other stable plastic wastes, i.e. polyethylene (PE),

polypropylene (PP) and polystyrene (PS). As a matter of

fact, whilst PE, PP and PS thermally react at quite low

temperatures, reducing the polymer chain [2,3], PVC

pyrolysis involves significant cross-linked reactions with

the formation of polyaromatic structures (possibly chlo-rinated) and a carbonaceous residue (char). Moreover

the process description becomes more complex because

chlorine needs to be introduced into the system addition

to carbon and hydrogen. The literature includes several

papers on pyrolysis and gasification of PVC.

However some scientists have reviewed the ki-

netic results of PVC thermal degradation, which have

been usually fitted in the literature to a potential ki-

netic model of the order

dá/dô = k f (á ) = k 0 f (á ) exp(-E /RT ).S. Moulay [4] has exposed that PVC degradation

takes place in a two steps process, dehydrochlorination

and a further degradation step yielding a solid residue

and a volatile fraction.

In this paper it was derived the analytical shape

of the kinetical curves for the two temperature stages of

the investigated process by means of Gropjanov-

Abbakumov,s method [5].

EXPERIMENTAL

Thermal degradation of poly(vinyl chloride) has

been studied during heating up to around 723 K at the

rate: dT/dô = 4 and 10 o/min using Cahn D-200 re-

cording microbalances in a stream of inert gas at the

flow rate 100 ml Ar/min. For the purpose of the identi-

fying the outgoing gases the isothermal analyses were

used, subjecting PVC samples 10 min. at the character-

istic temperatures. The analysis has been made by auto-

matic sampling gas chromatograph Hewlett-Packard GC

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Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010

252

5890 with FID and TCD detectors and the Porapak P

packed column (i.d 2 mm, length 2 m). At the same

time the outgoing gas was analyzed continually on mass

spectrometer VG GAS Analysis LTD.

RESULTS AND DISCUSSION

A standard PVC was thermally treated in inert

atmosphere, up to 723 K, using a heating rate of 4 and 10o/min. The thermal degradation occurs in two steps to

nearly 89 % weight loss. In Fig.1 the dynamic TG curves

of PVC in Ar atmosphere (100á ) and two different heat-

ing rates are presented, but in Fig. 2, the dependences of

the degraded sample (á) on the temperature (T) are shown.

The first degradation step lasting up to 573 K, leads to a

weight loss of about 65%, but the second one, occurring

up to 723 K, results in nearly 85% weight loss as a whole.

The liberated gas products as a result of the thermal deg-

radation have been determined by gas chromatography

and mass spectrometry. These products are presented in

Fig.3 and Fig.4, consequently.

The main gas products released from PVC are

methane, ethane, ethyne, hydrogen, HCl, Cl2, 1-butene

and benzene, appeared as a result of decomposition and

the recombination of the gas products in two steps of

the process. The description of the degradation process

is difficult, because of the complexity of the chemical

reactions carried out at the same time. In this work, the

characterization of the rate of weight loss is made by

overall kinetic parameters determined on the basis of

the weight loss curves. The determination of PVC deg-

radation kinetics is commonly carried out by mathemati-

cal treatment of the TG curve, or series of curves ob-

tained under different temperature-ramped conditions.

In order to obtain the formal kinetic param-

eters which explain two steps of the underlying process

comprising TG curve, we have adopted Gropjanov-

Abbakumovs method which could analyze groups of

reactions in the separated steps, by using two different

heating rates.

Using the mentioned method, the data of Figs.

1 and 2 as well as the equation

dá/dô = K f (á ) = K 0 f (á ) exp(-E /RT )

where: dá/dô percentage of the degraded sample in

unite time, K 0

entropy factor independent on tem-

perature, f (á ) function of the decomposition degree,

depending on the control mechanisms of the reaction,

E energy of activation, R gas constant, T tempera-

ture, the energy of activation can be determined [5]:

E = R[ln( d α /d τ )2 - ln(d α /d τ )]1

(1/ T 1) - (1/ T 2)0

20

40

60

80

100

0 200 400 600

T, C

(100-a% 4 C/min 10 C/min

0

20

40

60

80

100

0 200 400 600

T, C

=,%

4 C/min

10 C/min

Fig. 1. TG curves of PVC in inert atmosphere.

Fig. 2. Degradation polytherms of PVC.Fig. 3. Gas chromatography analysis of the thermally treated

PVC.

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253

In our case the value of the activation energy for

the temperature region up to around 573 K is 107,033

kJ/mol, but for the region starting from 643 up to 723 K,

the value is 212,840 kJ/mol which is in good correlation with the results of the authors Troitskii et al. [6] and

Zaikov et al. [7]. The values dá/dô have been determined

by using the relationship: dá/dô=( dá/dT) × ( dT/dô).

The value K 0

is determined as tg(a) from the

liner plots of (dá /dô ) / exp(-E /RT ) versus f (á ) which

passes through the origin of coordinates: Fig. 5 (a,b)

and Fig. 6 (a,b). Concerning the first temperature re-

gion, the values on the ordinate is calculated by the

relationship: 9,4/exp(-107033/RT).10-12 for the heating

rate of 4o/min, but for the heating rate of 10o/min, the

valid relationship is as following: 25,33/exp(-1070330/

RT).10-13. For the second temperature region, the val-

ues on the ordinate are calculated by the two relation-ships, concerning the two different heating rates, 4 and

10oC/min: 1,319/exp(-212840/RT).10-19 and 3,298/exp(-

212840/RT).10-19, respectively.

For the first region, by the heating rate of 4o/

min, there are two values for the magnitude K 0 : 3,125.1012

as a result of the validity of the model relationships for

f (á ): (1á )4. For the heating rate of 10o/min, the value

of K 0 is: 3,11.1013 depending on the model relationship

validity: (1 á )4. In the temperature region starting from

Fig. 4. Mass spectrometry analysis of the thermally treated PVC.

0

1

2

3

0 0,2 0,4 0,6 0,8 1

(1-= )"

94e1

RT. 112

0

5

10

15

20

25

30

35

40

0 0,5 1 1,5(1-= )

"

23e

1

R1

13

Fig. 5. Dependence of (dá/dô ) / exp(-E /RT ) on f (α ) for the first region, with the heating rate of 4 o/min (a) and

10 o/min (b).

a) b)

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Journal of the University of Chemical Technology and Metallurgy, 45, 3, 2010

254

643 up to 723 K, the values of K 0 depending on the heat-

ing rates of 4 and 10o/min are: 9,11.1019 and 1,48.1020.

The analytical shape of the kinetic curve f (á ) is: (1 á )6.

Depending on the mechanism of the degradation process

the analytical shape of the kinetic curve f (á ) for the tem-perature interval up to around 573 K is presented with

the model equation f (á )=(1á )4, but for the temperature

region from 643 up to around 723 K, is given by the

model equation: f (á ) =(1á )6 (Table 1). The both model

equations reveal the kinetic controlled mechanisms of

the exampled process on macro level. The values of the

pre-exponential factor K 0 , dependent on the slope of the

degradation curves, are expressed different for the two

examined steps of degradation.

CONCLUSIONSThe thermal degradation of PVC was analyzed

in this paper on the basis of the released gas products as

well as on the basis of a phenomenological model that

describes the kinetics of the two separate temperature

stages of the process. By means of the thermogravimetry

and Gropjanov-Abbakumovs non-isothermal method

for the investigation of the formal kinetic of PVC ther-

mal degradation, the classical kinetics parameters like

energy of activation and preexponential factor for the

characteristic temperature regions have been determined.

The analytical models of the kinetic curves for the two

characteristic regions have been given. The adequacy of

distinct equations in different temperature regions has

been determined.

REFERENCES

1. S.Chattopadhyay, G. Madras, Polymer Degradation

and Stability, 78, 2002, 519-524.

2. E. Ranzi, M. Dente, T. Faravelli, G.Bozzano, S.Fabini,

R. Nava, V. Cozzani, L. Tognotti, J. Anal. Appl. Pyrol.,

40-41, 1999, 305-319.

3. T. Faravelli, G. Bozzano, C. Scassa, M. Perego, S. Fabini,

E. Ranzi, M. Dente, J. Anal. Appl. Pyrol., 52, 1999,

87-103.4. S. Moulay, Progress in Polymer Science, 35, Issue 3,

2010, 303-331.

5. V.M. Gropjanov, V.G. Abbakumov, Him. i him. tehnol.

18, 2, 1975, 202.

6. B.B.Troitskii, L.S.Troitskaya, Int. J. Polym. Mater.,

41, 1998, 285.

7. G.E. Zaikov, K.Z. Gumargalieva, T.V. Pokholok, Y.V.

Moiseev, V.G.Zaikov, Polym. Plast. Technol. Eng.,

39, 2000, 567.

0

10

20

30

0 0,0005 0,001 0,0015 0,002 0,0025

(1-α )6

1 , 3

1 9 / e

x p ( - 2 1 2 8 4 0 / R T ) . 1 0 - 1 9

Fig. 6. Dependence of (dá/dô ) / exp(- E / RT ) on f (á ) for the second region, with the heating rate of 4o /min (a) and 10 o /min (b)

0

20

40

60

80

0 0,001 0,002 0,003

(1-α )6

3 , 2

9 8 /

e x p ( - 2 1 2 8 4 0 / R T ) . 1 0 - 1 9

Temperature interval K 0 K .f(α)

up to 573 K ν= 4oC

.min

-1

ν= 10oC

.min

-1

(1-α)4

(1-α)4

3,12.10

12

3,11.10

13

3,12.10

12⋅exp(-107033/RT)⋅ (1-α)

4

3,11.10

13⋅exp(-107033/RT)⋅ (1-α)

4

643-723 K ν= 4oC

.min

-1

ν= 10oC

.min

-1

(1-α)6

(1-α)6

9,11.10

19

1,48.10

20

9,11.10

19⋅exp(-212840/RT) ⋅ (1-α)

4

1,48.10

20⋅exp(-212840/RT) ⋅ (1-α)

4

Table1. The values of K 0and the equations of Kf( á ) for two thermal regions.

a) b)

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