Polymer Degradation and Stability 36 (1992) 85-89

Kinet!cs of polymer degradation involving the sphttlng off of small molecules: Part 7

Thermooxidative dehydrochlorination of PVC

Peter ~imon Department of Physical Chemistry, Faculty of Chemical Technology, Slovak Technical University, Radlinsk~ho 9, 812 37

Bratislava, Czechoslovakia

(Received 1 February 1991; accepted 18 February 1991)

High-conversion kinetic runs of the thermooxidative dehydrochlorination of PVC have been treated employing a theory developed previously. Satisfactory agreement between theory and experiment has been achieved. The kinetic parameters obtained by the non-linear least-squares method show that the activation energy and the pre-exponential factor for random elimination of HCI from PVC are identical with those for an inert atmosphere. Oxygen catalyzes the initiation step of the zip reaction by interaction with degraded parts of the polymer chain; the process is very complex. The rate constant for the catalyzed initiation is proportional to the square root of the mole fraction of oxygen present in the dehydrochlorination atmosphere. Oxygen also affects the zip growth, the zip length decreasing with increasing oxygen pressure.

INTRODUCTION

Due to its practical importance, the thermal dehydrochlorination of PVC in an oxidative atmosphere has been extensively studied. It has been found that the process is autocatalytic, 1-3 a distinct increase in the rate of dehydrochlorina- tion being observed above 6500-7400ppm of oxygen in the dehydrochlorination atmosphere. 4,5 In comparison with dehydrochlorination in an inert atmosphere, the polymer is less intensely discolored, indicating that shorter polyene se- quences are formed. 6 These most characteristic features are well recognized, but many important details about the mechanism of the process remain unknown. To the present, only Valko has given a kinetic scheme for the high-conversion thermooxidative degradation of PVC and checked it by experiment. 7

In Parts 1-3 of this series, 8-~° a method of derivation of kinetic equations for the degrada- tion of polymers which occurs by elimination of low-molecular compounds has been established

Polymer Degradation and Stability 0141-3910/92/$05.00 © 1992 Elsevier Science Publishers Ltd.

and used for the treatment of PVC de- hydrochlorination in an inert atmosphere. Later, the method was extended to autocatalytic mechanisms of degradation H'12 and applied to the study of PVC dehydrochlorination in an HCI atmosphere.13 The aim of this paper is to employ the theory for treatment of the kinetic curves of high-conversion thermooxidative dehydro- chlorination of PVC published in refs 1-3.

SIMULATION OF THE KINETIC CURVES

85

For the simulation of kinetic curves, the same procedure is used as in Parts 3 and 6 of this series 1°,13, i.e. the whole set of kinetic runs measured for various temperatures is treated simultaneously. The rate constant of random elimination, B, and the average zip length, m, are expressed by eqns (1) and (2) in Part 3. The ratios of the rate constants of the catalytic and random activations are given by eqns (1) and (2) in Part 6.

Models of both the immediate and gradual zip growths 11'12 are tested to fit the experimental

86 Peter ~imon

data. In the case of the immediate zip growth, the kinetic equations are given by eqns (7) and (8) in Part 4 (ref. 11) and the parameters for the minimization of the sum of squares between experimental and calculated values of the conversion for chosen time intervals are A, E,, AA, AE~, AA~, AE~, Po and Xm. For the gradual zip growth, the kinetics are described by eqns (13) and (14) in Part 5 (ref. 12) and the minimized parameters are A, E,, AA~, AE~, AAa, AEt), Po and x,,,. The meaning of the individual parameters is given in Part 6.

RESULTS AND DISCUSSION

We have examined the possibility of the occurrence of the mechanisms of immediate and gradual zip growths n'~2 as well as the possibility of the catalysis of initiation by zips as expressed by eqn (9) in Part 4. For the latter case, the fit was better in all cases for the autocatalytic effect proportional to conversion. The catalysis of initiation by zips is thus improbable.

The sum of squares for the model of gradual zip growth 12 is lower by 10-30% than the sum for the model of immediate zip growth, n This result is rather surprising since the immediate zip growth has been unequivocally shown to occur in both inert and HCI atmospheres m3 and one would expect that this mechanism occurs also in thermooxidative PVC dehydrochlorination. A possible explanation for the difference between the expectation and the result may be the fact that catalysis of the initiation reaction is a process which is too complex to be expressed by the simple relationship given by eqn (2) in Part 4. An attempt was therefore made to express the transformation between the number of reaction steps and time as a modification of the relationship in the form

dn/N = B(1 + ~x a) dt (1)

where a is an exponent. Equation (1) was then combined with eqns (1) and (5) in Part 4, obtaining the modified kinetic equations (2) and (3) for the immediate zip growth:

dx 1 - (1 _p)m dt B(1 +flX")(Xm X) (2)

P

dr) - " = B(1 + fix")(1 - p ) dt

(3)

For the value a = 3/2, the sum of the squares is only about 50% of its original value for a = 1; for example, from ref. 1 data the standard deviation of a mesh point decreases from 0-034 (a = 1) to 0.020 ( a = 3 / 2 ) . For a = 3 / 2 , the sum of the squares is lower than that for the model of gradual zip growth. This leads to the conclusion that the mechanism of immediate zip growth occurs also in the thermooxidative degradation of PVC. The catalysis of the initiation reaction is such a complex process that eqn (2) in Part 4 represents only a rough approximation of its kinetics.

Equation (1) with a = 3 / 2 represents only another estimation of the course of catalysis of the initiation step. Little information is available to allow one to propose an equation correspond- ing fully to reality. However, the parameters obtained from the calculations depend on the equation used, as can be demonstrated by using eqns (2) and (3) for the treatment of experimental data. Increase of the exponent a leads to a decrease of the parameter fl (decrease of both AA~ and AEt0 and increase of m (higher AA, lower AE,). For a > 3/2, the fit does not improve, but m increases dramatically. The parameters A and E, remain practically constant. A possibility of escaping from the complicated situation caused by the obscurity of the mechanism is to use the kinetic equations from Part 4 with a = 1. However, considering the parameters fl and m, it must be kept in mind that their absolute values can be dubious and that only their trends should be analyzed. The results of the calculations are listed in Table 1. For the sake of comparison, the results of the treatment of data from previous work are also given in Table 1 for the values a = 3/2 and a = 2. As Fig. 1 shows, the agreement between the experimen- tal and calculated kinetic runs is satisfactory, but it is not as excellent as in previous c a s e s . 1°'13

Table 1 shows that the pre-exponential factor and activation energy of random elimination of HCI from PVC chains are not sensitive to the choice of equation used for the estimation of the course of autocatalysis. Their values lie in the intervals log(A/s-1) = 12.8-13-3 and E, = 163- 179kJmo1-1. Scattering of these values is not great and the average values log(A/s -1) = 13.0 and Ea = 170 kJ mo1-1 coincide perfectly with the values obtained for the inert and HC1 atmospheres. 1°,13 This suggests that oxygen does

Kinetics of polymer degradation--Part 7

Table 1. Kinetic parameters for the thermooxidative d e h y d r o c h l o r i n a t i o n of PVC

87

Atmosphere log(A/s 1) Ea A logA AE,, l og (AJs l) Ea Po Xm Ref. (kJ mol -l) (kJ mol 1) (kJ mol -~)

Oxygen 12.99 179-1 -4-07 -45-3 11-75 144.6 0.051 0.914 1 Oxygen (a = 3/2) 13-24 180-6 -3-50 -46-6 9.73 128.0 0.028 0-913 1 Oxygen (a = 2) 13-27 185.4 -3-63 -54-4 9.60 131.5 0.004 0-897 1 Oxygen 13.25 167.2 -3-63 -36-7 14.99 170.5 0.020 0-933 2 Air 12.82 162.8 -3 .69 -37-5 11.27 137.4 0-011 0-973 2 Air 12.77 171.9 -1-03 -24-0 14.39 176.7 0.012 0-970 3

not affect the random elimination of HCI from PVC chains.

The values of the pre-exponential factor and activation energy of the catalyzed initiation, A s and Ea, have been calculated using eqns (3) and (4) in Part 6. As the catalyzed initiation is a complex process, A s and Ea are quantities corresponding to the process as a whole. The parameter fl depends on temperature. As Table 1 shows, some values of Eo are lower than Ea and others are slightly higher. Thus, according to eqn (1) in Part 6, in some cases fl decreases with increasing temperature and in other cases it slowly increases. If the catalyzed activation were a simple autocatalytic reaction, Ea should be less than Ea. The fact that this relationship is not valid in all cases indicates again that the catalysis is a complex process in which some reaction step has an activation energy higher than Ea. This conclusion is supported by Minsker et al.14 who have observed an increasing ratio of the rates of

0

x

I

= - . 04 x

- . 0 8

. 0 8

.04 • zx =

A x

÷

+ ÷

g o © ~ Q 0

,. a A A

X X ~ m

x m

I= I3

= x exp

Fig. 1. Deviations between the calculated and experimental values of conversion for the dehydrochlorination of PVC in an oxygen atmosphere) Temperatures: + , 200°C; O,

210°C; O, 220°C; x , 230°C; A, 240°C; I-q, 250°C.

catalyzed and random initiation with increasing temperature for dehydrochlorination in an atmosphere with a mole fraction of oxygen of 0.21.

The parameter # depends also on the content of oxygen in the dehydrochlorination atmos- phere; its value is higher in pure oxygen than in air. The kinetic curves for PVC dehydrochlorina- tion with various oxygen contents in the carrier gas at 230°C, measured by Talamini and Pezzin, ~ offer the possibility of analyzing the dependence of fl on the partial pressure of oxygen. We treated these curves individually using eqns (7) and (8) in Part 4. In order to decrease the number of variables (increasing reliability of the results obtained), the values of those parameters which should not depend on the oxygen content, i.e. B, Po and x,,, have been kept constant. Their values were kept at B = 0-0089 h -1, Po = 0.051 and x,, = 0.914 (data from Table 1). The sum of the squares for each curve was minimized with respect to fl and m, and the results of these calculations are given in Fig. 2. Figure 3 demonstrates that fl depends linearly on the square root of the mole fraction of oxygen in the dehydrochlorination atmosphere. This is in agreement with Minsker et al. 14 who found that the rate of catalyzed initiation is proportional to the square root of the oxygen pressure. A linear relationship between the rate constant of dehydrochlorination and the square root of the oxygen pressure has also been reported by Talamini and Pezzin.

The average zip length, m, decreases with increasing temperature. The values of AA and AEa are less negative than those for the inert atmosphere. ~° Consequently, according to eqn (2) in Part 3, the dependence of m on temperature is less steep in comparison with dehydrochlorination in an inert atmosphere. The values of m are within the range of those for the inert atmosphere. The dependence of m on the mole fraction of oxygen is illustrated in Fig. 2,

88 Peter ~imon

m t 16 160

12 ,L ~ ~ 120

8 80

4 4O

O. .2 .4 .6 .8 1.

x02

Fig. 2. Dependence of m and fl on the mole fraction of oxygen in the dehydrochlorination atmosphere (Xo2).

which demonstrates that a small amount of oxygen greatly shortens the zip length which then remains approximately constant. These results indicate that oxygen affects the zip growth. Concerning m, the values obtained are of low reliability so that the conclusion about the influence of oxygen on the zip growth could be erroneous. However, it is supported by the experimental finding that the rate of zip propagation is proportional to oxygen pressure. 14

(

160

120

80

40

0. .2 .4 .6 .8 1.

Fig. 3. Dependence of fl on the square root of the mole fraction of oxygen in the dehydrochlorination atmosphere.

The shortening of zip length under the influence of oxygen is in accordance with the proposal of Abbas and S6rvik that oxygen attack occurs preferentially at allylically activated sites, thus terminating the growing zips. 5 This is also obvious from the fact that there is less severe discoloration in PVC degraded under oxygen or air. If the rate of propagation is accelerated by oxygen and shorter zips are formed, then, as eqn (2) in Part 3 shows, the rate of termination must be accelerated even more.

For the initial rates, when x---~ 0 and p ---> 0, the kinetic equation given by eqn (7) in Part 4 reduces to the form

dx dt n m ( x m - x ) (4)

This is the first-order reaction rate equation with an effective rate constant given by eqn (11) in Part 2. The effective activation energy and pre-exponential factor determined from the initial rates are expressed by eqns (4) and (5) in Part 3. Provided that the polymer does not contain pre-oxidized structures able to catalyze the zip initiation, the less negative AEa and shorter zip length should result in higher activation energy and pre-exponential factors than in an inert atmosphere. This has been observed in our laboratory.15

The treatment of thermooxidative de- hydrochlorination of PVC presented here prov- ides a quantitative evaluation of the process. The kinetic parameters obtained suggest that oxygen does not affect the random elimination of HCI from PVC chains. The catalyzed zip initiation is a very complex process. The reaction order of 1/2 with respect to oxygen indicates that it could be a radical chain reaction; this is in accordance with the results of other authors. 4-7 The presence of oxygen also affects the zip length m. The series authors believe that the conclusions made here and the values of the kinetic parameters will be helpful in elucidating the mechanisms of degradation and stabilization of PVC.

REFERENCES 1. Talamini, G. & Pezzin, G., Makromol. Chem., 39

(1960) 26. 2. Lis~, M. & Varga, S., Chem. Zvesti, 14 (1960) 14. 3. Guyot, A. & Benevise, J. P., J. Appl. Polym. Sci., 6

(1962) 489. 4. Varbanskaya, R. A. & Pudov, V. S., Vysokomol.

Soedin, BZ3 (1981) 211.

Kinetics of polymer degradation--Part 7 89

5. Abbas, K. B. & S6rvik, E. M., J. Appl. Polym. Sci., 17 (1973) 3577.

6. lv,~n, B., Kelen, T. & Tiid6s, F., Degradation and stabilization of PVC. In Degradation and Stabilization of Polymers, Vol. 2, ed. H. H. G. Jellinek. Elsevier, 1989.

7. Valko, L., J. Polym. Sci. C, 16 (1967) 545, 1979. 8. Simon, P., Poly. Deg & Stab., 29 (1990) 155. 9. Simon, P. & Valko, L., Poly. Deg. & Stab., 29 (1990)

253.

10. Simon, P., Gatial, A. & Valko, L., Poly. Deg. & Stab., 29 (1990) 263.

11. ~imon, P., Poly. Deg. & Stab., 35 (1992) 45. 12. Simon, P., Poly. Deg. & Stab., 35 (1992) 157. 13. Simon, P. & Valko, L., Poly. Deg. & Stab., 35 (1992)

249. 14. Minsker, K. S., Berlin, A. A. & Abdullin, M. 1.,

Vysokomol. Soedin, !116 (1974) 439. 15. Oremusov~i, J., Influence of metal stearates on the PVC

thermal stability. PhD thesis, Slovak Technical Univer- sity, Bratislava, 1989.