Pyrolysis of polypropylene

P\ROLYSIS

OF POL\-PROPYLElYE

II *. KINEIKS

OF DEGIUDATION

I?(XFtODtXTIOS Reently

ctied

out

it has been shown [I] that the overall isothermally.

obey5

the

kinetic

law

da,‘dr = k( 1 _ =)[I _ (1 _ a,=*]l. z setting 1 -a=(coshIp)-:‘.* one obtains da;‘dz = k( 1 - a)tanh j3 In these expressions the parameters k and b are or&- functions of the temperature_

This kinetic law. already verified elsewhere (21. has been established assuming that the pyrol_vsis proceeds through a three-step mechanism: (a) a random initiation reaction: (b) a depol>merization reaction: and (c) a trrmination reaction by disproportionation or combination of radicalsThe purpose of this kinetic study was to check the accuracy of this mechanism and this kinetic law for the overall decomposition of polyprop>-IeIlC.

E\;PERIM ENTAL

TECHSIQL-ES

The thermogravimetric experiments were carried out either at constant temperature (isothermal methods) between 660 and 700 K or dynamically at constant heating rates in the range 12.5-100 K h-’ with an accuraq- of 5%. N-c used a Setaram B 60 thermobalance equipped with a temperature programming device. As in the stud_\-of pol_vprop_\-enedecomposition products [3]. we used two samples: 85% isotsctic (IPP). and 50% atactic polymer fAPP)_ The initial mass of pol>mcr subjectd to the pyrolysis was 120 2 1 mg and the helium flow-rate ~-as 43 1 h- i_

THERSfOGR~\‘IMJZI-R\-

AT COSSTAST

HEATISG

RATES

Simultaneous recording of the weight loss. of its derivative and of temperature versus time enabled da_/dTgraphs to be plotted as a function of T for the degradation of IPP. As can be seen in Fig. 1 and Table 1. dar!‘dT passes through a maximum which decreases with increasing heating rate- whereas the conversion. am_ and the temperature. T,. increase [4]_ On the basis of these resuhs. we tried to determine the actktion energy (E) and pre-exponential factor (k,). From the many available numerical methods u-e chose the method of Sharp and Wentworth [5]. u-ho plotted h$k. I IdT

asa

+ (i-a)

1

function of l/E and that of Kissinger [6]. who took into account onl_v the temperature maximum. r,. corresponding to the maximum rate:

673

648

6S8

723

Likewise. we plotted

which is 3 peculiar TABLE

form of the Sharp and \Ventworth equation restricted

1

Hearing rate

da (yj+

-10: (K-‘)

r, (K.,

=-a

O-651 0_6sO 0-W

(S h-‘) 1’

3.73

25 50 75

‘3.74 3Al 332

671 6S2 692 xx!

100

322

705

O-678 0.X%

at

TABLE 2 Results obticd

constant

bF various intcrprewioa methods

hc;tting rate. 0. to the v&ws of the pxnmcters

L-

These expressions are obt;tined by m&ins equation of the decomposition reaction da jz=$l

k

(da..‘dT),.

cc, rmd

n = 1 in the gcnernl

rate

-a)”

or in the derivative with respect to T. d’a..idr’. The results obtained these panmeters rue @-en in Table2 These v&es of E and k, are obGously not in cpood agreement. but agreement is not worse than in the literature. zts can b+ seen in Table% In order to overcome these discrep~cies. due either to the misuse of first-order kinetic law or to the presence of atactic polypropylene in IPP. carried out isothermal pyrolysis experiments.

PYROL\-SIS OF AT.MXC

for tht tht we

POL\-PROP\-LESE

In order to stablish the influence of APP content on the decomposition mte of IPP. thermogrwimetric rtnalyis of both samples H-S czwried out at TABLE 3 Rcticu- of some astivaticxaenergy data in Iituaurt

the same tcmperatur~ 678 K. and the results obtained are shoxn in Fig. Z It is clear that the APP sample decomposes faster than IPP and at a lower temperature. which agrees uith literature results ~13.14]. Further, after the decomposition of the atactic fraction (50%). the rates for both samples are then identical with the rate of decomposition of the isotsctic polymer. Sioreover. we decomposed APP at constant heating Antes and obsen-ed _mter rats of decomposition than nith IPP. The Sharp and Wentworth method cleari_\-reveals the e.xisttnce of a two-step reaction (Fig.3). The frost step occurs at IOU- temperature and corresponds to the atactic fraction with an accik-srtioaeneqc-c of 226 kJ mol-I. The second step. at higher temperature. corresponds to 50% of isotacticpolymer uith an activation encw of 330 kI mol-I. This difference could not be detected during the decomposition of IPP because the atactic percentage in the IPP was lower (15%). Thus we obtained an intermediate \-alue of 305 kJ mol-’ for the activation energy. Therefore. to achieve a greater precision using only the kinetic lau- of IPP pyrolysis. we must remol-e the effects of the APP fraction. defining a neu-

-8

-9

mol

- 10

f

ccxwersion q:

u= ;

u - 0.15

OS5

and a new rate:

da,_

1 dr ----0-B

da

dr

rlO’(K-‘)

-3

109

It should be noted that these improvements do not change the preceding rem&s obtained by means of dynamic thermogratimtuic ana&sis. as drr, --=--_1 dT I-ai

da dT

1 1~

Hence the kinetic law contxmed is questionable_Henceforth a rwnablz kizxtic law may bc proposed only as a remit of an isorhermal kinetic investigation if the abowz improvements w invoh-ed, Therefore. rhe same vdms of E and k,. u-hatcver the method of interpremion. xnusfbe obtained

u-hen applied to dynamic thermo_gavimetricdata.

a

I!0

ISOTHERLsL+L

EXPERIMESTS

Fig.3 shows the \-ariation of IPP conversion with time at temperatures between 665 and 696 IL In Fig. 5 dni’dr is plotted versus Q and da,/dr versus a, (the curves are the same. oniy the numerical vaIues of the ordinate chanse).

dc,

dt

= 10

t

6

5

L

3

d 2

I 1 0’

Fi& 5. lmariation of tht ovcraU raw of tht

reactionaith conversion

These cun-es allow three conclusions to be drawn: (a! p~-rolysis is complete in this temperature range: (b) the du,.-‘dr cuxws pass throi!gh a maximum for \-alues of a, between 0.26 and 0.34: and (c! for xxlues of a; higher than 0.75. da,;‘dr varies almost line&_\-. These conclusions are similar to those drawn in prc\ious work on polystyrene degradation [ 11. The results are treattd later by the abox-e numerical methods. InrerpreItzrion of the e.rperimenlui dara

A mathematical

$+‘(I =k(l

study of the kinetic law

-a,,[1

- (1

2 -uJ

i

3

-_

-a,)tanh&

applied to the IPP fraction leads to the following msin concltions: rate passes through a maximum at c&=1-

1 bil

1.3) the

‘6

I I

whose superior value is 1 - l,;.‘rE = O-393: and {b, at the end of tht rczction. the tanh j3, term approaches unity. so that the rate varies almost linearl_v with cc,.

Qualitatively_ both points (a) and !b) have been vtrifitd. Further. the extrapolation of the linear part of the thtrmo_gram for a, = 0 givts the rate constant. k. the activation enere and the pre-exponential factor. kc,. From Fig. 6, whcrc In k is represtntcd as a function of 1. T. rhz foIlewing relationship is obtained: In k = 2.97 - 1o’s exp

I

-

33.510 T

]

The values of E and k, are 280 zcl mol-’ and about 3 - 10’” s-?‘. respstivcly. It is also possible to determine the parameter b. as for polystyene. fro-m the \xlues of (da,j’dr), and k introduced into the equation

This expression has been deduced from the rate equation and the expression of ai. both at the maximum. The intermediate detailed calculation being l&t aside. the following expression is obtained:

ii2

-6 I t

-8

i 1

As for pal>-styene. b decrezws vrith incresing tempenture. The “actiwtion ener& is &-en by E& = - 8.630- S-32 = - 72 lil mol- ’ srnd the numeric4 v&xs of h have the same magnitude ;is for polystyrene. for t-mple b = 0.40 ;It GS IL.

Sonc of the preceding numerical vslue of k snd b prow that the mechanism of p_\-rol_vSsof polyprop_\-ltne is the same s that of polystyreneAs the trmsfer rextion is supposed not to be t&en into account. the xwiation of these pxxmetcrs uith temperature must be prowd in the foilowing vvq-- The kinetic I~u- requires the rrrte constants of *the &men-k,= depropag&on ruction: rextions: k,, = Mndom initiation reaction: k:= termination reaction- It cm be shown thst the expressions for k and b 3re 2k,,l-;

k=[ k’: b=

1 -1=

k,

P(T)

where F/, and p(T) are the molar volume (the volume of palmer containing :V molecules of monomer) of pol>-prop_vltne and the yield of the P>TO~JS~S reaction, respectivelyThese expressions of k and b allowed us to write theoretical values of E and E, as functions of the actktion eneqies of the elementary steps:

EI.=E:-E= AccordinS to Mita [lS]. E,, should be of the same magnitude as the C-C bond energ- in &tactic pol\prop~lene (348 kJ mol- ‘) and E2 should be of the magnitude of the sum of the actkation energ- and the enrhaipy \tiation of p&propylene polymerization {about 105 kI mol-’ )_ E,_ the activaian energ- for termination by combination of rdicals_ is about 20 k.l mol-‘_ These L-alues lead to the values E = 264 kJ mol- ’ and Es = - SS Id mol- ‘_ which are veq close to those obtained experimentally (X0 and --I2 kJ mol-‘). Thus. the assumptions for the whole mechanism are verified. Second inreqwetation of rkmoyacimerric rath L’X COEWJ.PIZ %eaCns~rakes If the interpretation of the results obtained by the Sharp and 1Ventworth method takes into 3ccount the expression of tanh 13, = [l - <1 - 0: F’J’ ’ varying with a, and temperature. the calculated values of E and k, are 253 kl mol-’ and -r5- 10’” s-’ . respectively-. as shoun by the equation k=

25-

lO’%p

(

-

30.430 T

)

In other respects. the Kissinger method cannot be improved term. So. & pmiously. u-e used the Sharp and H’cntworth maximum rate and plotted

versus 1/T. k=l_S-

by the tanh r3, method zt the

and obtained

1oX6cxp

(

-

30.120 T

).

which compares well vrith the preceding equation, It must be emphasized that the difference between thgst results and the first ones is related to the variation of tanh /3,, from 0.76 when 0 = 12.5 K h-’ to 0.65 when += 100 K h- ‘_ This variation has just been taken into account whereas it had not been previousl>-. Then. putting IZ= 1 implied tanh /3,,= 1.

TabIe4 gives all the results obtained for E and k. according to the interpretation methods applied to the dynamic thermogravimetric experiments. The last row in Table4 gives the values obtained by the isothermal methods_ This table indicates that the improved results are far less scattered than the first ones obtained u-hen tanh j$, had not been taken into account- Of course. the non-isothermal values are different from the isothermal vaIues. but an error of 15 kJ mol-* in E. k being kept constant for a given temperature. induces a corresponding error in k,. which may reach the first power of ten, Then. a11 the values reported can be considered as being in good ageemtnt and probab& the correct values are E = 265 2 15 kJ mol- ’ and k, --3_ loss.?1 s- ‘_ The activation energ- is undoubtedly ver\- close to the v_aIue found previousIF b? means of the equation E=

ii E’,-

E,)

+ Ed

Finali_v. w-t can sag that the p_\-rolysisof IPP is governed by a mechanism simihr to that for polystyrene. This mechanism is basicall_\-a depolymctition as the principal products obtained are compounds with 3n carbon atom&

TABLE 4 of E and k, obtained b,\-different

V&es

methods E (W

Method and U-uta-ortb

1

Sharp acd W&woA ra:c Kissicoa =In---

X-,R E

applied zt the maximum of

E RL

Skarp ad

N-entwortb cmrcc~cd by tanh 8.

Sharp ad

Wentworth corrected by tanh 8,

app2icdat the mzximumof rate Isxkrmal

mct!wd

mol-‘)

kc is-:)

REFERExcEs