Kinetic study of the thermal oxidation of polypropylene

ELSEVIER

PII:

Polymer Degradation and Stabihiy 57 (1997) 23 I-240 0 1997 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0141.3910/97/$17.Ml

SO141-3910(96)00167-X

Kinetic study of the thermal oxidation of polypropylene Laurence Achimsky, Ludmila Audouin*

& Jacques Verdu

ENSAM, 151 Bd. de l’H6pita1, 75013 Paris, France (Received

3 June 1996)

The thermal oxidation of non-stabilized isotactic polypropylene was studied in air at 80, 90, 100 and 150°C using gravimetric and IR spectrophotometric determinations (hydroxyls and carbonyls). A kinetic model in which radical production results only from unimolecular hydroperoxide decomposition was developed. This model predicts that: l

l

l

the induction time depends only on the rate constant of hydroperoxide decomposition. It must be almost independent of the polypropylene source (for low content of catalytic impurities and low initial hydroperoxide concentration) and its apparent activation energy must be about 100 kJ mol-‘; the induction times relative to, respectively, weight, carbonyl and hydroxyl gain must be in the ratio 2:3:3; ‘the steady state’ oxidation rate must be independent of initiation rate constant.

The experimental data on oxidation kinetics in the temperature range study confirm all these predictions. 0 1997 Elsevier Science Limited

1 INTRODUCTION

An interesting peculiarity of these processes is that the corresponding initiation rate is almost constant, which simplifies considerably the kinetic analysis. l The participation of metallic impurities (M), for instance’

There is a large amount of literature on the mechanisms and kinetics of low temperature thermal oxidation of hydrocarbon polymers but, in spite of that, there is no general consensus on a mechanistic and kinetic scheme. Considering the problem only from the point of view of kinetics, it appears that practically each reaction step is a source of disagreement. In the case of initiation there are a large variety of proposed mechanisms, including for instance: l

Polymer

l

Monomer

M + 0, (M...O,)

unit-oxygen PH + 0,

P’

interaction +

P’+

HO;

This latter process can involve many mechanisms which have been reviewed by Kamiya and Niki.’ * To whom all correspondence

should

+

(M...O,)

+ PH +

complex

M + P’ + HO;

The strong spatial heterogeneity of oxidation observed, for instance, by chemiluminescence imaging on polypropylene powder* and films’ could result from some heterogeneous dispersion of metallic impurities derived, for instance, from polymerisation catalysts. It can be noticed that in all the cases where the above mechanisms would be the predominant radical sources, oxidation is normally expected to proceed at a constant rate or to be more or less auto-retarded. In fact, the thermal oxidation of ‘pure’ hydrocarbon polymers at low temperatures

thermolysis PH +

under

be addressed. 231

232

L. Achimsky

(typically below 400 K) displays a pseudoinduction period whose reality has been quesand a well marked autotioned recently,” accelerated character. This is, no doubt, linked to the increasing importance of branching reactions linked to hydroperoxide decomposition by a unimolecular mechanism:’ POOH or by a bimolecular POOH

+

POOH

OH’

mechanism”.’

+ POOH

Other mechanisms for instance

PO’+

+ have

PO’ + PO; + H,O been

also proposed,”

+ PH -+ PO’ + P’ + H,O

It is reasonable to suppose that these branching processes become the predominant radical source after a more or less long initial period. At least two types of hydroperoxides of different reactivities could be involved in initiation.’ Propagation is also a subject of great discussion, especially as far as reaction homogeneity is concerned. The problems of large scale heterogeneity linked, for instance, to the diffusion control of the reaction in thick samples or to the insolubility of oxygen in the crystalline phase,“’ will be ignored here and the discussion will be focused on small-scale heterogeneities. One of the main causes of heterogeneity is the occurrence of a competitive intramolecular propagation” which has been widely discussed in the case of polypropylene. Another possible cause is the low mobility of peroxy macro radical which would favour a local propagation from an ‘infectious’ centre.’ It is worth noting that both causes of heterogeneity must favour bimolecular rather than unimolecular POOH decomposition. Celina and George” showed that the growth of oxidized domains due to local propagation could be the cause of the auto-accelerated character of oxidation kinetics. In this domain. the key question is: ‘Is it allowable to use the conventional kinetic approach derived from liquid phase chemistry in the study of solid state polymer oxidation?’ There are also many points of discussion regarding termination. It is generally assumed that termination occurs essentially from the bimolecular combination of peroxyls but, according to Billingham,” there is a general agreement that experimental kinetic data on this reaction seem to be inconsistent with this assumption.

et al. The literature of the last few years reveals clearly a genera1 tendency to reject the classical kinetic approach for the reasons indicated above. Despite that, it seemed interesting to revisit ‘closed loop’ mechanistic schemes in which only POOH decomposition is a significant source of radicals. These schemes had been previously studied, for instance by Karpukhin and Slobodetskaya” in the case of photo-oxidation for the unimolecular case, and by Tobolsky et ~1.’ in the case of thermal oxidation. However, it seems that they were never used as tools to validate the choice of a given thermal oxidation mechanism. Surprisingly, it appeared in our study that the scheme involving unimolecular POOH decomposition is qualitatively and quantitatively in good agreement with the literature data on the pseudo-induction period of the thermal oxidation of isotactic PP in solid state.” The aim of the present work is to try to introduce new criteria for the validity or nonvalidity of the ‘closed loop’ mechanistic scheme with unimolecular POOH decomposition. The discussion will be made on the basis of experimental results obtained by IR spectrophotometry and gravimetry on non-stabilized isotactic polypropylene samples in the 80-150°C temperature range. 2 THEORY It has been found recently’” that many important features of polypropylene (PP) thermal oxidation could be explained by a ‘closed loop’ mechanistic scheme in which the unique source of radicals is the unimolecular hydroperoxide decomposition: POOH P’+O, PO;+PH PO;+ PO;

LY P’ (radicals) PO; PO,H + P’ inactive products + 0,

+ + + +

k, 6 k; k,,

In the above cited paper,” it was shown that the hydroperoxide concentration y varies as fol1ows:

where 7

y, = s.cr=k,[PH] 7 and 6 = 1 - (y,,ly,) G

Kinetic study of the thermal oxidation of polypropylene

233

initiation and termination that in stationary state k6[ POO’]’

reactions.

We know

= ak,[ POOH]

so that finally dx dt tiy

Fig. 1. Shape the method

tiw

tix

= @k,[ POOH]

and $

= qk,[POOH]

t

of the oxidation kinetic curves, schematic of of determination of the pseudo-induction time

where

@ and * are constants.Thus

t..

dx -=@k,!$ where y,, is initial the hydroperoxide concentration. In the following, only the case of reasonably ‘clean’ samples, such as y0
dt

7

and x=

Since x=0,

Jo’@k7$(l

-exp-

T)2dt

for t=O we find:

(2)

For all the species formed in oxidation reactions, the kinetic curves will have the shape shown schematically in Fig. 1. In all the cases, the duration of the induction period will be determined at the intersection of the tangent at the inflection point or of the asymptotic straight-line with the abscissa axis. It has been shown in a previous articleI that

tiy=

1 k(21n267

l)i.e.tiy=

k7t

2

- exp - k,t

I (6)

In the same way,

k,t-3+4exp-

k,t 2

- exp - k,t I

(7)

0.386 k,

Thus, it appears that both OH and C=O kinetic curves must display the same induction time

for low initial POOH concentration. Let us now consider the formation of stable especially carbonyl and oxidation products, hydroxyl (non-hydroperoxide) compounds. In the above scheme, they can be formed only during initiation or termination steps. Thus, if we call x the hydroxyl concentration and z the carbonyl concentration, we have: dx dt

= &k,[POOH]

+ &k6[POO’]*

(3)

s

= $ k [POOH] ”

+ $ hk 6[POO’12

(4)

dt

k,t-3+4exp-

where &, $,, q5h and tib correspond to the yields of the corresponding products in, respectively,

ti, = t,, =

Jk,

It is noteworthy that tti (or tiz) is about 9 times longer than the induction time of hydroperoxides (ti,)* Experimentally, it is not easy to separate the contributions of the hydroperoxides from the other OH containing compounds, so it can be reasonably supposed that x is influenced by the POOH concentration. In this case, it could be expected that the induction time of x would be shorter than the carbonyl (z) one. A possible method of checking this is given by the calculation of the concentration of the species being studied at the end of the initiation period of carbonyls tiz = 3/k,.

L. Achimskv

234

et al. initiation molecule:

We obtain:

event

POOH [POOH],

= $

(1 - exp - $ )’ = 0.6 $ 7

[C=O],=@$[4exp-

+

ation of a water

is lost by --$ PC

t P’

OH’+PH-+

7

joration

Thus, the rate of oxygen polymer is:

-cxp-31

7

w21

~ = 0.84@ ;

into the

k7[POOH]

= - k,[O,][F

dt

IH’

7 + ;k,[PC [OH],=*$[4exp-

5

-exp-3

(10)

1

7

The concentration obtained from:

4)841$rd k;

WI

~

It is reasonable to suppose that @ and q are of the order of magnitude of unity. Thus, at the end of the induction period, i.e. for t = t,,= t,; the concentrations of termination products C=O and OH are presumably higher than the POOH concentration. It was of interest to calculate the pseudostationary rates C=O, OH and the maximum rate of POOH build-up. In the case of OH and C=O this corresponds to the slope of the asymptote. In the case of POOH, it corresponds to the slope at the inflection point of the curve

k[ PO;l[

The resolution leads to: $

of

(,1;;;’ 2

- f 2

- k,t + ___ 2

X exp -

diGI ___ dt

f

(lloa;

+ k,[PO;]’

1+2a) D2] - k,/2 1

k,PJ - k,

7

2

X exp -

By replacing

2

e expression

for

021

k,t

2

1

021

+ :

a2

exp - k,t

Je obtain

[P’] in eqn

dt

(8)

mperatures are l-4 mol ll’ ” and

2ff)

- 4

w21 p=-= - k,[O,][P’]

equation

X exp - k,t

[P’]= It thus appears that the maximum rate of carbonyl and hydroxyl build-up is noticeably higher than that of POOH. Let us now consider the oxygen absorption. According to this model, it can be represented by the following differential equation:

be

(11) -ential

this

Typical values at mode1 k2 = 10” mol 1-l s ‘,I’ [O,] = k,=10m3 s-’ ‘).” at 373 K. Thus, k,[O,] >> k,, so P’ simplifies as follows:

and

can

k,[ POOH]

= - k,[O,][l

dt

+

[P’]=

adicals

of

I

k,

,-

u

k,t 2

1

weight gain.

Ul”2l ~

dt

= -

k2[02][ P’] + ak,[POOH]

It can be assumed

that

one oxygen

atom

(9)

per

[02]=-

gt+2$(, 7

-exp-

!$I

7

(in mol ll’).

Kinetic study of the thermal oxidation of polypropylene

The corresponding induction period is thus: ti, = 2/k,. The corresponding weight gain rate is thus:

enthalpy of fusion AH,=86 J g-l. The IR spectrum does not indicate any spectral anomaly except very small peaks at 1720 and 3400 cm-‘. Their absorbance values correspond approximately to concentrations of 0.02 mol kg-’ for carbonyls and 0.06 mol kg-’ for hydroxyls. No absorption is observable in the UV spectrum, whereas strong bands at 22.5 and 280 nm, linked to the presence of a phenolic antioxidant, were observed in non-extracted films.

i.e. in terms of the mass fraction (W): dW -=-dt considering ll’.

32

a2

900 k,

235

1 - exp - $

3.2 Exposure

that the PP has a density of 900 g

Isothermal ageing tests were performed in air at 80, 90, 110 and 150 f 1°C in ventilated ovens. 3.3 Characterisation

3 EXPERIMENTAL

A Brucker IFS 28 FIIR spectrophotometer was used for infrared measurements. Examples of the spectral changes are presented in Figs 2 and 3. The absorbance values were determined at 3410 and 3550 cm-’ (OH groups with distinct hydrogen bonds) and 1714 cm-’ (carbonyl/carboxyl). In a first approach, the concentration of these species was calculated using the following molar absorptivity values:

3.1 Material The material under study was an industrial isotactic homo polymer PP sample supplied by ATOCHEM. Films of 100 f 20 ,zrn thickness were prepared from granules by moulding between Teflon plates. Additives, soluble impurities and low molecular weight amorphous PP were extracted from a mixture of ethanol-chloroform-hexane (1:1:4) in a soxhlet over 30 h. The initial characteristics of the samples were then determined after 48 h drying in vacuum at 30°C. The density was 913 f 6 g ll’ against 909 g ll’ for the starting granules. The melting point was 161°C and the

E+~ = 300 M-’ cm-’ Ref. 18 eOH= 70M-’ cm-’ Ref. 19, 20

0.8

0.6

3.00 2.00 1.I5 1.50 1.25 1.00 Initial

0

I 3750

I 3700

I 3650

I 3600

I 3550

I

3500

I 3450

I 3400

I

I

3350

3300

3:50

I

I

3200

3150

I 3100

Wavenumber cm-’ Fig. 2. Infrared

spectra

of hydroxyl

domain

at 150°C for different

oxidation

times.

I 3050

L. Achimsky

236

et al.

2.50 -

2.25 -

:/^

4.30

2.00 -

‘\ i

3.00 I .I5 .

--/

2.00

\

\ .32

1.50.

/

1.75

/ \\ ‘l\

I so

8 2 0 8

1.25

8 Q

‘\\ \

r

1.25

',

1.00

1.00

\

‘\ ’

/I

--

Initial

il

/

0.75 1

/’

/\,

iI,

0.50

0.25

0-

I= I

I

I875

1900

I 1800

1825

18iO

I 1750

I

1715

I 1725

Wavenumber

Fig. 3. Infrared

spectra

of carbonyl

domain

In a rigorous quantitative analytical study, these values would be questionable, but at this level of our investigation, they are sufficient because they give a good order of magnitude. The films were weighed using a Perkin Elmer AD-2Z micro balance with relative precision of 10mm4. 4 RESULTS 4.1 Mass increase The kinetic curves of mass changes are presented in Fig. 4. They display a noticeable difference with theoretical curves because a very important and non-predicted mass loss process takes place

/

!

A00

I675

lQ50

1625

I

1600

1575

1550

cm-’

at 150°C for different

oxidation

times.

in the final period of exposure. Globally, their shape is the same as previously found by Tudos et al.” and by Rychla et ~1.~~ To determine the corresponding duration of the pseudo-induction period, we shall assume that the tangent at the inflection point is not very different from the theoretical asymptote of the mass gain curve. In other words, it is assumed that weight loss becomes significant only beyond the inflection point. Then, according to our definition of induction period, the equation of the tangent at the inflection point would be: w = r,, (t - t, ,,> The values Table 1.

of

Y,, and

t,,,. are

reported

in

4.2 OH and C=O groups

_1t-i

: 111111/

0.1

i

!

t;;,;;I

Time

Fig. 4. Changes

, 10

1

of weight

‘kiil 100

: /

,tittJ 1000

(hours)

at 80. 90. 1 IO and 150°C.

The changes of IR spectrum in the domains of interest are illustrated in Figs 2 and 3. The kinetic curves of carbonyl and hydroxyl build-up are shown in Figs 5 and 6. They have similar shapes as theoretical curves except for their final part where the concentrations tend to stabilize or decrease whereas the model predicts a continuous linear increase. This feature is no doubt related to the previously found mass loss process.

Kinetic study of the thermal oxidation

of polypropylene

237

Table 1. Pseudo-induction times I~,, ti, and tiz, pseudo-stationary rates r,, r, and r,, calculated values t,/t,, apparent activation energies E, for polypropylene auto-oxidation at 80,90,110 and 150°C

T (“C)

t,, (h)

tu fh)

t,z(h)

80 90 110 150 E, (kJ/mol)

254 114.5 10.3 0.40 117

357 136 14.5 0.43 120.5

383 150 18 1.32 101.5

r, X IO'(s-l) r, X 10"(mol kg-’ SC’) 1.35 3.3 19.7 282 95

1..58 2.11 9.54 79.1 73

5.1 Comparison data

x = x0 + r,(t - tix) for OH groups and z = z0 + rz(t - tiz) for C = 0 groups where x0 and z0 are the initial concentrations of OH and C=O groups, respectively, t,, and tiz are the corresponding induction times, and r,, r, are the slopes of the linear part. The values of these parameters are also listed in Table 1.

1 Time

Fig. 5. Changes

10 (hours)

100

in hvdroxvl concentration a 150°C.

7.35 9.1 52.3 851 83.5

t&

t*A.

0.93 0.91 0.81 1.19 -

1.4 1.19 1.41 1.1 -

5 DISCUSSION

The linear part can be modelled by:

0.1

r, X lo7(mol kg-’ s-‘)

and

and t,/t,,,

1000

at 80, 90. 110 and

of our results with literature

In our previous article,14 the pseudo-induction time was graphically determined on kinetic curves of PP oxidation found in the literature and the corresponding points were placed in an Arrhenius graph. The fact that most of the points are close to a single straight-line and the value of the corresponding apparent activation energy (E=93.7 kJ mall’) can be considered as a strong argument in favour of the proposed ‘unimolecular closed loop scheme’. The same data are reported in Fig. 7 together with the experimental points obtained here. The existence of a single ‘universal’ straight-line (for reasonably clean samples) seems to be confirmed. The only difference with the preceding result is a slighty higher activaton energy (110.5 kJ mall’) in better agreement with theoretical considerations relative to POOH decomposition, see below. 5.2 Rate constant k, for unimolecular

POOH

decomposition

According to the theory, we expect t,lti, - 1 and ti,ltiw - 1.5. The induction time data in Table 1 0

1

1 / p

0.8 +

M

L

T=90”C T=l 10°C T=150”C

.A-

E

0.6 t-

2.2 10’

0.1

1 Time

Fig. 6. Changes

in carbonyl

10 (hours)

concentration 150°C.

100

1000

at 80, 90, 110 and

2.4 10’

2.6 10’ l/T

2.8 10-l (K’)

3 10-l

;

3.2 10’

Fig. 7. Arrhenius plot of induction time for polypropylene auto-oxidation from the following literature data (expansion of Fig. 5 in Ref. 14): (0) Ref. 14; (+) Ref. 21, (Cl) Ref. 23, (x) Ref. 24, ( + ) Ref. 25, ( n) Ref. 26, (A) Ref. 27, (V) Ref. 28, (0) our results given here.

L. Achimsky

238 Table 2. Kinetic data determined T (“c) 80 90 110 150

k,, x IOh (S ‘)

k,, x IO” (S ‘)

k,; X IO” (S ‘)

2.19 4.85 53.9 1389

2.33 6.13 57.5 1938

2.18 556 46.3 631.3

from the experimental u x IO7 (mol’”

rate

It can be determined

constant

a

from gravimetric

1“” s ‘)

results presented in Table 1 k,/v’k,

data that:

k7,v is a rate constant

determined

from

mot ““.s I”)

2.07 3.24 7.93 2Y.9

@

cp

4.4 2.9 I.8 1.4

1-9 I.1 0.8 0.49

the weight loss data. The results are listed in Table 2. The apparent activation energy E, is 106 kJ molt ‘. Since

we expect E, = E, + ; E, - ; E, provided that the radical yield a of POOH decomposition is independent of temperature in the conditions under study. If we take E, = ET,,, (apparent activation energy determined from weight gain data), we obtain E, - i

E, = 47.5 kJ mol

’

The reaction which limits the radical yield cy is a cage disproportionation of the radical pair resulting from POOH thermolysis, i.e. \ /

CH-OOH

L

I>

CH-O*+OH'] \ -/

C=O+H,O

In the case of PP, if tertiary POOH groups predominate, this reaction cannot occur. So, it can be assumed in a first approach, that LY= 2, which allows for the calculation of k,k; “2 whose values are listed in Table 2. They range between 2 x lo-’ and 30 x 10-5 ]I/2 mol-‘!’ s-l/?. 5.4 Steady state As has been shown in the theory section, the oxidation rate in the linear part (steady state) of the curves is proportional to a2/k, whatever the chosen criterion. It is interesting to remark that r_z

where

X IO’ (1”’

9.12 21.2 173 3319

lead to ratios of experimental values of induction times that are very close to the theoretical ones. The equality of induction times of OH (til) and C=O (tlz) build-up cannot be considered as a strong proof because it is possible to imagine a great diversity of mechanistic schemes in which both kinetics would be sharply linked. In contrast, the fact that induction times determined from IR data and those determined from gravimetric data are in the ratio of 3/2 cannot result from a coincidence. The values of k, determined from the values of ti are listed in Table 2. As expected, they are very close to one another, except perhaps at 150°C. where they vary of f 50% around an average value of 1.3 X 10. ’ s ~I. The corresponding apparent activation energies are E7= 120.5 kJ mol-’ (tlX), E,; = 101.5 kJ mol-’ (t,._) and E,, = 117 kJ mol-’ (t,,,,). As expected, they are very close to one another but significantly lower than the values obtained with the model compounds.‘7.2” In fact, the exact values of k, and the corresponding activation energy E, are expected to vary with the eventual catalytic effect of metallic impurities. Gijsman et ~1.“”have experimentally shown that the induction time is a decreasing function (and thus k, is an increasing function) of the concentration of titanium ions present in catalyst residues. Thus, small derivations from a hypothetical single ‘universal’ straight line In ti =f(T ‘) (Fig. 7) are not surprising. They correspond logically to variations in the catalytic impurities concentrations from sample to sample. The presence of catalytic impurities also explains, at least partially. why the activation energy determined in industrial PP samples is lower than for the model compounds. 5.3 Composite

et al.

a2 k:[ PH]‘a = k, k,

239

Kinetic study of the thermal oxidation of polypropylene

The

following

characteristics

are

thus

expected: apparent activation energy 1. A corresponding twice (E, - Ed2) i.e. about 95 kJ mall’. The experimental values determined from r,,*r, and E,(r,) =95 kJ mall’, rZ are, respectively, E,(r,) = 73 kJ mall’ and E,(r,) = 83.5 kJ mall’. From oxygen absorption measurements, Gijsman et aL9 obtained E,(r,,) = 80 kJ mall’. 2. The steady-state rate r, is expected to be independent of the initiation rate constant k,. This is well confirmed by the work of Gijsman.30 As previously reported, the induction time appears to depend sharply of the concentration of titanium ions. In contrast, the rate is independent of the ‘steady-state’ titanium ions concentration. Similar results were also found from chemiluminescence measurements by Celina and George:2 kinetic curves corresponding to distinct powder granules differ by their pseudo-induction time but are parallel in their maximum rate.

5.5 OH/C=0

ratio

The OH ‘yield’ Q, and the CO ‘yield’ decrease

continuously when the temperature increases. Thus, the OH/C=0 molar ratio remains constant when the temperature increases, which can be interpreted in terms of competition between hydrogen abstraction and p scission for the PO’ radicals resulting from POOH decomposition or escaping from the cage after a bimolecular combination of POO’ radicals. PO’ + PH PO’

+ --a

POH + P’ P=O + P’

k 71 k 72

here indicate that, presumably, E,, > E,,. aspect will be developed in a future article.

This

6 CONCLUSIONS From the experimental results reported here or available in the literature, the following set of arguments was obtained in favour of the ‘closed loop’ mechanistic scheme with unimolecular POOH decomposition applied to the thermal oxidation of non-stabilized isotactic polypropylene in air, in the 80-150°C temperature range. 1. The pseudo-induction times are close to universal values, almost independent of the polymer source for reasonably clean samples. 2. The apparent activation energy for induction times is about 100 kJ mall’ which corresponds to POOH unimolecular decomposition. 3. The weight increase (oxygen build-up), carbony1 and hydroxyl growth are characterized by induction times in the ratio of 2/3/3. 4. The activation energy for the steady-state oxidation rate is independent of the chosen structural variable and is about 80-90 kJ mall’. It is in good agreement with previously published data on oxygen absorption kinetics. rate is independent of 5. The steady-state initiation rate, i.e. of the presence of catalytic impurities which affect, on the contrary, induction times. All these characteristics are predicted by the proposed model. Thus, despite a marked heterogeneity at morphological scale,24.‘o the probable presence of many types of hydroperoxides’ and the presence of at least two P’ radicals of different reactivity: -7’-CHy CH3

(P)

and

- FH - CH2’

(p”),

CH3

We see thus that, in a first approach

dPH1 dt d[C = 0]

=

k71Dw k 72

dt In other words, the trend of the variation of the ratio (OH/C=O) will be given by the sign of (E,, - E,,), E,, and E,, being the apparent activation energies of, respectively, hydrogen abstraction and p scission. The results reported

P’resulting from normal propagation events and P” resulting of p scission of tertiary alkoxyls. The ‘closed loop’ scheme with unimolecular initiation seems to give a good overall description of the PP oxidation process between 80 and 150°C. Our research program included tests at temperatures lower than 80°C but their results were not reported here because a significant change of kinetic regime is observed between 80 and 70°C. The results reported here cannot be extrapolated to temperatures lower than 80°C.

L. Achimsky

240 A study of hydroperoxyde progress and will be published

build-up shortly.

is

in

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