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Initiated oxidation and auto-oxidation of polyethylene in trichlorobenzene solution
INITIATED OXIDATION AND A U T O - O X I D A T I O N OF P O L Y E T H Y L E N E IN T R I C H L O R O B E N Z E N E S O L U T I O N
M. IRING, T. KELEN Pc F. TOD6S
Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest, Hungary (Received: 18 September, 1978)
ABSTRACT
Thermal auto-oxidation and initiated oxidation (by dicumyl peroxide) of low density polyethylene were studied in trichlorobenzene solution. The rate dependence on the pressure of oxygen, temperature and concentrations of initiator and polymer have been investigated. Oxidation of the initiator itself has also been studied. The experimental results obtained are well interpreted by highly simplified mechanisms. Extrapolation from the rate data obtained in relatively dilute solutions at 160 °C to the rate of oxidation in the molten phase gave surprisingly good agreement with the previously measured experimental value.
1.
INTRODUCTION
Oxidation of polyolefins in solution has not been much studied. Some of the most important work in this field was reported by Dulog et al., 1 Denisov et al. "--'~ and Bawn and Chaudri. 1s The process has been studied by most people in the molten phase, because the data thus obtained are more or less directly applicable to the characterisation of the structural stability of the polymers under the conditions of processing. This led us some years ago to initiate a thorough investigation of the oxidation of polyolefins in the solid and molten phase. The thermal oxidation of low density polyethylene (PE) in the molten phase and of isotactic polypropylene (PP) in solid phase was studied. The effect of the role of transport on the rate of thermal oxidation under various experimental conditions was investigated and conversion and rate dependence of the critical thickness of polymer films were found for both PE and pp.5 The rate of oxygen uptake of both polymers increases with the concentration of 297 Polymer Degradation and Stability 0141-3910/79/0001-0297/$02.25 © Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
298
M. 1raNG, T. ICEt.EN,F. T~2D6S
oxygen. For PE the rate rises only slightly above 250 torr with further increase of pressure while for PP it is proportional to the 0-8th power of oxygen pressure over the whole region studied, i.e. even at 760 torr. 6 In the process, about half of the oxygen consumed by the polymer is bound in the condensed phase as hydroperoxide, hydroxy and oxo groups. Hydroperoxides are active intermediates in the process and their concentration versus time plots exhibit maxima. The end products of oxidation are accumulated autocatalytically.7 In the course of PE oxidation no measurable amount of alcoholic hydroxyl groups is incorporated in the polymer. At high conversion the oxygen is present almost quantitatively in the form of carbonyl groups of which a considerable proportion are carboxy, s The other half of the oxygen absorbed leaves the polymer in the form of volatile decomposition products of which about 90 ~ is water. A small portion of the oxygen appears in low molecular weight organic fragments, mainly oxo compounds. This is predominantly acetaldehyde in the case of PE, and acetone in the case of pp.7.9.1o The molecular weights of both polymers decrease considerably during the process. The average number of chain scissions depends linearly on the oxygen uptake and very slightly on reaction conditions such as temperature and pressure. 1L The average numbers of chain scissions and carboxy groups formed are nearly identical. 12 In the initiated oxidation of PE and PP, hydroperoxide (HP) groups are formed on the polymer chain; both polymers suffer fragmentation in this process. The ratios of the average number of incorporated HP groups to that of scissions are nearly identical for both polymers. There is, however, a characteristic difference between the two polymers: in the case of PE carbonyl groups are also incorporated in the chain in equal proportion with fragmentation, while with PP no carbonyl formation could be observed. The decomposition of HP groups built in during initiated oxidation was found to be a complex process in both polymers. The overall change in HP concentration involves several parallel processes, the rates and orders of which seem to depend upon the topochemistry of HP groups. The rate of decomposition of polyethylene hydroperoxide shows, for example, a correlation with the carbonyl content of the sample. ~3"1a During our extensive work in the field of thermal oxidation of polyolefins in the solid phase, we have become convinced that it is difficult to obtain kinetic relationships which describe the oxidation process quantitatively. The reason for this is partly that in the solid phase degradation, the morphologies of the original polymers may be different, or the reaction sometimes becomes diffusion controlled. These factors greatly limit the reproducibility of measurements. ~5.16 On t.he other hand, a simplified kinetic scheme is generally applied which describes the mechanism of liquid phase oxidation of low molecule hydrocarbons but does not describe the solid phase oxidation of macromolecules in all its complexity.16 In order to eliminate these problems, we studied the oxidation of polyolefins in
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
299
solution in trichlorobenzene in parallel with our experiments in the molten and solid phases.
2.
EXPERIMENTAL
2. l. M a t e r i a l s
Purified low density polyethylene (PE) powder was used. 7 1,2,4-trichlorobenzene (TCB), purified by extraction with sulphuric acid was subsequently distilled. Dicumyl peroxide (DC P) was recrystallised from ethanol/water, 72: 28 w/w, melting point 38 °C. 2.2. Oxidation The apparatus used is illustrated in Fig. 1. PE powder was weighed into the reaction vessel shown in Fig. l(a). In the case of auto-oxidation, the appropriate amount of solvent, and in that of initiated oxidation, of stock solution of initiator in TCB, was added to the sample. The reaction vessel was then connected to the oxidation device (Fig. 1(b)). The reaction vessel (4) and the empty reference vessel (3) were evacuated at room temperature and together with the burette (I) filled with oxygen at the required pressure through stopcocks (7 and 8) by opening the needle valve (2). The vessels were then placed in the thermostat (5) preheated to the temperature required. The reaction vessel was shaken by a stirrer motor connected to the eccentric sheave (or wheel) (6). After 10 minutes preheating (during which the PE dissolved in the TCB) the vessels were connected to the outlets of the measuring burette (l). During measurements, oxygen pressure in the reaction vessel decreases 2
_
, o
Fig. 1.
7
893
/85/ b
Oxidation apparatus; (a) reaction vessel, (b) line-diagram of oxygen absorption device.
300
M. I R I N G ,
T. KELEN, F. TOI~S TABLE 1 Thermal oxidation
Initiated oxidation
150 + 170 300 + 800
lO0 + 130 40 + 800
0.07 + 2-80
0.07 + 2-30
--
0"01. I0 - z + 8 " 5 . 1 0 - :
Temperature (T. °C) Oxygen pressure (P, tort) Polymer concentration ([RH], tool monomer unit/litre) Dicumyl peroxide concentration (1, mol/litre)
z.10'(m o_____,)
2o] W,.o//b 15
10"
100
200
t (rain)
300
Fig. 2. Change in oxygen uptake versus time; (a) auto-oxidation of PE, 160°C, 760 tort, [P,H] = 0,56 mol/lkre; (b) initiated oxidation of PE, 120°C, 760tort, [RH] = 0.56mo[/litre, [DCP]o = 1-48. [0--" mol/litre; (c) oxidation of DCP, 120°C, 760tort, [DCP]o = 1.48.10-z mol/litre.
INITIATED OXIDATION A N D A U T O - O X I D A T I O N OF P O L Y E T H Y L E N E
301
as oxygen is consumed and the level of the differential manometer of the measuring burette changes as indicated by an arrow in the figure. The ori~nal pressure (the level of the differential manometer) is restored by decreasing the volume of the corrugated teflon membrane connected to the burette and filled ~fith mercury. The volume contraction is proportional to the rise in the mercury level and can be read from the burette (10) (0.1 ml gradation). Solvent vapour is condensed in the condenser (9) thermostatted at 25 °C. The gas burette is also thermostatted at this temperature. 2.3. Experimental conditions Overall oxygen uptake (Z) was studied under the experimental conditions shown in Table 1.
3.
EXPERIMENTAL
RESULTS
3.1. Absorption of oxygen Auto-oxidation. Oxygen uptake versus time of auto-oxidation of 160°C is illustrated in Fig. 2 (curve a). The kinetics of thermal oxidation in solution is described by an S-shaped curve similar to that obtained in the molten phase,
Wz,o .1os ( rn_.___O2min-' ) 50
J /f ,s
30
f
o.
20
I
1,
Y
10 0
|
0
2
4
6
8
10
: .1o2( off ) Fig. 3. Dependence of the initial rate of oxygen uptake on initiator concentration, 120°C. 760 torr: a initiated oxidation of PE b oxidation of DCP.
302
M. IRING, T. KELEN, F. TI~II~S
although curve a has not attained the maximum rate. Maximum rates from kinetic curves obtained in thermal oxidation will be subsequently designated by W z . ~ Initiated oxidation. Oxygen absorption in initiated oxidation is illustrated in Fig. 2 (curve b) by a Z versus t plot obtained at 120°C. Curve c in Fig. 2 illustrates the oxygen uptake of DCP in the absence of polymer but under otherwise identical conditions. The rates of initiated oxidation are determined by the initial slopes of these curves (Wz.o). The dependence of this rate on the concentration of DCP is shown on a linear scale in Fig. 3, and on a logarithmic scale in Fig. 4 (curves a). Data
,og[ o /
•g l . t ,a "a
I
-1 -2
Fig. 4.
/
"
#
!
/
/
4-
-
0
1
T h e data of Fig. 3 plotted on a logarithmic scale.
for the oxidation of DCP are also illustrated in both figures (curves b). From Fig. 4, it may be calculated that the apparent reaction order of the initiated oxidation of PE with respect to the initiator concentration is about 0"7, while that of the oxidation of DCP is about 1.0. 3.2. Dependence of reaction rate on the temperature The temperature dependence of auto-oxidation (150-175°C) and initiated oxidation of PE in solution as well as that of oxidation of the initiator (100--130 °C) were also investigated. The Arrhenius-type relationships which can be derived from the experimental data will be discussed in Section 4 (Evaluation of Results).
INITIATED OXIDATION AND ALr'I'O"OXIDATION OF POLYETHYLENE
303
3.3. Dependence of the reaction rate on oxygen pressure The initial rate of initiated oxidation is independent of oxygen pressure above approximately 200 torr. The pressure dependence of auto-oxidation could only be studied above 200 torr (at lower pressures, TCB boils above 150 °C) and the rate of auto-oxidation was algo found to be independent of oxygen pressure in this region. 3.4. Dependence of the reaction rate on polymer concentration Curve a of Fig. 5 shows the dependence of the maximum rate of auto-oxidation at 160 °C on polymer concentration, while curve b illustrates the initial rate of initiated oxidation at 120°C (I o = 1-48.10 -2 mol/litre). In curve b, the rate of oxidation of Wz " 10s
('-~2
rain-'
) /
100"
~7
/
/ / /
.~.
5o
/
./i
/
u/.
/ b
. . . .
o
i
2
)
Fig. 5. Dependence of the rate of oxygen uptake on polymer concentration; (a) maximum rate of auto-oxidation of PE, 160°C, 760torr; (b) initial rate of initiated oxidation of PE, 120°C, 760torr, [DCP]o = 1.48.10 -z mol/litre.
DCP itself is also shown (point C) at [RH] = 0). The rate of initiated oxidation shows some slight dependence on the concentration, but there is strong concentration dependence in auto-oxidation.
4.
EVALUATIONOF RESULTS
The initiated oxidation of PE is a process of high complexity in which various elementary reactions are combined such as oxidation of the initiator, initiated oxidation itself and auto-oxidation of the polymer. These processes can be separately described by the following simplified reaction steps•
304
M. IRING, T. KELEN, F. TUDOS
(a) Oxidation o f dicumyl peroxide
Dicumyl peroxide (I) decomposes into cumyloxy radicals which, upon fast isomerisation, further decompose and yield methyl (R') radicals. These radicals, in the presence of sutficient oxygen instantaneously form peroxide radicals: CH 3
CH3
I
CH3
1
I
Ph--C~O---O---C--Ph
I
I
CH 3
CH3
k", 2Ph--C---C"
(1)
I CH3
CH 3
I ph--CmO.
, Ph--C--CH 3 + C H 3-
I
LI
CH 3
O
C H 3' + 0 2
.~ C H 3 0 2 .
(2)
(3)
Some of the radicals may combine to form inert end products: CH3
CH 3
t
I
Ph--C--O. + CH 3.
, Ph--C--O--CH 3
I
I
CH3
CH3
(4)
If the probability of radicals reacting in this way is I - f ' , then only the fractionf', of the radicals formed actually initiates oxidation. Since the thermal decomposition of the initiator is the rate determining process in dicumyl peroxide oxidation, the above steps can be summarised in the following 'stoichiometric' equation i(+2f,O2 )
k, , product + 2f'R'O 2"
(5)
If the system contains only dicumyl peroxide, the peroxide radical presumably does not consume more oxygen in an inert solvent and undergoes bimolecular termination. The amount of oxygen formed in the termination reaction has not been taken into account in this simplified treatment. Thus the rate of oxygen absorption in the system containing only the initiator is the following: W z = 2f'kaI = 2f'kaI o e-*"
(6)
where I 0 represents the initial concentration of dicumyl peroxide. The Arrheniustype relationship found experimentally based on the initial rate of DC P oxidation is: f ' k a = 2.78.101'~ exp ( - 31,350/RT) min- l
(7)
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
305
Using eqn. (7) it may be calculated that the value of f ' k d at I I0°C is 6-1. t 0 - 6 s - i . Considering the fact that the value determined in different solvents for the rate constant of thermal decomposition of DCP is (8-11). 10 - 6 S - l t7 the approximate value of f ' is 0.6-0.8, which seems physically acceptable. The experimentally determined activation energy is also in rather good agreement with the literature (~34kcal/mol). This indicates that the assumption of thermal decomposition of the initiator as the rate determining step of DCP oxidation is a rather good approximation. This assumption is also supported by the fact that the rate of oxygen absorption is proportional to the first power of the DCP concentration.
(b) Initiated oxidation of PE The process is initiated by R'O 2 radicals formed in reaction (3):
(8)
R'O 2. + RH -, R'O2H + R. R. + 02 --, RO 2.
(9)
It has been assumed that the concentration of hydroperoxide formed in reaction (8) is not significant in the initial stage of PE oxidation and that in the presence of sufficient amounts of hydrocarbon and oxygen both reactions are very fast. Thus the initiation of PE oxidation by dicumyl peroxide can be described, on combining eqns. (5), (8) and (9), by the following stoichiometric equation: I(+4f'O2)
~" , product + 2f'RO2"
(10)
Since degenerative chain-branching, which is very important in auto-oxidation, is not important in the early stages of initiated oxidation, at an adequately high rate of initiation only chain propogation steps need be taken into account: RO 2. + RH - - ~ product + R. R- + 0 2
, RO 2.
(11) (12)
i.e. at sufficiently high oxygen pressure: RO 2- + RH(+ 02)
k:, product + RO 2"
(13)
The process terminates in the bimolecular reaction of propagating radicals: RO2. + RO2.
k, , product
(14)
The amount of oxygen formed in the termination reaction is neglected in this present very simplified treatment.
M. IRING, T. KELEN, F. TODOS
306
Thus in the steady state,
d[R02"] dt
= 2f'kdI -- k,[RO2"] z = 0
(15)
Thus,
2f•kdI
(16)
The rate of oxygen absorption is given by:
W z = 4f'kal + -~k44x/2f'kaI[RH]
(17)
Thus the dependence of the initial rate of initiated oxidation on the initiator and hydrocarbon concentrations is given by,
Wz'° = 4f'kd
~.
~
[RH]°
x/~o
(18)
in which I o is the initial concentration of initiator. The plot of experimental data according to (18) is shown in Fig. 6. It is clear that this relationship applies over a wide range of initiator and polymer concentrations (see Section 2.3). T h e f ' k d value (1.04.10 - 3 rain - ~) determined from the intercept is in good agreement with that calculated from relation (7). The k2/x/-~a value calculated from the slope is 9.4.10- 3 (mol- * litre min- t)l _,. Applying eqns. (7) and (18), experimental data on the temperature dependence of Wz.o leads to the following Arrhenius-relationship for the constant k2/x//k.~: k2
,_-- = 1-02.10 s . e x p ( - 12,600/RT)(mol- t litre min- t)~/2
(19)
x/k, The activation energy seems to be reasonable compared with other (literature) data.
(c) Auto-oxidation of PE In the absence of initiator, the initiation step is probably the direct reaction between oxygen and hydrocarbon: R H + 0 , - - , R . + H O 2.
(20)
HO_," + RH ~ H.,O 2 + R-
(21)
R. + 0 2 ---, RO 2"
(22)
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
307
WZ~ . 10'~n-1]
;o
16
12 ~+
I
4f
iI I
0
0
5
5
20
[RH][~I_]~ Fig. 6.
Plot of initial rate of initiated oxidation of PE according to (18), 120°C, 760tort; .. [DCP]o = 1.48.10 -z [RH]o varying; + , [RH]o = 0"56 mol/litre, [DCP]o varying.
or, in combined form RH + 02 ( + RH + 20.,)
k,., product + 2RO_,-
(23)
In the description of the oxidation of PE in the molten phase 6 the chain propagation step was given as follows: RO 2 - + R H R-+O 2
k:,, R O 2 H + R .
(24)
, RO 2-
(25)
~'.. RO2H + RO2"
(26)
or, in a combined form: RO 2- + R H ( + 02)
On the basis of our experiments in solution we assume, however, that in addition to reaction (26), direct isomerisation of polymer peroxy radicals (i.e. without hydroperoxide formation) into oxygen-containing end-products (e.g. acids) may
308
M. IRING, T. KELEN, F. TLrDOS
take place. The process involves simultaneous chain scission and formation of hydrocarbon radicals and can be written in combined form analogously with (13): RO z - + R H ( + 0 2 )
k?:, p r o d u c t + R O z.
(27)
That is, only the rate constants are different (k 2 = kzl + k22 ). Chain branching is described by a relationship already used in our previous work: 6 RO2H [+ (1 + f ) R H + 2fO2]
k, , product + 2fRO 2.
(28)
where f denotes the coefficient characterising the cage effect of hydroperoxide decomposition. Termination takes place in a bimolecular reaction analogous to (14); the oxygen formed is again neglected. Oxygen absorption in a system containing no initiator is thus described by the following equation:
Wz =
3k,[O2][RH ] + k2[RO2"I[RH] + 2fk3[RO2H]
(29)
and the steady state expression for radical concentration is as follows: d[RO2' ] = 2kt[O2][RH ] + 2fk3[ROzH ] - k~[RO2"] 2 = 0 dt
(30)
For the estimation of rate dependence on polymer concentration, the process may be divided into two stages: (A) In the initial stage of oxidation, in which initiation takes place predominantly in reaction (23) and the radical yield due to chain branching can be neglected, eqn. (30) leads to,
_ /2kltO21
[RO2"] -,. q
k,
Hl
(31)
and
Wz~3kl[Oz][RH]+ k 2 ~ k - ~ O 2] [RH] 3/2
(32)
That is, in this stage the apparent reaction order of oxygen absorption with respect to polymer concentration ranges between 1 and 1.5. (B) In the stage in which the rate of oxidation is maximal, the production of radicals takes place mainly during chain branching, and the radical yield due to
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
309
reaction (23) is negligible. In this stage the concentration of hydroperoxide reaches a maximum value and
d[ROzH] at
= kzt[RH][RO2-]
-
k3[ROzH ] =
0
(33)
Thus, by neglecting (23), (30) gives: / 2 f k 3 [ROzH] [RO2"I ~ k/ ~
(34)
It follows from (33) and (34) that, 2 2 [RO2HI = ~[fk21
[RH]2
~
(35)
and
[RO2"] -~ ~fk21 [RH]
(36)
The rate of oxygen absorption is: Wz ~ (k, + 2fk21) 2fk21 [RH] 2 k,
(37)
Wz, max • 10~rI---~ min-"l 150
100
50
0 Fig. 7.
Of '/
/
I-
fD /
3
Plot of maximum rate of auto-oxidation of PE according to (37), 160~C, 760torr.
310
M. IRING, T. KELEN, F. TODOS
T h a t is, in this stage the a p p a r e n t r e a c t i o n o r d e r o f the process with respect to p o l y m e r c o n c e n t r a t i o n is 2. E x p e r i m e n t a l d a t a are r e p r e s e n t e d by this r e l a t i o n s h i p as shown in Fig. 7. T h e d a t a o b t a i n e d at 160°C a n d p l o t t e d versus [RH] in Fig. 5 are presented here versus [RH ]2 a n d give a linear d e p e n d e n c e . By e x t r a p o l a t i o n o f the slope ( I . 2 5 . 1 0 - ~ t o o l - t litre r a i n - i) to p o l y m e r c o n c e n t r a t i o n c o r r e s p o n d i n g to the m o l t e n phase (32-93 mol/litre), we o b t a i n 0.135 tool l i t r e - i r a i n - t for the rate o f oxygen u p t a k e which is in very g o o d a g r e e m e n t with the e x p e r i m e n t a l l y m e a s u r e d value (0.107). This indicates t h a t the m e c h a n i s m s o f the a u t o - o x i d a t i o n o f PE in T C B s o l u t i o n and in the m o l t e n p h a s e at 160°C are not m u c h different. Based on o u r e x p e r i m e n t a l d a t a for the t e m p e r a t u r e d e p e n d e n c e o f the m a x i m u m rate o f o x y g e n u p t a k e in a u t o - o x i d a t i o n , a n d by the use o f (37), we o b t a i n the following A r r h e n i u s - t y p e r e l a t i o n s h i p : (k 2 + 2fk21) 2jk21- = 4"26. 101v e x p ( - 4 2 , 6 0 0 / R T ) ka
mol -~ litre min -1
(38)
The difference in the a c t i v a t i o n energies (in m o l t e n phase o x i d a t i o n : ~ 35 kcal) indicates t h a t there m a y be some differences.
REFERENCES
1. L. DULOG, E. RADLMANNand W. KERN, Makr. Chem., 60, 1 (1963). 2. N. V. ZOLO'rOVAand E. T. DENISOV,Vv~. SqC.d, B, 12, 866 (1970). 3. Ju. B. SmLov and E. T. DENISOV, Vys. Soed. A, 16, 662 (1974). 4. P. A. [VANCHENKO.E. T. DENISOVand V. V. Ki.(hRtTONOv,'Kinetika i Katali-. 12. 492 (1971). 5. M. IRING,S. LASZL6-HEDVlG,T. KELENand F. T0o6s, Thermal analysis. Proceedings 4th ICTA. Budapest, 2, 127 (1974). 6. T. KELEN, M. [RINGand F. TOo6s, Europ. Polym. J., 12, 35 (1976). 7. M. IRING.T. KELENand F. Tf2t~Ss, Makr. Chem., 175/'2, 467 (1974). 8. M. [RING, S. LS,SZL6-HEDvtG, K. BARAB~XS,T. KELENand F. TOl:ffJs, Europ. Polym. J., 14,439 (1978). 9. K. B.-XRAB.~S,M. IRING,T. KELENand F. TOD6s,;J. Polym. Sci., Syrup. No. 57, 65 (1976). 10. K. BARAB,~S,M. IRING, S. L~SZLO-HEDVIG,T. KELEN and F. Tt~'o6s,'Europ. Polym. J., 14, 405 (1978). I 1. M. IRING,S. LiXSZLb-HEDVlG,T. KELEN,F. TOD6S,'L. FE'ZES,G. SAMAIand G. BODOR,J. Pol. Sci., Syrup. No. 57, 55 (1976). 12. M. IRING, F. Ti)t)6s, Zs. FODO~,and T. KELEN(to be published). 13. M. [RING,T. KELE,'q,F. TOD6Sand S. L,kSZLO-HEDVlG,J. Polym. Sci., Syrup. No. 57, 89 (1976). 14. S. L~.SZLO-Hr:DVtG,M. IRING,G. B~.LIN'r,T. KELENand F. TOD6S, Magy. K~m. F., 83, 258 (1977). 15. A. L. BUCHACHENKO,Specificity of oxidation of polyolefins in the solid phase, Main lecture in the 15th Prague Microsymposium on Macromolecules, Prague July 21-24 (1975). 16. L. R~,CH and S. S. STIVALA,Autooxidation of hydrocarbons andpol.votefins, 7. New York, Marcel Dekker Inc. (1969). 17. V. L. ANTONOVSKII,Organicheskie Perekisnye Iniciatory, lzd. Khimia, Moskva (1972). 18. C. E. H. BAWNand S. A. CHAUD~, Polymer, 9, 113, 123 (1968).
M. IRING, T. KELEN Pc F. TOD6S
Central Research Institute for Chemistry of the Hungarian Academy of Sciences, Budapest, Hungary (Received: 18 September, 1978)
ABSTRACT
Thermal auto-oxidation and initiated oxidation (by dicumyl peroxide) of low density polyethylene were studied in trichlorobenzene solution. The rate dependence on the pressure of oxygen, temperature and concentrations of initiator and polymer have been investigated. Oxidation of the initiator itself has also been studied. The experimental results obtained are well interpreted by highly simplified mechanisms. Extrapolation from the rate data obtained in relatively dilute solutions at 160 °C to the rate of oxidation in the molten phase gave surprisingly good agreement with the previously measured experimental value.
1.
INTRODUCTION
Oxidation of polyolefins in solution has not been much studied. Some of the most important work in this field was reported by Dulog et al., 1 Denisov et al. "--'~ and Bawn and Chaudri. 1s The process has been studied by most people in the molten phase, because the data thus obtained are more or less directly applicable to the characterisation of the structural stability of the polymers under the conditions of processing. This led us some years ago to initiate a thorough investigation of the oxidation of polyolefins in the solid and molten phase. The thermal oxidation of low density polyethylene (PE) in the molten phase and of isotactic polypropylene (PP) in solid phase was studied. The effect of the role of transport on the rate of thermal oxidation under various experimental conditions was investigated and conversion and rate dependence of the critical thickness of polymer films were found for both PE and pp.5 The rate of oxygen uptake of both polymers increases with the concentration of 297 Polymer Degradation and Stability 0141-3910/79/0001-0297/$02.25 © Applied Science Publishers Ltd, England, 1979 Printed in Great Britain
298
M. 1raNG, T. ICEt.EN,F. T~2D6S
oxygen. For PE the rate rises only slightly above 250 torr with further increase of pressure while for PP it is proportional to the 0-8th power of oxygen pressure over the whole region studied, i.e. even at 760 torr. 6 In the process, about half of the oxygen consumed by the polymer is bound in the condensed phase as hydroperoxide, hydroxy and oxo groups. Hydroperoxides are active intermediates in the process and their concentration versus time plots exhibit maxima. The end products of oxidation are accumulated autocatalytically.7 In the course of PE oxidation no measurable amount of alcoholic hydroxyl groups is incorporated in the polymer. At high conversion the oxygen is present almost quantitatively in the form of carbonyl groups of which a considerable proportion are carboxy, s The other half of the oxygen absorbed leaves the polymer in the form of volatile decomposition products of which about 90 ~ is water. A small portion of the oxygen appears in low molecular weight organic fragments, mainly oxo compounds. This is predominantly acetaldehyde in the case of PE, and acetone in the case of pp.7.9.1o The molecular weights of both polymers decrease considerably during the process. The average number of chain scissions depends linearly on the oxygen uptake and very slightly on reaction conditions such as temperature and pressure. 1L The average numbers of chain scissions and carboxy groups formed are nearly identical. 12 In the initiated oxidation of PE and PP, hydroperoxide (HP) groups are formed on the polymer chain; both polymers suffer fragmentation in this process. The ratios of the average number of incorporated HP groups to that of scissions are nearly identical for both polymers. There is, however, a characteristic difference between the two polymers: in the case of PE carbonyl groups are also incorporated in the chain in equal proportion with fragmentation, while with PP no carbonyl formation could be observed. The decomposition of HP groups built in during initiated oxidation was found to be a complex process in both polymers. The overall change in HP concentration involves several parallel processes, the rates and orders of which seem to depend upon the topochemistry of HP groups. The rate of decomposition of polyethylene hydroperoxide shows, for example, a correlation with the carbonyl content of the sample. ~3"1a During our extensive work in the field of thermal oxidation of polyolefins in the solid phase, we have become convinced that it is difficult to obtain kinetic relationships which describe the oxidation process quantitatively. The reason for this is partly that in the solid phase degradation, the morphologies of the original polymers may be different, or the reaction sometimes becomes diffusion controlled. These factors greatly limit the reproducibility of measurements. ~5.16 On t.he other hand, a simplified kinetic scheme is generally applied which describes the mechanism of liquid phase oxidation of low molecule hydrocarbons but does not describe the solid phase oxidation of macromolecules in all its complexity.16 In order to eliminate these problems, we studied the oxidation of polyolefins in
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
299
solution in trichlorobenzene in parallel with our experiments in the molten and solid phases.
2.
EXPERIMENTAL
2. l. M a t e r i a l s
Purified low density polyethylene (PE) powder was used. 7 1,2,4-trichlorobenzene (TCB), purified by extraction with sulphuric acid was subsequently distilled. Dicumyl peroxide (DC P) was recrystallised from ethanol/water, 72: 28 w/w, melting point 38 °C. 2.2. Oxidation The apparatus used is illustrated in Fig. 1. PE powder was weighed into the reaction vessel shown in Fig. l(a). In the case of auto-oxidation, the appropriate amount of solvent, and in that of initiated oxidation, of stock solution of initiator in TCB, was added to the sample. The reaction vessel was then connected to the oxidation device (Fig. 1(b)). The reaction vessel (4) and the empty reference vessel (3) were evacuated at room temperature and together with the burette (I) filled with oxygen at the required pressure through stopcocks (7 and 8) by opening the needle valve (2). The vessels were then placed in the thermostat (5) preheated to the temperature required. The reaction vessel was shaken by a stirrer motor connected to the eccentric sheave (or wheel) (6). After 10 minutes preheating (during which the PE dissolved in the TCB) the vessels were connected to the outlets of the measuring burette (l). During measurements, oxygen pressure in the reaction vessel decreases 2
_
, o
Fig. 1.
7
893
/85/ b
Oxidation apparatus; (a) reaction vessel, (b) line-diagram of oxygen absorption device.
300
M. I R I N G ,
T. KELEN, F. TOI~S TABLE 1 Thermal oxidation
Initiated oxidation
150 + 170 300 + 800
lO0 + 130 40 + 800
0.07 + 2-80
0.07 + 2-30
--
0"01. I0 - z + 8 " 5 . 1 0 - :
Temperature (T. °C) Oxygen pressure (P, tort) Polymer concentration ([RH], tool monomer unit/litre) Dicumyl peroxide concentration (1, mol/litre)
z.10'(m o_____,)
2o] W,.o//b 15
10"
100
200
t (rain)
300
Fig. 2. Change in oxygen uptake versus time; (a) auto-oxidation of PE, 160°C, 760 tort, [P,H] = 0,56 mol/lkre; (b) initiated oxidation of PE, 120°C, 760tort, [RH] = 0.56mo[/litre, [DCP]o = 1-48. [0--" mol/litre; (c) oxidation of DCP, 120°C, 760tort, [DCP]o = 1.48.10-z mol/litre.
INITIATED OXIDATION A N D A U T O - O X I D A T I O N OF P O L Y E T H Y L E N E
301
as oxygen is consumed and the level of the differential manometer of the measuring burette changes as indicated by an arrow in the figure. The ori~nal pressure (the level of the differential manometer) is restored by decreasing the volume of the corrugated teflon membrane connected to the burette and filled ~fith mercury. The volume contraction is proportional to the rise in the mercury level and can be read from the burette (10) (0.1 ml gradation). Solvent vapour is condensed in the condenser (9) thermostatted at 25 °C. The gas burette is also thermostatted at this temperature. 2.3. Experimental conditions Overall oxygen uptake (Z) was studied under the experimental conditions shown in Table 1.
3.
EXPERIMENTAL
RESULTS
3.1. Absorption of oxygen Auto-oxidation. Oxygen uptake versus time of auto-oxidation of 160°C is illustrated in Fig. 2 (curve a). The kinetics of thermal oxidation in solution is described by an S-shaped curve similar to that obtained in the molten phase,
Wz,o .1os ( rn_.___O2min-' ) 50
J /f ,s
30
f
o.
20
I
1,
Y
10 0
|
0
2
4
6
8
10
: .1o2( off ) Fig. 3. Dependence of the initial rate of oxygen uptake on initiator concentration, 120°C. 760 torr: a initiated oxidation of PE b oxidation of DCP.
302
M. IRING, T. KELEN, F. TI~II~S
although curve a has not attained the maximum rate. Maximum rates from kinetic curves obtained in thermal oxidation will be subsequently designated by W z . ~ Initiated oxidation. Oxygen absorption in initiated oxidation is illustrated in Fig. 2 (curve b) by a Z versus t plot obtained at 120°C. Curve c in Fig. 2 illustrates the oxygen uptake of DCP in the absence of polymer but under otherwise identical conditions. The rates of initiated oxidation are determined by the initial slopes of these curves (Wz.o). The dependence of this rate on the concentration of DCP is shown on a linear scale in Fig. 3, and on a logarithmic scale in Fig. 4 (curves a). Data
,og[ o /
•g l . t ,a "a
I
-1 -2
Fig. 4.
/
"
#
!
/
/
4-
-
0
1
T h e data of Fig. 3 plotted on a logarithmic scale.
for the oxidation of DCP are also illustrated in both figures (curves b). From Fig. 4, it may be calculated that the apparent reaction order of the initiated oxidation of PE with respect to the initiator concentration is about 0"7, while that of the oxidation of DCP is about 1.0. 3.2. Dependence of reaction rate on the temperature The temperature dependence of auto-oxidation (150-175°C) and initiated oxidation of PE in solution as well as that of oxidation of the initiator (100--130 °C) were also investigated. The Arrhenius-type relationships which can be derived from the experimental data will be discussed in Section 4 (Evaluation of Results).
INITIATED OXIDATION AND ALr'I'O"OXIDATION OF POLYETHYLENE
303
3.3. Dependence of the reaction rate on oxygen pressure The initial rate of initiated oxidation is independent of oxygen pressure above approximately 200 torr. The pressure dependence of auto-oxidation could only be studied above 200 torr (at lower pressures, TCB boils above 150 °C) and the rate of auto-oxidation was algo found to be independent of oxygen pressure in this region. 3.4. Dependence of the reaction rate on polymer concentration Curve a of Fig. 5 shows the dependence of the maximum rate of auto-oxidation at 160 °C on polymer concentration, while curve b illustrates the initial rate of initiated oxidation at 120°C (I o = 1-48.10 -2 mol/litre). In curve b, the rate of oxidation of Wz " 10s
('-~2
rain-'
) /
100"
~7
/
/ / /
.~.
5o
/
./i
/
u/.
/ b
. . . .
o
i
2
)
Fig. 5. Dependence of the rate of oxygen uptake on polymer concentration; (a) maximum rate of auto-oxidation of PE, 160°C, 760torr; (b) initial rate of initiated oxidation of PE, 120°C, 760torr, [DCP]o = 1.48.10 -z mol/litre.
DCP itself is also shown (point C) at [RH] = 0). The rate of initiated oxidation shows some slight dependence on the concentration, but there is strong concentration dependence in auto-oxidation.
4.
EVALUATIONOF RESULTS
The initiated oxidation of PE is a process of high complexity in which various elementary reactions are combined such as oxidation of the initiator, initiated oxidation itself and auto-oxidation of the polymer. These processes can be separately described by the following simplified reaction steps•
304
M. IRING, T. KELEN, F. TUDOS
(a) Oxidation o f dicumyl peroxide
Dicumyl peroxide (I) decomposes into cumyloxy radicals which, upon fast isomerisation, further decompose and yield methyl (R') radicals. These radicals, in the presence of sutficient oxygen instantaneously form peroxide radicals: CH 3
CH3
I
CH3
1
I
Ph--C~O---O---C--Ph
I
I
CH 3
CH3
k", 2Ph--C---C"
(1)
I CH3
CH 3
I ph--CmO.
, Ph--C--CH 3 + C H 3-
I
LI
CH 3
O
C H 3' + 0 2
.~ C H 3 0 2 .
(2)
(3)
Some of the radicals may combine to form inert end products: CH3
CH 3
t
I
Ph--C--O. + CH 3.
, Ph--C--O--CH 3
I
I
CH3
CH3
(4)
If the probability of radicals reacting in this way is I - f ' , then only the fractionf', of the radicals formed actually initiates oxidation. Since the thermal decomposition of the initiator is the rate determining process in dicumyl peroxide oxidation, the above steps can be summarised in the following 'stoichiometric' equation i(+2f,O2 )
k, , product + 2f'R'O 2"
(5)
If the system contains only dicumyl peroxide, the peroxide radical presumably does not consume more oxygen in an inert solvent and undergoes bimolecular termination. The amount of oxygen formed in the termination reaction has not been taken into account in this simplified treatment. Thus the rate of oxygen absorption in the system containing only the initiator is the following: W z = 2f'kaI = 2f'kaI o e-*"
(6)
where I 0 represents the initial concentration of dicumyl peroxide. The Arrheniustype relationship found experimentally based on the initial rate of DC P oxidation is: f ' k a = 2.78.101'~ exp ( - 31,350/RT) min- l
(7)
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
305
Using eqn. (7) it may be calculated that the value of f ' k d at I I0°C is 6-1. t 0 - 6 s - i . Considering the fact that the value determined in different solvents for the rate constant of thermal decomposition of DCP is (8-11). 10 - 6 S - l t7 the approximate value of f ' is 0.6-0.8, which seems physically acceptable. The experimentally determined activation energy is also in rather good agreement with the literature (~34kcal/mol). This indicates that the assumption of thermal decomposition of the initiator as the rate determining step of DCP oxidation is a rather good approximation. This assumption is also supported by the fact that the rate of oxygen absorption is proportional to the first power of the DCP concentration.
(b) Initiated oxidation of PE The process is initiated by R'O 2 radicals formed in reaction (3):
(8)
R'O 2. + RH -, R'O2H + R. R. + 02 --, RO 2.
(9)
It has been assumed that the concentration of hydroperoxide formed in reaction (8) is not significant in the initial stage of PE oxidation and that in the presence of sufficient amounts of hydrocarbon and oxygen both reactions are very fast. Thus the initiation of PE oxidation by dicumyl peroxide can be described, on combining eqns. (5), (8) and (9), by the following stoichiometric equation: I(+4f'O2)
~" , product + 2f'RO2"
(10)
Since degenerative chain-branching, which is very important in auto-oxidation, is not important in the early stages of initiated oxidation, at an adequately high rate of initiation only chain propogation steps need be taken into account: RO 2. + RH - - ~ product + R. R- + 0 2
, RO 2.
(11) (12)
i.e. at sufficiently high oxygen pressure: RO 2- + RH(+ 02)
k:, product + RO 2"
(13)
The process terminates in the bimolecular reaction of propagating radicals: RO2. + RO2.
k, , product
(14)
The amount of oxygen formed in the termination reaction is neglected in this present very simplified treatment.
M. IRING, T. KELEN, F. TODOS
306
Thus in the steady state,
d[R02"] dt
= 2f'kdI -- k,[RO2"] z = 0
(15)
Thus,
2f•kdI
(16)
The rate of oxygen absorption is given by:
W z = 4f'kal + -~k44x/2f'kaI[RH]
(17)
Thus the dependence of the initial rate of initiated oxidation on the initiator and hydrocarbon concentrations is given by,
Wz'° = 4f'kd
~.
~
[RH]°
x/~o
(18)
in which I o is the initial concentration of initiator. The plot of experimental data according to (18) is shown in Fig. 6. It is clear that this relationship applies over a wide range of initiator and polymer concentrations (see Section 2.3). T h e f ' k d value (1.04.10 - 3 rain - ~) determined from the intercept is in good agreement with that calculated from relation (7). The k2/x/-~a value calculated from the slope is 9.4.10- 3 (mol- * litre min- t)l _,. Applying eqns. (7) and (18), experimental data on the temperature dependence of Wz.o leads to the following Arrhenius-relationship for the constant k2/x//k.~: k2
,_-- = 1-02.10 s . e x p ( - 12,600/RT)(mol- t litre min- t)~/2
(19)
x/k, The activation energy seems to be reasonable compared with other (literature) data.
(c) Auto-oxidation of PE In the absence of initiator, the initiation step is probably the direct reaction between oxygen and hydrocarbon: R H + 0 , - - , R . + H O 2.
(20)
HO_," + RH ~ H.,O 2 + R-
(21)
R. + 0 2 ---, RO 2"
(22)
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
307
WZ~ . 10'~n-1]
;o
16
12 ~+
I
4f
iI I
0
0
5
5
20
[RH][~I_]~ Fig. 6.
Plot of initial rate of initiated oxidation of PE according to (18), 120°C, 760tort; .. [DCP]o = 1.48.10 -z [RH]o varying; + , [RH]o = 0"56 mol/litre, [DCP]o varying.
or, in combined form RH + 02 ( + RH + 20.,)
k,., product + 2RO_,-
(23)
In the description of the oxidation of PE in the molten phase 6 the chain propagation step was given as follows: RO 2 - + R H R-+O 2
k:,, R O 2 H + R .
(24)
, RO 2-
(25)
~'.. RO2H + RO2"
(26)
or, in a combined form: RO 2- + R H ( + 02)
On the basis of our experiments in solution we assume, however, that in addition to reaction (26), direct isomerisation of polymer peroxy radicals (i.e. without hydroperoxide formation) into oxygen-containing end-products (e.g. acids) may
308
M. IRING, T. KELEN, F. TLrDOS
take place. The process involves simultaneous chain scission and formation of hydrocarbon radicals and can be written in combined form analogously with (13): RO z - + R H ( + 0 2 )
k?:, p r o d u c t + R O z.
(27)
That is, only the rate constants are different (k 2 = kzl + k22 ). Chain branching is described by a relationship already used in our previous work: 6 RO2H [+ (1 + f ) R H + 2fO2]
k, , product + 2fRO 2.
(28)
where f denotes the coefficient characterising the cage effect of hydroperoxide decomposition. Termination takes place in a bimolecular reaction analogous to (14); the oxygen formed is again neglected. Oxygen absorption in a system containing no initiator is thus described by the following equation:
Wz =
3k,[O2][RH ] + k2[RO2"I[RH] + 2fk3[RO2H]
(29)
and the steady state expression for radical concentration is as follows: d[RO2' ] = 2kt[O2][RH ] + 2fk3[ROzH ] - k~[RO2"] 2 = 0 dt
(30)
For the estimation of rate dependence on polymer concentration, the process may be divided into two stages: (A) In the initial stage of oxidation, in which initiation takes place predominantly in reaction (23) and the radical yield due to chain branching can be neglected, eqn. (30) leads to,
_ /2kltO21
[RO2"] -,. q
k,
Hl
(31)
and
Wz~3kl[Oz][RH]+ k 2 ~ k - ~ O 2] [RH] 3/2
(32)
That is, in this stage the apparent reaction order of oxygen absorption with respect to polymer concentration ranges between 1 and 1.5. (B) In the stage in which the rate of oxidation is maximal, the production of radicals takes place mainly during chain branching, and the radical yield due to
INITIATED OXIDATION AND AUTO-OXIDATION OF POLYETHYLENE
309
reaction (23) is negligible. In this stage the concentration of hydroperoxide reaches a maximum value and
d[ROzH] at
= kzt[RH][RO2-]
-
k3[ROzH ] =
0
(33)
Thus, by neglecting (23), (30) gives: / 2 f k 3 [ROzH] [RO2"I ~ k/ ~
(34)
It follows from (33) and (34) that, 2 2 [RO2HI = ~[fk21
[RH]2
~
(35)
and
[RO2"] -~ ~fk21 [RH]
(36)
The rate of oxygen absorption is: Wz ~ (k, + 2fk21) 2fk21 [RH] 2 k,
(37)
Wz, max • 10~rI---~ min-"l 150
100
50
0 Fig. 7.
Of '/
/
I-
fD /
3
Plot of maximum rate of auto-oxidation of PE according to (37), 160~C, 760torr.
310
M. IRING, T. KELEN, F. TODOS
T h a t is, in this stage the a p p a r e n t r e a c t i o n o r d e r o f the process with respect to p o l y m e r c o n c e n t r a t i o n is 2. E x p e r i m e n t a l d a t a are r e p r e s e n t e d by this r e l a t i o n s h i p as shown in Fig. 7. T h e d a t a o b t a i n e d at 160°C a n d p l o t t e d versus [RH] in Fig. 5 are presented here versus [RH ]2 a n d give a linear d e p e n d e n c e . By e x t r a p o l a t i o n o f the slope ( I . 2 5 . 1 0 - ~ t o o l - t litre r a i n - i) to p o l y m e r c o n c e n t r a t i o n c o r r e s p o n d i n g to the m o l t e n phase (32-93 mol/litre), we o b t a i n 0.135 tool l i t r e - i r a i n - t for the rate o f oxygen u p t a k e which is in very g o o d a g r e e m e n t with the e x p e r i m e n t a l l y m e a s u r e d value (0.107). This indicates t h a t the m e c h a n i s m s o f the a u t o - o x i d a t i o n o f PE in T C B s o l u t i o n and in the m o l t e n p h a s e at 160°C are not m u c h different. Based on o u r e x p e r i m e n t a l d a t a for the t e m p e r a t u r e d e p e n d e n c e o f the m a x i m u m rate o f o x y g e n u p t a k e in a u t o - o x i d a t i o n , a n d by the use o f (37), we o b t a i n the following A r r h e n i u s - t y p e r e l a t i o n s h i p : (k 2 + 2fk21) 2jk21- = 4"26. 101v e x p ( - 4 2 , 6 0 0 / R T ) ka
mol -~ litre min -1
(38)
The difference in the a c t i v a t i o n energies (in m o l t e n phase o x i d a t i o n : ~ 35 kcal) indicates t h a t there m a y be some differences.
REFERENCES
1. L. DULOG, E. RADLMANNand W. KERN, Makr. Chem., 60, 1 (1963). 2. N. V. ZOLO'rOVAand E. T. DENISOV,Vv~. SqC.d, B, 12, 866 (1970). 3. Ju. B. SmLov and E. T. DENISOV, Vys. Soed. A, 16, 662 (1974). 4. P. A. [VANCHENKO.E. T. DENISOVand V. V. Ki.(hRtTONOv,'Kinetika i Katali-. 12. 492 (1971). 5. M. IRING,S. LASZL6-HEDVlG,T. KELENand F. T0o6s, Thermal analysis. Proceedings 4th ICTA. Budapest, 2, 127 (1974). 6. T. KELEN, M. [RINGand F. TOo6s, Europ. Polym. J., 12, 35 (1976). 7. M. IRING.T. KELENand F. Tf2t~Ss, Makr. Chem., 175/'2, 467 (1974). 8. M. [RING, S. LS,SZL6-HEDvtG, K. BARAB~XS,T. KELENand F. TOl:ffJs, Europ. Polym. J., 14,439 (1978). 9. K. B.-XRAB.~S,M. IRING,T. KELENand F. TOD6s,;J. Polym. Sci., Syrup. No. 57, 65 (1976). 10. K. BARAB,~S,M. IRING, S. L~SZLO-HEDVIG,T. KELEN and F. Tt~'o6s,'Europ. Polym. J., 14, 405 (1978). I 1. M. IRING,S. LiXSZLb-HEDVlG,T. KELEN,F. TOD6S,'L. FE'ZES,G. SAMAIand G. BODOR,J. Pol. Sci., Syrup. No. 57, 55 (1976). 12. M. IRING, F. Ti)t)6s, Zs. FODO~,and T. KELEN(to be published). 13. M. [RING,T. KELE,'q,F. TOD6Sand S. L,kSZLO-HEDVlG,J. Polym. Sci., Syrup. No. 57, 89 (1976). 14. S. L~.SZLO-Hr:DVtG,M. IRING,G. B~.LIN'r,T. KELENand F. TOD6S, Magy. K~m. F., 83, 258 (1977). 15. A. L. BUCHACHENKO,Specificity of oxidation of polyolefins in the solid phase, Main lecture in the 15th Prague Microsymposium on Macromolecules, Prague July 21-24 (1975). 16. L. R~,CH and S. S. STIVALA,Autooxidation of hydrocarbons andpol.votefins, 7. New York, Marcel Dekker Inc. (1969). 17. V. L. ANTONOVSKII,Organicheskie Perekisnye Iniciatory, lzd. Khimia, Moskva (1972). 18. C. E. H. BAWNand S. A. CHAUD~, Polymer, 9, 113, 123 (1968).