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New aspects of the photooxidation of polyolefins
Polymer Photochemistry 5 (1984) 313-331
New Aspects o[ the Photooxidation of Polyolefins
G. G e u s k e n s , F. D e b i e , M. S. K a b a m b a a n d G. N e d e l k o s Universit6 Libre de Bruxelles, Campus Plaine 206/1, B-1050 Brussels, Belgium
ABSTRACT A quantitative study of the photooxidation of an ethylene-propylene copolymer has been performed. Three topics are discussed: the influence of previous thermal oxidation on the rate of photooxidation ; the balance of all chemical reactions involved in the photooxidation ; and the use of computer simulation to test the consistency of the usually proposed mode of action of hindered amine light stabilizers.
INTRODUCTION Despite many papers published on the photooxidation of polyethylene (PE) and polypropylene (PP) the detailed mechanism of the degradation of polyolefins has not yet been clearly established and several important questions still remain unanswered. Among these are: (1) What is the relative importance of ketones and hydroperoxides in the initiation of photooxidation? (2) What is the balance of all possible reactions involved in photooxidation? (3) What is the mode of action of hindered amine light stabilizers? The main reason for such a situation is probably that PE and PP 313 Polymer Photochemistry 0144-2880/84/$3-00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Northern Ireland
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are semi-crystalline polyolefins. Therefore they are neither perfectly transparent nor completely soluble at room temperature and photooxidation is restricted to amorphous regions in the polymer. All this makes a quantitative study fairly difficult. In order to avoid some experimental problems and to have a new look at the photooxidation of polyolefins we have studied, as a simple model, an ethylenepropylene copolymer (EPM) supplied by Essochem Europe as VistaIon 404. This is completely amorphous at room temperature and can be easily purified by dissolution and precipitation. Clear films have been obtained by solution casting and then irradiated in air with the 3 1 0 n m radiation of a fluorescent sunlamp. Reliable quantitative results make quantitative correlations possible. Moreover a computer simulation has been used to determine on a semi-empirical basis the relative importance of all reactions involved in the photooxidation mechanism or responsible for the inhibition of the oxidation when stabilizers are used. This new approach to the problem provides a powerful technique not only to test the consistency of a possible mechanism but also to extrapolate the obtained data to very long exposure times, well beyond the experimental limit.
I N F L U E N C E OF A P R E V I O U S T H E R M A L O X I D A T I O N O N T H E R A T E OF P H O T O O X I D A T I O N As E P M is heated in the presence of air at 180°C hydroperoxides are produced. They can be quantitatively analyzed by an iodometric method based on the reaction: 1 R O O H + 31- + 2H ÷ --~ I3 + H 2 0 + R O H Their concentration first increases with heating time, goes through a maximum and then decreases (Fig. 1). The same behaviour is observed at 150 and 130°C but the maximum concentration is higher as the temperature decreases and the time required to reach the maximum is longer: 15 min at 180°C, 1 h at 150°C and 4 h at 130°C. Such curves result from competition between production and destruction of hydroperoxides. Initially, as their concentration is low, they are mainly isolated and decompose by a monomolecular mechanism: R O O H ~ RO" + O H "
(1)
New aspects of the vhotooxidation of polyolefins
lO'2mol
kg"!
315
/,-
///I / ,/
i
/
/
C
I
L
I
I
i
1
2
3
a
5
I
h
Fig. 1. Concentration of hydroperoxides in EPM heated in air at various temperatures. The decomposition of one hydroperoxide molecule generates two radicals. Both can initiate photooxidation and generate one new h y d r o p e r o x i d e each (more as a chain reaction is initiated). T h e r e f o r e hydroperoxides are p r o d u c e d faster than they decompose and start to accumulate. T h e y then t e n d to become h y d r o g e n - b o n d e d and to decompose by a bimolecular mechanism: ROOH : --~ RO" + H 2 0 + ROO" HOOR
(2)
As two molecules are d e c o m p o s e d only two radicals are generated and the rate of production thus decreases c o m p a r e d with the rate of destruction. A t high local concentrations induced decomposition also Occurs:
RO" + R O O H ~
R O H + ROO"
(3)
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and the global reaction (2) + (3) becomes: 3 R O O H ---> R O H + H 2 0 + 2ROO"
(4)
This also contributes to a decrease of the relative importance of production with regard to destruction of hydroperoxides. Last but not least, at high local concentrations peroxy radicals produced close together are more likely to recombine than to propagate a chain reaction: /10"
-0
~1 +
.~C~ H
O~ R
+ O2
(5)
HJO~R
It should be noted that in this last reaction ketones are produced without any scission of the polymer main chain. Nevertheless, chain scissions occur during the thermal oxidation of EPM (Fig. 2). The molecular weight drops rather quickly initially (faster at 180°C than at 150°C and 130°C) and then remains practically constant at a value independent of temperature. It is noteworthy M v 10 5 5
130°C 3
o
2
IBO*C 1
h
I ~ . 2. Viscosimetry-average molecular weight of EPM heated in air at various temperatures.
New aspects of the photooxidation of polyolefins
317
that the decrease of the molecular weight occurs precisely during the initial period of the thermal oxidation w h e n the concentration of hydroperoxides increases. Chain scissions are thus clearly associated with the m o n o m o l e c u l a r decomposition of hydroperoxides and the /3-scission of alkoxy radicals is the most likely mechanism: O I --CH2--C--CH2-I R
0 II ~ --CH2--C + I R
"CH2--
(6)
If R = C H 3 , m e t h y l k e t o n e s are p r o d u c e d at a time w h e n the formation of ketones in the middle of the chains by reaction (5) is not important. Indeed, although the carbonyl absorption in the infra-red steadily increases with heating time (Fig. 3) a progressive shift of the m a x i m u m f r o m 1725 cm -1 (methyl ketones) to 1720 c m -1 (dialkyl ketones) is observed.
CC:0 102 COOHI03
h
ols
;
,:s
Fig. 3. Concentration (mol kg-~) of ketones (Q) and hydroperoxides (+) in EPM heated in air at 180°C.
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The problem is now to evaluate the relative importance of ketones and hydroperoxides in the initiation of the photooxidation of EPM that has been previously oxidized at 180°C for different periods of time. Usually the increase of absorption in the carbonyl region of the infra-red spectrum is taken as a measure of the rate of photooxidation. However, this is not a suitable technique since different products contribute to the absorption (ketones, carboxylic acids, etc.). The most accurate and reliable method is to measure the rate of oxygen absorption as previously oxidized EPM films are exposed to U V light. After a very short initial period it remains practically constant and can be easily measured. The results are plotted in Fig. 4 as a function of the previous heating time at 180°C in the presence of air. Figure 4 indicates that the rate of oxygen absorption first increases (starting from a value that is not zero even for a purified EPM sample because it still contains a small but measurable amount of hydroperoxides), then decreases slightly before increasing steadily afterwards. It is remarkable that the shape of the experimental curve looks very much like a superposition of the ketone and hydroperoxide concentration curves (Fig. 3) if the scales are adjusted in order to make the initial and final values of the rate of oxygen absorption coincide with the initial concentration of hydroperoxides and the final C00H103 Cc..O 102 .8
ro 2
./,
/
,, / COO
"',
CO0H -"--.
0.5
I
1.5
2
Fig. 4. R a t e of oxygen absorption (in arbitrary units) in E P M irradiated in air at 310 nm as a function of duration of previous thermal oxidation at 180°C.
New aspects of the photooxidation of polyolefins
319
concentration of ketones, respectively. This clearly suggests that in slightly oxidized samples where no or very few ketones are present photooxidation is initiated by hydroperoxides according to reaction (1) whereas in m o r e extensively oxidized samples, w h e r e the concentration of hydroperoxides is very low, ketones play a dominant role. T h e Norrish type I reaction is p r o b a b l y responsible for the initiation of the photooxidation in this latter case: R ~ - - C O - - R 2 --~ Ri + C O + R2
(7)
Taking into account the m e a s u r e d concentrations of oxidation products o n e finds that, on an equal concentration basis, hydroperoxides are a b o u t 150 times m o r e efficient than ketones for initiating the photooxidation. This ratio is not only d e p e n d e n t on q u a n t u m yields of reactions (1) and (7) but also on the probability of b o t h types of c h r o m o p h o r e s absorbing the light. This is m e a s u r e d by the absorption coefficients. With the absorption coefficient of hydroperoxides being a b o u t 0.3 litres mol -~ cm -a at 310 nm, 2 and the q u a n t u m yield of reaction (1) a b o u t 0-5 in a p o l y m e r matrix, 3 one can estimate that the q u a n t u m yield of the Norrish type I reaction of ketones having an absorption coefficient of 5 litresmo1-1 cm ~ at 310 nm is approximately 2 x 10 4. This is quite a reasonable order of magnitude for m a c r o k e t o n e s in a p o l y m e r matrix since it is o n e o r d e r of magnitude smaller than in solution. 2 It can thus b e concluded that both ketone and hydroperoxide groups can initiate the photooxidation of polyolefins and that their relative importance d e p e n d s on the extent of oxidation of the sample.
COMPUTER SIMULATION OF THE P H O T O O X l D A T I O N MECHANISM As an E P M film is irradiated at 310 nm various oxidation products are f o r m e d most of which have already b e e n d e t e c t e d in P E and PP. T h e chemical reactions responsible for the occurrence of these products are usually known from m o d e l c o m p o u n d studies. H o w e v e r , the balance of all possible chemical reactions involved in the p h o t o oxidation of polyolefins has never b e e n established up to now. B e c a u s e E P M provides a much b e t t e r opportunity for a quantitative study than semi-crystalline polyolefins, this c o p o l y m e r seems to b e a
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~Eg
f
RCOR+ ROH_+ 02 I
\
\,
',\
'
CH3COCH3
I
I ~ . 5. Photooxidation mechanism of EPM.
New aspects of the photooxidation of polyolefins
321
good model for a computer simulation of a consistent mechanism of photooxidation. All the compounds that have been detected in E P M are shown in Fig. 5 and are connected by well-established chemical reactions. These account for: (1) The formation of hydroperoxides R'+ 02 ~
ROO"
(8)
ROO" + R H ~ R O O H + R"
(9)
(2) The photolysis of hydroperoxides, according to reaction (1). (3) The formation of alcohol, water and peroxides RO" + R H ~ R O H + R"
(10)
"OH+ R H ~ H z O + R "
(11)
2RO" ~ R O O R (4) (5)
(12)
T h e formation of ketones inside a polymer chain, according to reaction (5) T h e photolysis of ketones, according to the Norrish type I reaction (reaction (7)) and the Norrish type II reaction R - - C O - - C H z - - C H E - - C H - - ~ R--CO---CH3 + CH2~--m-CH-J I R R (13)
O n e reaction that has been discovered recently results in the production of carboxylic acids by the photolysis of hydroperoxides forming a complex with ketones 4"s /
~+H- • -O~I II ~5~/O- •/ ..C 8 + R/O
R
\R
~ O--R R/
H ~ O I
(14)
R
It should be pointed out that the /3-scission of alkoxy radicals generated from hydroperoxides (reaction (6)) has been deliberately omitted in the photooxidation scheme although it is m e n t i o n e d by most authors for P E and PP. Indeed, we have observed that the vacuum photolysis at 365 n m of
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a previously photooxidized E P M sample results in the photolysis of hydroperoxides without the formation of any ketone. 4 T h e increase of the carbonyl absorption is actually due to the carboxylic acids p r o d u c e d by reaction (14). H o w e v e r , the /3-scission is expected to b e c o m e m o r e important at higher temperatures, 2 in particular in the thermal oxidation. Figure 5 provides the clue to a c o m p u t e r simulation of the p h o t o oxidation of E P M if probabilities are assigned to all reactions in order to get the best fit with experimental results, some of which are collated in Table 1. This can be achieved on a semi-empirical basis. T h e principle of the m e t h o d is that a given a m o u n t of unidentified radicals is first injected into the system. These give rise to a sequence of reactions (Fig. 5), some of which generate new free radicals that are in turn injected into the system. The n u m b e r of turns is proportional to time but the time-scale has to be adjusted by comparison with experimental results. O n e can safely assume that alkyl radicals react quantitatively with oxygen (reaction (8)) and that peroxy radicals very quickly abstract a h y d r o g e n a t o m f r o m the p o l y m e r (reaction (9)) or disproportionate (reaction (5)). A l t h o u g h reaction (5) is actually a bimolecular reaction it can be considered as a p s e u d o - m o n o m o l e c u l a r o n e because it involves one pair of peroxy radicals that have been p r o d u c e d close t o g e t h e r in the p o l y m e r matrix and it is reasonable to assume that the fraction of peroxy radicals that occurs as pairs is i n d e p e n d e n t of the total concentration of peroxy radicals. The abstraction of h y d r o g e n
TklllZ 1 Photooxidation of EPM at 310 nm in Air (Concentration in tool litre-l) J
t(h) ROOH R - - C O - - R (xlO-:) (x10 -2)
CH2~C~
RCOOH
100 170 240 280 320
2.0 × 1 0 _ 2 a 3-5 X 1 0 - 2 7.0×10 -2 9.7 x 10-2 1"6x 10-1
2"5 X 1 0 - 2 4.0x 10-2 7-0 × 10-2 1-4x 10-1
1-4 2-4 3.3 3"8 4-3
1"0 2.4 4-2 4-8 5"4
__
a Vinyl unsaturations are already present in the polymer before irradiation.
New aspects of the photooxidation of polyolefins
323
(reaction (9)) is also p s e u d o - m o n o m o l e c u l a r because of the large excess of R H . T h e balance b e t w e e n reactions (9) and (5) is the key step that regulates the kinetics of photoxidation. Fortunately, it can b e easily d e t e r m i n e d since the products of these reactions, hydroperoxides and ketones, respectively, have b e e n quantitatively analyzed. A f t e r a few trials, it b e c o m e s clear that in E P M both reactions occur with approximately equal probabilities. Of course this is likely to be different in P E and P P and may be a cause of quantitative differences in the rate of photooxidation of polyolefins. A close a d j u s t m e n t of the calculated curves to the experimental results (Fig. 6) requires that 15% of the hydroperoxide groups are p h o t o l y z e d to give free radicals (reaction (1)) in the experimental conditions used and that the relative probability of this reaction and the Norrish type I reaction of ketones (reaction (7)) is 1 5 0 : 1 in a g r e e m e n t with data presented in the first part of this paper. It is also required that the Norrish type I and type II reactions of ketones (reactions (7) and (13)) occur with q u a n t u m yields in the ratio 1 : 5 0
20
10"2M 10"2M
15
10
:C:O
•
lOO
"
+
200
-
00H
300
Fig. 6. Calculated curves and experimental results for the photooxidation of EPM (the presence of a small amount of vinyl unsaturations in the initial sample has not been taken into account in the computer simulation).
324
G. Geuskens et al.
as suggested by published data. 6 O n e last important parameter concerns reaction (14), the probability of which increases with the concentration of ketones in the vicinity of hydroperoxide groups. It is taken as zero at the beginning of the reaction but the photolysis of hydroperoxides forming a complex with ketones becomes four times more important than the photolysis of the other hydroperoxides (reaction (1)) at the end of the period investigated. This is due to their increased concentration and their higher absorption coefficient. 5 T h e excellent agreement between calculated curves and experimental results (Fig. 6) obtained with a reasonable set of parameters selected from the literature and sometimes by trial and error strongly supports the mechanism proposed. This in particular indicates that the Norrish type II reaction of ketones (reaction (13)) as measured from the production of terminal unsaturations is initially the main chain scission process (the Norrish type I reaction being negligible) but that reaction (14) progressively becomes more and more important. A n o t h e r role of the computer simulation, besides testing the mechanism, is to permit extrapolation up to very long exposure times. Indeed, the time-scale of Fig. 7 extends to 1000 h irradiation (roughly equivalent to 20 years in normal weathering conditions),
CH2= C C,
O0H
J
200
t.00
600
BOO
Fig. 7. Calculated curves for the photooxidation of E P M ( - - - - , reached on Fig. 6).
I000
experimental limit
New aspects of the photooxidation of polyolefins
325
well beyond the experimental limit of about 300 h when the thin films used tear and become unserviceable. Figure 7 indicates that the rates of both chain-scission reactions (13) and (14) progressively increase, whereas hydroperoxides accumulate with an approximately constant rate and the concentration of ketones reaches a plateau as they are destroyed by reactions (7) and (14) as fast as they are produced by reaction (5).
M O D E OF ACXION OF ]HIINIIEIRFIPI A M I N E LIGHT STABHJIZERS
Some hindered amines are very efficient light stabilizers in polyolefins. The best known example is probably Tinuvin 770
--t~rl2) 8--~.....O
In PP, the time required to reach a given level of oxidation, measured by the carbonyl absorption, is extended by a factor of about 10 as 0 . 2 w t% Tinuvin 770 is used as an additive, v The behaviour of this stabilizer is, however, very peculiar since it provides almost complete protection during an apparent induction period, roughly proportional to the additive concentration, and then loses its efficiency in such a way that the rate of photooxidation becomes practically the same as that of an unstabilized sample. The m o d e of action of hindered amine light stabilizers (HALS) is not yet clearly established although most specialists admit that the nitroxide radicals, formed by oxidation of the amine, act as radical scavengers and are regenerated by the following sequence of r e a c t i o n s 7-9
~ N O ' + R" ~ ~ N O R "... N OR+ /
ROO" ~ / NO" + R O O R
(15) (16)
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G. Geuskens et al.
where R" and ROO" are alkyl and peroxy macroradicals likely to initiate or to propagate the oxidation of the polymer. The mechanism of oxidation of HALS to nitroxide radicals is not known but two hydroperoxides are probably involved 1° and the substituted h y d r o x y l a m i n e j N O R is a likely intermediate since it is also an efficient photostabilizer. 7 One possibility is j~NH + ROOH ~NOR
+ H20
~ N O R + R O O H ~ ~ N O " + RO" + R O H
(17) (18)
The scope of this paper is not to provide experimental results to support or invalidate some of the reactions mentioned above but to test by computer simulation whether the mechanism proposed is compatible with published data and what fraction of active radical species has to be scavenged to give the observed protective effect. For this purpose, we have added reactions (15), (16), (17) and (18) to the photooxidation scheme of Fig. 5. The new scheme is presented in Fig. 8. After a few trials it becomes obvious that reactions (15)-(18) can not provide a good fit of calculated curves to experimental results. Depending on the efficiencies assumed for reactions (15) and (16), the rate of photooxidation can be slowed down at will but the shape of the experimental curves (induction period followed by an oxidation as fast as in the absence of stabilizer) is never obtained. Such a result requires that one intermediate in the regenerating sequence of reactions (15) and (16) is progressively lost or becomes inactive. One can, for instance, suggest that a fraction of the "~/NOR /groups becomes 'trapped' in an unfavourable environment that makes any further encounter with a peroxy radical unlikely. This could be due to the morphology of the polymer or to its chemical structure if inactive ~ N O R groups are located in a fully oxidized region where no mole peroxy radicals are generated. Although the progressive transformation of ] N O R groups into an inactive form is hypothetical and still requires"experimental support, it provides a good agreement between calculated curves and experimental results (Fig. 9) if one further assumes that 1% of hydroperoxide groups react according to reaction (17) and 5% according to reaction (18) to generate nitroxide radicals. The latter have to
New aspects of the photooxidation of polyolefins PRODUCTION OF NO'
327
REGENERATION OF NO'
I I
IRo. .o.I
NOR I
> "INACTIVE"
1
RCOR + ROH + O~ I
SEE FIG,5
SEE FIG,5
Fig. 8. Hypothetical m o d e of action of H A L S used as a basis for c o m p u t e r simulation.
is i 1°-2 M
iO
0
i
h m
L
i
m
Fig. 9. Inhibition of the carboxylic acid formation during the photooxidation of E P M . Calculated curves for various concentrations of H A L S (mol kg -~ x 103).
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G. Geuskens et al.
scavenge one-third of the alkyl radicals (reaction (15)) and /~"]qOR groups have to react with one-quarter of the peroxy radicals (reaction (16)) if the observed efficiency of HALS is to be obtained. These values are rather high but could be accounted for by the preferential location of HALS and derivatives in partially oxidized regions of the polymer where the photooxidation is most likely initiated (by the photolysis ,of hydroperoxides and ketones). A progressive loss of activity of N O R groups compatible with the length of the induction • J . . . period observed experimentally can. be justified if only 1 o'/o i s transformed into an inactive form each time reaction (15) occurs while 99% regenerate nitroxide radicals by reaction (16). The curves of Fig. 9 are quite similar in shape to experimental curves obtained in pp,7 and the induction period is roughly proportional to the concentration of HALS. Figure 10 indicates that not only the HALS itself but also the substituted hydroxylamine ~ N O R and the nitroxide radical ~ N O " are efficient stabilizers. O n / a n equal concentration basis, the efficiency follows the sequence NO'> )NOR>~NH as experimentally observed. Since the agreement between calculated curves and experimental results looks quite good, computer simulation can be used to get a deeper insight into the
15 10-2M
NH 10
ff
NOR
/
NO"
h 500
1000
1500
Fig. 10. Comparativeefficienciesof HALS (NH) and the derived substituted hydroxylamine (NOR) and nitroxide radical (NO') as inhibitors of the photooxidation of EPM (on an equal concentration basis: 1× 10-SM). Calculated curves for the formation of carboxylicacids (-- -, experimentalcurve for the unstabilizedsample).
New aspects of the photooxidation of polyolefins
10-3M
in
329
/I'"
"'-.
1ooo
2000
3OOO
Fig. 11. Concentration of various intermediates derived from HALS during the photooxidation of EPM. Calculated curves for an initial HALS concentration of 3 x 10-3 M. Concentration of carboxylic acids in arbitary units (for the actual scale see Fig. 9.). mechanism of the protection due to H A L S . T h e concentrations of the various intermediates can b e calculated as a function of time. Figure 11 shows that starting with an initial H A L S concentration of 3 × 10-3M (0"15 w t % Tinuvin 770) a b o u t 7 0 % is rapidly transformed into the active form of " ~ N O R . Nitroxide radicals are p r o d u c e d m o r e slowly and their concentration b e c o m e s stationary at a b o u t 5 × 10 -4 M in a g r e e m e n t with the experimental o r d e r of magnitude. T h e concentration of 'inactive' " ~ N O R groups progressively increases and, therefore, after reaching a maximum, the concentration of 'active' ~ . . O R groups starts decreasing. As it drops to zero the nitroxide radicals are no longer regenerated and their concentration also drops. A t that m o m e n t photooxidation starts at the same rate as in the unstabilized sample; it is the end of the induction period. A l t h o u g h this part of the paper is based exclusively on c o m p u t e r simulations, it provides s o m e important conclusions concerning the m o d e of action of H A L S . The mechanism p r o p o s e d by m a n y scientists (reactions (15) and (16)) is compatible with the experimental results b u t the radical scavengers have to b e fairly efficient (more than expected from model compounds) ix and o n e has to assume that o n e intermediate, the ~ N O R group, b e c o m e s progressively inactive.
330
G. Geuskens et al.
Experimental support for this suggestion should be looked for and it is likely that a better understanding of the inactivation mechanism will open new possibilities for stabilization that obviate such a drawback.
CONCLUSIONS This paper demonstrates that both hydroperoxides and ketones produced during previous thermal oxidation can initiate the photooxidation of EPM. On an equal concentration basis the former are much more efficient but in extensively oxidized samples ketones play a dominant role anyway. The relative importance of hydroperoxides and ketones is not only dependent on the extent of oxidation of the polymer but also on its chemical structure. Because most ketones that accumulate during thermal oxidation are formed inside the polymer chain by a mechanism that requires at least one secondary peroxy radical it can be expected that ketones are much more important in P E than in PP, for instance. A computer simulation based on a reasonable choice of parameters provides a good fit of calculated curves to experimental results and support the photooxidation mechanism proposed for EPM. This indicates that two reactions contribute to the main chain scission and thus to the deterioration of the physical and mechanical properties: the Norrish type II reaction of ketones and the photolysis of hydroperoxides forming complexes with ketones. This last reaction produces carboxylic acids. The key step that regulates the rate of the photooxidation is the competition for peroxy radicals between hydrogen abstraction and disproportionation. It opens two parallel lanes for the reaction: one, going through hydroperoxides, generates new free radicals and makes it faster whereas the other, going through ketones, slows it down because free radicals are consumed and few are produced. In EPM both lanes are followed with about equal probabilities but in PP where most peroxy radicals are tertiary, disproportionation is much less likely than in P E for instance. This could be the reason why PP photooxidizes much faster than PE. Computer simulation opens up new vistas over the mode of action of HALS. The mechanism usually proposed cannot justify the experimental results unless one intermediate is progressively lost or becomes inactive. This last assumption, if it finds experimental support,
New aspects of the photooxidation of polyolefins
331
could draw new research lines likely to improve the efficiency of HALS.
REFERENCES 1. Geuskens, G., Bastin, P., Lu Vinh, Q. and Rens, M., Polym. Degr. and Stab., 3 (1980-81) 295. 2. Geuskens, G. and David, C., Pure Appl. Chem., 51 (1974) 233. 3. Geuskens, G., Baeyens-Volant, D., Delaunois, G., Lu Vinh, Q., Piret, W. and David, C., Europ. Polym. J., 14 (1978) 299. 4. Geuskens, G. and Kabamba, M. S., Polym. Degr. and Stab., 4 (1982) 69. 5. Geuskens, G. and Kabamba, M. S., Polym. Degr. and Stab., 5 (1983) 399. 6. Hartley, G. H.-and Guillet, J. E., Macromolecules, I (1968) 165. 7. Hodgeman, D. K. C., Developments in polymer degradation--4, Grassie, N. (ed.), Applied Science Publishers Ltd, London, 1982. 8. Carlsson, D. J., Garton, A. and Wiles, D. M., Developments in polymer stabilization--I, Scott, G. (ed.), Applied Science Publishers Ltd, London 1980. 9. Shlyapintokh, V. Ya. and Ivanov, V. B., Developments in polymer stabilization--5, Scott, G. (ed.), Applied Science Publishers Ltd, London, 1982. 10. Carlsson, D. J., Chan, K. H., Durmis, J. and Wiles, D. M., J. Polym. Sci. Chem. Ed. 20 (1982) 575. 11. Grattan, D. W., Carlsson, D. J. and Wiles, D. M., Polym. Degr. and Stab., 1 (1979) 69.
New Aspects o[ the Photooxidation of Polyolefins
G. G e u s k e n s , F. D e b i e , M. S. K a b a m b a a n d G. N e d e l k o s Universit6 Libre de Bruxelles, Campus Plaine 206/1, B-1050 Brussels, Belgium
ABSTRACT A quantitative study of the photooxidation of an ethylene-propylene copolymer has been performed. Three topics are discussed: the influence of previous thermal oxidation on the rate of photooxidation ; the balance of all chemical reactions involved in the photooxidation ; and the use of computer simulation to test the consistency of the usually proposed mode of action of hindered amine light stabilizers.
INTRODUCTION Despite many papers published on the photooxidation of polyethylene (PE) and polypropylene (PP) the detailed mechanism of the degradation of polyolefins has not yet been clearly established and several important questions still remain unanswered. Among these are: (1) What is the relative importance of ketones and hydroperoxides in the initiation of photooxidation? (2) What is the balance of all possible reactions involved in photooxidation? (3) What is the mode of action of hindered amine light stabilizers? The main reason for such a situation is probably that PE and PP 313 Polymer Photochemistry 0144-2880/84/$3-00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Northern Ireland
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are semi-crystalline polyolefins. Therefore they are neither perfectly transparent nor completely soluble at room temperature and photooxidation is restricted to amorphous regions in the polymer. All this makes a quantitative study fairly difficult. In order to avoid some experimental problems and to have a new look at the photooxidation of polyolefins we have studied, as a simple model, an ethylenepropylene copolymer (EPM) supplied by Essochem Europe as VistaIon 404. This is completely amorphous at room temperature and can be easily purified by dissolution and precipitation. Clear films have been obtained by solution casting and then irradiated in air with the 3 1 0 n m radiation of a fluorescent sunlamp. Reliable quantitative results make quantitative correlations possible. Moreover a computer simulation has been used to determine on a semi-empirical basis the relative importance of all reactions involved in the photooxidation mechanism or responsible for the inhibition of the oxidation when stabilizers are used. This new approach to the problem provides a powerful technique not only to test the consistency of a possible mechanism but also to extrapolate the obtained data to very long exposure times, well beyond the experimental limit.
I N F L U E N C E OF A P R E V I O U S T H E R M A L O X I D A T I O N O N T H E R A T E OF P H O T O O X I D A T I O N As E P M is heated in the presence of air at 180°C hydroperoxides are produced. They can be quantitatively analyzed by an iodometric method based on the reaction: 1 R O O H + 31- + 2H ÷ --~ I3 + H 2 0 + R O H Their concentration first increases with heating time, goes through a maximum and then decreases (Fig. 1). The same behaviour is observed at 150 and 130°C but the maximum concentration is higher as the temperature decreases and the time required to reach the maximum is longer: 15 min at 180°C, 1 h at 150°C and 4 h at 130°C. Such curves result from competition between production and destruction of hydroperoxides. Initially, as their concentration is low, they are mainly isolated and decompose by a monomolecular mechanism: R O O H ~ RO" + O H "
(1)
New aspects of the vhotooxidation of polyolefins
lO'2mol
kg"!
315
/,-
///I / ,/
i
/
/
C
I
L
I
I
i
1
2
3
a
5
I
h
Fig. 1. Concentration of hydroperoxides in EPM heated in air at various temperatures. The decomposition of one hydroperoxide molecule generates two radicals. Both can initiate photooxidation and generate one new h y d r o p e r o x i d e each (more as a chain reaction is initiated). T h e r e f o r e hydroperoxides are p r o d u c e d faster than they decompose and start to accumulate. T h e y then t e n d to become h y d r o g e n - b o n d e d and to decompose by a bimolecular mechanism: ROOH : --~ RO" + H 2 0 + ROO" HOOR
(2)
As two molecules are d e c o m p o s e d only two radicals are generated and the rate of production thus decreases c o m p a r e d with the rate of destruction. A t high local concentrations induced decomposition also Occurs:
RO" + R O O H ~
R O H + ROO"
(3)
316
G. Geuskens et al.
and the global reaction (2) + (3) becomes: 3 R O O H ---> R O H + H 2 0 + 2ROO"
(4)
This also contributes to a decrease of the relative importance of production with regard to destruction of hydroperoxides. Last but not least, at high local concentrations peroxy radicals produced close together are more likely to recombine than to propagate a chain reaction: /10"
-0
~1 +
.~C~ H
O~ R
+ O2
(5)
HJO~R
It should be noted that in this last reaction ketones are produced without any scission of the polymer main chain. Nevertheless, chain scissions occur during the thermal oxidation of EPM (Fig. 2). The molecular weight drops rather quickly initially (faster at 180°C than at 150°C and 130°C) and then remains practically constant at a value independent of temperature. It is noteworthy M v 10 5 5
130°C 3
o
2
IBO*C 1
h
I ~ . 2. Viscosimetry-average molecular weight of EPM heated in air at various temperatures.
New aspects of the photooxidation of polyolefins
317
that the decrease of the molecular weight occurs precisely during the initial period of the thermal oxidation w h e n the concentration of hydroperoxides increases. Chain scissions are thus clearly associated with the m o n o m o l e c u l a r decomposition of hydroperoxides and the /3-scission of alkoxy radicals is the most likely mechanism: O I --CH2--C--CH2-I R
0 II ~ --CH2--C + I R
"CH2--
(6)
If R = C H 3 , m e t h y l k e t o n e s are p r o d u c e d at a time w h e n the formation of ketones in the middle of the chains by reaction (5) is not important. Indeed, although the carbonyl absorption in the infra-red steadily increases with heating time (Fig. 3) a progressive shift of the m a x i m u m f r o m 1725 cm -1 (methyl ketones) to 1720 c m -1 (dialkyl ketones) is observed.
CC:0 102 COOHI03
h
ols
;
,:s
Fig. 3. Concentration (mol kg-~) of ketones (Q) and hydroperoxides (+) in EPM heated in air at 180°C.
318
G. Geuskens et al.
The problem is now to evaluate the relative importance of ketones and hydroperoxides in the initiation of the photooxidation of EPM that has been previously oxidized at 180°C for different periods of time. Usually the increase of absorption in the carbonyl region of the infra-red spectrum is taken as a measure of the rate of photooxidation. However, this is not a suitable technique since different products contribute to the absorption (ketones, carboxylic acids, etc.). The most accurate and reliable method is to measure the rate of oxygen absorption as previously oxidized EPM films are exposed to U V light. After a very short initial period it remains practically constant and can be easily measured. The results are plotted in Fig. 4 as a function of the previous heating time at 180°C in the presence of air. Figure 4 indicates that the rate of oxygen absorption first increases (starting from a value that is not zero even for a purified EPM sample because it still contains a small but measurable amount of hydroperoxides), then decreases slightly before increasing steadily afterwards. It is remarkable that the shape of the experimental curve looks very much like a superposition of the ketone and hydroperoxide concentration curves (Fig. 3) if the scales are adjusted in order to make the initial and final values of the rate of oxygen absorption coincide with the initial concentration of hydroperoxides and the final C00H103 Cc..O 102 .8
ro 2
./,
/
,, / COO
"',
CO0H -"--.
0.5
I
1.5
2
Fig. 4. R a t e of oxygen absorption (in arbitrary units) in E P M irradiated in air at 310 nm as a function of duration of previous thermal oxidation at 180°C.
New aspects of the photooxidation of polyolefins
319
concentration of ketones, respectively. This clearly suggests that in slightly oxidized samples where no or very few ketones are present photooxidation is initiated by hydroperoxides according to reaction (1) whereas in m o r e extensively oxidized samples, w h e r e the concentration of hydroperoxides is very low, ketones play a dominant role. T h e Norrish type I reaction is p r o b a b l y responsible for the initiation of the photooxidation in this latter case: R ~ - - C O - - R 2 --~ Ri + C O + R2
(7)
Taking into account the m e a s u r e d concentrations of oxidation products o n e finds that, on an equal concentration basis, hydroperoxides are a b o u t 150 times m o r e efficient than ketones for initiating the photooxidation. This ratio is not only d e p e n d e n t on q u a n t u m yields of reactions (1) and (7) but also on the probability of b o t h types of c h r o m o p h o r e s absorbing the light. This is m e a s u r e d by the absorption coefficients. With the absorption coefficient of hydroperoxides being a b o u t 0.3 litres mol -~ cm -a at 310 nm, 2 and the q u a n t u m yield of reaction (1) a b o u t 0-5 in a p o l y m e r matrix, 3 one can estimate that the q u a n t u m yield of the Norrish type I reaction of ketones having an absorption coefficient of 5 litresmo1-1 cm ~ at 310 nm is approximately 2 x 10 4. This is quite a reasonable order of magnitude for m a c r o k e t o n e s in a p o l y m e r matrix since it is o n e o r d e r of magnitude smaller than in solution. 2 It can thus b e concluded that both ketone and hydroperoxide groups can initiate the photooxidation of polyolefins and that their relative importance d e p e n d s on the extent of oxidation of the sample.
COMPUTER SIMULATION OF THE P H O T O O X l D A T I O N MECHANISM As an E P M film is irradiated at 310 nm various oxidation products are f o r m e d most of which have already b e e n d e t e c t e d in P E and PP. T h e chemical reactions responsible for the occurrence of these products are usually known from m o d e l c o m p o u n d studies. H o w e v e r , the balance of all possible chemical reactions involved in the p h o t o oxidation of polyolefins has never b e e n established up to now. B e c a u s e E P M provides a much b e t t e r opportunity for a quantitative study than semi-crystalline polyolefins, this c o p o l y m e r seems to b e a
320
G. Geuskens et al.
~Eg
f
RCOR+ ROH_+ 02 I
\
\,
',\
'
CH3COCH3
I
I ~ . 5. Photooxidation mechanism of EPM.
New aspects of the photooxidation of polyolefins
321
good model for a computer simulation of a consistent mechanism of photooxidation. All the compounds that have been detected in E P M are shown in Fig. 5 and are connected by well-established chemical reactions. These account for: (1) The formation of hydroperoxides R'+ 02 ~
ROO"
(8)
ROO" + R H ~ R O O H + R"
(9)
(2) The photolysis of hydroperoxides, according to reaction (1). (3) The formation of alcohol, water and peroxides RO" + R H ~ R O H + R"
(10)
"OH+ R H ~ H z O + R "
(11)
2RO" ~ R O O R (4) (5)
(12)
T h e formation of ketones inside a polymer chain, according to reaction (5) T h e photolysis of ketones, according to the Norrish type I reaction (reaction (7)) and the Norrish type II reaction R - - C O - - C H z - - C H E - - C H - - ~ R--CO---CH3 + CH2~--m-CH-J I R R (13)
O n e reaction that has been discovered recently results in the production of carboxylic acids by the photolysis of hydroperoxides forming a complex with ketones 4"s /
~+H- • -O~I II ~5~/O- •/ ..C 8 + R/O
R
\R
~ O--R R/
H ~ O I
(14)
R
It should be pointed out that the /3-scission of alkoxy radicals generated from hydroperoxides (reaction (6)) has been deliberately omitted in the photooxidation scheme although it is m e n t i o n e d by most authors for P E and PP. Indeed, we have observed that the vacuum photolysis at 365 n m of
322
G. Geuskens et al.
a previously photooxidized E P M sample results in the photolysis of hydroperoxides without the formation of any ketone. 4 T h e increase of the carbonyl absorption is actually due to the carboxylic acids p r o d u c e d by reaction (14). H o w e v e r , the /3-scission is expected to b e c o m e m o r e important at higher temperatures, 2 in particular in the thermal oxidation. Figure 5 provides the clue to a c o m p u t e r simulation of the p h o t o oxidation of E P M if probabilities are assigned to all reactions in order to get the best fit with experimental results, some of which are collated in Table 1. This can be achieved on a semi-empirical basis. T h e principle of the m e t h o d is that a given a m o u n t of unidentified radicals is first injected into the system. These give rise to a sequence of reactions (Fig. 5), some of which generate new free radicals that are in turn injected into the system. The n u m b e r of turns is proportional to time but the time-scale has to be adjusted by comparison with experimental results. O n e can safely assume that alkyl radicals react quantitatively with oxygen (reaction (8)) and that peroxy radicals very quickly abstract a h y d r o g e n a t o m f r o m the p o l y m e r (reaction (9)) or disproportionate (reaction (5)). A l t h o u g h reaction (5) is actually a bimolecular reaction it can be considered as a p s e u d o - m o n o m o l e c u l a r o n e because it involves one pair of peroxy radicals that have been p r o d u c e d close t o g e t h e r in the p o l y m e r matrix and it is reasonable to assume that the fraction of peroxy radicals that occurs as pairs is i n d e p e n d e n t of the total concentration of peroxy radicals. The abstraction of h y d r o g e n
TklllZ 1 Photooxidation of EPM at 310 nm in Air (Concentration in tool litre-l) J
t(h) ROOH R - - C O - - R (xlO-:) (x10 -2)
CH2~C~
RCOOH
100 170 240 280 320
2.0 × 1 0 _ 2 a 3-5 X 1 0 - 2 7.0×10 -2 9.7 x 10-2 1"6x 10-1
2"5 X 1 0 - 2 4.0x 10-2 7-0 × 10-2 1-4x 10-1
1-4 2-4 3.3 3"8 4-3
1"0 2.4 4-2 4-8 5"4
__
a Vinyl unsaturations are already present in the polymer before irradiation.
New aspects of the photooxidation of polyolefins
323
(reaction (9)) is also p s e u d o - m o n o m o l e c u l a r because of the large excess of R H . T h e balance b e t w e e n reactions (9) and (5) is the key step that regulates the kinetics of photoxidation. Fortunately, it can b e easily d e t e r m i n e d since the products of these reactions, hydroperoxides and ketones, respectively, have b e e n quantitatively analyzed. A f t e r a few trials, it b e c o m e s clear that in E P M both reactions occur with approximately equal probabilities. Of course this is likely to be different in P E and P P and may be a cause of quantitative differences in the rate of photooxidation of polyolefins. A close a d j u s t m e n t of the calculated curves to the experimental results (Fig. 6) requires that 15% of the hydroperoxide groups are p h o t o l y z e d to give free radicals (reaction (1)) in the experimental conditions used and that the relative probability of this reaction and the Norrish type I reaction of ketones (reaction (7)) is 1 5 0 : 1 in a g r e e m e n t with data presented in the first part of this paper. It is also required that the Norrish type I and type II reactions of ketones (reactions (7) and (13)) occur with q u a n t u m yields in the ratio 1 : 5 0
20
10"2M 10"2M
15
10
:C:O
•
lOO
"
+
200
-
00H
300
Fig. 6. Calculated curves and experimental results for the photooxidation of EPM (the presence of a small amount of vinyl unsaturations in the initial sample has not been taken into account in the computer simulation).
324
G. Geuskens et al.
as suggested by published data. 6 O n e last important parameter concerns reaction (14), the probability of which increases with the concentration of ketones in the vicinity of hydroperoxide groups. It is taken as zero at the beginning of the reaction but the photolysis of hydroperoxides forming a complex with ketones becomes four times more important than the photolysis of the other hydroperoxides (reaction (1)) at the end of the period investigated. This is due to their increased concentration and their higher absorption coefficient. 5 T h e excellent agreement between calculated curves and experimental results (Fig. 6) obtained with a reasonable set of parameters selected from the literature and sometimes by trial and error strongly supports the mechanism proposed. This in particular indicates that the Norrish type II reaction of ketones (reaction (13)) as measured from the production of terminal unsaturations is initially the main chain scission process (the Norrish type I reaction being negligible) but that reaction (14) progressively becomes more and more important. A n o t h e r role of the computer simulation, besides testing the mechanism, is to permit extrapolation up to very long exposure times. Indeed, the time-scale of Fig. 7 extends to 1000 h irradiation (roughly equivalent to 20 years in normal weathering conditions),
CH2= C C,
O0H
J
200
t.00
600
BOO
Fig. 7. Calculated curves for the photooxidation of E P M ( - - - - , reached on Fig. 6).
I000
experimental limit
New aspects of the photooxidation of polyolefins
325
well beyond the experimental limit of about 300 h when the thin films used tear and become unserviceable. Figure 7 indicates that the rates of both chain-scission reactions (13) and (14) progressively increase, whereas hydroperoxides accumulate with an approximately constant rate and the concentration of ketones reaches a plateau as they are destroyed by reactions (7) and (14) as fast as they are produced by reaction (5).
M O D E OF ACXION OF ]HIINIIEIRFIPI A M I N E LIGHT STABHJIZERS
Some hindered amines are very efficient light stabilizers in polyolefins. The best known example is probably Tinuvin 770
--t~rl2) 8--~.....O
In PP, the time required to reach a given level of oxidation, measured by the carbonyl absorption, is extended by a factor of about 10 as 0 . 2 w t% Tinuvin 770 is used as an additive, v The behaviour of this stabilizer is, however, very peculiar since it provides almost complete protection during an apparent induction period, roughly proportional to the additive concentration, and then loses its efficiency in such a way that the rate of photooxidation becomes practically the same as that of an unstabilized sample. The m o d e of action of hindered amine light stabilizers (HALS) is not yet clearly established although most specialists admit that the nitroxide radicals, formed by oxidation of the amine, act as radical scavengers and are regenerated by the following sequence of r e a c t i o n s 7-9
~ N O ' + R" ~ ~ N O R "... N OR+ /
ROO" ~ / NO" + R O O R
(15) (16)
326
G. Geuskens et al.
where R" and ROO" are alkyl and peroxy macroradicals likely to initiate or to propagate the oxidation of the polymer. The mechanism of oxidation of HALS to nitroxide radicals is not known but two hydroperoxides are probably involved 1° and the substituted h y d r o x y l a m i n e j N O R is a likely intermediate since it is also an efficient photostabilizer. 7 One possibility is j~NH + ROOH ~NOR
+ H20
~ N O R + R O O H ~ ~ N O " + RO" + R O H
(17) (18)
The scope of this paper is not to provide experimental results to support or invalidate some of the reactions mentioned above but to test by computer simulation whether the mechanism proposed is compatible with published data and what fraction of active radical species has to be scavenged to give the observed protective effect. For this purpose, we have added reactions (15), (16), (17) and (18) to the photooxidation scheme of Fig. 5. The new scheme is presented in Fig. 8. After a few trials it becomes obvious that reactions (15)-(18) can not provide a good fit of calculated curves to experimental results. Depending on the efficiencies assumed for reactions (15) and (16), the rate of photooxidation can be slowed down at will but the shape of the experimental curves (induction period followed by an oxidation as fast as in the absence of stabilizer) is never obtained. Such a result requires that one intermediate in the regenerating sequence of reactions (15) and (16) is progressively lost or becomes inactive. One can, for instance, suggest that a fraction of the "~/NOR /groups becomes 'trapped' in an unfavourable environment that makes any further encounter with a peroxy radical unlikely. This could be due to the morphology of the polymer or to its chemical structure if inactive ~ N O R groups are located in a fully oxidized region where no mole peroxy radicals are generated. Although the progressive transformation of ] N O R groups into an inactive form is hypothetical and still requires"experimental support, it provides a good agreement between calculated curves and experimental results (Fig. 9) if one further assumes that 1% of hydroperoxide groups react according to reaction (17) and 5% according to reaction (18) to generate nitroxide radicals. The latter have to
New aspects of the photooxidation of polyolefins PRODUCTION OF NO'
327
REGENERATION OF NO'
I I
IRo. .o.I
NOR I
> "INACTIVE"
1
RCOR + ROH + O~ I
SEE FIG,5
SEE FIG,5
Fig. 8. Hypothetical m o d e of action of H A L S used as a basis for c o m p u t e r simulation.
is i 1°-2 M
iO
0
i
h m
L
i
m
Fig. 9. Inhibition of the carboxylic acid formation during the photooxidation of E P M . Calculated curves for various concentrations of H A L S (mol kg -~ x 103).
328
G. Geuskens et al.
scavenge one-third of the alkyl radicals (reaction (15)) and /~"]qOR groups have to react with one-quarter of the peroxy radicals (reaction (16)) if the observed efficiency of HALS is to be obtained. These values are rather high but could be accounted for by the preferential location of HALS and derivatives in partially oxidized regions of the polymer where the photooxidation is most likely initiated (by the photolysis ,of hydroperoxides and ketones). A progressive loss of activity of N O R groups compatible with the length of the induction • J . . . period observed experimentally can. be justified if only 1 o'/o i s transformed into an inactive form each time reaction (15) occurs while 99% regenerate nitroxide radicals by reaction (16). The curves of Fig. 9 are quite similar in shape to experimental curves obtained in pp,7 and the induction period is roughly proportional to the concentration of HALS. Figure 10 indicates that not only the HALS itself but also the substituted hydroxylamine ~ N O R and the nitroxide radical ~ N O " are efficient stabilizers. O n / a n equal concentration basis, the efficiency follows the sequence NO'> )NOR>~NH as experimentally observed. Since the agreement between calculated curves and experimental results looks quite good, computer simulation can be used to get a deeper insight into the
15 10-2M
NH 10
ff
NOR
/
NO"
h 500
1000
1500
Fig. 10. Comparativeefficienciesof HALS (NH) and the derived substituted hydroxylamine (NOR) and nitroxide radical (NO') as inhibitors of the photooxidation of EPM (on an equal concentration basis: 1× 10-SM). Calculated curves for the formation of carboxylicacids (-- -, experimentalcurve for the unstabilizedsample).
New aspects of the photooxidation of polyolefins
10-3M
in
329
/I'"
"'-.
1ooo
2000
3OOO
Fig. 11. Concentration of various intermediates derived from HALS during the photooxidation of EPM. Calculated curves for an initial HALS concentration of 3 x 10-3 M. Concentration of carboxylic acids in arbitary units (for the actual scale see Fig. 9.). mechanism of the protection due to H A L S . T h e concentrations of the various intermediates can b e calculated as a function of time. Figure 11 shows that starting with an initial H A L S concentration of 3 × 10-3M (0"15 w t % Tinuvin 770) a b o u t 7 0 % is rapidly transformed into the active form of " ~ N O R . Nitroxide radicals are p r o d u c e d m o r e slowly and their concentration b e c o m e s stationary at a b o u t 5 × 10 -4 M in a g r e e m e n t with the experimental o r d e r of magnitude. T h e concentration of 'inactive' " ~ N O R groups progressively increases and, therefore, after reaching a maximum, the concentration of 'active' ~ . . O R groups starts decreasing. As it drops to zero the nitroxide radicals are no longer regenerated and their concentration also drops. A t that m o m e n t photooxidation starts at the same rate as in the unstabilized sample; it is the end of the induction period. A l t h o u g h this part of the paper is based exclusively on c o m p u t e r simulations, it provides s o m e important conclusions concerning the m o d e of action of H A L S . The mechanism p r o p o s e d by m a n y scientists (reactions (15) and (16)) is compatible with the experimental results b u t the radical scavengers have to b e fairly efficient (more than expected from model compounds) ix and o n e has to assume that o n e intermediate, the ~ N O R group, b e c o m e s progressively inactive.
330
G. Geuskens et al.
Experimental support for this suggestion should be looked for and it is likely that a better understanding of the inactivation mechanism will open new possibilities for stabilization that obviate such a drawback.
CONCLUSIONS This paper demonstrates that both hydroperoxides and ketones produced during previous thermal oxidation can initiate the photooxidation of EPM. On an equal concentration basis the former are much more efficient but in extensively oxidized samples ketones play a dominant role anyway. The relative importance of hydroperoxides and ketones is not only dependent on the extent of oxidation of the polymer but also on its chemical structure. Because most ketones that accumulate during thermal oxidation are formed inside the polymer chain by a mechanism that requires at least one secondary peroxy radical it can be expected that ketones are much more important in P E than in PP, for instance. A computer simulation based on a reasonable choice of parameters provides a good fit of calculated curves to experimental results and support the photooxidation mechanism proposed for EPM. This indicates that two reactions contribute to the main chain scission and thus to the deterioration of the physical and mechanical properties: the Norrish type II reaction of ketones and the photolysis of hydroperoxides forming complexes with ketones. This last reaction produces carboxylic acids. The key step that regulates the rate of the photooxidation is the competition for peroxy radicals between hydrogen abstraction and disproportionation. It opens two parallel lanes for the reaction: one, going through hydroperoxides, generates new free radicals and makes it faster whereas the other, going through ketones, slows it down because free radicals are consumed and few are produced. In EPM both lanes are followed with about equal probabilities but in PP where most peroxy radicals are tertiary, disproportionation is much less likely than in P E for instance. This could be the reason why PP photooxidizes much faster than PE. Computer simulation opens up new vistas over the mode of action of HALS. The mechanism usually proposed cannot justify the experimental results unless one intermediate is progressively lost or becomes inactive. This last assumption, if it finds experimental support,
New aspects of the photooxidation of polyolefins
331
could draw new research lines likely to improve the efficiency of HALS.
REFERENCES 1. Geuskens, G., Bastin, P., Lu Vinh, Q. and Rens, M., Polym. Degr. and Stab., 3 (1980-81) 295. 2. Geuskens, G. and David, C., Pure Appl. Chem., 51 (1974) 233. 3. Geuskens, G., Baeyens-Volant, D., Delaunois, G., Lu Vinh, Q., Piret, W. and David, C., Europ. Polym. J., 14 (1978) 299. 4. Geuskens, G. and Kabamba, M. S., Polym. Degr. and Stab., 4 (1982) 69. 5. Geuskens, G. and Kabamba, M. S., Polym. Degr. and Stab., 5 (1983) 399. 6. Hartley, G. H.-and Guillet, J. E., Macromolecules, I (1968) 165. 7. Hodgeman, D. K. C., Developments in polymer degradation--4, Grassie, N. (ed.), Applied Science Publishers Ltd, London, 1982. 8. Carlsson, D. J., Garton, A. and Wiles, D. M., Developments in polymer stabilization--I, Scott, G. (ed.), Applied Science Publishers Ltd, London 1980. 9. Shlyapintokh, V. Ya. and Ivanov, V. B., Developments in polymer stabilization--5, Scott, G. (ed.), Applied Science Publishers Ltd, London, 1982. 10. Carlsson, D. J., Chan, K. H., Durmis, J. and Wiles, D. M., J. Polym. Sci. Chem. Ed. 20 (1982) 575. 11. Grattan, D. W., Carlsson, D. J. and Wiles, D. M., Polym. Degr. and Stab., 1 (1979) 69.