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Photo-stabilising action of a polymeric hindered piperidine compound in polypropylene film: Influence of processing
PolymerDegradationandStability4 (1982) 161 166
PHOTO-STABILISING ACTION OF A POLYMERIC HINDERED PIPERIDINE COMPOUND IN POLYPROPYLENE FILM: INFLUENCE OF PROCESSING
NORMAN S. ALLEN & ALAN PARKINSON
Department of Chemistry, John Dalton Faculty of Technology, Manchester Polytechnic, Chester Street, Manchester M1 5DG, Great Britain Received: 5 October, 1981)
A BSTRA C T
The ef/ect of a phenolic antioxidant on the photo-stabilismg performance of a polymeric hindered piperidine compound in polypropylene has been examined using infra-red and ESR spectroscopic techniques. Processing history is shown to play a dominant r61e in controlling the photo-stabilising performance of these systems. Whilst the antioxidant gave enhanced performance, in most cases its effect is antagonistic. The ESR results suggest that maximum stabilisation is associated with the conversion of the amine to the substituted hydroxylamine and not the nitroxyl radical.
INTRODUCTION
The mode of operation of hindered piperidine light stabilisers and their interaction with antioxidants have attracted considerable interest in recent years. 1 The stabilising effectiveness of these hindered piperidines almost certainly depends on their ability to form a stable nitroxyl radical which then scavenges macro-radicals (P.) produced during photo-oxidation: 1-4 ~" 7
N--H
hv AH [Ol
•
~ /
N--O"
P
•
/"
N--O--P
(1)
The nitroxyl radicals are believed to be produced through a stoichiometric reaction of the amines with hydroperoxides 5'6 although recent work 7 on model systems and in polymers suggests this is not the case. In fact, the importance of the nitroxyl radical as an active macro-radical trap appears to be in some doubt 7"8and there now 161 Polymer Degradation and Stability 0141-3910/82/0004-0161/$02.75 England, 1982 Printed in Great Britain
© Applied Science Publishers Ltd,
162
NORMAN S. ALLEN, ALAN PARKINSON
appears to be more emphasis on the r61e of the back reaction of the substituted hydroxylamine with peroxy radicals:7'9 \
\
/ N - - O - - P + PO 2.
~ PO2P + / N - - O .
(2)
Recently we produced some evidence to show that conversion of the nitroxyl radical to the substituted hydroxylamine is an important process, although the effects of processing on the light stability of the hindered piperidine examined (bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate) gave conflicting results, a° To establish the importance of mechanisms (1) and (2) above, we have resorted to the use of another hindered piperidine stabiliser but of the polymeric type, with the structure (I):
CH. CH3~CH3CH3~CH
CH 3
N
CH 3
CH 3
N
(I) ---N--(CH2) 6 - - N
."
N..fN NH--C(CH3)2CH2--C(CH3)3 Certainly, with this type of stabiliser both the light stability and ESR results agree on both mechanisms. We have also examined the interaction of this stabiliser with a phenolic antioxidant since previous work has shown the involvement of antagonism. 1o, 1
EXPERIMENTAL
Materials Polypropylene powder containing no commercial additives was supplied by ICI (Petrochemicals and Plastics Division) Ltd, Great Britain. The light stabiliser, Chimassorb 944, with the structure (I), and the antioxidant, pentaerythritol-tetrafl-(4-hydroxy-3,5-di-t-butylphenyl) propionate (Irganox 1010), were supplied by Ciba-Geigy (UK) Ltd, Manchester. The additives were solvent blended into polypropylene powder at 0.1 ~ w / w concentration using dichloromethane as the solvent. The solvent was allowed to evaporate overnight. The additives were also processed into polypropylene at 200 °C for 10, 20 and 30 min using a Brabender Plasticorder (Duesburg, West Germany).
163
PHOTOSTABILISATION OF POLYPROPYLENE
Films ~400 ~m thick were then prepared by pressing the polymer between sheets of aluminium foil at 200°C for 1 min followed by quench cooling in water.
Irradiation All the films were exposed in a Microscal Apparatus (Microscal Ltd, London), utilising a 500 W high pressure mercury/tungsten fluorescent lamp at Manchester Polytechnic and in a S E P A P 70:07 unit utilising a 1000 W xenon/mercury lamp at the Universit6 de Clermont II. Both units operate at low relative humidity < 20 and 50 °C.
Rates of photo-oxidation Rates of photo-oxidation of the polymer films were monitored by infra-red spectroscopy using the well established carbonyl index method. 1"3'5-11 Spectra were recorded using Perkin-Elmer spectrometer models 457 (Manchester Polytechnic) and 682 (Universit6 de Clermont II).
ESR spectra ESR spectra were recorded at room temperature, using a Brucker ER2000 spectrometer, at the Universite de Clermont II.
RESULTS AND DISCUSSION
The rates of photo-oxidation of all the stabilised polypropylene films in the Microscal unit are compared in Figs 1 to 3. The most interesting feature of these results is that whilst all the antioxidant control films exhibit an induction period
//
O.I
005 z u
I
[
I00 200 IRRADIATION
3(~)0
4 0I 0
510
6 0L0
TIME, hrs
Fig. 1. Carbonyl index versus time of irradiation in the Microscal unit for polypropylene film (~400/~m thick) containing 0-1%w/w Irganox 1010 after O - - 0 m i n , E]--10min, A 20min and 30 rain of processing at 200 °C.
///
164
N O R M A N S. A L L E N , A L A N P A R K I N S O N
04
/ /
04
0.0 5
0-05
z O ,¢ ¢J
5~)O IRRADIATION
I 0 0I 0
I 15OO
T I M E , hrs
Fig. 2. Carbonylindexversustime of irradiation in the Microscal unit for polypropylene film (~400/~m thick) containing 0-1 ~ow/w of Chimassorb 944 after O--0min, [] 10min, A--20min and V--30min of processing at 200 °C.
I
500 IR R A D I A T I O N
I
IOOO
IS~O
T I M E , hrs
Fig. 3. Carbonylindexversustime or irradiation in the Microscal unit for polypropylene film (~400 pm thick) containing 0-1 ~o w/w each of Irganox 1010+ Chimassorb 944 after O--0 min, U]--10min, A--20min and ~--30min of processing at 200°C.
prior to the formation of carbonyl groups (Fig. 1), all the hindered piperidine stabilised films show a very slow growth in carbonyl from the onset of irradiation (Figs 2 and 3). The relative stabilities of the films may be more easily compared by the embrittlement data shown in Table 1. These values correspond to the time to 0.06 carbonyl units. It is seen that the light stability of the Irganox 1010 films decreases after 10 and 20 minutes' processing and then dramatically increases again after 30min. These results follow closely the expected growth and decay in hydroperoxide concentration under these processing conditions. 1°'12. The light stabilities of the hindered piperidine stabilised films show quite a different pattern of behaviour. In this case there was a dramatic increase in stability up to 20 minutes' processing, followed by a decrease after 20 min. These results also follow the growth and decay of hydroperoxides during processing and clearly suggest that there is a favourable interaction between the hydroperoxides and the hindered piperidine to give a stable nitroxyl radical. The hindered piperidine, Tinuvin 770, shows a decrease in light stability with increasing hydroperoxide concentration. The light stability results of the combination of the hindered piperidine, Chimassorb 944, and the antioxidant, Irganox 1010, show that there is a strong interaction between them. Whilst the results show that the presence of the antioxidant gives an improvement in * See Note Added in Proof (p. 166).
165
P H O T O S T A B I L I S A T I O N OF P O L Y P R O P Y L E N E
TABLE 1 TIME TO 0"06 CARBONYL INDEX FOR CHIMASSORB 944/IRGANOX 1010 SAMPLES DURING IRRADIATION
Additive (0.1% w/w each)
Processing time (rain) 10 20
0 Irganox 1010 Chimassorb 944 Chimassorb 944 + Irganox 1010
480 850 1150
370 1000 1040
30
390 1350 1300
440 1200 1400
Film thickness ~ 400 #m.
light stability, its presence is clearly antagonistic. Similar results were obtained in the S E P A P exposure unit. An examination of the polymer films by ESR spectroscopy shows a n u m b e r o f interesting features which correlate closely with the light stability results. The effect of processing on the nitroxyl radical concentration is shown in Fig. 4. In the absence of antioxidant it is seen that, over a 20-min processing period, there is a decrease in the nitroxyl radical concentration. All the commercial hindered piperidine comp o u n d s contain some o f the corresponding nitroxyl radical. 1,1o In this case it would appear that initially the nitroxyl radical is trapping macroalkyl radicals during processing to form the substituted hydroxylamine, confirming reaction (1), and this gives rise to a marked improvement in light stability. Thus, the hydroxylamine appears to be an effective stabiliser. A further 10 minutes' processing results in an increase in the nitroxyl radical concentration due to reaction of the hydroxylamine with peroxy radicals by reaction (2). In this case, the light stability decreases. The presence of the antioxidant reduces the initial concentration of the nitroxyl radical. After 10 minutes' processing the nitroxyl radical concentration is reduced, but, on
>-2~
I0
-
_z
~
<
~
z
d
~ I
20
PROCESSING
3~ T I M E j rains
Fig. 4. ESR signal intensity of the nitroxyl radical versus processing time at 200°C for polypropylene film ( ~ 400 #m thick) containing C)--0" 1% Chimassorb 944 and Q--0.1% Irganox 1010. A typical ESR spectrum of the nitroxyl radical of Chimassorb 944 is shown ( ).
uJ
~
ENT
500 I
iRRADIATION
[000 I
TIME,
1500 1
HAS
Fig. 5. Change in ESR signal intensity of the nitroxyl radical for polypropylene film (~ 400 /~m thick) containing 0.1% Chimassorb 944 (0 min of processing) during irradiation in the Microscal unit.
166
NORMAN S. ALLEN, ALAN PARKINSON
further processing, the presence of the antioxidant appears to stabilise the hydroxylamine and this is reflected by an improvement in light stability. The stabilisation by the antioxidant is evidently due to the ability of the phenol to react competitively with the peroxy radicals, lo,13 A typical ESR spectrum of the nitroxyl radical of this stabiliser is also shown in Fig. 4 and, as expected, it possesses a typical triplet character with an mT value of 2.009. The change in nitroxyl radical concentration during irradiation is shown in Fig. 5. As reported for other hindered piperidines, the concentration increases sharply during the very early stages of irradiation, 1.2.6,v,14 but, in this case, there is also a slow growth towards embrittlement followed by a very slow decrease well beyond the induction period. Clearly, the behaviour of this polymeric stabiliser is quite different from that of, say, Tinuvin 770, in commercial polypropylene film. It is also much less efficient and this is reflected by its inefficiency in inhibiting the growth of carbonyl products during irradiation. ACKNOWLEDGEMENTS
The authors wish to thank Professor J. Lemaire of the Universit6 de Clermont I! for helpful discussions and the use of his equipment. The authors also thank the NATO Scientific Division for a travel grant (No. 8780/D1) in support of this work. REFERENCES 1. N. S. ALLEN, in Developments in polymer photochemistry. Vol. 2 (N. S. Allen (Ed)), Applied Science Publishers Ltd, London. Chapter 7 (1981). 2. Y. B. SnILOVand E. T. DENISOV, Vysokomolek. Soedin, A16, 2313 (1974). 3. A. R. PATEL and J. J. USILTON, Advances in Chemistry Series No. 169 (D. L. Allara and W. L. Hawkins (Eds)), Am. Chem. Soc., Washington, D.C. Chapter 10, 116 (1978). 4. E. G. ROSANTSEVand V. D. SCHOLLE, Synthesis, 190 (1971). idem--ibid. 401 (1971). 5. K. B. CHAKRABORTYand G. SCOTT, Chemy Ind., 237 (1978). 6. N. S. ALLEN, Makromol. Chemie, 181, 2413 (1981). 7. J. SEDLAR,J. PETRUJ,J. PACand A. ZAnRADNICKOVA,Eur. Polym. J., 16,659 idem--ibid. 663 (1980). 8. D. W. GRATTAN, D. J. CARLSONand D. M. WILES, Polym. Deg. & Stab., 1, 59 (1979). 9. K. B. CHARRABORTYand G. SCOTT, Polymer, 21,252 (1980). 10. N. S. ALLEN, J. L. GARDETTEand J. LEMAIRE,Polym. Photochem., 1, 111 (1981). 11. N. S. ALLEN, Polym. Deg. & Stab., 2, 129 (1980). 12. K. B. CnAgRABORTYand G. SCOTT, Polymer, 18, 98 (1977). 13. J. POSPISIL,in Developments in polymer photochemistry. Vol. 2 (N. S. Allen (Ed)), Applied Science Publishers Ltd, London. Chapter 3 (1981). 14. G. BALINT,A. ROCKENBAUER,T. KELEN, F. TUDOSand L. JOCKAY,Polym. Photoehem.,l, 139 (1981).
NOTE ADDED IN PROOF R e c e n t h y d r o p e r o x i d e a n a l y s i s i n o u r l a b o r a t o r y o n c o n t r o l P P film s h o w s t h a t m a x i m u m [POOH] is a c h i e v e d a f t e r 10 m i n p r o c e s s i n g a t 2 0 0 °C.
PHOTO-STABILISING ACTION OF A POLYMERIC HINDERED PIPERIDINE COMPOUND IN POLYPROPYLENE FILM: INFLUENCE OF PROCESSING
NORMAN S. ALLEN & ALAN PARKINSON
Department of Chemistry, John Dalton Faculty of Technology, Manchester Polytechnic, Chester Street, Manchester M1 5DG, Great Britain Received: 5 October, 1981)
A BSTRA C T
The ef/ect of a phenolic antioxidant on the photo-stabilismg performance of a polymeric hindered piperidine compound in polypropylene has been examined using infra-red and ESR spectroscopic techniques. Processing history is shown to play a dominant r61e in controlling the photo-stabilising performance of these systems. Whilst the antioxidant gave enhanced performance, in most cases its effect is antagonistic. The ESR results suggest that maximum stabilisation is associated with the conversion of the amine to the substituted hydroxylamine and not the nitroxyl radical.
INTRODUCTION
The mode of operation of hindered piperidine light stabilisers and their interaction with antioxidants have attracted considerable interest in recent years. 1 The stabilising effectiveness of these hindered piperidines almost certainly depends on their ability to form a stable nitroxyl radical which then scavenges macro-radicals (P.) produced during photo-oxidation: 1-4 ~" 7
N--H
hv AH [Ol
•
~ /
N--O"
P
•
/"
N--O--P
(1)
The nitroxyl radicals are believed to be produced through a stoichiometric reaction of the amines with hydroperoxides 5'6 although recent work 7 on model systems and in polymers suggests this is not the case. In fact, the importance of the nitroxyl radical as an active macro-radical trap appears to be in some doubt 7"8and there now 161 Polymer Degradation and Stability 0141-3910/82/0004-0161/$02.75 England, 1982 Printed in Great Britain
© Applied Science Publishers Ltd,
162
NORMAN S. ALLEN, ALAN PARKINSON
appears to be more emphasis on the r61e of the back reaction of the substituted hydroxylamine with peroxy radicals:7'9 \
\
/ N - - O - - P + PO 2.
~ PO2P + / N - - O .
(2)
Recently we produced some evidence to show that conversion of the nitroxyl radical to the substituted hydroxylamine is an important process, although the effects of processing on the light stability of the hindered piperidine examined (bis(2,2,6,6-tetramethyl-4-piperidinyl) sebacate) gave conflicting results, a° To establish the importance of mechanisms (1) and (2) above, we have resorted to the use of another hindered piperidine stabiliser but of the polymeric type, with the structure (I):
CH. CH3~CH3CH3~CH
CH 3
N
CH 3
CH 3
N
(I) ---N--(CH2) 6 - - N
."
N..fN NH--C(CH3)2CH2--C(CH3)3 Certainly, with this type of stabiliser both the light stability and ESR results agree on both mechanisms. We have also examined the interaction of this stabiliser with a phenolic antioxidant since previous work has shown the involvement of antagonism. 1o, 1
EXPERIMENTAL
Materials Polypropylene powder containing no commercial additives was supplied by ICI (Petrochemicals and Plastics Division) Ltd, Great Britain. The light stabiliser, Chimassorb 944, with the structure (I), and the antioxidant, pentaerythritol-tetrafl-(4-hydroxy-3,5-di-t-butylphenyl) propionate (Irganox 1010), were supplied by Ciba-Geigy (UK) Ltd, Manchester. The additives were solvent blended into polypropylene powder at 0.1 ~ w / w concentration using dichloromethane as the solvent. The solvent was allowed to evaporate overnight. The additives were also processed into polypropylene at 200 °C for 10, 20 and 30 min using a Brabender Plasticorder (Duesburg, West Germany).
163
PHOTOSTABILISATION OF POLYPROPYLENE
Films ~400 ~m thick were then prepared by pressing the polymer between sheets of aluminium foil at 200°C for 1 min followed by quench cooling in water.
Irradiation All the films were exposed in a Microscal Apparatus (Microscal Ltd, London), utilising a 500 W high pressure mercury/tungsten fluorescent lamp at Manchester Polytechnic and in a S E P A P 70:07 unit utilising a 1000 W xenon/mercury lamp at the Universit6 de Clermont II. Both units operate at low relative humidity < 20 and 50 °C.
Rates of photo-oxidation Rates of photo-oxidation of the polymer films were monitored by infra-red spectroscopy using the well established carbonyl index method. 1"3'5-11 Spectra were recorded using Perkin-Elmer spectrometer models 457 (Manchester Polytechnic) and 682 (Universit6 de Clermont II).
ESR spectra ESR spectra were recorded at room temperature, using a Brucker ER2000 spectrometer, at the Universite de Clermont II.
RESULTS AND DISCUSSION
The rates of photo-oxidation of all the stabilised polypropylene films in the Microscal unit are compared in Figs 1 to 3. The most interesting feature of these results is that whilst all the antioxidant control films exhibit an induction period
//
O.I
005 z u
I
[
I00 200 IRRADIATION
3(~)0
4 0I 0
510
6 0L0
TIME, hrs
Fig. 1. Carbonyl index versus time of irradiation in the Microscal unit for polypropylene film (~400/~m thick) containing 0-1%w/w Irganox 1010 after O - - 0 m i n , E]--10min, A 20min and 30 rain of processing at 200 °C.
///
164
N O R M A N S. A L L E N , A L A N P A R K I N S O N
04
/ /
04
0.0 5
0-05
z O ,¢ ¢J
5~)O IRRADIATION
I 0 0I 0
I 15OO
T I M E , hrs
Fig. 2. Carbonylindexversustime of irradiation in the Microscal unit for polypropylene film (~400/~m thick) containing 0-1 ~ow/w of Chimassorb 944 after O--0min, [] 10min, A--20min and V--30min of processing at 200 °C.
I
500 IR R A D I A T I O N
I
IOOO
IS~O
T I M E , hrs
Fig. 3. Carbonylindexversustime or irradiation in the Microscal unit for polypropylene film (~400 pm thick) containing 0-1 ~o w/w each of Irganox 1010+ Chimassorb 944 after O--0 min, U]--10min, A--20min and ~--30min of processing at 200°C.
prior to the formation of carbonyl groups (Fig. 1), all the hindered piperidine stabilised films show a very slow growth in carbonyl from the onset of irradiation (Figs 2 and 3). The relative stabilities of the films may be more easily compared by the embrittlement data shown in Table 1. These values correspond to the time to 0.06 carbonyl units. It is seen that the light stability of the Irganox 1010 films decreases after 10 and 20 minutes' processing and then dramatically increases again after 30min. These results follow closely the expected growth and decay in hydroperoxide concentration under these processing conditions. 1°'12. The light stabilities of the hindered piperidine stabilised films show quite a different pattern of behaviour. In this case there was a dramatic increase in stability up to 20 minutes' processing, followed by a decrease after 20 min. These results also follow the growth and decay of hydroperoxides during processing and clearly suggest that there is a favourable interaction between the hydroperoxides and the hindered piperidine to give a stable nitroxyl radical. The hindered piperidine, Tinuvin 770, shows a decrease in light stability with increasing hydroperoxide concentration. The light stability results of the combination of the hindered piperidine, Chimassorb 944, and the antioxidant, Irganox 1010, show that there is a strong interaction between them. Whilst the results show that the presence of the antioxidant gives an improvement in * See Note Added in Proof (p. 166).
165
P H O T O S T A B I L I S A T I O N OF P O L Y P R O P Y L E N E
TABLE 1 TIME TO 0"06 CARBONYL INDEX FOR CHIMASSORB 944/IRGANOX 1010 SAMPLES DURING IRRADIATION
Additive (0.1% w/w each)
Processing time (rain) 10 20
0 Irganox 1010 Chimassorb 944 Chimassorb 944 + Irganox 1010
480 850 1150
370 1000 1040
30
390 1350 1300
440 1200 1400
Film thickness ~ 400 #m.
light stability, its presence is clearly antagonistic. Similar results were obtained in the S E P A P exposure unit. An examination of the polymer films by ESR spectroscopy shows a n u m b e r o f interesting features which correlate closely with the light stability results. The effect of processing on the nitroxyl radical concentration is shown in Fig. 4. In the absence of antioxidant it is seen that, over a 20-min processing period, there is a decrease in the nitroxyl radical concentration. All the commercial hindered piperidine comp o u n d s contain some o f the corresponding nitroxyl radical. 1,1o In this case it would appear that initially the nitroxyl radical is trapping macroalkyl radicals during processing to form the substituted hydroxylamine, confirming reaction (1), and this gives rise to a marked improvement in light stability. Thus, the hydroxylamine appears to be an effective stabiliser. A further 10 minutes' processing results in an increase in the nitroxyl radical concentration due to reaction of the hydroxylamine with peroxy radicals by reaction (2). In this case, the light stability decreases. The presence of the antioxidant reduces the initial concentration of the nitroxyl radical. After 10 minutes' processing the nitroxyl radical concentration is reduced, but, on
>-2~
I0
-
_z
~
<
~
z
d
~ I
20
PROCESSING
3~ T I M E j rains
Fig. 4. ESR signal intensity of the nitroxyl radical versus processing time at 200°C for polypropylene film ( ~ 400 #m thick) containing C)--0" 1% Chimassorb 944 and Q--0.1% Irganox 1010. A typical ESR spectrum of the nitroxyl radical of Chimassorb 944 is shown ( ).
uJ
~
ENT
500 I
iRRADIATION
[000 I
TIME,
1500 1
HAS
Fig. 5. Change in ESR signal intensity of the nitroxyl radical for polypropylene film (~ 400 /~m thick) containing 0.1% Chimassorb 944 (0 min of processing) during irradiation in the Microscal unit.
166
NORMAN S. ALLEN, ALAN PARKINSON
further processing, the presence of the antioxidant appears to stabilise the hydroxylamine and this is reflected by an improvement in light stability. The stabilisation by the antioxidant is evidently due to the ability of the phenol to react competitively with the peroxy radicals, lo,13 A typical ESR spectrum of the nitroxyl radical of this stabiliser is also shown in Fig. 4 and, as expected, it possesses a typical triplet character with an mT value of 2.009. The change in nitroxyl radical concentration during irradiation is shown in Fig. 5. As reported for other hindered piperidines, the concentration increases sharply during the very early stages of irradiation, 1.2.6,v,14 but, in this case, there is also a slow growth towards embrittlement followed by a very slow decrease well beyond the induction period. Clearly, the behaviour of this polymeric stabiliser is quite different from that of, say, Tinuvin 770, in commercial polypropylene film. It is also much less efficient and this is reflected by its inefficiency in inhibiting the growth of carbonyl products during irradiation. ACKNOWLEDGEMENTS
The authors wish to thank Professor J. Lemaire of the Universit6 de Clermont I! for helpful discussions and the use of his equipment. The authors also thank the NATO Scientific Division for a travel grant (No. 8780/D1) in support of this work. REFERENCES 1. N. S. ALLEN, in Developments in polymer photochemistry. Vol. 2 (N. S. Allen (Ed)), Applied Science Publishers Ltd, London. Chapter 7 (1981). 2. Y. B. SnILOVand E. T. DENISOV, Vysokomolek. Soedin, A16, 2313 (1974). 3. A. R. PATEL and J. J. USILTON, Advances in Chemistry Series No. 169 (D. L. Allara and W. L. Hawkins (Eds)), Am. Chem. Soc., Washington, D.C. Chapter 10, 116 (1978). 4. E. G. ROSANTSEVand V. D. SCHOLLE, Synthesis, 190 (1971). idem--ibid. 401 (1971). 5. K. B. CHAKRABORTYand G. SCOTT, Chemy Ind., 237 (1978). 6. N. S. ALLEN, Makromol. Chemie, 181, 2413 (1981). 7. J. SEDLAR,J. PETRUJ,J. PACand A. ZAnRADNICKOVA,Eur. Polym. J., 16,659 idem--ibid. 663 (1980). 8. D. W. GRATTAN, D. J. CARLSONand D. M. WILES, Polym. Deg. & Stab., 1, 59 (1979). 9. K. B. CHARRABORTYand G. SCOTT, Polymer, 21,252 (1980). 10. N. S. ALLEN, J. L. GARDETTEand J. LEMAIRE,Polym. Photochem., 1, 111 (1981). 11. N. S. ALLEN, Polym. Deg. & Stab., 2, 129 (1980). 12. K. B. CnAgRABORTYand G. SCOTT, Polymer, 18, 98 (1977). 13. J. POSPISIL,in Developments in polymer photochemistry. Vol. 2 (N. S. Allen (Ed)), Applied Science Publishers Ltd, London. Chapter 3 (1981). 14. G. BALINT,A. ROCKENBAUER,T. KELEN, F. TUDOSand L. JOCKAY,Polym. Photoehem.,l, 139 (1981).
NOTE ADDED IN PROOF R e c e n t h y d r o p e r o x i d e a n a l y s i s i n o u r l a b o r a t o r y o n c o n t r o l P P film s h o w s t h a t m a x i m u m [POOH] is a c h i e v e d a f t e r 10 m i n p r o c e s s i n g a t 2 0 0 °C.