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Photostabilising action of hindered piperidine compounds in polypropylene
Po|~m~r P h o m c h e ~
1 (1981) 243-259
PHOTOSTABILISING ACTION OF HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYLENEt
NORMANS. ALL~+~
Department of Chemistry, John Dalton Faculty of Technology, Manchester Polytechnic, Chester Street, ManchesterM1 5GD, Great Britain (Received: 3 June, 1980)
ABSTRACT Hindered amine compounds derived from piperidine are a relatively new addition to the range of commercial ultra-violet stabilisers for polymers (bulk and fibres). In comparison with other types of stabiliser, the hindered piperidines have proved to be by far the most effective systems. They do not appear to operate by mechanisms of optical screening or by excited state quenching. Essentially, their stabilising effectiveness depends upon their ability to form a stable nitroxyl radical, which then scavenges alkyl radicals produced during photo-oxidation. In polyole]ins they also inhibit the photo-reactions of carbonyl chromophores and react with hydroperoxides. This paper reports on the multifunctional behaviour of these commercially important stabiliser systems and their interactions with antioxidants. INTRODUCTION For outdoor applications many plastics materials have to be adequately protected against the harmful effects of the ultra-violet portion of sunlight that reaches the sudace of the earth, i.e. wavelengths from 290--400 nm. 1 This is particularly so for three commercially important polyolofins, polythenc (low and high density), polypropylene and poly(4-methylpent-l-ene). Of the many classes of compounds that have been developed as photostabilisers the relatively new hindered amines based on piperidine have proved to be the most effective,t-4 Of these, the hindered piperidine derivative t Presented at the 2nd InternationalConferenceon the 'Stabilisationand ControlledDegradation of Polymers',June 2--4th, Lucerne, 1980. 243 Polymer Photochemistry 0144-2880/81/0001-0243/$2-50~Applied Science Publishers Ltd, England, 1981 Printed in Northern Ireland
244
S. # I I ~ N
NORMAN
(structure (I)), known commercially as Tinuvin 770, has attracted widespread interest. The stabilising effect of these hindered piperidines is believed to depend upon their ability to form a stable nitroxyl radical, which then scavenges macroradicals (P') produced during photo-oxidation, l~s as in reaction scheme (1). Recently, Chakraborty and Scott, 7 attributed the photostabilising effectiveness of the hindered piperidine compound (I), in low density CH3
COC(CH2)~
~I--N
CHa CH3 (13
(1)
k~,,a
polyethylene, with its ability m react with hydroperoxides in the polymer to form a stable nitroxyl radical (reaction (2)).
~ ~
- - H
+ POOH
a,
N" + PO" + H 2 0
(2)
0" + "OH ,
N - - - O - - - ( O H + p.
Their evidence for this mechanism was based on the observation that the presence of a hydroperoxide decomposer, nickel diethyldithiocarbamate, antagonised the stabilising action of the hindered piperidine compound. In another mechanism, postulated by Grattcn et al.,s the nitroxyl radical is believed to form a hydrogen-bonded complex with hydroperoxides (for example, reaction (3)). To determine the validity of the mechanism illustratedin
HINDERED PIPERIDINE C O M P O U N D S
245
IN POLYPROPYLENE
reaction (1), the effect of photo-oxidation on the hindered piperidine stabiliser
N--O'---P--H + POOH
• ~N---O'--POOH
+ P--H
(3)
/ x
(structure (I)) is compared with that of a stable piperidine-N-oxy radical (structure (II)) in polypropylene using ESR spectroscopy. To demonstrate the C~H3CH3
7
\
/\
//~CH
2OH
CH3 CH3 (I13 multifunctional behaviour of these hindered piperidine compounds, the effect of thermally generated hydroperoxides and added anthraquinone have been examined on the photostabilising performance of compound (I) in polypropylene, using ESR spectroscopy. In this study, the effect of the hindered piperidine compound (I) and the model N-oxy radical (II), on the photochemical behaviour of anthraquinone in a model solvent, n-hexane, using flash photolysis and luminescence spectroscopy has also been examined. Anthraquinone was selected for study since its photochemical reactions in solution and for that matter, polymers, are well understood. 9"1° One of the major primary photochemical processes in the photo-oxidation of commercial polypropylene is isomerisation of a, unsatarated carbonyl impurities by the process shown in reaction (4), 1~ followed by the well-known I
I
h~
I
I
I/
Norrish Type I and II reactions. In this study the effect of both hindered piperidine compounds (I) and (II) have been examined on reaction (4). Finally, in view of the results of Chakraborty and Scott7 the effects of some typical primary and secondary thermal antioxidants have also been examined on the photostabilising effectiveness of the hindered pipeddine compound (I). To assist with the interpretation of these results a novel reaction between the N-oxy radical (II) and a typical phenolic antioxidant, (Irganox 1010), is described.
246
N O R M A N S. AI.I.I~.N
EXPERIMENTAL
Polypropylene powder containing no commercial additives was supplied by ICI (Plastics Division) Ltd. The antioxidants---Irganox 1010 (pentaerythritol-tetra/3-(4-hydroxy-3,5-di-tert-butylphenol) proportionate), Ciba-Geigy Ltd; Topanol CA (3-methyl-6-tert-butylphenol and crotonaldehyde condensate), ICI Ltd; Goodrite 3114 (tris(4-hydroxy-3,5-di-tert-butylbenzyl-isocyanurate), Goodrich Co Ltd; Weston 618 (distearyl pentaerythrityl diphosphite), Weston Chemicals; DLTDP (dilauryl thio-dipropionate), ICI Ltd; and light stabiliser Tinuvin 770 (bis[2,2,6,6-tetramethyl-4-piperidinyl]sebacate), Ciba-Geigy Corp---were supplied by their respective manufacturers. The compound 4hydroxyl-2,2,6,6-tetramethylpipeddine-N-oxy was purchased from the Eastman Kodak Co. Anthraquinone was purchased from BDH Chemicals and was recrystallised twice from ethanol. All solvents used were of standard laboratory grade or Analar quality. The compounds were solvent blended into the polypropylene powder using dichloromethane as a solvent. The solvent was allowed to evaporate overnight followed by pressing the powders at 200°(2, for I rain, into film 200-300/~m thick. A control sample was similarly prepared. Samples of polypropylene powder were heated at 120°C in an air oven for various periods of time up to I h. The stabiliser, Tinuvin 770, was then solvent blended into the thermally oxidised polypropylene powders at a 0.1% w/w concentration and treated as described above. Samples of Tinuvin 770 and Irganox 1010 (0.1% w/w of each) were also processed into polypropylene powder at 200°C for 10, 20 and 30 rain using a Brabender Plastic.order (Duesburg, W. Germany) followed by pressing into film, as described above. The antioxidant, Irganox 1010, was also oxidised to its quinone form by treatment with lead dioxide. Irganox 1010 (1 g) dissolved in toluene (50 nil) was shaken for 2-3 h with lead dioxide (5 g). After filtration the blue coloured filtrate (a stable phenoxy radical) was then bubbled vigorously with air for I h to oxidise the radical to the yellow quinone hydroperoxide. The toluene was then evaporated off and the product taken up in n-decane.
Hydroperoxide analysis The hydroperoxide concentrations in the thermally oxidised polymer powders were estimated using the standard iodometric method described earlier by Carlsson and Wiles. 12
Photo-oxidation The polymer films were irradiated in a Xenotest-150(GmbH) set-up for simulated sunlight exposure conditions out-of-doors ( k ' s > 3 0 0 n m ) (40°C, 50% relative humidity). The polymer films were flexed periodically to test for
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYI..ENE
247
embrittlement. The rates of photo-oxidation of the polymer films were also checked using the well established carbonyl index method (1710cm-~). A Perkin-Elmer Model 157G, infra-red absorption spectrometer was used for this purpose.
Spectroscopic measurement Fluorescence and phosphorescence spectra were obtained using a double grating (1200 lines mm -1) Hitachi Perkin-Elmer MPF-4 spectrofluorimeter equipped with two red-sensitive, R-446F, photomultiplier tubes. Ultra-violet absorption spectra were obtained using a Perkin-Elmer Model 554 spectrometer equipped with a microprocessor. Flash photolysis experiments were carried out in nitrogen (<5 ppm 02) saturated n-hexane using a micro-second apparatus with a photoflash of 300 J and a half-life of 10 ms. ESR spectra were obtained using a Jeol FE-3X spectrometer. RESULTS AND DIgCUgglON
Identification and behaviour of the N-oxy radical Figures 1 and 2 show the ESR spectra of the polymer films containing Tinuvin 770 and the N-oxy radical compound, before and after irradiation. From Fig. 1 it can be seen that Tinuvin exhibits a stable ESR spectrum before irradiation. During irradiation there is a marked increase in the intensity of the ESR spectrum, but after about 4000 h it attains an almost stationary steady-state intensity. The intensity of the ESR spectrum of the N-oxy radical compound markedly decreases during irradiation (Fig. 2) but, as found for the Tinuvin 770, the spectra attains a low steady-state intensity after about 4000 h. Also, at this low concentration, the spectrum of the N-oxy radical is resolvable into its three characteristic lines, similar to that for Tinuvin 770. At the relatively high concentration of N-oxy radical employed in this work the presence of the rigid polymer matrix will impair 'free-spin' of the radical 11 producing a distorted spectrum. These results clearly show that the radical generated during photostabilisation of the polymer by Tinuvin 770 is a nitroxyl radical. This is in agreement with the earlier postulate, 2-6 in which the nitroxyl radical is an essential intermediate formed during the reaction.
Effect on a, O-unsaturated carbonyl groups The light stabilising efliciencies of the Tinuvin 770 and model N-oxy radical at 0.25% w/w concentration in polypropylene are compared in Fig. 3. It is seen that both compounds inhibit carbonyl development in the polymer up to 4000 h of irradiation. Figure 4 shows the effect of irradiation on the intensity of the fluorescence
248
NORMAN S. ,~LL~N r
Ii /i
! i,f/
L/
3~vo
3 7o GAUSS
Fig. 1. E S R spectrum of his (2,2,6,6-tetramethyl-4-piperidinyl)sebacate (0.25%) in polypropylene film at r o o m temperature. ( ) A = 5 x 100; ( - - - ) A = 4 x 1000 before and (. . . . . ) A = 5 x 100 after 4000 h of irradiatiofl in a Xenotest-150. A = amplitude of the signal.
and phosphorescence excitation spectra of the a, /3-unsaturated carbonyl impurity groups in the polymer (the fluorescence and phosphorescence emissions originate from enone and dienone impurities respectively).1"11 Prior to irradiation it is seen that the presence of the piperidine compounds has no significant effect on the intensity of the excitation spectra. Thus, protective mechanisms involving optical screening and/or excited state quenching cannot be operative with either Tinuvin 770 or the N-oxy radical. After about 150h of irradiation the unstabilised polymer film began to embrittle. During the irradiation period prior to embrittlement there is a gradual reduction in the intensity of the emissions from the a, O-unsaturated carbonyl groups. Only during the period of embrittlement is the conversion of unsaturated carbonyl to saturated carbonyl groups complete (Fig. 4). 11 In contrast, no significant change is observed in the intensity of the emissions from the a, 13-unsaturated carbonyl groups in the polymer films containing either the Tinuvin 770 or N-oxy radical compound, over the irradiation period studied (Fig. 3).
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYI.ENE
249
I
,j
f
~"
/
I
•
f...J"
V
3 6,
3J6,
3~9
GAUSS
Fig. 2. ESR spectrum of 4-hydroxy-2,2,6,6-tetra-methylpiperidine-N-oxy (0.25%) in polypropylene film at room temperature. ( ) A = 10 before and ( - - - ) A = 10, (. . . . ) A = 1000, after 4000 h of irradiation in a Xenotest-150. A = amplitude of signal.
0"2
O.I z
Z O m {Z U
I Iooo IRRADIATION
2E~:)o TIME
3~)oo
~)
(HRS)
Fig. 3. Rate of photo-oxidation of polypropylene film in a Xenotest-150 containing O- no additives, ~ - 0 . 2 5 % 4-hydroxy-2,2,6,6-teU'amethylpiperidine-N-oxy and 0 - 0.25% bis (2,2,6,6tetramethyl-4-piperidinyl) sebacate.
250
NORMAN S. ALI.RN
A
6
WAVELENGTH, nm Fig. 4. Typical fluorescence (A) and phosphorescence (B) excitation spect~ of the a, /3unsaturated carbonyl impurities in polypropylene film; containing ( ) no additives before irradiation and 0.25% of the hindered piperidine compounds (1) and (If) before and after 4000 h of irradiation and ( - - - ) no additives after 150 h of irradiation.
It is evident from the results that inhibition of the photolysis of the a, l~-unsaturated carbonyl impurities in the polymer is one important mechanism by which the hindered piper/dine stab/liser operates. Essentially, there are two mechanisms by which the hindered piper/dine stab/liser could function here, of which the second appears more likely. In the first mechanism, the hindered hv
+ H I
•
+
(5)
II
O
I
(6)
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYLENE
251
piperidine stabiliser could effectively inhibit the photolysis of the a, /3unsaturated carbonyl impurities by donating the labile hydrogen from the nitrogen to the V-carbon radical site, thus preventing the isomerisation from occurring, as shown in reaction scheme (5). In the second mechanism, the nitroxyl radical generated from the amine could react with the T-carbon radical intermediate, formed during the isomerisation process (see reaction (6)).
Effect of thermally generated hydroperoxides The hydroperoxide concentrations in the thermally oxidised polypropylene powders are plotted as a function of time in Fig. 5. It is seen that under the experimental conditions employed here the hydroperoxide concentration increases to a maximum at 30 min heating and then decreases to a minimum at 50 rain. This behaviour agrees with the earlier findings of Chakraborty and Scott. lz Under the heating conditions employed here no macrocarbonyl groups were formed as determined by infra-red spectroscopy. Also shown in Fig. 5 are the nitroxyl radical concentrations produced on incorporating 0-1% of the hindered piperidine stabiliser into the thermally oxidised polymer samples. It is seen that both the hydroperoxide and nitroxyl radical concentration-plots match exactly, confirming the stoiehiometric reaction scheme (2), proposed by Chakraborty and Scott. 7 Table 1 compares the light stabilities of the thermally oxidised polypropylene films, containing the hindered piperidine stabiliser, Tinuvin 770. Interestingly, the ultra-violet embrittlement time decreases towards maximum hydroperoxide 12
m
Z
d
Z 0 U
O'4
lZ
~ o., 0
HEATING T I M E AT 1 2 0 ° C , MIN
Fig. 5. Comparison of the hydroperoxide groups concentration (g/Hue) in thermally oxidised polypropylene powders (Q) against time of heating (rain) with the nitroxTl radical concentration (relative intensity) produced on adding Tinuvin 770 to the thermally oxidised polymers (0) and heat pressing into film at 200eC.
252
N O R M A N S. AI.IIRN
TABLE 1 ULTRA-VIOLET EMBRrI'IT.EME, NT TIMES FOR PRE-THERMALLY
OXlDISED (120"C) POLYPI~Ot'YI..EN~~ 0"1% TmUVIN770
STAB~IS~ WITH
Heating time (120°C) (rain)
Embrittlement times (h)
0 10 20 30 40 50 60
3,300 3,200 2,800 2,500 2,700 2,800 3,100
concentration from 3,300 to 2,500 h and then increases with decreasing hydroperoxide concentration from 2,500 to 3,100 h. This rather interesting and unusual behaviour in light stability appears to follow very closely the prehydroperoxide content of the polymer films. Also, despite the severe thermal oxidation that the polymer samples had undergone, high photostability was still achieved. These results clearly demonstrate the importance of reaction scheme (2) during processing and the efficiency of the generated nitroxyl radical to scavenge macroradical species in the polymer.
Effect of a quinone Anthraquinone is a very efficient photosensitiser for polyolefins 1't4 and this is demonstrated by the embrittlement data shown in Table 2 below. It is also seen from the Table that the addition of the hindered piperidine stabiliser, Tinuvin 770, markedly inhibits the photosensitising effect of the anthraquinone. In hydrocarbon polymers and s o l v e n t s 9"1°'a4 anthraquinone is known to undergo a primary photochemical process of hydrogen-atom abstraction to give the semi-quinone radical (AH'), e.g. A
hv A*
P--H
(7)
AH'+P"
where P--H is the polymer and A* is the photo-excited triplet state of anthraquinone after it has undergone intersystem crossing from the lowest TABLE 2 EFrV_CT OF TnmYV~ 770 ON arm PHOrOSm~SmSINO ACTION OF ANTHRAQUINOI~ IN POLYPROPYLENE
Additive None 0.05% anthraquinone 0-05% anthraquinone+0.1% Tinuvin 770 0-1% Tinuvin 770
Embrittlement times (h) 180 40 450 3,300
H I N D E R E D P I P E R I D I N E C O M P O U N D S IN POLYPROP'YI..,ENE
253
excited singlet state. The semi-quinone radical (AH') can abstract another hydrogen atom from the substrate to give the corresponding hydroquinone (AH2), e.g. AH" + P - - H
> AH2 + P"
(8)
Alternatively A H can undergo a disproportionation reaction with another semi-quinone radical to give one molecule of hydroquinone (AH2) and one molecule of the original quinone, e.g. 2AH"
> A + AH 2
(9)
The formation of the semi-quinone radical (AH') is demonstrated by the flash photolysis results shown in Fig. 6. Flash photolysis of anthraquinone(anaerobic n-hexane)(10-SM) results in strong transient absorption with a wavelength maximum at 370 nm, normally associated with the semi-quinone radical. 9"1° Further, following one flash the solution exhibited a strong, visible, blue fluorescence due to the formation of the hydroquinone (AH2). Addition of the hindered piperidine stabiliser (10-4M) had little effect on the intensity of the transient absorption and hydroquinone formation. This is not unexpected since the amine will readily give up its hydrogen atom on the nitrogen to the photo-excited triplet anthraquinone (reaction (10)). However, addition of the stable N-oxy radical compound (10-aM) markedly inhibits transient absorption (Fig. 6) and hydroquinone formation. The latter result was confirmed by the observation of no fluorescence emission after a single flash. In this case the
03
0"2
,~
0.1
o
0 0
_J
0 W A V E L E N G T H , nm
Fig. 6. Transient absorption spectra produced in the flash photolysis of nitrogen saturated 10-SM solutions of anthraquinone containing: G- no stabiliser; 0 - 10-4 Tinuvin 770; and ®- 1 0 - ~ 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy in n-hexane.
NORMAN
254
S. AI.I.I~M
i $+ ~ - - U
(lO)
' hi-I" + ~ N "
nitroxyl radical must react rapidly with the semi-quinone radical (AH') as soon as it is formed to give the corresponding hydroxylamine (see reaction (11)).
, A+~/~N---O---H
(11)
The nitroxyl radical had no effect on the intensity or lifetime of the phosphorescence emission from anthraquinone. Thus, quenching of the photoexcited triplet state of anthraquinone by the nitroxyl radical cannot be operative here. The results above clearly demonstrate that the stabilising action of the hindered pipefidine compound must be produced by the formation of the nitroxyl radical. The radical, once formed, is an extremely efficient radical scavenger, reacting preferentially with alkyl and macroalkyl radicals to give a substituted hydroxylamine6 as shown in reaction (12). A number of
N--O" + P"
(12)
workers have shown that the nitroxyl radical is continually regenerated in a stabilising cycle through hydroperoxy radical scavenging by a substituted hydroxylamine, as shown in reaction (13). In fact, during photo-oxidation of
+ PO~
/~OH
+ PO~
' ~---O"
+ POOP
, ~N--O" +PO2H
(13)
(14)
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYLENE
255
8
I--<[ ..J uJ n" ,,¢ O £3
6
4
2
n" I.-
,z,
2~ IRRADIATION
Fig. 7.
,6o TIME
, HRS
Build-up of nitroxyl radical concentration with increasing exposure time in a Xenotest150, for polypropylene film containing 0.05% anthraquinone and 0.1% Tinuvin 770.
the stabilised polymer, containing anthraquinone there is a very rapid linear growth in the concentration of nitroxyl radicals, even up to embritflement (Fig. 7). This could be explained on the basis of reaction (13) where the hydroxylamine produced in reaction (11) could react with any hydroperoxy radicals in the polymer to form the nitroxyl radical, as illustrated in reaction (14).
Elyect of antioxidants The light stabilising effectiveness of Tinuvin 770 alone, and in combination with various primary and secondary antioxidants at 0.1% is demonstrated by the embrittlement data in Table 3. It can be seen that all the antioxidants TABLE 3 EMBRITTLEMENT
TIMES FOR
POLYPROPYLENE
FILMS CONTAINING
770/A~nOXn)Abrr COMBINATIONS IRRADIATED IN A X~qO'mST- 150 Additives
(0.1% each) None Tinuvin Tinuvin Tinuvin Tinuvin Tinuvin Tinuvin Tinuvin
770 770+Irganox 1010 770+Topanol CA 770+Goodrite 3114 770+Weston 618 770 + DLTDP 770+Goodritc 3114 +Weston 618 Tinuvin 770 + Goodrite 3114 + DLTDP
Embritdement times
Or) 150 2,900 2,100 2,450 2,600 2,600 2,000 2,500 2,250
256
NOaMANS. ALImN
antagonise, to varying extents, the stabilising action of the hindered piperidine stabiliser. Of the three primary antioxidants studied here, Irganox 1010 gave the strongest antagonistic effect. In the case of the two secondary antioxidants DLTDP was the stronger. The effect of the hindered piperidine stabiliser/antioxidant combinations on the photolysis of the luminescent a, /3-unsaturated carbonyl impurities in the polymer was also examined. The results are similar to those shown in Fig. 4. Only at the onset of embrittlement is there a marked decrease in the intensity of the fluorescence and phosphorescence emissions from the a, /3-unsaturated carbonyl groups in the stabilised films, comparable with that of the embrittled unstabilised polymer film. Chakraborty and Scott 7 attributed the antagonistic effect of transition metal dialkyl dithio-carbamates on the photostabilising performance of hindered piperidines to their ability to destroy hydroperoxides; thus preventing the conversion of the amine to the nitroxyl radical, as illustrated in reaction scheme (2). Thus, by analogy, the antagonistic effects of the antioxidants observed here may also be due to their ability to destroy hydroperoxides. This is certainly true for the secondary antioxidants which will react directly with hydroperoxides initially present in the polymer) s On the other hand, primary antioxidants do not directly decompose hydroperoxides, suggesting that some other interaction could be responsible for the antagonism. That this would appear to be so, is shown by the results in the next section.
Interaction of N-oxy radical with phenolic anfioxidants Of the antioxidants examined in the previous section, the strongest antagonist, Irganox 1010, was selected for further study here. Figure 8 shows the ultraviolet absorption spectrum of polypropylene film containing a mixture of the N-oxy radical and Irganox 1010 (0-1% w/w of each). It is seen that the polymer possesses an intense absorption band, with a wavelength maximum at 315 nm, which is absent for the films containing the additives alone. For example, Irganox 1010 exhibits a weak absorption band with wavelength maximum at 270 and 280 nm, while the N-oxy radical exhibits virtually no absorption apart from a very weak, broad band centred at about 240 nm. Clearly, there must be a strong reaction between the N-oxy radical and the phenolic antioxidant during thermal treatment of the polymer. According to Ganem 16 N-oxy radicals are capable of oxidising aliphatic alcohols to ketones by the general reaction (15). Thus, by analogy it is possible
~
m
O
•
+ RR'CHOH
) ~N--O---H
+ RR'C-----O+H" (15)
257
H I N D E R E D P I P E R I D I N E COMPOUNDS IN P O L Y P R O P Y L E N E
I-S
1.0
\ \.
\
o..~t. e,
C ,q
~h~
2~o
3ool
3~o
WAVELENG TH,am
Fig. 8. Ultra-violet absorption spectra of polypropylenefilms containing: 0.1% Irganox 1010
( ); 0.1% 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy(---); 0-1% Irganox 1010+0.1% Tinuvin 770 (--.--); and 0-1% Irganox 1010+0.1% 4-hydroxy-2,2,6,6-tetramethylpiperidine-Noxy (--x x), after solventblendingand pressinginto filmcomparedwith that of Irganox 1010 in n-decane after oxidationto the quinoneby lead dioxide (.......... ) that the N-oxy radical is oxidising the phenolic antioxidant to the corresponding quinone as shown in reaction (16). This appears to be confirmed by the results illustrated in Fig. 8, where the absorption spectrum of the quinone form of Irganox 1010, made synthetically, matches that produced in the polymer. The resonance stabilised quinone radical produced in reaction (16) will react further with oxygen to form a hydroperoxide, is Interestingly, examination of
~N--O"
+H O ~
, ~--O---H
+O
~
(16)
the polymer film by ESR spectroscopy showed the concentration of the N-oxy radical to be little changed after the reaction. It would appear, therefore, that in this case the N-oxy radical is acting as a catalyst for the oxidation of the phenolic antioxidant to the quinone. The hydroxylamine must be an unstable intermediate and reverts back to the N-oxy radical. No such absorption, however, was observed on solvent-blending Ixganox 1010 with Tinuvin 770 and pressing into film (Fig. 8). Only on processing did such a reaction occur, with the hindered piperidine stabiliser as shown by the
258
NOR~ANS.
~J~N
I.C
0.5
a
C
WAVELENG T Hp n m
Fig. 9. Ultra-violet absorption spectra of polypropylene film containing: 0.1% Irganox 1010+
0-1% Tinuvin 770 after processing for 10rain ( ), 20min (------) and 30rain (. . . . --) at 200°C. results in Fig. 9. Here it is seen that a similar absorption at 315 run also appears, with increasing intensity, after processing a mixture of Tinuvin 770 and Irganox 1010, (0.1% w/w of each) for 10, 20 and 30 min at 200°C. In this case the nitroxyl radical generated during processing must catalytically oxidise the antioxidant to its quinone form. These quinone products are known to be photo-active 1'x5 and can sensitise photo-oxidation of the polymer by a number of different mechanisms, e.g. hydrogen-abstraction, and hydroperoxide and singlet oxygen formation. Clearly their formation, either during processing or, perhaps, as photochemical intermediates, could lead to the antagonistic effects described above.
CONCLUSIONS
These results clearly show that the nitroxyl radical is an extremely powerful radical scavenger. Further, the hydroxylamine is an important intermediate in the photostabilisation of polyolefms by hindered piperidine compounds. The results also demonstrate that no one mechanism can fully account for the high photo-protective efficiency of these hindered piperidine stabilisers in commercial polyolefins. Finally, it must be pointed out that despite the antagonistic effects of both primary and secondary thermal antioxidants on the photostabilisation of the
HINDERED PIPERlDINE COMPOUNDS IN POLYPROPYLENE
259
polymer by hindered piperidine compounds, their presence in the polymer is, nevertheless, essential in order to maintain process stabilisation.
REFERENCES 1. McK~I.I~ , J. F. and &tiFf, N. S., Photochemistry of man-made polymers, Applied Science Publishers Ltd, London, 1979, 308 pp. 2. I-I~1~R, H. J. and BLATlWI~, H. R., Pure and Appi. Chem., 36 (1973) 141. 3. USlLTON,J. J. and PATEL, A. R., in: AdtJances in chemistry series, No. 169 Chpt. 10, (1978) 116, AIa.ARA, D. L. and HAWKINS,W. L. (eds.), American Chemical Soc/ety, Washington DC. 4. CAmXSON,D. J. and WILES, D. M., J. Macromol. Sci., Rev. Makromoi Chem., C14 (1976) 155. 5. ROSANTSEV,E. G. and SCHOLLE,V. D., Synthesis, (1971) 190, idem-ibid (1971) 401. 6. SmLOV,Y. B. and DENISOV,E. T., Vysokomol Soedi~ M 6 (1974) 2313. 7. CHAKRABORTV,K. B. and ScoTr, G., Chem. & Ind., London, (1978) 237. 8. GRATr~, D. W., REDDOCK,A. H., CAm_SSON,D. J. and Wn~-% D. M., J. Polym. Sci., Polym. Letts., Ed., 16 (1978) 143. 9. BmlX~E,N. K. and PORTER, G., Proc. Roy. $oc., London, A224 (1958) 259. 10. MCI(~.H~ , J. F., Rad. Res. Revs., 3 (1971) 141. 11. AL~~.N, N. S., HOMER, J. and McKgH ~'~, J. F., J. Appl. Polym. Sci., 22 (1977) 2261. 12. CARLSSON,D. J. and WB.F.S, D. M., Macromol., 2 (1969) 583; idem-ibid, 597. 13. CMAKRABORTY,K. B. and Sctyrr, G., Polymer, l g (1977) 99. 14. A H ~ , N. S., McI(m.i ~d~, J. F. and PaOTOPAPPAS, S. A., J. Appl. Polym. Sci., 22 (1978) 1451. 15. SCOTT,G., Atmospheric oxidation and anti-oxidants, Elsevier, Amsterdam, 1965. 16. GANEM,B., J. Org. Chem., 40 (1975) 1998.
1 (1981) 243-259
PHOTOSTABILISING ACTION OF HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYLENEt
NORMANS. ALL~+~
Department of Chemistry, John Dalton Faculty of Technology, Manchester Polytechnic, Chester Street, ManchesterM1 5GD, Great Britain (Received: 3 June, 1980)
ABSTRACT Hindered amine compounds derived from piperidine are a relatively new addition to the range of commercial ultra-violet stabilisers for polymers (bulk and fibres). In comparison with other types of stabiliser, the hindered piperidines have proved to be by far the most effective systems. They do not appear to operate by mechanisms of optical screening or by excited state quenching. Essentially, their stabilising effectiveness depends upon their ability to form a stable nitroxyl radical, which then scavenges alkyl radicals produced during photo-oxidation. In polyole]ins they also inhibit the photo-reactions of carbonyl chromophores and react with hydroperoxides. This paper reports on the multifunctional behaviour of these commercially important stabiliser systems and their interactions with antioxidants. INTRODUCTION For outdoor applications many plastics materials have to be adequately protected against the harmful effects of the ultra-violet portion of sunlight that reaches the sudace of the earth, i.e. wavelengths from 290--400 nm. 1 This is particularly so for three commercially important polyolofins, polythenc (low and high density), polypropylene and poly(4-methylpent-l-ene). Of the many classes of compounds that have been developed as photostabilisers the relatively new hindered amines based on piperidine have proved to be the most effective,t-4 Of these, the hindered piperidine derivative t Presented at the 2nd InternationalConferenceon the 'Stabilisationand ControlledDegradation of Polymers',June 2--4th, Lucerne, 1980. 243 Polymer Photochemistry 0144-2880/81/0001-0243/$2-50~Applied Science Publishers Ltd, England, 1981 Printed in Northern Ireland
244
S. # I I ~ N
NORMAN
(structure (I)), known commercially as Tinuvin 770, has attracted widespread interest. The stabilising effect of these hindered piperidines is believed to depend upon their ability to form a stable nitroxyl radical, which then scavenges macroradicals (P') produced during photo-oxidation, l~s as in reaction scheme (1). Recently, Chakraborty and Scott, 7 attributed the photostabilising effectiveness of the hindered piperidine compound (I), in low density CH3
COC(CH2)~
~I--N
CHa CH3 (13
(1)
k~,,a
polyethylene, with its ability m react with hydroperoxides in the polymer to form a stable nitroxyl radical (reaction (2)).
~ ~
- - H
+ POOH
a,
N" + PO" + H 2 0
(2)
0" + "OH ,
N - - - O - - - ( O H + p.
Their evidence for this mechanism was based on the observation that the presence of a hydroperoxide decomposer, nickel diethyldithiocarbamate, antagonised the stabilising action of the hindered piperidine compound. In another mechanism, postulated by Grattcn et al.,s the nitroxyl radical is believed to form a hydrogen-bonded complex with hydroperoxides (for example, reaction (3)). To determine the validity of the mechanism illustratedin
HINDERED PIPERIDINE C O M P O U N D S
245
IN POLYPROPYLENE
reaction (1), the effect of photo-oxidation on the hindered piperidine stabiliser
N--O'---P--H + POOH
• ~N---O'--POOH
+ P--H
(3)
/ x
(structure (I)) is compared with that of a stable piperidine-N-oxy radical (structure (II)) in polypropylene using ESR spectroscopy. To demonstrate the C~H3CH3
7
\
/\
//~CH
2OH
CH3 CH3 (I13 multifunctional behaviour of these hindered piperidine compounds, the effect of thermally generated hydroperoxides and added anthraquinone have been examined on the photostabilising performance of compound (I) in polypropylene, using ESR spectroscopy. In this study, the effect of the hindered piperidine compound (I) and the model N-oxy radical (II), on the photochemical behaviour of anthraquinone in a model solvent, n-hexane, using flash photolysis and luminescence spectroscopy has also been examined. Anthraquinone was selected for study since its photochemical reactions in solution and for that matter, polymers, are well understood. 9"1° One of the major primary photochemical processes in the photo-oxidation of commercial polypropylene is isomerisation of a, unsatarated carbonyl impurities by the process shown in reaction (4), 1~ followed by the well-known I
I
h~
I
I
I/
Norrish Type I and II reactions. In this study the effect of both hindered piperidine compounds (I) and (II) have been examined on reaction (4). Finally, in view of the results of Chakraborty and Scott7 the effects of some typical primary and secondary thermal antioxidants have also been examined on the photostabilising effectiveness of the hindered pipeddine compound (I). To assist with the interpretation of these results a novel reaction between the N-oxy radical (II) and a typical phenolic antioxidant, (Irganox 1010), is described.
246
N O R M A N S. AI.I.I~.N
EXPERIMENTAL
Polypropylene powder containing no commercial additives was supplied by ICI (Plastics Division) Ltd. The antioxidants---Irganox 1010 (pentaerythritol-tetra/3-(4-hydroxy-3,5-di-tert-butylphenol) proportionate), Ciba-Geigy Ltd; Topanol CA (3-methyl-6-tert-butylphenol and crotonaldehyde condensate), ICI Ltd; Goodrite 3114 (tris(4-hydroxy-3,5-di-tert-butylbenzyl-isocyanurate), Goodrich Co Ltd; Weston 618 (distearyl pentaerythrityl diphosphite), Weston Chemicals; DLTDP (dilauryl thio-dipropionate), ICI Ltd; and light stabiliser Tinuvin 770 (bis[2,2,6,6-tetramethyl-4-piperidinyl]sebacate), Ciba-Geigy Corp---were supplied by their respective manufacturers. The compound 4hydroxyl-2,2,6,6-tetramethylpipeddine-N-oxy was purchased from the Eastman Kodak Co. Anthraquinone was purchased from BDH Chemicals and was recrystallised twice from ethanol. All solvents used were of standard laboratory grade or Analar quality. The compounds were solvent blended into the polypropylene powder using dichloromethane as a solvent. The solvent was allowed to evaporate overnight followed by pressing the powders at 200°(2, for I rain, into film 200-300/~m thick. A control sample was similarly prepared. Samples of polypropylene powder were heated at 120°C in an air oven for various periods of time up to I h. The stabiliser, Tinuvin 770, was then solvent blended into the thermally oxidised polypropylene powders at a 0.1% w/w concentration and treated as described above. Samples of Tinuvin 770 and Irganox 1010 (0.1% w/w of each) were also processed into polypropylene powder at 200°C for 10, 20 and 30 rain using a Brabender Plastic.order (Duesburg, W. Germany) followed by pressing into film, as described above. The antioxidant, Irganox 1010, was also oxidised to its quinone form by treatment with lead dioxide. Irganox 1010 (1 g) dissolved in toluene (50 nil) was shaken for 2-3 h with lead dioxide (5 g). After filtration the blue coloured filtrate (a stable phenoxy radical) was then bubbled vigorously with air for I h to oxidise the radical to the yellow quinone hydroperoxide. The toluene was then evaporated off and the product taken up in n-decane.
Hydroperoxide analysis The hydroperoxide concentrations in the thermally oxidised polymer powders were estimated using the standard iodometric method described earlier by Carlsson and Wiles. 12
Photo-oxidation The polymer films were irradiated in a Xenotest-150(GmbH) set-up for simulated sunlight exposure conditions out-of-doors ( k ' s > 3 0 0 n m ) (40°C, 50% relative humidity). The polymer films were flexed periodically to test for
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYI..ENE
247
embrittlement. The rates of photo-oxidation of the polymer films were also checked using the well established carbonyl index method (1710cm-~). A Perkin-Elmer Model 157G, infra-red absorption spectrometer was used for this purpose.
Spectroscopic measurement Fluorescence and phosphorescence spectra were obtained using a double grating (1200 lines mm -1) Hitachi Perkin-Elmer MPF-4 spectrofluorimeter equipped with two red-sensitive, R-446F, photomultiplier tubes. Ultra-violet absorption spectra were obtained using a Perkin-Elmer Model 554 spectrometer equipped with a microprocessor. Flash photolysis experiments were carried out in nitrogen (<5 ppm 02) saturated n-hexane using a micro-second apparatus with a photoflash of 300 J and a half-life of 10 ms. ESR spectra were obtained using a Jeol FE-3X spectrometer. RESULTS AND DIgCUgglON
Identification and behaviour of the N-oxy radical Figures 1 and 2 show the ESR spectra of the polymer films containing Tinuvin 770 and the N-oxy radical compound, before and after irradiation. From Fig. 1 it can be seen that Tinuvin exhibits a stable ESR spectrum before irradiation. During irradiation there is a marked increase in the intensity of the ESR spectrum, but after about 4000 h it attains an almost stationary steady-state intensity. The intensity of the ESR spectrum of the N-oxy radical compound markedly decreases during irradiation (Fig. 2) but, as found for the Tinuvin 770, the spectra attains a low steady-state intensity after about 4000 h. Also, at this low concentration, the spectrum of the N-oxy radical is resolvable into its three characteristic lines, similar to that for Tinuvin 770. At the relatively high concentration of N-oxy radical employed in this work the presence of the rigid polymer matrix will impair 'free-spin' of the radical 11 producing a distorted spectrum. These results clearly show that the radical generated during photostabilisation of the polymer by Tinuvin 770 is a nitroxyl radical. This is in agreement with the earlier postulate, 2-6 in which the nitroxyl radical is an essential intermediate formed during the reaction.
Effect on a, O-unsaturated carbonyl groups The light stabilising efliciencies of the Tinuvin 770 and model N-oxy radical at 0.25% w/w concentration in polypropylene are compared in Fig. 3. It is seen that both compounds inhibit carbonyl development in the polymer up to 4000 h of irradiation. Figure 4 shows the effect of irradiation on the intensity of the fluorescence
248
NORMAN S. ,~LL~N r
Ii /i
! i,f/
L/
3~vo
3 7o GAUSS
Fig. 1. E S R spectrum of his (2,2,6,6-tetramethyl-4-piperidinyl)sebacate (0.25%) in polypropylene film at r o o m temperature. ( ) A = 5 x 100; ( - - - ) A = 4 x 1000 before and (. . . . . ) A = 5 x 100 after 4000 h of irradiatiofl in a Xenotest-150. A = amplitude of the signal.
and phosphorescence excitation spectra of the a, /3-unsaturated carbonyl impurity groups in the polymer (the fluorescence and phosphorescence emissions originate from enone and dienone impurities respectively).1"11 Prior to irradiation it is seen that the presence of the piperidine compounds has no significant effect on the intensity of the excitation spectra. Thus, protective mechanisms involving optical screening and/or excited state quenching cannot be operative with either Tinuvin 770 or the N-oxy radical. After about 150h of irradiation the unstabilised polymer film began to embrittle. During the irradiation period prior to embrittlement there is a gradual reduction in the intensity of the emissions from the a, O-unsaturated carbonyl groups. Only during the period of embrittlement is the conversion of unsaturated carbonyl to saturated carbonyl groups complete (Fig. 4). 11 In contrast, no significant change is observed in the intensity of the emissions from the a, 13-unsaturated carbonyl groups in the polymer films containing either the Tinuvin 770 or N-oxy radical compound, over the irradiation period studied (Fig. 3).
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYI.ENE
249
I
,j
f
~"
/
I
•
f...J"
V
3 6,
3J6,
3~9
GAUSS
Fig. 2. ESR spectrum of 4-hydroxy-2,2,6,6-tetra-methylpiperidine-N-oxy (0.25%) in polypropylene film at room temperature. ( ) A = 10 before and ( - - - ) A = 10, (. . . . ) A = 1000, after 4000 h of irradiation in a Xenotest-150. A = amplitude of signal.
0"2
O.I z
Z O m {Z U
I Iooo IRRADIATION
2E~:)o TIME
3~)oo
~)
(HRS)
Fig. 3. Rate of photo-oxidation of polypropylene film in a Xenotest-150 containing O- no additives, ~ - 0 . 2 5 % 4-hydroxy-2,2,6,6-teU'amethylpiperidine-N-oxy and 0 - 0.25% bis (2,2,6,6tetramethyl-4-piperidinyl) sebacate.
250
NORMAN S. ALI.RN
A
6
WAVELENGTH, nm Fig. 4. Typical fluorescence (A) and phosphorescence (B) excitation spect~ of the a, /3unsaturated carbonyl impurities in polypropylene film; containing ( ) no additives before irradiation and 0.25% of the hindered piperidine compounds (1) and (If) before and after 4000 h of irradiation and ( - - - ) no additives after 150 h of irradiation.
It is evident from the results that inhibition of the photolysis of the a, l~-unsaturated carbonyl impurities in the polymer is one important mechanism by which the hindered piper/dine stab/liser operates. Essentially, there are two mechanisms by which the hindered piper/dine stab/liser could function here, of which the second appears more likely. In the first mechanism, the hindered hv
+ H I
•
+
(5)
II
O
I
(6)
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYLENE
251
piperidine stabiliser could effectively inhibit the photolysis of the a, /3unsaturated carbonyl impurities by donating the labile hydrogen from the nitrogen to the V-carbon radical site, thus preventing the isomerisation from occurring, as shown in reaction scheme (5). In the second mechanism, the nitroxyl radical generated from the amine could react with the T-carbon radical intermediate, formed during the isomerisation process (see reaction (6)).
Effect of thermally generated hydroperoxides The hydroperoxide concentrations in the thermally oxidised polypropylene powders are plotted as a function of time in Fig. 5. It is seen that under the experimental conditions employed here the hydroperoxide concentration increases to a maximum at 30 min heating and then decreases to a minimum at 50 rain. This behaviour agrees with the earlier findings of Chakraborty and Scott. lz Under the heating conditions employed here no macrocarbonyl groups were formed as determined by infra-red spectroscopy. Also shown in Fig. 5 are the nitroxyl radical concentrations produced on incorporating 0-1% of the hindered piperidine stabiliser into the thermally oxidised polymer samples. It is seen that both the hydroperoxide and nitroxyl radical concentration-plots match exactly, confirming the stoiehiometric reaction scheme (2), proposed by Chakraborty and Scott. 7 Table 1 compares the light stabilities of the thermally oxidised polypropylene films, containing the hindered piperidine stabiliser, Tinuvin 770. Interestingly, the ultra-violet embrittlement time decreases towards maximum hydroperoxide 12
m
Z
d
Z 0 U
O'4
lZ
~ o., 0
HEATING T I M E AT 1 2 0 ° C , MIN
Fig. 5. Comparison of the hydroperoxide groups concentration (g/Hue) in thermally oxidised polypropylene powders (Q) against time of heating (rain) with the nitroxTl radical concentration (relative intensity) produced on adding Tinuvin 770 to the thermally oxidised polymers (0) and heat pressing into film at 200eC.
252
N O R M A N S. AI.IIRN
TABLE 1 ULTRA-VIOLET EMBRrI'IT.EME, NT TIMES FOR PRE-THERMALLY
OXlDISED (120"C) POLYPI~Ot'YI..EN~~ 0"1% TmUVIN770
STAB~IS~ WITH
Heating time (120°C) (rain)
Embrittlement times (h)
0 10 20 30 40 50 60
3,300 3,200 2,800 2,500 2,700 2,800 3,100
concentration from 3,300 to 2,500 h and then increases with decreasing hydroperoxide concentration from 2,500 to 3,100 h. This rather interesting and unusual behaviour in light stability appears to follow very closely the prehydroperoxide content of the polymer films. Also, despite the severe thermal oxidation that the polymer samples had undergone, high photostability was still achieved. These results clearly demonstrate the importance of reaction scheme (2) during processing and the efficiency of the generated nitroxyl radical to scavenge macroradical species in the polymer.
Effect of a quinone Anthraquinone is a very efficient photosensitiser for polyolefins 1't4 and this is demonstrated by the embrittlement data shown in Table 2 below. It is also seen from the Table that the addition of the hindered piperidine stabiliser, Tinuvin 770, markedly inhibits the photosensitising effect of the anthraquinone. In hydrocarbon polymers and s o l v e n t s 9"1°'a4 anthraquinone is known to undergo a primary photochemical process of hydrogen-atom abstraction to give the semi-quinone radical (AH'), e.g. A
hv A*
P--H
(7)
AH'+P"
where P--H is the polymer and A* is the photo-excited triplet state of anthraquinone after it has undergone intersystem crossing from the lowest TABLE 2 EFrV_CT OF TnmYV~ 770 ON arm PHOrOSm~SmSINO ACTION OF ANTHRAQUINOI~ IN POLYPROPYLENE
Additive None 0.05% anthraquinone 0-05% anthraquinone+0.1% Tinuvin 770 0-1% Tinuvin 770
Embrittlement times (h) 180 40 450 3,300
H I N D E R E D P I P E R I D I N E C O M P O U N D S IN POLYPROP'YI..,ENE
253
excited singlet state. The semi-quinone radical (AH') can abstract another hydrogen atom from the substrate to give the corresponding hydroquinone (AH2), e.g. AH" + P - - H
> AH2 + P"
(8)
Alternatively A H can undergo a disproportionation reaction with another semi-quinone radical to give one molecule of hydroquinone (AH2) and one molecule of the original quinone, e.g. 2AH"
> A + AH 2
(9)
The formation of the semi-quinone radical (AH') is demonstrated by the flash photolysis results shown in Fig. 6. Flash photolysis of anthraquinone(anaerobic n-hexane)(10-SM) results in strong transient absorption with a wavelength maximum at 370 nm, normally associated with the semi-quinone radical. 9"1° Further, following one flash the solution exhibited a strong, visible, blue fluorescence due to the formation of the hydroquinone (AH2). Addition of the hindered piperidine stabiliser (10-4M) had little effect on the intensity of the transient absorption and hydroquinone formation. This is not unexpected since the amine will readily give up its hydrogen atom on the nitrogen to the photo-excited triplet anthraquinone (reaction (10)). However, addition of the stable N-oxy radical compound (10-aM) markedly inhibits transient absorption (Fig. 6) and hydroquinone formation. The latter result was confirmed by the observation of no fluorescence emission after a single flash. In this case the
03
0"2
,~
0.1
o
0 0
_J
0 W A V E L E N G T H , nm
Fig. 6. Transient absorption spectra produced in the flash photolysis of nitrogen saturated 10-SM solutions of anthraquinone containing: G- no stabiliser; 0 - 10-4 Tinuvin 770; and ®- 1 0 - ~ 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy in n-hexane.
NORMAN
254
S. AI.I.I~M
i $+ ~ - - U
(lO)
' hi-I" + ~ N "
nitroxyl radical must react rapidly with the semi-quinone radical (AH') as soon as it is formed to give the corresponding hydroxylamine (see reaction (11)).
, A+~/~N---O---H
(11)
The nitroxyl radical had no effect on the intensity or lifetime of the phosphorescence emission from anthraquinone. Thus, quenching of the photoexcited triplet state of anthraquinone by the nitroxyl radical cannot be operative here. The results above clearly demonstrate that the stabilising action of the hindered pipefidine compound must be produced by the formation of the nitroxyl radical. The radical, once formed, is an extremely efficient radical scavenger, reacting preferentially with alkyl and macroalkyl radicals to give a substituted hydroxylamine6 as shown in reaction (12). A number of
N--O" + P"
(12)
workers have shown that the nitroxyl radical is continually regenerated in a stabilising cycle through hydroperoxy radical scavenging by a substituted hydroxylamine, as shown in reaction (13). In fact, during photo-oxidation of
+ PO~
/~OH
+ PO~
' ~---O"
+ POOP
, ~N--O" +PO2H
(13)
(14)
HINDERED PIPERIDINE COMPOUNDS IN POLYPROPYLENE
255
8
I--<[ ..J uJ n" ,,¢ O £3
6
4
2
n" I.-
,z,
2~ IRRADIATION
Fig. 7.
,6o TIME
, HRS
Build-up of nitroxyl radical concentration with increasing exposure time in a Xenotest150, for polypropylene film containing 0.05% anthraquinone and 0.1% Tinuvin 770.
the stabilised polymer, containing anthraquinone there is a very rapid linear growth in the concentration of nitroxyl radicals, even up to embritflement (Fig. 7). This could be explained on the basis of reaction (13) where the hydroxylamine produced in reaction (11) could react with any hydroperoxy radicals in the polymer to form the nitroxyl radical, as illustrated in reaction (14).
Elyect of antioxidants The light stabilising effectiveness of Tinuvin 770 alone, and in combination with various primary and secondary antioxidants at 0.1% is demonstrated by the embrittlement data in Table 3. It can be seen that all the antioxidants TABLE 3 EMBRITTLEMENT
TIMES FOR
POLYPROPYLENE
FILMS CONTAINING
770/A~nOXn)Abrr COMBINATIONS IRRADIATED IN A X~qO'mST- 150 Additives
(0.1% each) None Tinuvin Tinuvin Tinuvin Tinuvin Tinuvin Tinuvin Tinuvin
770 770+Irganox 1010 770+Topanol CA 770+Goodrite 3114 770+Weston 618 770 + DLTDP 770+Goodritc 3114 +Weston 618 Tinuvin 770 + Goodrite 3114 + DLTDP
Embritdement times
Or) 150 2,900 2,100 2,450 2,600 2,600 2,000 2,500 2,250
256
NOaMANS. ALImN
antagonise, to varying extents, the stabilising action of the hindered piperidine stabiliser. Of the three primary antioxidants studied here, Irganox 1010 gave the strongest antagonistic effect. In the case of the two secondary antioxidants DLTDP was the stronger. The effect of the hindered piperidine stabiliser/antioxidant combinations on the photolysis of the luminescent a, /3-unsaturated carbonyl impurities in the polymer was also examined. The results are similar to those shown in Fig. 4. Only at the onset of embrittlement is there a marked decrease in the intensity of the fluorescence and phosphorescence emissions from the a, /3-unsaturated carbonyl groups in the stabilised films, comparable with that of the embrittled unstabilised polymer film. Chakraborty and Scott 7 attributed the antagonistic effect of transition metal dialkyl dithio-carbamates on the photostabilising performance of hindered piperidines to their ability to destroy hydroperoxides; thus preventing the conversion of the amine to the nitroxyl radical, as illustrated in reaction scheme (2). Thus, by analogy, the antagonistic effects of the antioxidants observed here may also be due to their ability to destroy hydroperoxides. This is certainly true for the secondary antioxidants which will react directly with hydroperoxides initially present in the polymer) s On the other hand, primary antioxidants do not directly decompose hydroperoxides, suggesting that some other interaction could be responsible for the antagonism. That this would appear to be so, is shown by the results in the next section.
Interaction of N-oxy radical with phenolic anfioxidants Of the antioxidants examined in the previous section, the strongest antagonist, Irganox 1010, was selected for further study here. Figure 8 shows the ultraviolet absorption spectrum of polypropylene film containing a mixture of the N-oxy radical and Irganox 1010 (0-1% w/w of each). It is seen that the polymer possesses an intense absorption band, with a wavelength maximum at 315 nm, which is absent for the films containing the additives alone. For example, Irganox 1010 exhibits a weak absorption band with wavelength maximum at 270 and 280 nm, while the N-oxy radical exhibits virtually no absorption apart from a very weak, broad band centred at about 240 nm. Clearly, there must be a strong reaction between the N-oxy radical and the phenolic antioxidant during thermal treatment of the polymer. According to Ganem 16 N-oxy radicals are capable of oxidising aliphatic alcohols to ketones by the general reaction (15). Thus, by analogy it is possible
~
m
O
•
+ RR'CHOH
) ~N--O---H
+ RR'C-----O+H" (15)
257
H I N D E R E D P I P E R I D I N E COMPOUNDS IN P O L Y P R O P Y L E N E
I-S
1.0
\ \.
\
o..~t. e,
C ,q
~h~
2~o
3ool
3~o
WAVELENG TH,am
Fig. 8. Ultra-violet absorption spectra of polypropylenefilms containing: 0.1% Irganox 1010
( ); 0.1% 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxy(---); 0-1% Irganox 1010+0.1% Tinuvin 770 (--.--); and 0-1% Irganox 1010+0.1% 4-hydroxy-2,2,6,6-tetramethylpiperidine-Noxy (--x x), after solventblendingand pressinginto filmcomparedwith that of Irganox 1010 in n-decane after oxidationto the quinoneby lead dioxide (.......... ) that the N-oxy radical is oxidising the phenolic antioxidant to the corresponding quinone as shown in reaction (16). This appears to be confirmed by the results illustrated in Fig. 8, where the absorption spectrum of the quinone form of Irganox 1010, made synthetically, matches that produced in the polymer. The resonance stabilised quinone radical produced in reaction (16) will react further with oxygen to form a hydroperoxide, is Interestingly, examination of
~N--O"
+H O ~
, ~--O---H
+O
~
(16)
the polymer film by ESR spectroscopy showed the concentration of the N-oxy radical to be little changed after the reaction. It would appear, therefore, that in this case the N-oxy radical is acting as a catalyst for the oxidation of the phenolic antioxidant to the quinone. The hydroxylamine must be an unstable intermediate and reverts back to the N-oxy radical. No such absorption, however, was observed on solvent-blending Ixganox 1010 with Tinuvin 770 and pressing into film (Fig. 8). Only on processing did such a reaction occur, with the hindered piperidine stabiliser as shown by the
258
NOR~ANS.
~J~N
I.C
0.5
a
C
WAVELENG T Hp n m
Fig. 9. Ultra-violet absorption spectra of polypropylene film containing: 0.1% Irganox 1010+
0-1% Tinuvin 770 after processing for 10rain ( ), 20min (------) and 30rain (. . . . --) at 200°C. results in Fig. 9. Here it is seen that a similar absorption at 315 run also appears, with increasing intensity, after processing a mixture of Tinuvin 770 and Irganox 1010, (0.1% w/w of each) for 10, 20 and 30 min at 200°C. In this case the nitroxyl radical generated during processing must catalytically oxidise the antioxidant to its quinone form. These quinone products are known to be photo-active 1'x5 and can sensitise photo-oxidation of the polymer by a number of different mechanisms, e.g. hydrogen-abstraction, and hydroperoxide and singlet oxygen formation. Clearly their formation, either during processing or, perhaps, as photochemical intermediates, could lead to the antagonistic effects described above.
CONCLUSIONS
These results clearly show that the nitroxyl radical is an extremely powerful radical scavenger. Further, the hydroxylamine is an important intermediate in the photostabilisation of polyolefms by hindered piperidine compounds. The results also demonstrate that no one mechanism can fully account for the high photo-protective efficiency of these hindered piperidine stabilisers in commercial polyolefins. Finally, it must be pointed out that despite the antagonistic effects of both primary and secondary thermal antioxidants on the photostabilisation of the
HINDERED PIPERlDINE COMPOUNDS IN POLYPROPYLENE
259
polymer by hindered piperidine compounds, their presence in the polymer is, nevertheless, essential in order to maintain process stabilisation.
REFERENCES 1. McK~I.I~ , J. F. and &tiFf, N. S., Photochemistry of man-made polymers, Applied Science Publishers Ltd, London, 1979, 308 pp. 2. I-I~1~R, H. J. and BLATlWI~, H. R., Pure and Appi. Chem., 36 (1973) 141. 3. USlLTON,J. J. and PATEL, A. R., in: AdtJances in chemistry series, No. 169 Chpt. 10, (1978) 116, AIa.ARA, D. L. and HAWKINS,W. L. (eds.), American Chemical Soc/ety, Washington DC. 4. CAmXSON,D. J. and WILES, D. M., J. Macromol. Sci., Rev. Makromoi Chem., C14 (1976) 155. 5. ROSANTSEV,E. G. and SCHOLLE,V. D., Synthesis, (1971) 190, idem-ibid (1971) 401. 6. SmLOV,Y. B. and DENISOV,E. T., Vysokomol Soedi~ M 6 (1974) 2313. 7. CHAKRABORTV,K. B. and ScoTr, G., Chem. & Ind., London, (1978) 237. 8. GRATr~, D. W., REDDOCK,A. H., CAm_SSON,D. J. and Wn~-% D. M., J. Polym. Sci., Polym. Letts., Ed., 16 (1978) 143. 9. BmlX~E,N. K. and PORTER, G., Proc. Roy. $oc., London, A224 (1958) 259. 10. MCI(~.H~ , J. F., Rad. Res. Revs., 3 (1971) 141. 11. AL~~.N, N. S., HOMER, J. and McKgH ~'~, J. F., J. Appl. Polym. Sci., 22 (1977) 2261. 12. CARLSSON,D. J. and WB.F.S, D. M., Macromol., 2 (1969) 583; idem-ibid, 597. 13. CMAKRABORTY,K. B. and Sctyrr, G., Polymer, l g (1977) 99. 14. A H ~ , N. S., McI(m.i ~d~, J. F. and PaOTOPAPPAS, S. A., J. Appl. Polym. Sci., 22 (1978) 1451. 15. SCOTT,G., Atmospheric oxidation and anti-oxidants, Elsevier, Amsterdam, 1965. 16. GANEM,B., J. Org. Chem., 40 (1975) 1998.