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Influence of thiobisphenols on the photo-oxidation of low density polyethylene
Polymer Degradation and Stability 6 (1984) 17-29
Influence of Thiobisphenols on the Photo-Oxidation of Low Density Polyethylene
L. Jirfi6kovd-Audouin, J. F. Bory, J. F. Farrenq & J. Verdu D6partement Mat6riaux, ENSAM, 15 i Bd. de rH6pital, 75640 Paris CEDEX 13, France
& J. Pospi~il Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague 6, Czechoslovakia (Received: 21 December, 1982)
ABSTRACT The influence oJ thiobisphenols, some products o/their thermo-oxidative transjormation and model compounds on the photo-oxidation oJ low density polyethylene (Ld PE) has been studied. Thiobisphenols arejound to have a pro-oxidant effect in the early period oj exposure, which increases with their concentration. On the other hand, the carbonyl build up was retarded in a later irradiation period. Similar effects have been observed with some thiobisphenol transjormation products, bis( 3,5di-tert, butyl-4-hydroxybenzyl) sulphide and its oxidation products, sulphoxide and sulphone, respectively, and model 4,4'-methylenebis(2methyl-6-tert, butylphenol). Dodecyl 3,3'-thiodipropionate retarded the earbonyl build up during the whole period oJ ultraviolet irradiation. Kinetic data show that a concentration oj 0.1% by weight oJ thiobisphenols is the most effective in retarding the carbonyl build up in Ld PE 17 Polymer Degradation and Stability 0141-3910/84/$03"00 .~ Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain
18
L. Jir&dkov&-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil films. TheJormation of non-alcoholic oxygenated structures seems to be a
specific effect of the sulphur containing compounds under study. INTRODUCTION Antioxidants of the thiobisphenol or bis(4-hydroxybenzyl) sulphide type efficiently protect polyolefins against thermooxidative degradation 1,z Their high efficiency is due to the intramolecular synergism of the phenol and sulphide moieties which display antioxidant actions via different mechanisms. Experimental studies in conditions modelling the thermal oxidation, with isolation of reaction products, 3 or studies of the antioxidant activity in the catalytic decomposition of hydroperoxides, 4'6 clearly established the two main routes of antioxidant action: reaction of the phenolic part of the molecule with peroxy radicals interrupts the propagation of the chain autoxidation, whereas the sulphur of the bisphenol bridge decomposes hydroperoxides, which originated in the thermo-oxidation, to nonradical products. Phenols have only weak or no activity at all in photo-stabilization, 7,8 probably for two reasons, namely rapid consumption due to the high rate of RO" and RO~ formation in short kinetic chains and their natural photo-sensitivity. 9 On the other hand, hydroperoxide decomposers having sufficient mobility in the polymer matrix, and good photostability, can be efficient stabilizers against photo-chemical oxidation in which hydroperoxides play a key r61e. 9 The thiobisphenols, which are both hydroperoxide decomposers and radical traps, should be able to protect the polyolefines during their whole life from processing to exposure to atmospheric ageing, but it remains to be established that no detrimental side reactions occur. Published data on the influence of thiobisphenols on polyolefin photo-oxidation are somewhat contradictory, both a photo-sensitizing effect 1° and an inhibiting effect (higher than for phenols) 11 having been reported. This paper reports a study of the effect of various thiobisphenols, some of their transformation products (i.e. sulphoxides and sulphones), and model compounds containing only one of the antioxidant active functions (phenolic or sulphidic), on the photo-oxidation of Ld PE films in conditions similar to those used in practice, namely at low concentration (0.1 to 0-5 ~ by weight) under irradiation in the solar ultraviolet range (2 > 300 nm).
Influence o f thiobisphenols on the photo-oxidation o f low density polyethylene
19
EXPERIMENTAL Substrates
Two types o f non-stabilized Ld PE f r o m C d F Chimie were used: FB 3003 with a melt index o f 0.3 (type A) and FB 5005 with a melt index of 0.5 (type B). Antioxidants
C o m p o u n d s I to XIII (Scheme 1) were c h r o m a t o g r a p h i c a l l y pure. Blends o f Ld PE and a p p r o x i m a t e l y 0-1, 0.25 and 0 . 5 ~ by weight o f the antioxidants were pr epar e d by mixing at 165-170°C between hot cylinders for 8 min. Films were obtained by compression m oul di ng between glass plates at 170 °C unde r the following conditions: preheating at 0-1 M P a for 2.5rain; compression at 0 . 4 M P a for 4min. The film thicknesses ( about 0.015cm) were determined by means o f the BeerL a m b e r t law using the infra-red band absorbance at 1895cm -1 (absorbance per unit thickness = 2 c m - 1 ) . This band was used as an internal standard for quantitative s p e c t r o p h o t o m e t r i c determinations. The an tio x id an t concent r a t i ons (expressed in Tables 1 and 2 in moles per TABLE 1 Concentrations (c) of Antioxidants I-IV and the Percentage Loss During Processing in Ld PE Type A Films; Initial (rico) and Stationary (r~co) Rates of Carbonyl Formation and Rates of Formation of OH Groups (ron) During the UV Irradiation of Ld PE Type A Films Additire
I
II III IV
Ld PE" Ld PEb a Non-processed. b Processed.
C × 104 M litre i
Per cent loss
rico x 103 h-1
r~co × 103 h-1
ro, x 103 h 1
8.3 30.0 60"0 1.7 2.0 18.0 43.5 85.5
690 67.8 53-5 93'0 91"6 30.0 32.6 33.7
7-6 12.9 17.4 6"7 6-7 13.7 18.2 21-7 5.3 4.0
13.0 8.3 11.3 7.2 21 '0 8.0 10.3 17-0 22-l 18.7
--------1"3 0-7
20
L. Jir&6kovh-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. PospEil
,o s4o, I
HO
SO2
II
OH
III
IV
HO-S-C-OH V
Vl
HO'~CH2"~OH X
Xl
Xll
S(CHzCHzCOOClzH25)z XlII Scheme 1.
Influence of thiobisphenols on the photo-oxidation of low density polyethylene
21
TABLE 2 As for Table 1 for Antioxidants V-XIII and Ld PE Type B Films Additive
V VI VII
VIII IX X
XI XII
XIII Ld PE a Ld PE b
C x 104
Per cent
G c o x 103
r o n x 103
M litre- 1
loss
h- 1
h- 1
h- 1
8-8 21-0 9.8 22.2 43.3
67.3 19.0 50.3 53.7 55.4 -69.0 39.7 27.0 22.3 44.0 77.6 83.7 80.2 15.0
6.1 6.8 9.8 10.6 14.4 9.1 3.6 3.3 8.3 10-6 3.8 3.8 3.0 6.8 3.0 0.0 3.0
7.7 6.7 12.7 7-7 7-3 9.7 3.7 11.0 13-3 16.0 13.0 15.7 15.3 17.3 5.0 10.7 17.0
0.1 0.2 0.6 0.3 0-2 0.1 0.4 0.4 0.7 0.8 0.3 1.0 0.5 0.6 -0-7 0.9
5.7 16.3 49-7 105.7 15.9 11.1 20-2 49.0 15.3
ric o x 103
a Non-processed. b Processed.
litre (M litre-1)) in films were measured by infra-red and ultraviolet spectrophotometry using previously determined molar absorptivities.12 Variations of + 1 0 ~ were observed in concentration values for films molded from the same blend. Their averages and percentages of consumption during processing are reported in Tables 1 and 2. Photo-oxidation The photochemical reactor used has been described previously. 13 It is equipped with a fluorescent lamp emitting between 300 and 450 nm with an intensity maximum at 365nm. The light intensity was 3-5 x 1015 photons per square centimetre per second and the temperature was 40 _ 2 °C. The infra-red bands at 1720cm -1 (carbonyls) and 3430cm -1 (intermolecularly bonded hydroxyls) were monitored during exposure using a Perkin-Elmer 580 spectrophotometer.
22
L. Jir6dkov6-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
For a given absorption band i, at time t, we determined the normalized absorbance (Ai) r A i is expressed in c m - 1. (Ai) t
(OOi)t- 2 x (OOi)t l DO1895cm-i
(DO~)t is the optical density of the band i, measured by the baseline method, and l the film thickness. The reaction rates AAi/At are expressed in h - 1 c m - 1. The initial reaction rates, rico, were determined for the interval 0-66 h, the stationary reaction rates, rsco, for 150-300h, and the average oxidation rates, roll for hydroxyl groups for the whole exposure time.
RESULTS A N D DISCUSSION The influence of processing on the photo-oxidation of Ld PE is shown in Fig. 1. For type A, which has an initially higher carbonyl content, processing results in a decrease in the photo-oxidation rate, whereas the type B Ld PE behaves in the opposite manner. It is interesting to note that no change in the initial part of the carbonyl curve (within experimental error) is observed after processing. The type A
A I,
i
3
2
1
l
0
lO0
I
200
I
h
300
Fig. 1. Change in absorbance, A~ of carbonyl groups at 1720cm-1 with time (h) of ultraviolet irradiation: (a) Ld PE type A films, • non-processed; • processed; (b) Ld PE type B films, O processed, A non-processed.
Influence of thiobisphenols on the photo-oxidation ojlow density polyethylene
23
Ld PE was used in the study ofantioxidants I-IV (Table 1), and type B for compounds V-XIII (Table 2). The influence of 4,4'-thiobis(2-methyl-6-tert. butylphenol) (I), at three concentrations, on the photo-oxidation of Ld PE films is shown in Fig. 2. A clear pro-oxidative effect can be observed early in the exposure (0-66 h). This effect increases with the thiobisphenol concentration. On the other hand, the photo-oxidation rate decreases continuously with exposure time and becomes slower than the control sample after less than 100 h. Both features, namely the initial pro-oxidant effect increasing with concentration and self-retarded kinetics, are also displayed by 2,2'thiobis(4-methyl-6-tert.butylphenol) (IV) and bis(3,5-di-tert.butyl-4hydroxybenzyl) sulphide (VII) for three concentrations, c l - c 3, and by the thiobisphenol, VI (the isomer of I and IV), at concentration c 1. The thermo-oxidative transformation products of thiobisphenol, I, sulphoxide, II, and sulphone, III, have been found to have a very different influence on Ld PE photo-oxidation. Carbonyl development was not influenced by the presence of the sulphone, III, whereas significant lowering of the oxidation rate was observed with the sulphoxide, II (Fig. 3) at approximately the same concentration (Table 1). Dithiobisphenol, V, another product of the thermo-oxidation of I, had an
A
3
H Y
2
0
a
I
//
I
I
I00
200
I
300 h Fig. 2. Changes of absorbance, A, of carbonyl groups at 17200m- x of Ld PE films stabilized with 4,4'-thiobis(2-methyl-6-tert.butylphenol) (I) with time of ultraviolet irradiation: • c = 8 . 3 x l 0 - 4 M l i t r e - I , • c = 3 0 . 0 x 10-~Mlitre-1; Z~ c = 6 0 . 0 x 10 -4 M litre- 1; _ _ _ non-stabilized Ld PE type A.
24
L. Jir~6kov&-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
/
A
/
/o I I I
i//,I~
0
100
200
h
300
Fig. 3. Change of absorbance, A, of carbonyl groups at 1720cm -~ of Ld PE films stabilized with A , 4,4'-sulphinylbis(2-methyi-6-tert.butylphenol)(II) c = 1.7 x 10-4r~ litre -1 and O , 4,4'-sulphonylbis(2-methyl-6-tert.butylphenol) (III) c = 2 . 0 x 10-4M litre-1 with time of ultraviolet irradiation; . . . . non-stabilized Ld PE type A.
analogous effect on the Ld PE photo-oxidation to the thiobisphenols previously discussed. The non-sulphidic thermo-oxidation product of I, i.e. 3,3'-di-tert. butyl-5,5'-dimethyl-4,4'-biphenyldiol (XI) had a relatively weak influence in comparison with the thiobisphenols (Table 2). The sulphoxide, VIII, and sulphone, IX, are the model oxidation products of bis-(3,5-di-tert.butyl-4-hydroxybenzyl) sulphide (VII). The sulphoxide, VIII, acts like the thiobisphenol, displaying pro-oxidation and retardation effects, whereas no sensitization was observed with sulphone, IX, whose retardation efficiency is remarkable (Table 2). The large difference in behaviour between sulphones derived from sulphides I and VII therefore reveals the important r61e of the methylene bridges between the sulphur atom and aromatic nuclei. The model compounds were studied in order to separate the effects of the phenolic part and the sulphidic bridge of the antioxidants on the polymer photo-sensitization. 4,4'-Methylenebis(2-methyl-6-tert.butylphenol) (X) exhibits similar behaviour to thiobisphenol, I, despite the presence of the methylene group instead of sulphur. A comparison of Figs 2 and 4 illustrates the similarity of the photo-oxidation kinetics. However, both characteristics,
Influence of thiobisphenols on the photo-oxidation of low density polyethylene
0
1()0
2 0I 0
h
25
t 300
Fig. 4. Change of absorbance, A, of carbonyl groups at 1720cm- 1 of Ld PE films stabilized with 4,4'-methylenebis(2-methyl-6-tert.butylphenol)(X) with time of ultraviolet irradiation: • c = 16-3 x 10-4Mlitre -1, • c = 4 9 ' 7 x 10-4Mlitre -1, © c = 105"7 x 10-4Mlitre-~; . . . . non-stabilized Ld PE type B,
namely the initial pro-oxidant effect and the autoretardation, are significantly different. The aromatic non-phenolic model, XII, i.e. diphenyl sulphide, does not significantly influence the carbonyl build up (Table 2). However, we observed a loss of 80 % of XII during sample processing, probably due to its high volatility and perhaps to its poor solubility in the PE matrix. Thus, the rapid migration, rather than its chemical consumption, can explain the complete disappearance of the diphenyl sulphide (XII) after 4 hours' exposure. The effect of sulphides was checked with a non-aromatic but compatible compound, the didodecyl 3,3'-thiodipropionate (XIII) (DLTP), which was used in concentration c 1. The lowest concentration was chosen to avoid interference from the ester group of the additive (1735 cm - 1) in the polyethylene carbonyl measurements (1720 cm - 1). Under these conditions, both peaks can be clearly distinguished. D L T P retards the carbonyl development from the beginning of irradiation and does not display any pro-oxidative effect. Comparison of D L T P effects in concentrations c 2 and c a leads to the same conclusions (Fig. 5). The values of initial (rico) and stationary (rsco) photo-oxidation rates
L. Jirddkovd-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
26
4~
-
I
,
I
l
4~
l
l
¢
I
I
310 h
Fig. 5. Comparison of the development of absorption bands of carbonyl groups in the region 1650--1800cm -1 of Ld PE type B films stabilized with the didodecyl 3,3'thiodipropinate (XIII) (c 1 = 15.3 x 10- 4 u litre- 1, c2 = 46.5 x 10- 4 M litre- 1, c3 = 88'7 x 10- 4 M litre- 1) after (a) 0 and (b) 310 h of ultraviolet irradiation; (c) non-stabilized Ld PE at the same conditions.
are summarized in Tables 1 and 2. The periods for calculation of these rates (0-66 h and 150-300 h, respectively) were chosen arbitrarily to demonstrate the complex kinetic behaviour of these systems. It is, however, interesting to note that the initial period closely corresponds to the total disappearance of the antioxidant, as reported elsewhere. 14 It seems, therefore, that, despite a lack of fundamental data on mechanisms and 'true' kinetic parameters, a discussion on relationships between thiobisphenol structure and photo-chemical behaviour can be initiated on the basis of a comparison of the ric o and r,c o values. The dependence of ric o on antioxidant concentration illustrates the similarity of effects of the three sulphur-containing phenols, I, IV and VII (Figs 6 and 7). 2,2'-Thiobis(4-methyl-6-tert. butylphenol) (IV) was the most active as pro-oxidant. The effect of methylenebisphenol, X, is smaller: ric o increases linearly with concentration. It can be deduced from measurements on samples containing the additive concentration, cl, that the non-sulphidic bisphenols X and XI are weaker photo-sensitizers than corresponding thio compounds I, IV, V or VI (Tables 1 and 2).
Influence of thiobisphenols on the photo-oxidation of low density polyethylene
/
20
27
~
/
jf
/
10
5
I
0
I
20
i
I
i
40
60
80 CxlO4
Fig. 6. Change of rates of the formation of carbonyl groups at 1720cm -1 with concentration of antioxidants during ultraviolet irradiation of Ld PE type A films: • rico, 0 r,co; -- 4,4'-thiobis(2-methyl-6-tert.butylphenol) (I), - - - - 2,2'-thiobis(4-methyl-6tert.butylphenol) (IV).
r~103 L
I/
0
20
40
60 80 4 C~10
Fig. 7. Change of rates of formation of carbonyl groups at 1720 c m - 1 with antioxidant concentrations during ultraviolet irradiation of Ld PE films: • rico, 0 rsco; bis(3,5di-tert.-butyl-4-hydroxybenzyl) sulphide (VII), - - 4,4'-methylenebis (2-methyl-6tert.butylphenol) (X).
28
L. Jirddkov~-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
Other evidence of the similarity of the effects of the thiobisphenols being studied is given by the dependence of rsco on antioxidant concentration (Figs 6 and 7). All the curves display a minimum in the range of low concentrations (cl for IV and c2 for the less soluble I and VII). Taking this minimum as a stability criterion, it may be concluded that these phenolic sulphides are the most efficient as photo-stabilizers, the methylene-bisphenol, X, being less active. The position of this minimum, depends, of course, on the choice of periods for rsco calculation, so that such a criterion cannot be used quantitatively. However, it may be supposed by analogy that the low values of rsco for the sulphur-containing compounds II, V, VI, VIII and IX studied only in c a concentrations are consequences of the fact that their concentrations in the films (5- 20 x 10-4 i litre- 1) are probably near to the optimal values indicated by Figs 6 and 7. Thus, even ifcarbonyl formation rates provide interesting information, they are not necessarily absolute criteria of photo-stability. For this reason, the mean rates of hydroxyl formation have also been determined (roz measured at 3430cm-1). Despite a wide scattering of data, some clear tendencies are shown in Tables 1 and 2. The thiobisphenols I, IV and VI and the sulphur-containing oxidation products of I, i.e. II, III and V, as well as DLTP, are efficient suppressors of hydroxyl formation. The influence of VII, VIII and IX, in which a methylene group separates the sulphur atom and aromatic nuclei, is less. However, for VII, rondecreases with the antioxidant concentration. No effect was observed in the case of methylenebisphenol, X. It may therefore be supposed that the orientation of the oxidation reactions towards the formation of non-alcoholic oxygenated structures is a specific effect of the sulphur-containing compounds being studied. No interpretation can be made in the case of diphenyl sulphide, XII, because of its rapid migration, as previously shown. Biphenyldiol, XI, seems to have an inhibition effect but the lack of complementary data does not allow any interpretation. The mechanisms of antioxidant sensitisation will be examined in a later paper. REFERENCES 1. L. Jir~i~kov~iand J. Pospigil, Eur. Polym. J., 8, 75 (1972). 2. L. Jirfi6kovfi and J. Pospi~il, Eur. Polym. J., 9, 71 (1973).
Influence of thiobisphenols on the photo-oxidation of low density polyethylene 29 3. L. Jir/t~kovh and J. Pospi~il, Angew. Makromol. Chem., 66, 95 (1978). 4. L. Jir~6kov~i, Jelinkov~t, J. Rotschov~i and J. Pospi~il, Chem. Ind. (London), 384 (1979). 5. J. G. Scott and M. F. Yusoff, Eur. Polym. J., 16, 497 (1980). 6. A. J. Bridgewater and M. D. Sexton, J. S. C., Perkin II, 530 (1978). 7. W. Matrekey and F. H. Winslow, Am. Chem. Soc., Polym. Prep., 16, 606 (1975). 8. D. J. Kieysworth and D. G. M. Wood, Int. Conf. on Weathering and Degradation of Polymers, Plastics and Rubber Institute, London 1977, Preprint E2. 9. D. J. Carlsson and D. M. Wiles, J. Macromol. Sci-Rev., Maerom. Chem. C14, 155 (1975). 10. F. H. Winslow, W. Matreyek and A. M. Trozzolo, SPE Journal, 28, 19 (1972). 11. Yu. I. Temchin, E. E. Burmistrov, L. A. Skripko, R. S. Burmistrova, Yu. V. Kokhanov, M. A. Gushchina and E. G. Rozantsev, Vysokomol. Soyedin., AIS, 1038 (1973). 12. L. Jir~t~kova, J. Pospi~il and J. Verdu, Analysis 10, 1966 (1982). 13. A. Huvet, J. Philippe and J. Verdu, Eur. Polym. J., 14, 709 (1978). 14. L. Jir~i~kov~-Audouin, J. Pospi~il and J. Verdu, Polym. Deg. Stab., 6 (1984). (In press.)
Influence of Thiobisphenols on the Photo-Oxidation of Low Density Polyethylene
L. Jirfi6kovd-Audouin, J. F. Bory, J. F. Farrenq & J. Verdu D6partement Mat6riaux, ENSAM, 15 i Bd. de rH6pital, 75640 Paris CEDEX 13, France
& J. Pospi~il Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 16206 Prague 6, Czechoslovakia (Received: 21 December, 1982)
ABSTRACT The influence oJ thiobisphenols, some products o/their thermo-oxidative transjormation and model compounds on the photo-oxidation oJ low density polyethylene (Ld PE) has been studied. Thiobisphenols arejound to have a pro-oxidant effect in the early period oj exposure, which increases with their concentration. On the other hand, the carbonyl build up was retarded in a later irradiation period. Similar effects have been observed with some thiobisphenol transjormation products, bis( 3,5di-tert, butyl-4-hydroxybenzyl) sulphide and its oxidation products, sulphoxide and sulphone, respectively, and model 4,4'-methylenebis(2methyl-6-tert, butylphenol). Dodecyl 3,3'-thiodipropionate retarded the earbonyl build up during the whole period oJ ultraviolet irradiation. Kinetic data show that a concentration oj 0.1% by weight oJ thiobisphenols is the most effective in retarding the carbonyl build up in Ld PE 17 Polymer Degradation and Stability 0141-3910/84/$03"00 .~ Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain
18
L. Jir&dkov&-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil films. TheJormation of non-alcoholic oxygenated structures seems to be a
specific effect of the sulphur containing compounds under study. INTRODUCTION Antioxidants of the thiobisphenol or bis(4-hydroxybenzyl) sulphide type efficiently protect polyolefins against thermooxidative degradation 1,z Their high efficiency is due to the intramolecular synergism of the phenol and sulphide moieties which display antioxidant actions via different mechanisms. Experimental studies in conditions modelling the thermal oxidation, with isolation of reaction products, 3 or studies of the antioxidant activity in the catalytic decomposition of hydroperoxides, 4'6 clearly established the two main routes of antioxidant action: reaction of the phenolic part of the molecule with peroxy radicals interrupts the propagation of the chain autoxidation, whereas the sulphur of the bisphenol bridge decomposes hydroperoxides, which originated in the thermo-oxidation, to nonradical products. Phenols have only weak or no activity at all in photo-stabilization, 7,8 probably for two reasons, namely rapid consumption due to the high rate of RO" and RO~ formation in short kinetic chains and their natural photo-sensitivity. 9 On the other hand, hydroperoxide decomposers having sufficient mobility in the polymer matrix, and good photostability, can be efficient stabilizers against photo-chemical oxidation in which hydroperoxides play a key r61e. 9 The thiobisphenols, which are both hydroperoxide decomposers and radical traps, should be able to protect the polyolefines during their whole life from processing to exposure to atmospheric ageing, but it remains to be established that no detrimental side reactions occur. Published data on the influence of thiobisphenols on polyolefin photo-oxidation are somewhat contradictory, both a photo-sensitizing effect 1° and an inhibiting effect (higher than for phenols) 11 having been reported. This paper reports a study of the effect of various thiobisphenols, some of their transformation products (i.e. sulphoxides and sulphones), and model compounds containing only one of the antioxidant active functions (phenolic or sulphidic), on the photo-oxidation of Ld PE films in conditions similar to those used in practice, namely at low concentration (0.1 to 0-5 ~ by weight) under irradiation in the solar ultraviolet range (2 > 300 nm).
Influence o f thiobisphenols on the photo-oxidation o f low density polyethylene
19
EXPERIMENTAL Substrates
Two types o f non-stabilized Ld PE f r o m C d F Chimie were used: FB 3003 with a melt index o f 0.3 (type A) and FB 5005 with a melt index of 0.5 (type B). Antioxidants
C o m p o u n d s I to XIII (Scheme 1) were c h r o m a t o g r a p h i c a l l y pure. Blends o f Ld PE and a p p r o x i m a t e l y 0-1, 0.25 and 0 . 5 ~ by weight o f the antioxidants were pr epar e d by mixing at 165-170°C between hot cylinders for 8 min. Films were obtained by compression m oul di ng between glass plates at 170 °C unde r the following conditions: preheating at 0-1 M P a for 2.5rain; compression at 0 . 4 M P a for 4min. The film thicknesses ( about 0.015cm) were determined by means o f the BeerL a m b e r t law using the infra-red band absorbance at 1895cm -1 (absorbance per unit thickness = 2 c m - 1 ) . This band was used as an internal standard for quantitative s p e c t r o p h o t o m e t r i c determinations. The an tio x id an t concent r a t i ons (expressed in Tables 1 and 2 in moles per TABLE 1 Concentrations (c) of Antioxidants I-IV and the Percentage Loss During Processing in Ld PE Type A Films; Initial (rico) and Stationary (r~co) Rates of Carbonyl Formation and Rates of Formation of OH Groups (ron) During the UV Irradiation of Ld PE Type A Films Additire
I
II III IV
Ld PE" Ld PEb a Non-processed. b Processed.
C × 104 M litre i
Per cent loss
rico x 103 h-1
r~co × 103 h-1
ro, x 103 h 1
8.3 30.0 60"0 1.7 2.0 18.0 43.5 85.5
690 67.8 53-5 93'0 91"6 30.0 32.6 33.7
7-6 12.9 17.4 6"7 6-7 13.7 18.2 21-7 5.3 4.0
13.0 8.3 11.3 7.2 21 '0 8.0 10.3 17-0 22-l 18.7
--------1"3 0-7
20
L. Jir&6kovh-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. PospEil
,o s4o, I
HO
SO2
II
OH
III
IV
HO-S-C-OH V
Vl
HO'~CH2"~OH X
Xl
Xll
S(CHzCHzCOOClzH25)z XlII Scheme 1.
Influence of thiobisphenols on the photo-oxidation of low density polyethylene
21
TABLE 2 As for Table 1 for Antioxidants V-XIII and Ld PE Type B Films Additive
V VI VII
VIII IX X
XI XII
XIII Ld PE a Ld PE b
C x 104
Per cent
G c o x 103
r o n x 103
M litre- 1
loss
h- 1
h- 1
h- 1
8-8 21-0 9.8 22.2 43.3
67.3 19.0 50.3 53.7 55.4 -69.0 39.7 27.0 22.3 44.0 77.6 83.7 80.2 15.0
6.1 6.8 9.8 10.6 14.4 9.1 3.6 3.3 8.3 10-6 3.8 3.8 3.0 6.8 3.0 0.0 3.0
7.7 6.7 12.7 7-7 7-3 9.7 3.7 11.0 13-3 16.0 13.0 15.7 15.3 17.3 5.0 10.7 17.0
0.1 0.2 0.6 0.3 0-2 0.1 0.4 0.4 0.7 0.8 0.3 1.0 0.5 0.6 -0-7 0.9
5.7 16.3 49-7 105.7 15.9 11.1 20-2 49.0 15.3
ric o x 103
a Non-processed. b Processed.
litre (M litre-1)) in films were measured by infra-red and ultraviolet spectrophotometry using previously determined molar absorptivities.12 Variations of + 1 0 ~ were observed in concentration values for films molded from the same blend. Their averages and percentages of consumption during processing are reported in Tables 1 and 2. Photo-oxidation The photochemical reactor used has been described previously. 13 It is equipped with a fluorescent lamp emitting between 300 and 450 nm with an intensity maximum at 365nm. The light intensity was 3-5 x 1015 photons per square centimetre per second and the temperature was 40 _ 2 °C. The infra-red bands at 1720cm -1 (carbonyls) and 3430cm -1 (intermolecularly bonded hydroxyls) were monitored during exposure using a Perkin-Elmer 580 spectrophotometer.
22
L. Jir6dkov6-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
For a given absorption band i, at time t, we determined the normalized absorbance (Ai) r A i is expressed in c m - 1. (Ai) t
(OOi)t- 2 x (OOi)t l DO1895cm-i
(DO~)t is the optical density of the band i, measured by the baseline method, and l the film thickness. The reaction rates AAi/At are expressed in h - 1 c m - 1. The initial reaction rates, rico, were determined for the interval 0-66 h, the stationary reaction rates, rsco, for 150-300h, and the average oxidation rates, roll for hydroxyl groups for the whole exposure time.
RESULTS A N D DISCUSSION The influence of processing on the photo-oxidation of Ld PE is shown in Fig. 1. For type A, which has an initially higher carbonyl content, processing results in a decrease in the photo-oxidation rate, whereas the type B Ld PE behaves in the opposite manner. It is interesting to note that no change in the initial part of the carbonyl curve (within experimental error) is observed after processing. The type A
A I,
i
3
2
1
l
0
lO0
I
200
I
h
300
Fig. 1. Change in absorbance, A~ of carbonyl groups at 1720cm-1 with time (h) of ultraviolet irradiation: (a) Ld PE type A films, • non-processed; • processed; (b) Ld PE type B films, O processed, A non-processed.
Influence of thiobisphenols on the photo-oxidation ojlow density polyethylene
23
Ld PE was used in the study ofantioxidants I-IV (Table 1), and type B for compounds V-XIII (Table 2). The influence of 4,4'-thiobis(2-methyl-6-tert. butylphenol) (I), at three concentrations, on the photo-oxidation of Ld PE films is shown in Fig. 2. A clear pro-oxidative effect can be observed early in the exposure (0-66 h). This effect increases with the thiobisphenol concentration. On the other hand, the photo-oxidation rate decreases continuously with exposure time and becomes slower than the control sample after less than 100 h. Both features, namely the initial pro-oxidant effect increasing with concentration and self-retarded kinetics, are also displayed by 2,2'thiobis(4-methyl-6-tert.butylphenol) (IV) and bis(3,5-di-tert.butyl-4hydroxybenzyl) sulphide (VII) for three concentrations, c l - c 3, and by the thiobisphenol, VI (the isomer of I and IV), at concentration c 1. The thermo-oxidative transformation products of thiobisphenol, I, sulphoxide, II, and sulphone, III, have been found to have a very different influence on Ld PE photo-oxidation. Carbonyl development was not influenced by the presence of the sulphone, III, whereas significant lowering of the oxidation rate was observed with the sulphoxide, II (Fig. 3) at approximately the same concentration (Table 1). Dithiobisphenol, V, another product of the thermo-oxidation of I, had an
A
3
H Y
2
0
a
I
//
I
I
I00
200
I
300 h Fig. 2. Changes of absorbance, A, of carbonyl groups at 17200m- x of Ld PE films stabilized with 4,4'-thiobis(2-methyl-6-tert.butylphenol) (I) with time of ultraviolet irradiation: • c = 8 . 3 x l 0 - 4 M l i t r e - I , • c = 3 0 . 0 x 10-~Mlitre-1; Z~ c = 6 0 . 0 x 10 -4 M litre- 1; _ _ _ non-stabilized Ld PE type A.
24
L. Jir~6kov&-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
/
A
/
/o I I I
i//,I~
0
100
200
h
300
Fig. 3. Change of absorbance, A, of carbonyl groups at 1720cm -~ of Ld PE films stabilized with A , 4,4'-sulphinylbis(2-methyi-6-tert.butylphenol)(II) c = 1.7 x 10-4r~ litre -1 and O , 4,4'-sulphonylbis(2-methyl-6-tert.butylphenol) (III) c = 2 . 0 x 10-4M litre-1 with time of ultraviolet irradiation; . . . . non-stabilized Ld PE type A.
analogous effect on the Ld PE photo-oxidation to the thiobisphenols previously discussed. The non-sulphidic thermo-oxidation product of I, i.e. 3,3'-di-tert. butyl-5,5'-dimethyl-4,4'-biphenyldiol (XI) had a relatively weak influence in comparison with the thiobisphenols (Table 2). The sulphoxide, VIII, and sulphone, IX, are the model oxidation products of bis-(3,5-di-tert.butyl-4-hydroxybenzyl) sulphide (VII). The sulphoxide, VIII, acts like the thiobisphenol, displaying pro-oxidation and retardation effects, whereas no sensitization was observed with sulphone, IX, whose retardation efficiency is remarkable (Table 2). The large difference in behaviour between sulphones derived from sulphides I and VII therefore reveals the important r61e of the methylene bridges between the sulphur atom and aromatic nuclei. The model compounds were studied in order to separate the effects of the phenolic part and the sulphidic bridge of the antioxidants on the polymer photo-sensitization. 4,4'-Methylenebis(2-methyl-6-tert.butylphenol) (X) exhibits similar behaviour to thiobisphenol, I, despite the presence of the methylene group instead of sulphur. A comparison of Figs 2 and 4 illustrates the similarity of the photo-oxidation kinetics. However, both characteristics,
Influence of thiobisphenols on the photo-oxidation of low density polyethylene
0
1()0
2 0I 0
h
25
t 300
Fig. 4. Change of absorbance, A, of carbonyl groups at 1720cm- 1 of Ld PE films stabilized with 4,4'-methylenebis(2-methyl-6-tert.butylphenol)(X) with time of ultraviolet irradiation: • c = 16-3 x 10-4Mlitre -1, • c = 4 9 ' 7 x 10-4Mlitre -1, © c = 105"7 x 10-4Mlitre-~; . . . . non-stabilized Ld PE type B,
namely the initial pro-oxidant effect and the autoretardation, are significantly different. The aromatic non-phenolic model, XII, i.e. diphenyl sulphide, does not significantly influence the carbonyl build up (Table 2). However, we observed a loss of 80 % of XII during sample processing, probably due to its high volatility and perhaps to its poor solubility in the PE matrix. Thus, the rapid migration, rather than its chemical consumption, can explain the complete disappearance of the diphenyl sulphide (XII) after 4 hours' exposure. The effect of sulphides was checked with a non-aromatic but compatible compound, the didodecyl 3,3'-thiodipropionate (XIII) (DLTP), which was used in concentration c 1. The lowest concentration was chosen to avoid interference from the ester group of the additive (1735 cm - 1) in the polyethylene carbonyl measurements (1720 cm - 1). Under these conditions, both peaks can be clearly distinguished. D L T P retards the carbonyl development from the beginning of irradiation and does not display any pro-oxidative effect. Comparison of D L T P effects in concentrations c 2 and c a leads to the same conclusions (Fig. 5). The values of initial (rico) and stationary (rsco) photo-oxidation rates
L. Jirddkovd-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
26
4~
-
I
,
I
l
4~
l
l
¢
I
I
310 h
Fig. 5. Comparison of the development of absorption bands of carbonyl groups in the region 1650--1800cm -1 of Ld PE type B films stabilized with the didodecyl 3,3'thiodipropinate (XIII) (c 1 = 15.3 x 10- 4 u litre- 1, c2 = 46.5 x 10- 4 M litre- 1, c3 = 88'7 x 10- 4 M litre- 1) after (a) 0 and (b) 310 h of ultraviolet irradiation; (c) non-stabilized Ld PE at the same conditions.
are summarized in Tables 1 and 2. The periods for calculation of these rates (0-66 h and 150-300 h, respectively) were chosen arbitrarily to demonstrate the complex kinetic behaviour of these systems. It is, however, interesting to note that the initial period closely corresponds to the total disappearance of the antioxidant, as reported elsewhere. 14 It seems, therefore, that, despite a lack of fundamental data on mechanisms and 'true' kinetic parameters, a discussion on relationships between thiobisphenol structure and photo-chemical behaviour can be initiated on the basis of a comparison of the ric o and r,c o values. The dependence of ric o on antioxidant concentration illustrates the similarity of effects of the three sulphur-containing phenols, I, IV and VII (Figs 6 and 7). 2,2'-Thiobis(4-methyl-6-tert. butylphenol) (IV) was the most active as pro-oxidant. The effect of methylenebisphenol, X, is smaller: ric o increases linearly with concentration. It can be deduced from measurements on samples containing the additive concentration, cl, that the non-sulphidic bisphenols X and XI are weaker photo-sensitizers than corresponding thio compounds I, IV, V or VI (Tables 1 and 2).
Influence of thiobisphenols on the photo-oxidation of low density polyethylene
/
20
27
~
/
jf
/
10
5
I
0
I
20
i
I
i
40
60
80 CxlO4
Fig. 6. Change of rates of the formation of carbonyl groups at 1720cm -1 with concentration of antioxidants during ultraviolet irradiation of Ld PE type A films: • rico, 0 r,co; -- 4,4'-thiobis(2-methyl-6-tert.butylphenol) (I), - - - - 2,2'-thiobis(4-methyl-6tert.butylphenol) (IV).
r~103 L
I/
0
20
40
60 80 4 C~10
Fig. 7. Change of rates of formation of carbonyl groups at 1720 c m - 1 with antioxidant concentrations during ultraviolet irradiation of Ld PE films: • rico, 0 rsco; bis(3,5di-tert.-butyl-4-hydroxybenzyl) sulphide (VII), - - 4,4'-methylenebis (2-methyl-6tert.butylphenol) (X).
28
L. Jirddkov~-Audouin, J. F. Bory, J. F. Farrenq, J. Verdu, J. Pospigil
Other evidence of the similarity of the effects of the thiobisphenols being studied is given by the dependence of rsco on antioxidant concentration (Figs 6 and 7). All the curves display a minimum in the range of low concentrations (cl for IV and c2 for the less soluble I and VII). Taking this minimum as a stability criterion, it may be concluded that these phenolic sulphides are the most efficient as photo-stabilizers, the methylene-bisphenol, X, being less active. The position of this minimum, depends, of course, on the choice of periods for rsco calculation, so that such a criterion cannot be used quantitatively. However, it may be supposed by analogy that the low values of rsco for the sulphur-containing compounds II, V, VI, VIII and IX studied only in c a concentrations are consequences of the fact that their concentrations in the films (5- 20 x 10-4 i litre- 1) are probably near to the optimal values indicated by Figs 6 and 7. Thus, even ifcarbonyl formation rates provide interesting information, they are not necessarily absolute criteria of photo-stability. For this reason, the mean rates of hydroxyl formation have also been determined (roz measured at 3430cm-1). Despite a wide scattering of data, some clear tendencies are shown in Tables 1 and 2. The thiobisphenols I, IV and VI and the sulphur-containing oxidation products of I, i.e. II, III and V, as well as DLTP, are efficient suppressors of hydroxyl formation. The influence of VII, VIII and IX, in which a methylene group separates the sulphur atom and aromatic nuclei, is less. However, for VII, rondecreases with the antioxidant concentration. No effect was observed in the case of methylenebisphenol, X. It may therefore be supposed that the orientation of the oxidation reactions towards the formation of non-alcoholic oxygenated structures is a specific effect of the sulphur-containing compounds being studied. No interpretation can be made in the case of diphenyl sulphide, XII, because of its rapid migration, as previously shown. Biphenyldiol, XI, seems to have an inhibition effect but the lack of complementary data does not allow any interpretation. The mechanisms of antioxidant sensitisation will be examined in a later paper. REFERENCES 1. L. Jir~i~kov~iand J. Pospigil, Eur. Polym. J., 8, 75 (1972). 2. L. Jirfi6kovfi and J. Pospi~il, Eur. Polym. J., 9, 71 (1973).
Influence of thiobisphenols on the photo-oxidation of low density polyethylene 29 3. L. Jir/t~kovh and J. Pospi~il, Angew. Makromol. Chem., 66, 95 (1978). 4. L. Jir~6kov~i, Jelinkov~t, J. Rotschov~i and J. Pospi~il, Chem. Ind. (London), 384 (1979). 5. J. G. Scott and M. F. Yusoff, Eur. Polym. J., 16, 497 (1980). 6. A. J. Bridgewater and M. D. Sexton, J. S. C., Perkin II, 530 (1978). 7. W. Matrekey and F. H. Winslow, Am. Chem. Soc., Polym. Prep., 16, 606 (1975). 8. D. J. Kieysworth and D. G. M. Wood, Int. Conf. on Weathering and Degradation of Polymers, Plastics and Rubber Institute, London 1977, Preprint E2. 9. D. J. Carlsson and D. M. Wiles, J. Macromol. Sci-Rev., Maerom. Chem. C14, 155 (1975). 10. F. H. Winslow, W. Matreyek and A. M. Trozzolo, SPE Journal, 28, 19 (1972). 11. Yu. I. Temchin, E. E. Burmistrov, L. A. Skripko, R. S. Burmistrova, Yu. V. Kokhanov, M. A. Gushchina and E. G. Rozantsev, Vysokomol. Soyedin., AIS, 1038 (1973). 12. L. Jir~t~kova, J. Pospi~il and J. Verdu, Analysis 10, 1966 (1982). 13. A. Huvet, J. Philippe and J. Verdu, Eur. Polym. J., 14, 709 (1978). 14. L. Jir~i~kov~-Audouin, J. Pospi~il and J. Verdu, Polym. Deg. Stab., 6 (1984). (In press.)