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Antioxidant activity of some derivatives of 9-thiabicyclo[3,3,1]nonane in rubber and rubber vulcanizates
Polymer Degradation and Stability 37 (1992) 1-5
Antioxidant activity of some derivatives of 9-thiabicyclo[3,3,1]nonane in rubber and rubber vulcanizates J.-M. Herdan, Liana Cira Institute of Technological Engineering, Research and Refinery Design, R-2000 Ploie~ti, Romania
Maria Giurginca & G. Ivan Research Institute for Rubber and Plastics Processing, R-79628 Bucharest, Romania
(Received 1 September 1990; accepted 7 February 1991) Three new derivatives of 1,5-dimercapto-9-thiabicyclo[3,3,1]nonanecontaining alkylhydroxybenzyl groups bound to mercaptanic sulfur have been synthesized. The compounds have been tested in elastomers with various degrees of unsaturation, in order to correlate their protecting activity with their structure.
INTRODUCTION Derivatives of hindered phenols containing sulfur have been used as antioxidants for many years in a variety of products including polymers, oils, pharmaceuticals and cosmetics. Their outstanding efficiency is assigned to their internal synergism which consists of the action of the hindered hydroxyl as a chain breaker of auto-oxidation, and of the thioether groups as hydroperoxide decomposers. Derivatives of 9-thiabicyclo[3,3,1]nonane containing hindered phenolic groups were synthesized first by Stephen L2 and later by others 3,4 but their antioxidant efficiency in various substrates was not reported until more recently. Scott and coworker¢ -7 reported on the behavior of 3,5-di-tert-butyl-4-hydroxybenzylmercaptane and its S-alkylated derivatives in diene rubbers, emphasizing that: they can be grafted on elastomer macromolecules during processing and/or vulcanization of the rubber compounds, thus opening the way to the use of sulfur bearing antidegradants for improving the protection of elastomers. The present work is devoted to new derivatives of 9-thiabicyclo[3,3,1]nonane containing dialkyl-
hydroxybenzylthiol antioxidants.
groupings
for
use
as
EXPERIMENTAL New derivatives of 9-thiabicyclo[3,3,1]nonane have been synthesized by applying the following series of reactions: NH2 CI ~ ~ C] S / - ~ H2N+ Cl
S~C(NH2)2' II-- --] / S/']
cl
II
NH2 (1)
-el
NH2
I
C--S (21
~-~
S---C
i
NH2 He"
K
(2) SH
2
J.-M. Herdan et al.
OH
tBu tBu
!+
,
~~q~H2--S
HO
R
~ ,
CH2N(CH3)
R
| ! R = CH3 | | | R = tBu
OH I+
tBu~CH2N(CH3)2
)
OH tBu J.
(3)
C H 2 ~ H
S
~
OH (4)
tBu
tBu
IV tBu
Table 1. Characteristics of the new compounds Product
II !11 IV
Melting point (°C)
Sulfur content (%) Theoretical
Found
100-102 98-100 >230
18.2 15.9 15.9
17-7 15.2 15.4
using equation (5), 8
Infrared spectrum Vo. (cm -l) 3450 3630 3425; 3610
These new compounds have been characterized by spectral data and by the determination of the sulfur content (Table 1). The antioxidant efficiency of compounds II-IV was evaluated in two groups of rubbers: (i) rubbers with high unsaturation, namely natural rubber (NR) types RSS 1, and styrene-butadiene rubber (SBR 1500) type Carom SBR 1500 (Petrochemical Combine Borze~ti, Bocau, Romania); (ii) a rubber with low unsaturation, namely ethylene-propylene-diene terpolymer (EPDM) with 40% propylene and 5% diene, type Terpit C (Petrochemical Combine Pite~ti, Romania). The antioxidants (2 phr) were introduced into the purified rubbers. In order to evaluate the protecting efficiency of the new compounds, four testing methods were applied. (i) Determination of the kinetic characteristics--induction period and propagation period----deduced from the formation of oxygen bearing groups with time, recorded by the band at 1720 cm -1 in the infrared spectrum, Thermo-oxidative degradation was carried out at 100°C for highly unsaturated rubbers and at 130°C for low unsaturated rubbers, using a Perkin-Elmer 577 spectrograph. (ii) Determination of the gel content, initially and after 72h of aging at 100 or 130°C. (iii) Evaluation of the viscosity retention index (VRI)
%VRI = M__VV× ~ 100 ME
(5)
where ME and MVf are the initial and final Mooney viscosities at 100°C, after 72h of thermo-oxidative aging at 100 or 130°C. (iv) Testing of the physical properties of the protected vulcanizates after 72 h of aging at 70 or 90°C in an oven with air circulation. Standard recipes were used in preparing the compounds on a standard laboratory mill type WNR-2 (Troester KG, Hannover, Germany).
RESULTS AND DISCUSSION Behavior of elastomer + antioxidant systems during thermo-oxidative degradation Compounds l l - l V were added (2phr) to solutions of purified rubber and films cast on KBr windows were used for recording infrared spectra in order to evaluate the formation and Table 2. Behavior of protected rubbers in thermo-oxidative degradation Protecting agent Unprotected II II1 IV
NR °
SBR 1500 °
EPDM b
ti
tp
ti
to
ti
to
1 153 200 3
5 235 55 34
25 165 170 20
15 80 80 17
45 110 130 2
25 130 150 10
a Hours at 100°C. Hours at 130°C. 6, Initiation time; tp, propagation time.
Derivatives of 9-thiabicyclo[3,3,1]nonane in rubber changes of the content of the carbonyl/carboxyl groups (v = 1720 cm -~) during accelerated aging. The induction periods and propagation periods recorded in Table 2 emphasize the superior protection given by compounds ll and Ill; compound IV does not give protection to highly unsaturated rubbers and seems actually to have a pro-oxidant effect on low unsaturated rubber. The absence of the protecting activity of compound IV may be explained by the presence of an ortho-benzyl .structure which is unable to give a stable quinonemethide, which is an indispensable intermediate structure in the reaction of hindered phenolic antioxidants with peroxy radicals. 9 At the same time it should be mentioned that compounds II and l l l exhibit a marked antioxidant efficiency, superior to that of other phenolic antioxidants tested in rubber matrices. "' The stability of rubbers containing compounds ! ! - I V was evaluated by VRI (Table 3) and by gel content (Table 4). In this type of testing, the nature of the rubber is of particular importance. Table 3. Viscosity retention index (%VRI) for various protected rubbers Protecting agent
NR"
S B R 1500"
EPDM b
Unprotected II I!! IV
36 60 52 43
140 128 105 93
108 105 107 105
Table 4. Change in gel content (%) for protected and unprotected rubbers
NR
Protecting agent
Initial gel content
Gel content after aging~ 24 h
48 h
72 h
Unprotected 11 II! IV
1-3 5.8 1.5 2.7
11.5 0.0 11.8 10.4
15,5 0.0 19,0 15-8
2(I.3 6-2 20-8 16.9
SBR 1500 Unprotected !I !!1 IV
0.0 0.0 0-0 0.0
7-1 0-0 0.0 6-5
19-0 8-4 0-0 11.7
20.8 13.0 2.0 15-0
EPDM
2.3 0.3 0.4 0.8
3.8 4.5 0.9 1-0
4.7 4-6 1-2 3-8
8.7 6.3 6-5 5-3
"Aging EPDM.
Unprotected n I11 IV
Thus, NR exposed to thermo-oxidative degradation exhibits a tendency to viscosity reduction and such a tendency is inhibited by the addition of compounds I f - I V , the most prominent effect being induced by compounds II and 111; the presence of the same antioxidants leads to a diminution of the gel content at certain stages of aging. The decrease in the viscosity is a result of scission of the macromolecules while the formation of gel is a consequence of branching and cross-linking processes. In this respect, 11 is the most efficient product able to decrease both scission and branching or cross-linking processes. In SBR 1500, thermo-oxidative aging leads to an increase of viscosity and of gel content, indicating that branching and cross-linking processes prevail and addition of compound Ill almost completely eliminates these processes. In presence of product IV, :as well as branching/cross-linking, some chain scission also takes place. In EPDM, only a slight increase of gel content occurs, without much change of viscosity. Antioxidant Ill is able to prevent the crosslinking process for rather a long time. This may be attributed to the intrinsic stability of the elastomer itself as well as to the moderate conditions of the thermo-oxidative degradation used in the experiments.
Behavior of rubber vulcanizates in thermooxidative degradation
~' A f t e r aging for 72 h at 100°C. b A f t e r aging for 72 h at 130°C.
Rubber
3
at 100°C for N R and SBR; aging at 130°C for
Stress-strain characteristics of a vulcanizate obtained from a standard recipe based on NR (Table 5) seem to be independent of the antioxidant used for its protection. The retention of the main tensile characteristics after aging in an oven with circulating air shows that antioxidants I | and If| give protection comparable with phenolic antioxidants bearing a disulfide bridge.
A possible mechanism of antioxidant action of 9-thiabicyclo[3,3,1]nonane derivatives The antioxidant activity of certain 9thiabicyclo[3,3,1]nonane derivatives, particularly compounds II and Iii should be explained by their ability to react with peroxy radicals. Thus, antioxidant III loses reactive phenolic hydrogen
J.-M. Herdan et al.
4
Table 5. Retention of main tensile characteristics of natural rubber vulcanizates ° containing various antioxidants Antioxidant type Tensile characteristics
Ab
Bb
Cb
11
The diquinone structure VII may be able to react with another peroxy radical as well as undergo scission to give other species (IX, X, XI, Xll) with antioxidant capability (reactions (8)-(10)):
111
tBu
Initially Tensile strength (MPa) Elongation at break (%)
22.8 700
23.2 710
24-1 690
23.8 710
R O 0 ~
23.2 700
VII
,
H 2 C~/ ~
Retention of tensile characteristics after aging 3 days at 70°C
O"
(8)
tBu VII!
Tensile strength (%) Elongation at break (%)
84 82
82 78
95 88
94 91
86 77
Retention of tensile characteristics after aging 3 days at 90°C
, --S--~
VII
Tensile strength (%) Elongation at break (%)
67 70
70 62
80 69
85 78
+ S---CH
O"
73 62
tBu IX
X
a Test recipe, mass parts: NR 100, stearic acid 0-5, zinc oxide 5, 2-mercaptobenzothiazole 0.5, sulfur 3.5, antioxidant 2. b A: 2,2'-methylene-bis(4-methyi,6-tert-butyiphenol); B: 4,4'-thio-bis(2,6-tert-butylphenol); C: 4,4'-dithio-bis(2,6-tert-butyiphenol).
(9)
VIi
'--S~__S.
+
atoms to form a phenoxy diradical: ili
2ROO'
X!
~'
tBu
tBu tBu{~H2--S~-s
OtBu@H--CH~tBuO
CH2~O" v
tBu
tBu (6)
This diradical V incorporates reactive methylene hydrogen atoms which must be able to react with other peroxy radicals to form radical structure VI which can rearrange to form the quinonoid structure VII:
(1o)
Xll
The thienyl radicals X and XI formed in reactions (9) and (10) can participate in grafting reactions during thermo-oxidative processes which take place during storage of the rubbers or during processing of the rubber compounds. At the same time, the divalent sulfur present in the structure of the new antioxidants make them
V
~
2ROO
tBu tBu
•
H~
S Vl
tBu tBu
CH -O tBu
s
tBu VII
tBu
(7)
Derivatives of 9-thiabicyclo[ 3,3,1]nonane in rubber
capable of reacting with hydroperoxides:
m+Roo
exemplified in reactions (11)-(13) for the sulfur existing in the bridge of the thiabicyclononane.
1
CONCLUSIONS
Xlll
,
+
ROH
(11)
S-xIv
At high t e m p e r a t u r e , the sulfoxide X I V should be able to generate a sulfenic acid X V by rearrangement:
xlv
.e~, - - S ~ s _ _
-
5
(12)
xv
Three new derivatives of 9-thiabicyclo[3,3,1]nonane have been synthesized and their antioxidant activity tested in high and low unsaturated rubbers. C o m p o u n d s II and Ill, which have hindered phenolic units with - - S - - - C H 2 - - groups in the p a r a position, exhibit higher antioxidant efficiency as revealed by a diminution of the chain scission and branching/cross-linking processes. In natural rubber and natural rubber vulcanizates, the derivatives of 9-thiabicyclo[3,3,1]nonane exhibit an antioxidant effect comparable with hindered phenolic antioxidants bearing a disulfide bridge. The protecting activity of the c o m p o u n d s studied has been assigned to their reactivity with peroxy radicals as well as for hydroperoxides, which means that they exhibit an internal synergistic effect.
xv + ROOH REFERENCES
+ RO" + H20
(13)
XV!
The sulfenic acid XV, in its turn, is able to decompose hydroperoxides forming sulfoxyl and RO" radicals. The sulfoxyl radical can be easily oxidized to a sulfinic acid, then to a sulfonic acid by oxidant species present in the system, In this way, the antioxidants containing divalent sulfur act as h y d r o p e r o x i d e decomposers leading to non-radical species. ~l The same sulfur bearing intermediates are considered ~2 as free radical scavengers. Finally, it may be observed that in compounds I I - I V there are two other sulfur atoms able to react in ways very similar to those
1. Stephen, J. F., US Patent 4000112 (1976); CA, 86 (1977) 73646. 2. Stephen, J. F., FRG Patent 2 609 6513 (1976); CA, 86 (1977) 6103. 3. Jayme, J. J. & Askew, N. F., Belgium Patent 852716 (1977): CA, 89 (1978) 113772. 4. Whelan, E. L. & Jancis, E. N., US Patent 4221 702 (1980); CA, 94 (1981) 66704. 5. Farzaliev, V. M., Fernando, W. S. & Scott, G.. Eur. Polym. J., 14 (1978) 785. 6. Scott, G. & Yusoff, M. F., Eur. Polym. J., 16 (1980) 493. 7. Scott, G. & Sukarto, R., Eur. Polym. J., 20 (1984) 139. 8. Volintiru, T., et al. Revista de Chimie (Bucharest), 39 (1988) 1073. 9. Pospifil, J., Pure Appl. Chem., 36 (1973) 207. 10. Ivan, G. & Giurginca, M., Revista de Chimie (Bucharest), 37 (1986) 287. 11. Scott, G., In Development in Polymer Stabilizations6, ed. G. Scott, Applied Science, London, 1983, p. 29. 12. Lucki, J., Poly. Deg. & Stab., 11 (1985) 75.
Antioxidant activity of some derivatives of 9-thiabicyclo[3,3,1]nonane in rubber and rubber vulcanizates J.-M. Herdan, Liana Cira Institute of Technological Engineering, Research and Refinery Design, R-2000 Ploie~ti, Romania
Maria Giurginca & G. Ivan Research Institute for Rubber and Plastics Processing, R-79628 Bucharest, Romania
(Received 1 September 1990; accepted 7 February 1991) Three new derivatives of 1,5-dimercapto-9-thiabicyclo[3,3,1]nonanecontaining alkylhydroxybenzyl groups bound to mercaptanic sulfur have been synthesized. The compounds have been tested in elastomers with various degrees of unsaturation, in order to correlate their protecting activity with their structure.
INTRODUCTION Derivatives of hindered phenols containing sulfur have been used as antioxidants for many years in a variety of products including polymers, oils, pharmaceuticals and cosmetics. Their outstanding efficiency is assigned to their internal synergism which consists of the action of the hindered hydroxyl as a chain breaker of auto-oxidation, and of the thioether groups as hydroperoxide decomposers. Derivatives of 9-thiabicyclo[3,3,1]nonane containing hindered phenolic groups were synthesized first by Stephen L2 and later by others 3,4 but their antioxidant efficiency in various substrates was not reported until more recently. Scott and coworker¢ -7 reported on the behavior of 3,5-di-tert-butyl-4-hydroxybenzylmercaptane and its S-alkylated derivatives in diene rubbers, emphasizing that: they can be grafted on elastomer macromolecules during processing and/or vulcanization of the rubber compounds, thus opening the way to the use of sulfur bearing antidegradants for improving the protection of elastomers. The present work is devoted to new derivatives of 9-thiabicyclo[3,3,1]nonane containing dialkyl-
hydroxybenzylthiol antioxidants.
groupings
for
use
as
EXPERIMENTAL New derivatives of 9-thiabicyclo[3,3,1]nonane have been synthesized by applying the following series of reactions: NH2 CI ~ ~ C] S / - ~ H2N+ Cl
S~C(NH2)2' II-- --] / S/']
cl
II
NH2 (1)
-el
NH2
I
C--S (21
~-~
S---C
i
NH2 He"
K
(2) SH
2
J.-M. Herdan et al.
OH
tBu tBu
!+
,
~~q~H2--S
HO
R
~ ,
CH2N(CH3)
R
| ! R = CH3 | | | R = tBu
OH I+
tBu~CH2N(CH3)2
)
OH tBu J.
(3)
C H 2 ~ H
S
~
OH (4)
tBu
tBu
IV tBu
Table 1. Characteristics of the new compounds Product
II !11 IV
Melting point (°C)
Sulfur content (%) Theoretical
Found
100-102 98-100 >230
18.2 15.9 15.9
17-7 15.2 15.4
using equation (5), 8
Infrared spectrum Vo. (cm -l) 3450 3630 3425; 3610
These new compounds have been characterized by spectral data and by the determination of the sulfur content (Table 1). The antioxidant efficiency of compounds II-IV was evaluated in two groups of rubbers: (i) rubbers with high unsaturation, namely natural rubber (NR) types RSS 1, and styrene-butadiene rubber (SBR 1500) type Carom SBR 1500 (Petrochemical Combine Borze~ti, Bocau, Romania); (ii) a rubber with low unsaturation, namely ethylene-propylene-diene terpolymer (EPDM) with 40% propylene and 5% diene, type Terpit C (Petrochemical Combine Pite~ti, Romania). The antioxidants (2 phr) were introduced into the purified rubbers. In order to evaluate the protecting efficiency of the new compounds, four testing methods were applied. (i) Determination of the kinetic characteristics--induction period and propagation period----deduced from the formation of oxygen bearing groups with time, recorded by the band at 1720 cm -1 in the infrared spectrum, Thermo-oxidative degradation was carried out at 100°C for highly unsaturated rubbers and at 130°C for low unsaturated rubbers, using a Perkin-Elmer 577 spectrograph. (ii) Determination of the gel content, initially and after 72h of aging at 100 or 130°C. (iii) Evaluation of the viscosity retention index (VRI)
%VRI = M__VV× ~ 100 ME
(5)
where ME and MVf are the initial and final Mooney viscosities at 100°C, after 72h of thermo-oxidative aging at 100 or 130°C. (iv) Testing of the physical properties of the protected vulcanizates after 72 h of aging at 70 or 90°C in an oven with air circulation. Standard recipes were used in preparing the compounds on a standard laboratory mill type WNR-2 (Troester KG, Hannover, Germany).
RESULTS AND DISCUSSION Behavior of elastomer + antioxidant systems during thermo-oxidative degradation Compounds l l - l V were added (2phr) to solutions of purified rubber and films cast on KBr windows were used for recording infrared spectra in order to evaluate the formation and Table 2. Behavior of protected rubbers in thermo-oxidative degradation Protecting agent Unprotected II II1 IV
NR °
SBR 1500 °
EPDM b
ti
tp
ti
to
ti
to
1 153 200 3
5 235 55 34
25 165 170 20
15 80 80 17
45 110 130 2
25 130 150 10
a Hours at 100°C. Hours at 130°C. 6, Initiation time; tp, propagation time.
Derivatives of 9-thiabicyclo[3,3,1]nonane in rubber changes of the content of the carbonyl/carboxyl groups (v = 1720 cm -~) during accelerated aging. The induction periods and propagation periods recorded in Table 2 emphasize the superior protection given by compounds ll and Ill; compound IV does not give protection to highly unsaturated rubbers and seems actually to have a pro-oxidant effect on low unsaturated rubber. The absence of the protecting activity of compound IV may be explained by the presence of an ortho-benzyl .structure which is unable to give a stable quinonemethide, which is an indispensable intermediate structure in the reaction of hindered phenolic antioxidants with peroxy radicals. 9 At the same time it should be mentioned that compounds II and l l l exhibit a marked antioxidant efficiency, superior to that of other phenolic antioxidants tested in rubber matrices. "' The stability of rubbers containing compounds ! ! - I V was evaluated by VRI (Table 3) and by gel content (Table 4). In this type of testing, the nature of the rubber is of particular importance. Table 3. Viscosity retention index (%VRI) for various protected rubbers Protecting agent
NR"
S B R 1500"
EPDM b
Unprotected II I!! IV
36 60 52 43
140 128 105 93
108 105 107 105
Table 4. Change in gel content (%) for protected and unprotected rubbers
NR
Protecting agent
Initial gel content
Gel content after aging~ 24 h
48 h
72 h
Unprotected 11 II! IV
1-3 5.8 1.5 2.7
11.5 0.0 11.8 10.4
15,5 0.0 19,0 15-8
2(I.3 6-2 20-8 16.9
SBR 1500 Unprotected !I !!1 IV
0.0 0.0 0-0 0.0
7-1 0-0 0.0 6-5
19-0 8-4 0-0 11.7
20.8 13.0 2.0 15-0
EPDM
2.3 0.3 0.4 0.8
3.8 4.5 0.9 1-0
4.7 4-6 1-2 3-8
8.7 6.3 6-5 5-3
"Aging EPDM.
Unprotected n I11 IV
Thus, NR exposed to thermo-oxidative degradation exhibits a tendency to viscosity reduction and such a tendency is inhibited by the addition of compounds I f - I V , the most prominent effect being induced by compounds II and 111; the presence of the same antioxidants leads to a diminution of the gel content at certain stages of aging. The decrease in the viscosity is a result of scission of the macromolecules while the formation of gel is a consequence of branching and cross-linking processes. In this respect, 11 is the most efficient product able to decrease both scission and branching or cross-linking processes. In SBR 1500, thermo-oxidative aging leads to an increase of viscosity and of gel content, indicating that branching and cross-linking processes prevail and addition of compound Ill almost completely eliminates these processes. In presence of product IV, :as well as branching/cross-linking, some chain scission also takes place. In EPDM, only a slight increase of gel content occurs, without much change of viscosity. Antioxidant Ill is able to prevent the crosslinking process for rather a long time. This may be attributed to the intrinsic stability of the elastomer itself as well as to the moderate conditions of the thermo-oxidative degradation used in the experiments.
Behavior of rubber vulcanizates in thermooxidative degradation
~' A f t e r aging for 72 h at 100°C. b A f t e r aging for 72 h at 130°C.
Rubber
3
at 100°C for N R and SBR; aging at 130°C for
Stress-strain characteristics of a vulcanizate obtained from a standard recipe based on NR (Table 5) seem to be independent of the antioxidant used for its protection. The retention of the main tensile characteristics after aging in an oven with circulating air shows that antioxidants I | and If| give protection comparable with phenolic antioxidants bearing a disulfide bridge.
A possible mechanism of antioxidant action of 9-thiabicyclo[3,3,1]nonane derivatives The antioxidant activity of certain 9thiabicyclo[3,3,1]nonane derivatives, particularly compounds II and Iii should be explained by their ability to react with peroxy radicals. Thus, antioxidant III loses reactive phenolic hydrogen
J.-M. Herdan et al.
4
Table 5. Retention of main tensile characteristics of natural rubber vulcanizates ° containing various antioxidants Antioxidant type Tensile characteristics
Ab
Bb
Cb
11
The diquinone structure VII may be able to react with another peroxy radical as well as undergo scission to give other species (IX, X, XI, Xll) with antioxidant capability (reactions (8)-(10)):
111
tBu
Initially Tensile strength (MPa) Elongation at break (%)
22.8 700
23.2 710
24-1 690
23.8 710
R O 0 ~
23.2 700
VII
,
H 2 C~/ ~
Retention of tensile characteristics after aging 3 days at 70°C
O"
(8)
tBu VII!
Tensile strength (%) Elongation at break (%)
84 82
82 78
95 88
94 91
86 77
Retention of tensile characteristics after aging 3 days at 90°C
, --S--~
VII
Tensile strength (%) Elongation at break (%)
67 70
70 62
80 69
85 78
+ S---CH
O"
73 62
tBu IX
X
a Test recipe, mass parts: NR 100, stearic acid 0-5, zinc oxide 5, 2-mercaptobenzothiazole 0.5, sulfur 3.5, antioxidant 2. b A: 2,2'-methylene-bis(4-methyi,6-tert-butyiphenol); B: 4,4'-thio-bis(2,6-tert-butylphenol); C: 4,4'-dithio-bis(2,6-tert-butyiphenol).
(9)
VIi
'--S~__S.
+
atoms to form a phenoxy diradical: ili
2ROO'
X!
~'
tBu
tBu tBu{~H2--S~-s
OtBu@H--CH~tBuO
CH2~O" v
tBu
tBu (6)
This diradical V incorporates reactive methylene hydrogen atoms which must be able to react with other peroxy radicals to form radical structure VI which can rearrange to form the quinonoid structure VII:
(1o)
Xll
The thienyl radicals X and XI formed in reactions (9) and (10) can participate in grafting reactions during thermo-oxidative processes which take place during storage of the rubbers or during processing of the rubber compounds. At the same time, the divalent sulfur present in the structure of the new antioxidants make them
V
~
2ROO
tBu tBu
•
H~
S Vl
tBu tBu
CH -O tBu
s
tBu VII
tBu
(7)
Derivatives of 9-thiabicyclo[ 3,3,1]nonane in rubber
capable of reacting with hydroperoxides:
m+Roo
exemplified in reactions (11)-(13) for the sulfur existing in the bridge of the thiabicyclononane.
1
CONCLUSIONS
Xlll
,
+
ROH
(11)
S-xIv
At high t e m p e r a t u r e , the sulfoxide X I V should be able to generate a sulfenic acid X V by rearrangement:
xlv
.e~, - - S ~ s _ _
-
5
(12)
xv
Three new derivatives of 9-thiabicyclo[3,3,1]nonane have been synthesized and their antioxidant activity tested in high and low unsaturated rubbers. C o m p o u n d s II and Ill, which have hindered phenolic units with - - S - - - C H 2 - - groups in the p a r a position, exhibit higher antioxidant efficiency as revealed by a diminution of the chain scission and branching/cross-linking processes. In natural rubber and natural rubber vulcanizates, the derivatives of 9-thiabicyclo[3,3,1]nonane exhibit an antioxidant effect comparable with hindered phenolic antioxidants bearing a disulfide bridge. The protecting activity of the c o m p o u n d s studied has been assigned to their reactivity with peroxy radicals as well as for hydroperoxides, which means that they exhibit an internal synergistic effect.
xv + ROOH REFERENCES
+ RO" + H20
(13)
XV!
The sulfenic acid XV, in its turn, is able to decompose hydroperoxides forming sulfoxyl and RO" radicals. The sulfoxyl radical can be easily oxidized to a sulfinic acid, then to a sulfonic acid by oxidant species present in the system, In this way, the antioxidants containing divalent sulfur act as h y d r o p e r o x i d e decomposers leading to non-radical species. ~l The same sulfur bearing intermediates are considered ~2 as free radical scavengers. Finally, it may be observed that in compounds I I - I V there are two other sulfur atoms able to react in ways very similar to those
1. Stephen, J. F., US Patent 4000112 (1976); CA, 86 (1977) 73646. 2. Stephen, J. F., FRG Patent 2 609 6513 (1976); CA, 86 (1977) 6103. 3. Jayme, J. J. & Askew, N. F., Belgium Patent 852716 (1977): CA, 89 (1978) 113772. 4. Whelan, E. L. & Jancis, E. N., US Patent 4221 702 (1980); CA, 94 (1981) 66704. 5. Farzaliev, V. M., Fernando, W. S. & Scott, G.. Eur. Polym. J., 14 (1978) 785. 6. Scott, G. & Yusoff, M. F., Eur. Polym. J., 16 (1980) 493. 7. Scott, G. & Sukarto, R., Eur. Polym. J., 20 (1984) 139. 8. Volintiru, T., et al. Revista de Chimie (Bucharest), 39 (1988) 1073. 9. Pospifil, J., Pure Appl. Chem., 36 (1973) 207. 10. Ivan, G. & Giurginca, M., Revista de Chimie (Bucharest), 37 (1986) 287. 11. Scott, G., In Development in Polymer Stabilizations6, ed. G. Scott, Applied Science, London, 1983, p. 29. 12. Lucki, J., Poly. Deg. & Stab., 11 (1985) 75.