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Mechanisms of antioxidant action: Formation of antioxidant adducts with rubbers through sulphur by a mechanochemical procedure
PolymerDegradationandStability 4 (1982) 343-351
MECHANISMS OF ANTI OXIDANT ACTION: FORMATION OF ANTIOXIDANT ADDUCTS WITH RUBBERS THROUGH SULPHUR BY A MECHANOCHEMICAL PROCEDURE
G. SCOTT 8~ S. M. TAVAKOLI
Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, Great Britain (Received: 6 February, 1982)
ABSTRACT
It is shown that antioxidants containing a thiol group can be mechanochemically reacted with both natural rubber ( NR) and styrene-butadiene-rubber (SBR). Oxygen inhibits the reaction and the optimum temperature is in the region of 70 °C. Increase in the antioxidant concentration in the rubber leads to increasing levels of adduct Jormation. Evidence is presented to show that although a substantial part of the adduct isformed during mechanochemical treatment, about 20 % of the total binding occurs during the vulcanisation reaction.
INTRODUCTION
Previous publications have described the formation of antioxidant thiol adducts with natural rubber latices in the presence of free radical generators. 1 -6 It has also been disclosed 6 that adduct formation can be carried out in a shearing mixer and the application of this procedure to ABS has been described. 7 This is a particularly convenient method for the production of bound antioxidant masterbatch concentrates which can then be used as conventional additives during the normal compounding procedures. 5-7 The present investigation is concerned with the formation of polymer bound antioxidants by this procedure using some of the sulphur antioxidants ((I) to (VI)) investigated previously by the latex procedure.5 343 Polymer Degradation and Stability 0141-3910/82/0004-0343/$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
344
G. SCOTT, S. M. TAVAKOLI OH t B u ~
~
tBu
CH2SH (I) (BHBM)
N H - ~
NHCOCH2SH
(II) (MADA)
OH
OH
CHaOCOCH2SH
CH2S(CH2),COOR
tBu
(IfD
(IV) (a) n = 1, R~---H. (b) n = 2, R~H. (c) n = 1, R~------C4H9.
tBu
tBu
tBu
tBu (v) (a) n=l. (b) n=2.
tBu HO~~--CHEOCOCHES--S--CH2COOCH2~ tBu
(vl)
OH tBu
EXPERIMENTAL
Synthesis The synthesis of all an tioxidants has been described previously,2'5' 18 except that of (III) (3,5-di-tert-butyl-4-hydroxybenzyl mercaptoglycollate). This was prepared as follows. 0.10 moles of 3,5-di-tert-butyl-4-hydroxybenzyl alcohol in 200 ml dry toluene was mixed with 0.10moles of thioglycollic acid. A slow stream of deoxygenated nitrogen was passed through the flask and the mixture refluxed for tBu
FORMATION OF ANTIOXIDANT THIOL ADDUCTS IN RUBBERS
345
12 h with stirring and removal of water by a Dean and Stark apparatus. The product was repeatedly washed with distilled water to remove unreacted thioglycollic acid. After drying over M g S O 4, toluene was removed and the white waxy solid was extracted with hexane. After removal of hexane, the white waxy solid was titrated with iodine and was found to contain 100 ~o of thiol. Infra-red analysis showed the presence of phenolic OH, 3 6 1 5 c m - 1 ; - - S H , 2560cm -1 and ester carbonyl, 1730 cm - 1.
Materials Rubbers were prepared by coagulation and extraction of latices as described earlier; 5 NR was Qualitex A from W M Symmington and Sons Limited and SBR was a special grade (40 ~ solids) kindly provided by International Synthetic Rubber Limited without added antioxidant and with <0.1 ~o styrene. Vulcanising ingredients and formulations were as described earlier for NR. s In the case of SBR, 2 g/100 g sulphur was used and the compounding time was 10 min. Vulcanisation was carried out as described previously. 5 In the case of SBR, the vulcanisation time was 45min. Estimation of bound antioxidants was made using the phenolic hydroxyl index, A 3 6 4 0 c m - 1 / A 2 7 2 0 c m - 1 in the case of the hindered phenols, as described earlier s and the aromatic index A1595 c m - ~ / A 2 7 2 0 c m - 1 in the case of the aromatic amide (II) in NR. For the amide in SBR, the amide index, A3260 cm - 1/A 1940 cm - 1 was used. Straight line calibration curves (1-10 ~ ) were obtained in all cases. Mechanochemical reactions were carried out in a R A P R A torque rheometer at a rotor speed of 68 rpm. The rubbers were sheeted out on an open mill to form an envelope to enclose the antioxidant before it was placed in the torque rheometer. The processed rubbers were quenched in cold water to stop atmospheric oxidation.
RESULTS
Effect of temperature Viscosity of the polymer is a critical feature of mechanically initiated reactions in polymers since this is primarily responsible for the radical concentration by scission of the polymer chain under shear. 9'1° It has been shown that in the case of rubbers, mechano-oxidation involving chain scission decreases steadily up to 110°C 11 and is superseded by thermal oxidative degradation at higher temperatures. (See reference 12 for a general discussion of the chemical effects of strain on rubbers.) It was therefore of some importance to observe the efficiency of mechan ochemical binding of typical thiol antioxidants with temperature. Table 1 shows this for BHBM (I) and M A D A (II) processed in a closed mixer at 10g/100g rubber. The amide was consistently bound to a higher degree than the hindered phenol. Similar trends were observed at lower concentrations of antioxidant.
346
G. SCOTT, S. M. TAVAKOLI TABLE l EFFECT OF TEMPERATURE ON THE EXTENT OF MECHANOCHEMICAL ADDUCT FORMATION WITH BHBM AND M A D A IN STYRENE--BUTADIENE RUBBER (SBR) (CONCENTRATIONOF ANTIOXIDANTS10 g/100 g RUBBER;PROCESSING TIME 10 min)
Temperature (°C) 25 70 90
Extent ofbmdmg (%)* BHBM (1) MADA (tt) 43 40 36
52 58 50
* In the unvulcanised rubber. Estimated for BHBM using the ir phenolic hydroxyl index, A 3 6 4 0 c m - 1 / A2720 c m - 1 and for M A D A using the ir amide index, A3260 cm - 1/A 1940 cm - 1.
TABLE 2 EFFECT OF AIR ON THE MECHANOCHEMICAL REACTION OF M A D A (II) WITH NR. ANTIOXIDAN'T CONCENTRATION 10 g] 100 g RUBBER; TEMPERATURE, 70 °C; PROCESSINGTIME, 10min
Conditions
Extent of binding ( %)*
Full mixer Mixer one-third full
50 33
* In the unvulcanised rubber. Estimated for M A D A using the ir aromatic index, A 1595 cm - t/A2720 cm - t.
Effect of air It is known that oxygen is an effective trap for macroalkyl radicals formed by mechanochemical scission of polymers during processing.9 Table 2 shows the effect of allowing air into the shearing mixer by only partially filling the chamber for natural rubber. Similar effects were observed with phenolic thiols (I) and (III) (see Table 3). TABLE 3 EFFECT OF AIR ON THE MECHANOCHEMICAL BINDING OF THIOL ANTIOXIDANTS (I) AND (III). TEMPERATURE,[70°C;
ANT1OXIDANTCONCENTRATION,2 g/100 g; REACTIONTIME, 10 min
Conditions Full mixer Two-thirds full mixer One-third full mixer
Extent of binding ( %)*
(0
(m)
49 38 24
54 50 41
* Measured after vulcanisation.
F O R M A T I O N O F A N T I O X I D A N T T H I O L A D D U C T S IN RUBBERS
347
TABLE 4 EFFECT OF ANTIOXIDANT CONCENTRATIONON THE MECHANOCHEMICAL BINDING OF ANTIOXIDANTS (I) AND ( l i D . TEMPERATURE, 7 0 ° C ; REACTION TIME,
10 min; MIXERONE-THIRDFULL
Concentration (g/lOOg) 2 3 4
Extent of binding ( %)* (I) (Ill) 24 37 53
40 45 49
* Measured after vulcanisation.
EfJect of antioxidant concentration Table 4 shows that, in the presence of air, there is a steady increase in the level of binding with increase in antioxidant concentration. Similar results were obtained even on an open mill where higher shear rates can be achieved (see Table 5). However, the level of binding is even further reduced due to the more effective oxygen inhibition. Ten per cent and 20 o/ masterbatches of M A D A were made in NR by the mechanochemical procedure and the extent of binding was measured in the unvulcanised films. Table 6 shows the results obtained. TABLE 5 EFFECT OF ANTIOXIDANT CONCENTRATION ON THE MECHANOCHEMICAL BINDING OF ANTIOXIDANTS (I) AND (Ill) ON AN OPEN TWO ROLL MILL. TEMPERATURE, 2 5 ° C / 2 m i n ; NIP 4 × 1 0 - 3 i n
Concentration of antioxidant (g/ lOOg) 1 2 3
Extent of binding ( %)* (I) (II1) 21 29 32
22 28 38
* Measured after vulcanisation.
TABLE 6 EXTENT OF BINDING OF M A D A MASTERBATCHES
(1I)
IN N R
Concentration of an tioxidan t (g/lOOg)
Extent of binding ( %)*
10 20
50 44
* Measured before vulcanisation.
348
G . SCOTT, S. M. T A V A K O L I TABLE 7 EFFECT OF VULCANISATION ON THE BINDING OF IvlECHANOCHEMICALLY FORMED ADDUCT (I) MASTERBATCH IN S B R AT 2 5 ° C (SEE TABLE 1)
Concentration Extent of binding (%) of antioxidant Before After in vulcanisate vulcanisation vulcanisation 0.25 0.50 1.0 2.0
43 43 43 43
60 62 56 58
Effect of vulcanisation Incorporation of thiol antioxidants during normal compounding and vulCanisation does not lead to polymer bound antioxidants. 2'5 However, there is evidence 5 that latex reacted antioxidants become more effective after vulcanisation. A possible explanation for this is that further adduct formation occurs during vulcanisation when the antioxidant has been pre-reacted with the rubber. In order to evaluate this possibility, a 10 % mechanochemical masterbatch of BHBM (I) in SBR containing 43 ~o adduct was diluted without extraction during compounding with unstabilised SBR. The extent of binding was re-evaluated in the final vulcanisate. The results are shown in Table 7. Similar experiments were carried out with 10 % masterbatches of MADA (II) in both NR and SBR and the results obtained at an antioxidant concentration of 2 % in the final vulcanisate are shown in Table 8. TABLE 8 EFFECT OF VULCANISATION ON THE BINDING OF MECHANOCHEMICALLYFORMEDADDUCT (II) MASTERBATCH ( 1 0 % ) IN N R AND S B R AT 7 0 ° C (SEE TABLE 1)
Rubber
NR SBR
Extent of binding (%) Before After vulcanisation vulcanisation 50 58
68 74
Mechanochemical binding of antioxidant sulphides (IV) to (VD These antioxidants were investigated less thoroughly than the thiols (I) to (III). Table 9 summarises the results of the bound antioxidant level after vulcanisation in all cases. It is clear that although the effects of changing reaction conditions in general parallel the behaviour of the thiols (I) to (III), the level of binding under the same conditions is generally lower.
349
FORMATION OF ANTIOXIDANT THIOL ADDUCTS IN RUBBERS
TABLE 9 MECHANOCHEMICAL BINDING OF ANTIOXIDANT SULPH1DES MEASURED BY EXTRACTION AFTER VULCANISATION
Antioxidant
Rubber
Conditions
Conc. (g/lOOg)
Temperature T i m e (°C) (min)
Extent of binding
(700) IV(a) i V(a) IV(a) IV(b) IV(b) IV(b) IV(a) IV(a)
NR NR NR NR NR NR NR NR
TR(10) TR(10) TR(10) TR(10) TR(10) TR(10) TR(40) TR(20)
2 3 4 2 3 4 2 2
70 70 70 70 70 70 70 70
10 10 10 10 10 10 10 10
26 38 40 31 39 47 58 50
IV(b) IV(a) IV(a) IV(a) IV(b) IV(b) IV(b) IV(a) IV(a) IV(a) IV(a) IV(a) IV(a) IV(c) V(a) V(b) VI IV(a) IV(b)
NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR SBR SBR
TR(20) OM OM OM OM OM OM TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30)
2 I 2 3 1 2 3 2 2 2 2 2 2 2 2 2 2 2 2
70 25 25 25 25 25 25 50 70 90 70 70 70 70 70 70 70 70 70
10 2 2 2 2 2 2 10 I0 10 5 10 15 10 10 10 10 10 10
48 4 13 21 13 22 35 49 58 42 49 58 44 6 6 45 48 55 46
IV(b)
NR
TR(30)
2
70
l0
48
NR = Natural rubber. SBR = Styrene butadiene rubber. TR( ) = Torque rheometer (rubber charge). OM = Open two roll mill.
DISCUSSION
The results reported confirm that optimal conditions exist for mechano-initiated reactions in polymers. Most antioxidants show an optimal temperature which is limited by the solubility of the additive in the polymer at low temperatures (compare BHBM (I) which is highly soluble in hydrocarbons with MADA (II) which is much less soluble (Table 1)). To react effectively, the antioxidant must be present in the polymer matrix at the site of chain scission. The tendency towards lower binding at high temperatures is partly due to the reduced viscosity of the rubber but also to oxidation of the adduct, a reaction which leads to elimination of the sulphenic acid from the rubber. Oxygen inhibits the adduct formation, both by deactivating the
350
G. SCOTT, S. M. TAVAKOLI
initiating radicals and by forming hydroperoxides which are able to oxidise the sulphides to sulphoxides. The observations are consistent with Scheme 1.
Shear
R--R
~ 2R"
I II !
ASH
~,
~ RCH~HR' IO 2 H
I I i
02
ROO"
I
RH + AS.
R'
I
/
I
\
AS---C--C-
I I
R
H
(R .)
I I
~
RH
I
ASH
I I I I
I
R. + ROOH N ~ /
H
R'
I
I
AS-~--C--H
I
+ AS.
I
R H
! 0
II ROH + ASCH~CH2--R'
I
ASOH + RCH==CHR' Reaction scheme for mechanochemical adduct formation. Scheme 1. It will be shown in another publication 13 that the deliberate addition of hydroperoxides has a deleterious effect on the adduct behaviour of thiol adducts. In spite of the deleterious effect of oxygen on the binding reaction, increasing the concentration of thiol increases the yield of adduct (see Tables 4 and 6). This is also consistent with Scheme 1, since increase in ASH concentration allows it to compete more effectively with oxygen for the primary alkyl radicals. The formation of further adducts during vulcanisation cannot be fully explained on the basis of the present evidence. It is clear that it must involve secondary products since the parent thiols are not bound to any significant extent during vulcanisation. In general, the higher the level of initial adduct formation, the lower
FORMATION OF ANTIOXIDANT THIOL ADDUCTS IN RUBBERS
351
is the level of secondary binding. ~3 It seems possible that oxygenated products may be involved, including peroxides formed in the rubber during processing, but the chemistry of the vulcanisation process is too complex to permit a resolution of this problem at present. The ageing performance of rubbers based on the adduct rubbers described in this paper has been discussed elsewhere. 5
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. R. N. FERNANDO, G. SCOTT and J. E. STOCKEY, J. Rubb. Res. Inst. Sri Lanka, 54, 531 (1977). K. W. S. KULARATNEand G. SCOTT, Europ. Polym. J., 15, 827 (1979). G. SCOTT,Plastics and Rubber: Processing, 41 (June, 1977). G. SCOTT,Developments in polymer stabilisation--4 (G. Scott (Ed.)), Applied Science Publishers, London, p. 181 (1981). G. SCOTT and S. M. TAVAKOLI, Polym. Des. and Stab., 4, 279 (1982). G. SCOTT,Brit. Pat. 1 503501 (1974); US Pat., 4213892 (1980). M. GHAEMYand G. SCOTT, Polym. Des. and Stab., 3, 233 (1981). A. A. KATBABand G. SCOTT, Polym. Des. and Stab., 3, 221 (1980-81). M. PIKEand W. F. WATSON, J. Polym. Sci., 9, 229 (1952); W. F. WATSON, Trans. IRL 29, 32 (1953). R. J. CERESAand W. F. WATSOn, J. App. Polym. Sci., 1, 101 (1959). W. F. BussE and E. N. CUNNINGHAM,Proc. Rubb. Tech. Conf., p.288 (1938). G. SCOTT,Atmospheric oxidation and antioxidants, Elsevier, London and New York, p. 466 et seq. (1965). O. AJIBOYEand G. SCOTT, Polym. Des. and Stab. (In press.)
MECHANISMS OF ANTI OXIDANT ACTION: FORMATION OF ANTIOXIDANT ADDUCTS WITH RUBBERS THROUGH SULPHUR BY A MECHANOCHEMICAL PROCEDURE
G. SCOTT 8~ S. M. TAVAKOLI
Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, Great Britain (Received: 6 February, 1982)
ABSTRACT
It is shown that antioxidants containing a thiol group can be mechanochemically reacted with both natural rubber ( NR) and styrene-butadiene-rubber (SBR). Oxygen inhibits the reaction and the optimum temperature is in the region of 70 °C. Increase in the antioxidant concentration in the rubber leads to increasing levels of adduct Jormation. Evidence is presented to show that although a substantial part of the adduct isformed during mechanochemical treatment, about 20 % of the total binding occurs during the vulcanisation reaction.
INTRODUCTION
Previous publications have described the formation of antioxidant thiol adducts with natural rubber latices in the presence of free radical generators. 1 -6 It has also been disclosed 6 that adduct formation can be carried out in a shearing mixer and the application of this procedure to ABS has been described. 7 This is a particularly convenient method for the production of bound antioxidant masterbatch concentrates which can then be used as conventional additives during the normal compounding procedures. 5-7 The present investigation is concerned with the formation of polymer bound antioxidants by this procedure using some of the sulphur antioxidants ((I) to (VI)) investigated previously by the latex procedure.5 343 Polymer Degradation and Stability 0141-3910/82/0004-0343/$02.75 © Applied Science Publishers Ltd, England, 1982 Printed in Great Britain
344
G. SCOTT, S. M. TAVAKOLI OH t B u ~
~
tBu
CH2SH (I) (BHBM)
N H - ~
NHCOCH2SH
(II) (MADA)
OH
OH
CHaOCOCH2SH
CH2S(CH2),COOR
tBu
(IfD
(IV) (a) n = 1, R~---H. (b) n = 2, R~H. (c) n = 1, R~------C4H9.
tBu
tBu
tBu
tBu (v) (a) n=l. (b) n=2.
tBu HO~~--CHEOCOCHES--S--CH2COOCH2~ tBu
(vl)
OH tBu
EXPERIMENTAL
Synthesis The synthesis of all an tioxidants has been described previously,2'5' 18 except that of (III) (3,5-di-tert-butyl-4-hydroxybenzyl mercaptoglycollate). This was prepared as follows. 0.10 moles of 3,5-di-tert-butyl-4-hydroxybenzyl alcohol in 200 ml dry toluene was mixed with 0.10moles of thioglycollic acid. A slow stream of deoxygenated nitrogen was passed through the flask and the mixture refluxed for tBu
FORMATION OF ANTIOXIDANT THIOL ADDUCTS IN RUBBERS
345
12 h with stirring and removal of water by a Dean and Stark apparatus. The product was repeatedly washed with distilled water to remove unreacted thioglycollic acid. After drying over M g S O 4, toluene was removed and the white waxy solid was extracted with hexane. After removal of hexane, the white waxy solid was titrated with iodine and was found to contain 100 ~o of thiol. Infra-red analysis showed the presence of phenolic OH, 3 6 1 5 c m - 1 ; - - S H , 2560cm -1 and ester carbonyl, 1730 cm - 1.
Materials Rubbers were prepared by coagulation and extraction of latices as described earlier; 5 NR was Qualitex A from W M Symmington and Sons Limited and SBR was a special grade (40 ~ solids) kindly provided by International Synthetic Rubber Limited without added antioxidant and with <0.1 ~o styrene. Vulcanising ingredients and formulations were as described earlier for NR. s In the case of SBR, 2 g/100 g sulphur was used and the compounding time was 10 min. Vulcanisation was carried out as described previously. 5 In the case of SBR, the vulcanisation time was 45min. Estimation of bound antioxidants was made using the phenolic hydroxyl index, A 3 6 4 0 c m - 1 / A 2 7 2 0 c m - 1 in the case of the hindered phenols, as described earlier s and the aromatic index A1595 c m - ~ / A 2 7 2 0 c m - 1 in the case of the aromatic amide (II) in NR. For the amide in SBR, the amide index, A3260 cm - 1/A 1940 cm - 1 was used. Straight line calibration curves (1-10 ~ ) were obtained in all cases. Mechanochemical reactions were carried out in a R A P R A torque rheometer at a rotor speed of 68 rpm. The rubbers were sheeted out on an open mill to form an envelope to enclose the antioxidant before it was placed in the torque rheometer. The processed rubbers were quenched in cold water to stop atmospheric oxidation.
RESULTS
Effect of temperature Viscosity of the polymer is a critical feature of mechanically initiated reactions in polymers since this is primarily responsible for the radical concentration by scission of the polymer chain under shear. 9'1° It has been shown that in the case of rubbers, mechano-oxidation involving chain scission decreases steadily up to 110°C 11 and is superseded by thermal oxidative degradation at higher temperatures. (See reference 12 for a general discussion of the chemical effects of strain on rubbers.) It was therefore of some importance to observe the efficiency of mechan ochemical binding of typical thiol antioxidants with temperature. Table 1 shows this for BHBM (I) and M A D A (II) processed in a closed mixer at 10g/100g rubber. The amide was consistently bound to a higher degree than the hindered phenol. Similar trends were observed at lower concentrations of antioxidant.
346
G. SCOTT, S. M. TAVAKOLI TABLE l EFFECT OF TEMPERATURE ON THE EXTENT OF MECHANOCHEMICAL ADDUCT FORMATION WITH BHBM AND M A D A IN STYRENE--BUTADIENE RUBBER (SBR) (CONCENTRATIONOF ANTIOXIDANTS10 g/100 g RUBBER;PROCESSING TIME 10 min)
Temperature (°C) 25 70 90
Extent ofbmdmg (%)* BHBM (1) MADA (tt) 43 40 36
52 58 50
* In the unvulcanised rubber. Estimated for BHBM using the ir phenolic hydroxyl index, A 3 6 4 0 c m - 1 / A2720 c m - 1 and for M A D A using the ir amide index, A3260 cm - 1/A 1940 cm - 1.
TABLE 2 EFFECT OF AIR ON THE MECHANOCHEMICAL REACTION OF M A D A (II) WITH NR. ANTIOXIDAN'T CONCENTRATION 10 g] 100 g RUBBER; TEMPERATURE, 70 °C; PROCESSINGTIME, 10min
Conditions
Extent of binding ( %)*
Full mixer Mixer one-third full
50 33
* In the unvulcanised rubber. Estimated for M A D A using the ir aromatic index, A 1595 cm - t/A2720 cm - t.
Effect of air It is known that oxygen is an effective trap for macroalkyl radicals formed by mechanochemical scission of polymers during processing.9 Table 2 shows the effect of allowing air into the shearing mixer by only partially filling the chamber for natural rubber. Similar effects were observed with phenolic thiols (I) and (III) (see Table 3). TABLE 3 EFFECT OF AIR ON THE MECHANOCHEMICAL BINDING OF THIOL ANTIOXIDANTS (I) AND (III). TEMPERATURE,[70°C;
ANT1OXIDANTCONCENTRATION,2 g/100 g; REACTIONTIME, 10 min
Conditions Full mixer Two-thirds full mixer One-third full mixer
Extent of binding ( %)*
(0
(m)
49 38 24
54 50 41
* Measured after vulcanisation.
F O R M A T I O N O F A N T I O X I D A N T T H I O L A D D U C T S IN RUBBERS
347
TABLE 4 EFFECT OF ANTIOXIDANT CONCENTRATIONON THE MECHANOCHEMICAL BINDING OF ANTIOXIDANTS (I) AND ( l i D . TEMPERATURE, 7 0 ° C ; REACTION TIME,
10 min; MIXERONE-THIRDFULL
Concentration (g/lOOg) 2 3 4
Extent of binding ( %)* (I) (Ill) 24 37 53
40 45 49
* Measured after vulcanisation.
EfJect of antioxidant concentration Table 4 shows that, in the presence of air, there is a steady increase in the level of binding with increase in antioxidant concentration. Similar results were obtained even on an open mill where higher shear rates can be achieved (see Table 5). However, the level of binding is even further reduced due to the more effective oxygen inhibition. Ten per cent and 20 o/ masterbatches of M A D A were made in NR by the mechanochemical procedure and the extent of binding was measured in the unvulcanised films. Table 6 shows the results obtained. TABLE 5 EFFECT OF ANTIOXIDANT CONCENTRATION ON THE MECHANOCHEMICAL BINDING OF ANTIOXIDANTS (I) AND (Ill) ON AN OPEN TWO ROLL MILL. TEMPERATURE, 2 5 ° C / 2 m i n ; NIP 4 × 1 0 - 3 i n
Concentration of antioxidant (g/ lOOg) 1 2 3
Extent of binding ( %)* (I) (II1) 21 29 32
22 28 38
* Measured after vulcanisation.
TABLE 6 EXTENT OF BINDING OF M A D A MASTERBATCHES
(1I)
IN N R
Concentration of an tioxidan t (g/lOOg)
Extent of binding ( %)*
10 20
50 44
* Measured before vulcanisation.
348
G . SCOTT, S. M. T A V A K O L I TABLE 7 EFFECT OF VULCANISATION ON THE BINDING OF IvlECHANOCHEMICALLY FORMED ADDUCT (I) MASTERBATCH IN S B R AT 2 5 ° C (SEE TABLE 1)
Concentration Extent of binding (%) of antioxidant Before After in vulcanisate vulcanisation vulcanisation 0.25 0.50 1.0 2.0
43 43 43 43
60 62 56 58
Effect of vulcanisation Incorporation of thiol antioxidants during normal compounding and vulCanisation does not lead to polymer bound antioxidants. 2'5 However, there is evidence 5 that latex reacted antioxidants become more effective after vulcanisation. A possible explanation for this is that further adduct formation occurs during vulcanisation when the antioxidant has been pre-reacted with the rubber. In order to evaluate this possibility, a 10 % mechanochemical masterbatch of BHBM (I) in SBR containing 43 ~o adduct was diluted without extraction during compounding with unstabilised SBR. The extent of binding was re-evaluated in the final vulcanisate. The results are shown in Table 7. Similar experiments were carried out with 10 % masterbatches of MADA (II) in both NR and SBR and the results obtained at an antioxidant concentration of 2 % in the final vulcanisate are shown in Table 8. TABLE 8 EFFECT OF VULCANISATION ON THE BINDING OF MECHANOCHEMICALLYFORMEDADDUCT (II) MASTERBATCH ( 1 0 % ) IN N R AND S B R AT 7 0 ° C (SEE TABLE 1)
Rubber
NR SBR
Extent of binding (%) Before After vulcanisation vulcanisation 50 58
68 74
Mechanochemical binding of antioxidant sulphides (IV) to (VD These antioxidants were investigated less thoroughly than the thiols (I) to (III). Table 9 summarises the results of the bound antioxidant level after vulcanisation in all cases. It is clear that although the effects of changing reaction conditions in general parallel the behaviour of the thiols (I) to (III), the level of binding under the same conditions is generally lower.
349
FORMATION OF ANTIOXIDANT THIOL ADDUCTS IN RUBBERS
TABLE 9 MECHANOCHEMICAL BINDING OF ANTIOXIDANT SULPH1DES MEASURED BY EXTRACTION AFTER VULCANISATION
Antioxidant
Rubber
Conditions
Conc. (g/lOOg)
Temperature T i m e (°C) (min)
Extent of binding
(700) IV(a) i V(a) IV(a) IV(b) IV(b) IV(b) IV(a) IV(a)
NR NR NR NR NR NR NR NR
TR(10) TR(10) TR(10) TR(10) TR(10) TR(10) TR(40) TR(20)
2 3 4 2 3 4 2 2
70 70 70 70 70 70 70 70
10 10 10 10 10 10 10 10
26 38 40 31 39 47 58 50
IV(b) IV(a) IV(a) IV(a) IV(b) IV(b) IV(b) IV(a) IV(a) IV(a) IV(a) IV(a) IV(a) IV(c) V(a) V(b) VI IV(a) IV(b)
NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR SBR SBR
TR(20) OM OM OM OM OM OM TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30) TR(30)
2 I 2 3 1 2 3 2 2 2 2 2 2 2 2 2 2 2 2
70 25 25 25 25 25 25 50 70 90 70 70 70 70 70 70 70 70 70
10 2 2 2 2 2 2 10 I0 10 5 10 15 10 10 10 10 10 10
48 4 13 21 13 22 35 49 58 42 49 58 44 6 6 45 48 55 46
IV(b)
NR
TR(30)
2
70
l0
48
NR = Natural rubber. SBR = Styrene butadiene rubber. TR( ) = Torque rheometer (rubber charge). OM = Open two roll mill.
DISCUSSION
The results reported confirm that optimal conditions exist for mechano-initiated reactions in polymers. Most antioxidants show an optimal temperature which is limited by the solubility of the additive in the polymer at low temperatures (compare BHBM (I) which is highly soluble in hydrocarbons with MADA (II) which is much less soluble (Table 1)). To react effectively, the antioxidant must be present in the polymer matrix at the site of chain scission. The tendency towards lower binding at high temperatures is partly due to the reduced viscosity of the rubber but also to oxidation of the adduct, a reaction which leads to elimination of the sulphenic acid from the rubber. Oxygen inhibits the adduct formation, both by deactivating the
350
G. SCOTT, S. M. TAVAKOLI
initiating radicals and by forming hydroperoxides which are able to oxidise the sulphides to sulphoxides. The observations are consistent with Scheme 1.
Shear
R--R
~ 2R"
I II !
ASH
~,
~ RCH~HR' IO 2 H
I I i
02
ROO"
I
RH + AS.
R'
I
/
I
\
AS---C--C-
I I
R
H
(R .)
I I
~
RH
I
ASH
I I I I
I
R. + ROOH N ~ /
H
R'
I
I
AS-~--C--H
I
+ AS.
I
R H
! 0
II ROH + ASCH~CH2--R'
I
ASOH + RCH==CHR' Reaction scheme for mechanochemical adduct formation. Scheme 1. It will be shown in another publication 13 that the deliberate addition of hydroperoxides has a deleterious effect on the adduct behaviour of thiol adducts. In spite of the deleterious effect of oxygen on the binding reaction, increasing the concentration of thiol increases the yield of adduct (see Tables 4 and 6). This is also consistent with Scheme 1, since increase in ASH concentration allows it to compete more effectively with oxygen for the primary alkyl radicals. The formation of further adducts during vulcanisation cannot be fully explained on the basis of the present evidence. It is clear that it must involve secondary products since the parent thiols are not bound to any significant extent during vulcanisation. In general, the higher the level of initial adduct formation, the lower
FORMATION OF ANTIOXIDANT THIOL ADDUCTS IN RUBBERS
351
is the level of secondary binding. ~3 It seems possible that oxygenated products may be involved, including peroxides formed in the rubber during processing, but the chemistry of the vulcanisation process is too complex to permit a resolution of this problem at present. The ageing performance of rubbers based on the adduct rubbers described in this paper has been discussed elsewhere. 5
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
M. R. N. FERNANDO, G. SCOTT and J. E. STOCKEY, J. Rubb. Res. Inst. Sri Lanka, 54, 531 (1977). K. W. S. KULARATNEand G. SCOTT, Europ. Polym. J., 15, 827 (1979). G. SCOTT,Plastics and Rubber: Processing, 41 (June, 1977). G. SCOTT,Developments in polymer stabilisation--4 (G. Scott (Ed.)), Applied Science Publishers, London, p. 181 (1981). G. SCOTT and S. M. TAVAKOLI, Polym. Des. and Stab., 4, 279 (1982). G. SCOTT,Brit. Pat. 1 503501 (1974); US Pat., 4213892 (1980). M. GHAEMYand G. SCOTT, Polym. Des. and Stab., 3, 233 (1981). A. A. KATBABand G. SCOTT, Polym. Des. and Stab., 3, 221 (1980-81). M. PIKEand W. F. WATSON, J. Polym. Sci., 9, 229 (1952); W. F. WATSON, Trans. IRL 29, 32 (1953). R. J. CERESAand W. F. WATSOn, J. App. Polym. Sci., 1, 101 (1959). W. F. BussE and E. N. CUNNINGHAM,Proc. Rubb. Tech. Conf., p.288 (1938). G. SCOTT,Atmospheric oxidation and antioxidants, Elsevier, London and New York, p. 466 et seq. (1965). O. AJIBOYEand G. SCOTT, Polym. Des. and Stab. (In press.)