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A kinetic study on the action of rubber oxidation inhibitors—III. Kinetics of inhibited oxidation
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KH. S. BAGDASAR'YANa n d R. I. MILYUTINSKAYA
REFERENCES 1o S° N. ZHURKOV and T. P. SANFIROVA, Dokl. Akad. Nauk SSSR 101: 237, 1955 2. S. N. ZHURKOV a n d E. Ye. TOMASHEVSKII, Zh. tekh. fiz. 25: 66, 1955 3. S. N. ZHURKOV, Vestn. A.kad. Nauk SSSR, No. 11, 78, 1957 4. S. N. ZHURKOV and T. P. SANFIROVA, Fizika tverd, tela 2: 1035, 1960 5. S. N. ZHURKOV e~ al., Fizika tverd, tela 2: 1040, 1960 6. S. N. ZHURKOV and S. A. ABASOV, Vysokomol. soyed. 3: 441, 1961 7. S. N. ZHURKOV a n d S. A. ABASOV, Vysokomol. soyed. 3: 450, 1961 8. S. N. ZHURKOV and S. A. ABASOV, Fizika tverd, tela 4: 2184, 1962 9. S. N. ZHURKOV a n d S. A. ABASOV, Vysokomol. soyed. 4: 1703, 1962 10. N. GRASSI, K h i m i y a protsessov destruktsii polimerov, Izd. in lit., 1959 ll.B. G. ACHHEMMER, M. TRYON and G. M. KLINE, Kunststoffe, 49: 600, 1959
A KINETIC STUDY OF THE ACTION OF RUBBER OXIDATION INHIBITORS--IH. KINETICS OF INHIBITED OXIDATION* K H . S. B A G D A S A R ' Y A N a n d R . I . M I L Y U T I N S K A Y A V. L. Karpov Physico-Chemical I n s t i t u t e
(Received 18 July 1963)
IN A PREVIOUS communication [1] it had been shown that different inhibitors used in various concentrations, give rise to markedly different induction periods during the oxidation of rubber. These differences at times amount to two orders of magnitude. If all inhibitors were to react directly with the primary radicals initiating the oxidation chain, then the induction period v for all inhibitors would be the same and would be determined by the obvious expression Vin T ~ y x 0
where Vin is the initiation rate, Xo is the initial inhibitor concentration and v is a stoichiometric coefficient. The marked differences in the values of ~ found in experiment may be explained by different reasons. It is possible that weak inhibitors act principally as chain transmitters, that is they are incomplete intdbitors. It is possible that strong inhibitors have the special property of being regenerated after the act of chain rupture. In order to throw light on the different inhibitor efficiencies, an analysis of the kinetics of inhibited oxidation of rubber is given in the present work. The correspondence of theory with experimental data made it possible to express sufficiently well-founded considerations about the causes of the different inhibitor * Vysokomol. soyed. 6: No. 6, 1098-1103, 1964.
Rubber oxidation inhibitors--III
1209
efficieneies. In particular, the possibility follows from the theory of obtaining interesting data about the mectmnism of inhibitor activity by the method of investigating inhibited oxidation in the presence of oxidation initiators. The experiments carried out gave an interesting but unexpected result, requiring separate consideration and further investigation. KINETICS OF INHIBITED OXIDATION OF RUBBER
Let us find the kinetic expression for the induction period for oxidation in the presence of an inhibitor. For the sake of discussion we will consider that the same reactions take place in the induction period as during ordinary oxidation, and that they differ only in that a rupture of the reaction chains takes place as a result of the reaction of the radical RO 2 with the inhibitor XH. Elementary reactions Initiation-->R R + O2-->RO2 RO2-kRH-->Peroxide-kR f R0-->R
Peroxide --> ~ H0-->R R0~ ~- XH-->Products
Kinetic expressions Vin k1[R] [02] k2[ROs] [RH] k3[ROOR] kx[RO~]x
Here RH is the rubber, and x the concentration of inhibitor. The following system of kinetic equations describes the oxidation process in the induction period; /
d[R] dt
= Viu--kk2 [R02] [RH]q-2~k3 [ROOR]--]cl [R] [02],
d [RO~] dt
--k 1 [R] [02]--k2 [R02] [RH]--flk x [R02] x,
d [ROOR] =k2 [R02] [RH]--k3 [ROOR], dt
(1)
(2) (3)
d [02] d T = k l [R] [02],
(4)
dx dt --(1 -- J) kx [R02] x,
(5)
Where ~ is the coefficient of chain branching (see [2]), fl is the coefficient of inhibition (fl-=2 for full inhibition, fl=0 for full chain transmission), j is the coefficient of inhibitor regeneration (if the inhibitor is completely regenerated in subsequent reactions, then J-->l). By adding equations (1) and (2) we obtain: Vt. + 2yk3 [ROOR]--~kx [R02] x = o.
(6)
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KH. S. BAGDABAR'YANand R . I . MILYUTINSKAYA
From equations (2), (4), (5) we obtain dt
=
(1
)dr
Pkx~ +1
)
.
(7)
Integration of equation (7) with the conditions x-=-xq, A 0 2 = 0 leads to an equation connecting the oxygen uptake in the induction period with the consumption of inhibitor A O___~=_fifl [ L ° In x° + l - x ] xo 1 -- g x xd '
(8)
where Lo=k~[RH]/flkxx o is the ratio of the rate of chain growth to the rate of its rupture at the initial inhibitor concentration, that is, L 0 is the initial length of the reaction chain for the given initial inhibitor concentration. I f Lo= 0 then this means, t h a t the oxygen consumption is completely determined by the initiation reaction. In our experiments the duration of the induction p e r i o d was determined as the time to achieve certain threshold value (AO2)ind, after which the rate of oxygen uptake markedly rose. Since small inhibitor concentrations were used, the ratio (AO~)i,d/xo was always considerably greater than unity. Thus if (ziOn)in d were equal to 5 X 10-6 mole [2], the inhibitor concentration would be not greater than 0.1 weight % or 5 x 10-7 mole in 0.1 grams of polymer. Consequently the quantity (AO2)ind/Xo was not less than 10, and in some experiments reached a value of 1000. Since at the end of the induction period x/x o is small [1] (probably of the order 10-~-10-4), then from equation (8) it follows that the requirement (AOz)ind/X0>>1 can be fulfilled only ffone of the two following conditions is fulfilled: L0>> 1 (long reaction chains at the start o f the induction period) or ill(l--g) >> 1 that is 5--->1 (efficient regeneration of the inhibitor). For further analysis is it desirable to find the relationship between the average peroxide molecule lifetime 1/k a and the duration of the induction period r. I n reference [2] it was found t h a t ka=2.2 x 10-2 min -x at 80 ° C, and the activation energy for the decomposition was equal to ~21 kcal/mole. Hence we find that at 100 ° C I/ka=10 rain, whereas the induction periods under our experimental conditions lasted tens and hundreds of minutes [1]; in this way, 1/ka<
(9)
Equation (9) together with equations (5) and (6) gives
dx dt
(1-,~)
p
v,~
1--(2~k~[~]/~k,x)"
(lo)
From equation (10) it follows that with branching chains, induction perioda
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Rubber oxidation i n h i b i t o r s - - I I I
can occur only when the denominator in equation (10) is a positive quantity. Consequently, in any case
2yk 2 [RH] = 2yLo < 1.
(11)
]~]CxX0
Since 7 is equal to 0.35-1 [2], then L 0 cannot be considerably greater than rarity. Consequently, the condition (AO2)ina/Xo>>1 can be satisfied (see equation (8)) only if fl/(1--~)>>l, t h a t is ~->1 (effective regeneration of the inhibitor). From equation (9) it follows that as x is reduced the rate of consumption of inhibitor increases and tends to infinity as x~-2~,ke [RH]/fl]Cx:Xcr. The time to achieve the concentration Xcr m a y be considered as the duration of the induction period. By integrating equation (9) from x o to x¢r, we obtain
v oL
Xo
Since the inhibitor concentration is very small at the end of the induction period [1], x¢dx o is not greater than 0.1; hence
~x0 ~=(1-~) V~
(13)
It follows from equation (13) that the large differences in induction periods for the various inhibitors [1] are explained by differences in the quanity fill--(f). From equations (7) and (9), the following expression is obtained for the initial oxidation rate in the induction period
-~-)o=
p~Xo )~ t,
~
).
(14)
The fraction in the righthand part of equation (14) does not differ strongly from unity for typical inhibitors, and tends towards unity at high inhibitor concentrations. Thus the rate of oxygen uptake will tend towards the rate of initiation. * In this way, for high inhibitor concentrations, when the fractions in the enumerator and denominator of equation (14) are small, we obtain the following expression for the initial oxidation rate
~)0=
~ICxXo
A
(15)
Under these conditions very small steady state concentrations of rubber peroxide are to be expected, and this is confirmed by the data of Angert and Kuz'minskii [3], since the reaction of RO e radicals with rubber, leading to the * The possibility of similar processes taking place at high inhibitor concentrations [3] is not discussed by us.
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KH. S. BAGDASAR'YAN a n d R. I. 1VhLYUTINSKAYA
formation of peroxide molecules, is strongly suppressed b y the reaction of these radicals with the inhibitor. Let us consider in somewhat more detail the types of reactions which could lead to chain transfer and to regeneration of the inhibitor XH. These reactions are set out in Table 1, and the values of the coefficients ~ and (1--5) are shown for cases when after the reaction I only the reaction in question takes place. TABLE 1, SECONDARY REACTIONS DURING THE INHIBITED OXIDATION OF RUBBER
Reaction II XH+RO~'-+X" +ROOH II X'+C'w->X--CC-- w - II
IV
V
X'+
X'+
*
--C=C--
w-->w
--XC--C'--
w--HC--C=C--w---->
*
XR+--
--
~--C'--C=C--
X ' + wHC--C'-- w-~XH+ w--C=C-- w
~ --
fl
( 1-- g)
2
1 1 0 0
0 0 2
If reactions I I - V take place simultaneously after reaction I, then fl and (1--5) take on intermediate values. Of the two reactions, IV and V, leading to inhibitor regeneration, only reaction V leads to fl/(1--5)>> 1. INVESTIGATION OF INHIBITED OXIDATION IN THE PRESENCE OF OXIDATION INITIATOR
I n this way the characteristic kinetic feature of the rubber oxidation inhibitor is the value fl/(1--5), which is determined not only by the nature of the inhibitor, but also b y the type of rubber and the temperature. If the initiation rate is measured b y an independent method, then the value fl/{ 1--5) m a y be determined from the duration of the induction period according to equation (13). The use of an oxidation initiator, in principle opens up the possibility of simultaneously determining both Vtn, and also fl/(1--~). I n the presence of an oxidation initiator it is possible to obtain an expression for v, if Vln in equation (10) be replaced by Vin-+fkCoe-m where co is the initial initiator concentration, k is the decomposition constant of the initiator, ] is the efficiency of initiation. I f an initiator is selected which almost completely decomposes in the time of the induction period, then instead of equation (13) (13) we obtain ~0 .
z
.
Xo C0 .
(1--5) VIn
.
Vin x0
(16)
The dependence of z on Co/Xo makes it possible to determine the rate of steady state initiation Vin and the absolute value of fl/(1--~). To carry out this investigation a suitable oxidation initiator has to be selected. To do this the decomposition kinetics of several radical initiators were studied in rubber films. Benzene peroxide was unsuitable for the initiation of oxidation of rubber in
Rubber oxidation inhibitors--III
1213
the presence of inhibitors of the amine class, but it could be suitable for certain other classes, since it enters into a molecular reaction with these compounds. The rate of decomposition of the dinitrile of azoisobutyric acid in rubber films which was investigated by the manometric method, turned out to be very small; the decomposition constants at 80 and 100°C were equal to 0.47× 10-s and 3.3 × 10-5 sec -1, while in solution in benzene these constants were equal to 16× 10-~ and 160× 10-5 sec-h This initiator thus turned out to be unsuitable for our purpose. An a t t e m p t to use methylphenyltriazene as an oxidation initiator also turned out to be unsuccessful, since oxidation in the presence of this compound took place with a large induction period. Phenylazotriphenylmethane (PATM) is free of the defects mentioned. This compound is prepared following the method given in the literature [4]. The de-
x
8O0
60O
4OO
200
1
0
0.50
i
I.OA
Dependence of induction period (v) at 70°C on the molar ratio (A) initiator : inhibitor. Initiator, phenylazotriphenyl methane: 1--phenyl-fl-naphthylamine, c0=7.3× × 10-5 mole/1.; 2--phenothiazine, %= 7.1 × 10-~ mole/1. composition rate constant in rubber film, determined manometrically at 70°C, turned out to be equal to 6 × 10-4 sec -1 (time for half decomposition, 20 rain). One had to be satisfied t h a t the decomposition of P A T ~ in rubber film was not accompanied by any considerable cellular effect. For this purpose, the complete decomposition of PATM (4.9× 10-5 mole) was investigated in vacuum in the presence of diphenylpicrylhydrazyl (9.8 × 10-5 mole), which was introduced into the rubber film by the usual method. After the complete breakdown of the PATM, the diphenylpicrylhydrazyl turned out to be almost completely used up. Thus the radicals formed during the decomposition of PAT~[ in rubber films enter completely into radical reactions, t h a t is the cellular effect is practically
1214
K H . S. BAGDASAR'YAN a n d R. I. MILYUTINSKAYA
absent in this case. Rubber oxidation initiated by PATI~I takes place without an induction period and leads to a reproduceable steady state rate. Data are presented in Table 2 about the dependence of the duration of the induction period on the inhibitor concentration. TABLE 2. INDUCTION PERIOD IN THE OXIDATION OF RUBBER UNDER THE JOINT ACTION OF AN INITIATOR (PHENYLAZOTRIPHENYLMETHANE) AND INHIBITOR
Inhibitor
Concentration, mole/1. × 10-5
Co
2", 1Tlin
X0
Phenyl-fl-naphthylamine Ditto Phenothiazine
inhibitor
initiator
7"30 6"77 7.03 6"95 0.071 0"066 0"081
0 1'03 1.90 4.50 0 0'018 0"036
0 0.15 0.27 0.65 0 0.23 0.45
790 665 465 160 830 620 405
The time for half decomposition of the initiator is considerably less than ~. The dependence of tor on Co/Xo is shown in the Figure for two inhibitors, which differ in their effectiveness b y almost a factor of 100. Used in concentrations differing by a factor of 100 t h e y show an identical induction period amounting to approximately 800 min. Both straight lines have the same slope and intersect the abscissa axis at co/x o ~ 1. The results obtained does not agree with equation (16) from which it should follow t h a t the slope ought to be proportional to x 0. Agreement with experiment is obtained if one postulates t h a t the radicals formed from the initiator react directly with inhibitor molecules, reducing their number in the rubber by the number of initiator molecules introduced. I n this case x 0 in equation (13) ought to be replaced by x o - - c o. We then obtain
T - ( 1 - ~ ) Vi.\ which agrees with the experiment. I n this way, the initiator used did not make it possible to determine separately Vin and/?/(1--5). CONCLUSIONS
Investigation of the inhibited oxidation of sodium butadiene rubber at 70100°C led to the following conclusions: I) the duration of the induction period for different inhibitors differs b y a factor of not less t h a n 100; 2) the amount of oxygen absorbed in the induction period is 100-1000 times greater thaa~ the q u a n t i t y of inhibitor introduced.
Polymers with conjugated double b o n d s - - I I
1215
T h e kinetics of the i n h i b i t e d o x i d a t i o n of r u b b e r h a s b e e n discussed theoretically for t h e case of p r a c t i c a l interest, n a m e l y 1/ka
,~:ote added in proof. The conclusion about efficient inhibitor regeneration is based on the assumption that the true amount of oxygen absorbed in the induction period approximates to the threshold value z/(O2)ind, observed in experiment. I f the first quantity is considerably less than the second, it is necessary to determine the absolute value fl/(1--J) to explain the causes of different inhibitor efficiencies. Translated by G. MODLE.~" REFERENCES
1. Z. A. SINITSYNA and Kh. S. BAGDASAR'YAN, Sb: Khimicheskiye svoistva i modifikatsiya polimerov, Izd. Akad. Nauk SSSR, p. 272, 1964 2. Kh. S. BAGDASAR'YAN, Z. A. SINITSYNA and P. I. MILYUTINSKAYA, ibid, p. 265 3. L. G. ANGERT and A. S. KUZMINSKII, J. Polymer Sci. 32: 1, 1958 4. H. WIELAND, H. HOVE and K. B(iRNER, Liebigs Ann. Chem., 446, 31, 1925
ELECTRICAL AND MAGNETIC PROPERTIES OF POLYMERS WITH CONJUGATED DOUBLE BONDSmII. FLUORESCENCE OF POLYPHENYLACETYLENES * V. A. B E N D E R S K I I a n d P. A. STUNZHAS Institute for Chemical Physics, U.S.S.R. Academy of Sciences
(Received 18 July 1963) I N THE first c o m m u n i c a t i o n [1] t h e electrical c o n d u c t a n c e a n d electron spin r e s o n a n c e s p e c t r a of the f r a c t i o n a t i o n p r o d u c t s of p o l y p h e n y l a c e t y t e n e w i t h m o l e c u l a r weights (M) f r o m 590 u p to 1870 were i n v e s t i g a t e d . I t was s h o w n * Vysokomol. soyed. 6: No. 6, 1104-Ili0, 1964.
KH. S. BAGDASAR'YANa n d R. I. MILYUTINSKAYA
REFERENCES 1o S° N. ZHURKOV and T. P. SANFIROVA, Dokl. Akad. Nauk SSSR 101: 237, 1955 2. S. N. ZHURKOV a n d E. Ye. TOMASHEVSKII, Zh. tekh. fiz. 25: 66, 1955 3. S. N. ZHURKOV, Vestn. A.kad. Nauk SSSR, No. 11, 78, 1957 4. S. N. ZHURKOV and T. P. SANFIROVA, Fizika tverd, tela 2: 1035, 1960 5. S. N. ZHURKOV e~ al., Fizika tverd, tela 2: 1040, 1960 6. S. N. ZHURKOV and S. A. ABASOV, Vysokomol. soyed. 3: 441, 1961 7. S. N. ZHURKOV a n d S. A. ABASOV, Vysokomol. soyed. 3: 450, 1961 8. S. N. ZHURKOV and S. A. ABASOV, Fizika tverd, tela 4: 2184, 1962 9. S. N. ZHURKOV a n d S. A. ABASOV, Vysokomol. soyed. 4: 1703, 1962 10. N. GRASSI, K h i m i y a protsessov destruktsii polimerov, Izd. in lit., 1959 ll.B. G. ACHHEMMER, M. TRYON and G. M. KLINE, Kunststoffe, 49: 600, 1959
A KINETIC STUDY OF THE ACTION OF RUBBER OXIDATION INHIBITORS--IH. KINETICS OF INHIBITED OXIDATION* K H . S. B A G D A S A R ' Y A N a n d R . I . M I L Y U T I N S K A Y A V. L. Karpov Physico-Chemical I n s t i t u t e
(Received 18 July 1963)
IN A PREVIOUS communication [1] it had been shown that different inhibitors used in various concentrations, give rise to markedly different induction periods during the oxidation of rubber. These differences at times amount to two orders of magnitude. If all inhibitors were to react directly with the primary radicals initiating the oxidation chain, then the induction period v for all inhibitors would be the same and would be determined by the obvious expression Vin T ~ y x 0
where Vin is the initiation rate, Xo is the initial inhibitor concentration and v is a stoichiometric coefficient. The marked differences in the values of ~ found in experiment may be explained by different reasons. It is possible that weak inhibitors act principally as chain transmitters, that is they are incomplete intdbitors. It is possible that strong inhibitors have the special property of being regenerated after the act of chain rupture. In order to throw light on the different inhibitor efficiencies, an analysis of the kinetics of inhibited oxidation of rubber is given in the present work. The correspondence of theory with experimental data made it possible to express sufficiently well-founded considerations about the causes of the different inhibitor * Vysokomol. soyed. 6: No. 6, 1098-1103, 1964.
Rubber oxidation inhibitors--III
1209
efficieneies. In particular, the possibility follows from the theory of obtaining interesting data about the mectmnism of inhibitor activity by the method of investigating inhibited oxidation in the presence of oxidation initiators. The experiments carried out gave an interesting but unexpected result, requiring separate consideration and further investigation. KINETICS OF INHIBITED OXIDATION OF RUBBER
Let us find the kinetic expression for the induction period for oxidation in the presence of an inhibitor. For the sake of discussion we will consider that the same reactions take place in the induction period as during ordinary oxidation, and that they differ only in that a rupture of the reaction chains takes place as a result of the reaction of the radical RO 2 with the inhibitor XH. Elementary reactions Initiation-->R R + O2-->RO2 RO2-kRH-->Peroxide-kR f R0-->R
Peroxide --> ~ H0-->R R0~ ~- XH-->Products
Kinetic expressions Vin k1[R] [02] k2[ROs] [RH] k3[ROOR] kx[RO~]x
Here RH is the rubber, and x the concentration of inhibitor. The following system of kinetic equations describes the oxidation process in the induction period; /
d[R] dt
= Viu--kk2 [R02] [RH]q-2~k3 [ROOR]--]cl [R] [02],
d [RO~] dt
--k 1 [R] [02]--k2 [R02] [RH]--flk x [R02] x,
d [ROOR] =k2 [R02] [RH]--k3 [ROOR], dt
(1)
(2) (3)
d [02] d T = k l [R] [02],
(4)
dx dt --(1 -- J) kx [R02] x,
(5)
Where ~ is the coefficient of chain branching (see [2]), fl is the coefficient of inhibition (fl-=2 for full inhibition, fl=0 for full chain transmission), j is the coefficient of inhibitor regeneration (if the inhibitor is completely regenerated in subsequent reactions, then J-->l). By adding equations (1) and (2) we obtain: Vt. + 2yk3 [ROOR]--~kx [R02] x = o.
(6)
1210
KH. S. BAGDABAR'YANand R . I . MILYUTINSKAYA
From equations (2), (4), (5) we obtain dt
=
(1
)dr
Pkx~ +1
)
.
(7)
Integration of equation (7) with the conditions x-=-xq, A 0 2 = 0 leads to an equation connecting the oxygen uptake in the induction period with the consumption of inhibitor A O___~=_fifl [ L ° In x° + l - x ] xo 1 -- g x xd '
(8)
where Lo=k~[RH]/flkxx o is the ratio of the rate of chain growth to the rate of its rupture at the initial inhibitor concentration, that is, L 0 is the initial length of the reaction chain for the given initial inhibitor concentration. I f Lo= 0 then this means, t h a t the oxygen consumption is completely determined by the initiation reaction. In our experiments the duration of the induction p e r i o d was determined as the time to achieve certain threshold value (AO2)ind, after which the rate of oxygen uptake markedly rose. Since small inhibitor concentrations were used, the ratio (AO~)i,d/xo was always considerably greater than unity. Thus if (ziOn)in d were equal to 5 X 10-6 mole [2], the inhibitor concentration would be not greater than 0.1 weight % or 5 x 10-7 mole in 0.1 grams of polymer. Consequently the quantity (AO2)ind/Xo was not less than 10, and in some experiments reached a value of 1000. Since at the end of the induction period x/x o is small [1] (probably of the order 10-~-10-4), then from equation (8) it follows that the requirement (AOz)ind/X0>>1 can be fulfilled only ffone of the two following conditions is fulfilled: L0>> 1 (long reaction chains at the start o f the induction period) or ill(l--g) >> 1 that is 5--->1 (efficient regeneration of the inhibitor). For further analysis is it desirable to find the relationship between the average peroxide molecule lifetime 1/k a and the duration of the induction period r. I n reference [2] it was found t h a t ka=2.2 x 10-2 min -x at 80 ° C, and the activation energy for the decomposition was equal to ~21 kcal/mole. Hence we find that at 100 ° C I/ka=10 rain, whereas the induction periods under our experimental conditions lasted tens and hundreds of minutes [1]; in this way, 1/ka<
(9)
Equation (9) together with equations (5) and (6) gives
dx dt
(1-,~)
p
v,~
1--(2~k~[~]/~k,x)"
(lo)
From equation (10) it follows that with branching chains, induction perioda
1211
Rubber oxidation i n h i b i t o r s - - I I I
can occur only when the denominator in equation (10) is a positive quantity. Consequently, in any case
2yk 2 [RH] = 2yLo < 1.
(11)
]~]CxX0
Since 7 is equal to 0.35-1 [2], then L 0 cannot be considerably greater than rarity. Consequently, the condition (AO2)ina/Xo>>1 can be satisfied (see equation (8)) only if fl/(1--~)>>l, t h a t is ~->1 (effective regeneration of the inhibitor). From equation (9) it follows that as x is reduced the rate of consumption of inhibitor increases and tends to infinity as x~-2~,ke [RH]/fl]Cx:Xcr. The time to achieve the concentration Xcr m a y be considered as the duration of the induction period. By integrating equation (9) from x o to x¢r, we obtain
v oL
Xo
Since the inhibitor concentration is very small at the end of the induction period [1], x¢dx o is not greater than 0.1; hence
~x0 ~=(1-~) V~
(13)
It follows from equation (13) that the large differences in induction periods for the various inhibitors [1] are explained by differences in the quanity fill--(f). From equations (7) and (9), the following expression is obtained for the initial oxidation rate in the induction period
-~-)o=
p~Xo )~ t,
~
).
(14)
The fraction in the righthand part of equation (14) does not differ strongly from unity for typical inhibitors, and tends towards unity at high inhibitor concentrations. Thus the rate of oxygen uptake will tend towards the rate of initiation. * In this way, for high inhibitor concentrations, when the fractions in the enumerator and denominator of equation (14) are small, we obtain the following expression for the initial oxidation rate
~)0=
~ICxXo
A
(15)
Under these conditions very small steady state concentrations of rubber peroxide are to be expected, and this is confirmed by the data of Angert and Kuz'minskii [3], since the reaction of RO e radicals with rubber, leading to the * The possibility of similar processes taking place at high inhibitor concentrations [3] is not discussed by us.
1212
KH. S. BAGDASAR'YAN a n d R. I. 1VhLYUTINSKAYA
formation of peroxide molecules, is strongly suppressed b y the reaction of these radicals with the inhibitor. Let us consider in somewhat more detail the types of reactions which could lead to chain transfer and to regeneration of the inhibitor XH. These reactions are set out in Table 1, and the values of the coefficients ~ and (1--5) are shown for cases when after the reaction I only the reaction in question takes place. TABLE 1, SECONDARY REACTIONS DURING THE INHIBITED OXIDATION OF RUBBER
Reaction II XH+RO~'-+X" +ROOH II X'+C'w->X--CC-- w - II
IV
V
X'+
X'+
*
--C=C--
w-->w
--XC--C'--
w--HC--C=C--w---->
*
XR+--
--
~--C'--C=C--
X ' + wHC--C'-- w-~XH+ w--C=C-- w
~ --
fl
( 1-- g)
2
1 1 0 0
0 0 2
If reactions I I - V take place simultaneously after reaction I, then fl and (1--5) take on intermediate values. Of the two reactions, IV and V, leading to inhibitor regeneration, only reaction V leads to fl/(1--5)>> 1. INVESTIGATION OF INHIBITED OXIDATION IN THE PRESENCE OF OXIDATION INITIATOR
I n this way the characteristic kinetic feature of the rubber oxidation inhibitor is the value fl/(1--5), which is determined not only by the nature of the inhibitor, but also b y the type of rubber and the temperature. If the initiation rate is measured b y an independent method, then the value fl/{ 1--5) m a y be determined from the duration of the induction period according to equation (13). The use of an oxidation initiator, in principle opens up the possibility of simultaneously determining both Vtn, and also fl/(1--~). I n the presence of an oxidation initiator it is possible to obtain an expression for v, if Vln in equation (10) be replaced by Vin-+fkCoe-m where co is the initial initiator concentration, k is the decomposition constant of the initiator, ] is the efficiency of initiation. I f an initiator is selected which almost completely decomposes in the time of the induction period, then instead of equation (13) (13) we obtain ~0 .
z
.
Xo C0 .
(1--5) VIn
.
Vin x0
(16)
The dependence of z on Co/Xo makes it possible to determine the rate of steady state initiation Vin and the absolute value of fl/(1--~). To carry out this investigation a suitable oxidation initiator has to be selected. To do this the decomposition kinetics of several radical initiators were studied in rubber films. Benzene peroxide was unsuitable for the initiation of oxidation of rubber in
Rubber oxidation inhibitors--III
1213
the presence of inhibitors of the amine class, but it could be suitable for certain other classes, since it enters into a molecular reaction with these compounds. The rate of decomposition of the dinitrile of azoisobutyric acid in rubber films which was investigated by the manometric method, turned out to be very small; the decomposition constants at 80 and 100°C were equal to 0.47× 10-s and 3.3 × 10-5 sec -1, while in solution in benzene these constants were equal to 16× 10-~ and 160× 10-5 sec-h This initiator thus turned out to be unsuitable for our purpose. An a t t e m p t to use methylphenyltriazene as an oxidation initiator also turned out to be unsuccessful, since oxidation in the presence of this compound took place with a large induction period. Phenylazotriphenylmethane (PATM) is free of the defects mentioned. This compound is prepared following the method given in the literature [4]. The de-
x
8O0
60O
4OO
200
1
0
0.50
i
I.OA
Dependence of induction period (v) at 70°C on the molar ratio (A) initiator : inhibitor. Initiator, phenylazotriphenyl methane: 1--phenyl-fl-naphthylamine, c0=7.3× × 10-5 mole/1.; 2--phenothiazine, %= 7.1 × 10-~ mole/1. composition rate constant in rubber film, determined manometrically at 70°C, turned out to be equal to 6 × 10-4 sec -1 (time for half decomposition, 20 rain). One had to be satisfied t h a t the decomposition of P A T ~ in rubber film was not accompanied by any considerable cellular effect. For this purpose, the complete decomposition of PATM (4.9× 10-5 mole) was investigated in vacuum in the presence of diphenylpicrylhydrazyl (9.8 × 10-5 mole), which was introduced into the rubber film by the usual method. After the complete breakdown of the PATM, the diphenylpicrylhydrazyl turned out to be almost completely used up. Thus the radicals formed during the decomposition of PAT~[ in rubber films enter completely into radical reactions, t h a t is the cellular effect is practically
1214
K H . S. BAGDASAR'YAN a n d R. I. MILYUTINSKAYA
absent in this case. Rubber oxidation initiated by PATI~I takes place without an induction period and leads to a reproduceable steady state rate. Data are presented in Table 2 about the dependence of the duration of the induction period on the inhibitor concentration. TABLE 2. INDUCTION PERIOD IN THE OXIDATION OF RUBBER UNDER THE JOINT ACTION OF AN INITIATOR (PHENYLAZOTRIPHENYLMETHANE) AND INHIBITOR
Inhibitor
Concentration, mole/1. × 10-5
Co
2", 1Tlin
X0
Phenyl-fl-naphthylamine Ditto Phenothiazine
inhibitor
initiator
7"30 6"77 7.03 6"95 0.071 0"066 0"081
0 1'03 1.90 4.50 0 0'018 0"036
0 0.15 0.27 0.65 0 0.23 0.45
790 665 465 160 830 620 405
The time for half decomposition of the initiator is considerably less than ~. The dependence of tor on Co/Xo is shown in the Figure for two inhibitors, which differ in their effectiveness b y almost a factor of 100. Used in concentrations differing by a factor of 100 t h e y show an identical induction period amounting to approximately 800 min. Both straight lines have the same slope and intersect the abscissa axis at co/x o ~ 1. The results obtained does not agree with equation (16) from which it should follow t h a t the slope ought to be proportional to x 0. Agreement with experiment is obtained if one postulates t h a t the radicals formed from the initiator react directly with inhibitor molecules, reducing their number in the rubber by the number of initiator molecules introduced. I n this case x 0 in equation (13) ought to be replaced by x o - - c o. We then obtain
T - ( 1 - ~ ) Vi.\ which agrees with the experiment. I n this way, the initiator used did not make it possible to determine separately Vin and/?/(1--5). CONCLUSIONS
Investigation of the inhibited oxidation of sodium butadiene rubber at 70100°C led to the following conclusions: I) the duration of the induction period for different inhibitors differs b y a factor of not less t h a n 100; 2) the amount of oxygen absorbed in the induction period is 100-1000 times greater thaa~ the q u a n t i t y of inhibitor introduced.
Polymers with conjugated double b o n d s - - I I
1215
T h e kinetics of the i n h i b i t e d o x i d a t i o n of r u b b e r h a s b e e n discussed theoretically for t h e case of p r a c t i c a l interest, n a m e l y 1/ka
,~:ote added in proof. The conclusion about efficient inhibitor regeneration is based on the assumption that the true amount of oxygen absorbed in the induction period approximates to the threshold value z/(O2)ind, observed in experiment. I f the first quantity is considerably less than the second, it is necessary to determine the absolute value fl/(1--J) to explain the causes of different inhibitor efficiencies. Translated by G. MODLE.~" REFERENCES
1. Z. A. SINITSYNA and Kh. S. BAGDASAR'YAN, Sb: Khimicheskiye svoistva i modifikatsiya polimerov, Izd. Akad. Nauk SSSR, p. 272, 1964 2. Kh. S. BAGDASAR'YAN, Z. A. SINITSYNA and P. I. MILYUTINSKAYA, ibid, p. 265 3. L. G. ANGERT and A. S. KUZMINSKII, J. Polymer Sci. 32: 1, 1958 4. H. WIELAND, H. HOVE and K. B(iRNER, Liebigs Ann. Chem., 446, 31, 1925
ELECTRICAL AND MAGNETIC PROPERTIES OF POLYMERS WITH CONJUGATED DOUBLE BONDSmII. FLUORESCENCE OF POLYPHENYLACETYLENES * V. A. B E N D E R S K I I a n d P. A. STUNZHAS Institute for Chemical Physics, U.S.S.R. Academy of Sciences
(Received 18 July 1963) I N THE first c o m m u n i c a t i o n [1] t h e electrical c o n d u c t a n c e a n d electron spin r e s o n a n c e s p e c t r a of the f r a c t i o n a t i o n p r o d u c t s of p o l y p h e n y l a c e t y t e n e w i t h m o l e c u l a r weights (M) f r o m 590 u p to 1870 were i n v e s t i g a t e d . I t was s h o w n * Vysokomol. soyed. 6: No. 6, 1104-Ili0, 1964.