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Retardation of polymer oxidation by natural antioxidant gossypol: Part 1—Kinetics of oxygen consumption
Polymer Degradation and Stability 35 (1992) 141-146
Retardation of polymer oxidation by natural antioxidant gossypol: Part 1--Kinetics of oxygen consumption A. P. Mar'in, Yu. A. Shlyapnikov Institute of Chemical Physics, USSR Academy of Sciences, 117334, Moscow, USSR
E. S. Mametov & A. T. Dzhalilov Polytechnical Institute, 700011, Tashkent, USSR (Received 17 October 1990; accepted 3 December 1990)
The kinetics of oxygen consumption in the induction period of oxidation of polypropylene, polyethylene, and polyamide-12 has been studied. It has been shown that the dependence of maximum oxygen consumption rate on concentration is described by a curve with its minimum in the region of low gossypol concentrations. The effectivity of gossypol as an antioxidant is reduced in the polyethylene, polypropylene, and polyamide-12 series.
INTRODUCTION
been ascribed to the fact that gossypol and its derivatives can form strong chelate complexes with metal ions. Gossypol retards thermal and thermooxidative degradation of PVC and it inhibits dehydrochlorination in air more effectively than diphenylolpropane and 2,6-ditert-butyl-4-methylphenol2 In plasticized PVC gossypol also protects plasticizer from oxidation. 5 Oxygen consumption rate is an important characteristic of polymer oxidation. Yet there is very little information on oxygen consumption kinetics in the course of inhibited polymer oxidation. In Ref. 6 the authors have studied the kinetics of oxygen consumption by polyethylene in the presence of 2,2'-methylene-bis-(4-methyl6-tert-butylphenol) (MBP). They also have demonstrated that the curve showing the dependence of oxidation rate on antioxidant concentration passes through a minimum as antioxidant concentrations are close to critical ones. In this work we have studied the influence of gossypol as a natural antioxidant on the kinetics of oxygen consumption in the course of oxidation of polyethylene, polypropylene, and polyamide-12.
Gossypol (2,2'-di-(8-formyl-l,6,7-tri-hydroxy-5isopropyl-3-methyl-naphthyl)) H--C-~O OH
HO O-~C--H j H~C
C~H7
~
~
O
H H
C3H7
is a component of cotton-seed oil and is separated by purification as a by-product.1 It is known that gossypol is an inhibitor of radical polymerization2 and that it inhibits oxidation of various organic compounds. 1,3-5 The stabilizing effect of gossypol and gossypol anthranilate in combination with phenyl-flnaphthylamine on oxidation of synthetic rubber has been studied. 4 An interesting phenomenon has been revealed: the inhibiting effect of gossypol was increased when the polymers contained metals of variable valency. This has Polymer Degradation and Stability 0141-3910/91/$03-50 © 1991 Elsevier Science Publishers Ltd. 141
142
A . P . Mar'in et al.
EXPERIMENTAL
In this work isotactic polypropylene, low-density polyethylene, polyamide-12, and gossypol with a melting point of 178-180°C were used. The polymer powder was mixed with a certain amount of gossypol and alcohol. The mixture was dried in air and films were prepared by pressing in an inert atmosphere at 150°C for polyethylene, and 200°C for polypropylene and polyamide-12. Oxygen consumption was followed by studying the pressure change, volatile products being removed by solid potassium hydroxide. The volume of the reaction system was 12 c m 3, the polymer sample weighed 50 mg.
6
¢uI
7
01
E un
o x
RESULTS A N D DISCUSSION Figure 1 shows the kinetics of oxygen consumption by polypropylene in the presence of gossypol of different concentrations. As is seen from Fig. 1 the oxygen consumption kinetic curves have a complicated shape which is particularly notable with large concentrations of gossypol. At the beginning of oxidation the oxygen consumption rate increases, passes through a maximum, then decreases and sharply jumps at the end of oxidation. In this case the time corresponding to the beginning of fast oxidation (induction period) is increased with gossypol initial concentration. The dependence of the maximum oxygen consumption rate in the course of the induction
020 06 ~
l f
1
l 2
3
o /
0.12
~0.08 z
o
0"04
0
I
0"08 EGossypol]
i 0"10 ( t o o l kg -1)
Fig. 2. Dependence of maximum rate of oxygen consumption in the induction period of polypropylene oxidation on gossypol concentration at (1) temperature = 160°C, and (2) temperature = 200°C at Po2= 300 mm Hg. period on gossypol initial concentration is described by a curve with its minimum being in the region of 0 . 0 4 m o l k g -1 of gossypol. With large gossypol concentrations the rate is increased approximately in direct proportion to the antioxidant concentration in the polymer (Fig. 2). A similar phenomenon with polyethylene oxidation in the presence of MBP has been observed. 6 The picture observed may be explained by the fact that the recorded rate, Wo:, is equal to a sum of oxygen consumption of oxidation of the polymer itself, V¢~, and of direct oxidation of the inhibitor, WE, i.e. Wo~= W1 + W2. To estimate the value of W1 we postulate that the rate of formation of RO2 radicals is equal to their consumption rates. kl[R'][O2] = k2[RO2][RH]
I 200
I 400
I 600
Time (rain)
Fig. 1. Oxygen consumption curves of oxidation of polypropylene containing (1) 0-005, (2) 0.05, (3) 0.075, and
(4) 0.2 too! kg-1 of gossypol at temperature = 20ff'C and Po~= 300 mm Hg.
(1)
Neglecting the initiation rate in the absence of the inhibitor we have V¢~= kE[RO2][RH] If oxidation of one molecule of the inhibitor involves consumption of e oxygen molecules the
Retardation of polymer oxidation by gossypol: Part 1
oxygen consumption rate resulting from the inhibitor oxidation is W2 = ekoi x i x [O2]. The overall oxygen consumption rate is (i = [In]) Wo2 = ekoi x [O2]i + kE[ROE][RH]
According to Ref. concentration is
6
the
ROE
(2)
stationary
f~ko,[O2]i [ROE] = (1 - o ) k 3 i - a:(rk2[mH]
(3)
where f~, or, and cr are the yields of radicals in reactions (IH + O2), (RO2 + IH), and ( R O O H + RH) respectively, k3 is (ROE + IH) reaction rate constant. Equation (3) is meaningful if its denominator is positive, i.e. if i > i , where icr is the antioxidant critical concentration, equal to icr- teok2[RH] (1 - tr)k3
(4)
Substituting eqn (3) into eqn (2) and taking into account eqn (4) we have = ~ k2f[RH] e} ko,[O2]i Wo2 [k3(1 - o ) ( i - icr) +
(5)
Equation (5) describes a curve with a minimum. In this case, if the value of i is sufficiently large the oxygen consumption rate is directly proportional to the inhibitor concentration. Extrapolation of the straight part of the curve to the ordinate gives the minimum rate of oxygen consumption by the polymer itself. WI =
k2f~ko,[RH][O2]
k3(1
-
o)
143
properties are also observed although these changes are much slower. These changes may also lead to undesired consequences. In particular, a change in the polymer molecular mass in the induction period may lead to a change in polymer characteristics during its processing. The induction period is frequently understood as the time from the start of reaction to the moment when the measured property is changed to a certain value. In our case, polymer oxidation may also be characterized by an apparent induction period, 'l~'app that corresponds to the time when a certain small amount of oxygen (0.025molkg -1) is absorbed. The complete or true induction period, r, that corresponds to the time from the beginning of polymer oxidation to the moment when the reaction passes to the fast stage is, as a rule, greater or equal to rapp. The dependence of polypropylene oxidation induction period, determined in these two ways, on gossypol initial concentration is shown in Fig. 3. If the gossypol concentration is low the apparent and true induction periods are close to each other. The gossypol critical concentration (i.e. the one beyond which the induction period is rapidly increased) does not depend on the way in which the induction period is determined, the critical concentration being 3.5 × l 0 -3 mol kg -1 at 200°C. With high inhibitor concentrations the induction periods determined in various ways differ. The dependence of apparent induction period on gossypol concentration is described by a curve 600
n
=a
The slope of the straight line is koi x e x [02]. The value a is calculated on the basis of data shown in Fig. 2 and in this case it is 2 x 10-6mol kg -~ s -1, that is much greater than the value for polyethylene oxidation in the presence of MBP, one of the most efficient antioxidants: (a = 6 x 10 .7 mol kg -1 s-l). The difference may be explained by greater reactivity of gossypol towards molecular oxygen (i.e. by larger values of koi and f~) or by a lesser value of the rate constant k3. It is known that when the induction period is finished crucial changes in many properties of a polymer (molecular mass, strength, etc.) are observed and the sample breaks down. Within the induction period changes in the polymer
400
40 o
/
0
o' I
f
0.1
0.2
[Gossypol] (rnol kg -I) Fig. 3. Dependence of (1) apparent and (2) true induction periods of polypropylene oxidation on gossypol concentration at temperature = 200°C and Po2 = 300 m m Hg.
144
A. P. Mar'in et al. 600
D
400 .c_ E
2OO
0
I 2
I 3
4 + log i 0
Fig. 4. Dependence of induction period of polypropylene oxidation on gossypol concentration in r - log io coordinates at temperature = 200°C and Poe = 300 mm Hg.
with its maximum in the region of 0-070 . 1 m o l k g - L The polypropylene oxidation induction period is increased monotonously with antioxidant concentration, and, if the concentration is below the critical concentration the induction period is increased in direct proportion to log io (Fig. 4), that is the dependence of • on io may be described by the following equation: = ~cr + k~-~0.434
log(io/i,)
(6)
where ke, is the effective rate constant for antioxidant consumption ~'cr= ~"with io = icr.
Equation (6) was derived assuming that the antioxidant is consumed according to the first order law. 7 One of the reasons for the maximum in the ~'app- io curve is gossypol oxidation by oxygen and consequently an increase in the total oxygen consumption rate in the induction period with an increase in the initial gossypol concentration. The induction periods of polypropylene oxidation are increased in the presence of gossypol with decreasing temperature. In this case maxima on the l~app - - i o curves are preserved. Gossypol critical concentration is also changed with temperature. For example, it is 1 . 5 x 10 -3 mol kg -1 at 180°C and 2.2 x 10 -3 at 190°C. The activation energy of this process is 84 + 4 kJ mo1-1. On oxidation of a solid polymer the dependence of the oxygen consumption maximum rate on gossypol concentration is similar to that for a molten polymer. The rate of oxygen consumption by polymer, W1 is 7 x 10 -5 mol kg -~ s -~ at 160°C, and the activation energy of the process is 35 + 5 kJ mol -~ within 160-200°C. It is of interest to compare effectiveness of antioxidants of various molar mass in polypropylene (Table 1). For MBP, 2,2'methylene-bis (4,6-di-tert-butylphenol), gossypol, and the ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid and pentaerythritol an increase in the antioxidant molar mass results in an increase in critical concentration and a reduction in effectiveness and in the rate constant of its consumption.
Table 1. Parameters of polypropylene oxidation in the presence of various phenofic antioxidants ° Antioxidant 2,2'-Methylene-bis(4-methyl -6-tert-butylphenol)
Molecular mass
icr X 10 3 (mol k g - 1)
kc, × 10 4 (s- t)
Ref.
342
1"2
1"9
11
di-tert-butylphenol )
426
2"0
1"0b
12
2, 2'-Di-(8-formyl-l,6,7tri-hydroxy-isopropyl-3 -methyl-naphthyl)
518
3"5
1"0b
The ester of 3,5-di-tertbutyl-4-hydroxyphenylpropionic acid and penthaerythritol
1178
6"5
0"44 b
2,2'-Methylene-bis(4,6-
° Temperature = 200°C, Po2 = 300 mm Hg. b Calculated by eqn (6).
Retardation of polymer oxidation by gossypol: Part 1 According to eqn (4), icr depends on the values of o, k2 and k3. Let us assume that o is constant, i.e. the antioxidants almost equally influence the probability of degenerate chain branching. As k2 is a polymer characteristic, its value does not d e p e n d on either the nature or the concentration of antioxidant. Thus, a decrease in k3 must be a main reason for an increase in critical concentration with an increase in the antioxidant molecular mass. This may be due to a reduction of the antioxidant diffusion coefficient or to immobilization of antioxidants that may lead to a reduction in the n u m b e r of collisions of mobile molecules with immobile macroradicals, RO~. The value of k3 may also be influenced by a specific character of the distribution of antioxidant and oxygen in the polymer. According to the polymer zone model a polymer both in solid and molten state contains a set of zones of short range order violation, Z. 8'9 Oxygen, antioxidants and other low-molecular compounds are present predominantly in these zones and oxidation reactions also take place there. The population of zones where c o m p o u n d s of m e d i u m molecular mass, Z, can be sorbed is rapidly reduced with molecular mass (or molecule size), M, according to the following law: 10 [Z] = [Z]o e x p ( - 9)M)
(7)
where q~ is a constant. In a polymer there is a certain concentration of zones where oxygen molecules may occur, but the antioxidant molecules cannot be present. A n increase in the molar mass of the antioxidant leads to an increase in the fraction of zones inaccessible to it.
145
The increase in the fraction of zones where the chain termination cannot proceed will result in an increase in the length of the kinetic oxidation chain. It may also be considered as a reduction in the rate constant of the chain termination k3. At the same time direct oxidation of the inhibitor may occur only if in the same zone both the antioxidant molecule and at least one oxygen molecule are present. Figure 5 illustrates how the concentration of centres absorbing antioxidants (curve 1) and both antioxidant and oxygen (curve 2) calculated by eqn (7) depends on the antioxidant molecular mass. Let us assume that the polymer contains antioxidant of some constant concentration (straight line 3 of Fig. 5). By changing the molar mass of the antioxidant, that is by moving from left to right along the straight line, one will come to the AB region of Fig. 5 where antioxidant and oxygen may be present simultaneously. Concentration of such zone is reduced with an increase in the antioxidant molecular mass that leads inevitably to a reduction in the rate constant of antioxidant consumption. 6 kef~ = koi x (1 + const) The antioxidant concentration to the right of the B point exceeds the value of Z, i.e. the upper limit of solubility of the antioxidant, and the value of kett is independent of the antioxidant molecular mass. Thus, an observed dependence of effectiveness of phenolic antioxidants on molecular mass is in good agreement with our assumption. Let us compare effectiveness of gossypol in different polymers (Table 2). Gossypol in small concentrations most effectively retards oxidation of polyethylene while oxidation of polypropylene and polyamide-
0"014
Table 2. Parameters of polymer oxidation in the presence of gossypol*
2
A
Polymer ~ o.006
ic, x 103 (mol kg 1)
W !
I
i
400
500
600
Gossypoi concentration (mol kg -1)
I
700
M o l e c u l a r mass
Fig. 5. Dependence of concentration of centres antioxidant, and (2) both antioxidant and molecular mass of antioxidant calculated by Ref. 9. (3) Represents a certain constant concentration.
Time corresponding to absorption of 0.025 mol kg-' of oxygen (~app)
sorbing (1) oxygen on data from antioxidant
Polypropylene Polyethylene Polyamide-12
3-5 0.5 10
0.02
0.1
72 200 40
94 100 83
a Temperature -----200°C, Po2 = 300 mm Hg.
146
A . P. Mar'in et al.
12 are not so effectively retarded. This is seen from the values of critical concentrations and induction periods, ~'app. This difference m a y be due to different rate constants for chain propagation, k2, and chain termination, k3, in these polymers. If the gossypol content exceeds 0.1 mol kg-1 the difference in induction periods a m o n g these polymers disappears. In contrast to polyethylene containing only - C H ~ groups, the m o r e reactive tertiary carbon atoms present in polypropylene must lead to an increase in k2.This accounts for the increase in critical concentration of gossypol in transition from polyethylene to polypropylene. Polyamide contains the same --CH2 groups as polyethylene but the presence in polyamide of the polar - N H C O - group, capable of forming complexes with phenol must lead to a reduction in k3 and to an increase in the critical concentration of antioxidant. Thus, the behaviour of gossypol as a natural antioxidant in polymers is similar to that of o t h e r phenolic antioxidants.
REFERENCES 1. Markman, A. M. & Rzhekhin, V. P., Gossipol i yego proizvodnye (Gossypol and its Derivatives). Food Industry, Moscow, 1965. 2. Ismailov, I., Fatkhulayev, E., Karimova, M. M. & Kapitulla, I. I. Doil. An Izv. SSSR, (Proceeding of Oil & Fat Institute, USSR) U (1981) 36. 3. Slazina, G. A., Rzhekhin, V. P. & Goryacheva, N. Trudy VNIIZh, (Reports Academy of Sciences of Uzbek Republic, USSR) 25 (1965) 439. 4. Piotrovskii, K. B., Gromova, G. N., Ivanova, L. M. & Goldberg, A. O. Kauchuk i rezina (Natural and Synthetic Rubber), 3 (1975) 33. 5. Minsker, K. S., Abdullin, M. I. & Rakhimov, I. Plastmassy (Plastics), 5 (1987) 42. 6. Shlyapnikov, Yu.A., Kiryushkin, S. G., Tyuleneva, N. K.,:~.Iring, M. & Fodor, Zs. Polym. Bull., 19 (1988) 449. 7. Shlyapnikov, Yu.A. Develop. Polym. Stab., 5 (1981) 1. 8. Shlyapnikov, Yu.A. Makromol. Chem. (Macromol. Symp.), 27 (1989) 121. 9. Shlyapnikov, Yu.A. & Mar'in, A. P. Eur. Polym. J., 23 (1987) 623. 10. Gedraitite, G. B., Mar'in, A. P. & Shlyapnikov, Yu.A. Eur. Polym. J., 25 (1989) 39. 11. Shlyapnikov, Yu.A., Miller, V. B. & Torsuyeva, E. S. Izv. A N SSSR (Chem. Ser.,) 11 (1961) 1966. 12. Vasileyskaya, N. S., Livanova, N. M., Miller, V. B., Samarina, L. V. & Shlyapnikov, Yu.A. Izv. A N SSSR (Chem. Ser.,) 11 (1972) 2614.
Retardation of polymer oxidation by natural antioxidant gossypol: Part 1--Kinetics of oxygen consumption A. P. Mar'in, Yu. A. Shlyapnikov Institute of Chemical Physics, USSR Academy of Sciences, 117334, Moscow, USSR
E. S. Mametov & A. T. Dzhalilov Polytechnical Institute, 700011, Tashkent, USSR (Received 17 October 1990; accepted 3 December 1990)
The kinetics of oxygen consumption in the induction period of oxidation of polypropylene, polyethylene, and polyamide-12 has been studied. It has been shown that the dependence of maximum oxygen consumption rate on concentration is described by a curve with its minimum in the region of low gossypol concentrations. The effectivity of gossypol as an antioxidant is reduced in the polyethylene, polypropylene, and polyamide-12 series.
INTRODUCTION
been ascribed to the fact that gossypol and its derivatives can form strong chelate complexes with metal ions. Gossypol retards thermal and thermooxidative degradation of PVC and it inhibits dehydrochlorination in air more effectively than diphenylolpropane and 2,6-ditert-butyl-4-methylphenol2 In plasticized PVC gossypol also protects plasticizer from oxidation. 5 Oxygen consumption rate is an important characteristic of polymer oxidation. Yet there is very little information on oxygen consumption kinetics in the course of inhibited polymer oxidation. In Ref. 6 the authors have studied the kinetics of oxygen consumption by polyethylene in the presence of 2,2'-methylene-bis-(4-methyl6-tert-butylphenol) (MBP). They also have demonstrated that the curve showing the dependence of oxidation rate on antioxidant concentration passes through a minimum as antioxidant concentrations are close to critical ones. In this work we have studied the influence of gossypol as a natural antioxidant on the kinetics of oxygen consumption in the course of oxidation of polyethylene, polypropylene, and polyamide-12.
Gossypol (2,2'-di-(8-formyl-l,6,7-tri-hydroxy-5isopropyl-3-methyl-naphthyl)) H--C-~O OH
HO O-~C--H j H~C
C~H7
~
~
O
H H
C3H7
is a component of cotton-seed oil and is separated by purification as a by-product.1 It is known that gossypol is an inhibitor of radical polymerization2 and that it inhibits oxidation of various organic compounds. 1,3-5 The stabilizing effect of gossypol and gossypol anthranilate in combination with phenyl-flnaphthylamine on oxidation of synthetic rubber has been studied. 4 An interesting phenomenon has been revealed: the inhibiting effect of gossypol was increased when the polymers contained metals of variable valency. This has Polymer Degradation and Stability 0141-3910/91/$03-50 © 1991 Elsevier Science Publishers Ltd. 141
142
A . P . Mar'in et al.
EXPERIMENTAL
In this work isotactic polypropylene, low-density polyethylene, polyamide-12, and gossypol with a melting point of 178-180°C were used. The polymer powder was mixed with a certain amount of gossypol and alcohol. The mixture was dried in air and films were prepared by pressing in an inert atmosphere at 150°C for polyethylene, and 200°C for polypropylene and polyamide-12. Oxygen consumption was followed by studying the pressure change, volatile products being removed by solid potassium hydroxide. The volume of the reaction system was 12 c m 3, the polymer sample weighed 50 mg.
6
¢uI
7
01
E un
o x
RESULTS A N D DISCUSSION Figure 1 shows the kinetics of oxygen consumption by polypropylene in the presence of gossypol of different concentrations. As is seen from Fig. 1 the oxygen consumption kinetic curves have a complicated shape which is particularly notable with large concentrations of gossypol. At the beginning of oxidation the oxygen consumption rate increases, passes through a maximum, then decreases and sharply jumps at the end of oxidation. In this case the time corresponding to the beginning of fast oxidation (induction period) is increased with gossypol initial concentration. The dependence of the maximum oxygen consumption rate in the course of the induction
020 06 ~
l f
1
l 2
3
o /
0.12
~0.08 z
o
0"04
0
I
0"08 EGossypol]
i 0"10 ( t o o l kg -1)
Fig. 2. Dependence of maximum rate of oxygen consumption in the induction period of polypropylene oxidation on gossypol concentration at (1) temperature = 160°C, and (2) temperature = 200°C at Po2= 300 mm Hg. period on gossypol initial concentration is described by a curve with its minimum being in the region of 0 . 0 4 m o l k g -1 of gossypol. With large gossypol concentrations the rate is increased approximately in direct proportion to the antioxidant concentration in the polymer (Fig. 2). A similar phenomenon with polyethylene oxidation in the presence of MBP has been observed. 6 The picture observed may be explained by the fact that the recorded rate, Wo:, is equal to a sum of oxygen consumption of oxidation of the polymer itself, V¢~, and of direct oxidation of the inhibitor, WE, i.e. Wo~= W1 + W2. To estimate the value of W1 we postulate that the rate of formation of RO2 radicals is equal to their consumption rates. kl[R'][O2] = k2[RO2][RH]
I 200
I 400
I 600
Time (rain)
Fig. 1. Oxygen consumption curves of oxidation of polypropylene containing (1) 0-005, (2) 0.05, (3) 0.075, and
(4) 0.2 too! kg-1 of gossypol at temperature = 20ff'C and Po~= 300 mm Hg.
(1)
Neglecting the initiation rate in the absence of the inhibitor we have V¢~= kE[RO2][RH] If oxidation of one molecule of the inhibitor involves consumption of e oxygen molecules the
Retardation of polymer oxidation by gossypol: Part 1
oxygen consumption rate resulting from the inhibitor oxidation is W2 = ekoi x i x [O2]. The overall oxygen consumption rate is (i = [In]) Wo2 = ekoi x [O2]i + kE[ROE][RH]
According to Ref. concentration is
6
the
ROE
(2)
stationary
f~ko,[O2]i [ROE] = (1 - o ) k 3 i - a:(rk2[mH]
(3)
where f~, or, and cr are the yields of radicals in reactions (IH + O2), (RO2 + IH), and ( R O O H + RH) respectively, k3 is (ROE + IH) reaction rate constant. Equation (3) is meaningful if its denominator is positive, i.e. if i > i , where icr is the antioxidant critical concentration, equal to icr- teok2[RH] (1 - tr)k3
(4)
Substituting eqn (3) into eqn (2) and taking into account eqn (4) we have = ~ k2f[RH] e} ko,[O2]i Wo2 [k3(1 - o ) ( i - icr) +
(5)
Equation (5) describes a curve with a minimum. In this case, if the value of i is sufficiently large the oxygen consumption rate is directly proportional to the inhibitor concentration. Extrapolation of the straight part of the curve to the ordinate gives the minimum rate of oxygen consumption by the polymer itself. WI =
k2f~ko,[RH][O2]
k3(1
-
o)
143
properties are also observed although these changes are much slower. These changes may also lead to undesired consequences. In particular, a change in the polymer molecular mass in the induction period may lead to a change in polymer characteristics during its processing. The induction period is frequently understood as the time from the start of reaction to the moment when the measured property is changed to a certain value. In our case, polymer oxidation may also be characterized by an apparent induction period, 'l~'app that corresponds to the time when a certain small amount of oxygen (0.025molkg -1) is absorbed. The complete or true induction period, r, that corresponds to the time from the beginning of polymer oxidation to the moment when the reaction passes to the fast stage is, as a rule, greater or equal to rapp. The dependence of polypropylene oxidation induction period, determined in these two ways, on gossypol initial concentration is shown in Fig. 3. If the gossypol concentration is low the apparent and true induction periods are close to each other. The gossypol critical concentration (i.e. the one beyond which the induction period is rapidly increased) does not depend on the way in which the induction period is determined, the critical concentration being 3.5 × l 0 -3 mol kg -1 at 200°C. With high inhibitor concentrations the induction periods determined in various ways differ. The dependence of apparent induction period on gossypol concentration is described by a curve 600
n
=a
The slope of the straight line is koi x e x [02]. The value a is calculated on the basis of data shown in Fig. 2 and in this case it is 2 x 10-6mol kg -~ s -1, that is much greater than the value for polyethylene oxidation in the presence of MBP, one of the most efficient antioxidants: (a = 6 x 10 .7 mol kg -1 s-l). The difference may be explained by greater reactivity of gossypol towards molecular oxygen (i.e. by larger values of koi and f~) or by a lesser value of the rate constant k3. It is known that when the induction period is finished crucial changes in many properties of a polymer (molecular mass, strength, etc.) are observed and the sample breaks down. Within the induction period changes in the polymer
400
40 o
/
0
o' I
f
0.1
0.2
[Gossypol] (rnol kg -I) Fig. 3. Dependence of (1) apparent and (2) true induction periods of polypropylene oxidation on gossypol concentration at temperature = 200°C and Po2 = 300 m m Hg.
144
A. P. Mar'in et al. 600
D
400 .c_ E
2OO
0
I 2
I 3
4 + log i 0
Fig. 4. Dependence of induction period of polypropylene oxidation on gossypol concentration in r - log io coordinates at temperature = 200°C and Poe = 300 mm Hg.
with its maximum in the region of 0-070 . 1 m o l k g - L The polypropylene oxidation induction period is increased monotonously with antioxidant concentration, and, if the concentration is below the critical concentration the induction period is increased in direct proportion to log io (Fig. 4), that is the dependence of • on io may be described by the following equation: = ~cr + k~-~0.434
log(io/i,)
(6)
where ke, is the effective rate constant for antioxidant consumption ~'cr= ~"with io = icr.
Equation (6) was derived assuming that the antioxidant is consumed according to the first order law. 7 One of the reasons for the maximum in the ~'app- io curve is gossypol oxidation by oxygen and consequently an increase in the total oxygen consumption rate in the induction period with an increase in the initial gossypol concentration. The induction periods of polypropylene oxidation are increased in the presence of gossypol with decreasing temperature. In this case maxima on the l~app - - i o curves are preserved. Gossypol critical concentration is also changed with temperature. For example, it is 1 . 5 x 10 -3 mol kg -1 at 180°C and 2.2 x 10 -3 at 190°C. The activation energy of this process is 84 + 4 kJ mo1-1. On oxidation of a solid polymer the dependence of the oxygen consumption maximum rate on gossypol concentration is similar to that for a molten polymer. The rate of oxygen consumption by polymer, W1 is 7 x 10 -5 mol kg -~ s -~ at 160°C, and the activation energy of the process is 35 + 5 kJ mol -~ within 160-200°C. It is of interest to compare effectiveness of antioxidants of various molar mass in polypropylene (Table 1). For MBP, 2,2'methylene-bis (4,6-di-tert-butylphenol), gossypol, and the ester of 3,5-di-tert-butyl-4-hydroxyphenylpropionic acid and pentaerythritol an increase in the antioxidant molar mass results in an increase in critical concentration and a reduction in effectiveness and in the rate constant of its consumption.
Table 1. Parameters of polypropylene oxidation in the presence of various phenofic antioxidants ° Antioxidant 2,2'-Methylene-bis(4-methyl -6-tert-butylphenol)
Molecular mass
icr X 10 3 (mol k g - 1)
kc, × 10 4 (s- t)
Ref.
342
1"2
1"9
11
di-tert-butylphenol )
426
2"0
1"0b
12
2, 2'-Di-(8-formyl-l,6,7tri-hydroxy-isopropyl-3 -methyl-naphthyl)
518
3"5
1"0b
The ester of 3,5-di-tertbutyl-4-hydroxyphenylpropionic acid and penthaerythritol
1178
6"5
0"44 b
2,2'-Methylene-bis(4,6-
° Temperature = 200°C, Po2 = 300 mm Hg. b Calculated by eqn (6).
Retardation of polymer oxidation by gossypol: Part 1 According to eqn (4), icr depends on the values of o, k2 and k3. Let us assume that o is constant, i.e. the antioxidants almost equally influence the probability of degenerate chain branching. As k2 is a polymer characteristic, its value does not d e p e n d on either the nature or the concentration of antioxidant. Thus, a decrease in k3 must be a main reason for an increase in critical concentration with an increase in the antioxidant molecular mass. This may be due to a reduction of the antioxidant diffusion coefficient or to immobilization of antioxidants that may lead to a reduction in the n u m b e r of collisions of mobile molecules with immobile macroradicals, RO~. The value of k3 may also be influenced by a specific character of the distribution of antioxidant and oxygen in the polymer. According to the polymer zone model a polymer both in solid and molten state contains a set of zones of short range order violation, Z. 8'9 Oxygen, antioxidants and other low-molecular compounds are present predominantly in these zones and oxidation reactions also take place there. The population of zones where c o m p o u n d s of m e d i u m molecular mass, Z, can be sorbed is rapidly reduced with molecular mass (or molecule size), M, according to the following law: 10 [Z] = [Z]o e x p ( - 9)M)
(7)
where q~ is a constant. In a polymer there is a certain concentration of zones where oxygen molecules may occur, but the antioxidant molecules cannot be present. A n increase in the molar mass of the antioxidant leads to an increase in the fraction of zones inaccessible to it.
145
The increase in the fraction of zones where the chain termination cannot proceed will result in an increase in the length of the kinetic oxidation chain. It may also be considered as a reduction in the rate constant of the chain termination k3. At the same time direct oxidation of the inhibitor may occur only if in the same zone both the antioxidant molecule and at least one oxygen molecule are present. Figure 5 illustrates how the concentration of centres absorbing antioxidants (curve 1) and both antioxidant and oxygen (curve 2) calculated by eqn (7) depends on the antioxidant molecular mass. Let us assume that the polymer contains antioxidant of some constant concentration (straight line 3 of Fig. 5). By changing the molar mass of the antioxidant, that is by moving from left to right along the straight line, one will come to the AB region of Fig. 5 where antioxidant and oxygen may be present simultaneously. Concentration of such zone is reduced with an increase in the antioxidant molecular mass that leads inevitably to a reduction in the rate constant of antioxidant consumption. 6 kef~ = koi x (1 + const) The antioxidant concentration to the right of the B point exceeds the value of Z, i.e. the upper limit of solubility of the antioxidant, and the value of kett is independent of the antioxidant molecular mass. Thus, an observed dependence of effectiveness of phenolic antioxidants on molecular mass is in good agreement with our assumption. Let us compare effectiveness of gossypol in different polymers (Table 2). Gossypol in small concentrations most effectively retards oxidation of polyethylene while oxidation of polypropylene and polyamide-
0"014
Table 2. Parameters of polymer oxidation in the presence of gossypol*
2
A
Polymer ~ o.006
ic, x 103 (mol kg 1)
W !
I
i
400
500
600
Gossypoi concentration (mol kg -1)
I
700
M o l e c u l a r mass
Fig. 5. Dependence of concentration of centres antioxidant, and (2) both antioxidant and molecular mass of antioxidant calculated by Ref. 9. (3) Represents a certain constant concentration.
Time corresponding to absorption of 0.025 mol kg-' of oxygen (~app)
sorbing (1) oxygen on data from antioxidant
Polypropylene Polyethylene Polyamide-12
3-5 0.5 10
0.02
0.1
72 200 40
94 100 83
a Temperature -----200°C, Po2 = 300 mm Hg.
146
A . P. Mar'in et al.
12 are not so effectively retarded. This is seen from the values of critical concentrations and induction periods, ~'app. This difference m a y be due to different rate constants for chain propagation, k2, and chain termination, k3, in these polymers. If the gossypol content exceeds 0.1 mol kg-1 the difference in induction periods a m o n g these polymers disappears. In contrast to polyethylene containing only - C H ~ groups, the m o r e reactive tertiary carbon atoms present in polypropylene must lead to an increase in k2.This accounts for the increase in critical concentration of gossypol in transition from polyethylene to polypropylene. Polyamide contains the same --CH2 groups as polyethylene but the presence in polyamide of the polar - N H C O - group, capable of forming complexes with phenol must lead to a reduction in k3 and to an increase in the critical concentration of antioxidant. Thus, the behaviour of gossypol as a natural antioxidant in polymers is similar to that of o t h e r phenolic antioxidants.
REFERENCES 1. Markman, A. M. & Rzhekhin, V. P., Gossipol i yego proizvodnye (Gossypol and its Derivatives). Food Industry, Moscow, 1965. 2. Ismailov, I., Fatkhulayev, E., Karimova, M. M. & Kapitulla, I. I. Doil. An Izv. SSSR, (Proceeding of Oil & Fat Institute, USSR) U (1981) 36. 3. Slazina, G. A., Rzhekhin, V. P. & Goryacheva, N. Trudy VNIIZh, (Reports Academy of Sciences of Uzbek Republic, USSR) 25 (1965) 439. 4. Piotrovskii, K. B., Gromova, G. N., Ivanova, L. M. & Goldberg, A. O. Kauchuk i rezina (Natural and Synthetic Rubber), 3 (1975) 33. 5. Minsker, K. S., Abdullin, M. I. & Rakhimov, I. Plastmassy (Plastics), 5 (1987) 42. 6. Shlyapnikov, Yu.A., Kiryushkin, S. G., Tyuleneva, N. K.,:~.Iring, M. & Fodor, Zs. Polym. Bull., 19 (1988) 449. 7. Shlyapnikov, Yu.A. Develop. Polym. Stab., 5 (1981) 1. 8. Shlyapnikov, Yu.A. Makromol. Chem. (Macromol. Symp.), 27 (1989) 121. 9. Shlyapnikov, Yu.A. & Mar'in, A. P. Eur. Polym. J., 23 (1987) 623. 10. Gedraitite, G. B., Mar'in, A. P. & Shlyapnikov, Yu.A. Eur. Polym. J., 25 (1989) 39. 11. Shlyapnikov, Yu.A., Miller, V. B. & Torsuyeva, E. S. Izv. A N SSSR (Chem. Ser.,) 11 (1961) 1966. 12. Vasileyskaya, N. S., Livanova, N. M., Miller, V. B., Samarina, L. V. & Shlyapnikov, Yu.A. Izv. A N SSSR (Chem. Ser.,) 11 (1972) 2614.