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Retardation of polymer oxidation by natural antioxidant gossypol: Part 2—Abnormal effects of sulphide and other additives
Polymer Degradation and Stability 35 (1992) 229-233
Retardation of polymer oxidation by natural antioxidant gossypol: Part 2--Abnormal effects of sulphide and other additives A. P. Mar'in & Yu. A. Shlyapnikov Institute of Chemical Physics, USSR Academy of Sciences, 117334, Moscow, USSR &
E . S. M a m e t o v & A . T. Dzhalilov Polytechnical Institute, 700011, Tashkent, USSR (Received 17 October 1990; accepted 4 December 1990)
The effects of dilauryl thiodipropionate, heptadecane, and phenyl benzoate on polypropylene oxidation in the presence of gossypol have been studied. It has been demonstrated that dilauryl thiodipropionate increases the antioxidant effectiveness of gossypol, provided the latter is used in small concentrations. If gossypol is used in high concentrations its effectiveness is reduced by sulphide. In this case the curves of the dependence of induction period or of oxygen consumption rate on sulphide concentration will show both maxima and minima. It is supposed that these additives exert similar effects on the polymer structure and on the antioxidant distribution in the polymer.
INTRODUCTION
same centres that contain compound A leads to displacement of A from sorption centres.
As a reaction medium, polymers differ from low-molecular solvents. The molecules of a solute can with equal probability build a solvation shell in any part of the volume of liquid while in polymers, due to the immobility of macromolecular segments, the solute molecules utilize the zones in which the short-range order has already been violated. 1-4 The main part of the compound dissolved in polymer is present in such zones known as centres of sorption, Z. The molecules of low-molecular compounds dissolved in polymer differ in their mobility depending on their position in the polymer. In the centre of sorption the molecules are less mobile and less reactive as compared with the molecules beyond these centres. 4'5 The presence in a polymer of another substance, B, capable of being sorbed by the
A +Z~----AZ B + Z ~ BZ
(1)
A Z + B~---BZ + A
and to an increase in the portion of mobile molecules, A, not trapped by molecule sorption centres, and this, in turn, leads to an increase in the rate of reactions where this compound participates. 4,6 The effectivity of antioxidants in polymer is determined by competition of reaction IH + RO~ that leads to the kinetic chain termination (i.e. retardation of polymer oxidation) and of undesired reactions of oxidation of the antioxidant itself by oxygen (IH + O2---~-.-RO2) and its interaction with decomposing hydroperoxide groups. The I H + R O 2 reaction, where ROI is a practically immobile macroradical, would proceed if antioxidant molecules can move inside
Polymer Degradation and Stability 0141-3910/91/$03.50 © 1991 Elsevier Science Publishers Ltd. 229
230
A . P . Mar'in et al.
the polymer bulk. At the same time the light mobile oxygen molecules play a major part in the IH + 02 reaction. Therefore, one can anticipate that a change in antioxidant mobility will affect IH 4- PO~ reaction rate to a larger extent than the rates of side reactions. It has been shown earlier6 that the effectiveness of retardation of polyamide-12 oxidation by 2,2'-methylene-his (4-methyl-6-tertbuthylphenol) is increased by the introduction into the polymer of phenyl benzoate that does not influence the oxidation rate but is capable of increasing the concentration of mobile molecules at the antioxidant. In another study7 a reduction in antioxidant mobility has been achieved by pretreatment of molten polymer. In this case the concentration of antioxidant molecules bonded with polymer (i.e. in sorption centres) was increased. As a result there was a marked increase in critical (inactive) antioxidant concentration and a significant increase in duration of induction period at high antioxidant concentration on account of a reduction in the side reaction rates. In practice, an increase in antioxidant effectiveness is frequently achieved by using a mixture of two or more antioxidants. In this case one can observe a mutual enhancement of their effectiveness (synergism) when, at a constant total concentration, the mixture of antioxidants retards the oxidation to a greater extent or for a longer time than does each component taken separately. Synergism is typical for mixtures of radical acceptors (phenols and amines) and hydroperoxide group decomposers which are mainly organic sulphides and phosphites. In this paper the influence of dilauryl thiodipropionate, 'inert' heptadecane, and 'inert' phenyl benzoate on the effectivity of retardation of polypropylene oxidation by gossypol has been studied. In some experiments the influence of gossypol, dilauryl thiodipropionate, and heptadecane on phenyl benzoate vapour pressure, a characteristic of its mobility in the polymer, was also studied.
products by potassium hydroxide. The induction period, ~-, has been determined as the time of transition of the slow oxidation process into the fast one. The phenyl benzoate vapour pressure over the polymer was determined on a special thermostatted gas cell with quartz windows. Concentration of the substance in the vapour was determined spectrophotometrically.
RESULTS AND DISCUSSION If concentrations of gossypol and dilauryl thiodipropionate in polypropylene are small (0.01 mol kg -1 in all) one will observe synergism that manifests itself in an increase in the induction period (Fig. 1). More complicated relations were revealed on oxidation of polypropylene which contained gossypol and dilauryl thiodipropionate in large concentration. Thus, the induction period for oxidation of polypropylene containing 0-1 molkg -1 of gossypol increased with sulphide content, then it decreased and increased again (Fig. 2). With constant concentration of gossypol the oxygen absorption rate whose value was inversely related to the induction period was reduced in the regions where the induction period increased and vice versa (Fig. 3). While an increase in duration of the induction period in the presence of sulphide can be explained by destruction of part of the
80--
In this work we used polypropylene 'Moplen' with characteristic viscosity 1.53 dl g-1 (decalin, 135°C). Polymer oxidation was studied by using a vacuum system with absorption of volatile
0
c E 4O
20
o
EXPERIMENTAL
1
60
I
100
80
I 0
I 20
I
60 [Gossipol] I 40 ~Sulfide]
I
40 (%) I 60 ('1o)
I
20
0
I 80
I 100
15g. 1. Dependence of induction period of polypropylene oxidation on (1) the composition of gossypol-sulphide mixture, and (2) on gossypol concentration in the absence of sulphide. Temperature = 200°C, Po~ = 300 mm Hg.
Retardation of polymer oxidation by gossypol--Part 2 600
231
3
8 44 .E
E 400
I
T
~o 5 E
1
200 o
I
I I I 0-04 0"08 [A] (tool kg-I)
I
I
I
0.04
I
J
0.08
[PB] [HE)] (mol kg -1 )
10--
0 ×
¢8 e4
Fig. 2. Dependence of induction period of polypropylene oxidation with polypropylene containing 0.1molkg -1 of gossypol on the concentration of (1) sulphide, (2) heptadecane, and (3) phenyl benzoate. Temperature = 200°C, Po2= 300 mm Hg.
6 (
4
hydroperoxides in polypropylene, a decrease in ~cannot be explained in that way. One of the possible causes is variation in the properties of polymer as solvent for these additives. To check the validity of this assumption we studied oxidation of polypropylene in the presence of phenyl benzoate and heptadecane, which have no effect on the polymer oxidation rate. With constant concentration of gossypol ( 0 . 1 m o l k g -1) the dependence of induction period on both phenyl benzoate and heptadecane concentrations is the same as it is in the case of sulphide (Fig. 2). A change in the oxygen absorption rate within the induction period of oxidation in the presence of these additives is inversely related to a change in the induction period for r (Fig. 3). A similar picture can be observed on oxidation of a solid polymer as well. Similar relations were observed s during the investigation of low-molecular product formation on decomposition of polypropylene hydroperoxides as a function of the concentration of a plasticizer which was a polypropylene oligomer with molecular mass 300. This phenomenon was ascribed to polymer plasticization. The left-hand sides of the curves of r versus additive concentration are probably accounted for by displacement of gossypol molecules from sorption centres 6'9 resulting in an increase in their reactivity, while the shape of the curves that follows may be explained by a change in the structure of both molten and solid polymer due to plasticization. In this case gossypol itself may be a plasticizer. As has already been noted the mechanisms of polymer oxidation in the presence of gossypol
I 0
t
I
004
I
I
0.08
ER2sl (rnoL kg-b
Fig. 3. Dependence of oxygen consumption maximum rate in the induction period of oxidation of polypropylene containing 0.1 mol kg-1 of gossypol on concentration of (1) phenyl benzoate, (2) heptadecane, and, (3) sulphide. Temperature = 200°C, Po2 = 300 mm Hg. and other additives are not likely to be explained only by chemical reactions. The effect of additives on the polymer structure and their distribution in the polymer may be studied by using data on solubility of antioxidants and their models in the polymer. It is known that small concentrations of low-molecular additives can influence the polymer supermolecular structure, i.e. plasticize the polymer. Low-molecular compounds can also mutually displace each other from different zones of the polymer and reduce solubility. If these compounds are added in concentrations that exceed solubility they can extract other additives into a separate phase and reduce their concentration in the polymer. The solubility of a phenyl benzoate, the model compound in polypropylene at 60°C first increased and then decreased with an increase in concentration of gossypol added to the polymer. The same picture is observed if dilauryl thiodipropionate is present in the polymer along with gossypol (Fig. 4). It should be noted that addition of gossypol to polypropylene leads to some reduction in crystallinity of the polymer. An increase in the solubility of phenyl benzoate with gossypol or sulphide content is probably due to a decrease in
232
A . P . Mar'in et al.
o 14 O~ ,i
E
.
,0_,
O
I
I
O.OO8
I
I
O.O16
I
5 o
A 12 7
['Gossypol'] (tool kg-1) Fig. 4. Dependence of solubility of phenyl benzoate in polypropylene on gossypol concentration in (1) the absence, and (2) the presence of 0.005molkg -1 of sulphide. Temperature = 60°C.
E ~D
o 10
la-, crystallinity and, consequently, an increase in the volume of the polymer amorphous phase where the low-molecular substance is dissolved. A decrease in phenyl benzoate solubility in polymer may be due to displacement of part of dissolved phenyl benzoate from the sorption centres that can absorb both dissolved substances. 9'1° This postulate is confirmed by reduction in phenyl benzoate solubility that approximately corresponds to the amount of gossypol added to the polymer (0.02mol kg-1). Likewise, the low-molecular substhnces dissolved in polymer can reduce solubility of gossypol itself in the polymer. To determine the influence of gossypol and some other antioxidants on the structure of molten polymer we examined the equilibrium desorption of phenyl benzoate from polypropylene and polyethylene. The amount of the substance in the gas phase (vapour pressure over the polymer) was recorded with the aid of UV spectroscopy by using a sorption cell. We recorded the concentration of phenyl benzoate that had evaporated from polymer to the gas phase and was in equilibrium with phenyl benzoate dissolved in the polymer. The concentration of phenyl benzoate in the polymer environment is inversely related to its solubility in the polymer and, in a complicated way, depends on gossypol concentration (Fig. 5). This dependence passes through a maximum when the concentration is 0-015-0-03 mol kg -1 and through a minimum when it is 0-1molkg -1. Phenyl benzoate concentration in the gas phase may be increased when gossypol is added to polymer because part of the dissolved phenyl benzoate is displaced from sorption centres to thereby reduce its solubility in the polymer. However, a decrease
O
I
O.1 [Gossypol3 (tool kg-1)
I
0.2
Fig. 5. Phenyl benzoate concentration in the gas phase as a function of gossypol concentration in polypropylene at (1) 180°C, (2) 190°C, (3) 200°C, (4) 210°C, (5) 200°C in the presence of 0.05 mol kg- 1 sulphide. Initial phenyl benzoate concentration in polypropylene was 0-01 mol kg -1.
in phenyl benzoate concentration in the gas phase (decrease in vapour pressure over the polymer) by gossypol cannot be explained by these arguments. Therefore, we assume that the polymer structure changes under the influence of gossypol. The amount of phenyl benzoate in the environment is also increased when sulphide is added to gossypol-containing polymer, due to a reduction in phenyl benzoate solubility in the molten polymer. Displacement of phenyl benzoate from the polymer was also observed when heptadecane at various concentrations was introduced into a polymer with a constant gossypol content (Fig.
6). To determine the equilibrium solubility of gossypol in the polymer at elevated temperature we studied the kinetics of its solution in polypropylene films at 150 and 160°C. Gossypol equilibrium concentration in polymer was determined during 900-1500 h, and it was 0.35 molkg -1 at those temperatures. The exist-
Retardation of polymer oxidation by gossypol--Part 2
15
•~ 11
7' 0
005 [Heptadecane] (tool kg-1)
0.10
Fig. 6. Phenyl benzoate concentration in the gas phase as a
function of heptane concentration in polypropylene at 200°C in the presence of 0.01 mol kg -1 of gossypol. Initial phenyl benzoate concentration in polypropylene was 0-01 mol kg -1.
ence of regions where solubility does not depend on temperature is one of the peculiarities of solutions added to the polymer. 11 The solubility of gossypol in polypropylene considerably surpassed the limiting concentration (0.1 mol kg -1) that had been used in experiments on polymer oxidation; that is gossypol existed in the polymer as solution and not as separate phase. Since the vapour pressure of a low-molecular substance over its solution in a polymer depends on the amount of mobile molecules that can move inside the polymer and pass into the gas phase (it is only mobile molecules that participate in reactions with macroradicals) the reactivity of the substance dissolved in the polymer should
233
change in proportion to its vapour pressure over the polymer. Affected by the 'third' lowmolecular substance, mobility of antioxidant and phenyl benzoate should change according to similar laws. Therefore, a proportionality between phenyl benzoate vapour pressure and parameters that are characteristic of gossypol effectiveness is natural (see Fig. 1). Thus, the effects of gossypol, the natural antioxidant, can be explained by the general theory of inhibited oxidation of polymers, the physical factors that account for the polymer structure and distribution of gossypol and other additives in the polymer being taken into consideration.
REFERENCES 1. Barrer, R. M. & Barrie, J. M. J. Polym. Sci., 23 (1957) 331. 2. Pace, R. J. & Datyner, J. J. Polym. Sci. (Polym. Phys. Edn), 18 (1980) 1103. 3. Livanova, N. M., Mar'in, A. P., Ershov, Yu.A., Miller, V. B. & Shlyapnikov, Yu. A. Vysokomol. Soyed., B18 (1976) 410. 4. Mar'in, A. P. & Shlyapnikov, Yu. A. Dokl. Akad. Nauk SSSR, 215 (1974) 1160. 5. Assink, R. A. J. Polym. Sci. (Polym. Phys. Edn), 13 (1975) 1665. 6. Mar'in, A. P., Yazenko, I. V., Avetisyn, S. R. & Akutin, M. S. Dokl. Akad. Nauk SSSR, 2,53 (1980) 159. 7. Mar'in, A. P. & Shlyapnikov, Yu.A. Vysokomol. Soyed., B17 (1975) 582. 8. Gromov, B. A., Tarsuyeva, E. S. & Shlyapnikov, Yu.A. Vysokomol. Soyed., B14 (1972)607. 9. Shlyapnikov, Yu.A. Macromol. Chem. (Macromol. Symp.), 27 (1989) 121. 10. Shlyapnikov, Yu.A. & Mar'in, A. P. Eur. Polym. J., 23 (1987) 623. 11. Shlyapnikov, Yu.A. & Mar'in, A. P. Eur. Polym. J., 23 (1987) 629.
Retardation of polymer oxidation by natural antioxidant gossypol: Part 2--Abnormal effects of sulphide and other additives A. P. Mar'in & Yu. A. Shlyapnikov Institute of Chemical Physics, USSR Academy of Sciences, 117334, Moscow, USSR &
E . S. M a m e t o v & A . T. Dzhalilov Polytechnical Institute, 700011, Tashkent, USSR (Received 17 October 1990; accepted 4 December 1990)
The effects of dilauryl thiodipropionate, heptadecane, and phenyl benzoate on polypropylene oxidation in the presence of gossypol have been studied. It has been demonstrated that dilauryl thiodipropionate increases the antioxidant effectiveness of gossypol, provided the latter is used in small concentrations. If gossypol is used in high concentrations its effectiveness is reduced by sulphide. In this case the curves of the dependence of induction period or of oxygen consumption rate on sulphide concentration will show both maxima and minima. It is supposed that these additives exert similar effects on the polymer structure and on the antioxidant distribution in the polymer.
INTRODUCTION
same centres that contain compound A leads to displacement of A from sorption centres.
As a reaction medium, polymers differ from low-molecular solvents. The molecules of a solute can with equal probability build a solvation shell in any part of the volume of liquid while in polymers, due to the immobility of macromolecular segments, the solute molecules utilize the zones in which the short-range order has already been violated. 1-4 The main part of the compound dissolved in polymer is present in such zones known as centres of sorption, Z. The molecules of low-molecular compounds dissolved in polymer differ in their mobility depending on their position in the polymer. In the centre of sorption the molecules are less mobile and less reactive as compared with the molecules beyond these centres. 4'5 The presence in a polymer of another substance, B, capable of being sorbed by the
A +Z~----AZ B + Z ~ BZ
(1)
A Z + B~---BZ + A
and to an increase in the portion of mobile molecules, A, not trapped by molecule sorption centres, and this, in turn, leads to an increase in the rate of reactions where this compound participates. 4,6 The effectivity of antioxidants in polymer is determined by competition of reaction IH + RO~ that leads to the kinetic chain termination (i.e. retardation of polymer oxidation) and of undesired reactions of oxidation of the antioxidant itself by oxygen (IH + O2---~-.-RO2) and its interaction with decomposing hydroperoxide groups. The I H + R O 2 reaction, where ROI is a practically immobile macroradical, would proceed if antioxidant molecules can move inside
Polymer Degradation and Stability 0141-3910/91/$03.50 © 1991 Elsevier Science Publishers Ltd. 229
230
A . P . Mar'in et al.
the polymer bulk. At the same time the light mobile oxygen molecules play a major part in the IH + 02 reaction. Therefore, one can anticipate that a change in antioxidant mobility will affect IH 4- PO~ reaction rate to a larger extent than the rates of side reactions. It has been shown earlier6 that the effectiveness of retardation of polyamide-12 oxidation by 2,2'-methylene-his (4-methyl-6-tertbuthylphenol) is increased by the introduction into the polymer of phenyl benzoate that does not influence the oxidation rate but is capable of increasing the concentration of mobile molecules at the antioxidant. In another study7 a reduction in antioxidant mobility has been achieved by pretreatment of molten polymer. In this case the concentration of antioxidant molecules bonded with polymer (i.e. in sorption centres) was increased. As a result there was a marked increase in critical (inactive) antioxidant concentration and a significant increase in duration of induction period at high antioxidant concentration on account of a reduction in the side reaction rates. In practice, an increase in antioxidant effectiveness is frequently achieved by using a mixture of two or more antioxidants. In this case one can observe a mutual enhancement of their effectiveness (synergism) when, at a constant total concentration, the mixture of antioxidants retards the oxidation to a greater extent or for a longer time than does each component taken separately. Synergism is typical for mixtures of radical acceptors (phenols and amines) and hydroperoxide group decomposers which are mainly organic sulphides and phosphites. In this paper the influence of dilauryl thiodipropionate, 'inert' heptadecane, and 'inert' phenyl benzoate on the effectivity of retardation of polypropylene oxidation by gossypol has been studied. In some experiments the influence of gossypol, dilauryl thiodipropionate, and heptadecane on phenyl benzoate vapour pressure, a characteristic of its mobility in the polymer, was also studied.
products by potassium hydroxide. The induction period, ~-, has been determined as the time of transition of the slow oxidation process into the fast one. The phenyl benzoate vapour pressure over the polymer was determined on a special thermostatted gas cell with quartz windows. Concentration of the substance in the vapour was determined spectrophotometrically.
RESULTS AND DISCUSSION If concentrations of gossypol and dilauryl thiodipropionate in polypropylene are small (0.01 mol kg -1 in all) one will observe synergism that manifests itself in an increase in the induction period (Fig. 1). More complicated relations were revealed on oxidation of polypropylene which contained gossypol and dilauryl thiodipropionate in large concentration. Thus, the induction period for oxidation of polypropylene containing 0-1 molkg -1 of gossypol increased with sulphide content, then it decreased and increased again (Fig. 2). With constant concentration of gossypol the oxygen absorption rate whose value was inversely related to the induction period was reduced in the regions where the induction period increased and vice versa (Fig. 3). While an increase in duration of the induction period in the presence of sulphide can be explained by destruction of part of the
80--
In this work we used polypropylene 'Moplen' with characteristic viscosity 1.53 dl g-1 (decalin, 135°C). Polymer oxidation was studied by using a vacuum system with absorption of volatile
0
c E 4O
20
o
EXPERIMENTAL
1
60
I
100
80
I 0
I 20
I
60 [Gossipol] I 40 ~Sulfide]
I
40 (%) I 60 ('1o)
I
20
0
I 80
I 100
15g. 1. Dependence of induction period of polypropylene oxidation on (1) the composition of gossypol-sulphide mixture, and (2) on gossypol concentration in the absence of sulphide. Temperature = 200°C, Po~ = 300 mm Hg.
Retardation of polymer oxidation by gossypol--Part 2 600
231
3
8 44 .E
E 400
I
T
~o 5 E
1
200 o
I
I I I 0-04 0"08 [A] (tool kg-I)
I
I
I
0.04
I
J
0.08
[PB] [HE)] (mol kg -1 )
10--
0 ×
¢8 e4
Fig. 2. Dependence of induction period of polypropylene oxidation with polypropylene containing 0.1molkg -1 of gossypol on the concentration of (1) sulphide, (2) heptadecane, and (3) phenyl benzoate. Temperature = 200°C, Po2= 300 mm Hg.
6 (
4
hydroperoxides in polypropylene, a decrease in ~cannot be explained in that way. One of the possible causes is variation in the properties of polymer as solvent for these additives. To check the validity of this assumption we studied oxidation of polypropylene in the presence of phenyl benzoate and heptadecane, which have no effect on the polymer oxidation rate. With constant concentration of gossypol ( 0 . 1 m o l k g -1) the dependence of induction period on both phenyl benzoate and heptadecane concentrations is the same as it is in the case of sulphide (Fig. 2). A change in the oxygen absorption rate within the induction period of oxidation in the presence of these additives is inversely related to a change in the induction period for r (Fig. 3). A similar picture can be observed on oxidation of a solid polymer as well. Similar relations were observed s during the investigation of low-molecular product formation on decomposition of polypropylene hydroperoxides as a function of the concentration of a plasticizer which was a polypropylene oligomer with molecular mass 300. This phenomenon was ascribed to polymer plasticization. The left-hand sides of the curves of r versus additive concentration are probably accounted for by displacement of gossypol molecules from sorption centres 6'9 resulting in an increase in their reactivity, while the shape of the curves that follows may be explained by a change in the structure of both molten and solid polymer due to plasticization. In this case gossypol itself may be a plasticizer. As has already been noted the mechanisms of polymer oxidation in the presence of gossypol
I 0
t
I
004
I
I
0.08
ER2sl (rnoL kg-b
Fig. 3. Dependence of oxygen consumption maximum rate in the induction period of oxidation of polypropylene containing 0.1 mol kg-1 of gossypol on concentration of (1) phenyl benzoate, (2) heptadecane, and, (3) sulphide. Temperature = 200°C, Po2 = 300 mm Hg. and other additives are not likely to be explained only by chemical reactions. The effect of additives on the polymer structure and their distribution in the polymer may be studied by using data on solubility of antioxidants and their models in the polymer. It is known that small concentrations of low-molecular additives can influence the polymer supermolecular structure, i.e. plasticize the polymer. Low-molecular compounds can also mutually displace each other from different zones of the polymer and reduce solubility. If these compounds are added in concentrations that exceed solubility they can extract other additives into a separate phase and reduce their concentration in the polymer. The solubility of a phenyl benzoate, the model compound in polypropylene at 60°C first increased and then decreased with an increase in concentration of gossypol added to the polymer. The same picture is observed if dilauryl thiodipropionate is present in the polymer along with gossypol (Fig. 4). It should be noted that addition of gossypol to polypropylene leads to some reduction in crystallinity of the polymer. An increase in the solubility of phenyl benzoate with gossypol or sulphide content is probably due to a decrease in
232
A . P . Mar'in et al.
o 14 O~ ,i
E
.
,0_,
O
I
I
O.OO8
I
I
O.O16
I
5 o
A 12 7
['Gossypol'] (tool kg-1) Fig. 4. Dependence of solubility of phenyl benzoate in polypropylene on gossypol concentration in (1) the absence, and (2) the presence of 0.005molkg -1 of sulphide. Temperature = 60°C.
E ~D
o 10
la-, crystallinity and, consequently, an increase in the volume of the polymer amorphous phase where the low-molecular substance is dissolved. A decrease in phenyl benzoate solubility in polymer may be due to displacement of part of dissolved phenyl benzoate from the sorption centres that can absorb both dissolved substances. 9'1° This postulate is confirmed by reduction in phenyl benzoate solubility that approximately corresponds to the amount of gossypol added to the polymer (0.02mol kg-1). Likewise, the low-molecular substhnces dissolved in polymer can reduce solubility of gossypol itself in the polymer. To determine the influence of gossypol and some other antioxidants on the structure of molten polymer we examined the equilibrium desorption of phenyl benzoate from polypropylene and polyethylene. The amount of the substance in the gas phase (vapour pressure over the polymer) was recorded with the aid of UV spectroscopy by using a sorption cell. We recorded the concentration of phenyl benzoate that had evaporated from polymer to the gas phase and was in equilibrium with phenyl benzoate dissolved in the polymer. The concentration of phenyl benzoate in the polymer environment is inversely related to its solubility in the polymer and, in a complicated way, depends on gossypol concentration (Fig. 5). This dependence passes through a maximum when the concentration is 0-015-0-03 mol kg -1 and through a minimum when it is 0-1molkg -1. Phenyl benzoate concentration in the gas phase may be increased when gossypol is added to polymer because part of the dissolved phenyl benzoate is displaced from sorption centres to thereby reduce its solubility in the polymer. However, a decrease
O
I
O.1 [Gossypol3 (tool kg-1)
I
0.2
Fig. 5. Phenyl benzoate concentration in the gas phase as a function of gossypol concentration in polypropylene at (1) 180°C, (2) 190°C, (3) 200°C, (4) 210°C, (5) 200°C in the presence of 0.05 mol kg- 1 sulphide. Initial phenyl benzoate concentration in polypropylene was 0-01 mol kg -1.
in phenyl benzoate concentration in the gas phase (decrease in vapour pressure over the polymer) by gossypol cannot be explained by these arguments. Therefore, we assume that the polymer structure changes under the influence of gossypol. The amount of phenyl benzoate in the environment is also increased when sulphide is added to gossypol-containing polymer, due to a reduction in phenyl benzoate solubility in the molten polymer. Displacement of phenyl benzoate from the polymer was also observed when heptadecane at various concentrations was introduced into a polymer with a constant gossypol content (Fig.
6). To determine the equilibrium solubility of gossypol in the polymer at elevated temperature we studied the kinetics of its solution in polypropylene films at 150 and 160°C. Gossypol equilibrium concentration in polymer was determined during 900-1500 h, and it was 0.35 molkg -1 at those temperatures. The exist-
Retardation of polymer oxidation by gossypol--Part 2
15
•~ 11
7' 0
005 [Heptadecane] (tool kg-1)
0.10
Fig. 6. Phenyl benzoate concentration in the gas phase as a
function of heptane concentration in polypropylene at 200°C in the presence of 0.01 mol kg -1 of gossypol. Initial phenyl benzoate concentration in polypropylene was 0-01 mol kg -1.
ence of regions where solubility does not depend on temperature is one of the peculiarities of solutions added to the polymer. 11 The solubility of gossypol in polypropylene considerably surpassed the limiting concentration (0.1 mol kg -1) that had been used in experiments on polymer oxidation; that is gossypol existed in the polymer as solution and not as separate phase. Since the vapour pressure of a low-molecular substance over its solution in a polymer depends on the amount of mobile molecules that can move inside the polymer and pass into the gas phase (it is only mobile molecules that participate in reactions with macroradicals) the reactivity of the substance dissolved in the polymer should
233
change in proportion to its vapour pressure over the polymer. Affected by the 'third' lowmolecular substance, mobility of antioxidant and phenyl benzoate should change according to similar laws. Therefore, a proportionality between phenyl benzoate vapour pressure and parameters that are characteristic of gossypol effectiveness is natural (see Fig. 1). Thus, the effects of gossypol, the natural antioxidant, can be explained by the general theory of inhibited oxidation of polymers, the physical factors that account for the polymer structure and distribution of gossypol and other additives in the polymer being taken into consideration.
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