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Mechanisms of antioxidant action: The effect of galvinoxyl on the thermal oxidative stability of processed polypropylene
Polymer Degradation and Stability 5 (1983) 197-203
Mechanisms of Antioxidant Action: The Effect of Galvinoxyl on the Thermal Oxidative Stability of Processed Polypropylene Rouhalla Bagheri, Khirud B. Chakraborty & Gerald Scott Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham 34 7ET, Great Britain (Received: 25 June, 1982)
ABSTRACT The effectiveness of galvinoxyl (G. (1)) as a thermal antioxidant in polypropylene at 140°C is dependent on the prior processing treatment of the polymer. A characteristic fluctuation of antioxidant activity with time of processing is shown to be associated with the extent of conversion of galvinoxyl to hydrogalvinoxyl ( GH) in the polymer. It is concluded that the main stabilisation mechanism involved is deactivation of alkylperoxyl radicals (CB-D) by GH but secondary processes (CB-D and CB-A) involving G" may also make a contribution.
INTRODUCTION It was shown earlier 1'2 that galvinoxyl (I) is a highly effective melt stabiliser for polypropylene and that it is substantially converted to hydrogalvinoxyl (II) in the initial stages of the processing operation. tBu
tBu
tBu
tBu
(+e, +H) *0'
'
+
•
(-e, -11") tBu
tBu (I) G.
tBu
tBu (IIJ) GH
(1)
197 Polymer Degradation and Stability 0141-3910/83/0005-0197/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
Rouhalla Bagheri, Khirud B. Chakraborty, Gerald Scott
198
However, this reaction is reversible (reaction (1)). GH is a typical chainbreaking electron donor (CB-D) antioxidant and it is therefore of some interest to relate the subsequent thermal-oxidative and photo-oxidative stability of the polypropylene to the thermal history of the polymer during processing. EXPERIMENTAL The polypropylene films used in this work were obtained as described in an earlier paper. 2 Thermal ageing was carried out in a Wallace single cell oven with air passing over the polymer films at 2.5ft3/h. Carbonyl formation (1715cm -1) and decay of the antioxidant OH group (3640 cm- 1) in the polymer were followed by ir as described previously.2'3 RESULTS Figure 1 shows the oxidation (carbonyl formation) curves at 140 °C for polypropylene films compression moulded from polymer which had been 0.6 0.5 78 o
0.4
C~:yatrol
I30
20 10
5,15
~ 0.:3
i
0.2 0.1
• &~ 6
12
18
24 TIME. h
30
36
42
0 i 48
54
Fig. 1. T h e r m a l oxidation o f P P films c o n t a i n i n g galvinoxyl (G-) in a n air oven at 140°C. N u m b e r s o n curves are processing times in a closed mixer at 200°C; [G.] = 4 . 7 5 × 10 - 4 mol/100g.
60
199
Antioxidant action of galvinoxyl in polypropylene TABLE 1
Thermal Embrittlement Times at 140°C for Polypropylene Containing Galvinoxyl, G-, as a Function of Processing Time Processing time (min)
4.75*/200 +
5 10 15 20 30 Control(noG.)
57 49 57 48 33 0.5
Embrittlement time (h) 2.37"/190 ÷ 2.37*/200 +
26 22-5 26 21 12 0.5
-27 --18 0.5
* [G.] initially added to PP, x 104mol/100g. + Processing temperature, °C. p r o c e s s e d for various times at 200 °C. T h e e m b r i t t l e m e n t times at 140 °C for similar films processed with galvinoxyl for various times at two different c o n c e n t r a t i o n s a n d at two t e m p e r a t u r e s 2 are listed in T a b l e 1. T a b l e 2 lists the e m b r i t t l e m e n t times for similar films oxidised in an air o v e n at 110°C. T h e fluctuation in e m b r i t t l e m e n t time with processing time o b s e r v e d in Fig. 1 a n d T a b l e s 1 a n d 2 was r e p r o d u c i b l e ( + 2 h) a n d was f o u n d to relate closely to the f l u c t u a t i o n o f h y d r o g a l v i n o x y l ( G H ) c o n c e n t r a t i o n in the p o l y m e r after v a r i o u s processing times r e p o r t e d previously. TABLE 2
Thermal Embrittlement Time at 110 °C for Polypropylene Containing Galvinoxyl, G., as a Function of Processing Time Processing time (rain)
5 10 15 20 30 Control(noG-)
Embrittlement time (h) 4.75*/200 + 2"37"/190 +
672 600 672 576 274 4.5
* [G-] initially added to PP, x 104mol/100g. + Processing temperature, °C.
280 194 294 185 -4.5
200
Rouhalla Bagheri, Khirud B. Chakraborty, Gerald Scott 60
50
40
I'
e~ .4'
f
f f J f f f f f f f f f I
10
0
I
0
0.5
I
1.0
I
1.5
I
2.0
I
I
I
2.5
3.0
3.5
4.0
[GH), 104 mol/lO0 g
Fig. 2. Relationshipbetweenembrittlementtime of PP filmsin an air oven at 140°C and hydrogalvinoxyl[GH] concentration after processingin the presenceof G. (4.75 x 10-4 mol/100g) for various times in a closed mixer at 200°C. Figure 2 shows that an approximately straight line relationship exists between [GH] measured in the polymer after processing 2 and the embrittlement time (ET). The intercept on the ET axis (approximately 13h) is, however, much greater than the embrittlement time of unstabilised PP (approximately 0.5h), indicating that other antioxidant species may be contributing to the overall effect. Table 1 also shows that the higher the processing temperature and the higher the initial [G.], the better is the thermal oxidative stability of the fabricated film. This is again consistent with the amount of GH produced at different temperatures. 2 It was found possible 2 to monitor the decay of GH by following the infra-red absorption at 3640 c m - 1 during oven ageing. This is shown in Fig. 3 for the five different processing times. It can be seen that for any initial value of [GH], the latter decays at the same rate, although it may have been produced by processing for different times. The zero order
201
Antioxidant action of galvinoxyl in polypropylene
0.5
7
0.4
0.3
~
0.2
i
0.1
0
I 6
I 12
I I -18 24 THERMAL AGEING T I ~ , h
I
1
30
36
I 42
l 48
Fig. 3. Decayof GH hydroxylabsorbance(3640cm- 1)during thermaloxidativeageing (140°C/air oven) of PP processedwith G. (4.75 x 10-4 mol/100g) at 200°C in a closed mixer. Numbers on curves are processingtimes in minutes. plots suggest that the loss is surface area dependent and occurs predominantly by volatilisation of the antioxidant. In each case, the intercept on the time axis is just over 10 h longer than the embrittlement time (cf. Table 1). This is also the value of the intercept on the time axis in Fig. 2, suggesting again the presence of another antioxidant species.
DISCUSSION It has been shown previously 1'2 that galvinoxyl (G.) and hydrogalvinoxyl (GH) exist in equilibrium in polypropylene during the initial stages of processing in a shearing mixer at temperatures in the region of 200 °C (reaction (1)). Their relative concentrations depend on the relative concentrations of alkyl and alkylperoxyl radicals in the system. This is affected by both the applied torque acting on the polymer and the oxygen concentration in the mixer at any given time. 1,2 Consequently, samples removed from the mixer at intervals have different concentrations of both
202
Rouhalla Bagheri, Khirud B. Chakraborty, Gerald Scott
G. and GH. It is clear from the present studies that, under thermaloxidative conditions, GH is the antioxidant primarily involved in inhibition by the CB-D process (2). GH+ROO. , G.+ROOH (2) G. is a very much less effective antioxidant than GH in the presence of excess oxygen (e.g. in an open mixer during processing 2) or, as in the present case, when oxygen can readily diffuse into the sample, since it is rapidly and irreversibly destroyed under these conditions. 1 Nevertheless, the fact that the antioxidant activity of the GH/G. combination persists even after the GH concentration has been reduced to a very low level suggests that G. itself may be involved in the antioxidant function. It is known 2 that under oxidative conditions, G. is finally converted to 2,6-ditert-butyl benzoquinone (IV). This reaction, by analogy with reactions of other hindered phenols, almost certainly involves the trapping of alkylperoxyl radicals by the CB-D mechanism to give the unstable peroxy dienone (III) (reaction (3)). tBu
•
tBu
O-~,CH~ tBu
ROO,
tBu [" / OtBu ~
O , tBu~tBu
OOR CH ~ ) ' ~ [ tBuO"]
(3) [. tBu
tBu ]
O
(III)
(IV)
It seems likely that G. also possesses some CB-A activity under these conditions. Denisov 4 has shown that the ratio of alkyl to alkylperoxyl radicals (which determines the importance of the CB-A mechanism) is almost three orders of magnitude higher in polypropylene than it is in a liquid model hydrocarbon with a similar chemical structure at the same temperatures. This suggests that the CB-A process (4), and hence the CH 3 I
CH 3 I
G- + --C--CH 2 - CB-A GH + ~ C = C H - -
(4)
Antioxidant action of galvinoxyl in polypropylene
203
cyclical mechanism summarised in (1), may make a contribution to the stabilisation effect, although this is clearly less significant than it is in the polymer melt. z A similar conclusion has been reached for the CB-A antioxidant, 3,5,3',5'-tetra-tert-butyl stilbenequinone, an oxidation product of 2,6-ditert-butyl-4-methylphenol. 5 It is clear from the zero order kinetics of antioxidant depletion in polypropylene (see Fig. 3) that antioxidant loss occurs primarily by volatilisation. The G-/GH combination is of similar effectiveness to commercial thermal stabilisers for polypropylene 6 and, in view of the fact that physical loss of additives limits their effectiveness, it must be concluded that the system must have a high intrinsic antioxidant activity.
ACKNOWLEDGEMENTS We are grateful to the Ministry of Higher Education of Iran for a grant to one of us (R. B.). We are also grateful to ICI, Plastics Division, for the provision of unstabilised polypropylene.
REFERENCES 1. R. Bagheri, K. B. Chakraborty and G. Scott, Chem. and Ind., 865 (1980). 2. R. Bagheri, K. B. Chakraborty and G. Scott, Polym. Deg. and Stab., 4, 1 (1982). 3. K. B. Chakraborty and G. Scott, Polym. Deg. and Stab., 1, 37 (1979). 4. E. T. Denisov, Developments in polymer stabilisation--5 (G. Scott (Ed.)), Applied Science Publishers Ltd, London, p. 23 (1982). 5. G. Scott and M. F. Yusoff, Polym. Deg. and Stab., 3, 13 (1980-81). 6. G. Scott and M. F. Yusoff, Polym. Deg. and Stab., 3, 53 (1980-81).
Mechanisms of Antioxidant Action: The Effect of Galvinoxyl on the Thermal Oxidative Stability of Processed Polypropylene Rouhalla Bagheri, Khirud B. Chakraborty & Gerald Scott Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham 34 7ET, Great Britain (Received: 25 June, 1982)
ABSTRACT The effectiveness of galvinoxyl (G. (1)) as a thermal antioxidant in polypropylene at 140°C is dependent on the prior processing treatment of the polymer. A characteristic fluctuation of antioxidant activity with time of processing is shown to be associated with the extent of conversion of galvinoxyl to hydrogalvinoxyl ( GH) in the polymer. It is concluded that the main stabilisation mechanism involved is deactivation of alkylperoxyl radicals (CB-D) by GH but secondary processes (CB-D and CB-A) involving G" may also make a contribution.
INTRODUCTION It was shown earlier 1'2 that galvinoxyl (I) is a highly effective melt stabiliser for polypropylene and that it is substantially converted to hydrogalvinoxyl (II) in the initial stages of the processing operation. tBu
tBu
tBu
tBu
(+e, +H) *0'
'
+
•
(-e, -11") tBu
tBu (I) G.
tBu
tBu (IIJ) GH
(1)
197 Polymer Degradation and Stability 0141-3910/83/0005-0197/$03.00 © Applied Science Publishers Ltd, England, 1983. Printed in Great Britain
Rouhalla Bagheri, Khirud B. Chakraborty, Gerald Scott
198
However, this reaction is reversible (reaction (1)). GH is a typical chainbreaking electron donor (CB-D) antioxidant and it is therefore of some interest to relate the subsequent thermal-oxidative and photo-oxidative stability of the polypropylene to the thermal history of the polymer during processing. EXPERIMENTAL The polypropylene films used in this work were obtained as described in an earlier paper. 2 Thermal ageing was carried out in a Wallace single cell oven with air passing over the polymer films at 2.5ft3/h. Carbonyl formation (1715cm -1) and decay of the antioxidant OH group (3640 cm- 1) in the polymer were followed by ir as described previously.2'3 RESULTS Figure 1 shows the oxidation (carbonyl formation) curves at 140 °C for polypropylene films compression moulded from polymer which had been 0.6 0.5 78 o
0.4
C~:yatrol
I30
20 10
5,15
~ 0.:3
i
0.2 0.1
• &~ 6
12
18
24 TIME. h
30
36
42
0 i 48
54
Fig. 1. T h e r m a l oxidation o f P P films c o n t a i n i n g galvinoxyl (G-) in a n air oven at 140°C. N u m b e r s o n curves are processing times in a closed mixer at 200°C; [G.] = 4 . 7 5 × 10 - 4 mol/100g.
60
199
Antioxidant action of galvinoxyl in polypropylene TABLE 1
Thermal Embrittlement Times at 140°C for Polypropylene Containing Galvinoxyl, G-, as a Function of Processing Time Processing time (min)
4.75*/200 +
5 10 15 20 30 Control(noG.)
57 49 57 48 33 0.5
Embrittlement time (h) 2.37"/190 ÷ 2.37*/200 +
26 22-5 26 21 12 0.5
-27 --18 0.5
* [G.] initially added to PP, x 104mol/100g. + Processing temperature, °C. p r o c e s s e d for various times at 200 °C. T h e e m b r i t t l e m e n t times at 140 °C for similar films processed with galvinoxyl for various times at two different c o n c e n t r a t i o n s a n d at two t e m p e r a t u r e s 2 are listed in T a b l e 1. T a b l e 2 lists the e m b r i t t l e m e n t times for similar films oxidised in an air o v e n at 110°C. T h e fluctuation in e m b r i t t l e m e n t time with processing time o b s e r v e d in Fig. 1 a n d T a b l e s 1 a n d 2 was r e p r o d u c i b l e ( + 2 h) a n d was f o u n d to relate closely to the f l u c t u a t i o n o f h y d r o g a l v i n o x y l ( G H ) c o n c e n t r a t i o n in the p o l y m e r after v a r i o u s processing times r e p o r t e d previously. TABLE 2
Thermal Embrittlement Time at 110 °C for Polypropylene Containing Galvinoxyl, G., as a Function of Processing Time Processing time (rain)
5 10 15 20 30 Control(noG-)
Embrittlement time (h) 4.75*/200 + 2"37"/190 +
672 600 672 576 274 4.5
* [G-] initially added to PP, x 104mol/100g. + Processing temperature, °C.
280 194 294 185 -4.5
200
Rouhalla Bagheri, Khirud B. Chakraborty, Gerald Scott 60
50
40
I'
e~ .4'
f
f f J f f f f f f f f f I
10
0
I
0
0.5
I
1.0
I
1.5
I
2.0
I
I
I
2.5
3.0
3.5
4.0
[GH), 104 mol/lO0 g
Fig. 2. Relationshipbetweenembrittlementtime of PP filmsin an air oven at 140°C and hydrogalvinoxyl[GH] concentration after processingin the presenceof G. (4.75 x 10-4 mol/100g) for various times in a closed mixer at 200°C. Figure 2 shows that an approximately straight line relationship exists between [GH] measured in the polymer after processing 2 and the embrittlement time (ET). The intercept on the ET axis (approximately 13h) is, however, much greater than the embrittlement time of unstabilised PP (approximately 0.5h), indicating that other antioxidant species may be contributing to the overall effect. Table 1 also shows that the higher the processing temperature and the higher the initial [G.], the better is the thermal oxidative stability of the fabricated film. This is again consistent with the amount of GH produced at different temperatures. 2 It was found possible 2 to monitor the decay of GH by following the infra-red absorption at 3640 c m - 1 during oven ageing. This is shown in Fig. 3 for the five different processing times. It can be seen that for any initial value of [GH], the latter decays at the same rate, although it may have been produced by processing for different times. The zero order
201
Antioxidant action of galvinoxyl in polypropylene
0.5
7
0.4
0.3
~
0.2
i
0.1
0
I 6
I 12
I I -18 24 THERMAL AGEING T I ~ , h
I
1
30
36
I 42
l 48
Fig. 3. Decayof GH hydroxylabsorbance(3640cm- 1)during thermaloxidativeageing (140°C/air oven) of PP processedwith G. (4.75 x 10-4 mol/100g) at 200°C in a closed mixer. Numbers on curves are processingtimes in minutes. plots suggest that the loss is surface area dependent and occurs predominantly by volatilisation of the antioxidant. In each case, the intercept on the time axis is just over 10 h longer than the embrittlement time (cf. Table 1). This is also the value of the intercept on the time axis in Fig. 2, suggesting again the presence of another antioxidant species.
DISCUSSION It has been shown previously 1'2 that galvinoxyl (G.) and hydrogalvinoxyl (GH) exist in equilibrium in polypropylene during the initial stages of processing in a shearing mixer at temperatures in the region of 200 °C (reaction (1)). Their relative concentrations depend on the relative concentrations of alkyl and alkylperoxyl radicals in the system. This is affected by both the applied torque acting on the polymer and the oxygen concentration in the mixer at any given time. 1,2 Consequently, samples removed from the mixer at intervals have different concentrations of both
202
Rouhalla Bagheri, Khirud B. Chakraborty, Gerald Scott
G. and GH. It is clear from the present studies that, under thermaloxidative conditions, GH is the antioxidant primarily involved in inhibition by the CB-D process (2). GH+ROO. , G.+ROOH (2) G. is a very much less effective antioxidant than GH in the presence of excess oxygen (e.g. in an open mixer during processing 2) or, as in the present case, when oxygen can readily diffuse into the sample, since it is rapidly and irreversibly destroyed under these conditions. 1 Nevertheless, the fact that the antioxidant activity of the GH/G. combination persists even after the GH concentration has been reduced to a very low level suggests that G. itself may be involved in the antioxidant function. It is known 2 that under oxidative conditions, G. is finally converted to 2,6-ditert-butyl benzoquinone (IV). This reaction, by analogy with reactions of other hindered phenols, almost certainly involves the trapping of alkylperoxyl radicals by the CB-D mechanism to give the unstable peroxy dienone (III) (reaction (3)). tBu
•
tBu
O-~,CH~ tBu
ROO,
tBu [" / OtBu ~
O , tBu~tBu
OOR CH ~ ) ' ~ [ tBuO"]
(3) [. tBu
tBu ]
O
(III)
(IV)
It seems likely that G. also possesses some CB-A activity under these conditions. Denisov 4 has shown that the ratio of alkyl to alkylperoxyl radicals (which determines the importance of the CB-A mechanism) is almost three orders of magnitude higher in polypropylene than it is in a liquid model hydrocarbon with a similar chemical structure at the same temperatures. This suggests that the CB-A process (4), and hence the CH 3 I
CH 3 I
G- + --C--CH 2 - CB-A GH + ~ C = C H - -
(4)
Antioxidant action of galvinoxyl in polypropylene
203
cyclical mechanism summarised in (1), may make a contribution to the stabilisation effect, although this is clearly less significant than it is in the polymer melt. z A similar conclusion has been reached for the CB-A antioxidant, 3,5,3',5'-tetra-tert-butyl stilbenequinone, an oxidation product of 2,6-ditert-butyl-4-methylphenol. 5 It is clear from the zero order kinetics of antioxidant depletion in polypropylene (see Fig. 3) that antioxidant loss occurs primarily by volatilisation. The G-/GH combination is of similar effectiveness to commercial thermal stabilisers for polypropylene 6 and, in view of the fact that physical loss of additives limits their effectiveness, it must be concluded that the system must have a high intrinsic antioxidant activity.
ACKNOWLEDGEMENTS We are grateful to the Ministry of Higher Education of Iran for a grant to one of us (R. B.). We are also grateful to ICI, Plastics Division, for the provision of unstabilised polypropylene.
REFERENCES 1. R. Bagheri, K. B. Chakraborty and G. Scott, Chem. and Ind., 865 (1980). 2. R. Bagheri, K. B. Chakraborty and G. Scott, Polym. Deg. and Stab., 4, 1 (1982). 3. K. B. Chakraborty and G. Scott, Polym. Deg. and Stab., 1, 37 (1979). 4. E. T. Denisov, Developments in polymer stabilisation--5 (G. Scott (Ed.)), Applied Science Publishers Ltd, London, p. 23 (1982). 5. G. Scott and M. F. Yusoff, Polym. Deg. and Stab., 3, 13 (1980-81). 6. G. Scott and M. F. Yusoff, Polym. Deg. and Stab., 3, 53 (1980-81).