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Mechanistic aspects of the stabilization of polyamides by combinations of metal and halogen salts
Polymer Degradation and Stability 49 (1995) 127-133 @ 1995 Elsevier Science- Limited ELSEVIER
Printed in Northern Ireland. All rights reserved 0141-3910/95/$09.50
0141-3910(95)00065-8
Mechanistic aspects of the stabilization of polyamides by combinations of metal and halogen salts Koen Janssen, Pieter Gijsman* & Daan Tmnmers DSM Research, PO Box 18, 6160 MD Geleen, The Netherlands
(Received 9 September
1994; accepted 20 October 1994)
For aliphatic polyamides the most effective stabilizers against the long-term thermooxidative degradation are synergetic combinations of metal (mainly Cu and Mn) and halogen (mainly Br and I) salts. Their mechanism of action is still a topic of discussion. Experiments with a model compound for polyamide 46 (PA46) confirm that the combination of metal and halogen salts results in a synergetic mixture. It is shown that radical scavenging only cannot be responsible for the stabilizing action of mixtures of metal and halogen salts. The mechanism of action of combinations of metal and halogen salts is probably based on the metal ion catalyzed decomposition of hydroperoxides by the halogen salt.
1 INTRODUCTION
In comparison with polyolefins, polyamides have a high thermal resistance, which is the reason why they are mainly used in high-temperature environments. To be applicable, the polymers have to be stable at these high temperatures. The development of polyamides with a higher thermal resistance such as polyamide 46 (PA46)’ resulted in more interest in the thermooxidative stability of aliphatic polyamides. For unstabilized PA46 we have shown’ that due to a higher crystallinity and/or density of the amorphous phase the stability of this polymer is higher than the stability of PA66. We have also shown that the degradation chemistries of these two polymers are comparable. In both cases the degradation is caused by a radical oxidation mainly taking place at the N-vicinal methylene, in which reaction the of hydroperoxides plays an decomposition important role. For demanding applications aliphatic polyamides are stabilized. The most effective stabilizers are combinations of metal (mainly Cu and Mn) and halogen (mainly Br and I) salts. This is surprising because metal ions act as * Author to whom correspondence
should be addressed.
prodegradants in polyolefins, which is due to the catalytic decomposition of hydroperoxides by these transition metals according to the following mechanism: ROOH ROOH
Me”++
+ +
RO”
Me(“+‘)+-
+
OH-
+
ROO”
+
H’
+
ROO”
+
RO”
Me’“+“+
[Al
Me”+
PI
Me-/Me”+‘*
ROOH
2
+
+
H,O
I31
According to Emanuel and coworkers3 the electron transfer is preceded by the formation of a coordination complex between the metal ion and the hydroperoxide. The most active catalysts of hydroperoxide decomposition are derivatives of those metals, which are easily oxidized or reduced by a one-electron transfer, such as Cu, Co, Mn and Fe.4T5 In a number of systems transition metals can act as the antioxidant. This activity might be caused by two reactions:6 R” ROO”
M&f?)
+ +
Me”+
---, v
R+ ROO-
+
Me”+ +
Mel”+ 1)
[41
[51
In the case of the stabilization of polyamides with Cu ions it has been suggested that the
128
K. Janssen et al.
antioxidant effect is catalytic. Cu*’ oxidizes an alkyl radical to a carbonium ion, and the Cu’ formed in this reaction reduces alkylperoxy radicals:’ ri
?
of 152 “C (yield: 77% and purity: 99%). Analysis with GC-MS, IR and ‘H NMR confirmed the structure of the model compound (see Fig. l).For more details on the synthesis see Ref. 2. This compound does not sublime at 180°C (the temperature at which the experiments are done).
-CH,-C-N&H-
2.2 Chemicals
[81
It is also described that I- can reduce according to the following reaction?
Cu*+
All chemicals were used as received, but were crushed before adding to the model compound. CuI (98%), CuCl (99%), CuCl, (98%) Mn(Ac),.4H,O (99%) and CoCl, (97%) were purchased from Janssen Chimica. CuO (98%) and AgCl (99%) were purchased from Aldrich. ZnI, (98.5%), ZnCl, (97%), KI (99.5%) and CrCl, (98%) were purchased from Merck. 2.3
However, if this reaction causes the synergism between Cu and halogen salts, the most effective species has to be Cu+, and reaction (7) has to be, the reaction causing the antioxidant effectivity of Cu salts. However, this should result in a pronounced difference in activity between Cu’ and Cu*+, which is not the case.9 A better understanding of the synergism of Cu and halogen salts might lead to a further improvement of the long-term stability of aliphatic polyamides. Our study was done on a model compound which was shown to degrade comparably to PA46*, with several metal ions. A mechanism explaining the synergism between metal and halogen salts is postulated.
2 EXPERIMENTAL 2.1 Synthesis of the model compound
Experimental
setup
The setup consists of three degradation vessels, surrounded by a heated silicone oil jacket. The temperature in the vessels is kept constant at 180°C. All experiments were done with pure oxygen and 5 g of model compound, 0.09 mmol of metal salt and 1.1 mmol of potasium iodide. The stirring speed applied was 270 rpm. The pressure in each vessel was measured with a pressure transducer (Druck Ltd type PDCR 910 S/N 338692; accuracy: &0.07%). After each experiment gas samples were taken. In these samples the amounts of oxygen, CO and CO, were determined by gas chromatography (for more details see Ref. 10). In order to calculate the oxygen uptake, the volume of each vessel has to be known. This is determined volumetrically at room temperature with a gas burette filled with pure oxygen. For more details on the experimental setup see Ref. 2.
The model compound
(for structure see Fig. 1) is prepared from lauric acid (Janssen Chimica ~. 99.5%) and butane diamine (DSM) in m-xylene. The mixture is refluxed, during which water from the condensation is distilled off azeotropically. A crystalline powder is isolated with a melting point
? C, ,H23-C-NH-CH,CH,CH,CH,-NWC,
FEg. 1. Structure of model compound
0 II
-C, ,H,,
used.
3 RESULTS
AND
DISCUSSION
3.1 Oxygen uptake
Different copper salts were tested for their stabilizing action in the model compound during oxidation. Assuming that each molecule of the model compound reacts with one molecule of oxygen, 2212 mmol oxygen/kg compound can be supplied. For the unstabilized model compound
The stabilization of polyamides by combinations of metal and halogen salts
Time
(h)
Eig. 2. Oxygen uptake as a function of degradation time for the model compound (for structure see Fig. I), containing different copper salts: CuCl (0), CuCl, (A), CuO (W), Cu(OAc), (V), CUT (4) and reference (V).
this amount of oxygen uptake is almost reached (Fig. 2) after 20 h at 180 “C in pure oxygen. The stabilizing activity of the different Cu-salts depends on their counter ion (Fig. 2). CuCl does not stabilize at all, but also shows no catalytic effect on the oxidation of the compound, as in polyolefins. CuCl, shows a small stabilizing effect. Surprisingly CuO reduces the oxygen uptake to 1270 mmol/kg, while Cu(Ac),.H,O shows only half of the reference’s oxygen uptake. The oxygen uptake in the presence of CuI is the lowest, after 145 h, 565 mmol/kg. Supposing that the uptake follows a linear path, the uptake should be approximately 90 mmol/kg after 20 h. Thus, an increase in stabilizing effect is found from CuCl to CuCl, to CuO to Cu(Ac),.H,O to GUI. From these experiments it is concluded that 20 h of oxidation is sufficient to measure the effect of stabilizers. The synergism of Cu and halogen salts was studied with CuCl, CuCl,, CuO, CuI and KI (Fig. 1000
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‘/
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’
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129
3). Especially in the beginning KI has a it is small in stabilizing activity, although comparison to CuI. The synergisms between CuCl and CuCl, with KI are the largest, followed by CuO and KI. The smallest synergism is found between CuI and KI. However, this synergism might become more pronounced after longer oxidation times. From observation during the experiments it is known that a major part of the KI is not dissolved and lies on the bottom of the degradation vessel. So the effects of KI are due to very small amounts. The two most important routes to stabilize polymers against thermooxidative degradation peroxide are radical trapping and decomposition.l’ One of the possible synergisms between transition metals and halogen salts might be due to radical scavenging by the metal salt in its highest oxidation state, followed by a reaction of the metal in its lowest oxidation state with the halogen salt back to the metal ion in its highest oxidation state (reaction (9)). To check this postulated mechanism different metals (with and without a one-electron transfer) in combination with KI were evaluated for their stabilizing properties in the model compound (Fig. 4). A good comparison can only be obtained if the amount of metal salt used for the experiment is calculated on a mole scale. It is clear that there is an influence of the kind of metal on the oxidation rate of the model compound. All metal salts in combination with KI show a lower oxygen uptake than KI alone, indicating that each metal has an influence on the rate of oxidation. The following order has been found for the different metals
A ,’
,/
,’
m’
,,,./’
500
1:’ / //.’ I/’ f _/’ ,
fig. 3. Oxygen uptake the model compound different copper salts CuCl+ KI (o), CuCl, CUO +KI (A), CuI
as a function of degradation time for (for structure see Fig. 1) containing with and without KI: CuCl (V), (O), CuCl, +KI ( +), CuO (O), (V), CuI+KI (0), KI (4) and reference (M).
Fig. 4. Oxygen uptake as a function of degradation time for the model compound (for structure see Fig. 1) containing different metal salts in combination with KI: ZnClJKI (A), AgWKI (A), CrClJKI (Cl), ZnClJKI (0), ZnIJKI (V), Mn(Ac)JKI (O), CoClJKI (V), CuCl/KI ( + ), CuI/KI (m) and reference ($).
130
K. Janssen
(with increasing right):
oxygen
uptake
from
et al.
left to
CuI < CuCl < CoCl, < Mn(Ac), < ZnI, < ZnCl, < CrCl, < AgCl < SnCl, The experimental observation of the cobalt salt gives a possible indication of the role played by the metal. Normally CoCI, has a purple colour. When the model compound is melted, the metal salt turns blue and, a short time’ later, green. The blue colour is an indication of a tetrahedral surrounding of the metal in the 2 + oxidation state, while the green colour is ascribed to the same complex but with the metal ion in the 3 + oxidation state (Co2+ is easily oxidized to Co”’ in an oxygen atmosphere, certainly at 180 “C). These observations indicate that the metal forms a complex with the amide structures. This complexation may be important for the stabilization of the model compound. Because it is very unlikely that Zn and Ag salts take part in redox reactions it is obvious that mechanisms using a redox reaction cannot explain their stabilizing effect. However, these two metals do give rise to stabilization which is more pronounced than KI alone. Thus, radical scavenging is not very likely in the case of these metals. Hydroperoxide groups are now widely recognized as key intermediates in the oxidative degradation of many polymers.12*1’ Besides radical scavenging a second mechanism causing stabilization is hydroperoxide decomposition into non-radical products.” In several analytical methods KI is used to determine the amount of hydroperoxide in polymers quantitatively.‘4.‘5 In these determinations the reaction is accelerated by acids according to the following reaction: ROOH +
2
N-
+
2
I+-
->
ROik
/
i
I.,
followed by determination of the I*. Dulog16 reported an iodometric determination method for hydroperoxides in polypropylene with sodium iodide in the presence of a CuCl crystal. In this determination the CuCl probably catalyzes the decomposition of the hydroperoxide with I-. Thus the iodide ion is able to decompose hydroperoxides into non-radical products according to reaction (10). The relative importance of hydroperoxide decomposition versus radical scavenging for the stabilization mechanism of aliphatic polyamides the low effect of phenolic also explains
Fig. 5. Structure of the phenolic antioxidant
used.
antioxidants. The effect of a well known phenolic antioxidant for polyamides (for structure see Fig. 5) on the stability of the model compound indeed is small (Fig. 6). The relative importance of the stabilizing effect by radical scavenging or hydroperoxide decomposition depends on the kinetic chain length (number of propagation reactions before termination) of the oxidation. For polymers having a high kinetic chain length, radical scavenging will be more effective than hydroperoxide decomposition, while for polymers having a low kinetic chain length it is the other way around. Probably due to a lower oxygen solubility and/or oxygen diffusion coefficient” the kinetic chain length is lower for aliphatic polyamides18 than for polyolefins,l’ which results in a relatively higher importance of stabilization by hydroperoxide for decomposition for polyamides than polyolefins. This might also be the reason for the high luminescence intensity found for polyamides in comparison to polyolefins.20 Luminescence is believed to be due to the termination reaction of two peroxy radicals.‘l The above-mentioned shorter kinetic chain length for polyamides is probably due to a higher termination rate, which results in a higher luminescence intensity. 3.2 Formation
of gaseous products
After each experiment the gas phase was analyzed with GC. Determination of CO and CO, gave interesting results. 3.3 Formation
of carbon monoxide
Figure 7 shows the relation between oxygen uptake and amount of CO formed for the oxidations of the model compound with different copper salts with and without KI. It is clearly seen that the copper salt without KI gives a higher conversion of oxygen into CO than KI alone. Surprisingly the reference has the lowest
131
The stabilization of polyamides by combinations of metal and halogen salts
0
mne
20
16
12
4
0
24
,
ix
400
200
oxygen
Fig. 6. Oxygen uptake as a function
of degradation time for the model compound (for structure see Fig. 1) containing: phenolic antioxidant (for structure see Fig. 5) (A), CuI + KI (V) and reference (0).
conversion. Figure 8 is a presentation of the data for the different metal salts in combination with KI. Again the reference shows less CO formation than the metal salts. When the CO formation is monitored at an intermediate value for the oxygen uptake (approximately 600 mmol/kg) the following order is obtained (with increasing conversion of oxygen into CO): SnCl, < AgCl < CrCl, < ZnCl, < ZnI, < Mn(Ac), < CoCl, < CuI < CuCl Surprisingly the opposite order is found for the formation of carbon monoxide as for the oxygen uptake. So if the metal stabilizer in combination with KI suppresses the oxidation of the model compound, it also results in a higher conversion of oxygen into CO. 3.4 Formation of carbon dioxide Figures 9 and 10 show the results of the conversion of oxygen into carbon dioxide for the
LDtaks
600
Fig. 8. Formation of CO versus oxygen uptake for the model compound (for structure see Fig. 1) containing different metal salts in combination with KI: SnCl,/KI (A), AgCl/KI (A), CrClJKI (Cl), ZnClJKI (0), ZnI,/KI (V), Mn(Ac),/KI (O), CoClJKI (V), CuCl/KI ( + ), CuI/KI (a) and reference (t).
different stabilizers. The same observations can be made as for the CO formation. The presence of metal ions in the stabilizer results in the formation of more CO,. When the different metal salts in combination with KI are screened at an oxygen uptake of 6OOmmol/kg almost the same order is found as for the CO formation. Only ZnCl, and Mn(Ac), are exceptions in this respect, which can perhaps be ascribed to the formation of CO, from the stabilizer itself by decomposition of the acetate group. Calculation of the amount of CO, results in almost 40 mmol/kg, which is close to the difference found between Mn(Ac), and CoCl,. From the analysis of CO and CO, it is seen that a lot of oxygen is missing. This oxygen may be chemically bound to the model compound, but also a lot of oxygen may have reacted to form water. Indeed it is observed that after longer oxidation times water is condensed on cold spots in the degradation vessel. 800
500 .
D ,’
,’
,’
400
600-
,’
0
500
1500
1000 OxyQer
Lptake
2000
800
hn!olikd
4,
,D ,’,’ ,’ ,’
2500
hrnoi/kg,
Fig. 7. Formation of CO versus oxygen uptake for the model compound (for structure see Fig. 1) containing different copper salts with and without KI: CuCl (0), CuCl, (A), CuO (m), Cu(OAc), (V), CuI (A), CuCl+ KI ( + ), CuI + KI ( + ), KI (0) and reference (V).
9. Formation of CO2 versus oxygen uptake for the model compound (for structure see Fig. 1) containing different copper salts with and without KI: CuCl (0), CuCl, (A), CuO (W), Cu(OAc), (V), CuI (A), CuCl + KI ( + ), CuI + KI ( + ), KI (0) and reference (V).
Fig.
132
K. Janssen et al.
the oxygen will be consumed in the ‘normal’ oxidation,2 resulting in a lower conversion of oxygen into CO and CO,.
4 CONCLUSIONS
Fig. 10. Formation of CO, versus oxygen uptake for the model compound (for structure see Fig. 1) containing different metal salts in combination with KI: SnCl,/KI (A) AgCI/KI (a), CrCI,/KI (El), ZnCl,/KI (0), ZnI,/KI (V), Mn(Ac),/KI (O), CoC12/KI (v), CuCl/KI ( + ), Cur/K1 (m) and reference (4).
3.5 Postulation of the mechanism for the stabilization It is found that mixtures of metal ions with KI suppress the oxidation of the model compound. This is not only the case for salts of transition metals but also for salts of Ag and Zn. Thus the stabilizing effect cannot be due only to radical scavenging and is probably due to decomposition of hydroperoxides by KI, which is enhanced by metal ions. It is also found that the better a metal ion reduces the oxygen uptake the higher the conversion of oxygen into CO and CO,. These results can be explained by the following mechanism:
ii
O-OH +
p,-
+
;1w*
Me”’
[‘I;
C. ,tl,,-C-NH-Ck-CH2-Ch2-CH2-NrW: ‘3
+
Me”-
I_ -5:
, ,h_,
H I_
+
II,’
w
Experiments revealed that the mechanism postulated earlier (radical scavenging) cannot in itself be responsible for the stabilization of aliphatic polyamides, because metal salts which cannot take part in an oxidation process show good suppression of the oxidation of the model compound if they are used in combination with KI. The mechanism of stabilization is probably based on the decomposition of hydroperoxides. KI is responsible for the decomposition of hydroperoxides into non-radical products, whereas the metal salt will catalyze this decomposition reaction by complexation of the metal salt onto the amide and/or hydroperoxide group. It is confirmed that the combination of metal salts with KI results in a synergistic mixture.
REFERENCES 1. Harschnitz, R., Heather, P., Derks, W. & Leeuwendal, R., Kunsfstoffe, 88(11) (1990) 1272. 2. Gijsman, P., Tummers, D. & Janssen, K., Polym. Deg. Stub., 49 (1995) 121. 3. Emanuel, N. M., et al., Angew. Chem., 81(1969) 91. _ 4. Reich R. H. Kc Stivala, S. S., In Autooxidation of Hydrocarbons & Polyolefins, Dekker, New York, 1969,
p. 247 .5. Hansen, R. H., et al., J. Polym. Sci., 2A (1964) 587. 6. Scott, G., In Atmospheric Oxidation & Antioxidants, Vol. 1, ed. G. Scott, Elsevier Science Publishers, Amsterdam, 1993, pp. l-44. 7. Scott, G., In Atmospheric Oxidation & Antioxidants, Vol. 2, ed. G. Scott, Elsevier Science Publishers, Amsterdam, 1993, pp. 141-218. x. Gmelins Handbuch die Anorganische Chemie, Vol. 60, ed. E. H. Erich Pietsch, 1958, p. 389. 9. Voigt, J., In Die Stabilisiering de Kunststoffe Gegen Licht und W&me, Springer-Verlag, Berlin-HeidelbergNew York, 1966. 10. Gijsman, P., Hennekens, J. & Tummers, D., Polym. Deg. Stab., 39 (1993) 225. F., In Oxidation Inhibition in Organic Materials, Vol. 1, ed. J. Pospisil & P. P. Klemchuk,
11. Gugumus,
Good stabilizers will decompose the majority of the hydroperoxides formed. According to the above-described mechanism this decomposition reaction can lead to aldehydes, which oxidize fast. This oxidation yields CO, CO, and carboxylic acids. Thus, for good stabilizers high conversions of oxygen into CO and CO, are expected. With poorer stabilizers a bigger part of
CRC Press, Boca Raton, Florida, 1990, pp. 61-172. 12. Carlsson, D. J. & Wiles, P. M., J. Macromol. Sci. Rev. Macromol. Chem., Cl4 (1976) 67. J. L. & Lemaire, J., Polym. Photochem., 7 (1986) 409. 14. Carlsson D. J. & Lacoste, J., Polym. Deg. Stab., 32 (1991) 377.
13. Gardette,
15. Gijsman,
P., Hennekens,
J. & Vincent,
Polym. Deg. Stab., 42 (1993) 95.
J. A. J. M.,
The stabilization of polyamides by combinations of metal and halogen salts 16. Dulog, L. & Bleher, R., Makromol. Chem., Rapid Commun., 3 (1986) 153. 17. Billingham, N. C., In Oxidation Inhibition in Organic Materials, Vol. II, ed. J. Pospisil & P. P. Klemchuk, CRC Press, Boca Raton, Florida, 1990, pp. 249-97. 18. Lanska, B., In Lactam Based Polymers, Vol. I, ed. R. Puffr & V. Kubanek, CRC Press, 1991, pp. 262-
133
302. 19. Garton, A., Carlsson, D. J. & Wiles, D. M., Makromol. Chem., 181(1980) 1841. 20. George, G. A., Dew Polym. Deg., 3 (1981) 173. 21. Billingham, N. C., Then, E. T. H. & Gijsman, P., Polym. Deg Stab., 34 (1991) 263.
Printed in Northern Ireland. All rights reserved 0141-3910/95/$09.50
0141-3910(95)00065-8
Mechanistic aspects of the stabilization of polyamides by combinations of metal and halogen salts Koen Janssen, Pieter Gijsman* & Daan Tmnmers DSM Research, PO Box 18, 6160 MD Geleen, The Netherlands
(Received 9 September
1994; accepted 20 October 1994)
For aliphatic polyamides the most effective stabilizers against the long-term thermooxidative degradation are synergetic combinations of metal (mainly Cu and Mn) and halogen (mainly Br and I) salts. Their mechanism of action is still a topic of discussion. Experiments with a model compound for polyamide 46 (PA46) confirm that the combination of metal and halogen salts results in a synergetic mixture. It is shown that radical scavenging only cannot be responsible for the stabilizing action of mixtures of metal and halogen salts. The mechanism of action of combinations of metal and halogen salts is probably based on the metal ion catalyzed decomposition of hydroperoxides by the halogen salt.
1 INTRODUCTION
In comparison with polyolefins, polyamides have a high thermal resistance, which is the reason why they are mainly used in high-temperature environments. To be applicable, the polymers have to be stable at these high temperatures. The development of polyamides with a higher thermal resistance such as polyamide 46 (PA46)’ resulted in more interest in the thermooxidative stability of aliphatic polyamides. For unstabilized PA46 we have shown’ that due to a higher crystallinity and/or density of the amorphous phase the stability of this polymer is higher than the stability of PA66. We have also shown that the degradation chemistries of these two polymers are comparable. In both cases the degradation is caused by a radical oxidation mainly taking place at the N-vicinal methylene, in which reaction the of hydroperoxides plays an decomposition important role. For demanding applications aliphatic polyamides are stabilized. The most effective stabilizers are combinations of metal (mainly Cu and Mn) and halogen (mainly Br and I) salts. This is surprising because metal ions act as * Author to whom correspondence
should be addressed.
prodegradants in polyolefins, which is due to the catalytic decomposition of hydroperoxides by these transition metals according to the following mechanism: ROOH ROOH
Me”++
+ +
RO”
Me(“+‘)+-
+
OH-
+
ROO”
+
H’
+
ROO”
+
RO”
Me’“+“+
[Al
Me”+
PI
Me-/Me”+‘*
ROOH
2
+
+
H,O
I31
According to Emanuel and coworkers3 the electron transfer is preceded by the formation of a coordination complex between the metal ion and the hydroperoxide. The most active catalysts of hydroperoxide decomposition are derivatives of those metals, which are easily oxidized or reduced by a one-electron transfer, such as Cu, Co, Mn and Fe.4T5 In a number of systems transition metals can act as the antioxidant. This activity might be caused by two reactions:6 R” ROO”
M&f?)
+ +
Me”+
---, v
R+ ROO-
+
Me”+ +
Mel”+ 1)
[41
[51
In the case of the stabilization of polyamides with Cu ions it has been suggested that the
128
K. Janssen et al.
antioxidant effect is catalytic. Cu*’ oxidizes an alkyl radical to a carbonium ion, and the Cu’ formed in this reaction reduces alkylperoxy radicals:’ ri
?
of 152 “C (yield: 77% and purity: 99%). Analysis with GC-MS, IR and ‘H NMR confirmed the structure of the model compound (see Fig. l).For more details on the synthesis see Ref. 2. This compound does not sublime at 180°C (the temperature at which the experiments are done).
-CH,-C-N&H-
2.2 Chemicals
[81
It is also described that I- can reduce according to the following reaction?
Cu*+
All chemicals were used as received, but were crushed before adding to the model compound. CuI (98%), CuCl (99%), CuCl, (98%) Mn(Ac),.4H,O (99%) and CoCl, (97%) were purchased from Janssen Chimica. CuO (98%) and AgCl (99%) were purchased from Aldrich. ZnI, (98.5%), ZnCl, (97%), KI (99.5%) and CrCl, (98%) were purchased from Merck. 2.3
However, if this reaction causes the synergism between Cu and halogen salts, the most effective species has to be Cu+, and reaction (7) has to be, the reaction causing the antioxidant effectivity of Cu salts. However, this should result in a pronounced difference in activity between Cu’ and Cu*+, which is not the case.9 A better understanding of the synergism of Cu and halogen salts might lead to a further improvement of the long-term stability of aliphatic polyamides. Our study was done on a model compound which was shown to degrade comparably to PA46*, with several metal ions. A mechanism explaining the synergism between metal and halogen salts is postulated.
2 EXPERIMENTAL 2.1 Synthesis of the model compound
Experimental
setup
The setup consists of three degradation vessels, surrounded by a heated silicone oil jacket. The temperature in the vessels is kept constant at 180°C. All experiments were done with pure oxygen and 5 g of model compound, 0.09 mmol of metal salt and 1.1 mmol of potasium iodide. The stirring speed applied was 270 rpm. The pressure in each vessel was measured with a pressure transducer (Druck Ltd type PDCR 910 S/N 338692; accuracy: &0.07%). After each experiment gas samples were taken. In these samples the amounts of oxygen, CO and CO, were determined by gas chromatography (for more details see Ref. 10). In order to calculate the oxygen uptake, the volume of each vessel has to be known. This is determined volumetrically at room temperature with a gas burette filled with pure oxygen. For more details on the experimental setup see Ref. 2.
The model compound
(for structure see Fig. 1) is prepared from lauric acid (Janssen Chimica ~. 99.5%) and butane diamine (DSM) in m-xylene. The mixture is refluxed, during which water from the condensation is distilled off azeotropically. A crystalline powder is isolated with a melting point
? C, ,H23-C-NH-CH,CH,CH,CH,-NWC,
FEg. 1. Structure of model compound
0 II
-C, ,H,,
used.
3 RESULTS
AND
DISCUSSION
3.1 Oxygen uptake
Different copper salts were tested for their stabilizing action in the model compound during oxidation. Assuming that each molecule of the model compound reacts with one molecule of oxygen, 2212 mmol oxygen/kg compound can be supplied. For the unstabilized model compound
The stabilization of polyamides by combinations of metal and halogen salts
Time
(h)
Eig. 2. Oxygen uptake as a function of degradation time for the model compound (for structure see Fig. I), containing different copper salts: CuCl (0), CuCl, (A), CuO (W), Cu(OAc), (V), CUT (4) and reference (V).
this amount of oxygen uptake is almost reached (Fig. 2) after 20 h at 180 “C in pure oxygen. The stabilizing activity of the different Cu-salts depends on their counter ion (Fig. 2). CuCl does not stabilize at all, but also shows no catalytic effect on the oxidation of the compound, as in polyolefins. CuCl, shows a small stabilizing effect. Surprisingly CuO reduces the oxygen uptake to 1270 mmol/kg, while Cu(Ac),.H,O shows only half of the reference’s oxygen uptake. The oxygen uptake in the presence of CuI is the lowest, after 145 h, 565 mmol/kg. Supposing that the uptake follows a linear path, the uptake should be approximately 90 mmol/kg after 20 h. Thus, an increase in stabilizing effect is found from CuCl to CuCl, to CuO to Cu(Ac),.H,O to GUI. From these experiments it is concluded that 20 h of oxidation is sufficient to measure the effect of stabilizers. The synergism of Cu and halogen salts was studied with CuCl, CuCl,, CuO, CuI and KI (Fig. 1000
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I’
129
3). Especially in the beginning KI has a it is small in stabilizing activity, although comparison to CuI. The synergisms between CuCl and CuCl, with KI are the largest, followed by CuO and KI. The smallest synergism is found between CuI and KI. However, this synergism might become more pronounced after longer oxidation times. From observation during the experiments it is known that a major part of the KI is not dissolved and lies on the bottom of the degradation vessel. So the effects of KI are due to very small amounts. The two most important routes to stabilize polymers against thermooxidative degradation peroxide are radical trapping and decomposition.l’ One of the possible synergisms between transition metals and halogen salts might be due to radical scavenging by the metal salt in its highest oxidation state, followed by a reaction of the metal in its lowest oxidation state with the halogen salt back to the metal ion in its highest oxidation state (reaction (9)). To check this postulated mechanism different metals (with and without a one-electron transfer) in combination with KI were evaluated for their stabilizing properties in the model compound (Fig. 4). A good comparison can only be obtained if the amount of metal salt used for the experiment is calculated on a mole scale. It is clear that there is an influence of the kind of metal on the oxidation rate of the model compound. All metal salts in combination with KI show a lower oxygen uptake than KI alone, indicating that each metal has an influence on the rate of oxidation. The following order has been found for the different metals
A ,’
,/
,’
m’
,,,./’
500
1:’ / //.’ I/’ f _/’ ,
fig. 3. Oxygen uptake the model compound different copper salts CuCl+ KI (o), CuCl, CUO +KI (A), CuI
as a function of degradation time for (for structure see Fig. 1) containing with and without KI: CuCl (V), (O), CuCl, +KI ( +), CuO (O), (V), CuI+KI (0), KI (4) and reference (M).
Fig. 4. Oxygen uptake as a function of degradation time for the model compound (for structure see Fig. 1) containing different metal salts in combination with KI: ZnClJKI (A), AgWKI (A), CrClJKI (Cl), ZnClJKI (0), ZnIJKI (V), Mn(Ac)JKI (O), CoClJKI (V), CuCl/KI ( + ), CuI/KI (m) and reference ($).
130
K. Janssen
(with increasing right):
oxygen
uptake
from
et al.
left to
CuI < CuCl < CoCl, < Mn(Ac), < ZnI, < ZnCl, < CrCl, < AgCl < SnCl, The experimental observation of the cobalt salt gives a possible indication of the role played by the metal. Normally CoCI, has a purple colour. When the model compound is melted, the metal salt turns blue and, a short time’ later, green. The blue colour is an indication of a tetrahedral surrounding of the metal in the 2 + oxidation state, while the green colour is ascribed to the same complex but with the metal ion in the 3 + oxidation state (Co2+ is easily oxidized to Co”’ in an oxygen atmosphere, certainly at 180 “C). These observations indicate that the metal forms a complex with the amide structures. This complexation may be important for the stabilization of the model compound. Because it is very unlikely that Zn and Ag salts take part in redox reactions it is obvious that mechanisms using a redox reaction cannot explain their stabilizing effect. However, these two metals do give rise to stabilization which is more pronounced than KI alone. Thus, radical scavenging is not very likely in the case of these metals. Hydroperoxide groups are now widely recognized as key intermediates in the oxidative degradation of many polymers.12*1’ Besides radical scavenging a second mechanism causing stabilization is hydroperoxide decomposition into non-radical products.” In several analytical methods KI is used to determine the amount of hydroperoxide in polymers quantitatively.‘4.‘5 In these determinations the reaction is accelerated by acids according to the following reaction: ROOH +
2
N-
+
2
I+-
->
ROik
/
i
I.,
followed by determination of the I*. Dulog16 reported an iodometric determination method for hydroperoxides in polypropylene with sodium iodide in the presence of a CuCl crystal. In this determination the CuCl probably catalyzes the decomposition of the hydroperoxide with I-. Thus the iodide ion is able to decompose hydroperoxides into non-radical products according to reaction (10). The relative importance of hydroperoxide decomposition versus radical scavenging for the stabilization mechanism of aliphatic polyamides the low effect of phenolic also explains
Fig. 5. Structure of the phenolic antioxidant
used.
antioxidants. The effect of a well known phenolic antioxidant for polyamides (for structure see Fig. 5) on the stability of the model compound indeed is small (Fig. 6). The relative importance of the stabilizing effect by radical scavenging or hydroperoxide decomposition depends on the kinetic chain length (number of propagation reactions before termination) of the oxidation. For polymers having a high kinetic chain length, radical scavenging will be more effective than hydroperoxide decomposition, while for polymers having a low kinetic chain length it is the other way around. Probably due to a lower oxygen solubility and/or oxygen diffusion coefficient” the kinetic chain length is lower for aliphatic polyamides18 than for polyolefins,l’ which results in a relatively higher importance of stabilization by hydroperoxide for decomposition for polyamides than polyolefins. This might also be the reason for the high luminescence intensity found for polyamides in comparison to polyolefins.20 Luminescence is believed to be due to the termination reaction of two peroxy radicals.‘l The above-mentioned shorter kinetic chain length for polyamides is probably due to a higher termination rate, which results in a higher luminescence intensity. 3.2 Formation
of gaseous products
After each experiment the gas phase was analyzed with GC. Determination of CO and CO, gave interesting results. 3.3 Formation
of carbon monoxide
Figure 7 shows the relation between oxygen uptake and amount of CO formed for the oxidations of the model compound with different copper salts with and without KI. It is clearly seen that the copper salt without KI gives a higher conversion of oxygen into CO than KI alone. Surprisingly the reference has the lowest
131
The stabilization of polyamides by combinations of metal and halogen salts
0
mne
20
16
12
4
0
24
,
ix
400
200
oxygen
Fig. 6. Oxygen uptake as a function
of degradation time for the model compound (for structure see Fig. 1) containing: phenolic antioxidant (for structure see Fig. 5) (A), CuI + KI (V) and reference (0).
conversion. Figure 8 is a presentation of the data for the different metal salts in combination with KI. Again the reference shows less CO formation than the metal salts. When the CO formation is monitored at an intermediate value for the oxygen uptake (approximately 600 mmol/kg) the following order is obtained (with increasing conversion of oxygen into CO): SnCl, < AgCl < CrCl, < ZnCl, < ZnI, < Mn(Ac), < CoCl, < CuI < CuCl Surprisingly the opposite order is found for the formation of carbon monoxide as for the oxygen uptake. So if the metal stabilizer in combination with KI suppresses the oxidation of the model compound, it also results in a higher conversion of oxygen into CO. 3.4 Formation of carbon dioxide Figures 9 and 10 show the results of the conversion of oxygen into carbon dioxide for the
LDtaks
600
Fig. 8. Formation of CO versus oxygen uptake for the model compound (for structure see Fig. 1) containing different metal salts in combination with KI: SnCl,/KI (A), AgCl/KI (A), CrClJKI (Cl), ZnClJKI (0), ZnI,/KI (V), Mn(Ac),/KI (O), CoClJKI (V), CuCl/KI ( + ), CuI/KI (a) and reference (t).
different stabilizers. The same observations can be made as for the CO formation. The presence of metal ions in the stabilizer results in the formation of more CO,. When the different metal salts in combination with KI are screened at an oxygen uptake of 6OOmmol/kg almost the same order is found as for the CO formation. Only ZnCl, and Mn(Ac), are exceptions in this respect, which can perhaps be ascribed to the formation of CO, from the stabilizer itself by decomposition of the acetate group. Calculation of the amount of CO, results in almost 40 mmol/kg, which is close to the difference found between Mn(Ac), and CoCl,. From the analysis of CO and CO, it is seen that a lot of oxygen is missing. This oxygen may be chemically bound to the model compound, but also a lot of oxygen may have reacted to form water. Indeed it is observed that after longer oxidation times water is condensed on cold spots in the degradation vessel. 800
500 .
D ,’
,’
,’
400
600-
,’
0
500
1500
1000 OxyQer
Lptake
2000
800
hn!olikd
4,
,D ,’,’ ,’ ,’
2500
hrnoi/kg,
Fig. 7. Formation of CO versus oxygen uptake for the model compound (for structure see Fig. 1) containing different copper salts with and without KI: CuCl (0), CuCl, (A), CuO (m), Cu(OAc), (V), CuI (A), CuCl+ KI ( + ), CuI + KI ( + ), KI (0) and reference (V).
9. Formation of CO2 versus oxygen uptake for the model compound (for structure see Fig. 1) containing different copper salts with and without KI: CuCl (0), CuCl, (A), CuO (W), Cu(OAc), (V), CuI (A), CuCl + KI ( + ), CuI + KI ( + ), KI (0) and reference (V).
Fig.
132
K. Janssen et al.
the oxygen will be consumed in the ‘normal’ oxidation,2 resulting in a lower conversion of oxygen into CO and CO,.
4 CONCLUSIONS
Fig. 10. Formation of CO, versus oxygen uptake for the model compound (for structure see Fig. 1) containing different metal salts in combination with KI: SnCl,/KI (A) AgCI/KI (a), CrCI,/KI (El), ZnCl,/KI (0), ZnI,/KI (V), Mn(Ac),/KI (O), CoC12/KI (v), CuCl/KI ( + ), Cur/K1 (m) and reference (4).
3.5 Postulation of the mechanism for the stabilization It is found that mixtures of metal ions with KI suppress the oxidation of the model compound. This is not only the case for salts of transition metals but also for salts of Ag and Zn. Thus the stabilizing effect cannot be due only to radical scavenging and is probably due to decomposition of hydroperoxides by KI, which is enhanced by metal ions. It is also found that the better a metal ion reduces the oxygen uptake the higher the conversion of oxygen into CO and CO,. These results can be explained by the following mechanism:
ii
O-OH +
p,-
+
;1w*
Me”’
[‘I;
C. ,tl,,-C-NH-Ck-CH2-Ch2-CH2-NrW: ‘3
+
Me”-
I_ -5:
, ,h_,
H I_
+
II,’
w
Experiments revealed that the mechanism postulated earlier (radical scavenging) cannot in itself be responsible for the stabilization of aliphatic polyamides, because metal salts which cannot take part in an oxidation process show good suppression of the oxidation of the model compound if they are used in combination with KI. The mechanism of stabilization is probably based on the decomposition of hydroperoxides. KI is responsible for the decomposition of hydroperoxides into non-radical products, whereas the metal salt will catalyze this decomposition reaction by complexation of the metal salt onto the amide and/or hydroperoxide group. It is confirmed that the combination of metal salts with KI results in a synergistic mixture.
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p. 247 .5. Hansen, R. H., et al., J. Polym. Sci., 2A (1964) 587. 6. Scott, G., In Atmospheric Oxidation & Antioxidants, Vol. 1, ed. G. Scott, Elsevier Science Publishers, Amsterdam, 1993, pp. l-44. 7. Scott, G., In Atmospheric Oxidation & Antioxidants, Vol. 2, ed. G. Scott, Elsevier Science Publishers, Amsterdam, 1993, pp. 141-218. x. Gmelins Handbuch die Anorganische Chemie, Vol. 60, ed. E. H. Erich Pietsch, 1958, p. 389. 9. Voigt, J., In Die Stabilisiering de Kunststoffe Gegen Licht und W&me, Springer-Verlag, Berlin-HeidelbergNew York, 1966. 10. Gijsman, P., Hennekens, J. & Tummers, D., Polym. Deg. Stab., 39 (1993) 225. F., In Oxidation Inhibition in Organic Materials, Vol. 1, ed. J. Pospisil & P. P. Klemchuk,
11. Gugumus,
Good stabilizers will decompose the majority of the hydroperoxides formed. According to the above-described mechanism this decomposition reaction can lead to aldehydes, which oxidize fast. This oxidation yields CO, CO, and carboxylic acids. Thus, for good stabilizers high conversions of oxygen into CO and CO, are expected. With poorer stabilizers a bigger part of
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