- Email: [email protected]
SYNERGISM AND ANTAGONISM
431
Chapter 9 SYNERGISM AND ANTAGONISM GERALD SCOTT 1. FACTORS AFFECTING ANTIOXIDANT PERFORMANCE
Attempts to predict the practical performance of antioxidants in polymer systems from their behaviour in pure model compounds are rarely success ful. Many factors contribute to this and some of these, such as physical loss of antioxidants by volatilisation or leaching are now understood and have been discussed in Chapter 4. In many cases, however, lower than expected or higher than expected performance may be due to interference from some other chemicals, either adventitiously present or deliberately added to the system, which interact with the antioxidant. Most obvious of these are the pro-oxidants discussed in Volume I, Chapter 3. By increasing the oxidative stress they simply accelerate antioxidant destruction and hence shorten the induction period to the onset of rapid autoxidation. A familiar example of the effect of adventitious contaminants is the catalytic effect of transition metal ions on the homolytic breakdown of hydroperoxides which leads to more rapid destruction of chain breaking antioxidants (see Chapter 6). A similar phenomenon accounts for the relative ineffectiveness of chainbreaking antioxidants in a sulphur vulcanisate compared with raw rubber. Not only is the normal induction period observed in a pure hydrocarbon substrate shortened or removed completely but the rate of oxidation is also increased in the initial stages. This phenomenon is again due to the rapid introduction of radicals into the system by redox reactions between hy droperoxides and oxidised sulphur species (see Volume I, Chapter 5). Such effects may generally be predicted from a study of the non-sulphurated and sulphur modified substrates with and without antioxidant. Other antago nistic effects are more complex. For example, it was seen earlier (Chapter 3, Section 5) that hindered phenols are relatively ineffective under conditions of UV irradiation, due primarily to their photo-instability and in some cases, photo-prooxidant effects. Consequently, they antagonise with other antiox idants during UV exposure, but are protected by UV absorbers with which they effectively synergise. Phenolic antioxidants can also be readily de stroyed by agents such as ozone and singlet oxygen, in the latter case
432
GERALD SCOTT
through a charge-transfer intermediate [1,2]. This is of little value as an antioxidant process as is sometimes claimed since the proposed end product (I) is a typical hydroperoxide initiator. OH
OH
Ri-
~^
R
O,
^K^ > 1
W R,
+ -OOH
OOH I Hydroperoxides are themselves antagonists for some stabilisers. For example 2-hydroxybenzophenone UV absorbers vary considerably in their effectiveness depending on the processing conditions, and this has been shown to be due to the formation of hydroperoxides in the melt which rapidly destroy the UV absorber during exposure to UV light (see Chapter 8, Section 5.1.2). 2. SYNERGISTIC EFFECTS OF ANTIOXIDANTS AND STABILISERS
It is a very rare occurrence that two antioxidants simply give an additive effect in an oxidising substrate. If they did, replacement of a molar percent age of antioxidant A by antioxidant B would be predictable by the ideal straight line a'-b' in Fig. 1. In practice the result is generally either better or worse than that predicted. If better, this is synergism and a practical advantage but if it is worse, the result is antagonism and this particular combination has to be avoided if the antioxidant effectiveness is to be maximised [3]. Synergism is normally expressed as the percentage improve ment given by the combination of antioxidants over that expected on an additive basis and conversely, antagonism is the percentage decrease com pared with that expected (see Fig. 1). The development of synergistic "packages" has been one of the major accomplishments of stabilisation technology. It is very unlikely that any single-component stabilisation system would have been effective enough to
SYNERGISM AND ANTAGONISM
433
Synergism
A
Molar Ratio B/A
B
Fig. 1. Synergism and antagonism expressed as a function of the molar ratio of antioxidants A and B. protect the α-branched polyolefins [4]. Some very remarkable combination effects have been observed and in recent years the basic science underlying synergism has found some explanation in the organic and physical chemis try which has been discussed so far in this volume. The physical chemistry of antioxidants and stabilisers in polymers which is partly responsible for synergism has already been discussed in Chapter 4. In this Chapter, emphasis will be placed on the organic chemistry of syner gism. Three kinds of synergism can be distinguished [3]: (a) homosynergism in which two antioxidants acting by the same mechan ism interact, generally in a single electron transfer cascade; (b) heterosynergism where the antioxidants act by a different mechanism and hence complement one another; (c) autosynergism in which synergism results from two different functions in the same molecule. 2,1 Homosynergism The best known example of homosynergism is the regeneration of the tocopherols from their derived phenoxyls by ascorbic acid reduction (see Volume I, Chapter 1). Homosynergism has also been reported between phenolic antioxidants in hydrocarbons [3]. The effect is generally maximised when there is a complementation of steric hindrance in the phenols of the synergistic combination. Similar synergism is observed between aromatic amines and phenols where the phenol is sacrificed with regeneration of the amine [5] or possibly the derived nitroxyl. The sacrificial component of a synergistic mixture may have little antioxidant activity in its own right. Alkyl phosphite and phosphonate esters are
434
GERALD SCOTT
(2)
typical examples of weak antioxidants which, when used in combination with hindered phenols [6] considerably improve antioxidant performance and colour. It seems likely that they reduce back the phenoxyl before it can be irreversibly destroyed. The dialkyl monosulphides (II) are effective synergists with hindered phenols in polyolefins and there is little doubt that complementary CB-D and PD-C activities are primarily responsible for this phenomenon. XCH2CH2SCH2CH2X
II
However, De Jonge and Hope [7] have shown that in the case of phenols which give rise to relatively stable radicals (notably 2,6-diphenyl-4-methoxy phenol, III), regeneration of the phenol from phenoxyl, V, occurs at the expense of the monosulphides, IV (see Scheme 1). It is should be noted that the second regeneration step in Scheme 1 is closely related to the mecha nism discussed in Chapter 4 for the regeneration of hydrogalvinoxyl from galvinoxyl with elimination of a macroalkyl radical in the polymer chain. It is not clear how important is the homosynergistic mechanism shown in Scheme 1 compared with the competing peroxidolytic process known to occur (see Volume I, Chapter 5), since a variety of other oxidised products such as VIII, involved in the peroxidolytic activity of dialkyl monosulphides, (II) are also formed. O II XCH2CH2S SCH2CH2X
VIII
O 2.2 Heterosynergism Scheme 2 summarises the complementary mechanisms of antioxidant action. From this it follows that antioxidants which eliminate the main source of initiating radicals, the peroxide decomposers, extend the useful life of the chain-breaking antioxidants. Conversely, chain-breaking antioxi dants reduce the formation of hydroperoxides and hence protect the per oxide decomposers from decomposition. These cooperative effects are the basis of heterosynergism.
435
SYNERGISM AND ANTAGONISM
Ph
+ (R'OCOCH2CH2)2S IV
OMe
*
OH
xA^Ph 1 J III OMe
+ R'OCOCH2CH \
/
R'OCOCH=CH \
ΠΙ+
/
S VI
R'OCOCH2CH2
S VII
R'OCOCH2CH2 Scheme 1. Homosynergism between phenols and thiodipropionate esters [7].
CB-D
CHAIN-BREAKING MECHANISM
RO- + -OH
ROO-
PREVENTIVE MECHANISM CB—D -ROOH UVAMD
RH
PD-S PD-C UV Light Metal ions
Scheme 2. Mechanisms of antioxidant action [73]. CB-A, Chain-breaking acceptor. CB-D, Chain-breaking donor. PD-S, Stoichiometric peroxide decomposer. PD-C, Catalytic per oxide decomposer. UVA, UV Absorber. MD, Metal deactivator.
436
GERALD SCOTT
2.2.1 Thermal stabilisation based on CB-D/PD antioxidants Synergism between hindered phenols and sulphur compounds has been known for many years [8] and has been extensively studied in the polyolefins. Figure 2 shows the cooperative effects of several high molecular
0 DLTP
JL
_L
-L
JL
25 50 75 Composition of Stabiliser, %
100 Phenolic
Fig. 2. Synergism between DLTP and phenols in polypropylene at constant total concen tration in an air oven at 150°C.
1.
.™ r C -
(HO
Me OH
OH
2.
Me tBu 3.
HO
tBu CHCH,CH
OH).
(Reproduced with kind permission from Eur. Polym. J., Supplement 1969, p. 189).
SYNERGISM AND ANTAGONISM
300
-
/
\MBI+HDPA
\
MBT + HDPA
200
100
0.2
0.1
0.08
0.4
0.06
0.6
0.Θ
0.04
■A. 0.02
1 0 HDPA J
MBI/MBT
Concentration, mol/kg
Fig. 3. Synergism between p-hydroxydiphenylamine (HDPA) and sulphur compounds (MBI, MBT) in polypropylene at constant total concentration at 200°C. (Reproduced with kind permission from Eur. Polym. J., Supplement, 1969, p. 189).
weight phenols with dilauryl thiodipropionate [9] in polypropylene and Fig. 3, similar combinations of arylamine and two peroxide decomposers [9]. The optimum combination varies with the nature of both the phenol and the peroxide decomposer. The augmentation of one antioxidant by another is an effective way of using antioxidants efficiently, and is of great practical utility. Such antioxidant combinations have to be evaluated at various ratios to achieve maximal potential, since synergism may not be optimal at a 1:1 combination. TABLE 1 Synergism between ZnDEC (IXa) and a hindered phenol, 1076 (X) in the thermal stabilisation of polyolefins [101. (All concentrations 3*10 mol/100 g) Antioxidant
PP Embrittlement time (h at 140°C)
LDPE Induction period (h at 110°C)
None ZnDEC 1076 ZnDEC + 1076
0.5 32 58 165
10 110 110 300
438
GERALD SCOTT
Synergistic combinations of hindered phenols and zinc dialkyl dithiocarbamates (e.g. IXa) are particularly effective heat stabilisers for polyolefins [10] (see Table 1). The transition metal dithiocarbamates (e.g. IX(b) are also effective [8]. Unlike the hindered phenols, the effectiveness of the metal thiolate peroxide decomposers is very little affected by processing conditions [10]. OH tBu^^^^tBu R 2 NC^f
^ M
IX
An
ZnDEC, (a) R = Et, M = Zn, n = 2
CH 2 CH 2 COOR
NiDBC, (b) R = Bu, M = Ni, n = 2
1076j R =
^ j ^
O HOBP, R = C 8 H 17
/
/
c
\
^ O R
XI
HO 2.2.2 Synergistic UV stabilisers The thermal synergists discussed in the previous section are ineffective light stabilisers and it can be seen from Table 2 that a combination of a hindered phenol (1076, X) and a zinc dithiocarbamate are actually antago nists under photooxidative conditions. A combination of a UV absorber HOBP (XI, R=Oct) and zinc diethyl dithiocarbamate, ZnDEC (IX(a)) on the other hand, are synergistic [10]. Part of the explanation for this is evident from Table 2. It can be seen that the processing operation partially destroys the photoantioxidant activity of the UV absorber. Studies in model hydro carbons have also shown that 2-hydroxybenzophenones are rapidly de stroyed by UV light in the presence of hydroperoxides (see Chapter 8, Section 5.1.2). By destroying hydroperoxides during processing and sub sequently during UV irradiation the peroxide decomposer effectively ex tends the useful life of the UV absorber. However, ZnDEC and to a lesser extent NiDEC are destroyed by light, see Fig. 4, and part of the effect of the UV absorber is to protect the peroxide decomposers. Figure 4 also follows the decay of the two peroxide decomposers in the presence of HOBP and it was suggested that in this case the secondary peroxidolytic antioxidants formed from the dithiocarbamates protect the UV absorber from photooxidation [10]. Chakraborty and Scott [11] found that the optimal molar ratio of HOBP to ZnDEC was 2:1 in polypropylene (see Fig. 5). This underlines the
439
SYNERGISM AND ANTAGONISM
TABLE 2 Synergism between a UV absorber and antioxidants [10]. (Concentration, S^IQT4 mol/100 g) Stabiliser
Time to embrittlement (h) Processing time (min)
ZnDEC (IXa) HOBP (XI) 1076 (X) ZnDEC + HOBP 1076 + HOBP ZnDEC + 1076 Control, no additive
10
30
1400 2200 1800 >4000 «3500 1250 1200
1400 1600 1750 >4000 «3500 1250 900
Fig. 4. Change in the UV and visible absorption spectrum (Xmax at 330 nm of additives during the photooxidation of LDPE (processed 10 min at 150°C. Concentration of addi tives, 3X10"4 mol/100 g). 1, ZnDEC (330 nm); 2, NiDEC (330 nm); 3, NiDEC + HOBP (390-395 nm); 4, HOBP (330 nm); 5, ZnDEC + HOBP (330 nm); 6, NiDEC + HOBP (330 nm). (Reproduced with kind permission from Eur. Polym. J., 13 (1977) 1007).
general principle discussed above that it is not possible to assume that the optimum synergistic ratio is 1:1. Table 2 also shows that a hindered phenol synergises similarly with HOBP although not quite as effectively as the peroxide decomposers. Again there is evidence that the chain-breaking antioxidant can also protect the UV absorber during processing. However, Table 3 and Fig. 6, which compare the activity of synergistic mixtures at constant molar total concentration in
440
GERALD SCOTT
900r
80θ[
70θ[ JC
£ 60o| "c £ 50θ[
—
+* "ti
|
I I
400F
UJ
300l·
0
10
20
30
40
50
60
70
80
90
100
Mole fraction of ZnDEC(x100)
Fig. 5. Synergism between ZnDEC and HOBP in the photostabilisation of polypropylene. Total additive concentration βχΚΓ4 mol/100 g. (Reproduced with kind permission from Developments in Polymer Stabilisation, G. Scott (Ed.), Applied Science Publishers, 1983, p. 98).
PP [12] show that even a small proportion (20%) of phenolic antioxidants have a profound stabilising effect on the UV stability of HOBP. The corre spondence of the carbonyl formation curves and the HOBP decay curves (Fig. 6) suggests that the more powerful autosynergistic antioxidant pro tects the weaker antioxidant although doubtless this is only possible be cause the UV absorber protects the phenol from photolysis. Table 3 il lustrates the correspondence between embrittlement time and the rate of HOBP destruction in the polymer. Shlyapintokh et al. [13] have suggested that there may be a further reason for synergism between a UV absorber and a chain-breaking anti oxidant. They have pointed out that preferential destruction of antioxidant will occur in the surface of the polymer and that as a consequence, its concentration may drop below its critical level [14]. One role for the co-agent may be, therefore, to increase the rate of diffusion of the antioxidant from the polymer bulk to the surface, thus maintaining a greater than critical concentration in the "action zone". In a theoretical treatment of the effects of diffusion rate on stabilising effectiveness, Ivanov and Shlyapintokh have drawn the following conclusions [13]: (a) Light stability is proportional to sample thickness. (b) The higher the light intensity, the higher is the antioxidant concentra tion gradient.
441
SYNERGISM AND ANTAGONISM
+ 0.1
+0.2 HT o
+ 0.3
300
600
900
1200
1500
1Ö00
2100
Irradation Time, h
Fig. 6. Relationship between the rate of photooxidation (a)-(h) of polypropylene and the decay of the UV absorbance of HOBP (a')-(h') in the presence of BHBM-12 (XlVb, R=CieH37)*. Total additive concentration, 10"3 g/100 g. (a) control, no additive; (b) BHBM-12; (c), (c') 20% HOBP; (d), (dO 35% HOBP; (e), (e') 50% HOBP; (f) 65% HOBP, (g), (gO 80% HOBP; (h), (h') 100% HOBP. (Reproduced with kind permission from Polym. Deg. Stab., 2 (1980) 309). TABLE 3 Synergism between a UV absorber (HOBP) and hindered phenols (total concentration of synergistic mixture, 10 mol/100 g) Anti1076 oxidant (mol %) Te (h)
Syn (%)
(lO 4 ^ 1 )
—
35 50 65 80
750 1590 1350 1115 980 705
129 127 127
1.8 4.1
100
260
—
—
0 20
XIV(b)*
XIV(a)*
139 132
3.8 1.1 1.4
6.0
Te (h) 750 2100 1850 1547 1240 830 270
(104 h"1)
Te (h)
Syn (%)
ka
3.8 0.8
750 2350
—
3.8 0.8
176
1.1 1.6 2.9 3.7
1935 1600 1260 840
—
—
250
250 251 250 201 —
Syn (%) — 230 232 233 232
267
(10 4 IT1)
1.1 1.5 2.8 3.5 —
T e = Time to embrittlement; k& = First order rate constant for the decay of HOBP,
fWn m . QSu-Ea-MEà + toEà x i n f t (Ei-Ec) + (&2-Ec) whereas = embrittlement time of synergistic mixture; Ec - embrittlement time of control (no additive), E\ = embrittlement time of stabiliser 1; E2 - embrittlement time of stabil iser 2. ♦See Table 4.
442
GERALD SCOTT
(c) Diffusion synergism is dependent on the molar mass of the diffusant. (d) The extent of the synergism depends on the extinction coefficient of the UV absorber. (e) Maximal synergism is achieved at a 1:1 ratio. The fact that condition (e) is not met in Table 3 and Fig. 5 suggests that factors other than antioxidant diffusion may be involved in the synergism. However, as will be seen in the next section, some additives which have no stabilising activity in their own right can be synergistic with known photoantioxidants, thus eliminating chemical synergism 2.2.3 Synergism between photoantioxidants and antioxidant-inactive agents It was seen above that zinc dialkyldithiocarbamates are weak photoan tioxidants due to rapid photolysis of the metal complexes. It was found by Scott and co-workers [15,16] that aliphatic amine complexes of the zinc dithiocarbamates were about twice as effective as photostabilisers than the parent dithiocarbamates in polypropylene, although the amines alone had little UV stabilising activity. Three factors were found to be involved in the synergism: (i) The amine complexes were more resistant to photooxidation in poly propylene than the parent metal complex. This is shown typically in Fig. 7 for the complex of zinc diethyldithiocarbamate with diazabicyclooctane (DABCO, XII) which is an effective quenching agent for excited states [17-19]. It was suggested that DABCO might be involved in internal quenching of the excited state of the metal complex. The photooxidative stability of ZnDEC in organic solution was not significantly affected by the presence of DABCO, suggesting that quenching, if it occurs, is only impor tant in the associated amine complex.
A XII, DABCO
(ii) The DABCO complex was found to increase the rate of hydroperoxide decomposition by the ionic mechanism, see Fig. 8. (iii) Both DABCO and piperidine increase the equilibrium concentration of zinc dithiocarbamate in polypropylene (see Fig. 9). This purely physical effect is believed to be the most important reason for the synergism involv ing aliphatic amines which, unlike aromatic amines, have no chain-breaking antioxidant activity. The effect on antioxidant solubility is entirely con sistent with the effects of solubilising groups in the dithiocarbamates and 4-mercapto thiazolines discussed in Chapter 8 (Section 5.2). A similar conclusion has been reached by Efremkin and Ivanov [20] in a
443
SYNERGISM AND ANTAGONISM
0|
- 0.21 -0.4
\
\
N.
o
1-1
-0.6 L
S^ZnDEC-DABCO
\znDEC
N \ "s
c
-1
-0.8
L
\
-10
\
■ 50
I 1 11
v. N»
\
\
100
I
*^
150
1
^2 0J0
Irradiation Time, h
Fig. 7. Decay of UV absorbance of ZnDEC (285 nm) and ZnDEC-DABCO complex (285 nm) in polypropylene during photooxidation. Concentrations 6*10 mol/100 g. (Repro duced with kind permission from Developments in Polymer Stabilisation-6, G. Scott (Ed.), Applied Science Publishers, 1983, p. 101).
°1 o - 0.2 * I (Ω -0.4
I—·
$
CO - 0 . 6
3
\
^ \
Control (No add)
^ S .
I
Y
L
ZnDEC\ DABCO N
-0.8 Γ -1.0
V
<3
\\
\
S . ZnDEC N s
1
60
| 1
180 _l
120
1*
1 1 24 J
Irradiation Time, h
Fig. 8. First order plot for the decomposition offers-butyl hydroperoxide (TBH) by ZnDEC and its DABCO complex in chlorobenzene under UV irradiation. Metal complex concen trations, 10 M, [TBH], 10" M. (Reproduced with kind permission from Developments in Polymer Stabilisation-6, G. Scott (Ed.), Applied Science Publishers, 1983, p. 100). 1.2 1.0 0.8 0.6
2
4
6
8
10
Concentration x10, mol/lOOg
Fig. 9. UV absorbance at 285 nm of ZnDEC and its amine complexes as a function of additive concentration. (Reproduced with kind permission from Developments in Poly mer Stabilisation-6, G. Scott (Ed.), Applied Science Publishers, 1983, p. 101).
444
GERALDSCOTT
fi
.30öL o I
S.
β
/
\
/
\
\
I
§ 200 [■ / * /
\ \
^
1/
c
L/
1
" ioof
I
0
25
50
75
100%
Fig. 10. Induction periods to hydroxyl formation during photooxidation (at 300 nm) of isoprene/styrene block copolymer containing a mixture of 4-hydroxy-4'-benzeneazo(azobenzene) and tetrakis (2,2,6,6-tetramethyl-4-piperidinoxy)silane as a function of the fraction of HALS in the mixture. Film thickness, 170 mm, total stabiliser concentration, 2%. (Reproduced with kind permission from Developments in Polymer Stabilisation-8, G. Scott (Ed.), Elsevier Applied Science, 1987, p. 51).
study of the light stabilising effect of a 1:1 synergistic mixture of an 4-amino4'-nitroazobenzeneazobenzene and a hindered piperidine light stabiliser in an isoprene-styrene block co-polymer (see Fig. 10). It was not unambigu ously established however, that the synergism observed in this case is entirely due to mutual solubilisation [21], since anilines are weak anti oxidants and may be acting as synergists by a chemical mechanism. Shlyapintokh and co-workers [13] have suggested that quenching agents can also improve the light stability of phenolic antioxidants, thus extending their life under photooxidative conditions. They put forward a semi-quanti tative treatment of quenching based on the assumption that active radicals are produced by photolysis of the antioxidant [21]; (AH)! - ^ U
((AH)!·
02/RH
» nROO·
(3)
and that this process can be inhibited by quenching: (AH^ + S
> (AH)i + S
(4)
By making use of the Forster equation [22,23], the concentration, c, at which energy transfer to quencher and emission from the antioxidant are equally probable is given by Eqn (5); c-3/4Ä*
(5)
SYNERGISM AND ANTAGONISM
445
where R0 is the critical distance between the donor and the acceptor [21]. This suggests that, if the antioxidant and quencher are distributed uni formly in the polymer, then quenching will occur only at relatively high concentrations of both. It seems likely, however, in the case of the amine complexes discussed above that antioxidant and quencher are closely as sociated and that energy transfer may make a significant contribution to the synergism observed, although solubilisation in the polymer appears to be the main factor involved. Efremkin and Ivanov [20-24] have also studied the solubilising effects of additives on photoantioxidant activity. Thus, for example, a 1:1 w/w ratio of bismuth and zinc dialkyl dithiocarbamates at 0.2, 0.5 and 2% total concen tration gave synergistic effects of 30%, 80% and 30% respectively. A surfac tant (hydroxyethylated cetyl alcohol) improved synergism further, particu larly at higher concentrations of stabilisers. The two dithiocarbamates act by the same mechanism. Neither is a UV absorber and both are similarly photo-unstable. It is therefore difficult in this case to account for the synergism apart from physical causes. 2.3 Autosynergism Many examples were noted in the previous chapter of antioxidants that incorporate more than one function and are consequently more effective than can be accounted for by a single mode of action. A long established class of rubber antioxidants, the bis-phenol sulphides (e.g. XIII) owe their high activity to the presence of both a CB-D and a PD function in the same molecule [25]. The term autosynergism was coined [26] to describe syner gism within the same molecule. Some of the more important autosynergists that have been reported in the literature are listed in Table 4 with the most probable mechanisms involved. 2.3.1 Autosynergism involvingperoxidolytic (PD) mechanisms The incorporation of sulphur into antioxidants and UV absorbers leads to profound synergistic effects. Thus for example, the monosulphides XlVa and XlVb are seen in Table 5 to be approximately an order of magnitude more effective as thermal antioxidants in decalin and more than twice as effective in PP by oxygen absorption than 1076 (X). The advantage of the conven tional antioxidant in an open system is related to its physical behaviour Oower volatility, higher solubility, etc) and not to its inherent activity [30,53]. It has been shown [54] that the sulphur atom in XIV is slowly eliminated during oxidation with the formation of sulphur acids (see Scheme 3). The latter are effective catalysts for the non-radical decomposi tion of hydroperoxides [27]. The diphenylamines (XV) are also highly effec tive thermal antioxidants in rubbers and plastics, particularly when the alky group, R, is the polymer chain itself (see Chapter 5).
446
GERALDSCOTT
TABLE 4 Autosynergistic antioxidants Mechanism
Ref.
CB-D, PD-C
25
CB-D,
PD-C
27-29
XV
CB-D, PD-C
29-32
XVI
UVA, PD-C
29-32
XVII
UVA, PD-C
33,34-36
XVIII
UVA, CB-D, PD-C
10,16,37-44
XIX
UVA, CB-D
33,44-46
XX
UVA, CB-D
44-46
Thermal antioxidants tBu Me
HO^Vs-^OH Me
XIII
tBu
OH tBu ^ A ^ t B u
(a) R = H, BHBM
I T
,^τ,
ILJ CH2SR
^
TT
^TT^W^XÎV
(b)R = C12H25,BHBM-12
H f ~ \ N V~^V-NHCOCH2SR
UV Stabilisers /
O Vc-/
VoCH2CH2OCOCH2SR
HO R 2 NC:**
Ni
(RO)2pf
Ni
C4H9NH2 l
/ o
N i
C8H17 tBu Ho/ tBu
\
o C8H17 OEt yCH 2 PCT V Ni
447
SYNERGISM AND ANTAGONISM
TABLE 4 (Continuation)
Ni C=N / \ CH 3 OH J2 tBu
Mechanism
Ref.
XXI
UVA, CB-D, PD-S
34-36,47
XXII
CB-D, UVA
33
XXIII
MD, CB-D
48,49,86
XXIV
MD, CB-D
50
XXV
CB-D, MD
51
XXVI
PD-C, MD
52
XXVII PD-C, MD
52
O II
HO
c-o
tBu
tBu tBu
Metal Deactivators
cos>~ or OH II
o
H
,
,
H
O^KD^^O tBu
HO-/
yCH2CH2CONH-
tBu
0>
Zn
N I H
R 2 NC ^
S
-
Zn
For explanation of mechanism codes, see Scheme 2.
448
GERALDSCOTT
XIV(a)
, CH2SH
ROOH
CH2SOH
OH tBu
OH tBu
tBu
tBu ROOH
ROOH
CH 2 SR
II o
XIV(b)
OH tBu
OH tBu
tBu CH 2 S0 2 H PEROXIDE DECOMPOSERS + SO,
ROOH
S03(H2S04)
Scheme 3. Peroxidolytic antioxidant action of sulphur substituted phenols [54].
The use of autosynergistic phenols, XIV, in combination with conven tional UV absorbers gives additional photoantioxidant synergism in polyolefins [30] (see Fig. 6), rubber-modified polymers [30,55,56] and PVC [30,57]. Figure 11 compares the combination of HOBP and XIV(b) in polypropylene with a more conventional combination of HOBP and 1076. The presence of sulphur in the molecule almost doubles UV stability. Incorporation of sul phur into both chain-breaking antioxidant (XIV) and UV absorber (XVI) leads to a further increase in activity in ABS (see Table 6) [29]. It was seen in Chapter 3 that the most potent oxidation sensitiser in PVC is the combination of hydrogen chloride and hydroperoxides, both formed by
449
SYNERGISM AND ANTAGONISM
TABLE 5 Autosynergistic activity of sulphur-containing hindered phenols [30] Antioxidant
Induction period (h)
XIV(a) XTV(b) 1076
Dc 51.0 45.0 5.0
PPc 38.5 44.5 18.0
PPo 21.0 11.0 92.0
Dc, by oxygen absorption in decalin at 140°C; PPc, by oxygen absorption in polypropylene at 140°C; PPo, in polypropylene at 140°C in a forced air oven. TABLE 6 Synergism between autosynergistic antioxidant (XIV) and autosynergistic UV stabiliser (XVI) in ABS [30] Stabiliser
Concentration (mol/100 g)
XVI, R = ABS XVI, R = ABS XVI, R = ABS
3.0 6.0 3.0
+
XTV, R = ABS XVI, R = ABS
2.0 3.0
+
XIV, R = ABS
6.0
Time to embrittlement (h) 52 62
1
100
\
J 1
160
\
J
mechanical damage to the polymer during thermal processing. Most PVC stabilisers have the ability to scavenge HCl, but some, notably the thio tin stabilisers of which the dialkyl tin thioglycoUates (e.g. DOTG, XXVIII) are typical, destroy peroxides in a non-radical reaction. O (C 8 H 17 ) 2 Sn(SCH 2 COOR) 2 XXVIII, DOTG
0(
(C 4 H 9 ) 2 Sn
l
/
^CH || /CH
\
oc II o
XXIX, DBTM
450
GERALD SCOTT
This stabiliser class shows very similar behaviour to other sulphur antioxidants; initially it is a prooxidant but it becomes an antioxidant by conversion to sulphur acids [30,60-62]. The result of this is autosynergism between the HC1 scavenging and the peroxidolytic functions in the molecule. The thio tin stabiliser are therefore more effective heat stabilisers than the tin carboxylate stabilisers which do not contain sulphur; e.g. dibutyl tin maleate (DBTM, XXIX), see Chapter 3. Under photooxidative condition, however, they are photosensitisers due to the photolability of the interme diate sulphoxides (Scheme 4) which act as initiators for photooxidation. It should be noted that the reaction of liberated thiols with isolated double bonds removes another potential source of oxidative instability, but this is not an antioxidant process in the normal sense of the word.
Fig. 11. UV lifetimes of polypropylene films containing synergistic combinations of HOBP and phenolic antioxidants as a function of the molar proportion of the phenols. BHBM = XIV, R = H; BHBM-12 = XIV, R = C12H25; 1076 = X, R = C18H37; TBC = ter*-butyl-p-cresol. (Reproduced with kind permission from Eur. Polym. J., 16 (1980) 497).
451
SYNERGISM AND ANTAGONISM
Cooray and Scott [57-59] have shown that antioxidants containing a thiol group (e.g. Table 4, XIV, R=H) and UV stabilisers with similar substituents (e.g. Table 4, XVI, R = H) also add to monoenic unsaturation in PVC by the mechanism shown in Scheme 4. The resulting adducts are antioxidants when used in combination with a tin stabiliser. Both XIV and XVI are thermal stabilisers but XIV, R = H with DOTG leads to antagonism under photooxidative conditions (see Table 7) due to sulphoxide photolysis (Scheme 4) [30,61,62]. However, a combination of DBTM and the sulphur synergised hindered phenol, XIV, are photosynergistic. The sulphur-con taining UVA, XVI, also synergises effectively with DBTM. The role of sulphur in synergistic UV stabilisers has been discussed in some detail in Volume I, Chapter 5 and in Section 2.2.2 in the present chapter. The important principle that has emerged is that optimal synergis tic combinations rarely occur at a 1:1 molar ratio of the complementary functions. In the case of sulphur-containing antioxidants e.g. ZnDEC in combination with HOBP (Fig. 5), a 2:1 excess of HOBP is required to protect the peroxidolytic antioxidant from photolysis. Similarly, in the case of other sulphur-containing antioxidants (e.g. XIV), a similar excess is required (see Table 3). This points to a potential limitation in the use of autosynergists; namely that thefixedratio of the functional groups which results from their chemical combination in the same molecule may not be optimal. It is clearly possible to overload the synergistic combination with sulphur, as described above for XXVII and XVI. Furthermore, the optimal synergistic ratio for thermal oxidation may not be the same as for photooxidation and it may be necessary to compromise to obtain the best all-round balance of antioxidant performance [58,59]. In spite of the above reservation, there is good evidence that by careful design of the autosynergistic package, combinations of autosynergists may TABLE 7 Sulphur-containing stabilisers, XIV and XVI, as photoantioxidants in PVC. (Total con centration, 5.8*10 mol/100 g, synergistic optimum) [30] Stabiliser
Embrittlement time (h)
XXIX XXVIII XXIX + XIV XXVIII + XIV XXIX + XVI XXVIII + XVI Control (no antioxidant)
1000 460 1400 400 1850 1000 670
452
GERALDSCOTT
Cl R2Sn(SCH2COOR)2
o
HCl
l ROOH
■* R2SnCOOR + ROCOCH2SH
II
-CH=CH-
R2SnSCH2COOR
—CHCH2—
I
I
SCH2COOR
SCH2COOR ROOH
—CHCH2— R,SnSCH,COOR
—CHCH
PHOTOPROOXIDANTS
I
0=SCH 2 COOR
ROCOCH.SOH ROOH
ROCOCH2S02H HEAT STABILISERS (ANTIOXIDANTS) SO, + ROCOCH, Scheme 4. Autosynergistic PVC stabilisers [60-62].
give a better performance that comparable blends of conventional synergists. This is shown typically for a synergistic combination of XIV and XVI in ABS in Table 8 [63]. It can be seen that these stabilisers, each at 1 g/100 g are over four times more effective than three conventional antioxidant/stabiliser synergists containing the same functional groups each at 1 g/100 g. A contributory reason for the effectiveness of the autosynergist combination may be that the antioxidant functions have been targeted to the most vulnerable segment of the co-polymer (see Chapter 5). 2.3.2. Autosynergism involving metal deactivation Metal complexing agents, although they have some activity in neutralis-
SYNERGISM AND ANTAGONISM
453
TABLE 8 Comparison of photoantioxidant activity of polymer-bound autosynergists with a combi nation of conventional antioxidants containing the same functional group in ABS. (All concentrations, 1 g/100 g) [65] Antioxidant/Stabiliser
Embrittlement time (h)
BHT + HOBP + DLTP XIV + XVI
85 380
TABLE 9 Autosynergistic metal deactivators [51] Metal deactivate
tio (min)
Copper complex
X = NH, R = H X = NH, R = OH X = O, R = H X = 0, R = OH X = S, R = OH
6.9 235 9.9 60 27.4
— 1:2 — 2:1 —
tio = time to absorb 10 ml 02/fe.
ing the effects of transition metal ions, are not highly effective retarders of metal catalysed oxidation in polymers. Nevertheless, effective "metal deac tivators" have been developed for polymers which contain an additional antioxidant function [51,64] and many of them are exceptionally effective against copper ions. Table 9 compares the effects of phenyl benzimidazoles and related com pounds both with and without 2-hydroxy groups. It is evident that the phenolic function is essential to activity. However, the phenolic group does not need to be involved in metal complexation. For example in the commer cial copper deactivator, XXX, the complexing group is the bis-hydrazide function and the hydroxyl group operates entirely independently as a chainbreaking antioxidant.
454
GERALDSCOTT
tBu
tBu C H CH,CH XONHNHCOCH 22CH ι 2^η2
HO
2
- ^ OH tBu
tBu XXX
A number of sulphur-containing zinc complexes combine the activity of metal deactivators and peroxide decomposers. For example, zinc mercaptobenzimidazole (XXXI) has been used for many years as a copper deactivator in rubber [75] and the zinc dithiocarbamates (IX), dithiophosphates (XXXII) and xanthates (XXXIII) are also effective
a> L
ί Zn
1 2 H J XXXI
^S-
(RO)2P^
Zn
XXXII
2
R O < ^ 2:n S .2
XXXIII
These compounds are all antioxidants in their own right. Although they have been shown to undergo metathesis with copper salts, it is difficult to determine the relative importance of metal complexing and antioxidant activity. However, in some cases, notably the dithiocarbamates, the pre formed copper complexes are themselves antioxidants [76] and, as was seen in the last chapter, the nickel, copper and cobalt dithiocarbamates are effective photoantioxidants, suggesting that the antioxidant function of the ligand is more important than its ability to scavenge metal ions. 3. ANTAGONISM
Although synergism between antioxidants and stabilisers is fairly com mon, antagonism between antioxidants and stabilisers is relatively rare. Very few cases have been studied in thermal oxidation, but several practi cally important examples have been investigated in photostabilisation. 3.1 Antagonism Involving Peroxidolytic Antioxidants It was seen earlier that phenolic antioxidants are mediocre photoantiox idants in polymers and this was attributed to the conversion of phenols during oxidation to light sensitive peroxydienones and quinones. Chakr-
455
SYNERGISM AND ANTAGONISM
aborty and Scott [10] found that both zinc and nickel diethyldithiocarbamates (IX (a) and (b)) retarded the photooxidation of LDPE as did the phenolic antioxidant, 1076 (X). However, in both cases, the combination of metal complex and phenol were less effective (see Table 10). It was found that the rate of disappearance of both metal complexes in LDPE under photooxidative conditions was accelerated by the presence of the phenol (see Fig. 12), suggesting that either the phenol or its oxidation products sensi tised the photooxidation of the metals complexes. TABLE 10 Antagonism in UV stabilisation of LDPE. (Concentration of all additives, 3><10 mol/100 g) Antioxidant
Time to embrittlement (h)
None ZnDEC NiDEC HOBP 770 ZnDEC + 1076 NiDEC + 1076 NiDEC + 770
1200 1400 1800 2200 2250 1250 1580 1850
•S -0.6
100 200 Irradiation Time, h
300
Fig. 12. Change in UV absorbance (330 nm) of ZnDEC and NiDEC in the presence and absence of a phenolic antioxidant (1076) during photooxidation of LDPE. Concentrations 3X10"4 mol/100 g. 1, ZnDEC + 1076; 2, ZnDEC; 3, NiDEC + 1076; 4, NiDEC. (Reproduced with kind permission from Eur. Polym. J., 13 (1977) 1007).
456
GERALD SCOTT
A hindered piperidine, XXXIV, was also antagonised by NiDEC (see Table 10) and Tudos et al. found that the peroxide decomposer delayed the formation of nitroxyl [66]. Me Me HN
Me Me
\-OCO(CH2)8COO—ί
Me Me
ΝΗ
Me Me XXXIV, 770
Chakraborty and Scott [67] suggested that antagonism was due to a less efficient oxidation of the hindered piperidine by hydroperoxide to nitroxyl (see Chapter 9) due to competition from the peroxide decomposer:
>NH
ROOH ^ C ^
(6) Non-radical products
However, other workers have shown that thiyl radicals are trapped by nitroxyls [68,69] which may be reduced back to the parent amine in con sequence [69]: S \ Il N O + RoNC;· / §
S \ II > NOSCNRo 2 /
S \ Il > NH + HOSCNRo 2 (H20) / ^
(7)
It seems likely that both processes contribute to the antagonism. 3.2 Antagonism Involving Chain-Breaking
Antioxidants
A number of workers have shown that hindered amine photoantioxidants are less effective in the presence of hindered phenols [70-72]. Allen [72] has suggested that nitroxyl is removed from the system by reaction with the phenols and their derived phenoxyls, see Scheme 5. However, it is known that the hydroxylamines (XXXV) produced in this process are even more effective as photoantioxidants than the nitroxyls from which they are derived [73]. The coupled nitroxydienones (XXXVI) would almost certainly be photolysed further to the more stable quinones, XXXVII and XXXVIII, which are known to be effective sensitisers for polymer oxidation [74].
457
SYNERGISM AND ANTAGONISM
OH tBu
tBu
tBu
tBu
:NOH
>
-£■
XXXV CH,R
CH,R NO·
O
y
o
tBu^JL^tBu
tBu ^ J L / t B u
hv
I J
o
RCIÎ^ON^ XXXVI
XXXVIII
XXXVII
tBu \
R
/ tBu
R +
^NOH XXXV
Scheme 5. Possible mechanisms for antagonism between hindered amines (>NH) and hindered phenols. REFERENCES 1
B. Ranby and J.F. Rabec, Photodegradation, Photooxidation and Photostabilisation of Polymers, Wiley, 1975, p. 415. 2 T. Matsuura, N. Yoshimura, A. Nishinaga and I. Saito, Tetrahedron Lett. (1969) 1669; (1972) 4933. 3 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 203 et seq. 4 N.S. Allen, Ed., Degradation and Stabilisaton of Polyolefins, Applied Science PubUshers, London, 1983. 5 G.V. Karpukina, Z.K. Maizus and N.M. Emanuel, Dokl. Akad. Nauk SSSR, 152 (1963) 110; 182 (1968) 870. 6 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 210.
458 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
GERALD SCOTT
C.R.H.I. de Jonge and P. Hope, in: G. Scott (Ed.), Developments in Polymer Stabilisation-3, Applied Science Publishers, London, 1980, p. 21. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 295. G. Scott, Eur. Polym. J. Suppl. (1969) 189. K.B.Chakraborty and G. Scott, Eur. Polym. J., 13 (1977) 1007. K.B. Chakraborty and G. Scott, Polym. Deg. Stab., 1 (1979) 37. G. Scott and M.F. Yusoff, Polym. Deg. Stab., 2 (1980) 309. V.B. Ivanov and V.Ya. Shlyapintokh, in G. Scott (Ed.), Developments in Polymer Stabilisation-8, Elsevier Applied Science, London, 1987, p. 29. Yu.A. Shlyapnikov, in G. Scott, Editor, Developments in Polymer Stabilisation-5, Applied Science Publishers, London, 1982, p. 1. G. Scott, Pure Appl. Chem., 52 (1980) 365 S. Al-Malaika, K.B. Chakraborty and G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-6, Applied Science Publishers, London, 1983, p. 73. D. Bellus, in B. Ranby and J.F. Rabec (Eds), Singlet Oxygen, Reactions with Organic Compounds and Polymers, Wiley, 1978, p. 61. C.S. Foote, in B. Ranby and J.F. Rabec (Eds), Singlet Oxygen, Reactions with Organic Compounds and Polymers, Wiley, 1978, p. 135. R.H. Young and D.R. Brewer, in B. Ranby and J.F. Rabec (Eds), Singlet Oxygen, Reactions with Organic Compounds and Polymers, Wiley, 1978, p. 36. A.F. Efremkin and V.B. Ivanov, Polym. Photochem., 4 (1981) 179. E.L. Lovskaya, V.B. Ivanov and V.Ya. Shlyapintokh, Vysok. Soed., A27 (1985) 1589. Th. Forster, Z. Naturforsch., 4A (1949) 321. Th. Forster, Discuss. Faraday Soc, 27 (1959) 7. A.F. Efremkin and V.B. Ivanov, Vysok. Soed., B24 (1982) 662. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 287. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 214. V.M. Farzaliev, W.S.E. Fernando and G. Scott, Eur. Polym. J., 14 (1978) 785. K.W.S. Kularatne and G. Scott, Eur. Polym. J., 14 (1978) 835. G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-4, Applied Science Publishers, London, 1981, p. 181. G. Scott, in G. Scott, Editor, Developments in Polymer Stabilisation-6, Applied Science Publishers, London, 1983, p. 29. G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-8,1987, p. 209. G. Scott, Makromol. Chemie, Macromol. Symp., 28 (1989) 59 D.J. Carlsson, D.W. Grattan, T. Suprunchuk and D.M. Wiles, J. Appl. Polym. Sei., 22 (1978) 2217. R.P.R. Ranaweera and G. Scott, Chem. Ind., (1974) 774. R.P.R. Ranaweera and G. Scott, J. Polym. Sei., Polym. Lett. Ed., 13 (1975) 71. R.P.R. Ranaweera and G. Scott, Eur. Polym. J., 12 (1976) 591. S. Al-Malaika and G. Scott, Eur. Polym. J., 16 (1980) 709. S. Al-Malaika and G. Scott, Eur. Polym. Sei., 16 (1980) 503. S. Al-Malaika and G. Scott, Polymer, 23 (1982) 1711. S. Al-Malaika and G. Scott, Eur. Polym. J., 19 (1983) 235. S. Al-Malaika and G. Scott, in N.S. Allen (Ed.), Degradation and Stabilisation of
SYNERGISM AND ANTAGONISM
459
Polyolefins, Applied Science Publishers, London, 1983, p. 247. S. Al-Malaika, M. Coker and G. Scott, Polym. Deg. Stab., 22 (1988) 147. G. Scott, Macromol. Chem., Macromol. Symp., 22 (1988) 225. G. Scott, Macromol. Chem., Macromol. Symp., 27 (1989) 1. D.J. Carlsson and D.M. Wiles, J. Polym. Sei., Polym. Chem. Ed., 12 (1974) 2217. B. Felder and R. Schumacher, Angew. Macromol. Chem., 31 (1973) 35. L.L. Gervits, N.V. Zolotova and Ye.T. Denisov, Polym. Sei. USSR, A18 (1976) 468. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 214. 49 A.C. Mehta and T.R. Shishadri, J. Sei. Ind. Res., 18B (1957) 24. 50 H.S. Kuzminskii, V.D. Zaitseva and N.N. Lezhnev, Doklad. Acad. Nauk. SSSR, 125 (1959) 1057. 51 Z. Osawa in G. Scott, Editor, Developments in Polymer Stabilisation-7, Applied Science Publishers, London, 1984, p. 193. 52 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 402 et seq. 53 G. Scott and M.F. Yusoff, Eur. Polym. J., 16 (1980) 497. 54 G. Scott and R. Suharto, Eur. Polym. J., 20 (1984) 139. 55 W.S.E. Fernando and Scott, Eur. Polym. J., 16 (1980) 971. 56 M. Ghaemy and G. Scott, Polym. Deg. Stab., 3 (1981) 253. 57 B.B. Cooray and G. Scott, Eur. Polym. J., 16 (1980) 1145. 58 B.B. Cooray and G. Scott, Eur. Polym. J., 17 (1981) 379. 59 B.B. Cooray and G. Scott, Eur. Polym. J., 17 (1981) 385. 60 B.B. Cooray and G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation2, Applied Science Publishers, London, 1980, p. 53. 61 B.B. Cooray and G. Scott, Polym. Deg. Stab., 2 (1980) 35. 62 B.B. Cooray and G. Scott, Eur. Polym. J., 17 (1981) 233. 63. M. Ghaemy and G. Scott, Polym. Deg. Stab., 3 (1980-81) 405. 64 R.L. Hartless and A.M. Trozzolo, Coatings Plast. Preprints, USA, 3a (1974) 177. 65 M. Ghaemy and G. Scott, Polym. Deg. Stab., 3 (1980-81) 405. 66 F. Tudos, G. Balint and T. Kelen, in G. Scott (Ed.), Developments in Polymer Stabilisation-6, Applied Science Publishers, London, 1983, p. 121. 67 K.B. Chakraborty and G. Scott, Chem. Ind., (1979) 237. 68 R.I. Zhdaniv, V.A. Golubev, V.M. Gidd and E.G. Rozantsev, Izv. Akad. Nauk. SSSR, Ser. Khim., 10 (1970) 2390. 69 H. Dweik and G. Scott, Rubber Chem. Technol., 57 (1984) 735. 70 N.S. Allen, Makromol. Chem., 181 (1980) 2413; Polym. Photochem., 1 (1981) 243. 71 J. Sedlar, J. Marchai and J. Petruj, Polym. Photochem., 2 (1982) 175. 72 N.S. Allen, in N.S. Allen and J.F. Rabec (Eds), New Trends in the Photochemistry of Polymers, Elsevier Applied Science, London, 1985, p. 223. 73 G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-7, Applied Science Publishers, London, 1984, p. 65. 74 B. Ranby and J.F. Rabec, Photodegradation, Photooxidation and Photostabilisation of Polymers, Wiley, 1975, p. 303. 75 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 445 et seq. 76 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 402 et seq.
42 43 44 45 46 47 48
Chapter 9 SYNERGISM AND ANTAGONISM GERALD SCOTT 1. FACTORS AFFECTING ANTIOXIDANT PERFORMANCE
Attempts to predict the practical performance of antioxidants in polymer systems from their behaviour in pure model compounds are rarely success ful. Many factors contribute to this and some of these, such as physical loss of antioxidants by volatilisation or leaching are now understood and have been discussed in Chapter 4. In many cases, however, lower than expected or higher than expected performance may be due to interference from some other chemicals, either adventitiously present or deliberately added to the system, which interact with the antioxidant. Most obvious of these are the pro-oxidants discussed in Volume I, Chapter 3. By increasing the oxidative stress they simply accelerate antioxidant destruction and hence shorten the induction period to the onset of rapid autoxidation. A familiar example of the effect of adventitious contaminants is the catalytic effect of transition metal ions on the homolytic breakdown of hydroperoxides which leads to more rapid destruction of chain breaking antioxidants (see Chapter 6). A similar phenomenon accounts for the relative ineffectiveness of chainbreaking antioxidants in a sulphur vulcanisate compared with raw rubber. Not only is the normal induction period observed in a pure hydrocarbon substrate shortened or removed completely but the rate of oxidation is also increased in the initial stages. This phenomenon is again due to the rapid introduction of radicals into the system by redox reactions between hy droperoxides and oxidised sulphur species (see Volume I, Chapter 5). Such effects may generally be predicted from a study of the non-sulphurated and sulphur modified substrates with and without antioxidant. Other antago nistic effects are more complex. For example, it was seen earlier (Chapter 3, Section 5) that hindered phenols are relatively ineffective under conditions of UV irradiation, due primarily to their photo-instability and in some cases, photo-prooxidant effects. Consequently, they antagonise with other antiox idants during UV exposure, but are protected by UV absorbers with which they effectively synergise. Phenolic antioxidants can also be readily de stroyed by agents such as ozone and singlet oxygen, in the latter case
432
GERALD SCOTT
through a charge-transfer intermediate [1,2]. This is of little value as an antioxidant process as is sometimes claimed since the proposed end product (I) is a typical hydroperoxide initiator. OH
OH
Ri-
~^
R
O,
^K^ > 1
W R,
+ -OOH
OOH I Hydroperoxides are themselves antagonists for some stabilisers. For example 2-hydroxybenzophenone UV absorbers vary considerably in their effectiveness depending on the processing conditions, and this has been shown to be due to the formation of hydroperoxides in the melt which rapidly destroy the UV absorber during exposure to UV light (see Chapter 8, Section 5.1.2). 2. SYNERGISTIC EFFECTS OF ANTIOXIDANTS AND STABILISERS
It is a very rare occurrence that two antioxidants simply give an additive effect in an oxidising substrate. If they did, replacement of a molar percent age of antioxidant A by antioxidant B would be predictable by the ideal straight line a'-b' in Fig. 1. In practice the result is generally either better or worse than that predicted. If better, this is synergism and a practical advantage but if it is worse, the result is antagonism and this particular combination has to be avoided if the antioxidant effectiveness is to be maximised [3]. Synergism is normally expressed as the percentage improve ment given by the combination of antioxidants over that expected on an additive basis and conversely, antagonism is the percentage decrease com pared with that expected (see Fig. 1). The development of synergistic "packages" has been one of the major accomplishments of stabilisation technology. It is very unlikely that any single-component stabilisation system would have been effective enough to
SYNERGISM AND ANTAGONISM
433
Synergism
A
Molar Ratio B/A
B
Fig. 1. Synergism and antagonism expressed as a function of the molar ratio of antioxidants A and B. protect the α-branched polyolefins [4]. Some very remarkable combination effects have been observed and in recent years the basic science underlying synergism has found some explanation in the organic and physical chemis try which has been discussed so far in this volume. The physical chemistry of antioxidants and stabilisers in polymers which is partly responsible for synergism has already been discussed in Chapter 4. In this Chapter, emphasis will be placed on the organic chemistry of syner gism. Three kinds of synergism can be distinguished [3]: (a) homosynergism in which two antioxidants acting by the same mechan ism interact, generally in a single electron transfer cascade; (b) heterosynergism where the antioxidants act by a different mechanism and hence complement one another; (c) autosynergism in which synergism results from two different functions in the same molecule. 2,1 Homosynergism The best known example of homosynergism is the regeneration of the tocopherols from their derived phenoxyls by ascorbic acid reduction (see Volume I, Chapter 1). Homosynergism has also been reported between phenolic antioxidants in hydrocarbons [3]. The effect is generally maximised when there is a complementation of steric hindrance in the phenols of the synergistic combination. Similar synergism is observed between aromatic amines and phenols where the phenol is sacrificed with regeneration of the amine [5] or possibly the derived nitroxyl. The sacrificial component of a synergistic mixture may have little antioxidant activity in its own right. Alkyl phosphite and phosphonate esters are
434
GERALD SCOTT
(2)
typical examples of weak antioxidants which, when used in combination with hindered phenols [6] considerably improve antioxidant performance and colour. It seems likely that they reduce back the phenoxyl before it can be irreversibly destroyed. The dialkyl monosulphides (II) are effective synergists with hindered phenols in polyolefins and there is little doubt that complementary CB-D and PD-C activities are primarily responsible for this phenomenon. XCH2CH2SCH2CH2X
II
However, De Jonge and Hope [7] have shown that in the case of phenols which give rise to relatively stable radicals (notably 2,6-diphenyl-4-methoxy phenol, III), regeneration of the phenol from phenoxyl, V, occurs at the expense of the monosulphides, IV (see Scheme 1). It is should be noted that the second regeneration step in Scheme 1 is closely related to the mecha nism discussed in Chapter 4 for the regeneration of hydrogalvinoxyl from galvinoxyl with elimination of a macroalkyl radical in the polymer chain. It is not clear how important is the homosynergistic mechanism shown in Scheme 1 compared with the competing peroxidolytic process known to occur (see Volume I, Chapter 5), since a variety of other oxidised products such as VIII, involved in the peroxidolytic activity of dialkyl monosulphides, (II) are also formed. O II XCH2CH2S SCH2CH2X
VIII
O 2.2 Heterosynergism Scheme 2 summarises the complementary mechanisms of antioxidant action. From this it follows that antioxidants which eliminate the main source of initiating radicals, the peroxide decomposers, extend the useful life of the chain-breaking antioxidants. Conversely, chain-breaking antioxi dants reduce the formation of hydroperoxides and hence protect the per oxide decomposers from decomposition. These cooperative effects are the basis of heterosynergism.
435
SYNERGISM AND ANTAGONISM
Ph
+ (R'OCOCH2CH2)2S IV
OMe
*
OH
xA^Ph 1 J III OMe
+ R'OCOCH2CH \
/
R'OCOCH=CH \
ΠΙ+
/
S VI
R'OCOCH2CH2
S VII
R'OCOCH2CH2 Scheme 1. Homosynergism between phenols and thiodipropionate esters [7].
CB-D
CHAIN-BREAKING MECHANISM
RO- + -OH
ROO-
PREVENTIVE MECHANISM CB—D -ROOH UVAMD
RH
PD-S PD-C UV Light Metal ions
Scheme 2. Mechanisms of antioxidant action [73]. CB-A, Chain-breaking acceptor. CB-D, Chain-breaking donor. PD-S, Stoichiometric peroxide decomposer. PD-C, Catalytic per oxide decomposer. UVA, UV Absorber. MD, Metal deactivator.
436
GERALD SCOTT
2.2.1 Thermal stabilisation based on CB-D/PD antioxidants Synergism between hindered phenols and sulphur compounds has been known for many years [8] and has been extensively studied in the polyolefins. Figure 2 shows the cooperative effects of several high molecular
0 DLTP
JL
_L
-L
JL
25 50 75 Composition of Stabiliser, %
100 Phenolic
Fig. 2. Synergism between DLTP and phenols in polypropylene at constant total concen tration in an air oven at 150°C.
1.
.™ r C -
(HO
Me OH
OH
2.
Me tBu 3.
HO
tBu CHCH,CH
OH).
(Reproduced with kind permission from Eur. Polym. J., Supplement 1969, p. 189).
SYNERGISM AND ANTAGONISM
300
-
/
\MBI+HDPA
\
MBT + HDPA
200
100
0.2
0.1
0.08
0.4
0.06
0.6
0.Θ
0.04
■A. 0.02
1 0 HDPA J
MBI/MBT
Concentration, mol/kg
Fig. 3. Synergism between p-hydroxydiphenylamine (HDPA) and sulphur compounds (MBI, MBT) in polypropylene at constant total concentration at 200°C. (Reproduced with kind permission from Eur. Polym. J., Supplement, 1969, p. 189).
weight phenols with dilauryl thiodipropionate [9] in polypropylene and Fig. 3, similar combinations of arylamine and two peroxide decomposers [9]. The optimum combination varies with the nature of both the phenol and the peroxide decomposer. The augmentation of one antioxidant by another is an effective way of using antioxidants efficiently, and is of great practical utility. Such antioxidant combinations have to be evaluated at various ratios to achieve maximal potential, since synergism may not be optimal at a 1:1 combination. TABLE 1 Synergism between ZnDEC (IXa) and a hindered phenol, 1076 (X) in the thermal stabilisation of polyolefins [101. (All concentrations 3*10 mol/100 g) Antioxidant
PP Embrittlement time (h at 140°C)
LDPE Induction period (h at 110°C)
None ZnDEC 1076 ZnDEC + 1076
0.5 32 58 165
10 110 110 300
438
GERALD SCOTT
Synergistic combinations of hindered phenols and zinc dialkyl dithiocarbamates (e.g. IXa) are particularly effective heat stabilisers for polyolefins [10] (see Table 1). The transition metal dithiocarbamates (e.g. IX(b) are also effective [8]. Unlike the hindered phenols, the effectiveness of the metal thiolate peroxide decomposers is very little affected by processing conditions [10]. OH tBu^^^^tBu R 2 NC^f
^ M
IX
An
ZnDEC, (a) R = Et, M = Zn, n = 2
CH 2 CH 2 COOR
NiDBC, (b) R = Bu, M = Ni, n = 2
1076j R =
^ j ^
O HOBP, R = C 8 H 17
/
/
c
\
^ O R
XI
HO 2.2.2 Synergistic UV stabilisers The thermal synergists discussed in the previous section are ineffective light stabilisers and it can be seen from Table 2 that a combination of a hindered phenol (1076, X) and a zinc dithiocarbamate are actually antago nists under photooxidative conditions. A combination of a UV absorber HOBP (XI, R=Oct) and zinc diethyl dithiocarbamate, ZnDEC (IX(a)) on the other hand, are synergistic [10]. Part of the explanation for this is evident from Table 2. It can be seen that the processing operation partially destroys the photoantioxidant activity of the UV absorber. Studies in model hydro carbons have also shown that 2-hydroxybenzophenones are rapidly de stroyed by UV light in the presence of hydroperoxides (see Chapter 8, Section 5.1.2). By destroying hydroperoxides during processing and sub sequently during UV irradiation the peroxide decomposer effectively ex tends the useful life of the UV absorber. However, ZnDEC and to a lesser extent NiDEC are destroyed by light, see Fig. 4, and part of the effect of the UV absorber is to protect the peroxide decomposers. Figure 4 also follows the decay of the two peroxide decomposers in the presence of HOBP and it was suggested that in this case the secondary peroxidolytic antioxidants formed from the dithiocarbamates protect the UV absorber from photooxidation [10]. Chakraborty and Scott [11] found that the optimal molar ratio of HOBP to ZnDEC was 2:1 in polypropylene (see Fig. 5). This underlines the
439
SYNERGISM AND ANTAGONISM
TABLE 2 Synergism between a UV absorber and antioxidants [10]. (Concentration, S^IQT4 mol/100 g) Stabiliser
Time to embrittlement (h) Processing time (min)
ZnDEC (IXa) HOBP (XI) 1076 (X) ZnDEC + HOBP 1076 + HOBP ZnDEC + 1076 Control, no additive
10
30
1400 2200 1800 >4000 «3500 1250 1200
1400 1600 1750 >4000 «3500 1250 900
Fig. 4. Change in the UV and visible absorption spectrum (Xmax at 330 nm of additives during the photooxidation of LDPE (processed 10 min at 150°C. Concentration of addi tives, 3X10"4 mol/100 g). 1, ZnDEC (330 nm); 2, NiDEC (330 nm); 3, NiDEC + HOBP (390-395 nm); 4, HOBP (330 nm); 5, ZnDEC + HOBP (330 nm); 6, NiDEC + HOBP (330 nm). (Reproduced with kind permission from Eur. Polym. J., 13 (1977) 1007).
general principle discussed above that it is not possible to assume that the optimum synergistic ratio is 1:1. Table 2 also shows that a hindered phenol synergises similarly with HOBP although not quite as effectively as the peroxide decomposers. Again there is evidence that the chain-breaking antioxidant can also protect the UV absorber during processing. However, Table 3 and Fig. 6, which compare the activity of synergistic mixtures at constant molar total concentration in
440
GERALD SCOTT
900r
80θ[
70θ[ JC
£ 60o| "c £ 50θ[
—
+* "ti
|
I I
400F
UJ
300l·
0
10
20
30
40
50
60
70
80
90
100
Mole fraction of ZnDEC(x100)
Fig. 5. Synergism between ZnDEC and HOBP in the photostabilisation of polypropylene. Total additive concentration βχΚΓ4 mol/100 g. (Reproduced with kind permission from Developments in Polymer Stabilisation, G. Scott (Ed.), Applied Science Publishers, 1983, p. 98).
PP [12] show that even a small proportion (20%) of phenolic antioxidants have a profound stabilising effect on the UV stability of HOBP. The corre spondence of the carbonyl formation curves and the HOBP decay curves (Fig. 6) suggests that the more powerful autosynergistic antioxidant pro tects the weaker antioxidant although doubtless this is only possible be cause the UV absorber protects the phenol from photolysis. Table 3 il lustrates the correspondence between embrittlement time and the rate of HOBP destruction in the polymer. Shlyapintokh et al. [13] have suggested that there may be a further reason for synergism between a UV absorber and a chain-breaking anti oxidant. They have pointed out that preferential destruction of antioxidant will occur in the surface of the polymer and that as a consequence, its concentration may drop below its critical level [14]. One role for the co-agent may be, therefore, to increase the rate of diffusion of the antioxidant from the polymer bulk to the surface, thus maintaining a greater than critical concentration in the "action zone". In a theoretical treatment of the effects of diffusion rate on stabilising effectiveness, Ivanov and Shlyapintokh have drawn the following conclusions [13]: (a) Light stability is proportional to sample thickness. (b) The higher the light intensity, the higher is the antioxidant concentra tion gradient.
441
SYNERGISM AND ANTAGONISM
+ 0.1
+0.2 HT o
+ 0.3
300
600
900
1200
1500
1Ö00
2100
Irradation Time, h
Fig. 6. Relationship between the rate of photooxidation (a)-(h) of polypropylene and the decay of the UV absorbance of HOBP (a')-(h') in the presence of BHBM-12 (XlVb, R=CieH37)*. Total additive concentration, 10"3 g/100 g. (a) control, no additive; (b) BHBM-12; (c), (c') 20% HOBP; (d), (dO 35% HOBP; (e), (e') 50% HOBP; (f) 65% HOBP, (g), (gO 80% HOBP; (h), (h') 100% HOBP. (Reproduced with kind permission from Polym. Deg. Stab., 2 (1980) 309). TABLE 3 Synergism between a UV absorber (HOBP) and hindered phenols (total concentration of synergistic mixture, 10 mol/100 g) Anti1076 oxidant (mol %) Te (h)
Syn (%)
(lO 4 ^ 1 )
—
35 50 65 80
750 1590 1350 1115 980 705
129 127 127
1.8 4.1
100
260
—
—
0 20
XIV(b)*
XIV(a)*
139 132
3.8 1.1 1.4
6.0
Te (h) 750 2100 1850 1547 1240 830 270
(104 h"1)
Te (h)
Syn (%)
ka
3.8 0.8
750 2350
—
3.8 0.8
176
1.1 1.6 2.9 3.7
1935 1600 1260 840
—
—
250
250 251 250 201 —
Syn (%) — 230 232 233 232
267
(10 4 IT1)
1.1 1.5 2.8 3.5 —
T e = Time to embrittlement; k& = First order rate constant for the decay of HOBP,
fWn m . QSu-Ea-MEà + toEà x i n f t (Ei-Ec) + (&2-Ec) whereas = embrittlement time of synergistic mixture; Ec - embrittlement time of control (no additive), E\ = embrittlement time of stabiliser 1; E2 - embrittlement time of stabil iser 2. ♦See Table 4.
442
GERALD SCOTT
(c) Diffusion synergism is dependent on the molar mass of the diffusant. (d) The extent of the synergism depends on the extinction coefficient of the UV absorber. (e) Maximal synergism is achieved at a 1:1 ratio. The fact that condition (e) is not met in Table 3 and Fig. 5 suggests that factors other than antioxidant diffusion may be involved in the synergism. However, as will be seen in the next section, some additives which have no stabilising activity in their own right can be synergistic with known photoantioxidants, thus eliminating chemical synergism 2.2.3 Synergism between photoantioxidants and antioxidant-inactive agents It was seen above that zinc dialkyldithiocarbamates are weak photoan tioxidants due to rapid photolysis of the metal complexes. It was found by Scott and co-workers [15,16] that aliphatic amine complexes of the zinc dithiocarbamates were about twice as effective as photostabilisers than the parent dithiocarbamates in polypropylene, although the amines alone had little UV stabilising activity. Three factors were found to be involved in the synergism: (i) The amine complexes were more resistant to photooxidation in poly propylene than the parent metal complex. This is shown typically in Fig. 7 for the complex of zinc diethyldithiocarbamate with diazabicyclooctane (DABCO, XII) which is an effective quenching agent for excited states [17-19]. It was suggested that DABCO might be involved in internal quenching of the excited state of the metal complex. The photooxidative stability of ZnDEC in organic solution was not significantly affected by the presence of DABCO, suggesting that quenching, if it occurs, is only impor tant in the associated amine complex.
A XII, DABCO
(ii) The DABCO complex was found to increase the rate of hydroperoxide decomposition by the ionic mechanism, see Fig. 8. (iii) Both DABCO and piperidine increase the equilibrium concentration of zinc dithiocarbamate in polypropylene (see Fig. 9). This purely physical effect is believed to be the most important reason for the synergism involv ing aliphatic amines which, unlike aromatic amines, have no chain-breaking antioxidant activity. The effect on antioxidant solubility is entirely con sistent with the effects of solubilising groups in the dithiocarbamates and 4-mercapto thiazolines discussed in Chapter 8 (Section 5.2). A similar conclusion has been reached by Efremkin and Ivanov [20] in a
443
SYNERGISM AND ANTAGONISM
0|
- 0.21 -0.4
\
\
N.
o
1-1
-0.6 L
S^ZnDEC-DABCO
\znDEC
N \ "s
c
-1
-0.8
L
\
-10
\
■ 50
I 1 11
v. N»
\
\
100
I
*^
150
1
^2 0J0
Irradiation Time, h
Fig. 7. Decay of UV absorbance of ZnDEC (285 nm) and ZnDEC-DABCO complex (285 nm) in polypropylene during photooxidation. Concentrations 6*10 mol/100 g. (Repro duced with kind permission from Developments in Polymer Stabilisation-6, G. Scott (Ed.), Applied Science Publishers, 1983, p. 101).
°1 o - 0.2 * I (Ω -0.4
I—·
$
CO - 0 . 6
3
\
^ \
Control (No add)
^ S .
I
Y
L
ZnDEC\ DABCO N
-0.8 Γ -1.0
V
<3
\\
\
S . ZnDEC N s
1
60
| 1
180 _l
120
1*
1 1 24 J
Irradiation Time, h
Fig. 8. First order plot for the decomposition offers-butyl hydroperoxide (TBH) by ZnDEC and its DABCO complex in chlorobenzene under UV irradiation. Metal complex concen trations, 10 M, [TBH], 10" M. (Reproduced with kind permission from Developments in Polymer Stabilisation-6, G. Scott (Ed.), Applied Science Publishers, 1983, p. 100). 1.2 1.0 0.8 0.6
2
4
6
8
10
Concentration x10, mol/lOOg
Fig. 9. UV absorbance at 285 nm of ZnDEC and its amine complexes as a function of additive concentration. (Reproduced with kind permission from Developments in Poly mer Stabilisation-6, G. Scott (Ed.), Applied Science Publishers, 1983, p. 101).
444
GERALDSCOTT
fi
.30öL o I
S.
β
/
\
/
\
\
I
§ 200 [■ / * /
\ \
^
1/
c
L/
1
" ioof
I
0
25
50
75
100%
Fig. 10. Induction periods to hydroxyl formation during photooxidation (at 300 nm) of isoprene/styrene block copolymer containing a mixture of 4-hydroxy-4'-benzeneazo(azobenzene) and tetrakis (2,2,6,6-tetramethyl-4-piperidinoxy)silane as a function of the fraction of HALS in the mixture. Film thickness, 170 mm, total stabiliser concentration, 2%. (Reproduced with kind permission from Developments in Polymer Stabilisation-8, G. Scott (Ed.), Elsevier Applied Science, 1987, p. 51).
study of the light stabilising effect of a 1:1 synergistic mixture of an 4-amino4'-nitroazobenzeneazobenzene and a hindered piperidine light stabiliser in an isoprene-styrene block co-polymer (see Fig. 10). It was not unambigu ously established however, that the synergism observed in this case is entirely due to mutual solubilisation [21], since anilines are weak anti oxidants and may be acting as synergists by a chemical mechanism. Shlyapintokh and co-workers [13] have suggested that quenching agents can also improve the light stability of phenolic antioxidants, thus extending their life under photooxidative conditions. They put forward a semi-quanti tative treatment of quenching based on the assumption that active radicals are produced by photolysis of the antioxidant [21]; (AH)! - ^ U
((AH)!·
02/RH
» nROO·
(3)
and that this process can be inhibited by quenching: (AH^ + S
> (AH)i + S
(4)
By making use of the Forster equation [22,23], the concentration, c, at which energy transfer to quencher and emission from the antioxidant are equally probable is given by Eqn (5); c-3/4Ä*
(5)
SYNERGISM AND ANTAGONISM
445
where R0 is the critical distance between the donor and the acceptor [21]. This suggests that, if the antioxidant and quencher are distributed uni formly in the polymer, then quenching will occur only at relatively high concentrations of both. It seems likely, however, in the case of the amine complexes discussed above that antioxidant and quencher are closely as sociated and that energy transfer may make a significant contribution to the synergism observed, although solubilisation in the polymer appears to be the main factor involved. Efremkin and Ivanov [20-24] have also studied the solubilising effects of additives on photoantioxidant activity. Thus, for example, a 1:1 w/w ratio of bismuth and zinc dialkyl dithiocarbamates at 0.2, 0.5 and 2% total concen tration gave synergistic effects of 30%, 80% and 30% respectively. A surfac tant (hydroxyethylated cetyl alcohol) improved synergism further, particu larly at higher concentrations of stabilisers. The two dithiocarbamates act by the same mechanism. Neither is a UV absorber and both are similarly photo-unstable. It is therefore difficult in this case to account for the synergism apart from physical causes. 2.3 Autosynergism Many examples were noted in the previous chapter of antioxidants that incorporate more than one function and are consequently more effective than can be accounted for by a single mode of action. A long established class of rubber antioxidants, the bis-phenol sulphides (e.g. XIII) owe their high activity to the presence of both a CB-D and a PD function in the same molecule [25]. The term autosynergism was coined [26] to describe syner gism within the same molecule. Some of the more important autosynergists that have been reported in the literature are listed in Table 4 with the most probable mechanisms involved. 2.3.1 Autosynergism involvingperoxidolytic (PD) mechanisms The incorporation of sulphur into antioxidants and UV absorbers leads to profound synergistic effects. Thus for example, the monosulphides XlVa and XlVb are seen in Table 5 to be approximately an order of magnitude more effective as thermal antioxidants in decalin and more than twice as effective in PP by oxygen absorption than 1076 (X). The advantage of the conven tional antioxidant in an open system is related to its physical behaviour Oower volatility, higher solubility, etc) and not to its inherent activity [30,53]. It has been shown [54] that the sulphur atom in XIV is slowly eliminated during oxidation with the formation of sulphur acids (see Scheme 3). The latter are effective catalysts for the non-radical decomposi tion of hydroperoxides [27]. The diphenylamines (XV) are also highly effec tive thermal antioxidants in rubbers and plastics, particularly when the alky group, R, is the polymer chain itself (see Chapter 5).
446
GERALDSCOTT
TABLE 4 Autosynergistic antioxidants Mechanism
Ref.
CB-D, PD-C
25
CB-D,
PD-C
27-29
XV
CB-D, PD-C
29-32
XVI
UVA, PD-C
29-32
XVII
UVA, PD-C
33,34-36
XVIII
UVA, CB-D, PD-C
10,16,37-44
XIX
UVA, CB-D
33,44-46
XX
UVA, CB-D
44-46
Thermal antioxidants tBu Me
HO^Vs-^OH Me
XIII
tBu
OH tBu ^ A ^ t B u
(a) R = H, BHBM
I T
,^τ,
ILJ CH2SR
^
TT
^TT^W^XÎV
(b)R = C12H25,BHBM-12
H f ~ \ N V~^V-NHCOCH2SR
UV Stabilisers /
O Vc-/
VoCH2CH2OCOCH2SR
HO R 2 NC:**
Ni
(RO)2pf
Ni
C4H9NH2 l
/ o
N i
C8H17 tBu Ho/ tBu
\
o C8H17 OEt yCH 2 PCT V Ni
447
SYNERGISM AND ANTAGONISM
TABLE 4 (Continuation)
Ni C=N / \ CH 3 OH J2 tBu
Mechanism
Ref.
XXI
UVA, CB-D, PD-S
34-36,47
XXII
CB-D, UVA
33
XXIII
MD, CB-D
48,49,86
XXIV
MD, CB-D
50
XXV
CB-D, MD
51
XXVI
PD-C, MD
52
XXVII PD-C, MD
52
O II
HO
c-o
tBu
tBu tBu
Metal Deactivators
cos>~ or OH II
o
H
,
,
H
O^KD^^O tBu
HO-/
yCH2CH2CONH-
tBu
0>
Zn
N I H
R 2 NC ^
S
-
Zn
For explanation of mechanism codes, see Scheme 2.
448
GERALDSCOTT
XIV(a)
, CH2SH
ROOH
CH2SOH
OH tBu
OH tBu
tBu
tBu ROOH
ROOH
CH 2 SR
II o
XIV(b)
OH tBu
OH tBu
tBu CH 2 S0 2 H PEROXIDE DECOMPOSERS + SO,
ROOH
S03(H2S04)
Scheme 3. Peroxidolytic antioxidant action of sulphur substituted phenols [54].
The use of autosynergistic phenols, XIV, in combination with conven tional UV absorbers gives additional photoantioxidant synergism in polyolefins [30] (see Fig. 6), rubber-modified polymers [30,55,56] and PVC [30,57]. Figure 11 compares the combination of HOBP and XIV(b) in polypropylene with a more conventional combination of HOBP and 1076. The presence of sulphur in the molecule almost doubles UV stability. Incorporation of sul phur into both chain-breaking antioxidant (XIV) and UV absorber (XVI) leads to a further increase in activity in ABS (see Table 6) [29]. It was seen in Chapter 3 that the most potent oxidation sensitiser in PVC is the combination of hydrogen chloride and hydroperoxides, both formed by
449
SYNERGISM AND ANTAGONISM
TABLE 5 Autosynergistic activity of sulphur-containing hindered phenols [30] Antioxidant
Induction period (h)
XIV(a) XTV(b) 1076
Dc 51.0 45.0 5.0
PPc 38.5 44.5 18.0
PPo 21.0 11.0 92.0
Dc, by oxygen absorption in decalin at 140°C; PPc, by oxygen absorption in polypropylene at 140°C; PPo, in polypropylene at 140°C in a forced air oven. TABLE 6 Synergism between autosynergistic antioxidant (XIV) and autosynergistic UV stabiliser (XVI) in ABS [30] Stabiliser
Concentration (mol/100 g)
XVI, R = ABS XVI, R = ABS XVI, R = ABS
3.0 6.0 3.0
+
XTV, R = ABS XVI, R = ABS
2.0 3.0
+
XIV, R = ABS
6.0
Time to embrittlement (h) 52 62
1
100
\
J 1
160
\
J
mechanical damage to the polymer during thermal processing. Most PVC stabilisers have the ability to scavenge HCl, but some, notably the thio tin stabilisers of which the dialkyl tin thioglycoUates (e.g. DOTG, XXVIII) are typical, destroy peroxides in a non-radical reaction. O (C 8 H 17 ) 2 Sn(SCH 2 COOR) 2 XXVIII, DOTG
0(
(C 4 H 9 ) 2 Sn
l
/
^CH || /CH
\
oc II o
XXIX, DBTM
450
GERALD SCOTT
This stabiliser class shows very similar behaviour to other sulphur antioxidants; initially it is a prooxidant but it becomes an antioxidant by conversion to sulphur acids [30,60-62]. The result of this is autosynergism between the HC1 scavenging and the peroxidolytic functions in the molecule. The thio tin stabiliser are therefore more effective heat stabilisers than the tin carboxylate stabilisers which do not contain sulphur; e.g. dibutyl tin maleate (DBTM, XXIX), see Chapter 3. Under photooxidative condition, however, they are photosensitisers due to the photolability of the interme diate sulphoxides (Scheme 4) which act as initiators for photooxidation. It should be noted that the reaction of liberated thiols with isolated double bonds removes another potential source of oxidative instability, but this is not an antioxidant process in the normal sense of the word.
Fig. 11. UV lifetimes of polypropylene films containing synergistic combinations of HOBP and phenolic antioxidants as a function of the molar proportion of the phenols. BHBM = XIV, R = H; BHBM-12 = XIV, R = C12H25; 1076 = X, R = C18H37; TBC = ter*-butyl-p-cresol. (Reproduced with kind permission from Eur. Polym. J., 16 (1980) 497).
451
SYNERGISM AND ANTAGONISM
Cooray and Scott [57-59] have shown that antioxidants containing a thiol group (e.g. Table 4, XIV, R=H) and UV stabilisers with similar substituents (e.g. Table 4, XVI, R = H) also add to monoenic unsaturation in PVC by the mechanism shown in Scheme 4. The resulting adducts are antioxidants when used in combination with a tin stabiliser. Both XIV and XVI are thermal stabilisers but XIV, R = H with DOTG leads to antagonism under photooxidative conditions (see Table 7) due to sulphoxide photolysis (Scheme 4) [30,61,62]. However, a combination of DBTM and the sulphur synergised hindered phenol, XIV, are photosynergistic. The sulphur-con taining UVA, XVI, also synergises effectively with DBTM. The role of sulphur in synergistic UV stabilisers has been discussed in some detail in Volume I, Chapter 5 and in Section 2.2.2 in the present chapter. The important principle that has emerged is that optimal synergis tic combinations rarely occur at a 1:1 molar ratio of the complementary functions. In the case of sulphur-containing antioxidants e.g. ZnDEC in combination with HOBP (Fig. 5), a 2:1 excess of HOBP is required to protect the peroxidolytic antioxidant from photolysis. Similarly, in the case of other sulphur-containing antioxidants (e.g. XIV), a similar excess is required (see Table 3). This points to a potential limitation in the use of autosynergists; namely that thefixedratio of the functional groups which results from their chemical combination in the same molecule may not be optimal. It is clearly possible to overload the synergistic combination with sulphur, as described above for XXVII and XVI. Furthermore, the optimal synergistic ratio for thermal oxidation may not be the same as for photooxidation and it may be necessary to compromise to obtain the best all-round balance of antioxidant performance [58,59]. In spite of the above reservation, there is good evidence that by careful design of the autosynergistic package, combinations of autosynergists may TABLE 7 Sulphur-containing stabilisers, XIV and XVI, as photoantioxidants in PVC. (Total con centration, 5.8*10 mol/100 g, synergistic optimum) [30] Stabiliser
Embrittlement time (h)
XXIX XXVIII XXIX + XIV XXVIII + XIV XXIX + XVI XXVIII + XVI Control (no antioxidant)
1000 460 1400 400 1850 1000 670
452
GERALDSCOTT
Cl R2Sn(SCH2COOR)2
o
HCl
l ROOH
■* R2SnCOOR + ROCOCH2SH
II
-CH=CH-
R2SnSCH2COOR
—CHCH2—
I
I
SCH2COOR
SCH2COOR ROOH
—CHCH2— R,SnSCH,COOR
—CHCH
PHOTOPROOXIDANTS
I
0=SCH 2 COOR
ROCOCH.SOH ROOH
ROCOCH2S02H HEAT STABILISERS (ANTIOXIDANTS) SO, + ROCOCH, Scheme 4. Autosynergistic PVC stabilisers [60-62].
give a better performance that comparable blends of conventional synergists. This is shown typically for a synergistic combination of XIV and XVI in ABS in Table 8 [63]. It can be seen that these stabilisers, each at 1 g/100 g are over four times more effective than three conventional antioxidant/stabiliser synergists containing the same functional groups each at 1 g/100 g. A contributory reason for the effectiveness of the autosynergist combination may be that the antioxidant functions have been targeted to the most vulnerable segment of the co-polymer (see Chapter 5). 2.3.2. Autosynergism involving metal deactivation Metal complexing agents, although they have some activity in neutralis-
SYNERGISM AND ANTAGONISM
453
TABLE 8 Comparison of photoantioxidant activity of polymer-bound autosynergists with a combi nation of conventional antioxidants containing the same functional group in ABS. (All concentrations, 1 g/100 g) [65] Antioxidant/Stabiliser
Embrittlement time (h)
BHT + HOBP + DLTP XIV + XVI
85 380
TABLE 9 Autosynergistic metal deactivators [51] Metal deactivate
tio (min)
Copper complex
X = NH, R = H X = NH, R = OH X = O, R = H X = 0, R = OH X = S, R = OH
6.9 235 9.9 60 27.4
— 1:2 — 2:1 —
tio = time to absorb 10 ml 02/fe.
ing the effects of transition metal ions, are not highly effective retarders of metal catalysed oxidation in polymers. Nevertheless, effective "metal deac tivators" have been developed for polymers which contain an additional antioxidant function [51,64] and many of them are exceptionally effective against copper ions. Table 9 compares the effects of phenyl benzimidazoles and related com pounds both with and without 2-hydroxy groups. It is evident that the phenolic function is essential to activity. However, the phenolic group does not need to be involved in metal complexation. For example in the commer cial copper deactivator, XXX, the complexing group is the bis-hydrazide function and the hydroxyl group operates entirely independently as a chainbreaking antioxidant.
454
GERALDSCOTT
tBu
tBu C H CH,CH XONHNHCOCH 22CH ι 2^η2
HO
2
- ^ OH tBu
tBu XXX
A number of sulphur-containing zinc complexes combine the activity of metal deactivators and peroxide decomposers. For example, zinc mercaptobenzimidazole (XXXI) has been used for many years as a copper deactivator in rubber [75] and the zinc dithiocarbamates (IX), dithiophosphates (XXXII) and xanthates (XXXIII) are also effective
a> L
ί Zn
1 2 H J XXXI
^S-
(RO)2P^
Zn
XXXII
2
R O < ^ 2:n S .2
XXXIII
These compounds are all antioxidants in their own right. Although they have been shown to undergo metathesis with copper salts, it is difficult to determine the relative importance of metal complexing and antioxidant activity. However, in some cases, notably the dithiocarbamates, the pre formed copper complexes are themselves antioxidants [76] and, as was seen in the last chapter, the nickel, copper and cobalt dithiocarbamates are effective photoantioxidants, suggesting that the antioxidant function of the ligand is more important than its ability to scavenge metal ions. 3. ANTAGONISM
Although synergism between antioxidants and stabilisers is fairly com mon, antagonism between antioxidants and stabilisers is relatively rare. Very few cases have been studied in thermal oxidation, but several practi cally important examples have been investigated in photostabilisation. 3.1 Antagonism Involving Peroxidolytic Antioxidants It was seen earlier that phenolic antioxidants are mediocre photoantiox idants in polymers and this was attributed to the conversion of phenols during oxidation to light sensitive peroxydienones and quinones. Chakr-
455
SYNERGISM AND ANTAGONISM
aborty and Scott [10] found that both zinc and nickel diethyldithiocarbamates (IX (a) and (b)) retarded the photooxidation of LDPE as did the phenolic antioxidant, 1076 (X). However, in both cases, the combination of metal complex and phenol were less effective (see Table 10). It was found that the rate of disappearance of both metal complexes in LDPE under photooxidative conditions was accelerated by the presence of the phenol (see Fig. 12), suggesting that either the phenol or its oxidation products sensi tised the photooxidation of the metals complexes. TABLE 10 Antagonism in UV stabilisation of LDPE. (Concentration of all additives, 3><10 mol/100 g) Antioxidant
Time to embrittlement (h)
None ZnDEC NiDEC HOBP 770 ZnDEC + 1076 NiDEC + 1076 NiDEC + 770
1200 1400 1800 2200 2250 1250 1580 1850
•S -0.6
100 200 Irradiation Time, h
300
Fig. 12. Change in UV absorbance (330 nm) of ZnDEC and NiDEC in the presence and absence of a phenolic antioxidant (1076) during photooxidation of LDPE. Concentrations 3X10"4 mol/100 g. 1, ZnDEC + 1076; 2, ZnDEC; 3, NiDEC + 1076; 4, NiDEC. (Reproduced with kind permission from Eur. Polym. J., 13 (1977) 1007).
456
GERALD SCOTT
A hindered piperidine, XXXIV, was also antagonised by NiDEC (see Table 10) and Tudos et al. found that the peroxide decomposer delayed the formation of nitroxyl [66]. Me Me HN
Me Me
\-OCO(CH2)8COO—ί
Me Me
ΝΗ
Me Me XXXIV, 770
Chakraborty and Scott [67] suggested that antagonism was due to a less efficient oxidation of the hindered piperidine by hydroperoxide to nitroxyl (see Chapter 9) due to competition from the peroxide decomposer:
>NH
ROOH ^ C ^
(6) Non-radical products
However, other workers have shown that thiyl radicals are trapped by nitroxyls [68,69] which may be reduced back to the parent amine in con sequence [69]: S \ Il N O + RoNC;· / §
S \ II > NOSCNRo 2 /
S \ Il > NH + HOSCNRo 2 (H20) / ^
(7)
It seems likely that both processes contribute to the antagonism. 3.2 Antagonism Involving Chain-Breaking
Antioxidants
A number of workers have shown that hindered amine photoantioxidants are less effective in the presence of hindered phenols [70-72]. Allen [72] has suggested that nitroxyl is removed from the system by reaction with the phenols and their derived phenoxyls, see Scheme 5. However, it is known that the hydroxylamines (XXXV) produced in this process are even more effective as photoantioxidants than the nitroxyls from which they are derived [73]. The coupled nitroxydienones (XXXVI) would almost certainly be photolysed further to the more stable quinones, XXXVII and XXXVIII, which are known to be effective sensitisers for polymer oxidation [74].
457
SYNERGISM AND ANTAGONISM
OH tBu
tBu
tBu
tBu
:NOH
>
-£■
XXXV CH,R
CH,R NO·
O
y
o
tBu^JL^tBu
tBu ^ J L / t B u
hv
I J
o
RCIÎ^ON^ XXXVI
XXXVIII
XXXVII
tBu \
R
/ tBu
R +
^NOH XXXV
Scheme 5. Possible mechanisms for antagonism between hindered amines (>NH) and hindered phenols. REFERENCES 1
B. Ranby and J.F. Rabec, Photodegradation, Photooxidation and Photostabilisation of Polymers, Wiley, 1975, p. 415. 2 T. Matsuura, N. Yoshimura, A. Nishinaga and I. Saito, Tetrahedron Lett. (1969) 1669; (1972) 4933. 3 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 203 et seq. 4 N.S. Allen, Ed., Degradation and Stabilisaton of Polyolefins, Applied Science PubUshers, London, 1983. 5 G.V. Karpukina, Z.K. Maizus and N.M. Emanuel, Dokl. Akad. Nauk SSSR, 152 (1963) 110; 182 (1968) 870. 6 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 210.
458 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
GERALD SCOTT
C.R.H.I. de Jonge and P. Hope, in: G. Scott (Ed.), Developments in Polymer Stabilisation-3, Applied Science Publishers, London, 1980, p. 21. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 295. G. Scott, Eur. Polym. J. Suppl. (1969) 189. K.B.Chakraborty and G. Scott, Eur. Polym. J., 13 (1977) 1007. K.B. Chakraborty and G. Scott, Polym. Deg. Stab., 1 (1979) 37. G. Scott and M.F. Yusoff, Polym. Deg. Stab., 2 (1980) 309. V.B. Ivanov and V.Ya. Shlyapintokh, in G. Scott (Ed.), Developments in Polymer Stabilisation-8, Elsevier Applied Science, London, 1987, p. 29. Yu.A. Shlyapnikov, in G. Scott, Editor, Developments in Polymer Stabilisation-5, Applied Science Publishers, London, 1982, p. 1. G. Scott, Pure Appl. Chem., 52 (1980) 365 S. Al-Malaika, K.B. Chakraborty and G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-6, Applied Science Publishers, London, 1983, p. 73. D. Bellus, in B. Ranby and J.F. Rabec (Eds), Singlet Oxygen, Reactions with Organic Compounds and Polymers, Wiley, 1978, p. 61. C.S. Foote, in B. Ranby and J.F. Rabec (Eds), Singlet Oxygen, Reactions with Organic Compounds and Polymers, Wiley, 1978, p. 135. R.H. Young and D.R. Brewer, in B. Ranby and J.F. Rabec (Eds), Singlet Oxygen, Reactions with Organic Compounds and Polymers, Wiley, 1978, p. 36. A.F. Efremkin and V.B. Ivanov, Polym. Photochem., 4 (1981) 179. E.L. Lovskaya, V.B. Ivanov and V.Ya. Shlyapintokh, Vysok. Soed., A27 (1985) 1589. Th. Forster, Z. Naturforsch., 4A (1949) 321. Th. Forster, Discuss. Faraday Soc, 27 (1959) 7. A.F. Efremkin and V.B. Ivanov, Vysok. Soed., B24 (1982) 662. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 287. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 214. V.M. Farzaliev, W.S.E. Fernando and G. Scott, Eur. Polym. J., 14 (1978) 785. K.W.S. Kularatne and G. Scott, Eur. Polym. J., 14 (1978) 835. G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-4, Applied Science Publishers, London, 1981, p. 181. G. Scott, in G. Scott, Editor, Developments in Polymer Stabilisation-6, Applied Science Publishers, London, 1983, p. 29. G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-8,1987, p. 209. G. Scott, Makromol. Chemie, Macromol. Symp., 28 (1989) 59 D.J. Carlsson, D.W. Grattan, T. Suprunchuk and D.M. Wiles, J. Appl. Polym. Sei., 22 (1978) 2217. R.P.R. Ranaweera and G. Scott, Chem. Ind., (1974) 774. R.P.R. Ranaweera and G. Scott, J. Polym. Sei., Polym. Lett. Ed., 13 (1975) 71. R.P.R. Ranaweera and G. Scott, Eur. Polym. J., 12 (1976) 591. S. Al-Malaika and G. Scott, Eur. Polym. J., 16 (1980) 709. S. Al-Malaika and G. Scott, Eur. Polym. Sei., 16 (1980) 503. S. Al-Malaika and G. Scott, Polymer, 23 (1982) 1711. S. Al-Malaika and G. Scott, Eur. Polym. J., 19 (1983) 235. S. Al-Malaika and G. Scott, in N.S. Allen (Ed.), Degradation and Stabilisation of
SYNERGISM AND ANTAGONISM
459
Polyolefins, Applied Science Publishers, London, 1983, p. 247. S. Al-Malaika, M. Coker and G. Scott, Polym. Deg. Stab., 22 (1988) 147. G. Scott, Macromol. Chem., Macromol. Symp., 22 (1988) 225. G. Scott, Macromol. Chem., Macromol. Symp., 27 (1989) 1. D.J. Carlsson and D.M. Wiles, J. Polym. Sei., Polym. Chem. Ed., 12 (1974) 2217. B. Felder and R. Schumacher, Angew. Macromol. Chem., 31 (1973) 35. L.L. Gervits, N.V. Zolotova and Ye.T. Denisov, Polym. Sei. USSR, A18 (1976) 468. G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 214. 49 A.C. Mehta and T.R. Shishadri, J. Sei. Ind. Res., 18B (1957) 24. 50 H.S. Kuzminskii, V.D. Zaitseva and N.N. Lezhnev, Doklad. Acad. Nauk. SSSR, 125 (1959) 1057. 51 Z. Osawa in G. Scott, Editor, Developments in Polymer Stabilisation-7, Applied Science Publishers, London, 1984, p. 193. 52 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 402 et seq. 53 G. Scott and M.F. Yusoff, Eur. Polym. J., 16 (1980) 497. 54 G. Scott and R. Suharto, Eur. Polym. J., 20 (1984) 139. 55 W.S.E. Fernando and Scott, Eur. Polym. J., 16 (1980) 971. 56 M. Ghaemy and G. Scott, Polym. Deg. Stab., 3 (1981) 253. 57 B.B. Cooray and G. Scott, Eur. Polym. J., 16 (1980) 1145. 58 B.B. Cooray and G. Scott, Eur. Polym. J., 17 (1981) 379. 59 B.B. Cooray and G. Scott, Eur. Polym. J., 17 (1981) 385. 60 B.B. Cooray and G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation2, Applied Science Publishers, London, 1980, p. 53. 61 B.B. Cooray and G. Scott, Polym. Deg. Stab., 2 (1980) 35. 62 B.B. Cooray and G. Scott, Eur. Polym. J., 17 (1981) 233. 63. M. Ghaemy and G. Scott, Polym. Deg. Stab., 3 (1980-81) 405. 64 R.L. Hartless and A.M. Trozzolo, Coatings Plast. Preprints, USA, 3a (1974) 177. 65 M. Ghaemy and G. Scott, Polym. Deg. Stab., 3 (1980-81) 405. 66 F. Tudos, G. Balint and T. Kelen, in G. Scott (Ed.), Developments in Polymer Stabilisation-6, Applied Science Publishers, London, 1983, p. 121. 67 K.B. Chakraborty and G. Scott, Chem. Ind., (1979) 237. 68 R.I. Zhdaniv, V.A. Golubev, V.M. Gidd and E.G. Rozantsev, Izv. Akad. Nauk. SSSR, Ser. Khim., 10 (1970) 2390. 69 H. Dweik and G. Scott, Rubber Chem. Technol., 57 (1984) 735. 70 N.S. Allen, Makromol. Chem., 181 (1980) 2413; Polym. Photochem., 1 (1981) 243. 71 J. Sedlar, J. Marchai and J. Petruj, Polym. Photochem., 2 (1982) 175. 72 N.S. Allen, in N.S. Allen and J.F. Rabec (Eds), New Trends in the Photochemistry of Polymers, Elsevier Applied Science, London, 1985, p. 223. 73 G. Scott, in G. Scott (Ed.), Developments in Polymer Stabilisation-7, Applied Science Publishers, London, 1984, p. 65. 74 B. Ranby and J.F. Rabec, Photodegradation, Photooxidation and Photostabilisation of Polymers, Wiley, 1975, p. 303. 75 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 445 et seq. 76 G. Scott, Atmospheric Oxidation and Antioxidants, First Edition, Elsevier, Amster dam, 1965, p. 402 et seq.
42 43 44 45 46 47 48