Polymer Degradation and Stability 20 (1988) 181-202

Mechanistic Action of Phenolic Antioxidants in Polymers--A Review

Jan Pospi~il

Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia

(Received 2 September 1987; accepted 7 September 1987)

ABSTRACT

Hindered phenols are used as radical scavengers during fabrication, storage, processing and end-use of various polymers. Modern H M W antioxidants have very sophisticated molecular architectures, combining intrinsic chemical activity and the requirements of physical persistence. The mechanism of action has been discussed using the results of the elucidation of the chemical transformations which occur during the stabilisation process. Mechanisms responsible for co-operation phenomena, regenerative processes involved in homo- and heterosynergism, intramolecular co-operation in bifunctional phenolic antioxidants and co-operation pathways between hindered phenols and HALS are shown. Various catalytic species and some pigments are able to induce chemical transformation of hindered phenols and reduce their efficiency.

INTRODUCTION

Phenolic antioxidants confer excellent protection to thermoplastics, elastomer modified plastics and elastomers against deterioration of physical properties. Data published in the USA in the early 1980s indicate I that hindered phenols were the most favoured antioxidants. The substantial part of these has been used to stabilise polyolefins (PO) and styrene based plastics. From the total estimated consumption of almost 20 thousand tons of antioxidants for the stabilisation of plastics in the USA in 1983, 11"2 thousand tons were represented by hindered phenols. 1 The greatest amount of phenolic antioxidants was used for the stabilisation of PO. The severity of the environmental attack which causes degradation in the various stages of the life of PO is very different. Processing temperatures are generally in the

181 Polymer Degradation and Stability 0141-3910/88/$03"50 © 1988 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain

182 Jan Pospigil

TABLE 1 Practice in Polymer Stabilisation 2

Life stage of polymer Stabiliser Usual concentration (ppm)

Drying Antioxidant < 250 Storage Antioxidant < 250 Compounding Antioxidant 500-1000

Phosphite 500-1 000 Fabrication Antioxidant 500-1 000

Phosphite 500-1 000 End-use:

low stress

thermally stressed

weathering

The same as compounding and fabrication

Antioxidant 1 000-5 000 Thiosynergist 1 000-5 000 Phosphite 500-1 000 Antioxidant 0-1 000 Phosphite 500-1 000 Hindered amine 0-10000 Ultraviolet absorber 0-10 000

range 170-280°C. PO are exposed to these temperatures for a short period of time and the degradation is thermal and thermo-oxidative in character. During various end-uses, PO are exposed for extended time periods to oxygen at temperatures substantially lower than that characteristic of processing. During weathering, damage to PO caused by actinic solar light becomes important. Variability in the severity of attack determines the practical requirements of the concentration of hindered phenols and the simultaneous presence of other stabilisers to achieve the protection of PO. Data in Table 12 are indicative ('antioxidant' should be understood as 'phenolic antioxidant'). The concentration level of phenolic antioxidants given is an overall value: a more or less even distribution of antioxidants should be considered to occur in molten PO during processing. However, phenols, together with other stabilisers, are concentrated in amorphous domains in solid PO. The concentration of stabilisers in these domains is therefore higher than that given in Table 1 and all bimolecular interactions involved in the activity mechanisms are therefore favoured.

PHENOLIC COMPOUNDS AS ANTIOXIDANTS: GENERAL ASPECTS

Hindered phenols may be considered as classical antioxidants for polymers and have been the focus of intensive industrial and academic research for 35

Mechan&tic action of phenolic antioxidants & polymers 183

years. The basis of our present knowledge of the activity mechanism is very broad. Some characteristic phases in the research and development may be distinguished. Studies of relationships between the structure and activity are characteristic of the 1950s and early 1960s, kinetic elucidation was mostly done in the 1960s, the explanation of the role of chemical transformation was highlighted in the 1970s and profound studies of co-operation mechanisms in practical mixtures of different additives and of physical factors (e.g. of physical persistence and compatibility of components) were performed in the 1980s. There have always been reasons to reinterpret earlier data and to use this in the synthesis of antioxidants with optimum molecular architecture, either for general use or in some special applications.

Within the long-term chemical development of antioxidants, much effort has been devoted to the optimisation of testing methods and to a better understanding of differences between the activity mechanisms of phenols under natural and accelerated test conditions, to the exploitation of data obtained in accelerated tests for the prediction of the lifetime of stabilised polymers, to the elucidation of toxicological hazards involved in phenolics and, last but not least, to the optimisation of cost/performance relationships in technical practice.

Stabilisation requirements have become more demanding during the last two decades, due to the introduction and extended use of polymers with enhanced sensitivity to oxidation (for example, elastomer modified plastics or PO produced by new technologies, including LLDPE or vapour phase PP) and due to the application of polymers under more severe conditions. The commercial phenolic antioxidants of the last decade represent the success in meeting these new practical requirements. Phenols having molecular weights ___ 500 (HMW antioxidants) have been used at the expense of 2,6-di-tert butyl-4-methylphenol, the largest volume phenolic antioxi- dant. Even macromolecular phenolic antioxidants have been designed 1"or certain uses. In all these phenolic species, the mechanism of antioxidant activity may be described by basically similar pathways.

Hindered phenols are classified as the primary stabilisers of polymers. They terminate autoxidation chains by trapping oxygen centred (alkoxy, RO', and alkylperoxy, RO~) radicals and alkyl radicals R'. The rate of the propagation step of autoxidation is thus limited in oxidation processes having long kinetic chains. Phenols are characterised as chain breaking- (electron) donor (CB-D) antioxidants. 3 Individual steps involved in this mechanism have been described. 4 The simplest mechanistic explanation is based on kinetic measurements performed in oxidised liquid substrates with 2,6-di-tert butyl-4-methylphenol or its analogs. Deactivation (scavenging) of two RO2 radicals by one phenolic moiety has been postulated and the 'stoichiometric factor' 2 has been repeatedly assumed for each sterically

184 Jan Pospigil

hindered phenolic nucleus. However, deviations from the factor 2 have been observed. This is due to the active participation of some of the products formed from phenols (after the trapping of two RO2 radicals) during the subsequent polymer lifetime. 4- 6

The discussion of the mechanistic features of the antioxidant activity of phenols involves the chemistry of phenoxyls, O-centred radicals formed from phenolic compounds as a consequence of breaking the phenolic bond due to H-transfer to the chain propagating RO 2 radical or due to radiolytic processes. The formation of phenoxyls from different kinds of phenols has been confirmed experimentally under conditions modelling reactivity with RO 2 radicals 6 and, recently, by means of pulse radiolysis in oxidised hexadecane. 7 The primary steps of the radical reactivity of phenols and consecutive chemical transformations are a consequence of the intrinsic chemical activity of the phenolic molecule. The antioxidant mechanism involves the active participation of certain transformation products formed in situ from the original phenol in the degradation/stabilisation process. The observed activity of phenols is influenced by the presence of sensitisers or metal catalysts in the stabilised polymer and by the severity of the attacking environment.

FACTORS INVOLVED IN THE ANTIOXIDANT ACTIVITY OF PHENOLS

Aspects of intrinsic chemical activity

A careful interpretation of polar and steric substituent effects on the reactivity of the phenolic OH group and on the antioxidant efficiency determined by oxygen uptake tests in liquid hydrocarbons and PP in the absence of radiation and catalysts, revealed general rules for the architecture of molecules of phenolic antioxidant. 8 The most efficient phenols are 2,4,6-trisubstituted (sterically hindered phenols or cryptophenols). Less fre- quently, efficient 2,4,5-trisubstituted or even 2,4-disubstituted phenols are used. To control the relative ease of formation of the relevant phenoxyls in the oxidation with RO2 or 02, the optimum steric hindrance of the HO group by bulky substituents in positions 2 and 6 is of advantage. 9 For technical and economic reasons, the substitution most frequently used in practice is by either two tert butyl groups or a combination of one methyl and one tert butyl group. In multinuclear phenols, the ortho position to the OH group may be substituted by a bridge connecting phenolic moieties.

The chemical character of the substituent in position 4 is important for the reactivity of the phenolic OH group and often contributes to the stabilising

Mechanistic' action of phenolic antioxidants in polymers 185 mechanism. The substitution of position 4 by a methyl group or by a substituent joined to the phenolic moiety by means of a methylene group has been exploited in the commercial synthesis of efficient phenolic antioxi- dants. 1° These substituents are: (a), linear or cyclic groups of aliphatic or aromatic character, frequently containing functional groups with hetero- atoms O, N, P or S, e.g. ester, hydrazido or amido groups, 1,3,5-triazine or 1,3,5-triazine-(1H,2H,5H)-trione moieties, functional group forming salts of limited solubility, etc., (b) spacers connecting a hindered moiety with a polymer backbone, or (c) bridges connecting two or more phenolic moieties. All substituents contribute also to the physical persistence and compatibility of a phenolic antioxidant with the stabilised non-polar polymer. Thus, hindered phenols form the most chemically variegated class of antioxidants. Examples of typical efficient phenols used in polymers have been given. 2'1° The tert. butylated ester type phenols like Irganox 1010 (1, Ciba-Geigy) or Hostanox 03 (2, Hoechst) rank among the most efficient types. The optimum

(1)

H

OH

3~CH2 ~C(~OCH212

OH

(2)

molecular architecture of hindered phenolic molecules giving the required stabilising effect is influenced also by the structure of the host polymer, e.g. N,N'- 1,6-hexamethylenebis-3(3,5-di-tert. butyl-4-hydroxyphenyl)-propion- amide (3, Irganox 1098, Ciba-Geigy) is a very efficient stabiliser against thermal oxidation and discoloration of Nylons 6 and 66. For polyamides having long alkylene chains like Nylon 12, the ester type antioxidant 1 is more suitable.

7 ? HO-~CH2CH2CNH(CH2)6NHCCH2CH2-~OH (3)

186 Jan Pospigil

A comparison of multinuclear phenols of type 1 with the analogous mononuclear propionate ester type phenol (Irganox 1076, Ciba-Geigy) was made after a careful kinetic analysis. 1~ The equivalency of all phenolic moieties bound in one molecule in the deactivation of RO2 radicals was revealed in PP using melt flow index data. During the processing of PO, the stabilising efficiency of phenols is a function of the concentration of phenolic groups, regardless of the number of phenolic groups attached to a single molecule. The superiority of multinuclear phenols over mononuclear analogs has been explained by the increased physical persistency and by the important mechanistic fact that the trapping of the second radical RO 2 during the stabilisation process usually involves the unreacted phenolic group of the same molecule. Moreover, it seems that hydrogen abstraction from PP by phenoxyl (In')

In" 4- PP ~ InH + PP"

is more depressed in multinuclear than in mononuclear phenols. Due to the radical chain character of the oxidative degradation of

polymers, the concentration of oxidation products of polymers progress- ively increases. The efficiency of phenolic antioxidants is dependent on the concentration of oxidised species accumulated in the polymer and on the immediate concentration of antioxidant which is able to deactivate all the detrimental radical species. The phenomenon of a safe level of antioxidant has been considered quantitatively for many years and the concept of the 'critical antioxidant concentration' was formulated: 12 antioxidant is consumed during inhibited oxidation and its concentration drops to a certain ('critical') level. Below this level, which is considered to be decisive for the observed stabilisation effect, the rate of antioxidant consumption increases rapidly and autoxidation of the polymer becomes faster up to the failure of its useful properties.

This concept is, however, not generally applicable. A critical interpret- ation of a comprehensive set of data obtained with PP and LDPE shows explicitly ~ ~ that samples of stabilised PO may fail even if there is still a rather high concentration of efficient antioxidant present. These observations stimulated the formulation of a new concept called 'critical oxidation level'. This has been defined as 'the degree of oxidation of the polymer beyond which the stabilising system is no longer able to cope with the considerably enhanced oxidation rate induced by the oxidation products that have accumulated'. This oxidation level varies with the severity of the environmental attack. Experimental results are better accounted for by the critical oxidation level concept, especially at low concentrations of phenolic antioxidants, and by statistical considerations involving the heterogeneous character of PO.

Mechanistic action of phenolic antioxidants & polymers 187

Deviation from the critical oxidation level concept has been explained x~ by the semicrystallinity of PO. The kinetic models can no longer be valid, if at least one phenolic molecule is not present in each elementary amorphous domain which is attacked by oxidation. This is generally difficult to fulfil due to the random distribution of antioxidants in the elementary amorphous domains.

Influences of environment and impurities in polymers

Demanding environmental effects like increased temperature or intensive radiation limit the optimum performance of phenolic antioxidants in polymers. The physical effects have been reflected in the kinetics of the chain oxidation process. For example, radiation induced oxidation is character- ised by short kinetic chains. Chain propagating radicals RO~ are formed fast and the CB antioxidant, randomly distributed in the polymer matrix, scavenges only a part of the RO2 formed.

Radical scavenging during photo-oxidation to achieve better photo- stability of polymers is very important.la'L4 The phenolic part of some uv absorbers (UVA) contributes to RO~ scavenging only to a very small extent. Rather poor radical scavenging of UVA has been reported, 13 although transformation products characteristic of the reactivity of phenolic moieties of 2-hydroxyphenylbenzotriazole UVA with ROE have been isolated. ~5

The limited photo-stability of hindered phenols has been considered to be the main reason for their low efficiency during photo-oxidation. It may be due to the easy photo-cleavage of the OH group and to the formation of photo-active impurities from phenols. This may be improved by the use of 'photo-stable phenolic antioxidants'. Very favourable results were obtained with hindered phenols substituted in position 4 with an electron withdrawing group. ~3 H-transfer from the OH group is thus restricted and the formation ofdiene transformation products is slowed down. By a proper choice of the electron withdrawing group, even the photo-chemical reactivity of the system may be favourably exploited. This was achieved in systems represented by phenolic esters able to be transformed in situ via the photo-Fries rearrangement into UVA. Representatives of these photo-stable

phenolic antioxidants are Tinuvin 120 (4, R = ~ , Ciba-Geigy,

reference 13), Cyasorb uv 2908 (4, R = C16Ha3, American Cyanamide Co., reference 16) and phenolic imidazolidine (5,n = 1, reference 17). The favourable properties of 5 were lost in the absence of the electron withdrawing group, i.e. in 5, n = 0. Due to the modified inherent chemical reactivity of phenols 4 and 5, the H-transfer from OH is restricted mostly to

188 Jan Pospigil

reaction with RO 2. However, even these improved antioxidants do not stabilise photo-oxidised polymers without a proper combination with an efficient photo-stabiliser.

H O - ~ C O R (4)

[ 0 /CH2-~CH2OC-I--~ ( "~ X--OH (5)

Strong hydrogen bonds formed between phenolic groups belonging to different phenolic moieties of cyclic HMW phenol-formaldehyde conden- sates, 4-alkylcalix[n]arenes (6, n = a + 3, a = 1, 2, 3), may be responsible for the favourable properties of 6 in the photo-oxidation of polymers. 4-Alkylcalix[n]arenes (6, R = tert.alkyl C4.Sor8, n= 1, 3) are efficient antioxidants. ~8'19 In the concentration range 0.2--0-4% w/w, they possess high stabilising efficiency comparable with that of 2-hydroxy-4-octyloxy- benzophenone used at the same weight concentration. Calixarenes are partly oxidised by hydroperoxides during polyolefin processing or photo- oxidised by oxygen during PO weathering. Compounds with hydroxybenzo- phenone or arylsalicylate structures were identified among oxidation products 19 and are considered to be responsible for the high photo- stabilising effect of transformation products of 6.

R

CH ~ ~ ~ C H 2 R ~ - - o / H O ' / ~ ~ - R (6)

R

There is experimental evidence of change of the structure of sterically hindered phenols at high processing temperatures. Thermolytic dealkyl-

Mechanistic action of phenolic antioxidants in polymers 189

ation of the 2,6-di-tert.butyl-4-hydroxyphenyl moiety in propionate ester type antioxidants has been observed: small amounts of tert.butyl groups were cleaved at temperatures of 250-280°C and an antioxidant with a 2- tert.butyl-4-hydroxyphenyl moiety was formed. 2° The second tert.butyl group was released only very slowly and in trace amounts. The dealkylation process leading to phenolic structures having a less favourable chemical structure was checked by an analysis of phenolic products formed and by the evolution of isobutylene during dealkylation. The latter has been catalysed by acid impurities or fillers, i.e. classical Friedel-Crafts catalysts. The catalytic dealkylation of 2,6-di-tert.butyl-4-methylphenol resulting in a drop in the photolytic stability of this commercially important antioxidant has been reported recently by Allen et al. 21 The catalysis has been explained as a consequence of the presence of trace levels of iron in PP.

The detrimental effects of compounds of transition metals introduced into polymers with fillers, e.g. with asbestos, calcium carbonate or some kinds of carbon black, have practical implications. An increase in the number of kinetic chains due to the metal catalysis of hydroperoxide decomposition results in reduced stability of polymers. Even phenolic antioxidants with high intrinsic chemical efficiency cannot by themselves protect the polymer under these conditions. They have to be used in mixtures with stabilisers having the properties of metal deactivators. A phenolic compound having the structure of bis(3,5-di-tert.butyl-4-hydroxypropionyl)hydrazide (7, Irganox MD 1024, Ciba-Geigy) is an efficient stabiliser protecting PO in the presence of metallic impurities and has been used in combination with ester type phenolic antioxidant 1 for the stabilisation of PE communication cables used in contact with copper. 22

(7)

A combination of hindered phenolic antioxidants with an aromatic phosphite, e.g. tris(2,4-di-tert.butylphenyl)phosphite, is able to suppress the detrimental effect of metal impurities in carbon black 23 used in PP or in composites with EPDM.

Different residual levels of polymerisation catalysts represent another kind of detrimental metallic impurity in polyolefins produced by new technologies, as in gas phase PP or LLDPE. The metal induced deactivation of stabilisers is still not well understood, although attempts have been made to explain different phenomena involved in the catalytic oxidation of polymeric matrices from the point of view of the character of the impurities and the co-catalytic effect of humidity.

190 Jan Pospigil

The detrimental effect of residues of titanium/aluminium polymerisation catalysts on the photo-behaviour of hexadecyl 3,5-di-tert.butyl-4-hydroxy- benzoate (4, R = C16H33 ) have been demonstrated in LLDPE. 24 UV spectral measurements indicated a negative influence of residues of the same catalyst in gas phase PP on the photo-stability of 2,6-di-tert.butyl-4- methylphenol. 21 The latter was rapidly transformed into more strongly uv absorbing species. We may conclude that the sum of processes participating in the rapid deactivation of the initial phenolic structure via phenoxyl and a subsequent transformation of the latter into uv absorbing dienones is involved in this case.

Physical factors in polymer stabilisation

Polymers have been used in steadily increased amounts in more demanding applications. Physical losses of stabilisers under severe environmental attack are due to the low physical persistence of stabilisers, i.e. due to volatilisation or extraction. The practical consequence is a reduction of the concentration of stabilisers and a more rapid failure of the useful properties of polymers. The problem has been solved by the synthesis of HMW phenolic anti- oxidants having molecular weights >500. The elements of the high intrinsic chemical efficiency are fully exploited and a variety of mostly bi- to tetranuclear phenolic antioxidants have been commercialised. 2'~° More- over, different synthetic approaches have been exploited to produce a variety of macromolecular phenolic antioxidants. 25 These systems have, till now, had rather limited practical importance due to the high cost/ performance ratio and to problems arising from some unfavourable physical relationships in the polymeric matrix-additive system. Com- patibility problems are mainly involved. Experimental data concerning relationships between the molecular architecture of a macromolecular phenolic antioxidant and a polymer to be stabilised are still lacking.

It has been generally believed that rules governing activity mechanisms defined for low-molecular weight (LMW) and HMW phenolic antioxidants are valid also for macromolecular species. No mechanistic proof is available, however. Some macromolecular antioxidants are not well defined, especially systems prepared by polymer analogous reactions. 25 Deviations from intrinsic efficiency rules applied in LMW and HMW phenols have been observed in some macromolecular phenols, e.g. in alkylates of o-cresol with polybutadiene 26 and in similar systems prepared from other sterically unhindered phenols by alkylation with reactive polymers or high oligomers. The efficiency of antioxidants like this may be ascribed mainly to their physical persistence and good compatibility with the doped polymer.

Mechanistic action of phenolic antioxidants in polymers 191

MECHANISM OF ANTIOXIDANT ACTION OF PHENOLIC ANTIOXIDANTS: CHEMISTRY OF TRANSFORMATIONS

Phenoxyls are the first detectable radical species formed from phenolic antioxidants as a consequence of H-transfer to RO2 (RO') radicals. 4'6 They are also formed to some extent by oxidation with molecular oxygen at temperatures above 150°C, in sensitised photo-oxidation 27 and during the radiolysis of phenols. 7 The chemistry of phenoxyls is decisive for the consecutive transformations of phenols observed in oxidised polymers. Chemical transformations were explained in experiments performed mostly under model conditions and are an integral part of the antioxidant mechanism of phenols. Important pathways are summarised in Scheme 1

OH R1

RO~ R 2 InH

InH +

CH I

R 3

QM

O

H O O - - C H D

O. RI

• b In

O

g R 2 ~ O Rl

R 2 A r - - C H D

f Scheme !

O /R

CHD"

c~k~H D'

OH

R3--CH

R3--CH

OH H|n ' - |n 'H

T

ROj d

¢ -H

O

~' ~2~OO ~IR R O O - - C H D

O ~ Rl

R 3 - - C I

R 3 - - C

O StQ

192 Jan Posplgil

(InH = phenolic antioxidant, RX- - tert.butyl or methyl, R 2 = methyl or substituted methyl). Pathways (a) to (d) are involved: (a), bimolecular disproportionation of phenoxyl (In') regenerating InH and leading to a quinone methide (QM); (b), C--O coupling of In" with mesomeric cyclohexadienonyl (CHD') leading to aryloxycyclohexadienones (ArO-- CHD), and to QM and InH in the consecutive step; (c), C--C coupling of benzyl radicals formed via formal rearrangement of CHD', resulting in

phenolic dimers (HIn'--In'H); and (d), recombination of CHD" with a second RO2 to form alkylperoxycyclohexadienone (ROO--CHD). With the appropriate substitution (InH, R I = H, methyl or substituted methyl, R2= bulky substituent), a Scheme analogous to Scheme 1 and involving reactivity in position 2 may be drawn. A specific mechanistic feature favourable to the stabilising mechanism has been observed 11 with InH ( R I = tert.butyl, R2= CH2CH2C(O)OR4). The QM derivative may be formed either by the pathway shown in Scheme 1 or by a process shown in Scheme 2. This is an important mechanistic feature contributing to the

O" 0 OH

-ao2u O

CHECH2COR CHCH2COR CH O If fl CH--COR

Scheme 2

efficiency of hindered phenols substituted with the propionate group (like 1): the second RO 2 radical is scavenged by In" without formation of ROO-- CHD which is a thermo- or photo-initiator. The process has a CB- regenerative character due to the intramolecular rearrangement of the QM formed, which is substituted with a propionate group, into a hindered phenol substituted with a cinnamate group.

The relative importance of reactions (c) and (d) shown in Scheme 1 is dependent on the concentration of the reactants. Any of the bimolecular transformations of In" (reactions (a)-(c)) cannot be avoided in InH substituted in position 4 (or 2) with a methyl group or with a group attached to the phenolic moiety by means of a methylene group, Transformations result in all cases in the formation of conjugated dienone systems: QM, ArO--CHD and substituted stilbene quinones (StQ). The last of these is formed either in a consecutive oxidation of HIn'In'H by route (e), Scheme 1, or, together with HIn'In'H, from QM by route (f), Scheme 1. Dienoid compounds may be responsible for the local discoloration observed in polymers.

Mechan&tic action of phenolic antioxidants in polymers 193

Model experiments revealed that the formation of conjugated dienones is favoured by an increase in the concentration of InH. This simulates conditions in separated amorphous areas of semicrystalline PO where InH has accumulated. Due to the validity of the rule 'simili similia solvuntur', the transformation products of antioxidants are more soluble in these areas than in amorphous but antioxidant free areas. The migration of dienones is restricted and sites with concentrated photo-chemical impurities are created. We have not enough information regarding the solubility/compatibility phenomena of transformation products of phenols with the polymer matrix. Due to an increase in the molecular weight in C--C coupling products of the type HIn'In'H and in the relevant StQ, a restricted solubility should be expected. However, no substantial blooming and consecutive staining have been generally observed with dienones formed from efficient phenolic antioxidants.

Pathway (d), Scheme 1, is characteristic of the transformation which takes place in the presence of a relatively high concentration of RO2 radicals. This condition usually does not exist in the long-term use of polymers. Fortunately, the practical concentration of ROO--CHD is therefore beyond harmful limits. Under polymer processing conditions, ROO--CHD is thermolabile and is thermolysed in situ into substituted benzoquinones (BQ) and different phenolic species. 6

Another dienoid compound, viz., substituted hydroperoxycyclohexa- dienone (HOO--CHD, pathway (h), Scheme 1) is formed from hindered phenols at low concentration levels in the sensitised photo-oxidation of PO. 27'2s Its concentration drops after continuous irradiation. 27

Antioxidant aspects of the chemistry of phenoxyls may also be discussed in connection with the CB mechanism observed with some hydrolytically stable phosphites. It has been concluded 29 that a weak CB contribution of the latter is a consequence of the formation of a phenoxyl after the scavenging of the first RO 2 by the phosphite molecule. In" may couple to form a substituted biphenyldiol, an efficient CB--D antioxidant (Scheme 3).

O .

O \ ~ Ro~ o / P - - O R + o / P _ _ O . ~ O \

Scheme 3

194 Jan Pospigil

Phenolic antioxidants have been used in the gamma-radiation sterilis- ation or controlled modification of PO to suppress their post-gamma- irradiation deterioration during storage or application in air. Partial loss of the original phenolic structure due to irradiation has been reported. 3° Different 2,4- and 2,6-dialkyl- and 2,4,6-trialkylphenols containing tert.butyl and methylgroups were irradiated under model conditions to elucidate the character of the transformations. 3~ No isomerisation, dealkylation or realkylation has been observed, even after high doses. Products isolated after the irradiation of 2,6-dialkylphenols in hydrocarbon media (benzene, 2,2,4-trimethylpentane) and the kinetics of their formation indicate the transformation of InH via the respective In" and the consecutive C--O and C--C couplings. C-centred radicals R" were simultaneously formed from hydrocarbon solvents. Recombination of the former with the mesomeric form of In" results in the alkylation of the starting 2,6-dialkylphenol (Scheme 4). 2,4,6-Trisubstituted phenols, not able to be alkylated, do not recom- bine with R" and are considered to transform via phenoxyls according to Scheme 1.

OH OH

R I ~ R2 + R H ~ R 1 T M ~ / R 2

R Scheme 4

THE ROLE OF TRANSFORMATION PRODUCTS OF PHENOLIC ANTIOXIDANTS

Transformation products having general structures, HIn'In'H, QM, StQ, ArO--CHD, ROO--CHD or HOO--CHD (Scheme 1) cannot be considered to be inert compounds from the point of view of the thermal or photo-oxidative stability of polymers. The individual properties of these classes of transformation products were determined in independent model experiments. 4,6

A mechanistic explanation exists for the antioxidant supporting mechanisms of some of these compounds. Regeneration of the CB--D antioxidant activity via In" disproportionation and C--C coupling of CHD" (routes (a) and (c), Scheme 1), disproportionation (route (f), Scheme 1), or rearrangement of QM (Scheme 2) are involved. These processes should be complemented by a disproportionation reaction of In" with alkyl radical R" according to route (a), Scheme 5). 32 (Other examples of InH regeneration from In" are mentioned in the section dealing with intermolecular co-

Mechanistic action of phenolic antioxidants in polymers 195

Q M + R "

R ' + In' ~ , R~--CH-----CH2 + InH(In'H) Scheme 5

operation.) Scavenging of R" contributes to the stabilisation and has been characterised as a chain breaking-(electron) acceptor (CB--A) process. 3 This mechanism is autoco-operative to the CB--D process of phenols and has been considered to be operative with some transformation products having the structure of conjugated dienones, like substituted QM, StQ or BQ formed in situ. The autoco-operative mechanism should be effective mainly under limited access of oxygen, as in processing 33 or fatiguing of PO. The CB--A mechanism is exemplified for QM (route (b), Scheme 5) and accounts, in the first step, for the addition of R" to QM and for the formation of the respective phenoxyl. The second step leading to phenol In'H is analogous to the regeneration of In" via route (a), Scheme 5. Scavenging of radicals R" by QM may generate a polymer bound phenolic species. 25 Similarly to QM, substituted BQ (formed as consecutive products of phenol transformation, e.g. via thermolysis of ROO--CHD) are also efficient scavengers of R" radicals. 4

The intermolecular co-operation mentioned above which accounts for the supporting CB--A activity of transformation products formed in situ from CB--D antioxidants, cannot be expected to form an endless repeating cycle. Activity losses caused by side reactions and the formation of transformation products not involved in the autoco-operation are too serious, and the sum of the antioxidant active forms steadily drops.

Another potential factor which contributes to the loss of the CB--D efficiency of phenols due to transformations is the thermolysis or photolysis of ROO--CHD: 6'27 RO" radicals are formed (and demonstrated by means of EPR spectroscopy) and contribute to chain branching in the autoxidation of the polymer. However, the other part of the molecule is rearranged into BQ (in thermolysis) or cyclopentadienones (CPD, in photolysis). Both these structures impart retardation effects under model conditions. However, they are formed in stabilised polymers in too low concentrations, which has no practical impact on the final polymer stability.

O O R I ~ R2 R ~ f R2

\ C R

o 6 BQ CPD

196 Jan Posplgil

I N T R A M O L E C U L A R CO-OPERATION EFFECTS IN P H E N O L I C A N T I O X I D A N T S

Attempts have been made to modify the phenolic molecule by introducing centres which impart another kind of stabilising activity, like those with hydroperoxide which possess metal deactivating or light stabilising activities. Many systems of this kind have been experimentally synthesised. Only a little data on activity mechanisms have been reported.

It is difficult to synthesise an intramolecularly co-operating system having two or more different centres with a balanced efficiency correspond- ing to their individual intrinsic activity contributions extrapolated from monofunctional stabilisers. Most bifunctional systems therefore possess one functionality which is dominant under specific conditions of use and another functionality which intramolecularly supports the dominant function in a more or less concerted mechanism. Systems like this may be considered as autosynergistic. Phenolic sulphides having the structures of hindered 2,2'- or 4,4'-thiobisphenols, e.g. 4,4'-thiobis(2-methyl-6-tert.- butylphenol) (8, m = 1, n = 0) or its dithioanalog (8, m = 2, n = 0) are typical examples of antioxidants combining C B - - D and hydroperoxide

H O - ~ S m ( O ) ~ ~ O H (8)

CH3 CH3

OH

"CH3 (9)

SO3H

decomposing (HD) centres. Both stabilising mechanisms have been demonstrated experimentally by means of isolated and characterised transformation products. 34 The H D mechanism is evidently more liable to operate at polymer processing temperatures than at ambient temperatures. At the latter, the phenolic moiety contributes predominantly and the corresponding phenoxyls are formed. A detailed product and kinetic analysis has revealed the formation of phenolic sulphoxides (e.g. 8, m = 1, n = 1), sulphones (8, m = 1, n = 2), thiosulphonates (8, m = 2, n = 2) and sulphonic acids among the transformation products. The thermochemistry of different phenolic sulphoxides, e.g. 4,4'-sulphinylbis(2-methyl-6-tert.- butylphenol) (8, m = 1, n = 1) has been studied in detail. Together with thiosulphinates (8, m = 2, n = 1) these sulphoxides are considered to be the

Mechanistic action of phenolic antioxidants & polymers 197

precursors of peroxidolytic antioxidants. Acids like 3-tert.butyl-4-hydroxy- 5-methylbenzene sulphonic acid (9) or its 2-hydroxy isomer and sulphur dioxide were confirmed to possess a strong peroxidolytic efficiency. Co-operation of the sulphonyl group in 4,4'-sulphonylbis(2-methyl-6- tert.butylphenol) (8, m = 1, n = 2) with the hindered phenolic moiety has been confirmed to be the reason for the observed antioxidant efficiency of phenolic sulphone.

Unfortunately, no data are available on the mechanism of the activity and transformations of phenolic moieties in other important stabilisers containing combinations of a hindered phenolic moiety with another function, as in Irganox MD 1024 (7) or in bis(1,2,2,6,6-pentamethyl-4- piperidinyl)-2-n-butyl-2-(4-hydroxy-3,5-di-tert, butylbenzyl)malonate (10, Tinuvin 144, Ciba-Geigy), a combined photo-stabiliser.

OH

O C H 2 0 IP t LI

O C C C O

I I CH 3 CH3

(1o)

INTERMOLECULAR CO-OPERATION EFFECTS OF PHENOLIC ANTIOXIDANTS WITH OTHER STABILISERS

Mixtures of different stabilisers have been used to achieve the effective stabilisation of commercial polymers against degradation. Hindered phenols, typical CB- -D antioxidants, may be used in combination with stabilisers acting by the same mechanism to achieve a better final CB effect, or with stabilisers acting by an entirely different mechanism (i.e. with hydroperoxide decomposers, metal deactivators, UVA or hindered amine light stabilisers, HALS) to achieve a better complex protection. Various efficient systems have been developed in practice. Mechanistic features involved in these systems have been explained mostly using an empirical approach and taking into account known mechanisms of individual components. Some very important mechanistic studies were performed in model conditions.

198 Jan Pospigil

Regeneration of the phenolic antioxidant from the respective phenoxyl and/or transformation products having a conjugated dienoide structure, is the mechanistic feature mostly responsible for the CB--D efficiency in the stabiliser mixture (examples of the InH regeneration in the reactivity of In" and QM with radicals R" are given in Scheme 5). The mechanism of regeneration of a hindered phenolic antioxidant used in a mixture with an unhindered phenol has been explained 35 by hydrogen transfer from the less effective unhindered phenol to phenoxyl generated from the more effective (hindered) phenol, i.e. by differences in the reactivity of the phenoxyls involved. A similar concept has been used to explain the increased CB--D efficiency of mixtures of sterically hindered phenols and aromatic secondary amines 4'36 and was called homosynergism. 3

The effect of phenolic antioxidants used in combination with HD antioxidants (organic sulphides or phosphites), called heterosynergism 3 has been currently explained by a concentrated scavenging of RO~ and a non- radical decomposition of hydroperoxides. Due to this self-protecting mechanism, the activity period of InH is prolonged. However, product studies have revealed features of the regeneration of a hindered phenolic antioxidant from the relevant phenoxyl in co-operative systems containing di-tert.butylphosphonate 37 or dioctadecyl 3,3-thiodipropionate, 38 or from ROO--CHD and didodecyl 3,3-thiodipropionate. 39 Phenolic antioxidants are also regenerated as a result of the redox reactivity of quinoid transformation products with some compounding ingredients having the structures of aromatic thiols. 4°

Hindered phenols are only weak photo-stabilisers. Their radical scavenging efficiency is augmented in combination with UVA. Favourable co-operation improves the photo-stability of polymers and has been explained to some extent by the protection of the phenolic antioxidant from photolysis by UVA. Photo-degradation of polymers takes place near the polymer surface. The intensity of the light is decreased by UVA. This prevents the light absorption by chromophoric groups present in the polymer. Moreover, the excited chromophores may be deactivated by quenchers. Deactivation of light by photo-stabilisers is not complete, however. Autoxidation chains are still induced and must be terminated by the phenolic antioxidant. The activity of the UVA/InH system is dependent on the velocity at which the phenolic antioxidant is replenished by diffusion from the deeper-lying layers of the polymer to its surface. 14 This reveals that the chain-breaking efficiency of phenols, their photo-stability and physical properties allowing migration, are important mechanistic factors relevant to synergistic co-operation with UVA stabilisers.

The photo-stabilisation of articles having a high surface/volume ratio is of great commercial importance. Combinations of sterically hindered phenols

Mechanistic action of phenolic antioxidants in polymers 199

and of HALS based on piperidines are used and bring excellent results. The concentration of individual components is dependent on the polymer, shape of the article and environmental attack. 41 Both InH and HALS are chemically very reactive, and the chemistry of their transformations has been used to explain co-operative phenomena observed in practice. Although some important studies have been carried out in this direction, many aspects of the problem remain open.

Synergism was reported during oven ageing of HDPE and PP. The effect of the same stabiliser system, consisting of very efficient phenolic antioxidants and HALS, has, however, been reported to be antagonistic in the photoxidation of HDPE and pp.42,43 To explain the mechanism, the reactivity of starting stabilisers, derived free radicals (phenoxyls, cyclohexa- dienonyls, aminyls "~ / N ' , nitroxides ~/NO') and of molecular transform- ation products (quinonemethides, alkylperoxycyclohexadienones, O- alkylhydroxylamines ~/NOR and hydroxylamines )NOH) has been considered. 41'44 The integral Scheme 6 ( ) N H and subsequent species represent HALS and respective transformation products) involves the most important pathways to be discussed in connection with (a) the possibility of participation of the cyclical regeneration of the chain-breaking power of HALS via O-alkylhydroxylamine), (b) not fully clear phenol/nitroxide and phenoxyl/hydroxylamine interactions and (c) interactions of cyclohexa- dienonyl with nitroxide and amine (HALS). It should be considered that the phenolic antioxidant is transformed into In" mainly via its chain-breaking

~NH N. • HALS InH + / N

RO~ ~ N H

R 1 R 1

) N O ' + '~ ' /NNOH +

R 2 R 2

InH RO~

R" /NNOR A Scheme 6

0

X ( 0

< - R ~ ~ . R1

(Scheme 1)

200 Jan Posptgil

activity with RO~, and only an additional transformation may be due to the reactivity with nitroxides. The relative importance of these two transform- ation pathways (as well as of all other steps involving nitroxide reactivity taken into consideration) is strongly dependent on the structure of the phenolic antioxidant, on physical environmental factors (temperature, radiation, polarity effects) and on the relative concentration of all species involved in the system.

ACTIVITY OF PHENOLIC ANTIOXIDANTS IN THE PRESENCE OF PIGMENTS AND FILLERS

Pigments and fillers are widely used in polymers for a variety of reasons. They can have a significant influence, both favourable and unfavourable, on the thermal and light stability of polymers. There are many practical observations in this direction. An objective mechanistic explanation is mostly lacking and the processes are not completely understood.

The negative influence of organic yellow, orange and red pigments on the photo-stability of polymers was found 45 to be due to the sensitisation of singlet oxygen formation, initiation of polymer photo-oxidation by excited pigment chromophores, faster chemical transformation of stabilisers and reduction of the effective antioxidant concentration due to its adsorption on pigment surfaces. Photo-sensitised oxidation of hindered phenols was shown to cause rapid formation of H O O - - C H D (Scheme 1) and its consecutive transformation. 27

Other pigments, like uncoated titanium dioxide 46 and carbon black, 47 can react with hindered phenolic antioxidants, transform them via phenoxyls to coloured products and thus reduce the antioxidant power. It was reported that the decrease of the lifetime of PE doped with the phenolic antioxidant/carbon black system was very rapid at elevated temperatures, 48 in comparison with tests performed at ambient temperature. This indicates acceleration of chemical transformations of phenols in the presence of C- black due to temperature.

CONCLUSIONS

Present knowledge of the chemistry of hindered phenols in thermally and radiation induced oxidations allows us to explain most phenomena observed in stabilized polymers. Results obtained with hindered mono- nuclear phenols may be exploited in order to explain processes taking place with more sophisticated multinuclear HMW phenols containing other functional groups in the same molecule or macromolecular phenols with

Mechanistic action of phenolic antioxidants in polymers 201

different molecular architecture. New mechanistic features have been explained in recent years. However, some mechanistic problems impor tan t for practical stabilisation and for the synthesis o f more effective 'tailor- made ' phenolic antioxidants, remain open to further elucidation. This involves effects of fillers, pigments and catalytic metal impurit ies on phenols, the efficiency of intermolecularly co-operative phenol / l ight stabiliser systems and a series of problems arising f rom physical interactions between stabiliser(s) and the macromolecular matrix.

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