Polymers for Advanced Technologies Volume 3, p p . 443-455

REVIEW ARTICLE

Exploitation of the Current Knowledge of Antioxidant Mechanisms for Efficient Polymer Stabilization' Jan PospiSil lnstitute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6 , Czechoslovakia

ABSTRACT

New mechanistic knowledge of antioxidant and photo- antioxidant mechanisms is interpreted to meet the requirements of processing stability and durability of commercial plastics, rubbers and coatings in hostile environments. Factors reducing stabilizer activity and some ecological concerns dealing with polymer stabiliza- tion are included.

KEYWORDS: Antioxidant mechanisms, Photo-antioxidants, Polymer durability testing, Polymer stabi I ization

INTRODUCTION Physical and chemical consequences of processing degradation and long-term aging restrict the practical application of articles fabricated from unstabilized plastics, rubbers, multiphase and filled polymers, and resins. Requirements for proper stabilization are augmented due to the introduction of polymers synthesized by means of new generations of cata- lysts, rapid expansion of technologies exploiting more drastic processing conditions, and the increas- ing application of polymers in various important industrial areas under severe environmental con- ditions, where polymers are exposed to high temper- atures, intensive solar radiation and leaching media, as in the automotive, aerospace, communications, electronics, packaging and textile industries. The current knowledge of various modes and mecha- nisms of polymer degradation in a hostile environ- ment, and the respective reflection on polymer dura- bility and utility [l] creates a basis for a rational

Presented as an invited keynote lecture at the 33rd IUPAC Congress, Budapest, 17-22 August 1991 in the polymer section organized by Professor F. Tudos.

selection of appropriate stabilizers protecting against physical and chemical deteriogens. The current knowledge of structural factors governing inherent efficiency and proper physical properties may be considered as a key approach for the selection of efficient stabilizers.

2. CALL FOR AN EFFECTIVE EXPLOITATION OF STABILIZER ACTIVITY IN POLYMERS FOR ADVANCED APPLICATIONS A high inherent stability of polymers is very desir- able, but it is not a sufficient condition for the solution of durability problems. Besides trace amounts of defect structures and impurities present in commercial virgin polymers, new active function- alities are created and accumulate stepwise in de- localized domains, both during the processing and aging of polymers. The inherent sensitivity of any polymer to environmental deteriogens is thus aug- mented. This is a characteristic consequence of all degradation processes.

Other prodegradants are introduced with resi- dues of polymerization catalysts or by application fillers, pigments and/or reinforcing agents contami- nated with metallic impurities that have a catalytic character. Commercial polymers cannot therefore withstand environmental attacks without losing use- ful properties, and the application of stabilizers is mandatory to secure a long-term calamity-free perfor- mance under particular application conditions. Specific problems are implied in the stabilization of recycled polymers.

According to the spectrum of physical and chemical deteriogens attacking a polymer during its particular application, thermostabilizers, antioxi- dants, antifatigue agents, antiozonants, photo- antioxidants, light stabilizers, flame retardants and biostabilizers are used. Some of these such as the last

1042-7147/92/080443-13 $11.50 0 1992 by John Wiley & Sons, Ltd.

Received 20 1uly 1992 Revised 18 August 1992

444 I PospiSil

two classes, are used only for specific protection purposes. The other classes are of a more general interest.

Economic and environmental requirements are expressed in attempts to use more efficient stabi- lizers at lower concentration levels to achieve a better performance [Z]. Practical exploitation of theoretical knowledge of stabilization mechanisms contributes to this aim, as well as to the development of new important stabilizers. An understanding of the properties of stabilizer transformation products formed during polymer stabilization [3] and of the advantages offered by intermolecular and intra- molecular cooperation between different stabilizing moieties [4] is very stimulating.

The chemical transformation of antioxidants is a consequence of their active participation in the stabilization process. This is due to their higher reactivity with oxygen, oxygen-centered free radicals and hydroperoxides compared with the reactivity of the doped polymer matrix itself. This transformation may be called a “serviceable” one. Some products formed contribute to the integral stability effect by supporting complementary or regenerative mechan- isms [3, 51 (as in chain-breaking antioxidants). The chemical transformation is even a condition of an effective overstoichiometric activity of sulfur con- taining hydroperoxide decomposing antioxidants and photo-antioxidants [3]. However, an undesirable ”inactive” transformation also exists. This is repre- sented by photochemical depletion of antioxidants and by their reactivity with some other additives, metallic species, NO, and/or acid rain.

This paper covers topics arising from knowledge of the mechanistic behavior of antioxidants and photo-antioxidants, and is particularly focused on the current understanding of features of their chemi- cal reactivity under application conditions and/or exploitation of polymer used in highly demanding surroundings. Where applicable, reviews containing additional relevant references are reported in the paper.

3. CURRENT KNOWLEDGE 0 7 ANTIOXIDANT ACTIVITIES 3.1. Chain-breaking Antioxidants

Phenols and aromatic amines, the so-called primary antioxidants, are included in this stabilizer class that is characteristic of interference with the propagation step of autoxidation. Their principal activity involves scavenging of oxygen-centered radicals according to Scheme 1. InH represents the chain-breaking antioxi- dant (CB AO) and In‘ the respective free radical (phenoxyl, aminyl) [6]. ”Products” formed via con- secutive transformations of In’ participate mostly in

InH + RO; (RO’) - In’ + ROzH (ROH)

I Products

SCHEME 1

the stabilization process by their complementary and supporting effects, and knowledge of their chemistry contributes to the explanation of activity mechan- isms of individual types of CB A 0 [3].

3.1.1. Phenolic Antioxidants. Phenols represent the most chemically variegated class of antioxidants [5], with a well known chemistry of transformations during active participation in polymer stabilization [7]. “Products” in Scheme 1 are formed by dispropor- tionation of In’, C - 0 and C-C couplings of In’ and its mesomeric forms, or their recombination with other free radical species present in the system [2]. Typical classes of products (see Scheme 2) include derivatives of cyclohexadienone (1 = CHD, X = HO, HO,, R, RO, ROB a more complicated peroxidic moiety or a phenolic moiety), benzoquinone meth- ide (2 = QM, R3 = H, alkyl, phenolic moiety or the remaining part of a polyphenolic molecule), benzo- quinone (3) and various phenolic products formed from In’ [Z, 31.

Mixtures of compounds formed are very vari- egated according to reaction conditions. Products containing phenolic moieties participate in consecu- tive processes in which new reactive phenoxyls are repeatedly generated. It is very difficult to decipher the structure of free radicals and molecular products formed in minor amounts in the last phases of phenol transformations [8]. Products containing QM moieties are responsible for the discoloration of phenol-doped polymers. They represent a very reac- tive species participating in reversible photobleach- ing [Z], coupling processes [3], reactions with secondary antioxidants (see section 3.2.1), com- pounds having mercapto groups [9] and in trapping radicals R’, RO’ or RO; and RO,H [Z].

Recent product analyses (see Scheme 3) have explained the favorable contribution of structural factors in the propionate [lo] (e.g. 4), semi-hindered [Z, 31 (5) and cryptophenolic [Z] (6) types of antioxi- dants to the activity mechanisms. Intramolecularly and intermolecularly regenerated active species par- ticipate in the stabilization process. The formation of the two kinds of C-C coupling products from sterically hindered 2,6-ditert.butyl-4-methylphenol, an irreversible (7) and a reversible (8) process, and the mechanism of the conversion of the latter into generally reported form (7) was published recently

A new class of phenolic antioxidants, which react with both RO; and R’ radicals and interfere with the initiation and propagation steps of autoxi- dation, was commercialized recently [5]. The appli- cation of 2-tert.buty1-6-(3-tert.butyl-Z-hydroxy-5-

~ 4 1 .

1 2 3 SCHEME 2

Current Knowledge of Antioxidants / 445

Subst . Su bst . O=COR

4 5 6

7 8

R R

9 SCHEME 3

methylbenzyl)-4-methylphenyl acrylate (9, R = Me, Sumilizer GM, Sumitomo; Irganox 3052, Ciba-Geigy) is an important contribution to the practical stabili- zation of unsaturated hydrocarbon polymers at limited access to oxygen. The activity mechanism was proved in a product study [15]. Some discolo- ration imparted to polymers by a QM formed by oxidative C-C coupling of 9, R = Me, was reduced in improved product, Sumilizer GS, Sumitomo (9, R = tert.alkyl), where the possibility of QM forma- tion was eliminated [16].

Trapping of both types of radicals, R' and RO;, is mandatory during high heat and shear stress (melt processing) and fatiguing of polymers. The unique stabilization activity of 9 was proved in styrene- isoprene-styrene block copolymers using aerobic and anaerobic conditions [17]. Stabilizer 9 generally offers outstanding performance in butadiene-based polymers under anaerobic conditions, where tradi- tional phenols usually fail [15, 171.

3.1.2. Aromatic Amine Antidegradants. This unique class of stabilizers has been applied mainly to the stabilization of rubber vulcanizates, where their antioxidant, antiflex-crack and antiozonant proper- ties are fully exploited. Discoloration due to transfor- mation products of aromatic amines limits appli- cation in polyolefins and light-colored rubbers. Main commercialized structures include 4,4'-disubstituted diphenylamines, 6-substituted or oligomeric 2,2,4- trimethyl-1,2-dihydroquinolines, and N,N'-disubsti- tuted 1,4-phenylenediamines. All of these consist of RO; trapping CB AO. The last two structures impart antifatigue (R' trapping) activity, and derivatives of phenylenediamine are strong antiozonants. The

mechanisms of these antidegradan t processes were reviewed recently [18-201 and the role of transforma- tion products of aromatic amines was discussed in detail in reference [3]. The reactivities of nitroxides derived from various aromatic amines (12, 13; see Scheme 4) and those generated from hindered ali- phatic amines (HALS, 10) were used to explain differences in R' trapping effectivity. A comparison of stabilities of nitroxides 10-13 reveals [3] a drop of R'-trapping ability from 10 to 13, due to the increased reactivity of 12 and 13 in their mesomeric structures. Nitroxide 13, derived from 1,4-phenylenediamines, reacts only in its bis-nitrone mesomeric form, and a polymer-bound species 14 (a "secondary" nitroxide) is generated after scavenging of P . The P-trapping activity in the phenylenediamine series proceeds mainly via the respective 1,4-benzoquinonediimines (15), the principal transformation products of 1,4- phenylenediamines [18]. Ring alkylates, e.g. 16, formed from 15, are also polymer-bound species, similar to 14. Structures such as 14 and 16 also explain the formation of the so-called "nonextract- able" nitrogen in aged and aromatic amine-doped rubbers.

3.2. Hydroperoxide-decomposing Antioxidants and Their Combination with Chain-breakers The general activity mechanism of hydroperoxide- decomposing antioxidants (HD AO) is characteristic of SN2 replacement of the peroxidic 0-0 bond by nucleophilic compounds. Activated organic sulfides, phosphites or phosphonites are used commercially [5]. HD A 0 activity results in reduced initiation of chain oxidation. There are two different activity phases in the HD A 0 mechanism (Scheme 5): a stoichiometric one, resulting in the formation of oxidation products HD(0) and HD(O), (e.g. sulfox- ides, sulfones, phosphates and phosphonates), and an overstoichiometric one, characteristic of peroxi- dolytic products.

The chemistry of "activated" sulfides, having one or more hydrogens on the beta-carbon atom to the sulfur, is well understood [5, 211. Sulfoxide 17 formed in the first stoichiometric step decomposes

10 11 12 13

0' 1L 15 16

SCHEME 4

446 / PospiSil

RO2H Peroxidolytic Products

I * ROpH + HD - HD(0) + ROH 1 ROzH

HD(0)z + ROH

SCHEME 5

thermally into a sulfenic acid (18, Scheme 6), in a process that is accelerated by electron-withdrawing groups. This explains the high efficiency of esters of 3,3'-thiodipropionic acid. The latter forms part of various commercialized HD A 0 products [5]. 18 is either oxidized in consecutive steps into sulfinic and sulfonic acids, respectively (19, n = 2 and 3, respect- ively) or transformed into thiosulfinate 20, a source of thiosulfoxylic acid (21).

Protic acids 18,19 and 21 decompose R02H by a catalytic polar mechanism. Their activity is reduced by additives that have basic properties, such as calcium carbonate or stearate [21]. At a high ratio of sulfoxide 17 to S-acids, a hydrogen-bonded species, such as [RSO,H . . . OSRJ , reduces the activity of 18, 18, 19 and 21, and is responsible for an induction period observed in HD A 0 mechanisms. This may arise at temperatures lower than those necessary for thermolysis of 17, e.g. during stabilization at ambient temperatures. Moreover, transformation products of sulfides, i.e. RS(O),H, may induce homolysis of R02H. The free alkoxy radicals thus released are responsible for pro-oxidative effects:

R'02H + RS(O),H+ R'O' + RS'(O), + H20. (1)

Despite the high thermostabilizing effect of thioester-type HD AO, their odor-forming tendency inhibits their commercial importance.

Organic phosphites and phosphonites are pro- cessing stabilizers with HD A 0 activity. They improve the color of polymers. Weak metal- deactivating activity is considered with some of them, according to the following [22]:

M"' + yP(OR)3+M"'[P(OR)3],. (2)

R'02H I-* RS(O),H + R'OH

19 A I RS(0)R - S O H + Olefin

18

RS(0)SR + H 2 0 20

P(OAr)3 + RO; O=P(OAr)3 + RO' I

SCHEME 7

The R02H-reducing mechanism of alkyl, aryl, hin- dered aryl and cyclic phosphites has been explained in detail [5, 231 and generally follows Scheme 5. The respective phosphates are formed as the conse- quence. A supporting CB A 0 effect of hindered aromatic phosphites was explained recently [23] (Scheme 7, Ar = 2,4-ditert.butylphenyl). A release of a hindered phenoxyl ArO', participating in consecu- tive steps according to the mechanism characteristic of cryptophenols [2], is included.

New experiments performed with cyclic aroma- tic phosphites or sulfites have revealed [24] an efficient deactivation of R02H by direct reduction, according to Scheme 5; in addition, they enhance the HD A 0 effect due to acid-catalyzed decomposition of R02H (acids are formed via ester hydrolysis). The CB A 0 complementary effect is due to the release of free phenols after hydrolysis and/or peroxidolysis of the respective esters.

A great effort has been devoted to the improve- ment of the hydrolytic stability of phosphites. Structural improvements, such as the application of hindered aryl esters and/or built-in or blended weak bases such as tris(alkano1) amines, are an efficient solution [5].

3.2.1. Cooperation Between Chain-breaking and Hydroperoxide-decomposing Antioxidants. Many important recipes dealing with combinations of phe- nols with HD A 0 are available for various classes of polymers [lo]. Three mechanisms are involved [4] (Scheme 8): (a) a complementary mechanism (i.e. additivity of CB and HD activities); (b) a concerted supporting or self-protecting mechanism (i.e. protec- tion of CB A 0 from depletion with RO,H by means of HD AO); and/or (c) a regenerative mechanism (i.e. transformation products of CB A 0 are trans- ferred by HD A 0 or HD(O), to efficient chain- breakers In'H). The importance of individual pro- cesses is influenced by degradation conditions and stabilizer structures.

InH + RO; - In' + ROzH

/ a,bl HD Products +

c HD(O)n HD(O), + ROH

In'H 1

SCHEME 8 SCHEME 6

Current Knowledge of Antioxidants I 447

Kinetic measurements have confirmed the com- plentary and supporting effects resulting in syner- gism. Product studies have shown the self-protection and regenerative phenomena [4]. Recent mechanistic studies performed with hindered and semi-hindered phenolic antioxidants, and with cryptophenols hav- ing various molecular architectures, have revealed [2, 3, 51 that a more favorable synergistic effect in polypropylene was achieved with phenols having a lower sterical hindrance. The formation of an asso- ciation complex 22 (see Scheme 9) between 2-methyl- 6-tert.buty1-4-substituted phenols and thioester synergists has been proposed [12]. The respective association complex 22, allowing an in situ deactiva- tion of ROIH (Scheme 9), has been confirmed by IR measurements and contributes both to the comple- mentary and self-protecting effects.

A very strong efficiency was also measured with semi-hindered phenols of the 4,4'-iso- propylidenebisphenol series [13] and with crypto- phenols [25] combined with dialkyl sulfides and disulfides. A specific regenerative pathway due to the reactivity of unhindered cyclohexadienonyls (23, see Scheme 10) generated from cryptophenols (6) with thiyl (or sulfinyl) radicals created during ther- molysis of oxidation products formed from the thio- synergist [5] and leading to 24 was considered (Scheme 10, n = 0,l) as one of the possible contribut- ing mechanisms [25].

The formation of odor diminishes the excellent properties of sulfides used in combination with phenols. A new odorless thiosynergist with a pro- prietary structure was announced only recently [26].

Degradation connected with undesirable changes of physical and chemical properties takes place in polyolefins subjected to processing at high temperatures and shear. It has been shown [27] that a safe processing stability may be achieved in the temperature range up to 300°C, and that a high extruder output may be achieved with a combination of a physically persistent polynuclear phenolic antioxidant tetrakis[methylene 3-(3,5-ditert.butyl-4- hydroxyphenyl)propionate]methane (Irganox 1010, Ciba-Geigy), and a hydrolysis-resistant phosphite. This combination resulted in an enhanced service life of polyolefins in both thermal and weathering environments. However, a specific concentration ratio of both antioxidants should be used to reach the most effective stabilization. This is achieved by use of the following phenol/phosphite blends 1 : 2 for

2 2

,KH~) 2 ~ ( ~ ~ ~ 1 -Subst. 0 0 + 0 4 + ROH

cH3 i)R2

SCHEME 9

0 OH

+ R S ' ( R S * O ) ----+ - L b s t .

2 3

b b s t . 24

SCHEME 10

polypropylene and 1 : 4 for both HD-PE and LLD PE. The commercial blends Irganox B 215 (for PP) and B 561 (for PE) assure a successful formulation, even for stabilization of PP prepared by superactive catalysts.

Mechanistic product studies describing interac- tions in phenol-phosphite systems are very scarce. C-C coupling of QM derived from 2,6-ditert.butyl- 4-methyl-phenol catalyzed by the phosphite function has been reported [28]. An interesting explanation of the improvement of the color of polyolefins stabil- ized by phenols and phosphites was offered in references [22, 281. It includes the generation of colorless phosphates, phosphonates or yields from particular discoloring phenol transformation prod- ucts, 2,6-ditert.butyl-1,4-benzoquinone,2,6-ditert. butyl-1,4-quinonemethide and 3,5,3',5'-tetra-tert. butylstilbene-4,4' -quinone.

3.3. Photo-antioxidants

This class includes stabilizers that impart their efficient antioxidant activity to polymers stressed by O2 and actinic solar radiation. "Photostable" phenols such as aliphatic or aromatic esters of 4- hydroxybenzoic acid (25, see Scheme 11) and various 4-substituted 2,2,6,6-tetramethylpiperidines (26, R = H, alkyl) or analogous derivatives of 2,2,6,6- tetramethyl-3-piperazinone are typical representa- tives of free radical scavenging stabilizers, that are also active under conditions of photo-oxidation [ll].

Compounds of type 26 are generally called hin- dered amine light stabilizers (HALS) and surpass in their importance those type 25. HALS belong among the most fortunate developments in polymer stabi- lization. They were commercialized in the 1960s and their potential development is still in progress. HALS have been shown to impart protection against weathering to polymers in most applications. The performance is superior to that of other classes of light stabilizers. Many various HALS are now com- mercialized and available for polymer stabilization [Ill.

OH

O=C 0 R

&H*& 0 0

R R

25 26 SCHEME 11

448 I PospiSil

3.3.1. The Activity Mechanism of Hindered Aliphatic Amines. Intensive reserch has been devoted by distinguished research teams to deciphering the mechanism of this unique class of stabilizers. The results have revealed [29-321 that stable nitroxides (>NO)' are formed as the key intermediate from the secondary HALS (e.g. 26, R=H,>NH; Mark L-77, Asahi Denka Kogyo; Sanol LS 770, Sankyo; Tinuvin 770, Ciba-Geigy, as an example) due to the reactivity of the latter with hydroperoxides, peracids or acyl- peroxyls. Nitroxides are generated from hydroxyl- amine (>NOH) and 0-alkylhydroxylamine (>NOR), the respective transformation products of >NH, and also alkylperoxyl. The integral mechanism of HALS involves trapping of free radicals R', RO; , -C'(O) and/or -C(O)O;. Moreover, the principal chemical reactivities of >NH include the formation of associ- ates with oxygen, polyolefins and RO;, proposed recently [31], and complexation of transition metals [32]. Scavenging of macro-alkyls P' has been proved experimentally [29, 301. The >NOP formed is involved in the regenerative cycle >NO'

The chemistry of secondary HALS >NH also applies to tertiary HALS (>NR, R=alkyl, mostly methyl, e.g. 26, R=CH, Sanol LS 765. Sankyo; Tinuvin 765, Ciba-Geigy), according to the mechan- istic analyses of Gugumus [33, 341. Within experi- mental errors, N-methyl derivatives (>NCH,) exhibit the same performance in PP and PE as the corres- ponding secondary HALS [33]. A comparable perfor- mance was also shown with >NR, where R was saturated alkyl higher than methyl. The performance of tertiary HALS bearing benzyl groups or unsatur- ated alkyls on nitrogen varied depending on the PP batch used. Both mechanistic and kinetic elucidation showed that the comparable performance of the both types (>NH and >NCH,) of HALS may be attributed to a fast transformation of the >NCH, HALS into a formic acid salt of the corresponding secondary

>NOP.

- HALS:

0 RO;, (RO'), ROzH II

NCHj *)N+H2 - 0 C H (3) \

/ [ ~ N c H ~ . 04'

Free radical, RO,H and oxygen-HALS exciplex attacks on the labile hydrogen atoms of the N-methyl group during oven aging and photo-oxidation, res- pectively, are considered [34].

A great effort has been devoted to improving some disadvantages of secondary HALS, including the undesirable reduction of efficiency due to chemi- cal reactivities with acid moieties arising from the host (halogenated) polymer, polymerization catalyst residues in polyolefins, or some components of addi- tive, such as pigments, sulfur-containing HD A 0 or halogenated flame retardants. HALS bearing N-acyl groups (>NC(O)CH, type) did not exactly fulfil the expectancies for the improvement of properties in acid media. However, this was been achieved suc- cessfully with a novel class of HALS [35] that have the structure of 0-alkylhydroxylamines >NOR. The latter possess inherently low basicity (pY, = 4.2, com- pared with 8.5-9.7 and 6.5 for >NH or >NCH, and the oligomeric type >N-bridge-N< (see the later

structure 46), respectively) and suppresses reactivity with other formulation components. This class of stabilizers is represented by structure 26 (R = 0-is0-G HIT, Tinuvin 123, Ciba-Geigy). It should be anticipated that this kind of HALS will react in principle as a scavenger of dangerous RO; and RC(0)O; radials. This is in agreement with the observed mechanism in tested polymers [35].

Despite the excellent efficiency of HALS, charac- teristic of its involvement in the repeating regenera- tive cycle, some processes are known successively to reduce the efficiency, either by depletion of the parent HALS or of the respective >NO'. An undesir- able reduction in the photo-antioxidant activity results. The depletion involves [2, 3, 29, 30, 321 the formation of salts from HALS with any acid co- reactant present in the system, the formation of N-acyl derivatives, products of recombination of HALS with acyl radicals, and the photolytic destruc- tion of the piperidine cycle. Processes contributing to the diminished activity due to the reactivity with antioxidants are included in Section 3.3.2.

3.3.2. Cooperation Phenomena Between Hindered Amines and Antioxidants. Practical stabilization recipes profit from differences in the stabilization mechanisms of primary and secondary antioxidants and HALS. Commercial polymers are successfully stabilized by mixtures of high molecular weight (HMW) phenols, HD A 0 containing trivalent phos- phorus and HALS [ll]. Some aspects of the mechan- istic explanation of the observed efficiency phe- nomena are still vague. Antagonistic effects have been observed mostly with phenol-HALS mixtures in photoinitiated processes. Synergism has been reported during long-term heat aging. Some data may be extracted from model mechanistic studies. Depletion of part of the integral stabilizing activity is due to ineffective transformation of phenolic antioxi- dants via phenoxyls during photoprocesses [2, 41 (cf. section 4). Model experiments performed in heptane [4, 361 show that the pro-oxidative effect of various phenols diminishes the photostabilizing efficiency of HALS. Phenol transformation products having struc- tures of alkylperoxy (1, X = RO;) and hydroperoxy- cyclohexadienones (1, X = HOJ accelerated the 2- butylanthraquinone-sensitized photo-oxidation of heptane. These compounds also lowered the activity of HALS in accordance with their individual pro- oxidative contributions [36]. Benzoquinone 3 and 3,5,3',5'-tetratert-butyl-4,4'-stilbenequinone (StQ, 27) retarded the photo-oxidation of heptane. A sur- prisingly favorable enhancement of the stabilizing efficiency of secondary HALS (26, R = H) or tertiary HALS (26, R = CH,) was provided by a combination with StQ (27), the most common quinonemethinoide model of phenol transformation. This may account for a specific cooperative stabilizing effect of QM (27), expressed as the quenching of singlet molecular oxygen [37].

A direct reactivity of antioxidants with HALS and/or derived nitroxides >NO' represents another factor that changes the stabilizing effectiveness. From the point of view of the depletion of HALS,

Current Knowledge of Antioxidants I449

their application with sulfur-containing HD A 0 should be avoided. The chemical explanation of the antagonism thus produced reveals reactivity with various species formed via tranformation of sulfides [3]. Inefficient products are formed: compounds of the type >NS(O),R (n=1,2) from >NO' and thiyl (RS') or sulfinyl (RS'O) radicals or salts with S-protonic acids (19, n=1-3). The importance of interactions between transformation products formed from different components of the stabilizer mixture was thus proved by product studies. Up to now, no studies such as this are available to explain chemical interactions with trivalent phosphorus compounds. However, significant products have been isolated from HALS-phenol systems. Nitroxides >NO' derived from various HALS should be considered as the active reactant. They oxidize phenols similarly to other one-electron oxidation agents,

3 ~ 2 , RO;, RO', M"+, ;NO* InH - In' + Products (4)

hv , ' 0 2 , 3S*, NO2

and transform them into phenoxyls. Besides the consecutive transformation of phen-

oxyls known from the chemistry of phenolic anti- oxidants [2, 31, an additional reactivity with >NO' participates in product formation. Stilbenequinone (27) was isolated in crystalline form from reaction mixtures formed from 2,6-ditert.butyl-4- methylphenol (28) and various >NO' [29] via the phenolic intermediate 7 (Scheme 12). Nitroxides are reduced into the respective hydroxylamines >NOH (e.g. 26, R=OH). Various other monohydric and

OH r o

dihydric phenols are easily oxidized with nitroxides [2, 291. The complex reactivity was explained using 28 as a typical sterically hindered phenolic antioxi- dant. Oxidation by >NO' results in the respective phenoxyl 29a (or its mesomeric cyclohexadienonyl 29b) or the 4-hydroxybenzyl radical 29c, formed by rearrangement of 29b (Scheme 12). Radicals 29b and 29c react with >NO', and compounds 30 and 31 respectively are formed. Compound 31 was isolated in crystalline form [29] and represents proof of the formation of the benzyl radical 29c. The formation of 30 is reversible [2, 31. Increased reactivity possibili- ties of the C-C coupling product 7 in oxidation with >NO' result in the gradual formation of a mixture of products from the respective cyclohexadienonyl radi- cal and >NO' (compounds of the type 32). StQ (27) may be formed either directly from 7 or via 32.

The oxidation of phenolic antioxidants and of their phenolic coupling products by >NO', together with the low photostability of phenols, has to be considered seriously as a factor that diminshes the integral activity of phenol-HALS combinations. The formation of products from >NO' and phenoxyls also contributes actively to the HALS depletion process.

3.4. Intramolecular Cooperation in Bifunctional Antioxidants It has been believed that two different and well balanced functional moieties bound in one molecule provide stabilizers that impart a more complex form of protection. Both moieties involved are considered to participate according to their specific antioxidant

n O H 1

L H3

28 290 29b 29 c

H3C *Qp* ON< w C H$ N(

30

32 27

SCHEME 12

450 I PospiS.il

mechanisms, which are characteristic of the respect- ive monofunctional systems. However, it should be expected that one of the functionalities plays a major role, according to the character or intensity of deter- iogens. The participation of both active centers, e.g. CB and HD, in the stabilization process is mostly not entirely concerted. Nevertheless, the observed sup- porting or complementary effects are advantageous.

Various bifunctional stabilizers have been com- mercialized. Sterically hindered phenolic moieties are combined in most of them with sulfide, phos- phite, amide, hydrazide or hindered piperidine moieties [4]. Extensive mechanistic studies dealing with the chemistry of transformations of the involved moieties have been performed with various phenolic sulfides [2, 381. The results obtained with substituted isomeric 2,2'- and 4,4'- thio(dithio)bisphenols (33, see Scheme 13; n = 1,2, R = alkyl) allowed the explanation of the chemistry of the stabilization mechanism, and revealed a strong CB activity in autoxidized PP, and a very efficient HD A 0 activity due to the formation of thermolabile S-oxidation products and S-acids. DSC measure- ments reveal the thermolability of phenolic sulfox- ides (compound 34, m = 1, n = 0, is an example) and thiosulfinates (34, m = 1, n = 1). Transformations pro- ceeding on the sulfur atom due to the HD A 0 activity do not destroy the CB character of the remaining phenolic group in 34. A complex of trans- formation reactions of thiobisphenol 33 reveals the formation of sulfurless products and sulfur oxides [4]. The high HD activity of phenolic sulfones (e.g. 34, m = 2, n = 0) and the excellent peroxidolytic efficiency of dithiobisphenols (33, n = 2), two specific features in the chemistry of thiobisphenolic antioxi- dants, were explained as being a consequence of appropriate transformations of both species during the stabilization process [2, 41.

The excellent complex performance of other types of commercialized bifunctional antioxidants containing phenolic and sulfide moieties is a conse- quence of well-balanced intramolecular supporting and cooperative effects. Antioxidants 35 and 36 (see Scheme 14) have been selected as typical examples. Irganox 1035 (35, Ciba-Geigy), a thermostabilizer for polyolefins, combines the favorable mechanistic phenomena of propionate-type phenolic antioxi- dants and inherently activated thioesters. 2-Methyl- 4,6-bis(octylthiomethyl)phenol (36, Irganox 1520, Ciba-Geigy), an effective antioxidant for diene-based rubbers, contains two activated alkylthiomethyl groups in its molecule.

The antioxidative 3-(4-hydroxyphenyl)pro- pionate and 4,4'-thiobisphenolic moieties were

17

35 36

SCHEME 14

exploited in the efficient metal deactivators Irganox 1024 MD (37, Ciba-Geigy-see Scheme 15) and Hostanox VP OSP-1 (38, Hoechst), respectively. The ability to chelate metals is imparted into 37 and 38 by the polydentate character created by the fortunate molecular architectures, containing hydrazide and phosphite moieties, respectively.

Various systems combining phenolic CB A 0 moiety with a photo-antioxidant HALS moiety have been synthesized, and some of them have been commercialized. As an example, Sanol LS 2626 (39, Sankyo), an efficient stabilizer for polyolefins, can be chosen.

4. IMPACT OF ACTINIC SOLAR RADIATION AND ATMOSPHERIC POLLUTANTS ON ANTIOXIDANT CHEMISTRY The actinic part of the solar radiation (295-400 nm), together with aggressive atmospheric deteriogens generated by anthropogenic activities (nitrogen oxides NO,, ozone and acid rain), have a specific impact on both polymers and antioxidants. The photochemistry of polymers and the ozone aging of unsaturated elastomers are not the subjects of this paper. However, undesirable impacts on anti- oxidants should be mentioned, because they modify the final effects observed with stabilizer-doped polymers.

Phenolic antioxidants applied as a single addi- tive provide only very poor protection against photo- oxidation. This is not only due to the short kinetic chain length of the photo-oxidation process itself. Phenols (InH) are transformed into phenoxyls In' by direct photolysis, or as a consequence of chemical quenching of excited sensitizers (chromophores, see equation (4)) [2, 371. This inactive transformation,

37 38

33 34 SCHEME 13

39

SCHEME 15

Current Knowledge of Antioxidants I 451

0

4 0

0

0- 41

SCHEME 16

competing with RO;/ RO' scavenging (Scheme l), contributes to the depletion of activity, together with the formation of hydroperoxycyclohexadienones (1, X = HO,) from phenols and their photolysis, result- ing in pro-oxidative effects [36]. Compounds such as 1 ( X = HOZ, ROz) were confirmed to be photoactive impurities generated in polymeric matrix from vari- ous phenols. Some improvement in the photochemi- cal stability of phenols provides "photostable" phe- nols, esters of alkylated 4-hydroxybenzoic acid (25) 121 that are generally listed among light stabilizers.

The increasing concentration of NO, in the tro- posphere results in discoloration of polyolefins doped with phenolic antioxidants. Model experi- ments performed with 28 have revealed [2] the formation of 2,6-ditert.butyl-4-nitro-4-methyl-2,5- cyclohexadiene-1-one (40, R-methyl-see Scheme 16) and its consecutive transformation into quinone- methide (27). A reactivity such as this should also be considered for hindered phenols other than 28 that have a substituted methyl group in position 4. Another commercial antioxidant, 4,4'-thiobis (2- tert.butyl-5-methylphenol), was transformed by NO2 into brick red products [39]. It was shown recently [39], using experiments with Sumilizer GA-80 (Sumi- tomo), that 4-substituted semi-hindered phenols are transformed stepwise via oxidation and nitration in positions 3 and 4 into 7-tert.buty1-9-methyl-10-nitro- l-oxa-2,8-dioxospiro [4.5] decane-6,9-diene (41), imparting much lower discoloration than 40. This finding is an important improvement of stabilization of polyolefins for an NOz-contaminated environ- ment.

The reactivity of ozone with aromatic amines was studied in detail in connection with the anti- ozonant efficiency of the latter [18, 191. Although the "ozone scavenging theory", based on direct reac- tivity, cannot by itself satisfactorily explain all of the features of the antiozonant process, the present explanation suggests that the direct ozonation of amines is one of participating processes and there- fore belongs among "actively" contributing trans- formations of antidegradants to the stabilization process.

5. THE ACTIVITY OF ANTIOXIDANTS- IN POLYMERS CONTAMINATED WITH METALLIC IMPURITIES Specific attention has been paid to the stabilization of polymers contaminated with metallic impurities. The latter are mostly present in trace amounts as

residues of polymerization catalysts, or are intro- duced into polymers during processing and/or com- pounding with fillers, pigments or reinforcing agents. Metals may catalyze oxidation of the polymer matrix, oxidize some components of the stabilizing system and/or form complexes or salts with the latter [2]. A strong pro-oxidation risk exists in polymers used as insulating materials for wires and cables with metal conductors. Phenomena governing metal catalysis of polymer oxidation were elucidated thor- oughly in reference [40]. The catalysis occurs mainly by a redox mechanism, resulting in hydroperoxide homolysis [41]. The most active catalysts are ions of transition metals (Fe, Co, Mn and Cu). All experi- mental observations indicate that both hetero- geneous and homogeneous catalysis may occur, the latter being more effective. This implies that the catalytic effect has also been provided with metals and their oxides, due to their stepwise solubiliza- tion, resulting from the formation of metal carboxy- lates [41]. Efficient protection against the degrada- tion of metal-contaminated polymers has been based on the application of antioxidants and metal deacti- vators (MD) (41, 421. The principle of the activity of the latter has been based on an efficient conversion of a catalytically active metal species (L,M"+, L = ligand) into an inactive one, or at least into a less active metal complex ((MD)M"+), by means of a ligand exchange, according to the general Scheme 17 [41, 421. Some residual catalytic activity of the new complex (MD)M"+ having a rate constant k' (the latter must be much lower than the original constant k) has to be expected. Compounds 37 and 38 serve as examples of efficient MDs.

Metal deactivation itself does not represent a sufficient integral stabilization. Polymers must also be appropriately protected against radical chain oxi- dation, and the application of CB A 0 and/or their combinations with secondary A 0 is obligatory. It has been shown under model conditions (autoxida- tion of cyclohexene at 60°C) that even very efficient hindered phenols and aromatic amines, when used without MD, were not able to stabilize the hydro- carbon system filled with carbon black contaminated with metal impurities after the induction period [43].

Profitable stabilization recipes for polyolefins reinforced with asbestos or talcum, consisting of cooperative phenolic antioxidant - MD systems and considerably increasing the service lifetime of metal- contaminated systems, are available [42]. Combinations containing N,N'-disubstituted 1,4- phenylenediamine and MD provided more efficient protection in y-radiation-crosslinked PE (XLPE)/

L,M"+ + MD - (MD)M"+ + nL

l k 1 kt (Oxidation (Oxidation catalysis) catalysis)'

SCHEME 17

452 I PospiSil

4 2 43

SCHEME 18

copper matrix than analogous mixtures with phenols [44]. However, discoloration limits the commercial exploitation of aromatic amines in lightly pigmented polymers.

Copper(I1) ions, a dangerous polymer oxidation catalysts, also oxidize phenolic antioxidants [14]. A model reaction performed with 28 confirmed that the oxidation pathway generally proceeds according to the mechanism characteristics of sterically hindered phenols [2]. However, the process itself is very favorable for the generation of the "reversible" C-C coupling product 8 from the primarily formed phe- noxyl 29a. Compound 8 was isolated in yields up to 3 wt%. A kinetic study performed in chlorobenzene with 28 and copper(I1) acetate at 100°C reveals the formation of two C-C coupling products 7 and 8 from 28 from the beginning of the reaction. There is, however, an important difference in the fate of both the dimers in the consecutive phases of the process. The concentration of the "reversible" dimer 8 reaches a maximum (2.5-3%) after 10-12 hr of oxidation and then diminishes slowly. The "irreversible" dimer 7 accumulates in the mixture more slowly than 8, but continuously. In the later phases of the process, 7 is oxidized into StQ (27) (similarly as in Scheme 12). The beginning of the formation of benzoquinone (3) in the mixture coincides with the depletion of 8.

Results obtained with Cu(I1) ions indicate [14] that the chain-breaking activity of InH is depleted due to the oxidative activity of metal ions M"' according to equation (4). The active interference InH with radicals RO; (according Scheme 1) is thus restricted. Ions of other transitions metals, such as Mn, Co or Fe, that are present in the system in the higher valence state, have a similar effect, transform- ing phenols into phenoxyls and thus contributing to the "inactive" transformation of phenols. This mechanism explains the low activity of hindered phenols in metal catalyzed oxidations of hydrocar- bon polymers.

Catalyst technology in the production of poly- olefins has progressed during recent years. The increased productivity and selectivity of catalytst of the TiCl,/AlCl,/MgCl,/aromatic carboxylic ester

%A1 + 0 2 - RZAlOzR

I RzAlOR + O=P(OAr)3

P(0Ar)S SCHEME 19

type resulted in a reduction of the operation costs of polypropylene. However, the new technologies provide polyolefins from which catalyst residues and atactic polymer are no longer removed. Although the residual concentrations of catalysts of the third and the superactive third generations are low (residual elements in p.p.m.: Ti, 1.4-2.2; Al, 40-85; C1, 20-40; Mg, 10-13), their presence is associated with a diminished stability and an augmented tendency of PP to discoloration. The problem of the stabilization of polyolefins prepared by new generation catalysts was examined systematically in reference [45]. The results corroborate the conclusion that a multicompo- nent and polyfunctional combination consisting of conventional high-peformance primary antioxidants, secondary antioxidants, HALS and co-stabilizers (calcium stearate or its 1 : 1 mixture with hydrotalcite, 0.05-0.1 wt%) imparts processing, long-term thermal and UV stabilities, comparable in most aspects with those of the second generation PP. A slightly higher concentration of phenolic anti- oxidants is necessary to assure long-term thermal stability. Color development during processing was efficiently suppressed.

Acid components of catalyst residues increase the risk of corrosion of construction steel in equip- ment used for polymer processing. The acid catalysis of retro-Friedel-Crafts processes may also take place in used antioxidants. The application of co- stabilizers is of benefit in order to neutralize these dangerous acid compounds [45].

Some product studies clarify undesirable phenomena such as color formation and the reduc- tion of stabilizer efficiency due to chemical inter- actions of polymerization catalyst residues with stabilizers [2, 461. Titanium chloride residues trans- form phenolic antioxidants into discolored aryl tita- nates (e.g. 42, Scheme 18). The antioxidant efficiency is depleted, in a similar fashion the stepwise forma- tion of alkylaluminium phenoxides (e.g. 43).

Part of the alkyl phosphite antioxidant is con- sumed due to the reaction with alkylaluminium- derived peroxides (Scheme 19). An adduct of trialkyl- aluminium with 2,2,6,6-tetramethylpiperidine was identified [46].

Catalyst supports based on magnesium chloride, having properties of Lewis acids, catalyze retro- Friedel-Crafts reactions, resulting in partial dealkyl- ation of hindered phenolic antioxidants at process- ing temperatures [2]. This results in a change of stabilization mechanism. New stabilization concepts to deal with the residual catalyst components are

r 1

44 45 SCHEME 20

Current Knowledge of Antioxidants I 4 5 3

therefore desirable, as polymer stabilization technol- products (tyres, toys or various articles made from ogy is the key to the commercial success of new- rubber) into extracting media, including food or generation polymers. One of the solutions is the surface water. Data dealing with the migration of above-mentioned application of co-stabilizers to stabilizers from packaging materials into food simu- neutralize acid impurities [45]. lants and recommended extraction test methods are

of vital importance [47]. Public concern demands a

6 . ENVIRONMENTAL ACCEPTABILITY- AND PHYSICAL PERSISTENCE OF ANTIOXIDANTS

trustworthy determination of chronic and acute tox- icities and of physiological impacts arising from the application of stabilizers. The commercial interest is concentrated on the svnthesis of nontoxic stabilizers

Antioxidants and photo-antioxidants are not entirely harmless to human beings. Man comes into direct

with reduced extractibility and volatility. "

contact with stabilizers during various phases of their production and their blending with polymers. The use of stabilized food, cosmetics and Dharma-

6.1. Potential Physiolo@cal Influences of Antioxidants

ceuticals involves another direct administrGion. An The most dangerous effects may arise from impuri- indirect administration occurs by leaching of stabil- ties in some aromatic amine antidegradants. They izers from packaging materials and/or industrial are represented by carcinogenic 1- and 2-

r H

4 6 47

A : Q -NH-tert.C8H,7

b -N 0 LJ

4 3 H

4 8 49

50 SCHEME 21

454 / Pospisil

naphthylamines formed in trace amounts during the synthesis of N-phenyl-l(or 2)-naphthylamine or N,N’-bis(2-naphthyl)-1,4-phenylenediamine [18]. Although these compounds are very important for polymer stabilization, and new technologies provide very pure products, commercialization of both anti- degradants has had to be halted.

The role of phenolic antioxidants in biological systems is considered to be dual [2]. The conclusions of experiments describing the physiological impacts of hindered phenols are sometimes rather contradic- tory. Various pathways of cancer prevention have been reported [2]. However, an improper appli- cation, including administration of a high concentra- tion of phenols, may cause hypertrophy of the liver and even a tumorigenic action.

Considerable attention has been paid to the application of phenols of natural origin and of their synthetic analog to polymer stabilization. The search for an optimum commercial exploitation of gossypol (44, see Scheme 20) or a-tocopherol (45) is in progress.

Some complications arise from thioester syner- gists due to their organolepticity in odor-sensitive applications. An improved dialkyl sulfide synergist Amoxsyn 442 (Atochem), creating no objectionable odors, was mentioned recently [26]. Its structure is proprietary.

6.2. Physically Persistent Oligomeric and Polymeric Antioxidants and Photo-antioxidants

The high extractability and easy volatility limit exploitation of the inherent chemical efficiency of stabilizers in polymers used under demanding con- ditions and increase the risk of contamination of the environment. Great effort has been devoted to the synthesis of physically persistent stabilizers. This group involves so-called high molecular weight (HMW) stabilizers (having molecular weights approximately between 500 and 1000) and oligomeric or polymeric stabilizers. The latter two types are synthesized by polyreactions and reactions on polymers [48]. A chemically very variegated group of stabilizers has been described. The practical exploit- ation has been concentrated on oligomeric dihydro- quinolines and HALS. Chimassorb 944 (46a, Ciba-Geigy-see Scheme 21), Cyasorb UV 3346 (46b, American Cyanamid), Tinuvin 622 (47, Ciba-Geigy) or Uvasil299 (48, Enichem) are given as examples of a very fortunate commercial development of HALS. Polymeric stabilizer 49 combines properties of HALS and an effective UV absorber. Copolymer 50 (Chemi- gum HR, Goodyear Tire and Rubber) represents an excellent polymeric antioxidant for engineering applications of rubbers.

It has generally been accepted that the activity mechanism of oligomeric/polymeric stabilizers is similar to that of monomeric stabilizers bearing analogous functional moieties. However, physical phenomena such as migration or compatibility with the host polymer influence the final long-term stabil- izing effect. Knowledge of the physical relationships between the structure of stabilizer-functionalized

oligomers or polymers and the host polymer matrix is not sufficient up to now, and demands concen- trated research.

7. CONCLUSIONS Small amounts of properly chosen antioxidants and photo-antioxidants protect hydrocarbon polymers during processing and against environmental deter- iogens. The accumulated theoretical knowledge on stabilizer activities is useful for the optimization of stabilization recipes, to attain a favorable price/ performance ratio. Changes in stabilizing technolo- gies according to environmental aggressivity and the inherent sensitivity of polymeric systems to deterio- gens are in progress. Open problems to be solved include the commercial exploitation of the molecular architecture/inherent efficiency relationship, the need for a better understanding of the physical factors that affect the performance of polymeric stabilizers, and the stabilization of new-generation polymeric systems.

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