Die Angewandte Makromolekulare Chemie 176/177 (1990) 347-355 (3076)

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

PROPERTIES AND ROLE OF BENZOQUINONE IMINES IN POLYMER STABILIZATION

Jan PospiSil

Summary: Quinone imines are formed from aromatic amine antidegradants as a consequence of chemical transformations of the latter during the stabilization of hydrocarbon polymers. Quinone imines are deep coloured compounds and contribute to the antidegradant activity of the parent amines. The main interest is devoted to benzoquinone di- imines (BQDI), generated from N,N'-disubstituted l,4-phenylenedi- amines (PD). BQDI are antioxidants in unsaturated hydrocarbons, undergo hydrolytic deamination, addition and cyclization reactions, react with 2,6-dialkylphenols, thiols and C-centered radicals. PD are regenerated from BQDI in redox reactions. The active role of BQDI in antifatigue processes in rubbers has been explained using product analysis. BQDI account for staining and discoloring pro- perties of PD but are not harmful to the polymer stability.

INTRODUCTION

Stabilization of unsaturated elastomers and of most plastics against oxidative degradation initiated by thermal, catalytical, mechano- chemical and/or radiation processes is obligatory. Aromatic amine antidegradants (the term "amine" means an aromatic amine through- out the text) rank among the most efficient chain-breaking anti- oxidants (CB-AO). Some of them have also properties of strong anti- fatigue (AF) agents and antiozonants (AOZ), and even an appreciable

Paper presented at the 19th Colloquium of Danubian Countries on "Atmospheric Aging and Stabilization of Polymers" in Prague (Czechoslovakia), July 24 - 29, 1989

@ 1990 Hiithig & Wcpf Vcrlag, Easel Mw)3-3146/90/$03.00 347

metal deactivating ( M D ) efficiency was reportedl. In spite of the very useful multifunctional antidegradant properties of aromatic amines, especially of N,N'-disubstituted l,4-phenylene- diamines (PD), involving a broad application spectrum in mono- mers and various polymers, the technical application of amines has been mostly limited, due to their strong discoloration and staining properties, to the stabilization of general purpose rubber vulcanizates. The extent of the application of aromatic amines in plastics and rubber modified plastics is vely limited. Discoloration and staining together with toxicological objections are the main obstacle.

We concentrated our attention to the chemistry of coloured transformation products of amines. Several different well de- fined structural types of quinone imines should be mentioned in connection with the amine antidegradant chemistry2 : benzo- quinone monoimines (BQMI, 1) are formed by oxidation of di- phenylamine (DPA) or by hydrolytic deamination of some benzo- quinone diimines (BQDI, 2). The latter are products of the oxidation of N,N'-disubstituted 1,4-PD. Quinone imines having the cyclohexadiene ring substituted with additional aminogroups (e.g. 2) may be listed among Bandrowski's bases. In more com- plicated systems having structures close to those of various dyes, including precursors of aniline black, the quinone iminoide structure may also be part of a nitrogen heterocycle (e.g. 4, I). BQDI (2) are formed2 from the respective PD by oxidation with R O i radicals or ozonides in aged polymers or by direct oxidation with 02. A typical quinone iminoide impurity 2 was isolated from commercial IPPD3 and HPPD'.

d 1 2

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Q 0-N bN-@ N

NH,

4 5 6

A critical interpretation of all available data dealing with quinone imines makes possible a better understanding of the mechanism of aromatic amine antidegradant activity. The expla- nation of the AF mechanism of PD, of the hydrolytic instability of BQDI and reactivity with phenols are the main results based on an unambiguous identification of the respective reaction products.

BQDI: PROPERTIES AND INVOLVEMENT IN AGING OF POLYMERS

Quinone imines have characteristic absorptions in IR and W-VIS spectral regions5. It is however extremely difficult to use spectral analyses in the identification of BQDI in extracts of aged polymers. TLC and HPLC methods were used successfully in analyses of mixtures containing various PD with the respective BQD16. The latter form two geometric isomers (syn-, anti-)'. One isomer is readily transformed into the other, and vice versa. The isomerization is reversible and temperature dependent and an equilibrium mixture of isomers is always formed.

In spite of the attention devoted to the chemical reactivity of quinone imines5, data were lacking dealing with the chemistry of N,N'-disubstituted BQDI, important for the polymer stabiliz- ation. The stability of various BQDI in the solid state, as well as i.n a solution8, is dependent on the substitution on nitrogen. Derivatives containing aromatic substituents are always more stable than analogs substituted with secondary alkyls. Imvers- ible acid catalyzed transformations involving hydrolytic de- amination, substitution with fragments formed via hydrolysis or with other reactive species (amines, phenols, thiols) present in the reaction mixture, disproportionation and thermally induced cyclization take place together with redox reactions controlled

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by differences in the oxidation potentials of individual compo- nents of the reaction mixture. Due to the repetition of these individual processes and of their combinations,very rich reaction mixtures are formed. Analyses of mixtures containing relatively similar compounds, present mostly in very minor amounts, are very difficult. Generally, depending on the substitution of nitrogen atoms in BQDI, deamination resulting in the formation of BQMI and benzoquinone (BQ), Bandrowski's bases, compounds with structures of indophenols, indanilines and indamines and more complicated coloured quinone iminoid compounds, i.e.various poly conjugated systems are formed . It was found experimentally that the =N-sec-alkyl group is hydrolyzed very easily in com- parison with the =N-aryl group. The composition of the reaction mixture is always more complicated after the prolongation of the reaction time. It was shown' using two commercial antidegradants, IPPD and HPPD, that an easy hydrolysis must be alwas taken into account in systems containing one =N-sec-alkyl group. BQMI is the main reaction product. N,N'-Disec-alkyl-BQDI may be hydro- lyzed to BQ. Substituted phenazine ($) and fluorindine ( 5 ) were identified among the products. It was shown using suitable mo- dels that most of the transformation products of the starting BQDI are present in oxidized forms and only the respective simple PD is in the reduced form. 4-Hydroxy-DPA is regenerated by an analogous pathway from BQMI.

We may conclude that the types of irreversible transformations of BQDI mentioned above contribute essentially not only to the discoloration of polymers doped with PD, but also to the ob- served high antioxidant efficiency of PD. The process may be explained as a regeneration of PD (or of 4-hydroxy-DPA) via the reversible redox reactivity of BQDI (or BQMI) with subsequent transformation products of the latter in a weak acid and humid environment (the acidity may be provided by organic acids used as processing aids or by acid rain).

It was proved that BQDI accumulate during autoxidation of organic substrates doped with PD7, especially in the phase of strong re- tardation after an induction period (IP). Antidegradant ability of various BQDI was studied independently in detail during the oxidation of tetralin, cyclohexene, squalene and squalane . The retardation effect was augmented in polyunsaturated systems. Squalene (a model of IR) was oxidized with the highest rate in

9

9

10

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comparison with the other hydrocarbons under study and the in- fluence of BQDI on the oxidation process of the former was quantitatively different: all investigated BQDI imparted a di- stinct IP and a strong retardation effect after IP. The inhibi- tion effect of BQDI was concentration dependent. In the other three hydrocarbons, an efficient retardation of the oxidation was observed without any IP. Bandrowski's bases of type 2 re- veal also a strong antioxidant efficiency in squalene . It may be assumed that BQDI contribute actively to the stabilization process, but no generalization of the effect is possible, due to the important contribution of structure of the oxidized

3,lO hydrocarbon substrate. The result of model investigations explains a favourable effect of various BQDI on the aging characteristics of NR, SBR or IR vulcanizates.

We did not observe any regeneration of PD during the inhibition of oxidation of liquid hydrocarbons doped with BQDI (i.e. in an oxygen saturated system) lo. A nonspecif ied ESR signal was observed in IR doped with N,N'-DP-BQDIl1. We may assume that this signal should be ascribed to a semiquinone formed in a mixture of PD regeneration. The regeneration of PD (or 4-hy- droxy-DPA) from the respective BQDI (BQMI) was reported in NR at vulcanization temperatures 12'13. We should assume that a complex of factors contribute to the regeneration in this relatively complicated chemical system.

The supporting effect of BQDI to the antidegradant properties of PD in squalene and NR has been experimentally proved The radical trapping mechanism of the antidegradant action of BQDI is different from that of PD, and we consider that the antidegradant activity of BQDI is the main contribution to the strong AF activity of PD observed in rubber vulcanizates. The ability to scavenge radicals R' is one of the key mechanistic features in AF processes. Carbon centered radicals are formed during mechanochemical transformations of rubbers and actively participate in fatigue processes. The role of their deactivation is augmented in oxygen deficient systems.

Antioxidants like PD or 4-hydroxy-DPA do not react inself with . Model mixtures of the former with some efficient R' traps

(BQ, aliphatic nitroxides) do not show any synergism during the oxidation of 1-nonene. We guess that this phenomenon is due to the fact that the system PD/BQDI (formed from PD during the

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stabilization process) is more efficient RO;/R' scavenger than the above mentioned model mixtures. We consider therefore that the reactivity of BQDI with R' is a decisive process". A series of experiments was made to confirm the respective mechanism . The results are based on extensive product studies performed with easily hydrolyzable IP-BQDI18, N-phenyl-BQMI (product of the hydrolysis of the latter)16 and with the relatively stable DP-BQDI17. The reactivity of alkylradicals with cross-conjugated dienoid systems is characteristic of the mechanism of inhibition of the radical polymerization by means of BQ. In this case, the primary attack of R' is directed to the oxygen atom''. Ethers of hydroquinone are the main products. There is only a minor attack on the nucleus of BQ. We found l6-I8 that the reactivity of R' with the dienoid systems drops in the series BQ>BQMI>IP- -BQDI>DP-BQDI. The first two members of this series react with R' itself. The reactivity is however enhanced if a mixture of BQ1' or BQMI16 with their reduced forms is used. Both BQDI under study react with R' only in the presence of the respective PD, i.e. in the form of a semiquinone radical. Isolated reaction products confirm that 0-alkylate is the main product of BQMI, a mixture of ring-alkylates is the minor product16. Labile N-alkylates and stable phenylene ring 'alkylates are formed from both BQDI under study l7 ' 18. Thermolysis and acidolysis respecti- vely of N-alkylates yield the respective PD and contribute in this way to the regenerative mechanism of PD in the AF protec- tion. Ring alkylates should be considered as active polymer- bound antidegradants formed from BQDI and, at the same time, as a model of the vvnon-extractablevv nitrogen in rubbers doped with PD . BQDI are involved also in the mechanism of cooperation of PD with 2,6-dialkylphenols observed during the stabilization of ethylbenzene, gasoline and squalene. A product study with 2,6-di-tert-butylphenol and IPPD revealed2' that 2 , 6-di-tert- butyl-4-(4-phenylaminophenylimino)-2,5-cyclohexadiene-l-one (6) is formed together with phenolic coupling products in a weak acid environment and in the oxygen deficient system. Phenolic products were isolated in the form of quinoide compounds if the experiment was carried out in oxygen atmosphere. Compounds of the indaniline structure are formed also with 2-methyl-6-tert- butylphenol and 2,6-dimethylphenolI but their yield drops on

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15 the account of the formation of a cyclic condensation product . The latter decomposes readily to the starting PD: this process accounts for the PD regeneration via BQDI reactivity with phenols. It was proved experimentally21 that compound 5 behaves in squalene as an efficient antioxidant. The antioxidant effi- ciency drops gradually without depletion of 5: this accounts for a reactivity of aminyl derived from 5 with a labile hydro- gen atom from the oxidized substrate. As a consequence, 5 is re- generated.

BQDI used to be employed as vulcanizing agents for NR. It was found12 that - in contrast with the respective PD - quinone imines improve the scorch resistance of rubber mixtures. The activity was dependent on the type of the vulcanizing system used. An extensive study revealed12 that in sulfenamide accele- rated high sulfur vulcanizing system BQDI resemble conventional retarders of the nitrosodiphenylamine type. The properties of BQDI depend on the character of N,N'-disubstitution.Bandrowski's base 2 involving both conjugated BQDI system and the substituted sec-amino group (the latter has been generally considered as contributing negatively to the processing safety of rubbers in the presence of thiazole accelerators) reduces only very slightly the processing safety of NR vulcanization in the pre- sence of sulfenamide accelerator^'^. The crosslinking process, the temperature dependence of the latter and the physico-mech- anical properties of the vulcanizate were practically not in- fluenced by 2. This indicates that the BQDI system may partially paralyze the harmful effect of iminogroups.

The mechanism of interactions of BQDI (or BQMI) with vulcaniz- ation ingredients is not quite clear. A ractivity with thiols formed during the vulcanization process accelerated by sulfen- amides should be expected. This was proved by a reaction with 2-mercaptobenzothiazole . The major product was formed via 1,4-addition (this explains the delayed effect on crosslinking), the minor reaction pathway yields 2,2'-thiobis(benzothiazole) (taking part in the crosslinking) and the respective PD (the regeneration of the antidegradant activity).

13

CONCLUSIONS

The chemistry of quinone imines is involved in the inherent antidegradant efficiency of aromatic mines. Important experi-

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mental data were obtained with BQDI. (a) BQDI behave as antioxidants in highly unsaturated systems (squalene, diene based rubbers) while not affecting seriously the vulcanization process. (b) In a weak acid environment, BQDI are irreversibly chemically transformed, the respective PD is regenerated from BQDI in a re- versible redox process with transformation products of BQDI. The character of N-substituents in BQDI exerts a decisive effect on the BQDI reactivity. (c) The reactivity of BQDI and BQMI respectively with R' radicals explains the mechanism of the antifatigue activity of PD and the mode of formation of "nonextractable" nitrogen in aged rubbers. (d) The reactivity of BQDI with 2,6-dialkylphenols accounts for the formation of an antioxidant active indoaniline and of a cyclic product decomposing thermally under regeneration of PD. (e) Interactions of BQDI with vulcanization ingredients are not fully clear. A reactivity with thiols should be anticipated, accounting for the PD regeneration and influencing the cross- linking of rubbers. (f) BQDI and products of their consecutive transformations account for the staining and discoloration properties of PD but are not harmful to the rubber stability.

REFERENCES

J. PospiSil, Polymer stabilization and degradation, P.P.Klemchuk (Ed.), Am.Chem.Soc., Washington 1985, ACS Symp. Series 280, p. 157 J. PospiSil, 11th 1ntern.Conference on Advances in the Stabilization and Controlled Degradation of Polymers, Luzern 1989

L.Taimr, J. Pospigil, Angew.Makromol.Chem. 149 (1987) 119

L. Taimr, H. Pivcovl, J. PospiSil, Polym.Degr.Stab. 16 (1986) 159

P. Griinanger, Houben-Weyl, Methoden der organischen Chemie, 4th Ed., Vo1.7, Part 3b, Thieme Verlag, Stuttgart 1979, p. 233

J. Rotschovl, L. Taimr, J. PospiSil, J. Chromatogr. 216 (1981) 251

354

L. Taimr, J. Rotschovd, J. Pospisil, Chei. Ind. 1979, 413

J. Rotschovd, J. Pospisil, J.Chromatogr. 211 ( 1 9 8 1 ) 299

L. Taimr, J. Pospisil, Angew. Makromol. Chem. 92 ( 1 9 8 0 ) 5 3

lo J. Rotschovd, J. Pospisil, Collect. Czech. Chem. Commun. 47

l1 A.B. Raevskij, L.F. Kovrishka, Kautsch.Rezina 29 ( 1 9 7 0 ) No 3 , 9

12M.E. Cain, I.R. Gelling, G.I. Knight, P.M. Lewis, Rubber Ind.

( 1 9 8 2 ) 2501

- 9 ( 1 9 7 5 ) N o 6 , 216

l3 I. R. Gelling, G.I. Knight, Plast. Rubber Process. 2 ( 1 9 7 7 )

1 4 V . DuchdEek, A. Kuta, L. SoSkov6, L. Taimr, J. Rotschov6,

15L. Taimr, J. Pospigil, Angew. Makromol. Chem. E ( l 9 8 4 ) 1 8 1

1 6 L . Taimr, M. Prusikovd, V. Hanus, J. Pospisil, Angew. Makromol.

I7L. Taimr, M. Prusikovd, J. PospiE;il, Angew. Makromol. Chem.

"L. Taimr, M. Prusikovd, J. Pospisil, Angew. Makromol. Chem.,

"F.J.L. Aparicio, W.A. Waters, J.Chem.Soc. 1952, 4666

2oL. Taimr, J. Pospigil, Polym.Degr. Stab. 8 ( 1 9 8 4 ) 2 3

21L. Taimr, J. Pospisil, Polym.Degr. Stab. 8 ( 1 9 8 4 ) 6 7

N o 3 , 83

J. Pospigil, to be published

Chem. 156 ( 1 9 8 8 ) 9 1

- 1 6 9 ( 1 9 8 9 ) 37

submitted

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