Mechanisms of antioxidant action: The role of copper salts in the photostabilization of paint media

European Polymer Journal Vol. 16. pp. 1153 to 1158

0014-3057:80 1201-1153$0200'0

(3 Pergamon Press Lid 1980. Printed in Great Britain

MECHANISMS OF ANTIOXIDANT ACTION: THE ROLE OF C O P P E R SALTS IN THE PHOTOSTABILIZATION OF PAINT MEDIA F. RAST] and G. S c o w Department of Chemistry, University of Aston in Birmingham, Gosta Green. Birmingham B4 7ET. England

(Received 20 March 19801

Al~tr,~et--Verdigris, a copper pigment containing basic copper acetate hydrate, is an effective light stabilizer for linseed oil based paint media but gives rise to discolouration over a long period. The antioxidant process is shown to result in the formation of conjugated chromophores in the medium. A catalytic antioxidant mechanism involving redox reactions of the copper ions is proposed to account for the efficiency of the pigment as a light stabilizer.

INTRODUCTION Verdigris is a blue-green pigment used from earliest times as a pigment in paint media [1] until its use was made obsolete by the development of emerald green in the eighteenth century. Its presence in early paintings has always posed something of a problem due to the tendency of the original bright blue-green to turn brown with time [2]. The chernistry of this process has remained obscure since there is little evidence that the colour changes are associated with physical breakdown of the paint medium. The present investi. gation is therefore concerned with an examination of the effect of verdigris on the drying and photo-oxidation of linseed oil based paint media.

Estimation of hydroperoxides The following procedure was found to be most suitable in the presence of verdigris. 10 ml of 10",, Nal in isopropanol was added to 25 ml of 10",, (v/v) of glacial acetic acid in isopropanol to which 1 ml of the reaction mixture containing peroxide was added. After standing in the dark for 3 hr, the liberated iodine was titrated with standard sodium thiosulphate. O: absorption measurements under conditions of u.v. exposure, i.r. (AEH2, A ( ~ ) ) and dicarboxylic acid estimation were all carried out as described previously [3]. u.v. Spectra were recorded on a Perkin Elmer 137 spectrophotometer.

RESULTS

EXPERIMENTAL

Materials Linseed oil and verdigris were as described previously [3]. Chlorobenzene; general purpose (BDH) fractionally distilled over PzOs; b.p. 131.T/760mm Acetone. AR grade. Acetophenone, fractionally distilled; b.p. 201-202:/760 ram. a-Cumyl alcohol, recrystallized from pet ether (60/80); m.p. 34°. a-Methyistyrene, fractionally distilled, b.p. 165:; phenol, fractionally distilled, b.p. 182°. Cumene hydroperoxide was purified by the method of Kharasch [4]. b.p. 52-54°/0.1 m. Procedure GLC of the decomposition products of cumene hydroperoxide was carried out on a Pye 104 gas chromatograph with flame ionisation detector using the following conditions. Nitrogen purified by molecular sieve (UC.13X) was used as the mobile phase (40 ml/min). The following temperature program gave good separation of products: initial temperature, 100/12 rain; final temperature, 160°/5 rain, chart speed 26 ram/rain; detector temperature, 200°. Retention distances of pure compounds (cml were acetone, 2.2; chlorobcnzene, 8.0; 2-¢hloroethane, 11.7; ,y-methylstyrene, 13.1: acetophenone. 29.5: ~t-cumyl alcohol, 33.0; phenol, 39.6. A 5 ft column was used packed with 10-12% PEG adipate on 80-100 celite. GLC analysis of oxidative sossion products of hydrolysed linseed oil films has been described previously [3].

Effect of verdigris on the initial autoxidation of linseed oil The addition of verdigris (1 g/100g) to linseed oil led to an immediate change in the blue colour of the pigment to yellow-green with partial solution. Linseed oil under ambient conditions and without pigment absorbed little 02 during the first 2 hr and then autoaccelerated to give a linear rate of oxidation tsee Fig. 1). The addition of verdigris removed the induction period and gave a linear rate of oxidation over three times faster than the oil alone. After 20 hr the samples were exposed to u,v. light (S/B cabinet) and the rate of photooxidation was found to be greater for the linseed oil control than for the sample containing verdigris. Both samples were still liquid after 25 hr. Reaction of verdioris with cumene hydroperoxide Verdigris was found to accelerate the decomposition of cumene hydroperoxide in chloroform at 69 GLC examination of the products showed that they were almost entirely those expected on the basis of a homolytic breakdown of the hydroperoxide (~-cumyl alcohol, acetophenone and a-methyistyrenel. No phenol and only a trace of acetone, the ionic decomposition products, were detected

1153

1154

F. RAs'n and G. SCOTT

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FiB. 1. Effect of verdigtb on the rate of O2 absorptionin diffusedaylight and in the presenceof u.v. light (at 20°C).

Effect of verdigris on dried paint films Linseed oil films were formed on a NaC! disc by coating with linseed oil [3] and were allowed to stand in the laboratory for 7 days in normal daylight. The loss of weight of the films was then measured at inter. vals during photo-oxidation by measuring the log decay of the methylene group absorption, 1o8 (ACH2), at 2930 era- 1 as a function of exposure time [3]. The result for a verdigris pigmented (lYe) dried linseed oil film is compared with that for a non-pigmented control in Fig. 2. It is evident that the copper pigment retards the photo-oxidation of the film initially but, at the end of this stage, both rates became first order

and the rate was somewhat higher for the pigmented film than for the control in this stage of the photooxidation. A similar observation was made by following the decay of the ester carbonyl group (1740cm -1) over the same time (see Fig. 3). In this case, however, a well defined secondary induction period was evident. The initially dried films showed no absorption band in the region of 1640 era-t due to unsaturation but a band began to grow in this region and reached a maximum at 180 hr of u.v. irradiation. An examination of u.v. spectral changes over the same period showed an initial shift of the u.v. absorbante to'yards the visible region of the spectrum (see

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u.v.Exposuretime Icloys) Fig. 2. Decay of methylene group i,r. absorption at 2390cm-' (ACHz) in linseed oil films with and without verdigris.

Mechanisms of antioxidant action

1155

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Fig. 3. Decay of carbonyl (ester) i.r. absorption at 1740cm -I ( A ~ ) without verdigris.

Fig. 4) followed by a subsequent decrease in absorbance after 240 days of u.v. exposure.

OxMative scission products formed in verdigris piOmeritedfilms durin0 aoein0 Limced oil films without and with verdigris (60 g/100g) were aged in parallel in the diffuse dayl i ~ t of the laboratory and in a u.v. (S/B) cabinet and the amounts of carboxylic acids present after hydrolysis were measured by GLC [3]. The increase in C9 dicarboxyfic acid relative to a non-oxidizable saturated acid component of the system, Aag/ale, which is a measure of oxidative chain scission occurring in the paint medium, is recorded in Fig. 5 under the four sets of conditions. The retarding effect of verdigris in both daylight and in u.v. light is clearly evident.

in linseed oil films with and

Formation of conjugated unsaturation in verdigrispigmznted films during photo-oxidation Figure 4 shows the formation of a chromophore or chromophores absorbing above 30Onto. It is not possible to obtain information about the nature of these ¢hromophores from the dried film itself so an attempt was made to isolate the brown cotoured material by oxidising unsaturated acid in the presence of verdigris. Oleic acid containing verdigris (0.1 g/100g) was allowed to oxidize in diffuse daylight for 720 days. It was slowly converted to a dark brown mass. After hydrolysis with methanolic NaOH, the sodium salts were neutralized with HCI. The aqueous solution was extracted with ether to re, hove carboxylic acid scis. sion products. A brown gel separated from the

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Fig. 4. Spectral changes of drieH verdigris pigmented films during u.v. irradiation (numbers on curves are irradiation times, days).

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Fig. 5. Effect of verdigris on the rate of formation of C9 dicarboxylic acids in dried linseed oil in diffuse and u.v. light. aqueous phase and was found by i.r. to contain cis (910cm -~) and trans (960cm -~) unsaturation and conjugated carbonyl (1620-1680cm-t). A band at 1500-1550cm-t indicated the presence of unsaturation co-ordinated to copper. A similar absorption shift was observed for the double bond in 10-undecanoic acid (1640cm -t) to a triple absorption at 1510, 1590 and 16t0cm -t in the presence of verdigris. Similar shifts of ~ 9 0 c m - t have been observed by other workers [5"] on co-ordination of double bonds to copper.

The brown gel dissolved readily in chloroform and gave the u.v. absorption spectrum shown in Fig. 6, showing a succession of maxima at 243, 249, 254, 261, 268, 275, 286, 305, 320 rim.

DISCUSSION Verdigris consists of a complex mixture of copper acetate hydrate and basic copper acetate [6] dissolving in acetic acid to give a blue-green solution. In the present study verdigris was found to be insoluble in

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Mechanisms of antioxidant action pure linoleate esters but to be soluble in linoleic acid and commercial grades of linoleate esters (linseed oil) which contained free fatty acid. The pro-oxidant function observed in Fig. 1 results fro~ the well-known redox reactions between copper carboxylate and hydroperoxides [7] as indicated by the catalytic destruction of tert-butyl hydroperoxide to give free radical derived products. The antioxidant effect observed during the photooxidation of linseed oil and dried films made from it appears to be associated with the formation of the dark coloured product in the paint medium. This may be in part due to simple screening of u.v. light in thick films by the u.v. absorbing species but this cannot be the only explanation since the effect is evident even in very thin films where screening would be expected to have almost no effect.

Mechanism of the formation of conjuoation The u.v. spectrum of the dark brown gel dissolved in chloroform indicates the presence of conjugated unsaturation. Table 1, taken from the work of Sondhelmet and co-workers [8], shows the absorptions that might be expected in conjugated trienes, tetraenes and pentaenes. It would appear therefore that these species are all present in oleic acid photooxidized in the presence of verdigris. Some of the peaks observed (e.g. 261 rim) are not found in the simple trienes and may be associated with the presence of copper co-ordination complexes evidence for which was found in the i.r. spectra. Kochi [9] has found that alkyl radicals containing an available/~-hydrogen give alkene as a major and often exclusive product in reaction with cupric salts; other products are the anion radical adducts. It has been shown that the 5-carboxypentyl radical (1, RfffiH) gives co-hexenoic acid (III, Rffi=H) by reaction (I).

1157

ROCO(CH 2)6CH~ + "CCH==CHCH==CH(CH 2)4CH

(I)

O O.,

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\ (IV)

(V)

In the absence of Cu 2÷ these radicals vAll be readily oxidised to the corresponding dicarboxylic acid half ester (V). In the presence of copper ions, there will be a competition between Cu 2+ and 02 for (I) (see scheme (3)) leading to the regeneration of Cu 2+ in a cyclical mechanism. The overall process occurring in scheme (3) is the formation of unsaturation and hydroperoxide in a copper catalysed process (4).

ROCO(CH2)6(~H2

ROCO~CH2)6CH2OO"

(3) CB-D

ROCO(CH2hCHffiuCH2 + H ÷ + C u + -~Cu 2+ + ROCO{CH2)eCH2OOOverall reaction 2ROCO(CH2)oCH~

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Radicals related to (!) are formed abundantly during photolysis of conjugated carbonyl oxidation products of linoleate esters (III).

Two complementary antioxidant radical chain-breaking mechanisms [10"i, the chain-breaking-electron acceptor (CB-A) and chain=breaking-electron donor (CB-D), are involved in scheme (3) which amounts to a catalytic antioxidant process in which the effective

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antioxidant is continuously regenerated. Similar regenerative mechanisms have been observed recently with other classes of antioxidant. An important example is the continuous regeneration of nitroxyl radicals during thermal oxidation of low molecular weight hydrocarbons [11], during photo-oxidation of polyolefms [12, 13] and during mechano-oxidation of rubbers [13, 14].

Table I. u.v.maxima of conjugated polycoes H-{CH==CH~,-H [8] n

3 4 5

Absorbance of maximum (rim)

240

248

257

268 267

278 279

290 290

304 304

317

334

1158

F. Rgs'rl and G. Sco'rr

The formation of extended conjugation in paint media is associated with the continuation of the process outlined in scheme (3) and almost certainly involves prior co-ordination of ionic copper with the double bonds. It is clear from Figs 3 and 4 that the stabilization process is associated with the formation of conjugation and that the end of stabilization is characterized by the rapid disappearance of conjugation. This probably involves oxidative disruption of the conjugation by addition of alkylperoxy and alkoxy radicals to the double bonds as occurs in partially degraded PVC 1"15] and as in the case of PVC, this process is associated with crosslinking through the conjugated unsaturation [15]. It is remarkable however that this process can be retarded by the presence of as little as 1°~oof verdigris in the system.

REFERENCES

1. K. C. Bailey, The Eider Pliny's Chapters on Chemical Subjects. London (1929). 2. A. Kuhn, Stud. Conserv. 15, 12 (1970). 3. F. Rasti and G. Scott, Stud. Conserv. In press. 4. N. S. Kharasch. A. Fono and W. Nudenberg, J. org. Chem. 16, 113 (1951). 5. S. H. Kawaguchi and T. Ogura, Inoro. Chem. 5, 844 (1966). 6. J. Gauthier, PhD Thesis, University of Paris, 17, 27-43 (1958). 7. W. H. Richardson, J. Am. chem. Soc. 88, 975 (1966). 8. F. Sondheimer, D. A. Ban-Efraim and R. Wolovsky, J. Am. chem. Soc. 83, 1675 (1961). 9. J. K. Kochi, J. Am. chem. Soc. 84, 3271 (1962). tO. G. Scott, Atmospheric Oxidation and Antioxidants, Chapter 5. Elsevier, New York (1965). 11. T. A. B. M. Boisman, A. P. Blok and J. H. G. Frijns, Rec. Tray. Chem. Pays Bas 97, 310 (1978), 97, 313 (1978). Acknowledgements--We are grateful to the Trustees of the 12. K. B. Chakraborty and G. Scott, Polymer 21,252 (1980|. National Gallery for financial support of this work and to 13. G. Scott, S. Aft'. J: Chem. 32, 137 (1979). Mr G. Thompson, Dr J. Mills and the colleagues at the 14. A. Katbab and G. Scott, Chemy Ind. 573 (1980). National Gallery for helpful discussions. 15. Ref. ClO], p. 304.