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MECHANISM OF THE ACTION OF LIGHT STABILIZERS OF POLYMERS IN THE PHOTOSENSITIZED OXIDATION OF HYDROCARBONS COMMUNICATION 2. EFFECTIVENESS OF LIGHT STABILIZERS V. I. Gol'denbert, V. Ya. Shlyapintokh, L. M. Postnikov, and G. A. Sukhanov UDC 541.124 + 541.147 + 542.976.2 The protective action of light stabilizers of polymers in the photosensitized oxidation of hydrocarbons (diphenylmethane and ethylbenzene) is quantitatively determined by their ability to quench excited states, to inhibit oxidation reactions, and to absorb photochemically active light [i]. Therefore, the coefficients of absorption, quenching constants, and kinetic constants of inhibition can be used as objective characteristics of the effectiveness of the action of light stabilizers. The purpose of this work was to measure these con- stants and to determine the contribution of various mechanisms to the net effect of light stabilizers. EXPERIMENTAL The influence of light stabilizers on the rate of oxidation of diphenylmethane by molecular oxygen, photosensitized by benzophenone, was investigated. The reaction was conducted at the wavelength 365 at 70 ~ and an oxygen pressure of 1 atm. The rate of oxidation was measured according to the absorption of oxygen. The constants of inhibition ktJ~k 7 (kl~ and k 7 represent the rate constants of the reaction of the per- oxide radical with the light stabilizer and recombination of per oxide radicals, respectively) were determined according to the inhibition by light stabilizers of the dark reaction of the oxidation of diphenylmethane, initiated by decomposition of azobisisobutyronitrile. To determine the quenching constants kl2T , corrections for the absorption of light and inhibition were introduced into the experimentally measured rates of oxida- tion before the introduction of the light stabilizer wt (rate of initiation wit) and after the introduction of the light stabilizer w 4 (rate of initiation wl4). The rate w 2 at which the reaction would proceed in the absence of effects of quenching and inhibition was determined according to the experimentally measured rate w i and the coefficient of attenuation of light ~lCl z) siCi + e2C~(I -- e -(~'c,+~c~) (1) i~, -- j_ e_slC,l, (e is the absorption coefficient; C is the concentration; 1 is the thickness of the reaction vessel; the sub- script "1" pertains to the photosensitizer). The rate w 3 at which the reaction would proceed in the absence of the effect of inhibition was determined according to the experimentally measured rate w 4 and the inhibi- tion constant klJ~-k ~. The quenching constants were determined according to the Stern-Volmer equation from the dependence of the ratio wiJwla on the concentration of the light stabilizers. The details of the experimental procedure and substantiation of the methods of determining the constants were presented in communication 1 [1]. The values obtained for the quenching and inhibition constants are cited in Table 1. The contribution of various mechanisms of stabilization to the summary effect as a function of the concentration at various wavelengths of light inducing a photoreaction (Fig. la, b, and c) was calculated according to Table 1 data for two typical light stabilizers, tinuvin P and 2-hydroxy-4-octoxybenzophenone. The decrease in the rate of initiation of the photosensitized oxidation of diphenylmethane with increasing concentration of the light stabilizer under the experimental conditions (70 ~ po 2 = 1 atm, concentration of the photosensitizer 0.1 M, wit = 2.5 i0 -s mole liter -1 sec -1) was calculated. The absolute effectiveness of the light stabilizers was determined according to the formula Institute of Chemical Physics, Academy of Sciences of the USSR. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2232-2236, October, 1968. Original article submitted February 13, 1968. 2112

Mechanism of the action of light stabilizers of polymers in the photosensitized oxidation of hydrocarbons

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MECHANISM OF THE ACTION OF LIGHT STABILIZERS

OF POLYMERS IN THE PHOTOSENSITIZED OXIDATION

OF HYDROCARBONS

COMMUNICATION 2. EFFECTIVENESS OF LIGHT STABILIZERS

V. I. Gol'denbert, V. Ya. Shlyapintokh, L. M. Postnikov, and G. A. Sukhanov

UDC 541.124 + 541.147 + 542.976.2

The protective action of light stabilizers of polymers in the photosensitized oxidation of hydrocarbons (diphenylmethane and ethylbenzene) is quantitatively determined by their ability to quench excited states, to inhibit oxidation reactions, and to absorb photochemically active light [i]. Therefore, the coefficients of absorption, quenching constants, and kinetic constants of inhibition can be used as objective characteristics of the effectiveness of the action of light stabilizers. The purpose of this work was to measure these con- stants and to determine the contribution of various mechanisms to the net effect of light stabilizers.

E X P E R I M E N T A L

The influence of light s tabi l izers on the ra te of oxidation of diphenylmethane by molecular oxygen, photosensit ized by benzophenone, was investigated. The reac t ion was conducted at the wavelength 365 at 70 ~ and an oxygen p r e s s u r e of 1 atm. The ra te of oxidation was measured according to the absorption of oxygen.

The constants of inhibition k t J ~ k 7 (kl ~ and k 7 r ep resen t the ra te constants of the reac t ion of the pe r - oxide rad ica l with the light s tabi l izer and recombinat ion of per oxide rad ica ls , respect ively) were determined according to the inhibition by light s tabi l izers of the dark react ion of the oxidation of diphenylmethane, initiated by decomposi t ion of azobis isobutyroni t r i le . To determine the quenching constants kl2T , correc t ions for the absorpt ion of light and inhibition were introduced into the experimental ly measured r a t e s of oxida- tion before the introduction of the light s tabi l izer wt (rate of initiation wit) and after the introduction of the light s tabi l izer w 4 (rate of initiation wl4). The ra te w 2 at which the react ion would proceed in the absence of effects of quenching and inhibition was determined according to the experimental ly measured ra te w i and the coefficient of attenuation of light

~lCl z) siCi + e2C~ (I - - e -(~'c,+~c~) (1)

i~, - - j _ e_slC, l ,

(e is the absorption coefficient; C is the concentration; 1 is the thickness of the reac t ion vessel ; the sub- scr ip t "1" pertains to the photosensi t izer) . The ra te w 3 at which the reac t ion would proceed in the absence of the effect of inhibition was determined according to the experimental ly measured r a t e w 4 and the inhibi- tion constant klJ~-k ~. The quenching constants were determined according to the Stern-Volmer equation f rom the dependence of the ra t io wiJwla on the concentrat ion of the light s tabi l izers . The details of the experimental p rocedure and substantiation of the methods of determining the constants were presented in communicat ion 1 [1].

The values obtained for the quenching and inhibition constants a re cited in Table 1. The contribution of var ious mechanisms of stabil ization to the s u m m a r y effect as a function of the concentrat ion at var ious wavelengths of light inducing a photoreact ion (Fig. la, b, and c) was calculated according to Table 1 data for two typical light s tabi l izers , tinuvin P and 2-hydroxy-4-octoxybenzophenone. The dec rease in the ra te of initiation of the photosensit ized oxidation of diphenylmethane with increas ing concentrat ion of the light s tabi l izer under the experimental conditions (70 ~ po 2 = 1 atm, concentrat ion of the photosensi t izer 0.1 M, wit = 2.5 �9 i0 -s mole �9 l i ter -1 �9 sec -1) was calculated. The absolute effect iveness of the light s tabi l izers was determined according to the formula

Institute of Chemical Physics , Academy of Sciences of the USSR. Transla ted f rom Izves t iya Akademii Nauk SSSR, Ser iya Khimicheskaya, No. 10, pp. 2232-2236, October, 1968. Original ar t ic le submitted Feb rua ry 13, 1968.

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t~3~[ a Lg "~31 b 1C '~

, 3C, lk/iff..70~ , 3C, M 50" ' C,2" M.I~ Fig. 1. Dependence of the e f fec t iveness of the p ro - tec t ive act ion on the concentra t ion of the photos tab i - l i ze r , a) 2 -oxy-4-oe toxybenzophenone (k = 365 nm); b) 2-oxy-4-oc toxybenzophenone (k = 400 nm) ; c) t in- uv inP (~ =365nm): 1) (7)1 =wiI/wi4; 2) (2/)2 =wi2; 3) (~/)3 = wi2/wi3; 4) (Y)4 = wi3/wi4-

w~i w,: wi.~ w~l (2) Wi2 W~3 Wi4 Wi4

where wil/w12 is the dec r ea s e in the ra te of initiation on account of f i l t ra t ion Of light; w l j w l 3 in the dec rea se in the r a t e of init iation on account of quenching; and w13/wj4* in the dec rease on account of inhibition,

At sufficiently high concentra t ions of the light s t ab i l i ze r s considered (C ~ 10 -:~ M) a,~d the selected benzophenone concentra t ion, the absorp t ion of light is c lose to i00%. Under such conditions, according to Eq. (1), the r a t io is equal to

i~ w~2 ~ . 1 (3 ) = w~l - - i ~- (s2/eiC1) C2

The r a t i o

w, _ i (4)

varies with the concentration of the light stabilizer according to the same law. Consequently, the relative role of filtration and quenching depends upon the ratio between the constants ~2/81Ci and ki2T. In Fig. la the inhibition of the photoreaction conducted at the wavelength 365 nm is considered. In this case, for 2-hydroxy-4-octoxybenzophenone, ~2/~iCi ~ki2 ~', and the contributmns of the effects of filtration and quenching are approximately the same and do not vary with the light sensitizer concentration. ~rhen the wavelength is varied (~ = 400 nm) with the same light stabilizer, the effect of filtration becomes predomi- nant (Fig. Ib), since at 7 = 400 nm, the coefficient of absorption by the stabilizer is relatively higher than at ?~ = 365 nm. In general, ~2/~i ~ ki2 ~, and the relative contributions of the mechanisms of filtration and quenching vary with the concentration of the light stabilizer and depend upon the wavelength of the light pro-

ducing the photoreaction~

Figure ic considers the effect of inhibition, together with filtration of light and quenching, with

tinuvin P as an example. The relative contribution of inhibition increases with increasing stabilizer con-

centration, since the rate of initiation drops on account of filtration and quenching, and the oxidation chains

correspondingly become longer. As a result even of such a weak inhibitor as tinuvin 1 ~ -- i00 times weaker

than phenol -- at a concentration of 3 �9 10 -2 M, when the chain length reaches 160 units, the rate of oxidation

decreases by 2.8-fold.

Undoubtedly the results obtained can be qualitatively transferred to the light stabilization of polymers

as well. First of all we should mention that the ratio between the effectiveness of the action of various

light stabilizers, no matter what the method of characterizing their effectiveness may be, cannot serve as

an absolute characteristic. When the concentration is varied, the mechanism of the effect is changed, and

�9 The decrease in the rate on account of inhibition can be represented formally as the decrease in the rate of initiation.

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TABLE i, Quenching and Inhibition Constants for a Number

of Light Stabilizers in Diphenylmethanc (70 ~ po 2 = 1 arm)

Light s tabi l izer

2 -Hydroxy-4-dec ioxy- benzophenone

2- Hydr oxy-4- octyloxy- benzophenone

2-Hydr oxy- 4- methoxy- benz ophenone

2-Hydr oxy- 4- methyl - benzophenone

2,4-Dihydr oxybenzo- phenone

2,2' , 4 -Tr ihydroxybenzo- phenone

Salicylate Phenylsa l icyla te (salol) Phenyl e s t e r of 3 ,5 -d i -

chlorosa l ieyl ic acid 5 ,5 ' -b is (Phenylsa l icyla te)

sulfide

i, 1-Dicyano-2-methy l -2 - phenylethylene (TsN-I)

i, I-D icyano-2,2-diphenyl- ethylene (TsN-2)

I- Cyano- l-phenyl-2- br omophenylethylene (ATsB- I)

l-Cyano- i- car boethoxy- 190 2,2-diphenylethylene (TsE-2)

Other light s tab i l i ze rs

2 - (2 ' -Hydr oxy -5 ' -me th y l - 490 phenyt)benzotr iazole (tinuvin P)

4 ,6-Dibenzoylr esor cinol 90

K12r ( l i ters per mole), d e t e r - mined acco rd -

ing to inhibition of the photo-

reac t ion

Benzophenones

190

225

190

210

220

1400

Sal icylates

No quenching The same

140

510

Nitr i les

2OO

230

210

k13/~fk T, l i ter �9 mole -1 �9

sec-1

No inhibition

The same

fT Yf

f? W

4,15 �9 10 -3

5.2 ~ 10 -3

No inhibition The same

H N

W

. W

W

. -W

2.9 �9 10 -3

No inhibition

consequently, so is the re la t ive ef fec t iveness . The coefficients of absorption, quenching constants, and r a t e constants of inhibition can be used as absolute cha rac t e r i s t i c s . Under the condition that D s >> Dp (where D s and Dp a re the optical densi t ies of the light s tab i l izer and polymer , respec t ive ly) , r eg a rd l e s s of the values of the quenching constants, the mechanism of light f i l t rat ion becomes predominant . In o rder for the mechanism of quenching to become basic , the condition D s << Dp must be fulfilled. As has been shown in the v e r y in te res t ing studies [2], p r ec i s e ly under the conditions D s << Dp or D s < Dp, the protect ive action of var ious light s tab i l i ze rs is accompanied in po lymers chiefly according to the mechanism of quenching. Thus, the r e su l t s of this work, conducted on a model sys tem, agree with the r e su l t s obtained in the inves t i - gation of po lymers .

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It is interesting that many widely used light stabilizers of polymers, investigated in the work, either did not possess any inhibiting activity at all or proved to be exceptionally weak inhibitors (Table i}. This leads to the idea that intensification of the inhibiting ability of light stabilizers in general does not give any appreciable effect in the protection of polymers, especially if we consider that the stabilizers were created purely empirically, by selection and variation of effective structures. However, there are other facts. It is known, for example, that additions of antioxidants to polymers increase the effectiveness of the action of light stabilizers. Certain light stabilizers are also known, in particular, compounds of metals of variable valence, which possess very high inhibiting ability in model liquid-phase reactions [3]. Therefore, the question of the significance of the effect of inhibition for light stabilization of polymers will need further study.

The authors would like to thank R. S. Burmistrov and Z. G. Popov, who kindly provided the samples of n i t - r i l e s .

CONCLUSIONS

i. The mechanism of the action of light stabilizers of polymers depends upon their concentration and the wavelength of the exciting light. This dependence was explained quantitatively.

2. For a number of light stabilizers investigated, the quenching and inhibition constants we~e deter- mined~

1.

2. 3.

LITERATURE CITED

V. I. Gol'denberg, Vo Ya. Shlyapintokh, and L. M. Postnikov, Izvo Akad. Nauk SSSR, Ser. Khimo, 1483 (1968).

A. P. Pivovarov, Yu. Ao Ershov, and A. F. Lukovnikov, Plastmassy, No. I0, 7 (1966). E. M. Tochina, L. M. Postnikev, and V~ Ya. Shlyapintokh, Izv. Akad. Nauk SSSR, Ser. Khim., 71 (1968).

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