ESR study of polypropylene photostabilisation by sterically hindered piperidine derivatives

Po/~ner Photoc.hem/st~1 (1981) 139-152

ESR STUDY OF POLYPROPYLENE PHOTOSTABILISATION BY STERICALLY HINDERED PIPERIDINE DERIVATIVES

G. B~aa~rr, A. ROCKEr,mAtmR, T. KELEN, F. TODOS and L. JOKAY

Central ResearchInstitute for Chemistry of the Hungarian Academy of Sciences, Budapest, Hungary

ABSTRACT

E S R spectra of sterically hindered piperidine derivatives embedded in polypropylene are superpositions of signals arising from radicals of different mobility. Analysis of the spectra at different temperatures gives information on the mobility and distribution of radicals in solution and in polymer media. A t the temperature of irradiation, radicals are homogeneously distributed also in a polymer matrix. During irradiation the concentration of radicals first increases then, alter reaching a maximum, decreases. This intermediate characte~ is the result of simultaneous formation and reaction of nitroxyl radicals with radicals arising from polypropylene photo-oxidation. Maximum radical concentration is reached at higher dosage, and subsequent decrease in concentration is slower when hydroxy-benzophenone is applied together with the piperidine derivative. In case of joint application of the latter with a nickel complex, on the other hand, no radical formation can be detected in the polymer before, or in the course of irradiation.

INTRODUCTION

The stabilising effect of piperidine derivatives on polyolefins has been studied widely over the past few years. 1-4 In spite of this extensive research, their mode of action has not been entirely elucidated. Most authors 1"3"5generally agree that these types of compounds can not act as UV absorbers since they do not absorb light in the UV region. For this very reason these compounds cannot be quenchers because they cannot accept excitation energy present in this region of the spectrum. This behaviour has 139 Polymer Photochemistry O144-2880/81[O001-O139[$2.50©Applied Science Publishers Ltd, England, 1981 Printed in N o r t h e r n Ireland

140

c. BALIrCrEa"AL.

been confirmed experimentally by Alien, who found that phosphorescence of excited carbonyls did not change in the presence of hindered piperidine compounds during exposure. 5 Opinions greatly diverge, however, in the explanation of the excellent stabilising effect of these compounds. Some authors suggest that nitroxyl radicals, formed during the preparation of films containing piperidine derivatives2 or in their reaction with oxygen or other radicals, 6 exert the stabilising effect. In contrast to these, Allen doubts the presence of nitroxyl radicals or their importance in the photostabilising process. 5"7 In his opinion, piperidine derivatives hinder the isomerisation of ct-/3 unsaturated ketones and thus prevent their decomposition in light. 7 Grattan et al. 6 and Sedlar et al., a however, have found that an adduct is formed from nitroxyl radicals or parent hydroxylamine and hydroperoxides present in the polymer. Therefore, a local increase in the concentration of piperidines around the hydroperoxide may be assumed. The idea of complex formation with hydroperoxide has also been accepted by Allen et al., 5 whereas Gugumus9 assumes complex formation with the alkylperoxy radical. Some authors suggest the reaction of a nitroxyl radical with an alkyl radical, 2"5"1°'11but according to Carlsson et al. 12 and Sedlar et al. 13 this reaction is of minor importance due to the higher rate of reaction between the alkyl radical and oxygen. In this paper we compare ESR spectra of the tetramethyl-piperidine and nitroxyl radical in polypropylene and in solvent in order to prove that the radical formed from piperidine is of a similar character. Analysis of the spectra obtained at different temperatures gives information on the mobility and distribution of radicals in different media. Finally, we studied the change of radical concentration during the irradiation of polypropylene films containing nitroxyl radicals, tetramethyl-piperidine derivatives, and a mixture of the latter compound and a 2-hydroxy-benzophenone and Ni-dibutyl-dithio-carbamate.

EXPERIMENTAL

Materials P o l y p r o p y l e n e (PP): isotactic DAPLEN AT 10, supplied by Petrolchemie

Schwechat AG Austria, without any additives. Stabilisers: 2,2,6,6-tetramethyl-piperidino-sebacate (TMPS); 2,2,6,6tetramethyl-piperidin-N-oxy (TMPNO); nickel-dibutyl-dithio-carbamate (NiDBTC) (all produced in our laboratories); 2-oxy-4-octyloxy-benzophenone; Cyasorb UV 531 (Cy); supplied by Cyanamid Co.

ESR

S T U D Y OF P O L Y P R O P Y L E N E P H O T O S T A B I L I S A T I O N

141

Sample preparation The stabilisers .were compounded into the polymer in benzene solution. After evaporating the solvent, the polymer powder was pressed into films (100/~m thickness) in a nitrogen atmosphere at 170°C. All samples contained Topanol OC antioxidant (2,6-di-tert-butyl-4-methyl-phenol, supplied by ICI), in order to prevent thermal oxidation during film preparation.

Photo-oxidation Irradiation was performed with a Tungsram 1600 W Xenon lamp. The light was focused by elliptic mirrors onto the samples, mounted on a rotating drum in a thermostat. Considering the alternating light and dark periods, the dosage of irradiation on the samples in 24 h was about 2.5-3.5 Mlxh (mega lux hours). Average intensity of illumination: 3 x 10-7-5 x 1 0 - 7 Einstein s-1 c m - 2 .

ESR measurements These were carried out using a JEOL-JES-ME 3X spectrometer at X band with 100 kc/s field modulation. The temperatures of the samples were set by a JEOL temperature controlling unit with the accuracy of 1 degree.

RESULTS A N D DISCUSSION

Radical products o[ hindered piperidine derivatives Most authors dealing with this type of stabiliser have observed the formation of nitroxyl radicals from hindered piperidine derivatives during irradiation (see above). Allen and McKellar, 7 however, at one stage doubted the presence of this radical in irradiated polyolefins, due to the lack of discoloration (nitroxyl radicals are dark brown), and to the difference between the ESR spectra of polymers containing piperidine derivatives and those of the nitroxyl radical. In order to determine the character of the radical present in PP containing TMPS, we compared the ESR spectra of TMPS powder and its solution with those of the TMPNO-radical in solution, as well as ESR spectra of PP films containing TMPS and TMPNO. ESR spectra of TMPS powder, irradiated TMPS powder and TMPS in PP are shown in Fig. 1. Radicals can be detected in low concentration already in the initial stabiliser. Upon irradiation, a spectrum similar to that of PP containing TMPS can be observed (some slight increase in intensity can be observed and the concentration of radicals is by 2-3 orders of magnitude lower than that of TMPS in PP). The patterns of ESR spectra in Fig. 1 can be explained by the fact that some of the radicals are in a restrictive environment, and thus, each spectrum is a

G. BALINTET AL.

142

C

d

Fig. 1. (a) ESR spectra of TMPS powder, (b) irradiated TMPS powder, (c) 1.0 w% TMPS in PP, (d) 1.0w% TMPNO in PP. At the wings the third and fourth lines of Mn:MgO can also be seen, the distance between them is equal to 87.1 gauss. Amplification: (a)--180; (b)--100; (c)--710; (d)--50. Modulation: (a) and (b)--2 G; (c) and (d)--l-0 G. superposition of lines characteristic of mobile and immobile species. T h e difference between the spectrum of T M P S powder and that of the irradiated T M P S powder, as well as of T M P S in PP can be explained by the assumption that the concentration of immobile radicals is relatively higher in the latter two cases. This change can be seen from Fig. 1, where the second p e a k in the lower field (a) appears only as a shoulder in (b) and (c). T h e addition of T M P N O in PP results in a somewhat different E S R signal (Fig. ld). This can be explained by the fact that the majority of radicals (about 80%) are in a molecularly dispersed state. T h e mobility of these radicals is lower than in toluene (Fig. 2), but higher than in the case of TMPS in PP. In Fig. ld, a sharp singlet can be seen in the middle of the spectrum, which is due to molecularly non-dispersed radicals present in the polymer at about 20% concentration. 11

.J

Fig. 2. ESR spectra of TMPS dissolved in toluene; concentration: --0-1 g/ml (a) and that of TMPNO dissolved in toluene, at room temperature; concentration: -10 -4 g/ml (b). Amplification: (a)--1400; (b)---450. Modulation: (a)--0.5 G; (b)--0.05 G; (Mn: 0-5 G).

E S R STUDY OF POLYPROPYLENEPHOTOSTABILISATION

143

Mobility o[ the radicals

Quantitative determination of the mobility of radicals was performed by analysis of the E S R spectra taken at different temperatures, according to the method applied for the spin probe techniques. 14 Figures 3-5 show the E S R spectra of T M P N O radicals dissolved in toluene and PP, as well as that of TMPS in PP, at different temperatures. It can be seen that with a lowering of temperature the E S R patterns of T M P N O in toluene are similar to those of immobile radicals. On the other hand, at high temperatures mobility increases and similar spectra are obtained for radicals embedded in a solid polymer as for radicals dissolved in solution. The mobility of radicals can be characterised by the re-orientational frequency, which is the reciprocal of the rotational correlation time (%).14 The latter is the time required for the radical to rotate through an angle of one radian. The value of the rotational correlation time can be calculated by the equation "re = W o [ ( h o / h l ) 1/2 + ( h o / h _ l ) 1/2 - 2]C

where Wo is the peak-to-peak linewidth of the centre line of the E S R spectrum in Hz, and ho, hi and h_~ are the peak-to-peak amplitudes of the low-, middleand high-field lines. C is a constant depending on g and hyperfine tensor anisotropies and on given experimental conditions.

+J // J

Fig. 3. ESR spectra of TMPNO dissolved in toluene, at 60~C (a); at -100°C (b); and at -196°C (c). Amplification: (a)--220; (b)--160; (e)---400.Modulation: (a) and (b)--I G; (c)--5 G.

144

G. BALINT ET AL.

a

Jyj.-

C

d

"%-

-4-

Fig. 4. ESR spectra of TMPNO in PP, at 100°C (a); at 60°C (b); at 0°C (c); and at -100°C (d). Amplification: (a) and (b)--10 (Mn: 45); (c) and (d)--50. Modulation: (a) and (b)--2-5 G; (c) and

(d)--I G. The value of the C factor can be calculated from the expression: C =

41r,,/3 b2

where A-B b =4~--3 and A and B are principal values of the hyperfine tensor.

a

b

Fig. 5. ESR spectra of radicals formed from TMPS embedded in PP, at 100°C (a); at 60°C (b); at 0*C (c); and at -1960C (d). Amplification: (a)--36; (b)--100; (c)---630; (d)--40. Modulation: (a) and (b)--1 G; (c)--2 G; (d)--2.5 G.

ESR Values of A can lines in the spectra TMPNO-radical A Values of B can

STUDY OF POLYPROPYLENE PHOTOSTABILISATION

145

be determined from the half distance of the two extreme of completely immobile (frozen) radicals. W e obtained for = 32.28 gauss. be calculated from the following relationship: a = ½(A + 2B)

where a is the isotropic coupling constant (for TMPNO-radical a = 15-5 gauss, i.e. B = 7 . 1 1 gauss). Thus, for b and C we obtain: b = 2.96 x 108 s -1 C = 2.484

× 10 -16 s 2

T h e values obtained in gauss were transformed into frequency values (given in M Hz) by multiplication with the factor fl= g/0.71449. The re-orientational frequency values for T M P N O and radicals present in TMPS, both dissolved in toluene and in PP, are given in Table 1. From Table 1 it is seen that the mobility of radicals formed in TMPS embedded in PP is lower than that of T M P N O in the polymer. In other words, the temperature of the system has to be increased in order to reach the same mobility, as in the case of the T M P N O additive. The data also shows a quantitative difference between the mobility of radicals in the polymer and in the solvent. At elevated temperatures the logarithm of re-orientational frequency depends linearly on the reciprocal temperature given in K. These Arrhenius lines are suitable for estimation of the activation energy of re-orientation, if we assume that the tumbling of radicals in the polymer matrix is a typical rate process. Arrhenius representation of the re-orientational frequency is shown for T M P N O in toluene, and for TMPS both in toluene and in PP, in Fig. 6. Estimated activation energy values are given in Table 2. The values obtained for the activation energy of re-orientation are in good TABLE 1 RE-ORIENTATIONAL FREQUENCIES (S-1) OF N - O X Y RADICALS IN DIFFERENT ENVIRONMENTS

Temperature range °C 40-100 Room temperature 40--(-40) (-40)-(- 100) Below (-100)

TMPNO in toluene in PP 1012 1011 101°-109 2 x 108 3 × 107

3 × 108109 3 x 108 108 3 x 107 --

TMPS in toluene in PP --

108-109

3 × 101° 101°-109 ---

5 × 107 107 ---

146

G. B A L I N T E T AL.

25

~5 ~o~ K*

251

20

~5

5o

10~ K*

i./.t~ W

C

25

2O K" Fig. 6.

Arrhenius plot of the rotational frequency for TMPNO in PP (a); TMPS in PP (b); and TMPS in toluene (c).

agreement with Rabold's results, 14 who obtained 12.3 kcal/mole activation energy for the rotation of the tetramethyl-piperidine benzoate N-oxy radical (in the temperature range: 80-140°C) in PP, and 3.8 kcal/mole in methylcyclohexane. The difference in activation energies may result from the difference in radical dimensions and environmental parameters. TABLE 2 ACTIVATION ENERGY VALUES FOR RADICAL RE=ORIENTATION

Eaet

Sample

(kcal/mole)

Temperature

(kJ/mole) J

TMPNO in PP TMPS in toluene TMPS in PP

12.71 5.36 12.91

53.21 22.44 54.05

range (°C)

50-(+100) -30-(+22) 50-(+100)

ESR

S ~ r D V OF POLYPROPYLENE PHOTOSTABILISATION

AW Gauss

147

a

,oL \ -200 456

-tO0

0

I00 "C

4W Gauss

60 50

~ x I

:K. -2OO

4W Gauss

-I00

-Z6

j

I

0

100 °C

oX--

60

50

,%

~0

I I

I I

20 .200

Fig. 7.

~ -I00

t 0

I Tw ~3

I IOO °C

Plots of extreme separation versus temperature in the ESR spectra of TMPNO in toluene (a); in PP (b); and TMPS in PP (c).

148

G. BALINT ET AL. TABLE 3 T w VALUES FOR DIFFERENT SAMPLES

Additive

Environment

Temperature

(°c) TMPNO TMPNO TMPS

toluene PP PP

- 156 -46 +43

The mobility of radicals can be studied by another method suggested by Rabold) 4 Plotting the separation of the two outermost lines (AW) as a function of temperature, gives curves with a sharp change. The position and sharpness of the change depends on the type of the nitroxyl radical, as well as on the composition of the environment. We obtained such curves for TMPNO radical in toluene solution (a) and in PP (b), as well as for radicals formed in TMPS embedded in PP (c) (Fig. 7). Rabold defined as a point of reference the temperature at which A W = 50 gauss (Tw). This is approximately the point of maximum slope of the curves, and it may be assumed that at these Tw temperatures the radicals are in a similar restrictive environment. Comparison of the Tw values of different samples (Table 3) leads to the conclusion, that above these temperatures the nitroxyl radicals are not only uniformly dispersed in the medium, but their mobility is similar to that in solution.

Concentration change of radicals in the course of irradiation A rapid decrease in radical concentration can be observed during irradiation of PP originally containing 1.0 w% TMPNO-radical (Fig. 8), i.e. nitroxyl radicals are consumed in the course of irradiation. There are some data in the literature pointing to the existence of a reaction between nitroxyl and alkyl or alkoxy radicals. 2"5"1°'11 Therefore, it seems justified to assume that nitroxyl radicals stabilise the polymer by scavenging the chain propagating radicals, and thus interrupting polymer photo-oxidation. In the course of the irradiation of PP containing TMPS, radical concentration rapidly increases at the initial period of irradiation, then after a maximal value, a decrease can be observed, and the curve gradually levels out (Fig. 9). The induction period of carbonyl formation in the polymer is very long, during which the concentration of the initial TMPS decreases exponentially. The spontaneous rupture of the samples takes place at about 210 Mlxh, and during this time no carbonyl and hydroperoxide formation can be detected. TM The stabilising effect seems to be due to the transformation product of TMPS, i.e. to the N-oxy radical. Its concentration decreases only slowly even at very high doses: it can be assumed in this case that the nitroxyl radical may be regenerated from the product formed in the course of stabilisation. 16'~7

ESR

STUDY OF POLYPROPYLENE PHOTOSTABILISATION

149

:ok I

0

20

I

,{,0 Mlxh

Fig. 8. Relative concentration change of radicals during kTadiation by adding ]-0 w% TMPNO in PP.

The pattern of the E S R spectrum taken at room temperature does not alter during irradiation, i.e. the ratio of the two types of radicals (mobile and immobile) does not change, while their concentration increases and decreases. This may be attributed to the fact that irradiation is performed at about 70°C a n d - - a s has been d e m o n s t r a t e d - - a t this temperature the distribution of radicals is the same as in homogeneous solution. In an earlier paper 15 we showed that the joint addition of TMPS and O H - b e n z o p h e n o n e type Cyasorb U V 531 led to an increase in stability. Spontaneous rupture of the samples containing 1-0w% T M P S + 1 . 0 w % Cy

R ,~,0

2,0

i

50 Fig. 9.

ibo

75o

Mlxh

Relative concentration change of radicals during irradiation in case of PP containing 1-0 w% TMPS.

150

G. BJ~a~Ircr~r ~L.

occurred only at very high dosage (about 250 Mlxh; - 4 0 M l x h higher than observed for TMPS alone). The change in concentration of radicals formed in a PP sample containing 1.0 w% TMPS+ 1.0 w% Cy is shown in Fig. 10. Due to the presence of a good UV absorber, maximum radical concentration is reached at a higher dosage, and the decrease is slower than that observed when only TMPS was exposed. This may be one of the reasons for their higher stability in the case of the combined application of the two types of stabilisers. It may be assumed that the UV absorber retards all processes taking place upon the effect of UV light: including the formation and decay of radicals. The presence of a nickel complex beside TMPS on the other hand, does not result in enhanced stability. Spontaneous rupture of the polymer films containing 1.0w% T M P S + I . 0 w % Ni-DBTC was obtained with a dosage of 148 Mlxh. A similar antagonistic effect was found by Scott,2 who interpreted this effect to the decomposition of hydroperoxides by Ni-DBTC; their presence being essential for the formation of N-oxy radicals from TMPS. This seems to be supported by the fact that in the case of a polymer containing both TMPS and Ni-DBTC, only a very slight ESR signal can be detected before irradiation, and then it disappears during photo-oxidation. In contrast to this, we agree with Allen's opinion4 that the formation of N-oxy radicals is not exclusively the result of reaction with hydroperoxides. It has been stated that in TMPS powder containing no hydroperoxides the formation of nitroxyl radicals can also be observed in a very low concentration. On irradiation however, their concentration increases (Fig. 11). In agreement

R

2p

~o

0

Fig. 10.

I 50

~0 t

i 150

i 200 Mtxh

Relative concentration change of radicals during irradiation in case of PP containing 1-Ow% TMPS+ 1-Ow% Cy.

ESR STUDY OF POLYPROPYLENE PHOTOSTABILISATION

151

1,0

O,5

I

0 Fig. 11.

20

I

Mlxh 40

Relative concentration change of radicals formed in TMPS powder during irradiation.

with other workers 18"19 it would appear that radicals are formed from the Ni-DBTC before the formation of the Lewis acid, which is then supposed to react with hydroperoxides. These radicals may be trapped by N-oxy radicals formed from TMPS. After consumption of Ni-DBTC during irradiation, the ESR signal characteristic of N-oxy radicals formed from TMPS appears. At present it is not clear whether Ni-DBTC hinders the formation of nitroxyl radicals mainly via hydroperoxide decomposition, or if the nitroxyl radicals already formed are trapped by Ni-DBTC.

SUMMARY

Sterically hindered piperidine derivatives are oxidised already in air: in solution their ESR spectra show the triplet signal characteristic of nitroxyl radicals. The ESR spectra of piperidines embedded in the polymer show the following changes: the spectrum becomes a superposition of signals of different linewidth, due to the presence of both radicals in molecularly dispersed state and radicals of restricted mobility.

152

G. BALI~rrEr At..

By analysis of the mobility of radicals as a function of temperature it can be established, that at the temperature of irradiation---70°C--radicals are homogeneously distributed and their mobility is similar to that in solution. During irradiation the concentration of radicals first increases then, after reaching a maximum, decreases. This intermediate character of the kinetics of N-oxy radical concentration change may be attributed to the formation of radicals from TMPS and to their reaction with radicals arising from the oxidation of the polymer. By joint application of TMPS and Cyasorb U V 531, maximum radical concentration is reached at a later phase, and a slower decrease can be o b s e r v e d than if o n l y T M P S is applied, d u e to U V a b s o r p t i o n b y h y d r o x y benzophenone. T h e decrease in efficiency o b s e r v e d in the case of j o i n t a p p l i c a t i o n of T M P S a n d N i - D B T C , o n the other h a n d , m a y b e d u e to the fact that in this case n o significant radical c o n c e n t r a t i o n can b e o b s e r v e d in the p o l y m e r film before, or in the course of i r r a d i a t i o n . ACKNOWLEDGEMENT

We thank Dr .~. Rehfik for the preparation of stabilisers. REFERENCES 1. SHtaAPIgrOKH,V. J., IVANOV,V. B., KHVOSTACH,O. M., SHAPIRO,A. B. and ROZANTSEV,E. G., Dokl. Akad. Nauk. SSSR, 225 (1975) 1132. 2. CHAKRABORTY,K. B. and SCOTt, G., Chem. and Ind., 1978, 237. 3. GRATrAN,D. W., CARLSSON,D. J. and WILES,D. M., Polym. Degr. and Stab., 1 (1979) 69. 4. At.L~N,N. S., Polym. Degr. and Stab., 2 (1980) 129. 5. ALt EN, N. S., McKEtJ_~, J. F. and WILSON,D., Polym. Degr. and Stab, I (1979) 205. 6. GRATrAN,D. W., REDDOCH, A. H., CARI.SSON,D. J. and WILES, D. M., J. Polym. Sci., Polym. Letters Ed., 16 (1978) 143. 7. At.t.~N,N. S. and MCKmtAR, J. C., J. Appl. Polym. Sci., 22 (1978) 3277. 8 SEDLAR,J., PETRUJ, J., PAC, J. and NAVRATIL,i . , Polym. Comm., 21 (1980) 5. 9. GUGtrMIJS,F., In: Developments in polymer stabiUsation, Scott, G. (eel.), Vol. 1., Applied Science, London, 1979, p. 261. 10. SmLov, Ju. B. and DENISOV,E. T., Vysokomol. Soed., A16 (1974) 2313. 11. SOHMA,J., In: Developments in polymer degradation, Grassie, N. (ed.), Vol. 2, Applied Science, London, 1979, p. 99. 12. CARtSSON,n . J., GRATrAN,D. W., SUPRUNCHUK,T. and Wn.ES, D. M., J. Appl. Polym. Sci., 22 (1978) 2217. 13. SEDLAR,J., PETRUJ,J. and PAC, J., [I,IL syrup, on mechanism of degradation and stabilization of hydrocarbon polymers, Prague, 1979, Poster M 47. 14. RABOLD,G. P., J. Polym. Sci., A1,7 (1969) 1203. 15. BALI~rr,G., Krt~N, T., TOI~S, F. and REHAK, A., Polym. Bull., 1 (1979) 647. 16. FELDEn,B., ScI-rtJMACHER,R. and SrraK, F., Chem. and Ind., 1980, 155. 17. CARLSSON,D. J., GARTO~,A. and WILES, D. M., In: Developments in polymer stabilisation, Scott, G. (ed.), Vol. 1. Applied Science, London, 1979, p. 219. 18. HOLOSWORTH,J. D., SCOIT, G. and WILLIAMS,D., J. Chem. Soc., 1964, 4692. 19. HOWARD,J. A. and CHENmR, J. H. B., Can. J. Chem., $4 (1976) 390.