Photostabilization of ethylene-propylene-ethylidenenorbornene terpolymer by hindered amine compounds

Polymer Degradation and Stability 7 (1984) 205-220

Photostabilization of Ethylene-Propylene- Ethylidenenorbornene Terpolymer by Hindered

Compounds Amine

Maria Nowakowska

Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry, Jagiellonian University, Karasia 3, 30-060 Krak6w, Poland

(Received: 18 August, 1983)

ABSTRACT

The effect of bis(2,2,6,6-tetramethylpiperidinyl)sebacate ( ~ NH) and the stable bisnitroxyl radical ( ~ N O ' ) arising from it, on the 9,10- diphenylanthracene (DPA) sensitized photo-oxidation of ethylene- propylene-ethylidenenorbornene terpolymer (EPNB) has been ex- amined. It was found that the formation of hydroperoxide, which is a result of the photo-oxidation of EPNB, is inhibited by both compounds. The stabilizing action of the bisnitroxyl radical is considerably greater, however. The radical ~ NO" was found to be able to quench the singlet excited state of DPA. Both the compounds quench singlet oxygen, but the effectiveness of ~NO" is greater. The radical ~NO' , in its electronically excited state, induces hydroperoxide decomposition. The hydroxylamine formed in this process can participate in the inhibition of polymer photo- oxidation.

I N T R O D U C T I O N

In our previous study we showed that 9,10-diphenylanthracene (DPA) sensitizes the photo-oxidation of ethylene-propylene-ethylidenenorbornene terpolymer (EPNB).I This effect was believed to be connected

This paper was presented at the IUPAC 25th Microsymposium on Processing and Long Term Stabilities of Hydrocarbon Polymers, held in Prague from 18 to 21 July 1983.

205 Polymer Degradation and Stability 0141-3910/84/$03"00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

206 Maria Nowakowska

with reaction between singlet oxygen formed in the system and EPNB. Singlet oxygen is generated in the process of triplet state as well as singlet state quenching of DPA by oxygen, a The reaction of singlet oxygen with EPNB results in hydroperoxide formation.

Bis(2,2,6,6-tetramethylpiperidinyl)sebacate (Tinuvin 770) belongs to the class of sterically hindered amines (HALS) which are known to be very efficient photostabilizers in polymer systems.3 - s Recently the mechanism of the stabilizing action of HALS has been the subject of several papers.3 - 9 It was pointed out that the stabilizing effectiveness of HALS is closely connected with their ability to form stable nitroxyl radicals, s,9

HALS ( ~ N H ) incorporated in polymer systems are oxidized by hydroperoxides (ROOH), 1° peroxy radicals (RO~) 11 and singlet oxygen

+goon .~ ~ N O " (1)

) N H +RO~

+ 'Of

, > N O " (2)

~ N O " (3)

The formation of free nitroxyl radicals in polymers stabilized by HALS was confirmed experimentally.6' 1 a - 1 s When polypropylene was stabilized by the bifunctional stabilizer bis(2,2,6,6-tetramethyl-4-piperidinyl)- sebacate (Tinuvin 770) the corresponding mononitroxyl and bisnitroxyl 7 stable radicals were shown to be formed.

The nitroxyl radicals formed from HALS are well known to scavenge carbon-centred radicals (R') sa4,~6 which results in the formation of substituted hydroxylamine compounds

R'+ NO" • ) N O R (4)

which may contribute to the stabilization process by scavenging peroxyl radicals: 17

) N O R + R'O 2 ~ NO" + ROOR' (5)

The aim of this paper is to investigate the effect of bis(2,2,6,6- "~N tetramethylpiperidinyl)sebacate ( . / H) and its bisnitroxyl radical

( ~ N O ' ) on the 9,10-diphenylanthracene sensitized photo-oxidation of ethylene-propylene-ethylidenenorbornene terpolymer.

Photostabilization by hindered amine compounds 207

EXPERIMENTAL

Materials

The ethylene-propylene-ethylidenenorbornene terpolymer (EPNB) was kindly supplied by Professor O. Kramer, University of Copenhagen, Denmark. The molecular weight of the terpolymer (.~t w ) was 4 x 104. The ethylene and ethylidenenorbornene contents were about 50 ~o and 5 ~o by weight respectively. The terpolymer was purified by dissolving in cyclohexane and precipitating with methanol under a nitrogen atmosphere.

9,10-Diphenylanthracene (DPA; Merck) was used without further purification.

Bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate (Tinuvin 770; Ciba- Geigy Ltd) was purified by crystallization from ethyl alcohol.

Bis(2,2,6,6-tetramethyl- l-oxyl-4-piperidinyl)sebacate was prepared according to the procedure given by Son. is The ESR spectrum of the radicals was in good agreement with those presented in earlier literature. 15

Cumyl hydroperoxide (~,0t-dimethylbenzylhydroperoxide) was purified by vacuum distillation.

The solvents, hexane, benzene, cyclohexane, methyl alcohol and ethyl alcohol, were special grade for spectroscopy or Analar reagents, additionally distilled under atmospheric pressure.

The polymer films were prepared by pouring the appropriate amounts of EPNB solution in cyclohexane on quartz or NaC1 plates, according to the spectrophotometric method to be applied. The DPA, Tinuvin 770 and bisnitroxyl radicals were incorporated in the films by preparing a cyclohexane solution of the compounds at the required concentration and dissolving an appropriate amount of EPNB in this solution.

TLC aluminium sheets pre-coated with aluminium oxide (Merck, 60 F254, neutral, type E, layer thickness 0.2 mm) and suitable developing solvents 19 were used for the separation of products.

Apparatus

The polymer films and solutions were irradiated in air at atmospheric pressure with an ASH-400 medium-pressure lamp. A Carl Zeiss interference filter was used to isolate the radiation at 2 = 365 nm. The intensity of incident radiation at 2 = 365 nm was 2.2 x 10-9 einst cm-2


s-1 as determined by ferrioxalate actinometry. 2° The short-wavelength cut-off filter was used to isolate radiation at 2 > 315 nm.

The Xenon 150 lamp and Carl Zeiss interference filter were applied to isolate radiation at 2 = 436 nm.

UV-Vis and IR spectra of the samples were recorded with Zeiss Specord UV VIS and UR-20 instruments, respectively.

The fluorescence spectra of the DPA + ~ N H and DPA + ~ N O " were measured in benzene solution using a spectrofluorimeter produced by Cobrabid, Poland.

The viscosities of the terpolymer solutions in cyclohexane were measured with an Ubbelohde viscometer using a water thermostat at 20 + 0.1 °C.

RESULTS AND DISCUSSION

Irradiation of the EPNB + DPA film with light absorbed by DPA results in hydroperoxide formation. Figure 1 shows the absorption changes at ~7= 3400cm -1 of the EPNB + D P A as well as E P N B + DPA + Tinuvin 770 and EPNB + DPA + ~ N O " films (thickness = 30 × 10-4cm) observed during irradiation with light with 2 > 315nm. It can be observed that both Tinuvin and its bisnitroxyl radicals inhibit

A A , I I ~

I ,

so 100 1so 2oo t i r rad [h r s]

Fig. 1. The increase in absorption (AA) at 3400cm -1 o f E P N B + D P A (curve ]), EPNB + D P A + ~ N H (curve 2) and EPNB + D P A + ~ N O ' (curve 3) films as a funct ion o f t ime of i r radiat ion (fi lm thickness = 30 x ] 0 - 4 cm, [D PA]iniu= j = 5 x ] 0 - 3 too!

dm -3, [ jNH] in i~= 1 - [ ~ N O ' ]i,it,,l = 5 x 10 -3 mo ldm-3) .


photo-oxidation of EPNB sensitized by DPA. The bisnitroxyl action is stronger than that exhibited by the parent bisamine at the same initial concentration in the film.

The viscosity measurements illustrated in Fig. 2 indicate that irradiation of the EPNB + DPA solution in cyclohexane with light with 2 > 315 nm results in an increase in the viscosity of the system in the early stages of irradiation. This effect was explained by the formation of hydrogen bonds between hydroperoxide groups formed in the system. The hydroperoxides are the products of reaction between polymer and singlet oxygen generated in oxygen quenching of electronically excited states of DPA. The prolonged irradiation of the EPNB + DPA system

22

• 3 •

1B

14

10

. . . . ~o t [m' in] 60 IBO 3

Fig. 2. Changes of viscosity number of cyclohexane solutions of EPNB + DPA (curve 1), EPNB + DPA + ~ N H (curve 2) and EPNB + DPA + ~ N O ' (curve 3) ([EPNB] =0-12~ow/w, [DPA]i.itial = 3 × 10-Smoldm -3, [/NH]initial = [ j N O ]initial--

2 × 10-4moldm -3) during irradiation with light with 2 > 315 nm.


results in a considerable decrease of the viscosity. This effect is associated with the occurrence of secondary photodegradation processes. When Tinuvin 770 or its bisnitroxyl radicals at an initial concentration of 2 x 10 - 4 mol dm- 3 are present in the system the crosslinking observed for the EPNB + DPA system does not occur and the photodegradation process is strongly inhibited.

Thus the Tinuvin and its bisnitroxyl radicals interfere in the photophysical and/or photochemical processes which lead to hydroperoxide formation in the EPNB + DPA system.

Bearing in mind those processes which are of great importance in the early stages of DPA (A) sensitized photo-oxidation of EPNB (P) (see Scheme 1) we can define three main problems, which should be investigated,

~o ÷ h'~

Scheme l

-P0a ÷

.~N 0"÷ h9

(1) quenching of the singlet excited state of DPA by ~ N H and its bisnitroxyl radical ~ NO' ,

(2) quenching of singlet oxygen by ~ N H and ~ N O ' , (3) reaction of the bisnitroxyl radical ~ N O " in its electronically

excited state with hydroperoxides formed in the EPNB + DPA system.

(1) Quenching of the DPA singlet excited state by ~ N H and ~NO" The influence of Tinuvin 770and its bisnitroxyl radicals on the quantum yield of DPA fluorescence in benzene and cyclohexane solutions was investigated. It was found that Tinuvin does not influence the DPA fluorescence whereas bisnitroxyl radicals do. Figure 3A shows typical fluorescence spectra of DPA in benzene at various concentrations of ~ N O ' . Figure 3B shows the dependence of the ratio of the DPA fluorescence quantum yield in the absence (~0) and the presence (~) of quencher on the concentration of ~ NO' radical.


r -

3

L

. D L

A

I i i ,

o 380 ~o ~5o ~o 5~o ~o

2.0

B

1.5

1.0

. . . . ,

~ ~ ' 1~ ~ ,'5 '~;~0.~xi0,~o,~-~] Fig. 3. A, Fluorescence spectra of DPA in benzene solution in the presence of ~ N O " at various concentrations: 1, [ ~ N O ' ] = 0 ; 2, [ ~ N O ' ] = 6 x l 0 - 3 m o l dm-3; 3, [ ~ N O "] = 1-7 x 10- 2 mol dm - 3. B, Dependence of the ratio of the fluorescence quantum

yield of DPA in benzene solution on ~ N O ' concentration.


The dependence of O0/~ on [ ~ N O ' ] shown in Fig. 3B obeys the Stern-Volmer quenching law which is given by the following equation

O0 = 1 + kNoZs[)NO" ] (6) O

where • 0 and • are defined as above, Zs is the lifetime of the singlet excited state of the DPA molecule in the absence of quencher, kNo is the rate constant of DPA excited singlet state quenching by ~ NO' , and [ ) N O ] is the concentration of bisnitroxyl radicals.

The slope of the line in Fig. 3B gives the value of kNoZs. Using this experimental value and assuming Zs=7.2ns, 21 the rate constant for quenching of the singlet excited state of DPA by ~ NO" in benzene solution was calculated as

kso = (7.8 _ 0.1) x 109 dm 3 mol - x s- 1 (7)

This value is a little lower than that reported by Green and Singer 21 for DPA fluorescence quenching by di-tert-butylnitroxide free radicals (k = 9.2 x 109 dm 3 mol- 1 s- 1).

The process of quenching of the DPA singlet excited state by bisnitroxyl radicals (/~NO') lowered the quantum efficiency of singlet oxygen formation in the system.

The process"

1A* + ~ N O " *No IA ° + (~NO')* (8)

is competitive with oxygen quenching of the singlet excited state of D PA:

tA* + 302 'k°~ 3A* + xO~ (9)

It can be pointed out that the rate constant of oxygen quenching of the DPA singlet excited state in benzene is about 4.0 times greater than that found for bisnitroxyl radicals, kNo. In order to establish whether process (8) is really important in the system investigated, the values of quantum yields of DPA fluorescence quenching by oxygen in the absence (O~) and in the presence (O~ °) of ~ N O " at given concentrations should be calculated according to the following equations:

kq[O2] (10) = kq[O2] + 1/ s

kq[O2] (I)(~ 0 = kq[O2] "4- kNo[~NO ] + 1/z s (11)


Substituting the value of kNo calculated above, kq = 3.2 x 101°dm a tool-1 s-1 21 and z s = 7-2 x 1 0 - 9 s 21 in eqns (10) and ( l l ) the quantum yield of DPA fluorescence quenching by oxygen in the presence of bisnitroxyl radicals can be estimated. Figure 4 shows the dependence of ~ o as well as the ratios of ~ / ~ t ~ ° at atmospheric pressure of oxygen on ~NO" concentration.

The second effect, connected with radical quenching of the singlet excited state of DPA, i.e. the decrease of the total quantum yield of excited triplet state formation (OTNO), is also demonstrated in Fig. 4. The quantum yields of DPA triplet state formation, ~o and OT N°, were calculated according to the equations:

~o =~Osc + ~ (12)

~ o NO ~ o (13) = tl)ls c +

where

klsc (14) (I)°sc _ klsc + l/z s q- kq[O2]

kxsc (15) NO (I)Is C -- klsc + 1/z s q- kq[O2] + kNo[~NO']

and ~qO, ~qNO are given by the expressions (10) and (11). The following values were substituted: Olsc = klsc/(klsc + 1/Zs) = 0-12 (at [02] = 0, [NO' ]=0) , 2 Zs=7 .2x 10-9S , 21 k q=3 " 2x 10X°dm a mo1-1 s-X, 21 [02] = 1.43 x 10-amol d m - a , 2° kNo = 7.8 x 109dm 3 tool -1 s -1.

We can see that bisnitroxyl radicals (~ NO ") decrease considerably the quantum efficiency of singlet oxygen generation because they cause perturbation in both processes in which it is formed (decrease of (l)q and ~T). Curve 3 in Fig. 4 gives the dependence of quantum yield of singlet oxygen formation (~1o~) on bisnitroxyl radical concentration. The values of O1o, were calculated using the equation:

(I)10* = 0~O(~ O "{- (~)TNO(1)qT (16)

where

Oq~ = the quantum yield of singlet oxygen formation in the process of oxygen quenching of the excited triplet state of D PA, assumed to be equal to unity, 22

ot=kq/ (kq+k 'q)=0"5; 2 kq and k'q are the rate constants of the deactivation of the singlet excited state of DPA by oxygen, leading to the formation of the excited triplet state and the 107 or 3 0 2 molecule, respectively.


o.t.

25

0.2 " ~ 15

tO

0,1

5

0 , , , l ' ' ' ~o ' ' 100 200 300 4 [~N0"]" 10J [mo! din"

Fig. 4. Dependence on ~ N O " concentration of the quantum yields of the processes, DPA fluorescence quenching by oxygen in the presence o f ~ N O " (~v~Q °) (curve 1), DPA triplet state formation inthe presence of ~ N O " (~T °) (curve 2), singlet oxygen formation (@,~) (curve 3) and the ratios of quantum ~e]ds of DPA fluorescence quenching by oxygen (@~/@~o) and triplet state formation in the absence and in the presence of ~ N O "

On the basis of the results obtained it can be concluded that the quenching of the excited singlet state of DPA by bisnitroxyl radicals is an important mode of their stabilizing action in the EPNB + DPA system.

(2) Singlet oxygen quenching by ~ N H and ~NO" In the second part of this study the possibility of singlet oxygen quenching by Tinuvin ( ~ N H ) as well as by bisnitroxyl radicals ~NO" was investigated. Experiments were carried out on two different systems. In the first, the inhibiting effect of ~ N H or ~NO" on the consumption of diphenylisobenzofuran (DPBF) in benzene or carbon tetrachloride was investigated.


The inhibiting effect o f ) N H or ~ N O " on diphenylisobenzofuran or anthracene oxidation can be described in terms of the following photophysical and photochemical processes occurring in the systems:

i A o + h v , • IA * k% 3A * (17)

1A* 3A* (18) -~302 + 103"[-

3A, IA o (19)

10~ kd~' 302 (20)

k, IO 3 + IA o , AO 2 (21)

) N H ~ N O ' ( ~ N H ) * (22) k ~ "4 ~ 302 "4- iO~ + k7

~NO" (~NO')* (23)

where A represents the anthracene or diphenylisobenzofuran molecule. The ratio of the rate of consumption of A in the absence (V °) and in the

presence (V °) of the quencher ( ~ N H or ) N O ") is given by the equation:

V°/V Q = 1 + kq°[Q] (24) kr[A] + kd

where [Q]= [ ~ N H ] and kQq=kNq" or [Q] = [ ~ N O ' ] and kq o-- kqN°. Curve 1 in Fig. 5 shows excellent dependence of V°/V ° on quencher

concentration for the anthracene + Tinuvin + CC14 system. From the slope of this line and assuming values ofkr = 1.5 x 105 dm 3 mol- 1 s- 1,23 kd = 1.4 x 103 S- 1 24 and [A] = 1 x 10 -4 mol rim-a, the rate constant of singlet oxygen quenching by Tinuvin 770, NH kq , was calculated:

kq NH = (2 + 0.5) x 105 dm a mol- 1 s- 1 (25)

Curve 2 in Fig. 5 shows the dependence of V°/V ° on ~ N O " concentration in the DPBF + ) N O " + C 6 H 6 system. From the slope of this line and assuming values of kf = 7 x 10 s dm a mo1-1 s - l , 2s k d = 4 x 10gs -1 25 and [A] = 6 x 10-Smol dm -3 the rate constant of singlet oxygen quenching by bisnitroxyl radicals ~ N O ' w a s calculated:

k2 ° = (2 + 0.2) x 106 dm 3 mol- 1 s- t (26)


2.2

1

1.9

1.6

1.3

1.o

o i ' ' ; ' ' g ' ' r n ' ~ [o].lO'[rnot d Fig. 5. Graphs of the ratios of singlet oxygen acceptor consumption in the absence and in the presence of quencher in the two systems: 1, anthracene + Tinuvin 770 + CCI4; 2,

diphenylisobenzofuran + bisnitroxyl radicals + C6H 6.

Comparison of the values of kq NH and kq N° indicates that the nitroxyl radical is a better singlet oxygen quencher than Tinuvin. This result is in good agreement with that obtained by Bellu~ 26 for 2,2,6,6-tetramethylpiperidine and the corresponding nitroxides.

Figure 6 shows the dependence of the quantum yield of singlet oxygen quenching by ) N O " (curve 1) and ~ N H (curve 2) on concentration of quencher in the D PA + C 6H 6 system ([D PA] = 1 x 10- 4 mol dm- 3). The curves in Fig. 6 allow one to compare the effectiveness of Tinuvin and its bisnitroxyl radicals in the singlet oxygen quenching process. When Tinuvin at low concentration is introduced into the system ([~NH)_<I x 10-3mol dm -3) its participation in singlet oxygen quenching is rather low. In addition, taking into account the fact that


0.9

0.6 i 2 "~

0.3

o ' ' ' ' ' ' iO]=1¢[rnotd ']

Fig. 6. Dependence of the quantum yield of singlet oxygen quenching b y /N H " ((1)qNH) (line 1) and by ) N O " ( ~ o ) (line 2) on the concentration of quencher in the DPA + C6H 6

system ([DPA] = 1 x 10-4moldm-3).

~ N H is readily transformed to ) N O ' , it can be deduced that singlet oxygen is quenched almost entirely by nitroxyl radicals in real systems.

(3) Hydroperoxide decomposition by ~ NO" in its electronically excited slate Under our experimental conditions the E P N B + D P A + ~ N H or

NO" systems were irradiated with light with 2 > 315 nm. This light is absorbed mainly by the sensitizer (D PA). It was observed, however, that the bisnitroxyl radicals can also absorb light in the visible range (350-650nm). Figure7 shows the UV-Vis absorption spectra of bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate and its bisnitroxyl radical in hexane solution. It can be seen that Tinuvin 770 has one absorption band with a maximum at about 214rim (e = 2650dm 3 mol - 1 c m - 1). The absorption spectrum of the bisnitroxyl radical ( ~ N O ' ) consists of two bands; the first one with a maximum at 2 = 241 nm (e = 3360dm a mol - 1 cm- 1) and the second with a maximum at 2 = 480nm (e = 19dm 3 mol - 1 c m- 1). It was observed that the irradiation of bisnitroxyl radicals in hexane solution for 3 h with light with 2 = 436 nm causes no changes in their concentration. The irradiation of ~ NO" + cumyl hydroperoxide in hexane was then carried out. It was observed, using the TLC technique and a chemical method, 27'2a that electronically excited bisnitroxyl radicals

0.9,

3

250

LIJ L3 z < m oL o

m <

~® ~o ~o soo 6® ~. X[nr

218 M a r i a N o w a k o w s k a

. . . .

' ' 4~. ' ' }~ ' ' 32 26 2(} " 1 C

Fig. 7. The UV-Vis absorption spectra of bis(2,2,6,6-tetramethylM-piperidinyl)- sebacate ( c = 5 x 10-3moldm -3, layer thickness l=0.1cm, line 1) and bis(2,2,6,6- tetramethyl- l-oxyl-4-piperidinyl)sebacate (c = 5 x 10-, mol dm- 3, 1 = 0.5 cm, line 2;

c = 2 x 10 -3 mol dm -3, ! = 2cm, line 3) in hexane solution.

( ) NO' ) induced hydroperoxide decomposition followed by hydroxylamine formation.

It is worth noting that Sedl/~f and his coworkers 29 have shown that 2,2,6,6-tetramethylpiperidine and the corresponding nitroxyl free radical are not capable of decomposing hydroperoxide at ambient and slightly elevated temperature. Dulog and Bleher 3° found recently that 2,2,6,6- tetramethylpiperidine-N-oxyl reacts with hydroperoxide at elevated temperatures (70 ___ T ~ 80 °C).

Thus it can be expected that in polymer systems stabilized by HALS reaction occurs between hydroperoxides and nitroxyl radicals formed in the system. The hydroxylamine formed in such a system is a quite efficient inhibitor of photo- and thermal oxidation. Hydroxylamine reacts with peroxyl radicals and with hydroperoxides. 8 These reactions not only break the kinetic chain of polymer oxidation but also lead to the rapid accumulation of nitroxyl radicals ( ~ NO') in stabilized polymer:

~ N O H + PO 2 -~ ~NO" + POOH (27)

~ N O H + POOH ~ PO" + ~ N O + H20 (28)


CONCLUSIONS

Tinuvin 770 and its corresponding bisnitroxyl radical inhibit the photo- oxidation of EPNB terpolymer sensitized by DPA. The efficiency of ~NO" is considerably higher than that observed for the same initial concentration of ~ NH. This difference can be explained if one takes into account the following facts:

(1) ~ NO" has the ability to quench the excited singlet state of DPA whereas > N H has not,

(2) ~NO" as well as ~ N H quench singlet oxygen, but the rate constant of the process tO, + ~ N O " is ten times that for the process IO* + >NH,

(3) > NO" in its electronically excited state induces hydroperoxide decomposition accompanied by hydroxylamine formation. The latter compound can effectively participate in inhibition of the photo- and thermal degradation of polymer.

Since Tinuvin is easily oxidized to its bisnitroxyl radical in the polymer system, it seems probable that after some initial period of irradiation of the (EPNB + DPA + > N H ) system, the ~NO" radical takes over the stabilizing functions from Tinuvin.

These experimental results are represented in Scheme 2.

. . . .

P . . . . . ;

~ --,:~c~ . . . .

'A,+ hq -~---~.~'A" ~ k_...~ "-- J' ~"

, " IA ~( ~N 0,),,

Scheme 2

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Documents

Photostabilization of ethylene-propylene-ethylidenenorbornene terpolymer by hindered amine compounds