developments in the photo-oxidation and photo-stabilisation of polymers

Polymer Degradation and Stability 10 (1985) 97 125

Developments in the Photo-oxidation and Photo- stabilisation of Polymers*

Gerald Scott

Department of Chemistry, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, Great Britain

(Received: 26 October, 1983)

ABSTRACT

The strategic significance of hydroperoxides as initiators jor the photo- oxidation of hydrocarbon polymers has only recently become generally accepted by polymer photochemists. This has in turn led to the recognition that uv stabilisers are essentially antioxidants acting by the same mechanism as thermal antioxidants, but differing from them in their much greater stability under conditions of photo-oxidation.

Many of the most effective photo-antioxidants act catalytically and two general classes of catalytic photo-antioxidant have received most attention during the past 10 years."

1. The catalytic chain-breaking antioxidants are redox systems which oxidise alkyl radicals (CB-A) and reduce alkylperoxyl radicals (CB-D) in a cyclical repeating process. Nitroxyl radicals derived from commercial hindered amine light stabilisers (HA LS) are the most important CB-A/CB-D antioxidants, and some recent examples of new photo-antioxidants involving this principle are discussed.

2. The light-stable nickel thiolate antioxidants (particularly the dithiocarbamates and dithiophosphates) are slowly converted during photo-oxidation to lower molecular mass sulphur acids which are eff~'ctive catalysts j or peroxidolysis (PD-C).

New developments in this field are reviewed.

* This paper was presented at the Polymer Degradation Discussion Group Conference on Photooxidation and Photostabilisation of Polymers', held in Durham, Great Britain, on

8-9 September, 1983.

97 Polymer Degradation and Stability 0141-3910/85/$03'30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

98 Gerald Scott

INTRODUCTION

It has been known for many years that light has a deleterious effect upon the properties of polymers. Two processes have to be clearly distin- guished; photolysis and photo-oxidation. 'Weathering', the technological term used to describe the deterioration of polymer properties in the outdoor environment, involves a complex interaction of physical and chemical processes, but, of these, photo-oxidation is by far the most important.1 The presence of chromophores is essential to both photolysis and photo- oxidation and the deliberate introduction of light absorbing species, either as part of the polymer chain or as low molecular weight compounds incorporated into the polymer during processing, is now the basis of polymer films and fibres with short, but controllable, lifetimes in the outdoor environment. 2-4

THE CHEMISTRY OF PHOTO-OXIDATION

The earliest systematic studies of photo-oxidation were carried out by Bateman and Gee over thirty years ago. 5 These workers identified hydroperoxides as the primary photo-initiators in cyclohexene, a model compound related to the structural unit in rubbers. They showed that cyclohexene hydroperoxide (II) had a substantially higher absorbance in the uv spectral region between 290 and 560nm (see Fig. l) than cyclohexene (I) and Norrish and his co-workers later showed that the rates of decomposition of both tert-butyl hydroperoxide and cumene hydroperoxide were appreciable at 313 nm. 6'7 Both groups found a rapid build up of more strongly absorbing secondary products (for example, IIl) which were identified as carbonyl compounds formed by homolytic breakdown of the hydroperoxides.

H OOH

I 11

hv

0

lIl

(l)

Early investigations into the photo-stability of polyethylene established that it was much more susceptible to photo-oxidation than pure lower molecular weight alkanes. The latter have almost no absorbance

Photo-oxidation and photo-stabilisation of polymers 99

Fig. 1.

301 I.C

_J

-1.£

3oLeO 24oo 28oo 3 2 o o 3 6 o o

Ultraviolet absorbance ofcyclohexene and its hydroperoxide. (After reference 5, with permission.)

above 200nm whereas polyethylene, like most other commercial polymers, has a weak but significant absorbance above 300 nm. Ketonic carbony!, which absorbs at 1720 cm - ~ in the infra-red, was recognised as being a major contr ibutor to this uv absorbance. 8 Burnett e t al. 9 also noted the formation of hydroxyl absorbance at 1555 cm 1 which had the interesting characteristic that its intensity increased with increasing temperature at the expense of a broad hydrogen bonded hydroxyl absorbance at lower wave numbers. On prolonged heating at 150 °C, the sharp hydroxyl absorbance was irreversibly converted to a hydrogen bonded species. Although the full significance of this observation was not recognised at the time, related work on the association of hydroperoxides in solution 1° showed that this was consistent with the view that the sharp hydroxyl absorbance at 1555cm-1 was due to isolated hydroperoxide groups in polyethylene. 1 ~ More detailed investigations have shown 12 that a good correlation exists between the intensity of the 3555cm absorbance and hydroperoxide measured by a chemical analytical procedure in the polymer (see Fig. 2). This very convenient analytical relationship depends on the fact that, at the temperature of measurement, an equilibrium exists between free and associated hydroperoxides so that,

6 0

x

7

u 50

~2

I

0 4 0

X

~- 2o

Z

10 / i i i i i

10 20 30 40 50

H y d r o p e r ' o x i d e m e a s u r e d c h e m i o a t t y

(g t o o l g - 1 x 105)

0 0 60

1 O0 Gerald Scott

Fig. 2. Relationship between chemically measured hydroperoxide in LDPE and hydroperoxide index (A3555 cm- 1/A 1895 cm- '). (After reference 12, with permission.)

although the infra-red method measures only dissociated hydroperoxide, this is directly related to the total hydroperoxide in the system. The quantitative measurement of hydroperoxides in commercially produced polymers has led to the recognition of the importance of hydroperoxides in a variety of other polymers,13 including polypropylene,14'~ 5 rubber modified polystyrene ~6 (HIPS) and poly(vinyl chloride) (PVC). 17

The formation of hydroperoxides in polymers during processing is shown typically for LDPE in Fig. 312 but PP, HIPS and PVC show similar behaviour. 13Js That carbonyl compounds are secondary products is clearly demonstrated in Fig. 4 which shows the decomposition of thermally formed hydroperoxides during photo-oxidation and the associated formation of carbonyl compounds.19

Hydroperoxide photolysis leads to the rapid destruction of vinylidene unsaturation, present in the polymer after manufacture, and there seems little doubt that the presence of oxidation sensitive unsaturation is one of the main reasons why commercial polyethylene is less oxidatively stable

x

5 0

1 so

4o

i 30 165 o

8 20

8- 175

lO

0 10 20 30 4 0 50 60 70 80 90 Pr 'oces£ !qg t i m e ( rn [n)

Fig. 3. Formation of hydroperoxide in LDPE during processing. Numbers on curves are processing temperatures in an open mixer. C. indicates closed mixer. (After reference

12, with permission.)

lOO

(5)

c~

Fig. 4.

50

(30)

30 o

3

20

(30)

\ \

I loo

Ket rne -/(30)

Ketone ( 5 )

I ~l l I 200 300 400

T ime of i r r a d i a t i o n (hours)

Effect of irradiation time ( > 285 nm) on the formation and decay of functional groups in LDPE. Numbers in parentheses are processing times in minutes.

102 GeraM Scott

~ C H 2 ~ C ~ H 2 _ L6o-25o°c

II o, (RH) C H 2 IV

RO.

Cross-linked RO.

polymer gel

OOH I

~ C H - - C ~ C H z II

C H 2 V

~ h v (300 ram)

0 I

~ C H - - C ~ C H 2 - ( R O . ) II

C H 2 /

0 ~ OH II h

- - - C ~ C H 2 - + ~ C H - - C ~ H a - I1 IL

C H 2 C H 2

Vl

Scheme 1. Thermal formation and photo-chemical breakdown of photo-sensitising hydroperoxide groups in LDPE.

than might have been expected. The chemistry of the formation and subsequent reactions of the hydroperoxide chromophores in commercial LDPE are summarised in Scheme 1. There is evidence that the vinylidine group in IV, and probably the vinyl ketone (VI), produced by oxidation, are involved in subsequent addition reactions with the alkoxy radical, leading to the formation of cross-linked gel during the early stages of photo-oxidation. 19

Unequivocal evidence for the primary involvement of hydroperoxides, rather than derived carbonyl compounds, in the photo-initiation step in the photo-oxidation of polyethylene has been obtained by thermally destroying peroxides in LD PE films in argon before photo-oxidation.'2 Figure 5 shows that thermolysis leads to carbonyl in the polymer pro rata to the amount of hydroperoxides present after processing. Figure 612 compares the photo-oxidation rate of films processed for 30 min (curve 30) with similar films previously heated in argon for 20 h (curve 30 (HA)). It is clear that removal of hydroperoxides reduces the oxidation rate to that of a control which had been compression moulded in the essential absence of air and which also contained no hydroperoxides (curve 0). Irradiation of a similar film in argon, as expected, gave no significant carbonyl formation (curve 30 (HA, IA)).


2 4

2 0

1 6

1 2

"o r

5~ c" 0 8

©

Fig. 5.

Q 0 0 0 0 0---- 60 (4)

...,~ ,,~ w , . - - 3 0 (48)

11)

,o c , 6 o o-. ,or.... 5 (3) 0 10 20 30

T i m e o f heat ing in a rgon at 110o0 (h)

Change in carbonyl concentrat ion in processed L D P E on heating in argon at I I0°C. Numbers on curves are processing times (min) in an open mixer and those in parentheses are hydroperoxide concentrations (105gg- l ) . (After reference 12, with

permission.)

O () R.

~ H 2 C C H 2 _ _ h~ , __CH2C. CH2__ -...~b)

Very similar conclusions have been reached for ppl4 and PVC 17 and in all cases the initial rate of photo-oxidation, as measured by carbonyl formation, has been shown to be closely related to the hydroperoxides present after processing. Carbonyl compounds seem to be much less important than hydroperoxides during the initial stages of photo- oxidation although they play a part in photo-initiation, presumably by hydrogen abstraction (reaction 2(a)) and Norrish I photolysis (reaction 2(b)), during the later stages of photo-oxidation. 2°

OH I

~CH2C. CH z - + R.

O II

- - C H 2 C " + C H 2 -

(2)

104 Gerald Scott

/ ?

3O

25 30 ( H A ) / / /.~ /3o(c) 20

Z '%-

~ ~o ©

5

o i i

I O0 200 300 400 500 600 700 800 900

i r r ' a c l i a t i o r t i m e (h )

Fig. 6. Photo-oxidation (carbonyl generation) of LDPE. Numbers on curves are processing times at 150 °C. 0, compression moulded without processing; HA, heated in argon at 110°C/20 min; IA, irradiated in argon. (After reference 12, with permission.)

Polypropylene differs from polyethylene in that the concentration of hydroperoxides increases during photo-oxidation 15,21 (see Fig. 7). The square root of the hydroperoxide concentration was found to be linear with time, as required by conventional autoxidation kinetics involving bimolecular termination. 21

Although kinetic studies in polymers involve many assumptions, due to the heterogeneous nature of most polymers, rate constant data generally support the conclusion that hydroperoxides are much more important in the photo-initiation of oxidation in commercial polymers than other chemical processes such as carbonyl photolysis or sensitisation by catalyst residues, singlet oxygen or atmospheric contaminants. 21'22 Consequently, the discussion of uv stabilisation processes has recently moved away from uv absorption and photo-excited state quenching processes which, ten years ago, were believed to be the most important inhibition processes involved, 23 to antioxidant mechanisms, of which


l

o 60 t

E O ~ 4

§ 3o

@L

Fig. 7.

I

o 5

0 10 20 30 40 50 60 70 80 90

t_,o

-8

-e S

-4

-2

0 too

Irrodlohon t ,me { h )

Formation of hydroperoxides and carbonyl compounds in PP during ultraviolet irradiation. (After reference 15, with permission.)

peroxide decomposition and kinetic chain breaking have recently been shown to be much more significant in the overall process than was originally thought.

PHOTO-STABILISATION PROCESSES

Any understanding of the mechanism of photo-stabilisation processes must begin from a consideration of the mechanism of autoxidation itself, since photo-oxidation is just one facet of autoxidation, differing only in the nature of the initiation and, to a lesser extent, the termination steps. Scheme 2 outlines, in summary form, current knowledge of autoxidation and indicates how antioxidants and stabilisers may interfere with the most important processes involved. 24

Chain-breaking antioxidants which interfere with the primary autoxidation chain reaction fall into two complementary classes (Scheme 3). The chain-breaking donor (CB-D) antioxidants, which reduce alkylperoxyl radicals to the inactive anion, include the hindered phenols and aromatic amines. This is the most studied group of antioxidants since, in most autoxidising systems, alkyl peroxyl is the radical species present in excess in the system. However, it has been shown recently, that,

106 Gerald Scott

R H 0 2

• R O O . R O . + . O H

UVA MDQ . . . . ~

HEAT, LIGHT, METAL IONS

Preventive

PD-S PD -C

Scheme 2.

" " C B D

R O O H R H

Antioxidant mechanisms.

when the availability of oxygen at a macroalkyl radical site is restricted 25 (for example, in an internal mixer or in a screw extruder), the complementary chain-breaking acceptor process (CB-A) in which a macroalkyl radical is oxidised by the antioxidant to a carbenium ion (Scheme 3) may be as important, or even more important, than the CB-D mechanism•

Under conditions of limited oxygen supply, both CB-A and CB-D processes may operate together, provided the antioxidant (AH) has a 'stable' oxidised form and can form a redox couple with the reduced form (see Scheme 4). Many examples of catalytic antioxidants are now known in polymers 25 and they include stable phenoxy126 and nitroxy127 radicals in the melt stabilisation of polyolefins and stable nitroxyl radicals in the protection of rubbers against fatigue. 28 As will be seen below, some uv stabilisers act by a catalytic CB-A/CB-D cycle•

Of the other antioxidant mechanisms outlined in Scheme 2, the absorption of damaging uv light (UVA) and the decomposition of hydroperoxides to non-radical products, particularly by a catalytic


R °

~ ( -e )

R +

1 \ / H + / C = = C \ +

Chain-breaking acceptor (CB-A) mechanism

Scheme 3.

02 ROO.

l (+e)

ROO

ROOH

Chain-breaking donor (CB-D) mechanism

Complementary chain-breaking antioxidant mechanisms.

mechanism (PD-C), are particularly important preventive mechanisms in uv stabilisation. 24

Most known uv stabilisers do not act by a single mechanism but by a combination of mechanisms. Commercial uv stabilisers are generally classified into uv absorbers (UVAs), nickel complexes or quenchers (Q) and antioxidants (CB-A/CB-D) and synergists. Tables 1 to 3 show representative examples within these groups with their most probable mechanisms of action. Most members of the U VA class (Table 1) operate

~ / / \ /

R. / C = = C \

~ ~ RA

A" AH

ROOH ROO. Seheme 4. Generalised CB-A/CB-D antioxidant cycle.

108 Gerald Scott

TABLE 1 UV Stabilisers: UV Absorbers

Structure Commercial Code used Mechanism name in text of action

HO

, .sor UV 531

HO tBu

2. ~ N ) q ~ Tinuvin 327

tBu tBu

3.

/ - - - O /---- tBu tBu

UV 531 UVA,CB-D(Q)

UVA, CB-D(Q)

CB-D, UVA

primarily by absorbing uv light and re-emitting it as vibrational (thermal) energy, but, in addition, they have been shown to be able to scavenge radical species. 24 The last compound in Table 1 is particularly interesting because it contains a conventional hindered phenolic group typical of the thermal antioxidants and, although it is also a uv absorber, it almost certainly functions primarily as a uv stable chain-breaking antioxidant.

A wide variety of nickel complexes has been used in polyolefins and only a selection of these is shown in Table 2. Although the term 'quencher' is still widely used in the industry to describe their ability to deactivate photo-excited species, this mechanism is not now believed to be very important in practice since, in all cases, their antioxidant function has been shown to be much more important. The first three members of this series are typical of the class of uv stable peroxide decomposers (PD-C) and 4 and 5 are uv stable CB-D antioxidants. The presence of the nickel complex in all cases results in considerable uv screening activity but this is probably less important than their antioxidant properties (see below).

Conventional thermal antioxidants are not normally considered very effective uv stabilisers alone, but they are very often included in a uv stabiliser formulation as synergists. This is true of the typical hindered

TA

BL

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U

V S

tabi

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/%

,,

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1.

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)N~

.,s/

CN

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--

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C

PD-C

, U

VA

, C

B-D

(Q

) ~'~

./S.

.~

./.S

\ 2.

(R

O)2

P\s

..~

N~

...s

/P(O

R)2

N

iDR

P

PD-C

, U

VA

, C

B-D

(Q

) ~ ~.

S~

S-

"

3.

RO

C~.

S.f

iNI.

~..s

/CO

R

NiR

X

PD-C

, U

VA

, C

B-D

(Q

) ~:

~

tBu

tBu

~ O

Et

OE

t /

4.

HO

C

H2P

~.~.

O..~

Ni~

..o/P

CH

2..

. O

H

Irga

stab

200

2 C

B-D

, U

VA

(Q

) -~

tBu

tBu

,9~

H2N

Bu

0 N

i 0

5.

Cya

sorb

10

84

CB

-D,

UV

A (

Q)

UV

108

4

tOct

tO

ct

TA

BL

E 3

U

V S

tabi

liser

s: A

ntio

xida

nts a

nd S

yner

gist

s

Che

mic

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truc

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C

om

mer

cia

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ode

used

M

ech

an

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na

me

in t

ext

of

acti

on

Me

Me

Me

Me

I. H

N~

-~O

C

O(C

H2)

8C O

O--

~--

% H

Me

Me

Me

Me

OH

2.

tB

u~

tBu

CH

2CH

2CO

OC

1 a H

37

3. (

RO

CO

CH

2CH

2)2S

O

O

4. ~

\P

~'

~O

/

\OH

\ ~

/3

Tin

uvin

770

77

0 C

B-A

/CB

-D

Irga

nox

1076

10

76

DL

TP

DL

TP

CB

-D

(syn

ergi

st on

ly w

ith U

VA

)

PD-C

(s

yner

gist

only

with

UV

A)

PD-C

TN

PP

PD-S

(s

yner

gist

with

UV

A)

c~

e~

e~


phenol and thiodipropionate ester (compounds 2 and 3) and tris-aryl phosphite (compound 5) shown in Table 3. The hindered piperidine (compound 1), which will be discussed in detail below, is not an effective antioxidant but its derived nitroxyl radical has been shown to act by the CB-A/CB-D cycle 27 and its high activity is due to this species, formed either during processing or during photo-oxidation. Unlike all the effective light stabilisers listed in Tables 1 and 2, it does not absorb uv light, so that i t --and the derived nitroxyl radical--are uv stable. The cyclic phosphate ester (compound 4, Table 3) is also a powerful thermal antioxidant 29 and uv stabiliser 3° and, again, it has no significant UVA activity but acts by catalytically destroying hydroperoxides in a non- radical process.

The effectiveness of some typical uv stabilisers in polypropylene is compared in Fig. 8 at the same mass concentration (0.2 g/100 g).31 It can

× ~ o . ~

o[_ o

Fig. 8.

ZnDNC CONTROL i0

UV531 i C

A J J L L

200 4OO 600 8O0 lOGO 12OO IRRADIATION TIM~, ]%

Photo-oxidation of P P containing antioxidants as indicated on the curves (all at 0.2 g/100 g). (After reference 31, with permission.)

be seen that the thermal antioxidants 1076 and zinc dinonyldithiocarbamate (ZnDNC) have some uv stabilising activity, being almost as effective as the uv absorber UV531. The hindered piperidine, Tinuvin 770, is somewhat more effective than the nickel complex, 1084. The most effective stabiliser is nickel dinonyldithiocarbamate (NiDNC) which, like the zinc complex, shows an induction period to photo-oxidation associated with the complete absence of hydroperoxides during processing. Tinuvin 770, on the other hand, shows an initial period of

112 Gerald Scott

inactivity before the inhibition period commences and it has been shown that it cannot inhibit the formation of peroxides during processing.32

Figure 9 shows how the effectiveness of the uv stabilisers varies with increasing mass concentration. The thermal antioxidants 1076 and ZnDNC show almost no increase in activity with four-fold increase in concentration, whereas the uv absorber UV531 and the two nickel complexes, 1084 and NiDNC, show an almost linear increase with concentration. it has been shown that nickel dinonyldithiocarbamate is much more soluble than its lower homologues in the polymer.33 The latter are much less effective as uv stabilisers and it has been concluded that this is an important factor in determining the activity of a uv stabiliser which acts, at least in part, by uv absorption.

The differences in the behaviour of the “zinc and nickel dithiocarbamates is illustrated in Fig, IO. Superimposed on the induction

0 0.1 0.2 0.3 0.4 CONcF3?TRATION, g/loo g

Fig. 9. Effect of concentration on the ultraviolet stabilising efficiency of antioxidants in PP. (After reference 31, with permission.)


o . m . . . . . . . . .

C o n t r o l ~ o addltlve~

i i i ° g o . 3 - 0 . 4

i D E C N i D E C N i D E C 0 6 0 2.

c~ ...... . 9 . . . . . . . • , • . . ~_ j - l . o r: , o o 2 0 0 ~ o o 4 o o 500

] e r a d { , ~ t L o n t i ~ , e h ~

Fig. 10. Correlation of the decay of ultraviolet absorbance of ZnDEC (285 nm) and NiDEC (330nm) in LDPE with photo-oxidation induction period (initial additive

concentration, 3 x l 0-- 4 mol/100 g). (After reference 34, with permission.)

9 0 0 ~

800~

} 6oo;

w 4 0 0

3OO

2 0 0 . . . . . . . . . . . . . . . . . . . . . 0 10 20 30 4 0 50 60 70 8 0 9 0 100

Mole frochon ofZnDEC (x lO0)

Fig. 11. Synergism between UV 531 and ZnDEC in PP as a function of the mole fraction of ZnDEC in the mixture (total concentration, 6 x 10 4mol/100g). (After

reference 15, with permission.)

114 Gerald Scott

period curves are the decay curves for the photo-destruction of the complexes in the system. 34 It can be seen that the higher activity of the nickel complex is due to its higher uv stability. This can also be achieved by externally protecting the antioxidant from photo-destruction by using it in combination with a uv absorber. Figure 11 shows that zinc diethyl dithiocarbamate, which is a weak uv stabiliser in PP, shows synergistic activity with the uv absorber UV531. 34 The total concentration of stabilisers is kept constant in the system but, at a 3:1 ratio of UV531 to ZnDEC, the stabiliser system is almost twice as effective as UV531 alone.

,_o

c

0 . 0

- 0 . 4

- 0 . 7

- 0 . 6

- 0 . 8

(b)

( d )

(c)

(e

100 200 300 400 500 600 700 8 0 0

16

?

8

4

Irradiation time (h)

Fig. 12. Theef fec to fUV531on thedecayofNiDEC(absorbancea t330nm) inPPand the related induction period to photo-oxidation. (a) Decay of NiDEC. (b) Decay of NiDEC + UV 531. (c) Carbonyl formation, control. (d) Carbonyl formation, NiDEC. (e) Carbonyl formation, NiDEC + UV 531. (All concentrations, 3 × 10 -4 mol/100 g.) (After


I

One important function of the uv absorber is to protect the metal complex from photo-destruction. Synergism occurs even with the relatively photo- stable NiDEC (see Fig. 12) but the evidence suggests that not only does the uv absorber protect the dithiocarbamate from photo-oxidation, but the peroxide decomposer can also protect the uv absorber from the destructive effects of hydroperoxides during processing and photo- oxidation. 35


PEROXIDOLYTIC ANTIOXIDANTS (PD-C) AS ULTRAVIOLET STABILISERS

The above discussion has illustrated the effectiveness of the peroxide decomposers as uv stabilisers provided they are uv stable or are adequately protected from the effects of uv light. Recent studies of the mechanism of the transition metal dithiolates have suggested ways in which their activity might be considerably enhanced. 36

It is now well established that the metal dithiolates are the precursors of the effective catalytic peroxide decomposers. Peroxide decomposition takes place in a three-stage process (see Fig. 13)--an initial fast stage, a secondary induction period, followed by a third catalytic stage. Detailed

0 5 NiBX

0 I I ~ I

T i m e ( r a i n )

Fig. 13. Decomposition of cumene hydroperoxide (CHP), 10--2M in chlorobenzene at I I0°C in the presence of nickel dithiolates. (After reference 36, with permission.)

product studies 36'a7 have shown (see Fig. 14) that, in the case of NiDBP, Table 2, compound 2, R = nBu, the first rapid stage is associated with the formation of disulphide (see Scheme 5) which is then slowly converted to secondary nickel-free oxidation products of which the sulphonic acid, IX, is the most important since it readily loses SO2 to give the compound X which can be seen from Fig. 14 to be a relatively stable end product in the system. Figure 14 also shows that, although cumene hydroperoxide (CHP) is decomposed from the beginning of the reaction, phenol, the product of the ionic decomposition of CHP, does not start to be formed

116 Gerald Scott

/ / S \ . / S \ (RO)2 P \ S / NI \ S : P(OR)2

NiDRD

S O II II

(RO)2P__S__O H < Roon II

O IX

S S II II

ROOH ' ( R O ) 2 P - - S - - S - - P ( O R ) 2

VII, DRDS

R O O H

(RO)2 P : S \ S - - O H

II O

S II

(RO)2P__O H + SO 2 ROOH > S O 3

Scheme 5. X

VIII

Antioxidant mechanisms of nickel dialkyl dithiophosphates.

1.o ~

0.8 "1" U

PHENOL

i 1260 O-SO 2-

~ , ° °° ° •

X 0 580 • e%%%%.

o , \ , . o , 0 > . . . . I _ o , z

~N ~ k / / ~ " "OH ,I~. ' ~ o o.4 ,o~o i , \ ' ~ , \ I Z " ...... - o . , ~

z ' "

2 S=O L) o.2 - ......... 7. o.2 z O u

3.,~ L ~ " I o 0 5 10 15 20 25 30 35

REACTION TIME (MIN)

Fig. 14. Formation and decay of oxidation products of nickel dibutyldithiocarbamates (NiDBP, 2 x 10-*M) by cumene hydroperoxide (CHP, 10-2M) in chlorobenzene at 110 °C. Also shown are the decay curves for CHP ( . . . . . . ) and the formation of phenol

( - - - I~) . (After reference 36, with permission.)

Photo-oxidation and photo-stabilisation of polymers

TABLE 4 Effect of Oxidative Processing on the UV Stability of Polypropylene-

Containing Antioxidants and UV Stabilisers

117

Stabiliser Embrittlement time (h ) Closed mixer Open mixer

UV 531 835 295 MDRP (M = Ni, R = C4H9) 1 370 780 DRDS (R = iC3H7) 285 400 Control (no additive) 90 45

until after 10min when the intermediate sulphonic acid (IX) begins to break down. This clearly identifies the antioxidant activity with the sulphur acids formed from the nickel dithiophosphate by hydroperoxide oxidation. 3 6.3 7

Severe processing has a deleterious effect o n the nickel dithiophosphates (see Table 4) but, as might be expected from Scheme 5, it has a beneficial effect on the uv stabilising activity of the disulphide, VII. A combination of a uv absorber with NiDBP and the disulphide, DBDS, gives a synergistic stabiliser (see Table 5) but the activity of this system is powerfully augmented by oxidative processing. This is the kind of operation that the polymer manufacturer or polymer converter would normally never carry out since, as can be seen from the effect of oxidation processing in the control, it leads to a severe reduction in uv stability due to the formation of sensitising hydroperoxides. In the present instance,

TABLE 5 Effect of Oxidative Processing on the Activity of a Synergistic Combination

of Antioxidants

Stabiliser Concentration (g/lOOg)

Embrittlement time (h)

Normally Oxidatively processed processed*

Control (no additive) -- 90 DODS 0.1 "] NiDBP 0.1 ~ 2 100 UV 531 0.2

75

F> 4 500

*As described in British Patent No. 2, 117, 779, October, 1983.

118 Gerald Scott

however, the peroxides are used beneficially to convert the ant ioxidant precursor to the effective ant ioxidant during the processing operation. This is an interesting principle which is expected to find commercial application.

C H A I N - B R E A K I N G A N T I O X I D A N T S (CB-A)/CB-D) AS U L T R A V I O L E T STABILISERS

As was seen earlier, convent ional CB-D antioxidants are not efficient uv stabilisers because they are rapidly destroyed by uv light. This problem can be partially overcome by incorporat ing a uv stabilising group into the molecule which extends the durat ion of the ant ioxidant function. A new class of uv stable ant ioxidant has recently emerged which is uv stable because it does not absorb light at all. The best known commercial example of this is Tinuvin 770 (Table 3, c o m p o u n d 1), but the simpler analogues of this c o m p o u n d (e.g. X 1, R = H or Me) behave in exactly the same way and have been extensively investigated. 22'38'39

Me Me Me

Ro e. x - - ~ M e k - - ~ Me -

Me Me Me XI XII Kil l

The most striking feature of the behaviour of these compounds is their rapid disappearance during the very early stages of photo-oxidat ion with the format ion of the derived nitroxyl, X | l . 22 The nitroxyl itself is rather more effective as a uv stabiliser than the parent amine (see Fig. 15) and the hydroxylamine, XIII which is also formed during photo-oxidat ion, is more effective still 2~ (see also Table 6). The nitroxyl concentrat ion assumes an almost stationary state during the later stages of photo- oxidation (see Fig. 16), irrespective of the nature of the starting material, suggesting that a cyclical mechanism similar to that known to be involved during the processing of P P in the presence of hindered nitroxyl radicals is involved in the photo-stabil isation process. This is outlined in Scheme 4 in which A. = X | I and A H = X I I | . However, under photo-oxidative condit ions, it is known 22 that a macroalkylhydroxylamine may be formed as an intermediate, also as a transient, and two rather different

Photo-oxidation and photo-stabilisation oJpolymers l 19

.... / •

/ /

/

o I I I I I 1 Bo 3oo 4 so 60o 7so 9(x,

UV I R R A D I A T I O N TIM]£ (H,

Fig. 15. Photo-oxidation of P P as measured by the formation of carbonyl (1715 cm 1). 1. Control, no additive. 2. Tinuvin 770 (3 x 10-4mol/100g). 3. Bis-nitroxyl of Tinuvin 770 ( 3 x l 0 - 4 m o l / 1 0 0 g ) . 4. Xll, R = H ( 6 x l 0 - 4 m o l / 1 0 0 g ) . 5. Xll l , R = H

(6 x 10 4 mol/100 g). (After reference 27, with permission.)

O0

0 9 , 0

x

"7

7 . 5

6 . 0

4 . 5

c~ 3 . 0

b.

z

O ~ i I _ _

0 150 3 0 0 45}0 6 0 0 7 5 0

I R R A D I A T I O N TI~. IE, h

Fig. 16. Nitroxyl radical concentration in PP during ultraviolet exposure. 1. Tinuvin 770 {3 x 10-4mol/100g). 2. Xl | , R = H (6 x 10-4mol/100g). (After reference 27, with

permission.)

120 Gerald Scott

TABLE 6 Effectiveness of Tinuvin 770 and Related Oxidation Products as

UV Stabilisers for Polypropylene

Additice Embrittlement time (h)

None 90 Tinuvin 770

(Compound 1, Table 3) 750 Bis-nitroxyl of Tinuvin 770 960 XII 920 XIII 1 040

mechanisms have been proposed for the regeneration of nitroxyl from O- alkylhydroxylamine. The first involves the reaction of an alkylperoxyl radical with the alkylhydroxylamine (reaction (3)). 4°'41

C H 3 C H 3 I I

~ C H 2 - ROO. ___CCH2__ + ) N - - O . (3) I I

0 0 xn J I

N OR / \

XIV

The second 26'27 assumes that the formation of the macroalkylhydroxylamine is inherently reversible, particularly in the presence of light, and that the ultimate terminating step is the oxidation of the macroalkyl radical by nitroxyl (reaction (c), Scheme 6). Although primary alkylhydroxylamines which are not antioxidants are relatively stable, 42 secondary--and particularly tertiary--alkylhydroxylamines are rapidly converted in the presence of air to the nitroxyl radical 22 with the formation of oxidised products of the alkyl radical. Reactions (b) and (c) in Scheme 6 will therefore be in direct competition, and the ratio of the two processes will depend on the concentration of oxygen in the system and the strength of the C - - H bond on the/3-carbon atoms adjacent to the

H . H . H

h h j


(Xl) ~ N - - H ROOH). (a)

S c h e m e 6.

CH 3 CH 3 I I

-~ .CH 2 - (R.) ~ C = = C H - -

(x,,) )N--OH . ' ~ / (Xlll)

CH 3 f

• CCH2__ I

O I

N (xw) / \

f I I

ROOH ~ RO"

f ROOH R O O .

J

/

, , " . . . . _ _

Possible mechanisms involved in the catalytic antioxidant activity of nitroxyl radicals during the ultraviolet stabilisation of PP.

carbon centred radical. This C - -H bond is much weaker than a normal C- -H bond ( ~ 30 ~o) due to the delocalisation of the unpaired electron by hyperconjugation. Unsaturation has been shown to build up in PP during photo-oxidation in the presence of XI. This can be measured either by following the formation of the infra-red band at 1640cm- 1 (see Fig. 17) or by chemical estimation (catalytic iodination) which shows a linear correlation with the infra-red method. In a model substrate (methyl cyclohexane) both vinylene and vinylidine have been shown to be formed. 43 Although the nature of the nitroxyl regeneration step still remains to be resolved and must almost certainly involve the oxidation of free hydroxylamine, alkyl hydroxylamine, or both, by the peroxides formed in the cycle, there seems no doubt at all that reaction of macroalkyl radicals with nitroxyl is a key step. The occurrence of this reaction has considerable theoretical significance because, as was

0 . 2 0

2

0 . 1 6

z 0 . 1 ~

0 . 0 ~

0 . 0 4

122 Gerald Scott

0 1~0 30'0 ' ' 6 0 0 7150 4 5 0

IRRADIATION TIME, h

Fig. 17. Formation of unsaturation in PP containing ultraviolet stabilisers. 1. Tinuvin 770 (3 x 10-4mol/100g. 2. XII, R = H (6 x 10-*mol/100g). 3. Xil l , R = H

(6 x 10-4 mol/100 g). (After reference 27, with permission.)

mentioned earlier, it has been assumed in the .past that termination of autoxidation at normal oxygen pressures occurs almost exclusively by bimolecular reaction of alkylperoxyl (reaction (4)) and that the reaction of alkyl radicals (reactions (5) and (6)) can be ignored.

2ROO- • Disproportionation products (4)

ROO. + R. • ROOR (5)

2R- • R R (6)

This is certainly correct in most liquid hydrocarbons and Fig. 18, which is taken from the early studies of Bateman and Morris, 44 indicates the contribution of the three possible termination processes with increase in oxygen pressure for two oxidisable olefinic substrates at 318 K. It is evident that, even for the more oxidisable substrate, phytene, reaction (4) becomes the predominent process even at 100mm pressure. The autoxidation of an essentially saturated hydrocarbon, such as isopentane, will terminate by alkylperoxyl radical disproportionation to even lower pressures. Denisov 4s has calculated that [R.]/[ROO.] in this substrate is 5 × 10- 6 at 373 K and consequently a CB-A antioxidant would have to be several orders of magnitude more reactive towards alkyl radicals than oxygen (see Scheme 6, reactions (b) and (c)) if it were to be an effective inhibitor. However, Denisov has also shown that [PP-] / [PPOO.] = 2 × 10 -3 in PP at 371 K. This increase is probably sufficient to account


m

IOC

6C 8C X ~ 00"

4C

1 I0 I00

eo - .

o.

60

4o

2c

i 1o 1oo Oxygen pressure(turn)

Fig. 18. Contribution of alkylperoxyl and alkyl in termination reactions in the autoxidation of ethyl linoleate (top curve) and phytene (bottom curve) at 45 °C. (After


for the very high effectiveness of hindered amines and their oxidation products at low concentrations in the polymer. 46 (It has been shown that they are even uv stabilisers in a liquid hydrocarbon (methyl cyclohexane) at somewhat higher concentrations than normal.)

If further research confirms the generality of the alkyl radical trapping (CB-A) mechanism, this opens up the possibility of devising antioxidants based on other reversible redox catalysts. The combination of galvinoxyl and its reduced form (reaction (7)) is one such system which is known to be an effective melt stabiliser for PP. However, in the melt, the rate of radical formation is higher, and the concentration of oxygen is lower, than in photo-oxidation.

This redox combination does show a reasonable level of uv stabilising effectiveness (220h to embrittlement compared with 90h for the

124 GeraM Scott

tBu tBu tBu tBu

H

tBu tBu tBu tBu G. G H

unstabilised PP control). The evidence suggests that, in this case, as in the case of the aromatic nitroxyls, the operation of the CB-A/CB-D mechanism is limited by the inherent photo-sensitivity of the conjugated system. However, this should encourage the search for alternative redox antioxidants.

R E F E R E N C E S

I. G. Scott, Atmospheric oxidation and antioxidants, Elsevier, London and New York, 92 et seq. (1965).

2. J. E. Guillet, Polymers and ecologicalproblems. (Guillet, J. (Ed.)), Plenum Publishing Corporation (1973).

3. G. Scott, Polymers and ecological problems. (Guillet, J. (Ed.)), Plenum Publishing Corporation, 27 (1973).

4. D. Gilead and G. Scott, Developments inpolymer stabilisation--5. (Scott, G. (Ed.)), Elsevier Applied Science Publishers, London, 71 (1982).

5. L. Bateman and G. Gee, Proc. Roy. Soc., A195, 376, 391 (1948-9). 6. J. T. Martin and R. G. W. Norrish, Proc. Roy. Soc., A220, 322 (1953). 7. R. G. W. Norrish and M. H. Searby, Proc. Roy. Soc., A237, 464 (1956). 8. F. H. Rugg, J. T. Smith and R. C. Bacon, J. Polym. Sei., 13, 535 (1954). 9. J.D. Burnett, R. G. J. Millar and H. A. Willis, J. Polym. Sei., 15, 592 (1955).

10. L. Bateman and H. Hughes, J. Chem. Soc., 4595 (1952). 11. G. Scott, Atmospheric oxidation and antioxidants, Elsevier, London and

New York, 276 et seq. (1965). 12. K. B. Chakraborty and G. Scott, Europ. Polym. J., 15, 731 (1977). 13. G. Scott, Advances in Chemistry Series, 169, 30 (1976). 14. K. B. Chakraborty and G. Scott, Polymer, 18, 98 (1977). 15. K. B. Chakraborty and G. Scott, Polym. Deg. and Stab., 1, 37 (1979). 16. A. Ghaffar, A. Scott and G. Scott, Europ. Polym. J., 12, 615 (1976). 17. B. B. Cooray and G. Scott, Polym. Deg. and Stab., 3, 127 (1980-1981). 18. G. Scott, Developments in polymer degradation--1. (Grassie, N. (Ed.)),

Elsevier Applied Science Publishers Ltd, London, 205 (1977). 19. G. Scott, J. Polymer Sci. Symp. No. 57, 357 (1976). 20. C. H. Chew, L. M. Gan and G. Scott, Europ. Polym. J., 13, 361 (1977). 21. D. J. Carlsson, A. Garton and D. M. Wiles, Macromoleeules, 9, 695 (1976). 22. D. J. Carlsson, A. Garton and D. M. Wiles, Developments in polymer

stabilisation--l. (Scott, G. (Ed.)), Elsevier Applied Science Publishers Ltd, London, 219 (1979).


23. B. Ranby and J. F. Raber, Photodegradation, photooxidation andphoto- stabilisation ojpolymers. John Wiley and Sons, London and New York, 362 et seq. (1975).

24. S. A1-Malaika and G. Scott, Degradation and stabilisation ofpolyolefins. (Allen, N. S. (Ed.)), Elsevier Applied Science Publishers Ltd, London, 283 (1983).

25. S. A1-Malaika and G. Scott, Degradation and stabilisation ofpolyolefins. (Allen, N. S. (Ed.)), Elsevier Applied Science Publishers Ltd, London, 247 (1983).

26. R. Bagheri, K. B. Chakraborty and G. Scott, Chem. Ind., 865 (1980). 27. R. Bagheri, K. B. Chakraborty and G. Scott, Polym. Deg. and Stab., 4, 1

(1982). 28. A. A. Katbab and G. Scott, Europ. Polym. J., 17, 559 (1981). 29. K. J. Humphris and G. Scott, J. Chem. Soc., Perkin I1, 617 (1974). 30. G. V. Hutson and G. Scott, J. Polym. Sci. Part C, Syrup. No. 40, 67 (1973). 31. K. B. Chakraborty, G. Scott and W. R. Poyner, Plastics and Rubb.

Processing and Applications, 3, 54 (1983). 32. K. B. Chakraborty and G. Scott, Chem. and Ind., 237 (1979). 33. K. B. Chakraborty and G. Scott, Polym. Deg. and Stab. (In press.) 34. G. Scott, Pure and App. Chem., 52, 365 (1980). 35. K. B. Chakraborty and G. Scott, Europ. Polym. J., 13, 1007 (1977). 36. S. A1-Malaika, K. B. Chakraborty and G. Scott, Developments in polymer

stabilisation--6. (Scott, G. (Ed.)), Elsevier Applied Science Publishers Ltd, London, 73 (1983).

37. S. Al-Malaika and G. Scott, Polymer, 23, 1711 (1983). 38. V. Ya Shlyapintokh and G. B. Ivanov, Developments in polymer

stabilisation--5. (Scott, G. (Ed.)), Elsevier Applied Science Publishers Ltd, London, 41 (1982).

39. F. Tudos, G. Balint and T. Kelen, Developments inpolymer stabilisation 6. (Scott, G. (Ed.)), Elsevier Applied Science Publishers Ltd, London, 121 (1983).

40. G. A. Kovtun, A. Alexandrov and V. A. Goluev, lzv. Akad. Nauk SSSR, Set. Khim., 2197 (1974).

41. Y. B. Shilov and E. T. Denisov, Vysok. Soed., A16, 2313 (1974). 42. H. Berger, T. A. B. M. Bolsman and D. M. Brouwer, Developments in

polymer stabilisation 6. (Scott, G. (Ed.)), Elsevier Applied Science Publishers Ltd, London, I (1983).

43. K. B. Chakraborty and G. Scott, Polymer, 21,252 (1980). 44. L. Bateman and A. L. Morris, Trans. Farad. Soc., 49, 1026 (1953). 45. E. T. Denisov, Developments in polymer stabilisation--5. (Scott, G. (Ed.)),

Elsevier Applied Science Publishers Ltd, London, 23 (1982). 46. G. Scott, Developments in polymer stabilisation--7. (Scott, G. (Ed.)),

Chapter 2, Elsevier Applied Science Publishers Ltd, London (1984).

developments in the photo-oxidation and photo-stabilisation of polymers

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