Mechanisms of antioxidant action: Synergism between a 2-hydroxybenzophenone uv stabiliser and autosynergistic antioxidants in polypropylene

Poltmer Degradationand Stabditt 2 (1980) 309-319

MECHANISMS OF ANTIOXIDANT ACTION: SYNERGISM BETWEEN A 2 - H Y D R O X Y B E N Z O P H E N O N E UV STABILISER AND AUTOSYNERGISTIC ANTIOXIDANTS IN POLYPROPYLENE GERALD SCOTT & M. FAUZI YUSOFF

Department of Chemistry. University of Aston in Birmingham, Gosta Green. Birmingham B4 7ET. Great Britain (Received: 23 November, 1979)

ABSTRACT

Synergism between a uv stabiliser (2-hydroxy-4-octoxybenzophenone (I)) and phenolic antioxidants has been studied during the photo-oxidation of polypropylene. It has been found that hindered phenols containing benzylic sulphur (II) are more effective synergists at the same molar concentration than conventional hindered phenols and that optimum synergism occurs at high molar ratios (I/II). The autosynergistic phenols (II) are shown to protect the uv stabiliser against the efJects of hydroperoxides under photo-oxidative conditions by catalytically destroying them and scavenging radicals formed from them. The "uv absorber' (I) also appears to deactivate excited speciesJormed in the phot o-decomposition of oxidation products of (H).

INTRODUCTION

It has been shown that 2-hydroxy-4-octoxybenzophenone (HOBP (I)) is readily destroyed by hydroperoxides in auto-oxidisable substrates during uv exposure I and that this process, rather than direct photolysis, is primarily responsible for the durability of this class of uv stabiliser in polymers during service. The synergism observed between the transition metal dithiocarbamates and HOBP has been attributed in part to the ability of the catalytic peroxide decomposers (PD--C) to protect the uv stabiliser from hydroperoxides during processing and subsequent uv exposure. 2"3 The metal dithiocarbamates are, however, readily photolysed and a complementary aspect of the synergism observed between the two classes of antioxidant is the protection of the dithiocarbamates from photolysis during uv exposure by the more stable 'uv absorbers', z'3 309 Polymer Degradation and Stability 0141-3910/80/0002-0309/$02.25 © Applied Science Publishers Ltd, England, 1980 Printed in Great Britain

310

GERALD SCOTT, M. FAUZI YUSOFF

OH

~

OC8H17

(HOBP)

(I) Chain-breaking (CB--D) antioxidants also synergise with the 'uv absorbers 'z - '~ and it has recently been shown that autosynergistic phenolic sulphides (II(c)) are very effective synergists with 'uv absorbers' when they are molecularly dispersed by reaction with the polymer backbone. 5'6 OH tBu~]/tBu

CHzSR (11)

(a)

BHBM, R = H

(b) (c)

BHBM-12, R = C t 2 H 2 5 R = polymer

The purpose of the present investigation is to examine the effectiveness of BHBM and BHBM-12 as synergists with HOBP (1) in the uv stabilisation of polypropylene and to compare them with commercial hindered phenol antioxidants TBC (11I) and 1076 (IV).

(TBC)

OH

OH

CH 3

CH2CH2COOClsH37

(111)

(1076)

(IV)

EXPERIMENTAL

Materials Unstabilised polypropylene and the antioxidants used were all obtained as described previously.7 HOBP was a commercial product, UV531, obtained from American Cyanamid. Procedures The processing and fabrication of polymer films have been described previously. 7 In the present study, the polymer was processed for 5 min at 180°C and films (1.5 × 10-2cm) for uv exposure were obtained by compression moulding at 180°C.

I:il~, I,

[OO

L

20 80

~b G5

I

i

50 50

i

(,<, 35

80 20

i

i

1(]~) 0

ii~J|ta '~ IIOI~P Ino[~ '~. 'I'[~C

Rclati¢mship bctwec, IIOl'll' concentralion and uv a b s o r b a . c e a*t 330rim in polypropylcn¢ in the presence of TBC (total additive co,cc.tratio,. I 0 "~tool/100 g).

IO

12

1"1"1

"z

"4

-..I

2.,"

_x

.-4

"tl

©

:r

312

GERALD SCOTT, M. FAUZI YUSOFF

Ultra-violet exposures, uv absorbance decay and carbonyl formation were carried out as described previously.-' In order to establish that the antioxidants did not interfere with the uv absorbance of the uv absorber (H O B P), various molar ratios of antioxidants and HOBP were processed into PP (at total concentrations 10-3 mol/100 g) and good straight line plots were obtained for the absorbance ()'max 330 rim) used to follow the decay of HOBP during photo-oxidation (see Fig. 1). RESULTS

Figure 2 compares the effectiveness of the commercial hindered phenols, TBC (III) and Irganox 1076 (IV), with the autosynergistic phenolic thiol (II(a)) and phenolic 80O

700

600

HOBP

(I)

500

4oo

q-300

BHBM

1076

(IIa)

(IV)

200 -

t

BHBM-12

TSC

(:I)

(IIb)

1(30

lo t Concentration,

m o l / I O O g x iO 4

Fig. 2. Ultra-violet embrittlementtime of polypropylenefilms as a functionof additiveconcentration.

MECHANISMS OF ANTIOXIDANT ACTION

313

sulphide (ll(b)), which have previously been shown: to be the most effective antioxidants of this type in polypropylene. Like the commercial antioxidants ((III) and (IV)) and unlike the uv absorber ((I)), the phenolic sulphur compounds show only a small increase in effectiveness with concentration. All the antioxidants exhibit synergism in combination with HOBP. This is shown typically for 1076 and BHBM-12 in Figs 3 and 4 which also relate the growth of carbonyl in the polymer to the decay of the HOBP absorbance at 330 nm. The latter was shown by independent experiment to be linearly related to the actual concentration of HOBP (see Fig. 1). The analogous curves for TBC and BHBM were similar to those for 1076 and BHB M-12, respectively except for the lengths of the induction periods and the first-order rate constants for HOBP decay. These are compared in Table 1 for the four synergists at various molar ratios, as are the embrittlement times and the calculated synergistic effects at the same molar ratios. Figure 5 indicates that all the antioxidants show a synergistic optimum at low molar ratios of antioxidants but that autosynergistic sulphur-containing antioxidants are much more effective than the conventional hindered phenols.

DISCUSSION

It is clear from Figs 3 and 4 that a primary function of all the phenolic antioxidants is to inhibit the removal of HOBP during the induction period and to retard it during the first-order decay process. Both effects decrease as the molar rates of u: stabiliser to antioxidant decrease in the combination and (II) actually appears to become a sensitiser for HOBP destruction when present in excess (I):(!I) = 20-80. In spite of this, there is still synergism between the two components, even at low molar ratios, although this is lower when expressed as % synergism (see Table 1) and is of no practical importance when (I):(I1)= 1:3. At high (I):(II) ratios the synergism achieved is of practical and theoretical importance (see Fig. 5). The effect observed in the case of the sulphur-containing phenols is significantly greater than that found for the commercial antioxidants ((Ill) and (IV)) and is also greater than that observed with the more effective commercially available peroxide decomposers, the transition metal dithiocarbamates) It has been shown previously I that in the absence of substantial amounts of carbonyl (the situation found during the induction period in the inhibited photooxidation of polypropylene), HOB P is destroyed rapidly by the radical products of hydroperoxide photolysis. The phenolic sulphides have been shown to be among the most effective thermal antioxidants so far discovered, v due in part to their ability to destroy hydroperoxides in a non-radical process, r'9 It seems likely, therefore, that their protective effect on HOBP is due to this peroxidolytic activity which is superimposed on the chain-breaking activity which they share with the commercial hindered phenols ((III) and (IV)). Like other peroxide decomposers based on sulphides, the initial step in their action involves the formation of a sulphoxide.

4

(a)

300

i

9(b)

i

600

i

900

i

/

(f)

1200 I r r a d i d t i o n time, h

(e)

1500

i

(9)

I

I

I

I

i

18(30

2100

_____L

-O.4

-0.3

-0. 2

H rr

Fig. 3. Relationship between tile rate of photo-oxidation (a) to (11)of polypropylene and tile rate of decay of tile uv absorbance of tlO1:11"(a') to (h') ill the presence of 1076 (total additive concentration, 10- 3 mol/100 g). (a) Control (no additive); (b) 1076 only; (c) (c'), 20 70 H OBP; (d) (d'), 35 70 HOB P; (e) (e'), 5070 HOBP; (f) 6570 H O B P ; (g) (g'), 8070 HOBP; (h) (h'), 10070 HOBP.

0

Ol

~2

x

o

-0.1

©

,.<

> c7. N

© .-t .-.-I

t"

>

O 1"1"1

4~

L,J

o

(a)

3OO

/ (b)

(c')

90O

(d')

I I I I

h

15OO

(e')

(f)

(e)

2

GOO lrradlcitlon

12OO tt.~a,

18OO

2 IOO

(q)

-0.3

-0.2

o

Fig. 4. Relationship between tile rate of photo-oxidation (a)to(h)ofpolypropylene and the rateofdecay of the uvabsorbanceofHOIJP(a')to(h')in the presence of BHBM-12 (total additive concentration, 10- 3 g/100 g). (a) Control (no additive); (b) BHBM-12; (c) (c'), 20 ~ HOBP; (d) (d'), 35 ~'o HOBP; (e) (e'), 50~0 HOBP; (f) 6 5 ~ ItOBP; (g) (g'), 80 ~,,, HOBP; (h) (h'), 100~ HOBP.

d

"4 3

x"

,1 cf

o~

O.1

x

(g')

. . . . . . . . . . .

04

b

I

[

t.j~

La.a

>¢3 ,-4

z

6 X 5

"11

©

> 'Z

trl

750 1420 1170 910 680 500 230

0 20 35 50 65 80 100

-123 115 102 90 78 ---

3"8 1.3 1'7 2"3 7'1 8'3 --

HOBP/TBC Syn* k (%) (104~ - ' )

W h e r e : E, = E m b r i t t l e m e n t E~ = E m b r i t t l e m e n t E, = Embrittlement E 2 = Embrittlement

time time time time

of of of of

750 1590 1350 1115 980 705 260

T. (h~

synergistic mixture. control (no additive). a n t i o x i d a n t 1. a n t i o x i d a n t 2.

750 2100 1850 1547 1240 830 270

(hi

7"~

( E l - E,.) + ( E 2 -

E,.)

ka

3"8 0.8 1-1 1.6 2-9 3-7

(104h ' )

-230 232 233 232 176 . . . . .

(%)

Srn*

IIOBP/BIIBM

- (E, - E,.) + (E 2 - E,.)

3.8 1.1 1.4 1.8 4-1 6"0

(E~ - E,.)

-139 132 129 127 127 . . . . . .

IIOBP/I076 Svn* ka (%) (lO'*h-*)

S y n e r g i s m (~/,~) =

* S y n e r g i s m w a s c a l c u l a t e d u s i n g the f o r m u l a : 8

(IT~

(tool%)

Antioxidant

750 2350 1935 1600 1260 840 250

7'~. (hi

-267 250 251 250 201 -

3.8 0-8 I-I 1.5 2.8 3.5

tlOBP/BIIBM-12 Svn* ka ('~'/,) (iOah - ' )

TABLE 1 COMPARISON OF EMBRITTLEMENT TIMI,~S (TE) SYNERGISM (SYN) AND FIRST-ORDER RATE CONSTANTS FOR TIlE DECAY OF H O B P (ka) IN SYNI-RGISTIC SYSTEMS WIT}I ANTIOXIDAN'rS AT A TOTAL CONCENTRATION OF 10 ,3 t o o l / 1 0 0 g

0

b,I N ,<

0

t"rl >. rt~

o~

MECHANISMS OF A N T I O X I D A N T A C T I O N

317

2500

I

E3H B,'%4- t 2

I i

2000

1076 1500

e-

E 100(

E = "E

W 50(

OL

0 ....

2'5

, 50 Anttoxidant,

Fig. 5.

,

75

I 1 O0

tool %

Ultra-violet lifetimes of polypropylene films containing synergistic combinations of HOBP and phenolic antioxidant as a function of the molar proportion of antioxidants.

These are known to be readily photolysed to free radical products ~°'~t and may actually catalyse the photo-oxidation of some polymers. ~ The process which is outlined for BHBM-12 in Scheme (1) is reflected in the rapid initial rate ofcarbonyl formation in polypropylene containing this antioxidant alone (Fig. 4, curve (b)) and contrasts with the behaviour of Irganox 1076 (Fig. 3, curve (b)) and TBC (not shown) which are auto-accelerating in the normal way. It is clear, however, that in spite of this, an antioxidant is produced (as evidenced by the inflexion in the carbonyl formation curve (Fig. 4, curve (b)).

318

GERALD SCOTT, M. FAUZI YUSOFF

OH

OH

tBu~tBu

RO ~OH~' t B U ~ o B U l II

CH2SC12H25(I) i b / OH tBu.~tBu

/ r ~,,,

CH2SCI2H25 (dl~HOBP Non-radical products

(c) )

CH2SCI2H25(v) OH tBu~tBu ~(,~J

+ C12H2580"

"CH2T ' ¢x~'x~

tBu

(1)

tBu, ~ HO-~CH/CH2--@OH tBu

tBu

tBu

1,°

O = ~ C H - - C H ~ tBu

tBu ~=O tBu

(v]) The presence of HOBP in the system suppresses the photo-pro-oxidant reaction (Fig. 4, curve (d)) and is itself destroyed more rapidly (Fig. 4, curve (d')), indicating rapid removal of the triplet sulphoxide or its derived free radical products by HOBP (Scheme (1) (d)). The nature of this process is not at present understood and may also involve quenching of other photo-sensitive oxidation products such as the stilbenequinone (VI) which is a known photo-sensitiser. HOBP is known to be able to interact with triplet carbonyl, at least partially, by a sacrificial mechanism. 1 Table 1 and Fig. 5 illustrate the higher level of synergism that can be obtained with the autosynergistic (CB--D/PD--C) antioxidants than is possible with the simple CB--D antioxidants alone. Figures 3 and 4 show that, at the same concentration and molar ratio, BHBM-12 gives a longer induction period to the first-order decay of H O B P than does 1076(I V). Table 1 shows that the first-order rate constant for the

MECHANISMS OF ANTIOXIDANT ACTION

319

decay is also lower for the autosynergists than for the simple phenols at the same molar concentration. From Table 1 there appears to be an inverse correlation between the embrittlement times and the first-order rate constants for H O B P decay. It is somewhat surprising that such a small proportion of autosynergistic antioxidant can protect the uv absorber so effectively. However, it is known from earlier studies on other sulphur-containing antioxidants T M that they are converted most effectively to Lewis acid products when present in molar deficiency with respect to hydroperoxides present in the system. Furthermore, an excess of H O B P in the system is necessary in order to deactivate photo-excited products formed from the sulphides.

ACKNOWLEDGEMENTS

We are grateful to the National University of Malaysia for granting study leave to one of us (MFY) and to ICI (Plastics Division) Limited for the provision of unstabilised polypropylene.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

K. B. CHAKRABORTYand G. SCOTT, Europ. Polym. J., 15, 35 (1979). K. B. CHAKRABORrVand G. SCOTT, Europ. Polym. J., 13, 1007 (1977). K. B. CHAKRABORTYand G. SCOTT, Poly. Degrad. and Stab., 1, 37 (1979). G. SCOTT, Atmospheric oxidation and antioxidants, Elsevier. London and New York, pp. 216. 299 (1965). G. Scow, Plastics and rubber: Processing, 41 (June, 1977). G. SCOTT (Ed.), in De~'elopments in Polymer Stabilisation--l, Applied Science Publishers Ltd, London, 309 (1979). G. SCOTT and M. F. Yuso~, Europ. Polym. J., 16, 497 (1980). G. SCOTT, Atmospheric oxidation andantioxidants, Elsevier. London and New York, p. 203 et. seq. (1965). V. M. FARZALIEV.W. S. E. FERNANDOand G. ScoT-r, Europ. Polym. J., 14, 835 (1978). J. R. SHELTONand K. E. DAVtES, Int. J. Sulphur Chem.. 8, 217 (1973). B. B. COORA¥ and G. SCOTT, Poly. Degrad. and Stab. (in press). C. ARMSTRONGand G. SCOTT, J. Chem. Soc., 1747 (1971). C. ARMSTRONG, M. A. PLANT and G. SCOTT, Europ. Polym. J., 11, 161 (1975). C. ARMSTRONG, M. J. HUSBANDSand G. SCOTT, Europ. Polym. J.. 15, 241 (1979).