Mechanism of antioxidant action: Nature of transformation products of dithiophosphates—Part 1. Their role as antioxidants in polyolefins

Polymer Degradation and Stability 22 (1988) 147-159

Mechanism of Antioxidant Action: Nature of Transformation Products of Dithiophosphates Part 1. Their Role as Antioxidants in Polyolefins*

S. AI-Malaika, M. Coker & G. Scott Department of Chemical Engineering and Applied Chemistry, University of Aston, Aston Triangle, Birmingham B4 7ET, UK (Received 14 January 1988: accepted 22 January 1988)

A BSTRA C T

It is shown that the oxidation ~?/" nickel dialkyl dithiophosphates by to'droperoxides at room temperature, as measured by 31pNMR, yields thiophosphoo'l monosulphide and thionophosphoric acid as major products when the molar ratio ~?f hydroperoxide to the nickel complex is high. Processing ~['thiophosphoo,l disulphide concentrates in polypropylene is also shown to result predominantly in the .formation of the sulphur acid and thiophosphoo'l esters in various forms. It is also demonstrated that the incorporation of a synthesised sample of thionophosphoric acid in poO'propylene q~ords comparable melt- and UV-stabilisation to that of thiophosphoo'l disulphide.

INTRODUCTION Metal dialkyl dithiophosphates and their corresponding disulphides are used as additives in lubricating oils and as antioxidants (melt, thermal and photo-antioxidants) in a variety of hydrocarbon polymers. 1'2 Hydroperoxides which are formed in the polymer during processing have a significant effect not only on the ageing performance of the unstabilised polymer, but also on the antioxidants incorporated during processing since these may be * This paper was presented at the Polymer Degradation Discussion Group Conference held in the University of Aston, Birmingham, UK, on 2-4 September 1987. 147 Polymer Degradation and Stabilio, 0141-3910/88/$03.50 © 1988 Elsevier Science Publishers Ltd, England. Printed in Great Britain

148

s. Al-Malaika, M. Coker, G. Scott

partially oxidised to further transformation products which can play an important role in the antioxidant activity of the stabilising system. 1.3 5 The nickel O,O-dialkyl dithiophosphates have been shown 1'2'6- s to be oxidised initially to the corresponding disulphide, which, in turn, is slowly transformed into low molecular mass sulphur acids amongst other products. The purpose of this investigation is to examine the nature of transformation products (studied by 31p NMR) formed in reactions of dithiophosphates with hydroperoxides in a hydrocarbon solvent and in polypropylene. The antioxidant potential of a major transformation product of dithiophosphates, the thionophosphoric acid, in PP will also be investigated. -

EXPERIMENTAL Materials

Nickel O,O-di-butyl dithiophosphate (NiDBP) and bis-O,O-di-butyl thiophosphoryl disulphide (DBDS) were prepared by modified versions of procedures described by Chamberlain & Drago, 9 and Mikeska l° as follows. Both the nickel complex and the disulphides were prepared from the corresponding ammonium salt. The latter was prepared by bubbling ammonia gas into a hexane solution of dithiophosphoric acid (prepared by addition of P2S5, 1"5 mole, to butanol, 6 moles, under nitrogen for 3-30 h at 80°C) to give a white solid product which was recrystallised from toluene. To prepare the disulphide, DBDS, a solution of iodine (0.16moles) in 25% aqueous potassium iodide was added to a stirred solution of the ammonium salt (boiled with charcoal and filtered) until the iodine colour persisted. Excess iodine was removed by washing the organic layer with sodium thiosulphate solution (0"1 molar). Pure DBDS was produced, as a yellow viscous liquid, by stripping the resulting dried, filtered solution under vacuum. Spectral analyses of DBDS (IR, 31p NMR, HPLC) are shown in Fig. 1. The nickel salt was prepared by addition of an aqueous solution of nickel (II) chloride hexahydrate (0-13 mole) to a solution of the recrystallised ammonium salt until the mixture turned purple. The organic layer was extracted with diethyl ether and the solvent removed leaving a purple liquid with a melting point of 16°C. Infrared spectrometry showed (P)--O--C, 1060-900 c m - 1; (--P)--O--(C), 875-730 c m - 1; - - ~ S , 660 c m - l(d); --P--S--Ni, 560cm-1; and - - N i - - S , 350cm-1; 31p N M R showed only one peak with a chemical shift of 94ppm; the UV spectrum was characteristic of nickel dithiolates with a major absorption band at 316 nm and two other bands at 280 and 225 nm.

Mechanism of antioxidant action: Part 1

149

530

1060-730 o

i

3000

b/

2000

1600 1200 Wave NO. (cm -1 )

~

800

400

85. I

.---79.2 c/

~

19.04

16.50

Fig. !. Infrared (a), 31p N M R (b), and HPLC (c) analysis of di-butyl thiophosphoryl disulphide. Numbers on the NMR spectrum are chemical shifts in ppm and on the HPLC chromatogram are retention times in minutes. Insets in (b) and (c) are signals of the pure sample. Conditions of HPLC are: 5 ~m Spherisorb-ODS column (25cm x 0-64), 225 nm wavelength for detection, 25% deionised water/75% isopropanol as eluent and 1 ml/min flow rate.

The O, Oodi-butyl thionophosphoric acid (DBTA) was prepared according to a procedure described by Foss; ~ 3 ~p N M R showed only one peak with a chemical shift of 63ppm. The triester O,O,S-TBDTP was prepared by the alkylation of sodium dialkyl dithiophosphate with an alkyl bromide. ~2 The multiplicity of 3~p N M R peaks (see inset of Fig. 5) of this sample may be due to isomerisation of the alkyl group in the molecule. Spectroscopic grade cyclohexane (Fisons) and tert-butyl hydroperoxide (TBH) (ex Akzo Chemie) were used as supplied without further purification.

150

S. AI-Malaika, M. Coker, G. Scott

The polymer used in this study was unstabilised polypropylene supplied by Imperial Chemical Industries as Propathene HF22. The commercial stabilisers Irganox 1076, octadecyl-3-(3,5-di-tert-butyl-4-hydroxy phenyl) propionate (Ciba-Geigy) and Cyasorb UV531, HOBP, 2-hydroxy-4octoxybenzophenone (American Cyanamid) were used as supplied.

Techniques Oxidation of NiDBP by TBH (25°C) Solutions of TBH (0.3-3"0M) in cyclohexane were added to previously prepared 0.3M solutions of NiDBP in cyclohexane, such that the [TBH]/[NiDBP] ratio was varied between 1 and 10. The reaction in each case was very rapid, with concomitant precipitation of a green solid. After 24 h, each reaction mixture was filtered to remove the precipitate which was then thoroughly washed with cyclohexane and dried under vacuum at room temperature while the filtrate was subjected to 31p N M R analysis.

Product analysis The natures of phosphorus-containing transformation products obtained from reactions of NiDBP with TBH at different molar ratios were identified using 31p N M R spectroscopy. The soluble portion of each reaction mixture (after 24h) was examined on a Jeol Fx-9Q Fourier Transform N M R spectrometer operating at 36.20 MHz. All spectra were measured with noise decoupling of the phosphorus-hydrogen spin-spin coupling and chemical shifts were referenced to an external standard of 85% phosphoric acid. In most cases, absolute structural assignment of the 31p N M R signals was accomplished on the basis of their identity with that of synthesised authentic standards, and product % yields (Relative Abundance) are based on the normalisation of all peak signals to 100%. This technique was also used for product analysis of solvent extracts of polypropylene films containing concentrates of thiophosphoryl disulphide (DBDS). Table 1 gives a summary of the structures, chemical names and codes of the major transformation products. The precipitates which developed during the reactions were filtered off and analysed by infrared spectroscopy (Perkin-Elmer 599) using the KBr disc technique.

Polymer processing and testing Processing of polypropylene in the presence of the required amount of antioxidants was carried out in a R A P R A / H a m p d e n torque rheometer at 180°C in the presence of restricted (CM) and excess (OM) oxygen and films (250 ~m thick) were pressed as described previously, s Infrared spectroscopy

Mechan&m of antioxidant action: Part 1

151

TABLE 1 A Summary of N i D B P Transformation Products

Chemical shi/? (ppm}

Structural .formula

Chemical name

Code

94"

[{ROI2PSS]2Ni

Nickel di-butyl dithiophosphate

NiDBP

96"

(RO)2IRS)PS13- 1~,

O,O,S-TBDTP

85"

[IRO)zPSS]2

84.1" 79"

[RO)2PSS]2S 2 [(ROI2PS]2S

63"

(RO~2PSOH

60 54

(RO)2PSH 15 (RSj2(RO)PO TM ~'

28

{RO)z(RS)PO TM j6

21 ",h

(ROI2POSH

O,O,S-lri-butyl dithiophosphate Bis-/di-butyl thiophosphoryl disulphide Bis-/di-butyl thiophosphoryl tetrasulphide Di-butyl thiophosphoryl monosulphide Di-butyl thionophosphoric acid Di-butyl hydrogen thiophosphate O,S,S-tri-butyl dithiophosphate O,O,S-tri-butyl thiophosphate Di-butyl thiophosphoric acid

17

[I ROtePOS]_, ~7

0.9

(ROJsPO TM

Bis-/di-butyl phosphoryl disulphide O,O,O-tri-butyl phosphate

DBPD O,O,O-TBP

DBDS DBTE DBMS DBTA DBHP

O,S,S-TBDTP O,O,S-TBTP DBTA

"Assigned on the basis of its identity with that of an authentic standard (prepared in this study). bThis is an isomeric form of DBTA

(using Perkin-Elmer 599) was used to evaluate the performance of film samples exposed to UV-irradiation (using a sunlamp/Actinic blue lamp cabinet), while melt stability was assessed by determining the melt flow index of the processed polymer samples using a Davenport melt flow indexer at 230°C under a load of 2.16kg and through a standard die (2-095 mm). RESULTS Products of the oxidation of NiDBP by TBH The chemical changes which occur during the oxidation of NiDBP by TBH at different molar ratios, as measured by 31p NMR, are qualitatively illustrated in Fig. 2. It is clear that the oxidation of NiDBP by TBH leads mainly to the formation of signals with chemical shifts at lower fields than that of the original NiDBP. This is indicative of the formation of nickel-free thiophosphoryl species and some highly oxygenated phosphoruscontaining transformation products, particularly at high [TBH]/[NiDBP] molar ratios of 4-10 (see Fig. 2). It was shown earlier 6'7'18 that the major products of reactions of nickel dithiophosphates with hydroperoxides at high temperatures are the corresponding disulphide and thionophosphoric acid and that the proportion of the products formed is a function of the molar ratio of

S. Al-Malaika, M. Coker, G. Scott

152 II

[TBHI/INiDBPI =

I

= 4

ITBHI/INiDBPi 60

94

54 (4

14 i i NID BP

79

•, -

ill,.t ,~ i

I

i

{j t

i

94 ITBHIIINiDBPI = 2

79

i

ITBHIlINiDBP] = I0

I

21

I

63 79

{

Fig. 2. 3,p N M R spectra of products formed at the end of reactions o f N i D B P (0"3M) and T B H at differentmolar ratios in cyclohexanc at 25°C. Numbers on signa{s are chemical shifts in ppm.

the dithiophosphate to the peroxide. In this study a careful examination by 31p N M R of products formed by reaction of N i D B P with T B H (at room temperature) reveals, similarly, that the nature and proportion of transformation products is highly dependent on the ratio of NiDBP to TBH. Figure 3 shows the amount of undecomposed NiDBP at each [TBH]/NiDBP] molar ratio, and the changes in the relative abundance (% yield) of the major transformation products. It is clear that the thiophosphoryl monosulphide and disulphide (with chemical shifts of 79 and 85ppm respectively) predominate at lower [TBH]/[NiDBP] molar ratios. The formation of the sulphur acids in different isomeric forms (6 = 63, 60, 21) is, however, favoured at higher molar ratios and it is worth noting that the monosulphide, DBMS, remains as the major product of the reaction at all ratios. A small number ofa~P N M R signals (96, 54, 28, 17 and 0.9 ppm) of very low intensities also appeared at the highest molar ratio (see Fig. 2 and Table 1). These contribute only about 10% of the total products and are

Mechanism of antioxidant action: Part 1

153

i00

8O (a)

C3 ~3 M o~

=u 4o O oe

(b) 20

(d) 1

2

3

4

~o~

~o

5

6

7

8

9

iO

o~ [~] i b 1 ~ ]

Fig. 3. Major product yield after complete reaction of NiDBP with TBH in cyclohexane at 25~C at different molar ratios [TBH]/[NiDBP]. (a) NiDBP--94A (b) DBMS--796, [(RO)2 PS]2S (47%). (c) DBDS--856 (9%). (d) DBTA--(32%), (RO)2PSOH 646, (RO)2PSH--60 6, (RO)2POSH--21 6.

(a)

I

=

J

1

1

2000

1800

I

P

I

l

I

"T

i000

800

600

I

(b)

{

I 4000

3500

3000

2500

WAVE

1600

1 4 0 0 1200

T 400

200

N U M B E R (cm -I)

Fig. 4. Infrared spectra (KBr disc) of an authentic sample of NiSO4 6H20 (a) and of the green precipitate formed in the oxidation of N i D B P by TBH in cyclohexane at 25°C (b).

154

S. AI-Malaika, M. Coker, G. Scott

assigned to the triesters O,O,S-TBDTP, O,S,S-TBDTP and O,O,S-TBTP, the phosphoryl disulphide DBPD and the inorganic tributyl phosphate O,O,O-TBP (Table 1). The green precipitate isolated at the end of the reactions was identified as hydrated nickel sulphate (NiSO4 nH20) which did not melt or decompose up to 300°C, although bubbling was noted at 100°C. The formation of the nickel sulphate was further confirmed by comparing its infrared spectrum with that of an authentic sample (Fig. 4). Transformation products of D B D S concentrates in PP

Figure 5 shows the 31p N M R spectra of concentrated solvent extracts (methanol, 50°C) of polypropylene films containing concentrates (2.5%) of

96.4 !

CM)

.~][9s.[i s 96.4

94

94 (oM)

t

96.,

98.8 85184"1

63

54

q Fig. 5. 31p NMR spectra of solvent extracts of PP films containing 2.5% DBDS processed in a closed mixer (CM) and an open mixer (OM). Inset shows the 31p NMR spectrum of a synthesised sample of the tricster O,O,S-TBDTP.

Mechanism t~f antioxidant action: Part 1

155

DBDS. This figure clearly indicates that identical products are formed during processing of DBDS under conditions of both restricted (CM) and excess (OM) oxygen, and these are mainly the thionophosphoric acid, DBTA (63 ppm) and triesters in various forms (see Table 1), except that in the case of the oxidatively processed concentrate additional signals due to the disulphide (DBDS) at 85 ppm (residual or regenerated) and thiophosphoryl tetrasulphide (DBTE) at 84.1 ppm are also present. Antioxidant activity of DBTA in P P Thionophosphoric acid, DBTA, was shown previously v to be one of the stable end products which is always formed from N i D R P during its antioxidant function at high temperatures. In this study it was further 2000

~"

m PP

I--

z i,u =E

[] []

1000

[] [] []

i

[]

O

NiDBP I-IOBP DBOS DBTA DBDS+ HOBP DBTA+ HOBP

~

Fig. 6. Comparison of photoantioxidant effectiveness (CM, 10 min) of DBTA with that of the nickel complex, the disulphide and the UV absorber HOBP in PP. The synergistic effect of the acid (DBTA) and the disulphide (DBDS) with HOBP is also shown. Concentration of each additive is 0"2%.

S. Al-Malaika, M. Coker, G. Scott

156

demonstrated that this acid is also formed from thiophosphoryl disulphide when it is processed at high temperatures in PP. The efficiency of DBTA in stabilising polypropylene against photo- and ~elt-degradation was therefore studied and its activity compared with that of the nickel complex (NiDBP) and the disulphide (DBDS) at the same concentration (0"2%). Figure 6 shows that the sulphur acid (DBTA) is slightly more effective than the disulphide DBDS as a photo antioxidant and that both are much less effective compared with the corresponding nickel complex. It was shown previously 19 that nickel dithiolates are, in general, more photolytically stable than their corresponding disulphides and this must account for the better activity of the nickel complex in the polymer under conditions of UV irradiation. Combinations of the disulphide (DBDS) and the acid (DBTA) with the commercial UV absorber, HOBP, show synergistic behaviour and the overall photostabilising activity becomes comparable to that of the nickel complex. Figure 7 illustrates that the melt stabilising activity of DBTA is better than that of the disulphide, the nickel complex and the commercial melt stabiliser, Irganox 1076. 10

8

x

w a

• []

pp HOBP

[]

I R G A N O X 10

[] [] I

NI DBP DBDS DBTA

6

o, '"

4

0

Fig. 7. Comparison of melt stabilising effectiveness (CM, 10 min of DBTA with that of the nickel complex, the disulphide, the UV absorber HOBP and the commercial hindered phenol, Irganox 1076, in PP. Concentration of each additive is 0.2%.

Mechanism of antioxidant action: Part 1

157

DISCUSSION It was shown earlier 6- 8,1s, 19 that transformation products formed during the antioxidant function of dithiolates are mainly responsible for their effectiveness as stabilisers in polymers. It was further shown that the disulphide, which is one of the first transformation products of nickel dithiolates, is subsequently converted to further oxidation products, of which the sulphonic acid is most important since it loses SO2 readily, and, in the case of dithiophosphates, it gives thionophosphoric acid (e.g. DBTA) which is a stable end product. The results shown here clearly illustrate that thionophosphoric acid itself is an effective melt stabiliser and synergises effectively with UV stabilisers giving effective photostabiliser systems. DBTA is formed as a transformation product from both the nickel complex and the disulphide (see Figs 2 and 5). Scheme 1 outlines the chemical reactions which occur during the reactions of NiDBP with hydroperoxides and shows some of the important transformation products which were identified in this study by 31p NMR. Thionophosphoric acid (DBTA) which becomes more evident at higher [TBH]/NiDBP] molar ratios (Fig. 2) is most likely formed via reaction of thiophosphoryl radical [(RO)2PS2] with hydroperoxide (reactions (d)--if). Under the low temperature conditions used in this study for the reactions of the nickel complex with hydroperoxides, the formation of DBTA via oxidation of the disulphide (route (1)-(f) seems to be unlikely since it was shown earlier 2° that disulphides of dithioic acids are unable to effect thermal decomposition of TBH at temperatures below 70°C. However, in the polymer its formation during the high temperature processing operation is most likely to be through the latter route. It was found 2x that thionophosphoric acid (DBTA, chemical shift of 63ppm) also exists in another stable isomeric form, the thiophosphoric acid (DBTA, chemical shift of 21 ppm), see Fig. 2, and the concentration of each of the isomeric forms in the system is a function of the amount of hydroperoxide available. The detailed chemistry of thionophosphoric acid and thiophosphoryl disulphide in model compounds, examined by 31p N M R under conditions similar to those typically used in polymer processing, will be the subject of further communications in this series. Examination of the nature of transformation products of thiophosphates has thrown some light on their mode of antioxidant activity. It is clear from the results that products formed during the oxidation of the nickel thiolate by a hydroperoxide are distinctly similar to those formed during processing of the corresponding disulphide in polypropylene. Although the thiophosphoryl disulphides have been extensively reported 2'6- 9.2o,21 to be the first transformation product of metal thiolates during their reaction with

S. Al-Malaika, M. Coker, G. Scott

158

l/S\ / S \ (RO)2P Ni P(ORh \/ \// S S NiDBP

(a) ROOH

l

~S S%

(RO)2Px,.S_s/P(OR)2~

(RO)2P('~+ HO--Ni\s/P(OR)2+ RO"

DBDS

s

/

(RO' P// / '2 \S--OH II

~

/

s

,

2, ~ II --S--OH

'o'

/

~/\ /

I\

I..

/

/

/

s

\

r\ ~

/ I,~/.

,o,l.oo.

,.o +~o

'~'

/

O

//S\

~/ \

IRo21v.. SS

~

,.o~1~

\SR O,O,S-TBDTP

S

II

(RO)2P' + HO--Ni--S + 2ROH /\ IOI / \ ,

~ ,

S

o

II

,~,l..oo.,d,

,,,I \ H

~

~

s

II

s

~

NiSO4.nH20 \

(RO}2P--S--P(OR) 2 \ DBMS \

S

0, o S

II

(RO)2P--OH + SO2 DBTA

II

(RO)2P--OR O,O,O-TBTP

I

O II (RO)2P--SH Scheme I. Antioxidant mechanism of NiDBP. hydroperoxides, this study dearly shows that although the disulphide is always formed, the major product appears to be the monosulphide DMDS, even at higher [TBH]/[NiDBP] ratios. Thus reaction (i) in Scheme 1 appears to be the predominant reaction pathway. The formation of the sulphur acids which.is only evident with increasing hydroperoxide concentrations, no doubt results via the initially formed thiyl radical

Mechanism of antioxidant action: Part 1

159

(reactions (d)-(f) and at high temperatures via oxidation of the disulphide reactions (b), (l)-(f)).

ACKNOWLEDGEM ENTS We are grateful to Synthetic Chemicals for a grant to one of us (MC). We also thank Mr L. Fuller and Mrs A. Earle for their help in synthetic and H P L C work and M r J. Young and Dr F. Yate for helpful discussions during the course of the work.

REFERENCES 1. A1-Malaika, S., Chakraborty, K. B. & Scott, G. In Developments in Polymer Stabilisation--6, ed. G. Scott. Applied Science Publishers, London, 1983. 2. AI-Malaika, S. & Scott, G. In Degradation andStabilisation ofPolyolefins, ed. N. S. Allen. Applied Science Publishers, London, 1983. 3. lvanov, S. K. In Developments in Polymer Stabilisation--3, ed. G. Scott. Applied Science Publishers, London, 1980. 4. Scott, G. In Developments in Polymer Stabilisation~6, ed. G. Scott. Applied Science Publishers, London, 1983. 5. Vink, P. In Developments in Polymer Stabilisation--3, ed. G. Scott. Applied Science Publishers, London, 1980 6. AI-Malaika, S. & Scott, G., Europ Polym. J., 16 (1980) 503. 7. Al-Malaika, S. & Scott, G., Polym. Comm., 23 (1982) 1711. 8. A1-Malaika, S., Brit. Polym. J., 16 (1984) 301. 9. Chamberlain, C. S. & Drago, R. S., Inorg. Chem. Acta, 32 (1979) 75. 10. Mikeska, L., Lubricating Oils, US Patent 2,471 (19 September 1946), p. 115. 11. Foss, O., Acta Chemica Scandinavia, I (1947) 8. 12. Norman, G. R., Lesuer, W. M. & M astin, T. W., J. Am. Chem. Soc., 74 (1952) 161. 13. Coy, R. C. & Jones, R. B., Proceedings of ASLE/ASME Lub. Conf. Dayton, October 1979, pp. 77-9. 14. Marshall, G. L., Proceedings of the Inst. of Petr., London (Petro. anal. 81), (1983), p. 409. ! 5. Crutchfield, M. M., Dungan, C. H., Letcher, T. H., Mark, V. & Van Wazer, J. R., Compilations of 31P N M R data, Topics in Phosphorus Chemistry. Inter Sci. Pub., Vol. 5, 1967, 35-40. 16. Muller, N., Lauterbur, P. C. & Goldenson, J., J. Am. Chem. Soc., 78 (1956) 3557. 17. Kranczyk, E. & Skowronska, A., Phosphorus and Sulphur, 9 (1980) 189. 18. Al-Malaika, S. & Scott, G., Europ. Polym. J., 19 (1983) 235. 19. A1-Malaika, S. & Scott, G., Poly. Deg. and Stab., 5 (1983) 415. 20. Al-Malaika, S. & Scott, G., Polym. Comm., 24 (1983) 25. 21. AI-Malaika, S., Coker, M., Scott, G. & Smith, P., in preparation.