Polymer Degradation and Stability 19 (1987) 43 50

Stabilisation of EPDM at High Temperatures by Polymer-bound Antioxidants: Part 2--Mechanical

Property Change after Extraction During Compression Relaxation in Air

Gerald Scott & S. Mehdi Tavakol i

Polymer Group, Department of Molecular Sciences, Aston University, Birmingham B4 7ET, Great Britain

(Received 24 March 1987; accepted 8 April 1987)

A B S T R A C T

A polymer-bound antioxidant, MADA-B,* is considerably more ¢~ff~,ctive in E P D M in contact with a phosphate ester.[tuid than a conventional oligomeric antioxidant, Flectol H, as measured by mechanical properO' change. The trend is similar in a compression relaxation ( sealing /brce ) test in air in the absence of fluid at 150~'C.

INTRODUCTION

The behaviour of EPDM containing bound antioxidants in heat ageing tests (stress relaxation and mechanical property change) has been described in Part 1 of this paper. 1 In practice this rubber is frequently used in the form of seals, gaskets and hoses in contact with hydraulic fluids at high temperatures. An additional parameter, compression relaxation, is frequently as important as stress-strain characteristics under these conditions. 2 The effect of hydraulic fluids is to extract stabilising additives 3 with consequently accelerated change in stiffness and loss of sealing ability of the elastomer. The present investigation is concerned with a study of the effects of polymer-bound antioxidant MADA* compared with con- ventional commercial antioxidants under conditions of continuous or

* MADA is mercaptoacetylamido diphenylamine. I MADA-B is MADA chemically bound to the rubber.

43

Polymer De,~radation and Stability 0141-3910/87/$03"50 i~" Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain

44 GeraM Scott, S. Mehdi Tavakoli

intermittent contact with hydraulic fluids. It is known 4'5 that fluid contact leads to swelling which, under non-degradation conditions, reaches an equilibrium. Under oxidation conditions concomitant network destruction occurs with consequent increase in swelling. The combined effect of network destruction and swelling, therefore, has a profound effect on the vulcanisate due to plasticisation with associated property change (tensile strength, elongation at break) and stiffening or softening of the rubber. The changes in these properties are investigated in the present study using vulcanisates described in Part 1.1

EXPERIMENTAL

Skydrol LD-4 aircraft hydraulic fluid (Monsanto) was used for the fluid immersion tests. The preparation of EPDM containing bound antioxidants MADA-B and its vulcanisation has been reported in the previous paper. The oven ageing procedure was also as described earlier.

Fluid immersion tests

Typical gum and black vulcanisates cured with either an EV system (high accelerator, low sulphur) or by a sulphur-peroxide system 1 were examined after exposure to the hydraulic fluid. Two such tests were used:

(1) The sample was continuously and totally immersed in Skydrol in sample bottles closed to the atmosphere at 100°C and samples were removed at weekly intervals and the fluid was renewed. Mechanical properties of the samples were measured. Six replicate samples were used for each measurement.

(2) Cyclical fluid immersion-oven ageing test. Each cycle consisted of immersion in Skydrol at room temperature for 6 h followed by air oven ageing and property measurement every 48 h. The total time of ageing was for 4 weeks at 120°C. Mechanical properties were determined on six replicate samples for each measurement as described earlier, l

Sealing force measurements were carried out on 'O' rings by means of the Lucas Compression Stress Relaxometer v using the procedure recommended by Aston e t al . s

RESULTS

Sulphur-accelerator (semi-EV) vuicanisates

Table 1 shows the effect of continuous immersion in Skydrol on the aged properties of gum and black sulphur-accelerator vulcanisates and Table 2

Stabilisation of EPDM by polymer-bound antioxidants, Part 2 45

TABLE 1 Change in Mechanical Properties (%) of Sulphur-accelerator Cured EPDM

Vulcanisates after 4 Weeks' Continuous Immersion in Skydrol at 100°C

Vulcanisate Gum Black

EB TS Mlo o EB TS Mloo

Control - 4 2 -55 -35 - 3 9 - 4 8 -28 2% Flectol H - 3 5 - 4 4 - 2 9 -33 - 4 0 + 17 1% MADA-B (U) -13 -27 - 1 8 - 1 4 - 2 0 +2 1% MADA-B(E) - 1 0 -25 -11 - 1 6 - 2 4 - 4 2% MADA-B (U) -21 -29 - 2 2 - 1 9 -27 - 1 2 2% MADA-B (E) - 2 4 -31 - 2 9 - 2 2 - 2 0 - 16

u = Unextracted. E = Extracted before compounding and vulcanisation.

gives results for the s ame vulcanisa tes in the c o n t a m i n a t i o n test a f te r 4 weeks.

It is c lear tha t the two tests give c o m p a r a b l e results. The ma in difference

be tween them, as migh t be an t ic ipa ted , is tha t the c o n t i n u o u s i m m e r s i o n test

leads to decrease in m o d u l u s (chain scission) and increased swelling in m o s t

cases, whereas the c o n t a m i n a t i o n test gives ma in ly m o d u l u s increase due to cross- l inking. This reflects the fact tha t age ing takes place in the first case

p r e d o m i n a n t l y in the swollen s tate where c ross - l ink ing is reduced, whereas , in the latter, ageing occurs ove r a longer per iod in the absence o f swelling agent .

In b o t h tests, the p o l y m e r - b o u n d an t iox idan t is m o r e effective t han the

TABLE 2 Change in Mechanical Properties (%) of Sulphur-accelerator Cured EPDM

Vulcanisates after 4 Weeks in the Cyclical Contamination Test

Vulcanisate Gum Black

EB TS Mlo o EB TS Mlo o

Control - 5 2 -43 +35 -61 - 4 0 * 2% Flectol H -39 -31 +21 - 4 2 - 2 8 +71 1% MADA-B (U) -18 - 7 +8 -25 -11 +43 1% MADA-B (E) - 1 4 - 1 4 +4 -28 -13 +47 2% MADA-B(U) -22 - 1 9 +11 -31 - 1 6 +51 2% MADA-B (E) -28 -22 +13 -23 - 8 +39

* = Degraded. U = Uncxtractcd. E = Extracted before vulcanisation.

46 Gerald Scott, S. Mehdi Tavakoli

conventional oligomeric antioxidant and, as was observed previously in simple heat-ageing tests, ~ 1% of polymer-bound antioxidant appears to be more effective than 2% in inhibiting oxidative change in both oil contact tests. Possible reasons for this were discussed in Part 11 and are equally applicable in the present evaluation.

Sulphur-peroxide vulcanisates

The sulphur-peroxide vulcanisation system is believed to give a proportion of carbon-carbon cross-links 6 which are known to be more resistant to scission and reformation (reversion) than sulphur cross-links. Consequently, they might be expected to perform better under high temperature conditions, particularly under stress, and were compared in the present investigation with the more conventional semi-EV sulphur accelerator system described above.

Table 3 compares the vulcanisates after 6 weeks' continuous immersion in Skydrol at 100°C. This period was chosen because the 4 weeks used in the sulphur-accelerator system was insufficient to discriminate adequately between the formulations, thus confirming the above postulate that the network is somewhat more resistant to high temperature ageing than that produced by the sulphur-accelerator system. The effectiveness of the polymer-bound antioxidant in retarding this change is again evident. The same stabilising pattern is evident in the contamination test (Table 4) although here the rubbers underwent more extensive mechanical deterior- ation compared with the continuous immersion test and indeed the unstabilised controls were much more degraded than the corresponding sulphur-accelerator controls (Table 2). The stabilised formulations show the

TABLE 3 Change in Mechanical Propert ies (%) of Sulphur-peroxide Cured E P D M

Vulcanisates after 6 Weeks' Con t inuous Immers ion in Skydrol at 100°C

Vulcanisate Gum Black

EB TS Mlo o EB TS Mlo o

Contro l - 5 6 - 4 2 - 5 4 - 5 2 - 4 0 - 4 9 Flectol H (U) - 31 - 33 - 28 - 27 - 30 + 29 1% M A D A - B (U) - 1 4 - 1 0 - 1 1 - 1 0 - 9 + 1 5 1% M A D A - B (E) - 9 - 8 - 6 - 7 - 4 + 8 2 % M A D A - B (U) - 2 3 - 2 1 - 2 3 - 2 0 - 19 + 2 2 2% M A D A - B (E) - 1 8 - 1 3 - 1 5 - 1 5 - 1 1 + 1 8

U and E as in Table 1.

Stabilisation o f E P D M by polymer-bound antioxidants, Part 2 47

TABLE 4 Change in Mechanical Properties (%) of Sulphur-peroxide Cured EPDM

Vulcanisates after 4 Weeks in the Cyclical Contamination Test in Skydrol

Vulcanisate Gum Black

EB U TS MIO 0 EB UTS Mlo o

Control * * * * * * 2% Flectol H (U) - 2 8 - 3 0 -+-31 - 3 1 - 2 7 +49 1% MADA-B (U) - 1 7 - 1 7 +18 - 2 1 - 1 4 +34 1% MADA-B (E) - 1 3 - 1 4 +15 - 1 8 - 8 +30 2% MADA-B (U) - 2 4 - 2 4 +23 - 2 7 - 2 1 +43 2% MADA-B (E) - 2 0 - 2 0 +20 - 2 3 - 1 6 +38

U and E as in Table 1.

s a m e t r e n d s as in t he s u l p h u r - a c c e l e r a t o r v u l c a n i s a t e s a n d the p o l y m e r -

b o u n d a n t i o x i d a n t s a r e p a r t i c u l a r l y ef fec t ive in t h e b l a c k f i l led r u b b e r s .

A g a i n , b o t h se ts o f tes t r e su l t s c o n f i r m the s u p e r i o r i t y o f t h e p o l y m e r - b o u n d

a n t i o x i d a n t s o v e r t he c o n v e n t i o n a l h e a t - a g e i n g o l i g o m e r i c a n t i o x i d a n t ,

F l e c t o l H.

C o m p r e s s i o n stress re laxat ion m e a s u r e m e n t s

T h e s e a l i n g f o r c e o f s u l p h u r - a c c e l e r a t o r a n d s u l p h u r - p e r o x i d e E P D M

v u l c a n i s a t e s w a s d e t e r m i n e d a t 22°C in a n a i r oven . T h e r e su l t s a r e s h o w n in

T a b l e s 5 a n d 6. T h e s e r e su l t s p a r a l l e l t he p e r f o r m a n c e o f t he v u l c a n i s a t e s in

TABLE 5 Change in Sealing Force (%) of Sulphur-accelerator Cured EPDM Vulcanisates

after 72h at 150 C in Air

Vulcanisate Gum Black

22 C" 150 ('" 22 C a 150 C a

Control - 92 - 89 - 88 - 83 2% Flectol H (U) -71 - 6 8 - 6 9 - 6 6 1% MADA-B (U) - 6 6 - 6 3 - 6 4 - 6 0 1% MADA-B (E) - 6 0 - 5 7 - 5 6 -51 2% MADA-B (U) - 6 2 - 5 8 - 5 9 - 5 6 2% MADA-B (E) 65 - 6 0 - 6 3 - 5 8

" = Temperature of measurement. U = Unextracted. E = Extracted.

48 Gerald Scott, S. Mehdi Tavakoli

TABLE 6 Change in Sealing Force (%) of Sulphur-peroxide Cured EPDM Vulcanisates after 72 h at 150°C in Air. Measure-

ments Carried out at 150°C

Vulcanisate Gum Black

Control - 86 - 80 2% Flectol H (U) -70 -67 1% MADA-B (U) -61 -57 1% MADA-B (E) -53 -44 2% MADA-B (U) -56 -52 2% MADA (E) -58 -54

u, E as in Table 1.

the extraction tests except that the previously observed decrease in activity at higher concentration is not evident here. As might be anticipated, since the 'O' ring samples are much thicker than the samples used in normal heat ageing tests and since, in addition, there is no oil contact, the differences between the bound antioxidant and Flectol H are not so marked in this test. Somewhat more surprisingly there is little difference between sealing force loss measured at 22°C and that measured at 150°C. This implies that the stiffness (or modulus) of the rubber does not show appreciable variation over the temperature range considered and that severe temperature cycling may not be a serious cause for concern.

Only 150 ° sealing force measurements were carried out for the sulphur- peroxide vulcanisates (see Table 6) and the results obtained were almost identical to those in the sulphur-accelerator semi-EV system, both for the unstabilised control and for the stabilised formulations. There seems to be little advantage to be gained from the use of sulphur-peroxide vulcanisates with any of the antioxidants used.

D I S C U S S I O N

Both the softening observed in the Skydrol immersion test and the hardening resulting from short-term immersion in Skydrol followed by air ageing are due to oxidation of the rubber network. Both chain scission and cross-linking occur simultaneously in E P D M during heat ageing. It was seen in the previous paper that although chain scission occurs during continuous stress relaxation, the dominant process is cross-linking with hardening of the rubber under non-stressed conditions (see also reference 2). Swelling with Skydrol reduces the effect of oxidative cross-linking relative to chain- scission since the rate of the bimolecular cross-linking reaction is reduced by

Stabilisation o/' EPDM by polymer-bound antioxidants, Part 2 49

the presence of a 'diluent' whereas the unimolecular chain-scission reaction continues unchanged. Oxidative cross-linking is the primary cause of hardening and loss of other physical properties during static ageing but under conditions of continuous stress (continuous stress relaxation) permanent set of the rubber also occurs. It is a combination of permanent change in dimensions of the components together with increase in hardness which leads to the loss of sealing force. The sulphur-peroxide vulcanisate behaves very similarly to the sulphur-accelerator vulcanisate in compression stress relaxation, suggesting that non-oxidative reversion is not very important since the sulphur-peroxide vulcanisate would be expected to be more thermally stable than the sulphur-accelerator vulcanisate. The effects observed must therefore be primarily due to oxidation.

The compression stress relaxation test in an air oven is much less discriminating between different antioxidants than is the extension stress relaxation test I under similar conditions of temperature and time. The difference between the two tests lies primarily in the thickness of the rubber sample in the first test which reduces the rate of loss of antioxidant by migration and subsequent evaporation. However, the difference between the polymer bound antioxidant and the oligomeric antioxidant in the compression relaxation test is quite significant and, as has been seen, would be accentuated in an oil contact situation due to leaching of the antioxidants from the swollen surface layers.

CONCLUSIONS

(1) The polymer-bound antioxidant, MADA-B, is considerably more effective in both a semi-EV vu.lcanisate and in a sulphur peroxide vulcanisate than the oligomeric commercial antioxidant Flectol H in EPDM in contact with a phosphate ester fluid.

(2) MADA-B is, in general, more effective at 1 phr than at 2phr and solvent extraction appears to increase oxidative resistance.

(3) Fluid contact (both continuous immersion and contamination) discriminates much more effectively between antioxidants than does a thermal compression relaxation test (sealing force test). However, MADA-B is significantly more effective at 1 phr than Flectol H at 2 phr in the compression relaxation test at 150°C.

A C K N O W L E D G E M E N T S

This work has been carried out with the support of the Procurement Executive, Ministry of Defence. We are also grateful to Miss E. Kaye for

50 GeraM Scott, S. Mehdi Tavakoli

helpful discussions and to Dr D. K. Thomas. We also thank Mr M. J. Turner of Esso Chemicals Elastomeric Division for providing samples of EPDM.

R E F E R E N C E S

1. G. Scott and S. M. Tavakoli, Part 1. 2. H. A. Pfisterer and J. R. Dunn, J. Elastomers and Plastics, 7, 427 (1975). 3. D. K. Thomas, Developments in polymer stabilisation--1. (G. Scott (Ed.)), Applied

Science Publishers, London, 137 (1979). 4. E. Southern, Use of rubbers in engineering. (P. W. Allen, P. B. Lindley and R. A.

Payne (Eds)), MRPRA, (1967). 5. C. M. Blow, Int. Rubb. Conf., Brighton, May 1972, Paper B3. 6. E. Southern, Elastomers: Criteria for engineering behaviour, Chapter 6. (C.

Hepburn and R. J. W. Reynolds (Eds)), Applied Science Publishers, London (1979).

7. The Lucas Compression Stress Relaxometer, Wallace Equipment Manual, C8. 8. M. W. Aston, W. Fletcher and S. H. Morrel, Paper presented at the Fourth

International Conference on Fluid Sealing, Philadelphia, 5-9 May, 1969.