ARTICLE IN PRESS

Polymer Degradation and Stability xx (2006) 1e7www.elsevier.com/locate/polydegstab

+ MODEL

A novel method for monitoring chemical degradation ofcrosslinked rubber by stress relaxation under tension

Susanta Mitra*, Afshin Ghanbari-Siahkali 1, Kristoffer Almdal

The Danish Polymer Centre, Risø National Laboratory, POL-124, P.O. Box 49, Frederiksborgvej 399, DK-4000 Roskilde, Denmark

Received 2 January 2006; received in revised form 1 March 2006; accepted 2 March 2006

Abstract

This paper describes a novel test method for monitoring chemical degradation of a crosslinked rubber by stress relaxation under tension. Anaccelerated sulphur cured ethylene propylene diene rubber (EPDM) was subjected to stress relaxation under tension while exposing to 50%aqueous solution of nitric acid (HNO3). An experimental set up was designed and built in-house for this purpose. The tensile test specimenwas stretched to a constant elongation. The stress decay was monitored upon exposure to the harsh chemical. Stress decay was found muchfaster in the exposure media than in air. Decrosslinking was the main reason for quicker stress decay as observed by decrease in crosslinkdensity. To compare, experiments were conducted by exposing similar specimens in the same exposure media under unstressed conditions. Itwas found that monitoring chemical degradation under given stressed condition yielded relatively quicker yet reliable and reproducible results.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Stress relaxation; Chemical degradation; EPDM; Rubber; Crosslinking density; Accelerated test

1. Introduction

The importance of chemical degradation and its impact onformulating a rubber vulcanizate is evident from our previousstudies [1e3] and literature [4]. Industries that manufacturerubber products for chemical applications require a reliableyet easy test methodology to ascertain durability in serviceconditions. This, therefore, necessitates accelerated degra-dation tests on a laboratory scale that can monitor the perfor-mance of the rubber product in a harsh chemical environmenton a relatively shorter time scale. Rubber products, in general,are designed to withstand stresses of external and internal

* Corresponding author. GE Plastics, GE Global Research, GE India Technol-

ogy Centre (John F Welch Technology Centre), Polymer Science and Technol-

ogy, 122, Hoodi Village, EPIP Phase 2, Whitefield Road, Bangalore, Karnataka

560 066, India. Tel.: þ91 80 2503 2325; fax: þ91 80 2841 2111.

E-mail address: susanta.mitra@ge.com (S. Mitra).1 Present address: NKT Flexibles I/S, Priorparken 510, DK-2605 Broendby,

Denmark

0141-3910/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2006.03.002

origin depending on the service conditions. The external factorarises from the application condition whereas the internalstress may arise either during processing and/or degradation.Thus, the chemical reactions leading to degradation of rubbersusually take place in stressed condition and the extent of deg-radation is aggravated by the external stress in combinationwith temperature and harsh chemicals. Several new testmethods for adjudging durability of rubber products bymonitoring stress relaxation are proposed in the literature[5e11]. However, no report is available on the test methodvis-a-vis test equipment for observing chemical degradationin situ during stress relaxation of rubber particularly in harshacidic or alkaline chemical environments.

Over the years, rubber products (e.g., seals, gaskets,o-rings, hoses and cable insulation) are being used in variouschemical environments that could potentially undergo degra-dation. In practice, service conditions vary widely and areoften complex in the presence of both harsh chemicals andvarying temperatures. Since ‘‘maintenance free’’ productsare gaining popularity in the market, the demand for durability

ARTICLE IN PRESS

2 S. Mitra et al. / Polymer Degradation and Stability xx (2006) 1e7

+ MODEL

is also becoming stringent. In this situation, a reliable shorttime testing for estimating lifetime is becoming essential.The laboratory tests are usually being done on a relativelyfaster time scale using an exaggerated condition in order toextrapolate over the lifetime of the material in actual applica-tion. Several such standardized test methods are available inthe form of ASTM, ISO and BS Specifications. In this studythe rubber sample is strained in situ in harsh chemical environ-ments and stress decay is measured as a function of exposuretime. New equipment is indigenously built for this purpose anda new test methodology is proposed.

2. Equipment concept

The concept of stress relaxation under tension duringa chemical exposure condition is applied for fabricating thetest equipment. There are some distinct advantages in this ap-proach. First, the equipment is fabricated in such a manner thata standard dumbbell-shape specimen commonly used for rou-tine tensile testing could be employed. This avoids extra effortfor special sample preparation. The sample could be stretchedto any level up to 200% elongation. More importantly, thechemical attack by and large results in either chain scissionthrough attack on the polymer backbone and/or decrosslinking[1e3]. So, stress relaxation in tension can be presumed to giverelatively faster yet reliable results. The choice of stress relax-ation under tension in situ during chemical degradation couldbe counteracted by stress relaxation under compression sincesome rubber products like seals and gaskets are used in actualapplication condition which resembles with compressive stressrelaxation [6e9]. But the difficulty with stress relaxation un-der compression in chemical exposure conditions is that theuniform exposure of rubber samples to chemicals is hindered,and thus the choice of stress relaxation under tension is justi-fied by uniform exposure over the entire rubber specimen inthe chemical exposure medium.

3. Stress relaxation of rubber

When a constant strain is applied to a rubber sample cross-linked well above its gel point, the force required for maintain-ing the strain decreases with time. This behaviour is called‘‘stress relaxation’’. Both physical and chemical processescan cause the stress relaxation depending upon the time scale.At ambient conditions and/or short times, the stress relaxationpredominantly results from physical processes. At higher-temperatures or in degradative environments, and/or for longtime exposures, chemical processes dominate over physicalprocesses.

Three following processes can occur during stress relaxa-tion of a typical rubber vulcanizate:

a. Physical relaxation e which occurs due to relocation ofthe rubber chains and the fillers, when subjected to defor-mation. However, this happens within a very short span oftime right after deformation in a rubber vulcanizate that is

crosslinked well above its gel point (as observed in actualpractice).

b. Degradation e this is caused by heat, light, oxygen/air,and chemicals, which result in decrosslinking and/or chainscission. All of these would possibly reduce the counterforce during stress relaxation testing.

c. Crosslinking e breaking and subsequent rearrangement ofthe crosslinks and/or polymer backbone as well as combi-nation of oxygenated species (originated from degrada-tion) can form additional crosslinks. However, this doesnot contribute to ‘‘normal’’ stress relaxation as the newcrosslinks are formed in the load bearing condition.

Based on rubber elasticity theory, it is assumed that thestress s(t) is directly proportional to the network chain density,N(t) at time t, thus

sðtÞ=sð0Þ ¼ NðtÞ=Nð0Þ ð1Þ

where, N(0) is the network chain density of the rubber vulca-nizate before degradation (i.e., at time, t¼ 0).

Considering the simplest case (standard linear solid model)of continuous stress relaxation which is governed by

sðtÞ ¼ sðNÞ þ ðsð0Þ � sðNÞÞexp ð�t=tRÞ ð2Þ

where, s(N) is the residual equilibrium stress which is timeindependent at a very long time scale (t¼N) and tR is therelaxation time to achieve (1� 1/e) i.e., 0.63 of the initialstress value [12]. It is important to note that tR is not thetime that takes for a strained polymer chain to completelyuncoil and relax to its unstrained state.

4. Experimental

4.1. Materials

EPDM rubber used in this study (Nordel� IP 4520) con-tained 5-ethylidene-2-norbornene (ENB) as diene with a typi-cal monomer composition of ethylene/propylene/ENB as 50/45/5 by weight percentage. The pure rubber was characterizedfor monomer compositions (ethylene/propylene/ENB, wt%),mass average molar mass (133 kg/mol) and molar mass distri-bution (2.74), which is described in details elsewhere [1,2].

Compounding was done using rubber grade chemicals asper the recipe given in Table 1. Carbon black, anti-degradantsand other necessary ingredients were deliberately withheld forkeeping the rubber compound as simple as possible. The com-pound was then cured for 10 min at 170 �C using compressionmoulding technique. The rubber sheets were die cut to dumb-bell-shape tensile test specimens with dimensions (i.e., typi-cally of length 75 mm, width 13 mm, gauge length 25 mm,and thickness 2.5 mm) as per ISO 37: 1994 which were thenused for this study.

50% nitric acid (HNO3) solution was prepared by volumet-ric mixing of analytical grade of concentrated (w65%) HNO3

(AppliChem, GmbH) with Millipore water. The resultingaqueous acid solution was used as the exposure media.

ARTICLE IN PRESS

3S. Mitra et al. / Polymer Degradation and Stability xx (2006) 1e7

+ MODEL

4.2. Equipment fabrication and testing methodology

Fig. 1 shows the continuous stress relaxation equipmentdesigned for experiments in chemical degradation environ-ments. The chamber was made of Teflon coated steel in orderto be acid and base resistant. The main unit for stretching wasmade of Teflon and could be manoeuvred to adjust the stretch-ing limit. The gripping jaws were also made of Teflon coatedsteel. Tiny screws for holding the rubber samples tightly inplace inside gripping jaws were also made of Teflon. One grip-ping jaw was fixed and other was fastened to a corrosion resis-tant flexible steel wire. In order to avoid twisting or tilting of

Table 1

Compounding recipe of the EPDM rubber used for the present study

Compounding

ingredients

Chemical name Phr (parts

per hundred

parts of

rubber)

Function

Rubber ENBeEPDM 100.0 Rubber

S Sulphur 0.5 Curing agent

TMTD Tetramethylthiuram

disulphide

2.0 Accelerator

(sulphur donor)

MBTS Dibenzthiazyl disulphide

[2,20-dithiobis(benzothiazole)]

0.2 Accelerator

TBBS N-tert-butyl-

2-benzothiazolesulphenamide

1.0 Accelerator

ZnO Zinc oxide 3.0 Accelerator

Activator

St. A Stearic acid 0.5 Accelerator

Activator

Total 107.2

the sample during stretching, the moving gripping jaw wasguided through cutting slots. The corrosion resistant steelwire was then finally passed through a hole in the fixture at-tached to the load cell and could be tightened by actuatinga screw after the sample was stretched to a required level.A 5 kN load cell was employed for the measurement. Thiswas connected to a computer through a data acquisition box.The data were recorded on the computer after setting thetime intervals of choice (say 3 data points per minute). Theoutput data was obtained in volts. This could be convertedto load through a calibration technique. Standard weightswere used to calibrate with volts. Plotting loads used againstvolts observed thus generated a calibration curve. The linearfit of the data produces an equation correlating volts withload (Eq. (3)). Prior to testing, the thickness and width ofthe rubber sample were measured to calculate the cross sec-tional area of the original rubber sample, and thus nominalstress is calculated (i.e., load/area). In a typical experiment,the sample was set to a fixed elongation (i.e., strain). Approx-imately, 2e3 min were given to attain the equilibrium of thestretched sample. The chemical solution to be used as an ex-posure media was then poured into the testing chamber, andthe measurements were started through accumulation ofdata. Finally, the relaxation time tR and the residual equilib-rium stress s(N) after time t were estimated from the curvefitting algorithm available with standard worksheet software.

The same experiment was conducted in air (without anychemical exposure) for a definite period of time. tR ands(N) were determined similarly as mentioned above. Thisvalue was considered as the control value for the stressedsamples.

Fixed end

Movable end

Tension wireconnectedto load cell

Load cell

Sampleaged in unstressed

condition

Stressed samplein exposure media

Teflon part for guiding tension

Ageing chamber(Teflon coated)

Fig. 1. Equipment designed for stress relaxation in situ during chemical degradation.

ARTICLE IN PRESS

4 S. Mitra et al. / Polymer Degradation and Stability xx (2006) 1e7

+ MODEL

The ambient temperature during the experiment in a closedlaboratory was monitored through a thermometer and foundwithin 20� 2 �C during the entire course of experiments.A thermocouple could be attached to the system for monitor-ing and recording the temperature continuously during theexperiments.

4.3. Mechanical testing for unstressed sample

To compare the chemical degradation in stressed and un-stressed states, five tensile test samples were exposed to thesame exposure media for similar period of time (i.e., 1, 2, 3and 4 week/s) and characterized for change in mechanicalproperties (tensile strength and %elongation at break).Mechanical testing was conducted as per ISO 37:1994 onthe Instron universal test machine (Model 4303).

4.4. Crosslink density measurements

The crosslink density of the unstressed samples was evalu-ated according to the FloryeRehner equation [1]. Unstressedexposed samples with nearly equal dimensions and weightswere swelled in n-heptane at ambient temperature for 24 h(the equilibrium swelling time). The samples were then driedin an oven at 70 �C under vacuum for 12 h and then theweights of the samples were noted. The polymeresolvent in-teraction parameter (c) for calculations of crosslink densitywas chosen to be c¼ 0.38þ 0.083Vp where, Vp is the volumefraction of vulcanizate in the swollen gel [1e3]. For the un-stressed sample, the control sample was the unexposed one.

After stress relaxation testing for a particular time, all thesamples exposed both in air (treated as a control sample forstress relaxation) and in exposure media were kept inPetridish for 24 h to attain the ambient condition. After24 h, samples were swelled following the same proceduresas described for the unstressed sample.

5. Results and discussion

5.1. Stress relaxation in situ during chemicaldegradation

A typical calibration curve for converting the data fromvolts to load is made and obtained a linear equation (Eq. (3))

Load¼ ð�0:1611Þ þ ð0:4269Þ � volt ð3ÞThe correlation coefficient (R2¼ 0.99976) indicates the

sensitivity and the accuracy of the calibration method usedin this study.

Fig. 2 represents the normalised stress decay curve (both inair and in exposure media) for the cured EPDM rubber aftera constant stretching of 50% elongation. A rubber productlike seal, gasket or hoses which requires the chemical resis-tance for durability seldom experiences elongation morethan 50% in actual service conditions and this justifies ourchoice for setting the limit of stretching. It can be clearlyseen that the stress decay in the presence of the exposure

media takes place much faster than in the air. This phenome-non can be explained by the attack of the aqueous acid mediaon the monosulphidic, disulphidic, and/or polysulphidic cross-linking sites as found in our previous study [1e3]. The stressdecay in the exposure media continues with exposure time tillthe materials break completely at about 30 days. However, thestress decay attains a plateau almost after 20 days for thesample in air and it is possible that some physical (i.e., de-entanglement) as well as chemical modifications leading tostable network formation are equally responsible e.g., transfor-mation of polysulphide linkages into disulphidic and monosul-phidic linkages [12].

The curve fitting results using Eq. (2) are tabulated inTable 2. It is quite clear that residual equilibrium stress de-creases with increase in exposure time and no longer remainsas time independent. This clearly indicates the underlyingchemical modification of the crosslinked EPDM upon expo-sure to aqueous acidic solutions. The decrease in residual equi-librium stress is significant (by 34%) at the 1 week of exposurein comparison to that in air. Thereafter, the decrease in resid-ual equilibrium stress is less steep upon further exposure. Thiscould be due to the role of different types of crosslinks presentin the system under this stress relaxation in situ during chem-ical degradation. Chemically weaker crosslinks undergo acidinduced hydrolysis faster than relatively stable crosslinks.With increase in exposure time, more and more stable cross-links remain in the system and thus relatively gradual decreasein residual equilibrium stress is observed. This finding con-firms our postulation on the chemistry of acid induced degra-dation. Thus, under this chemical degradation environmentresidual equilibrium stress becomes a complex function ofexposure time and approaches to 0 as t / N. This is consis-tent with literature [12].

A mixture of different types of crosslinkings is formed ina typical sulphur cured rubber vulcanizate e.g., polysulphidic,disulphidic and monosulphidic along with some cyclic sulphurstructure especially when higher amounts of sulphur are usedin formulation [13]. In our case, however, efficient vulcaniza-tion system is employed (Accelerator to sulphur ratio >3) and

0 5 10 15 20 25 300,30

0,35

0,40

0,45

0,50

0,55

0,60

0,65

0,70

0,75

0,80

0,85

0,90

0,95

1,00

in Air

in 50%HNO3

Exposure time, Days

Fig. 2. Stress decay curves for EPDM crosslinked rubber in air and in 50%

HNO3 at a constant stretching to 50% elongation.

ARTICLE IN PRESS

5S. Mitra et al. / Polymer Degradation and Stability xx (2006) 1e7

+ MODEL

Table 2

Curve fitting results as per Eq. (2)

Sample stress relaxed

in media/time of exposure

s(0) (MPa) s(N) (MPa) tR (Days) Goodness of

fitting (c2)

Air/4 weeks 0.9713� 0.0001 0.7586� 0.0002 15.0006� 0.0291 0.0001

HNO3/1 week 0.9335� 0.0020 0.4989� 0.0022 7.0234� 0.0563 0.0002

HNO3/2 weeks 0.9291� 0.0005 0.4127� 0.0005 9.2213� 0.0190 0.0001

HNO3/3 weeks 0.9481� 0.0003 0.3813� 0.0004 12.6822� 0.0163 0.0001

HNO3/4 weeks 0.9304� 0.0004 0.2520� 0.0004 18.8280� 0.0238 0.0002

thus theoretically 20% of poly- and disulphidic linkages and80% of monosulphidic linkages are present as crosslinks[13]. In a typical sulphur cured rubber vulcanizate, the bonddissociation energy of polysulphidic crosslinks (CeSxeC) isw120 kJ/mol, disulphidic linkages (CeSeSeC) is w240 kJ/mol and monosulphidic linkage (CeSeC) is w270 kJ/mol[13]. This indicates that polysulphidic linkages will undergoacid induced hydrolysis faster than disulphidic and monosul-phidic linkages in a chemical degradation environment. Thisis exactly what we have noticed in this study. Hydrolysis ofdifferent types of crosslinks due to chemical degradation pos-sibly changes the relaxation rate with exposure time. This alsoexplains why relaxation time decreases drastically after 1week of exposure and increases thereafter with increase inexposure time.

Considering the above observation and explanation, the ap-plicability of Eq. (2) to this study is a matter of discussion.Whether or not the equation is valid, the stress decay (s(t)/s(0)) with exposure time (t) clearly reveals that chemical deg-radation proceeds faster in stressed condition. This equipmenthelps accelerating the ageing testing in a chemical degradationenvironment and produces reliable results within a relativelyshorter time scale. Moreover, the equipment vis-a-vis testmethodology is also capable of addressing the underlyingchemistry of degradation.

The repeatability and reliability of the results can be welladjudged by Fig. 3 which shows the overlapping of the respec-tive normalised stress curves for all the respective exposuretime and are well within the acceptance limit considering

0,0

0,2

0,4

0,6

0,8

1,0

28 days

21 days

14 days

7 days

Exposure time, Days 0 5 10 15 20 25 30

Fig. 3. Stress decay curves for EPDM crosslinked rubber for different exposure

time.

the uncertainty associated with the unavoidable flaws in thecuring and die cutting of rubber samples in industries.

5.2. Mechanical testing of the unstressed samples

Fig. 4 represents the plot of %retention of tensile strengthand %elongation of break values. It is noticeable from Fig. 4that the changes are well within experimental errors for allthe exposure times and thus requires prolonged time scale ofexperiment (e.g., for 12 weeks) [1e3]. Moreover, under un-stressed conditions, decrosslinking and/or chain scission oftenresults in oxygenated macro species containing oxygenatedfunctionalities (e.g., hydroxyl groups), which combines witheach other. All these parallel reactions make difficult to visu-alise a distinct trend in change in properties [1e3]. Moreover,tensile strength of a rubber vulcanizate is a complex functionof chemical nature of crosslinks and crosslink density. Undera given condition, tensile strength of a typical crosslinked rub-ber increases with increase in crosslink density up to an opti-mum extent and thereafter it decreases with further increasein crosslink density. Plausibly, the hydrolysis of crosslinkschanges the optimum level of crosslink density in our studyto facilitate the increase in tensile strength after 2 weeks of ex-posure in unstressed condition. This can be clearly seen bya slight upward turn in case of tensile strength from 1 weekto 4 weeks exposure. The reduction in tensile strength fromcontrolled to 1 week exposed sample was about by 15% andalmost half to the reduction (w34%) observed in the residual

0 5 10 15 20 25 3075

80

85

90

95

100

105

110

115

Tensile Strength

Elongation at break

Ret

enti

on o

f m

echa

nica

l pro

pert

y

Exposure time, days

Fig. 4. Change in mechanical properties for unstressed samples at different

exposure time.

ARTICLE IN PRESS

6 S. Mitra et al. / Polymer Degradation and Stability xx (2006) 1e7

+ MODEL

equilibrium stress value in stress relaxation in situ duringchemical degradation after 1 week of exposure (compared tostress relaxation in air). Decrease in crosslink density owingto chemical degradation on the other hand increases the rubberchain mobility compared to the controlled unexposed sampleand thus %elongation at break was found to increase withincrease in exposure time under the studied condition.

5.3. Crosslink density of the stressed and unstressedsamples

Fig. 5 represents the effect on crosslink density (measuredas 1/2Mc, where Mc is the number average molar mass of rub-ber chain in between two successive crosslinks) with exposuretime for the unstressed sample. The applicability of FloryeRehner equation for measuring the crosslink density ofstress-relaxed samples is subjected to discussion as the equa-tion is derived on the basis of Gaussian chains assumption[14,15]. Decrosslinking and/or chain scission due to chemicaldegradation under aqueous acidic condition in the stressedstate could lead to decrease in over all crosslink density. More-over, plausible combination of oxygenated species resultingfrom degradation under strained conditions could also formnew crosslinks which might have been non-load bearing dur-ing stress relaxation but could contribute to the degree ofswelling. However, considering the small extension ratio(w1.5), we assume that uniaxial stretching in the chemicaldegradation environment will keep the Gaussian approxima-tion valid especially after allowing the samples to attain theambient condition at the end of the stress relaxation [14,15].This reasonable assumption would allow us to measure the de-crease in crosslink density for all the stressed relaxed samplesusing the FloryeRehner equation. The decrease in crosslinkdensity for both stressed and unstressed samples is shown inFig. 5. The decrease in crosslinking density for the stressedsamples is more than that of the unstressed samples at theequivalent time of exposure. This reflects that the stressedconditions are more aggressive than unstressed conditionsand thus relatively quicker changes in residual equilibriumstress and relaxation time were observed.

0 5 10 15 20 25 307,27,47,67,88,08,28,48,68,89,09,29,49,69,8

10,010,210,4

Exposure time, days

Cro

sslin

k de

nsit

y (1

/2M

c)X

10-4

, mol

/g

Unstressed

Stressed

Fig. 5. Crosslink density for both stressed and unstressed samples measured at

different exposure time.

The value for the ‘0’ day exposed for stressed sample istaken from the sample after stress relaxation in air whilethat value for the unstressed is from the unexposed rubbersample. It is necessary to comment on the differences in thevalues between stressed and unstressed samples at the ‘0’day exposure. Very small difference is an indicative of thefact that the EPDM rubber vulcanizate used for this study con-tains relatively stronger crosslinks (presumably di- and mono-sulphidic), which did not appreciably rearrange or break uponstress relaxation in air at a constant 50% elongation.

6. Conclusions

Monitoring chemical degradation of a crosslinked rubberupon exposure to harsh chemicals in situ stress relaxation byour indigenous fabricated equipment and techniques seems tobe reliable for the shorter time scale of the experiments. The re-peatability and reproducibility of the equipment and data arealso demonstrated. Accelerated sulphur cured EPDM is usedbecause our previous studies show the resistance of this sampleunder aqueous acidic environment in unstressed conditions.Teflon coating of the chamber and the Teflon made extensionunit give the flexibility of using a variety inorganic/organic ag-gressive media. In fact, experiments could also be conducted atelevated temperature by placing the entire system suitably ina thermostat. The equipment can be refined further by addinga mechanical strain gauge arrangement for more accurate strainsetting. A thermocouple attachment could be another potentialadd-on feature for monitoring the temperature continuouslyduring the entire course of experiments.

This equipment shows a great potential for the research anddevelopment laboratory testing. The test method suggested inthis work is capable of adjudging the performance of a rubberproduct, which is to be designed for the optimum chemicalresistance in actual service conditions.

Acknowledgement

Financial support from MONEPOL (Danish Centre forthe Study of Polymer Degradation and Stability) a DanishResearch Agency funded centre contract (J.nr. 2000-603/4001-51) is gratefully acknowledged.

References

[1] Mitra S. Chemical degradation of specific uncrosslinked and crosslinked

rubbers upon exposure to specific non-absorbing aqueous chemicals.

PhD thesis, The Danish Polymer Centre, Dept. of Chemical Engineering,

Technical University of Denmark, Lyngby, Denmark; 2005.

[2] Mitra S, Ghanbari-Siahkali A, Kingshott P, Rehmeier HK, Abildgaard H,

Almdal K. Chemical degradation of crosslinked ethyleneepropyleneediene rubber in acidic environment. Part I. Effect on accelerated sulphur

crosslinks. Polym Degrad Stab 2006;91(1):69e80.

[3] Mitra S, Ghanbari-Siahkali A, Kingshott P, Rehmeier HK, Abildgaard H,

Almdal K. Chemical degradation of crosslinked ethyleneepropyleneediene

rubber in acidic environment. Part II. Effect of peroxide crosslinking in pres-

ence of a coagent. Polym Degrad Stab 2006;91(1):81e93.

[4] Grassie N, editor. Developments in polymer degradation e 6. London:

Elsevier Applied Science; 1985 [chapters 3 and 7].

ARTICLE IN PRESS

7S. Mitra et al. / Polymer Degradation and Stability xx (2006) 1e7

+ MODEL

[5] Murakami K, Tamura S. Chemorheological treatment of crosslinked

EPT. J Polym Sci Part A 1 Polym Chem 1971;9(2):423e9.

[6] Spetz G. Stress relaxation tests. 2nd ed. Sweden: Elastocon AB; 1999.

Technical report 98/1.

[7] Kearsley EA. A basis for predicting long-time behaviour from

short-time tests of geothermal sealants. J Test Eval 1983;11(4):

299e308.

[8] Gillen KT, Keenan MR, Wise J. New method for predicting lifetime

of seals from compressionestress relaxation experiments. Die Angew

Makromol Chem 1998;261e262(1):83e92.

[9] Brown RP, Bennett FNB. Compression stress relaxation. Polym Test

1981;2(2):125e33.

[10] Brown RP. Stress relaxation of rubbers in tension e the simple approach.

Polym Test 1980;1(1):59e64.

[11] Othman AB, Hepburn C. Stress relaxation measurement of rubber in

tension e a new technique. Polym Test 1992;11(1):47e59.

[12] Clarke SM, Elias F, Terentjev EM. Ageing of natural rubber under stress.

Eur Phy J E Soft Matter 2000;2(4):335e41.

[13] Akiba M, Hashim AS. Vulcanization and crosslinking in elastomers.

Prog Polym Sci 1997;22(3):475e521.

[14] Flory PJ, Erman B. Theory of elasticity of polymer networks. 3. Macro-

molecules 1982;15(3):800e6.

[15] Graessley WW. Polymeric liquids and networks: structure and properties.

Oxford: Garland Science (Taylor and Francis group); 2004.