SERVICELIFE PREDICTIONOF RUBBERCOMPONENTSUSEDIN ENGINEERINGAPPL1CATIONS- A Review

AbrahamPannikottuManager,PredictiveTesting

Akron RubberDevelopmentLaboratory, Inc.

January26, 2002

I. Introduction

A. DegradationAgents

B. DegradationMechanisms

1. Introduction

2. Thermo-oxidativedegradation

3. Thermaldecompositieon

4. Radiationdegradation

5. Ultraviolet light degradation

6. Ozonedegradation

C. Time DependentLimitations

1. Inductionperiod

2. Oxygendiffusion

3. Fluid transport

D. Parametersto Monitor Degradation

1. General

2. Tensilestress-strainproperties

3. Hardness

4. Stressrelaxation

5. Set

6. Dynamicstress-strainproperties

7. Volumechange

8. Otherproperties

9. Functionaltests

10. Chemicalanalysis

II. ComponentTesting

A. SimulatingService

1. General

2. Naturalenvironmentalexposure

3. Simulateddesignlife

B. Principlesof ComponentTesting

1. Introduction

2. When to testproducts

3. Designof producttests

Page1 of 14

4. Examplesof testrigs

5. Summary

III. AcceleratedTests

A. General

1. Purposeof acceleratedtests

2. Methodsof acceleration

B. DesigningAcceleratedTestProgramme

C. Effect of Temperature

1. Low temperature

2. Propertiesat servicetemperature

3. Thermalexpansion

4. Heatageing

D. Effect of Liquids

1. Generalprocedures

2. Standardliquids

3. Water

E. Effect of Gases

1. General

2. Exposureto ozone

3. Evaluationof cracking

F. Weathering

G. Fatigue

1. General

2. Heatbuild-uptests

3. Flex crackingandcut growth tests

4. Testsin tension

5. Non-standardmethods

H. Abrasion

1. General

2. Typesof abrasiontest

3. Abrasiontestconditions

4. Abrasiontestapparatus

5. Expressionof abrasiontestresults

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I. OtherDegradationAgents

1. Ionisingradiation

2. Electricalstress

J. ServiceConditions

1. General

2. Temperature

3. Solarirradiation

4. Otherfactors

K. PredictionTechniques

1. General

2. Standardisedprocedures

3. Modelsfor changeof parameterwith time

4. Environmentaldegradationtests

5. Arrheniusrelationship

6. Time/temperatureshift

7. Artificial weathering

8. lonisingradiation

9. Effect of liquids

10. Effect of gases

11. Creepandstressrelaxation

12. Set

13. Fatigue

14. Abrasion

15. Dynamicconditions

L. Limitations and Pitfalls in AcceleratedTesting

1. Limitations

2. Pitfalls

IV. Appendix

V. References

VI. Abbreviations

VII. Index

Page3 of 14

ABSTRACT

INTRODUCTION

Thetopic of ServiceLife Predictionis of both practical and scientificinterest. The servicelife ofa elastomersets limit to engineeringdesign. Therefore,Life Prediction of elastomersshould bepartof theengineeringdesignprocess.Elastomerpropertiesare sensitiveto heat,moisture,light,fluids and mechanical stressFigure 1. Elastomerscan undergo changesin propertieslargeenoughto causeproductfailure. Most elastomerparts, in engineeringapplications,are intendedto be in servicefor severalyears. Hence,theengineeror thedevelopingscientistcannotwait thatlong to evaluatetheagingprocessin actualserviceconditions. Thethreemajorengineeringtasksin elastomerapplicationsare to determinethe Shelf Life. Service Life, and Remaining Useful

LifePart already inuse.

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Elastomer Products

EnergyEnvironmental Factors

li/AcidSI0I

Solvent .ation

L1-ozone b [iht

WindLjJ..Corrosive * __ Stress

..____ Mechanical Stres!jdynamicMistand static

Hej

Environment DustSalt -.

Corrosion Water

Structural

Properties

Figure 1

Page5 of 14

Elastomericmaterialsare frequentlyusedunderseverethermal,chemical and mechanicalstressconditions Table 1. A major drawbackof elastomersis their tendencyto oxidize. Most ofthem becomeunstablein contactwith atmosphericoxygen. Elastomersare also affectedby lowand high temperature. The wide range of service conditions produceschangesin physical,chemical,thermodynamicand otherpropertiesof elastomersTable2. Theseserviceconditionscan changein an uncontrolledmanner.

ComprehensiveServiceLife Predictionstudiesshouldinvolve fundamentalchangesin physicalpropertiesof the material due to degradationmechanisms,inter-atomicbonding, microstructureandcrystal structures.Elastomerdegradationshould alsoinclude characteristicssuchasthe lossof plasticizer,separationof polymerfrom fillers, andsurfacefriction.

ServiceLife Prediction is clearly of great scientific interest and has attractedthe attention ofchemists,engineers,scientistsand solid statephysicists. Hence,it is not surprisingthat the topichashistorically been discussedform a varietyof different scientific approaches.Thereis a greatneedfor a comprehensiveLife Predictionmodel which incorporatesvariousapproaches.Oneofthe main challengesin developinga servicelife is that severalparametersto be consideredbothfrom the material and serviceenvironment. Many of theseparametersaredifficult to describepreciselyin a mathematicalmodel. The objectiveof this paperis to establisha practicalusablemethodfor quantitativelife prediction of elastomers. This paperdescribesmethodsof ServiceLife Predictionfor an 0-ring, a Fiber-reinforcedRubberPipeJoint anda ConveyerBelt Cover.

Table 1: Main FactorsResponsiblefor Degradation.Thermal MechanicalThermo-Oxidative HydrolyticPhoto ChemicalPhotooxidative High EnergyRadiationOzone

Table2: DominatingEventsduring Degradation.RandomChain Scission SubstitutionDepolymerization PlasticizerLossCrosslinking Filler BondingChangeSide GroupElimination

Page6 of 14

AkronRubber Development Laboratory, Inc.ARDL Methodology for Ouantitative LifePredictionof Elastomers

QuantitativeServiceLife Prediction of the elastomericcomponentis becomesan increasinglyimportantrequirementas elastomersare usedfor more critical engineeringapplications. Servicelife prediction methodology should include all processesthat may affect the function of theelastomeric component. ARDL’s approach is to: 1., select the predominant degradationprocessesand establish an appropriate accelerated aging test; 2., compare the failuremode/degradationprocessof lab samples with field sampling using chemical, physical andoptical techniquesTable 3; 3., establishthe failure rates using acceleratedlab tests; and 4.,extrapolaterates to theservicecondition to determineservicelife.

Table 3: Techniques

__________________________________

OpticalMicroscopeVideoMicroscope

Scanning ElectronMicroscopyFI-IRGCIMSDSCDMAPulseNMRTGA

CrosslinkDensity WetChemistrySpecificGravityMicroHardnessMicroHardnessDecay

DielectricConstantElectronSpinResonance

Page7 of 14

ARDL’s methodologyflowcharts are shownin Figures 2a and 2b. The first stepis to definethefunctionsof theelastomercomponent.Basedon thefunction,establisha failure criterion. Thefailure criterion may be an unacceptablechange in function and the changemay cause aparticular failure. Changesmay be stressrelaxation, creep, tear resistance,stiffness/moduluschange,swelling, dielectricproperties,dynamicproprieties,etc. Then,characterizeand identifythe underiiningmechanisminvolved in this change. Establishthe rateof changeby acceleratedlaboratorytest at different levels of severity and at different time intervals. It is important tokeepthe acceleratedtest condition similar to the servicecondition and perform the test at fourtemperatureshigherthan averageservicetemperature.

Material Characterization

‘JrFunctional Characterization of Material

4.Functional Characterization

‘I,of the Part

Identify the Predominant Failure Mode

Field Data Correlation

4.Service Life Prediction of thePartj

Figure2a

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Definition of ComponentFunction

chemicalcrack

Chri0Tafon

friction fluidfailure permeation

Evaluationof geometricalaspectsFunctionalDurability Test and Finite ElementAnalysis

rmaldiffusionanalysisnalysis

Contributionof eachprocessfor contributionsto ratefailure

FieldData

Life prediction of component

Figure 2b Basedon Reference6

Aging processin the acceleratedtest should be verified with "service aging" process. Theverification can be done by chemicalor physical evaluationof both field samplesand samplesfrom different levelsof acceleratedtest conditionsandtime intervals.

After the rateof degradationprocesshasbeen determinedusing acceleratedlaboratory testing,then therateat the servicetemperaturecanbe determinedby Arrheniusextrapolationor by time-temperaturesuperpositionextrapolation. It is importantto study the mechanismsof failure inserviceand correlatewith eachacceleratedagingtest. This will help to improvereliability of theServiceLife PredictionModel.

I. ArrheniusApproach

This approach of life prediction involves determining the rate of failure for a range oftemperaturesof interestandthen plotting theserateson an Arrheniustypeplot againstreciprocal

-IDefinition of FailureCriterion

Itemizeprocessescontributingto failure

Material teststo measurerateof eachprocess

fracture dynamicmechanics testing

stressanalysis

Page9 of 14

absolutetemperature.This approachconsidersthe failure processasa chemical reactionwherethe rateof reactionwill increaseastemperatureincreases.

ServiceLife = Ae ff1

Where:Ea is the activationenergyfor thefailure process

A. A is a constantdependingon typeof failure

To be able to useArrheniusequation,acceleratedtestingshouldbe usedto identify the"time-tofailure" for a minimumof four temperaturesabovetheaverageservicetemperature.Servicelifepredictioncanbe obtainedby plotting the logarithmof the "time-to-failure" versustheinverseofabsolutetemperature‘/‘- andextrapolatingthecurve suchthat it intersectsthestraight line

=Tserviceis theaverageservicetemperature.To estimatetheexpectedtime-to

sevice

failure, it is importantto usea materialpropertywhich featuressufficient rangeto assureareliabledeterminationat anyparticularstageof thepropertyduring both acceleratedagingandfield aging.

1. Time - Temperature SuperpositionApproach

This approachconsidersthe rateof deteriorationas a predominantlyviscoelasticbehavior. Thisapproachwill allow theinvestigatorto developa mastercurvefor servicelife predictionusingtheWLF-equationWilliams, Landel andFerry thegeneralform of which is:

c,T -

loga =

c2÷Tr-TWhere:

T is referencetemperaturesusuallyT shouldbe 50K aboveTg

aT is the shift factorC1 andC2 are materialconstantsoftenC1 = 17.44 andC2 = 51.6T is the test temperature

From the presentpoint of view, it must be notedthat by rule of thumb one decadeof test timeincreaseis equivalentto an increasein temperatureof approximately6-7°C. The main problemwith this approachis that polymerdegradationmay not follow strict viscoelasticbehavior. Theequation permits the shifting of datataken over a time rangeand at a variety of temperaturesalong the time axis, to createa mastercurve which extendsthe time axis by severalorder ofmagnitude.

Page10 of 14

CASESTUDIESCase1: Life Prediction of 0-Rings by Compression Stress RelaxationTesting

The two important propertiesof elastomeric seals are Compression Stress Relaxation andDynamic Response.Dynamic Responseis a measureof the ability of the seal to retract to itsoriginal shapeafter a constrainthas been removed. Rapid recoveryof an elastomericseal isimportantin which thecontactbetweenthesealandmating surfaceis momentarilybrokenduetovibration or dynamic motion. Dynamic propertiesof elastomerscan be measuredusing forcedynamics. Theforcedecayof elastomercomponentsunderconstantcompressivestrain is knownascompressionstressrelaxation. Thetest measuresthesealingforce exertedby a sealor 0-ringundercompressionbetweentwo platesFigure 3. This property is very importantwheresealsare relatively inaccessible,and the cost of replacementis too high. In such instances,lifeprediction is highly desirable. The automotive industry has focusedon the percentof retainedsealingforceasa functiontime in a compressionstressrelaxationtest.

Shouldbe parallelwhile themeasurementismade.

Glass Plates

The apparatususedat ARDL to study compressionstress relaxation is the Wykeham-Farrancecompressionstressrelaxationequipment. Cylindrical samplesare usedin accordancewith ISO3384. Compressionstressrelaxation is usedfor the service life prediction by measuringthesealingforcedecayasa functionof time and temperature.IRM 903 is usedasthe agingfluid. Across-sectionalview of thetestjig is shown in Figure 4. Thesecurvesare obtainedat 25 percentconstantcompressivestrain at three acceleratedaging temperatures100°C, 125°C and 150°C.Sixty percentsealingforce decayis selectedas the failure point. This failure point is developedbasedon an earlier study by ARDL using Leak test, CompressionStressRelaxationtesting,DynamictestingandAutomotive OEM Engineeringdata.

Load Cell

Plunger

Specimen

Figure3

Jig Housing

Page11 of 14

Field data was collected for silicone compound A, B and C. Chemical and physicalcharacterizationof field sampleand lab sampleswas usedto verify the mode of failure anddegradationprocess.

a TestParameter

CompressionStressRelaxation

Accelerated oven aging100°C, 150°C and 150°C

b Aging Fluid: IRM 903

c Test Condition:25% compression

d Failure Mode:

e FailureCriteria:

20% increasemax80%increasemax

Pilunger

Figure 4

1 Test:

a AcceleratedAging: at

Sample were aged inside jigs with

StressRelaxation

CompressStressRelaxationForceRetained,%

i DynamicProperties

40%

tan Sdynamicmodulus

Page12 of 14

2 ExtrapolationTechniques: ArrheniusandWLF

Results: Shown in Tables 4d and 4e based on Arrheniusextrapolationtechnique. The datain Tables4a, 4b and 4cwere used to make a mastercurve to determinea shiftfactor aT value. Calculatedvalues were not consistentwith published literature values. From this one canconcludethat viscoelasticmechanismsdo not determinetherate of failure. Table 4f shows dynamic modulus andtan S valuesof acceleratedagedsamples.

Table 4a: SiliconeCompoundA SealingForceRetained,%hours 100°C 125°C 150°C

0.5 100.0 100.0 100.048 40.5 38.0 36.5

168 34.5 33.0 28.5336 31.0 29.5 24.5840 29.5 25.0 21.5

1008 28.5 23.5 19.52000 26.5 22.5 18.55000 25.0 19.5 14.5

Table4b: Silicone CompoundB SealingForceRetained,%hours 100°C 125°C 150°C

0 100.0 100.0 100.048 81.0 75.5 71.5

168 69.5 62.5 56.5336 62.5 56.0 48.0840 58.0 51.0 44.0

1008 56.0 48.0 40.02000 54.0 45.5 36.55000 48.0 40.0 26.5

Figure 4c: SiliconeCompoundC Sealing forceretained,%hours 100°C 125°C 150°C

0 100.0 100.0 100.048 61.0 56.0 53.5

168 52.5 47.5 42.5336 48.0 41.5 36.5840 42.0 38.0 32.5

1008 41.5 35.5 29.52000 40.0 34.0 27.05000 37.5 28.5 20.5

Page13 of 14

Table 4d:

Pmperty A B CHardness,ShoreA 59 55 64Ulitiniate tensile,MPa 5.8 4.7 6.9CompressionSet, %22hrs/70°C 33 16 1422hrs/100°C 73 18 1722hrs/125°C 87 26 2222hrs/150°C 89 34 35

SealingForceRetained,%168 hrs/150°C 28.5 56.5 42.51008hrs/150°C F 48.0 29.5

FieldData F* G F***in1anljately **after 1 yearservice

Table4e:Compound A B CFieldData Early 5 years+ 1 year

ExtrapolatedArrhenius 61 hours 6 years 7 monthsWLF iN

otSuccessful

Table4f: DynamicModulus by DMA tan?150°C

Compound A B C0 10.5 0.22 9.5 0.20 12.5 0.24

48 18.9 0.25 10.8 0.21 13.8 0.25168 19.2 0.26 11.2 0.21 16.8 0.27336 19.8 0.27 12.8 0.22 22.8 0.28840 20.4 0.28 14.8 0.23 24.8 0.29

1008 20.4 0.28 16.9 0.24 25.8 0.30

Page14 of 14

Case2: Remaining Useful Life Determination of Elastomeric Pipe Joints

In this study, Arrheniusextrapolationtechniquewith Laboratoryacceleratedoven aging testingwas usedto establishthe Remain Useful Life of ElastomericPipe Joints. Failuremode of thispart wasidentified asoxidativedegradation.

b TestParameter

1 AcceleratedAging Test: Accelerated oven aging at 60°C,80°C and 100°C

2 Field Samples: a Inside section b Middle section cOutsidesection

3 ServiceDuration: Five years

4 AverageServiceTemperature: 45°C

5 MeasurementTemperature: DSC induction timemeasurement

FailureModeEvaluation: Dynamictesting - Dampingcoefficient N-sec/mm

II. Differential Scanning Calorimetry DSC ASTM D-3418-88

The samplewas heatedfrom 30°C to 300°C at 25°C per minute in helium for approximately10minutes. Thesamplewas allowedto equilibratefor 6 minutes. After equilibrating,thegaspurgewas switchedfrom helium to oxygen. DSC inductiontime was calculatedbasedon time fromintroductionof theoxygento completionof decomposition.

1 InductionTime Of Field Samples: 1 Outside layer= 28 minutes

2 Middle section = 44 minutes3 Inside layer = 38 minutes

DampingCoefficient N-sec/mmof field sample: 1 Outsidelayer = 38.88

2 Middle section = 2.57

3 Inside layer = 16.43

FailurePoint: Failurepoint is calculatedbasedon failedfield sample.Inductiontime = 13 minutesandDampingcoefficient= 51.5 N-sec/mm

ServiceUfe PredictionExtrapolationa Technique: ArrheniusApproach

2 Table Sa: InductionTime by DSChours 100 °C 80 °C 60 °C

0 45 45 454 35 41 42.516 18 32 36

Page15 of 14

48 6 25 34168 0 17 301000 0 4 242000 0 2 19

*Extrapolatedservicelife - 8 years.Therefortheremaininguseful life of this partwill be 3 years.

TableSbInductionTimeminutes

DampingCoefficientN-sec/mm

Partin UseInside 38 16.43Inner Case 44 2.57Outside 28 38.88

FailedPart Outside 13 51.5

Table Sc: DampingCoefficientby DMA N-sec/mmTeperature 100°C 80°C 60°C

0 1.58 1.58 1.5848 68.1 28.4 16.5

168 98.1 52.8 24.52000 failed 108.5 48.7

Page16 of 14

Case 3:Service Life Predictionof IndustrialConveyer BeltCovers:

Heavy, steelcable-reinforcedconveyerbelts arecommonlyusedin the coal industry.The failuremode of the belt cover in theparticularapplicationwas identifiedas hydrolysis with subsequentultimatetearfailure.

2. TestParameter

234

a AcceleratedTest: Fatigue testdynamic tensiontest

Aging Fluid: WaterTestTemperature: 60°C,Test: Five million cycles

Table6b: DampingCoefficientat 60 by DMA N-sec/mmCompoundA CompoundB

0 0.852 0.6841 million 1.048 0.8452 million 1.284 1.0483 million 0.748 1.1284 million failed 1.2325 million 0.8486 million failed

MeasurementTemperature:

at 10 Hz with 10%

80°C and 100°C

MTS Dynamic characterson dumbbell sampleat Sand2

a Properties:Dampingcoefficientb AverageServiceTemperature: 40°C

Table6a: Cyclesto FailureTemp

1008060

Extrapolated

Comp A120000700000

20000004,600,000

CompB180000

10000004000000

9,200,000

Page17 of 14

CONCLUSION

The above approachescan be applied to determinelife of elastomerscomponentsusedinengineeringapplications.However,it is importantto definefailure modeand failure mechanism.It is also importantto establishverification andcorrelationbetweenfield and lab samplesusingphysical and chemical techniques. The primary rate determiningmechanismof componentfailure can be predicted using the Arrhenius methodology.The Arrhenius method provides aquantitativedeterminationof the servicelife of elastomercomponentsin a particularapplication.Furtherresearchstudiesare requiredfor eachnew application. The servicelife predictioneffortconducted on elastomeric materials provides a good materials databasefor computer-aideddesign engineerswho in turn can usethe information to effectively model part durability, thusreducingtheneedfor complexandcostlyprototypetesting.

Service life prediction as an engineering tool is still in the infancyAlthough some successeshavebeen reported,more applicationresearchcontinuing interest showedby the automotive OEM’s is likely to leadelastomerservicelife prediction.

stage of development.is needed. Recentandto further advancesin

Page18 of 14

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5. A. Pannikottu,J.J.Leyden RubberandPlastic News 18 1995

6. A.N. Gent "EngineeringwithRubber"HanserPublisherDurability Chapter7 171 1992

7. K.C. Ludema "Friction, Wear, Lubrication" CRC Press Strength and DeformationPropertiesof Solids Chapter2 9 1979

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I