Relaxation phenomena of automotive TPEs By Abraham Pannlkollu,

Jerry J. Leyden and Michael S. Fulmer

Akron R1.1~ Otwe~nt LJDOriiOt'y Inc

Historically. the use o( thermuf>lastic elastomers in se"ere st'rvirf' en\'iron· menu has been limited by Tl'l::s inher· enl intrinsic properties.

The "soner" grad~• o( must TI'Es pos· seu neither high temperature· nor oil/solvent·resistance to displace the thermose t elastomers that ha'"e domi· nated severe serv ice applications (or many years.

TECHNICAl NOTEBOOK ~ted 11J Harald Htrzlic.b

With the development o( newer ther­moplastic materials and changing per· ceplions, however, it is evident that some TPEs, thermoplastic vulcanizates and co-polyesters are fmd ing expanding markets in these environments as the press toward TPEs with improved heal- and solvent-resistance continues to result in more re>ins in the market place which are performance competi­tive with TSEs.

One of the TSE mainslal's h.1< been automotive engine compartment appli­cations, particularly oil and transmis· sion seals-applic~lions "ofT limits" (or conventional TPEs.

Increasing under-the-hood tempera­lures have driven the upper end o( the required per(onnance temperature win­dow to 175' C (or certain elastomeric components.

TPEs continue Lo make inro:1ds inlo several underhood applications, usu~lly where specification requirements are typically a maximum of only 125' C. At the same lime, seals in contact with various fluids at these service tempera­lures further eliminate a number of TPEs from consideration for use as gas­kets, 0-rings and fluid-component seal· ing applications.

Since both temperature and Ouid ex· posure in combination represent

Fig. 1. Compression stress relaxation test Jig.

Force

r~:~~:': ._ ..... ~ .. ~~'~:E·;~~ri~i;;~~~ni~iri:t'-~~i~-~.q~:T1 In the aut.omolive industry, the most ~dely us~d technique t.o predict the

sealing capacity o( gasketing material is based on compression aet daLa which is obLained alter short-term aging in air or reference flu ids.'':' ·• : ·· ·· ··· • · · '

This is an indirect meuure o( sealing capacity ·and can easily be nilileading as a predictor o( the sealing performance of gaskets. ' . ' · •-. · ' ' · · ·

Recent increues in service temperature r equirements along with the use of SG-grade mot.or oils have created the need for .·. better means of meaauring the long-term performance of gasket materials. ' ' · · .. · · ' ·: : •' · : ' ::··

This paper report& compression alreas relaxation measurement-a on thenn<>­plastic elast.omer materials and how theae measurements compare with com-pression set data. . .· . · (· · · · · - . ;' ·

The report also outlines the trends of testing. gasket materials within the automotive industry and other sectora . . · ··: · · ' · :· ' · · . .. ·

. . .. · •. ~ . ,!: . .. . .

particularly difficult challenges (or TPE materials in sealing applications. it be­comes increasingly important to be able to predict the performance of new res­ins in both of these environments.

As the industry pursues the manufac­ture of these improved resins, appropriate perfo r mance evaluatio n tools continue to el'oll'e as well. His tori· cal single-point material property char­acterization testing, e.g., stress-strain , hardnes• and nexural properties, is u<eful when attempting to predict the life expecl~ncy o( a part. particularly in severe service. The advent of computer modeling and finite element analvsis as applied to thermoplastic parts is ·partie· ularly useful in identifying localized stress concentration areas, but is less useful when complicating and transient environmental variables are intro ­duced.

For several years now, manufacturers o( high performance TSEs, including Dow Corning STI and General Electric Co., as well as automotive manu(aclur· ers, most nol.:lbly Ford Motor Co. and

Fig. 3. Sealing Ioree decay.

General Motors Corp., hal'e been inves­tigating the use of compressinn stress­relaxation testi ng as a menns o( pre­dicting the service life of TSE seals in simulated service environments2·:t.

While most of this lest work has em­phasized higher service temperatures 050' C and 175' Cl, other TSE and TPE polymer manufactures, notably Shell Chemical Co., AES and Zeon Chemicals Inc., have developed an in­terest in testing for compression stress-r£1axalion at reduced tempera­lures (70' C to 125• Cl.

At the same lime, the Society of Automotive Engineers under the aus­pices of its CARS subcommittee on Long Term Aging, recently issued SAE J2236 which defines the continuous up­per temperature limit (or elastomeric materials based on long-term aging per­formance (1,008 hours)6•

Also forthcoming from Ford and GM are specification per(onnance standards for automotive seals based on compres· sion s\ress-relaxation testing, which most likely will replace or be added to

1oo1 ----..;;:::----- ----------------,

90

eo

~

" 70 0 ... "' 60 • ;; ..

!>() Vl ., ~ c •o ·;; -.; c: 30 1'-

20

10

0 0.1 100 1000 10000

Tlme, Hours

Fig. 4. Percent ol retained sealing Ioree.

Ttme 70'C 70' C 85'C 85'C IOO'C IOO' C Houu TPE 'I TPE 12 TPE II TPE 12 TPE 1' 1 TPE 1'2

0.5 100.0 100.0 100.0 100.0 100.0 100.0

22 •2.0 ~P.J 26.0 35.3 1C.O 27.6

72 37.t C3.7 20 .• 30.0 13.2 27.1

166 31.<& 37.6 1~.5 28.6 0.0 26.1

336 29 .• 33.6 18.0 25.0 0.0 2C.5

666 27.3 31.5 t 7.8 25.2 0.0 23.6

1000 25.2 31.8 13.0 2C.5 0.0 21.8

2000 25 .• 31.8 0.0 23.8 0.0 16.8

3000 25.6 33 .2 0.0 22.0 0.0 11.7

exi.sting compression set testing re­qu~rem<'nt.a lAST~ D 3951. ISOII3S compression slress· relaxation stan· dards have been in existence fur some tir11e now''·

Defining 'severity' The term "se,·ere sen·ice" as appl i<'d

to el~•tomeri~ materials is commonly assoc1ated With host ile tempe ratu res and chemical environment.•, either s1n­gly or more onen in combination.

For the manufacturers o( TPE compo­ne~t.a (or automotive underhood appli­caho_ns, the term "se,·erity" varies considerably, depending not only on the env~ronmenl.:ll variables but on the TPE polymer itself. In the family of a ll TPEs, severity varies considerablv

Consequently. service hfe predlction (or an individual TPE should be made only within near·rea•onable tempera­lures and environments.

Accelerated aging In 1990, Rapra Technology Ltd.'s R.P.

Brown 10 conducted a survey on the sta­tus o( methods for accelerated durabili­ty testing of polymers.

Input from 350 companies worldwide was solicited. Rapra concluded in part that, • ... normally single-point Ctests) are really only efTective as (quality as­surance) procedures ... · and that,· ... for thermal efTects, the only recognized pro­cedure is Arrhenius."

The classic An-hen ius equation. (d In kJidl=·C E!t21, can be modified to: log k = (E/2.303Rl OITJ + C Where: k =Specific acli,·ation rate E = Activation energy for the reaction R • Gas constant per gTam molecular weight T =Absolute temperature in Kelvin C = Mathematical constant

A straight line is produced when the log of a specific reaction rate is plotted against the reciprocal of absolute tem­perature since the above equation form is y • mx +b.

It has been empirically demonstrated that many reactions double or triple their rates for el'ery 10' C. As a result of both empirical observation and the

Continued on page 16

The authors Jerry J. Leyden, Abraham Pannikol·

lu and Michael S. Fulmer are officials at Akron Rubber Development Labora­tory Inc.

Leyden serves as president of the firm. He holds a bachelor's degree in chemistry from Ohio State Unh·ersity and an M.B.A. in management from Akron University. Prior to joining Ak­ron Rubber Development i n 1991, he held various technical and managerial positions at Midwest Elastomers Inc., Smithers Scientific Services Inc. and Firestone.

Pannikottu is manager of pred ictive testing. lle holds a bachelor's degree in mechanica l engineering (rom the Uni­versity o( South Gujarat in India and a master's degree in polymer science and physics from the University o( Akron.

Fulmer serves as manager of plastics testing at the Akron-ba•ed laboratory. Fulmer holds a bachelor's degree in life sciences from the University o f Akron. Before joining Akron Rubber Develop­ment, he worked in the physical testing and an alytical department-a at Chevron Chemical Co.'s polystyrene facihty.

Relaxation phenomena of underhood TPEs Co11/illl"ol {rum pnJir /.S :.hove lint":uily, if un~ can•fully l'huu~t'!'O n mconin~ful prupt•rty tlwl rt>latcs to •c,...·ire life, it 's pn»ihlc t.n pr~dod p~r· fomlance ot extr:qJ\•Inlt.•d ll.'nlp('ralurt.·s ba•cd on ob•e,...·ed rr•ull• al •cvcml a,·. lual lcmpcrnlurc•. Thi• technique ha• been u•cd s urrc .. rully fnr many year< when denling wilh t.h crrnll•~ll•ln•ln· mers in se"crc eawironml.'nt-..~ 11 " 11•

Gaskcting life expectancy The automotive industry has focused

on the perrenl or ret.aincd sealing force as a (unct.ion of time as the multipoint paramet.er (or useful service life predic· lion: When sealing force has decayed lo 2.5 percent. or the original sealing force, the seal is considered lo have failed.

The SlreSS decay o( polymer CompO· n ent.s under constant. compressive strain is known as compression stress~ relaxation. The lest measures lhe seal­ing force exerted by a seal or 0-ring un· der compression between t wo plates (See Fig. II. ll pro,;des definite infor· mation for t.he predict ion Of SCI'\'ice life by measuring the sealing force decay o( a sample as a function of lime, temper· ature and environment.

The ARDL t.esl apparatus used for the compression relaxation measure· ment.s is the ISO 3384 Wykeham Far­ranee de>; ce. This device measures t.he counter force exerted by a specimen maintained at a constant. strain be· tween two stainless steel plates inside t.he compression jig.

The instrument has a ''aricty of jigs for artommodaling test. pieces or 0· rings up t.o 100 mm outer diameter. Varioua ae,....;ce en,;ronment.s such as liquid, gas, or a mixture or liquid and

Carolina Knife offers

New Hot Cut Assembly

• designed specifically for conver· sion of rubber based textiles

• operates at 1500•f • woven products cut and sealed in

one step

Call Carolina Now! 1·800-520·5060

or (704) 253-6796 FAX: (704) 258·0693

Circle 172 on Fa11fax cord

~~~,.can he inlrotlun·d in to the KlninlcR" ~lt.•• •l rumpn·~siun j iJ.: nml mamt<lincd du rin~ n._:111l:: and h·~LIIIJ.:. A lypicul cros~·f'Cl'tiun vit•w of the cumprcssiun jiJ.: is shown in Fig. 2.

A typical •e:olin~ force d~rny ~roph of a TI'C: ~a>kel i• shnwn in t'i~ . 3. The•c curves were obtained :1l 25·pcrccnt con· s.t:1nt comprc~l"ivc !'~train al thrrc ncccl­cralt·d n~in~ tempera lurcH 170• C, 55• C and Joo• Cl. In this example, the •pcci· m('nS w~re n~<'d until a n arLitra ry "failure point." fur rclain~d scaling force is reached. i.e., when the scaling force decays lo 25 percent or the origina l un­aged scaling force. Scaling force rend· ing• may be found in Fig. 4 .

Fig. 5 is the Arrhenius se,...·ice life plot. from data obtained from the three decay plot.s from Fig. 3. The abscissa is t he reciprocal or the absolute tempera­lure, but. for convenience, lhe equiva· lent. Celsius tempera lure is sho .. ·n.

Compare lhe single-point. compres· sion sel dala on the two TI'Es (Fig. 6) with the multipoint. compression stress-relaxation curves CSee Figs. 7,8 and 91. I' ole lhal TPE ~2 docs nol t.ot.al· ly degrade a t. 100• C, but. that TPE U does. This data partially explains origi· nal equipment. manufacturers' prefer­ence (or the stress-relaxation dat.a.

Besides elevated temperature testing, the environment during compression stress-relaxation also can be nricd t.o obt.ain dala at low temperatures and/or in c:orrosi"e, oxidali\'e or fl uid environ· ment.s. An example or multi-media pre· dict.ed se,...;ce life cul'\•es from a recent TSE study is shown in Fig. 10.

Compression stress-relaxation t.esling is now underway al ARDL on a variety or elast.omeric seals in several "severe" environment.! for I ,008-hour agings. The result.s of a previous study were given in Denver at the ACS Rubber Di· ,·ision meeting in May 1993.

Predictive testing Dat.a t.o dale has shown two parlicu-

Circle 173 on Faslfax cord

Fig. 5. Arrhenlua aervlc e life prediction.

l~·r----------------------------------------------,

10000000

1000000

§ 100000

I 10000

i 1000 ~ .. " 100 ·~ .. en 10

0. 1

0.01 0 20 40 &o ao 100 120 160

t/T omperaturo, C

Fig. 6. Compression set (ASTM 0-395).

75.-------------------------------------------------~

70

65

( -- TPE#1 ....,_ TPE#2

30'~:::::...--r-----.-----,---~::===:;::::=~ eo 90 95 100 es Temperature C

70 75

Fig . 7. Compression stress relaxation a t 10• C.

-o eo .. -~ 70 ;; ct 60 .. u 0 50 u. r 40

"' .,; 30

';!. 20

( -- TPE#1 "*- TPE#2

10

01+-~-rTTrrn~-r,-rnTm--,-~~mT--~~~m-~~~~ O. t 10 100 1000 100CO

Time, Hours

Fig . 8. Compression stress relaxation at as• C.

lOOy-----~----------------~============~ 90 ( -e- TPE#1 _,.._ TPE#2

"0 eo ..

,!;

"' 70

;; ct 60 .. " 0 50 u.

"' ,!; ;;;

40

.. 30 en ~ 20

10

Torno, Hours

-.

-IB?lll!l%11 Fig. 9. Compression stress relaxation at 100" C.

100~---·,------------------~============~

[ -•- 1PE# 1 -. 1PE#2 l 90

'0 80 .. s 70

"' 'ii a: 60 .. u 0 50 u.

~ 40 .. .. ~0 (/)

j! 20 --.. 10

0 0.1 . 10000

Trne, Hours

Fig. 10. Service life in severe environments.

I 000000~! ===t:-10000

~ 1000

,; 3 100 " -~ .. 10 (/)

AT C

0.1

-·­OZC~E

OXJJA.TIVE

001+-----r--L~-----r----~----r----,-----r----~--~ 0 XI 40 60 80 100 120 140 160 ISO

1/Tempera1ure C

Jar benefit.s from a TPE perspecti\'e: • behavioral characteristics of ther­

moplastics under constant compressive strain; and

• the concept of maximum service temperature for TPE seals.

The thermoplastic behavior short term is consistent with compression set results obtained after short duration; lherearter the retained sealing force plateaus in contrast to TSE materials which show progressive deterioration.

A. expected, TPEs are more tempera­lure-sensitive than TSE materials, with a tendency to degrade beyond certain temperature limits. While this is a characteristic of thermoplastic male­rial, it can be used to establish the con· tinuous upper temperature for a given material application, pnrticularly when tested in the proper media.

Summary Compression stress-relaxation is the

luting methodology currently being used lo assess performance and predict the aervice life of thermoset elaslomeric seals in severe environment., and will most likely replace the single-point compression set test on aulomoli\'e material specifications.

As thermoplastic elastomer materials are developed for more severe-service applications, this testing melhudnlogy can be used to predict service life for TPE seals. It also can be used for rom· parative te•ling against TP~ controls or other TSEs already \xoing used in a porlicular applicnliun.

The life prediction methodology is soundly based on continuous multi­point stress- r elaxation coupled with clnssica l Arrhenius aging. Screening TPE materials utilizing this nppronch yields insight into long-term perform· a nee in eevere environmenLI.

References 1. <Ant. Or. Alan N .• f'd .• [nltin«-rint With Ru~ ~r; Hnw to Ouian Rubbe-r Compon~nt.,, tt.nAer Puhliahera/Odord UniYUSI\y Prua, s~w York tl992l. 2. GM CPC En~tinnrin~e 51.andard CME SR033 • .. Silicone Rubbu for Encine Cover Guketa," IMar<h 19921.

3 8un llnl. Or w ,JI ,am ~' • H u ~&•o· ll. '-" lll u.rn IJ. Ouin, Jamto~ £.TaLe. Al~tn 1. . Slo" um c:r•·.:••'> II , ca.: StiiCIIMf'll , WCuniJH""'"'''" S l rr •4 Hrl"'" ''"" I An1Hijlltf'll11111' o•• l (,,, t:,.,,,..,, ,. ,,. .. 1 "i. " '"" ' M otlo•· n11l11," :;At-: J•,,~., l~l'llll:t!'. tJ.',.Ioru,.ry Ht:t.l t

4 Furd L111(1Mo't'flnt.( M11lf't1AI !'l"'\'rH' IIIinl l W!' ~: M ~l)4!tU·/\, -nuutiiii' IIIC""'" I r\ ~~~~ I c.,\ ~~:•• nnl rJ n .. d.,rd, rur l Jt,.,., ,. tAnt , 11 •..:11 !"ttf'IIKih, 1. .. . T""'l""'"turt' H•·••!ll .otnt. U Hm~ • ~ IVuirr, Ur Chnrlu I', t"\ " '· Moon '""'" ('lu•nural Cn 1/\ESI, ~eumrrr""''''" r-;,, ... , H .. l"'"''''" 1\•·41 ~lf'A•u"' of Sfoalma \•v•C'•ty: lh .. M ... r and 1'1"" ''"~ ,..,. .... , •O..C 11. jQH)tJ

6. SAF. J4!2:1f;, "~t....,nd:ud Mt•lho -1 fur llf-tNmon tn,.: \unhnunua ll l•r-•r Tt•mjto.• rt~tur,. lt•••taUttl('l' nf E lr• •· t nme"," t l 994!1 SAt-: W111 rro· n•IAI"'. I'" 7 Urtt••h SLnn~nrd IIS~I:J, Jlnr\ A4:.!, J:to(:l , ·~'··!h­

od• of Tr11t1nR \'uknmtrd Hubl ,.•r J'nrt 1\ 42 Ut•tf•r· mmatlon u( Sttt'!'I~·Hchu.at•on,"' ti!JR:I t

It IStl SuH•douJ :lJ~'~_. , '"'RuhW r , Vulcanuf'd-Df'· t..•r m ' " "' '"" ,,( St'""""' )VI,., a tton tn Comp~tu~ton at Aml .. ,.nt llltt<i ,. , ~~l,.utlrd T,.mp••rt t U'Yiil." tl YAA\ ~· ISU S t u .. durt.l toll~•ii. ' "Uu /.l,..r, Vult' lllll ro•d ur Tho•fi i\H jl l ~t • l•r- l lt•t t"fnllnfttl•on ur ConljJtf'•••on Sttl' .. ., Ho•lu ' "''"" 'H '''Io!'" l." 1)!#1{;1,

)I) Hru• n , H: f' , K•f"" TrC'hn nln~~:.v Ltd , L'n•tt-d )\ u• .:d""'· ":-;"'"'' ur ·"'"tu• ufTr• t Mc-th•..d• r .. , A••••lo·nt lt••t Dur.olult t,\ Tr~lll•li: l'ol)mc.'r Tf'~lii iK, \'ul lU l~l '. f'l' J.:tu J J l '.u~n . f\,.uC'•• I; 11nd ~•nrc., \harlrs C ., •Nf'w l.dr J'r,.fi u·twu T .. rhnufUf' Tf'IIU St"aiJ •n s,.,,.,, ~·rvu•r t:n~""''"'"'nt.A." l:hllllumt'ncs, t~hy J989t, ~~~ :!11-=:! Jl lttrl••; . Arthu r W. "' 111 /. Jn,.ltlulf' nf Puhnll'r T··rhn .. l.ot.: \ , l't\. ~,\IJfiFAt ""lll l uf tht' Currf'nt S tan· d;ortl• r .• , s,,. .•• H·•hua t•nn "'""'"U'"nlc.'IILA tn Cum· 1''"'44 11 •11 r .. , Huhlwr s," l'ol;mt't Tra t tntt. \'ul. 6 I I!JI'iti t. 1'., ~~ •. )11!'1

.-------------·-----

We speah your Rubber and Plastics Process Language

t u: r .. t .·. • .- ... , • C' .-·.

2~:' -~:- :----~ E:;·.:::- f?.s :: == ·---r.a -- _ .. 4

Put FACTS pnx:css a>ntrol in ~'>e calendering. c.'-:n:sion. slll'et. 5lm, CXJating, oon"crting as weU as mi:ci:1g ind~<trics to worl; for you:

Integrate )'OUr equipment, r.:~easurcment, gaugi~g and cootrol with field-proven FACTS systems.

FACTS pro\' en systems are configured UJ your ~pedfic nffiis and can be reconfigured and expanded, allowing each syst.er.:1 UJ adapt lo your ch:tnging environment.

Speciali•u in Prorru Automation and Control

Call today for our FREE demo disk package.

2iJi Front Sttf'tt • Cuyahn1=a fal: 1 1Akron 1 O H ""22 1 2l6·928·2JJ2 • Fu: 2l6·92S·JOI8

Circle 17 4 on Fastfax cord

Circle 175 on faJIFax cord