70

Analysisof theproductionof spongerubberprofilesa b

Arndt Kremers,*AnnetteKrusche,EdmundHaberstroh

Institut fur KunststoffverarbeitungIKV, Pontstraße49,D-52062Aachen,GermanyFax:+49-(0)241-8888 262;E-mail: kremer@ikv.rwth-aachen.de

Intr oductionThe production of rubberprofiles on continuousvulcani-sation units belongsto the most important processes ofthe rubber industry. The annualworld wide volume ofsealingprofilesamountsto app.150kt/a only for automo-tive industry.[1] Unique performance properties in thefields of weather resistance, physical properties andprocessability are attributed to the automotive seals. Tofulfil thesedemandsthey mostly are a combination ofsolid and cellular materials.[2–3] Since the beginning ofthe 1990’s, ethylene-propylene-dieneelastomer(EPDM)hasbeenincreasingly usedfor automotive sealsandpro-files, as it complies with the required properties.Although the foamed material represents only a minorpartof theprofile, its production constitutesan importantpartof theEPDM application technology, which requiresa high level of know-how. All aspects of EPDM com-pounding in generalare included in this manufacturingprocess. The increasing product performance attributesrequiredfor thespongebodyseals,like theproduction ofcomplex configurationsto precise tolerance, a tempera-ture independent load deflectionanda smooth skin, leadto a complicate processdue to the large number of pro-cessvariablesduring fabrication and additionally due totheformulation effects.

In this paper the influencesof various parameters onproduct propertiesarediscussed.

In a large number of rheological and vulcametricaltests,the parametersare determined which describethespecific material properties of the compound. Withrespectto the practicability of the results,the investiga-tions are performed on a rheometerthat allows non-iso-thermalmeasurements.A mathematicaldescription of thevariationof thetemperatureprofile in theseal duringpro-cessingis usedto find a temperatureprofile for the rheo-logical measurement that is similar to theproduction pro-cess.This is to make sure that the vulcametrical para-metersare measured underconditionssimilar to proces-sing conditions. Furthermore,the mechanical propertiesof sealsmanufactured on an industrial production plantfor automotive sealsaredetermined.The objectivetargetis to find correlation betweenthe vulcametrical para-metersof thecompoundandthequality relevant mechan-ical properties of the spongerubber profile, so that thespongerubberextrusionbecomesmorecontrollable.

Spongerubber technologyAlthough automotive body sealscanbe manufacturedbya number of elastomerslike EPDM, NR, SBR,CR, NBRor LSR, EPDM is the most important polymer for thespongerubberproduction world wide.[3]

The manufacturing processof the EPDM seals canbedividedinto threetypical steps:1. Mix ing operation:The quality of the dispersion and

distribution of all ingredients is important for the finalproduct quality, especially for the smoothsurfaceoftheprofile.

Full Paper: EPDM spongerubberprofilesfor automotiveindustry are becomingincreasinglycomplex.As there isstill little knowledgeandprocessunderstanding,extensiveinvestigationshavebeendoneat the Institute of PlasticsProcessingto describethe influence of the rheologicalmaterial properties,the compoundcomponentsand theprocessparametersduringtheextrusion,blowing andvul-canisationon the profile quality. Thus the manufacturergainsa betterprocesscomprehension.In connectionwithimprovedtestmethods,this leadsto higherproductqual-ity andreducedproductioncosts.

Macromol.Mater. Eng.2000, 284/285 i WILEY-VCH Verlag GmbH, D-69451 Weinheim2000 1438-7492/2000/0112–0070$17.50+.50/0

a Presented at the 20th Plastics Technology Colloquium, IKVAachen,Germany, March22-24,2000.

b Publishedin differentform in RubberWorld 2000, 221(5)andin GummiFaserKunstst.2000, 53, 12.

Characteristic valuesof theproductionplant.

Macromol.Mater. Eng.2000, 284/285, 70–75

Analysisof theproductionof spongerubberprofiles 71

2. Extrusion:The temperature setting must becontrolledto avoid an early startof the degradation of the blow-ing agent.

3. Curing: The vulcanisationand simultaneousblowingreactions have the most important influence on thequality of thefinal seal.

There is still little knowledge about the optimumchoice of curing technology. Liquid curing medium(LCM), hot air or microwave (ultra high frequency(UHF)) curingbelong to thecommonsystems.Oneof themost important systemsworld wide is the UHF/hot aircombination. It leadsto a fast andhomogeneousheatingof evenvery complex shaped profiles within a shortdis-tancein theUHF unit. Thefollowing hotair systemkeepsthe profile on curing temperatureto completethe curingandblowing reaction.

Materials and experimental equipmentThe investigations describedbelow are concentrated onthespongerubberpartof thesealing andtheUHF curingprocess. Basedon the industrial processed compound A,thecompoundvariationsA1–A4 areusedfor the investi-gations(Fig.1).Theaim is a temporally better co-ordina-tion of the vulcanisationandthe blowing agentdegrada-tion.

The following exemplary resultsof the measurementare shown in Fig. 2. The variations of the prescriptionhaveinfluenceon thescorchtime aswell ason thevulca-nisationrate (quotedby a mechanical comparison valuein N/min) causedby the changesin the accelerator sys-

tem. The decreaseof DPG (A1) and the increaseCBSaccelerator(A3) raisethe vulcanisationrate.A reductionof the MBT accelerator content lowers the vulcanisationrate (A4).

The uncuredcompoundsareinvestigatedon the rotaryshearcure-meterMDR (Moving Die Rheometer)2000 P,Alpha Technologies, Akron. The cure-meter enables tomeasurethe changesof the pressure in the test chamberduring thetests.[4, 5] Non-isothermalmeasurementsarerea-lised at heatingratescorresponding to those in the laterprocess.However, thekind of heattransfer into themate-rial is different in the cure-meter(thermal conductivity)from the process(UHF). In the following investigationsthe courseof the torque, which is a measure of the com-pound viscosity, and the gradient of the torque, standingfor thevulcanisation rate,aremainly takeninto account.

For correlationpurposes, rectanglesamples of the dif-ferent compoundsare produced on an industrial produc-tion line, described in Fig. 3. The manufacturedprofilesareanalysedwith regardto their physicalandmechanicalproperties.

Calculation of the temperature profile in theUHF unitFirst the temperatureprofile for the rheologicalandvul-cametrical measurements is calculated. Relating to themicrowavetheory the powerdensity pw absorbed by thecompoundcan be calculatedaccording to the followingEq. (1) (Theusednomenclatureis summarisedat theendof thepaper):[6, 7]

Fig. 1. Formulation of thecompounds.

Fig. 2. Vulcanisation rate of the different compounds(MDR2000P).

Fig. 3. Characteristic valuesof theproductionplant.

72 A. Kremers,A. Krusche,E. Haberstroh

pw � 2p NE2 N e0 N er N tan d �1�In this equatione0 is theelectricfield constant.Thefre-

quency f and the electric field E are parameters of theequipment,whereasthe relativedielectric constanter andthe dissipation factor tand arematerial parametersof thespongerubbercompound, which depend on the tempera-ture. The temperature-dependentvalues are taken fromFedtkeandIppen.[8,9] Due to the energy balancethe tem-peraturedifferencecausedby theabsorbed UHF poweriscalculatedby thefollowing Eq. (2):[10]

_Q� _mN cp NDc with : _m� q N m NA �2�After relating Eq. (2) to the volume of the material in

combination with Eq. (1) the temperaturedifferencecanbecalculatedasfollows:

Dc � t NE2 N2pf N e0 N er N tan d

cp N q�3�

If er tand is constant, the rise of temperature in theUHF unit is directly proportional to the residencetime(Fig. 4, curve1). Theconsiderationof themeasured tem-peraturedependence[9] leadsto curves2 (er tand increaseswith the temperature)andcurve 3 (er tand decreaseswiththetemperature).

Measurements of the heating-up behaviour of the testcompound verify the validity of sucha simplifying pre-sumption.

MeasurementsAt first thevulcametricalproperties of all compoundsareinvestigated on a MDR 2000P. Afterwards the producedprofiles are analysedwith regard to their mechanicalproperties. The heatingrates for the test compounds arecalculatedon the baseof Eq. (3), the necessary materialparametersandtheresidencetime in theUHF unit. Heat-ing-up ratesbetween10 and30 K/min canbe realisedin

the used cure-meter. These are however smaller thanthoseappearing in theprocessin theUHF unit.

The rubbercompoundin thecavity of the rheometer isshearedby a rotor (sinusoidaloscillation). Due to themeasurement of its maximum torque as a function oftime andelevated temperaturethe information regardingthevulcanisationandblowing behaviour andtheviscositycanbeobtained.Starting from a minimal initial value, theelastictorque S9 increasesup to a maximumendvalue, assoonas the curing reaction begins.The minimal elastictorque S9min is a characteristic value for the compound.Simultaneously, theblowing agent degradesandthepres-sure in the cavity increases reaching a maximum Pmax.Thevery moment of a significant increaseof thepressureis measured as a fold of the torque.This value can bemarkedasSdeg andis characteristic for the blowing reac-tion, like themaximum pressure Pmax.

Compressionset testsand tensile testsare performedwith all profiles. Thecompressionset,thepressure resili-ence,the elongation at break, the breakingstrengthandthe specific dissipation are measured. In the followingdiscussionof the results only density, compression setandthespecificdissipationaretakeninto consideration.

Testresultsand interpr etation

Vulcametricalinvestigations

The development of the pressure during the rheologicalmeasurements is characteristic for the blowing process.Theanalysisof themaximum pressureis shown in Fig. 5.Thepressuresincreasewith anychangeof thecompoundfrom compound A up to A3 and decrease at A4, due totheearlierbeginof thevulcanisation. This means that theblowing agentdecomposeseasier, if the vulcanisationisless advanced;a fact that is verified by the pressuredevelopment in thecavity of compound A2. In contrasttothe basic compound, the increaseof the activator zincoxide(ZnO) leadsto a well increaseof pressure.This canbe explained by the smaller value of S9min, becausethecompound A2 shows no changein comparison to A1

Fig. 4. Temperature differenceof a rubber compoundin themicrowave(UHF) (qualitative).

Fig. 5. Influenceof the formulationsof the compoundson theprocesson thepressurein thecavity (MDR 2000P).

Analysisof theproductionof spongerubberprofiles 73

regardingother measured values, like scorch time andmaximumvulcanisationvelocity.

Furtherrheological investigations, which arenot intro-ducedin the next, allow the following conclusion: Thelower the viscosity of the material is, the easieris thedecomposition of theblowing agent.Theresults showtheinfluence on the changesof the accelerator system,thezinc oxide andthe heatingrates on the measurable rheo-logical parameters.

Physicalandmechanicalinvestigations

The results of the investigations show a substantialdependencebetweendensity andheatingrate.An increas-ing heating-uprate leadsto a lower density. The heatingrate, which is performed during the production in theUHF unit, hasa moresignificant influenceon the reach-abledensityof the product thanthe changesin the com-pound (Fig. 6). Mechanical properties show similarresults.

The graphical analysis of the compression set withregard to the different compoundversionsis shown inFig. 7. The mostobvious changecanbe recognised for aslow heating-up: The compressionset decreasesfrom Aup to A3 and increasesat compound A4. Due to thechangesin the formulations the scorch time is raised.Thus, the blowing agentcan be discomposed easierandbiggercells accrue.Simultaneously, the spongewill tendto showcells that aremainly open,so that the compres-sion setis reduced.Only for compound A4, that showsareduction of the scorch time, the pressure deformationrest increases. Thus,the variationsof the formulation donot lead to an improved product, which mainly shouldhavea closedcell structure.Thevariationof theheating-up rate (Fig. 8) shows, that higher heating rates alsodegradethe compressionset,which means the cell struc-tureis mainly open.

Fig. 9 shows the densities relatedto the compressionset for all compoundsand heating-uprates. This figuremeans a rank contrastregardingthe investigations whicharedescribedin ref.[1] But onehasto takeinto considera-tion that ref.[1] refers to a sponge rubber with predomi-nantly closedcell structure,while in Fig. 9 thecell struc-tureseemsto bemainly open.

Sponge rubberwith a low density mainly consists ofopen cells.At suchcell structuresa gasexchangeis quitesimple into both directions (during load and after dis-

Fig. 6. Correlationbetweenthe densityof the profiles andtheheatingratein theUHF Unit.

Fig. 7. Compressionsetrelatingto thevariationsof compoundA (T = 238C, t = 72h).

Fig. 8. Compressionsetrelatingto theheatingratein theUHFunit (T = 238C, t = 72h).

Fig. 9. Correlationbetweencompressionsetanddensityof thespongerubberprofile (T = 238C, t = 72 h).

74 A. Kremers,A. Krusche,E. Haberstroh

charge). The result is a lower value for the compressionsetfor lower densities.If it is possible to find anaccelera-tor systemfor the EPDM, which produceswith low den-sity several closedcell structures,a reverteddependencearises:A higher compressionset at low density and alower compressionsetat high density.

Summarising, for sponge rubber production it shouldbe noticed that a smaller density is easily obtainedbyincreasing the heating-up rateon the onehand,while ontheotherhandtherebymoreopencellsaccrue.

Secluding, the specific dissipation Dw is discussed(Fig. 10). The specific dissipation work is determinedinthe short-term tensile strength test according to DIN53 504.[11] During tensile load of the test piece,work isdoneat the sponge rubbersample. This energy correlateswith the areabelow the loading curve. Corresponding,thesample absorbsenergy againduringdischarging (areabelow the discharging curve). The difference of bothareasis interpretedas the specific dissipation Dw. Thedissipation is characteristic for the curing density of thesample.The lower the specific dissipation, the better isthe elasticity andthusalsothe curing density. The speci-fic dissipationshows thesmallest valueswhen high heat-ing rates are performed, which corresponds to the highcuringdensity.

The described characteristic material parametersandthe properties of the profiles canbe usedto form properregression models.Whenit is possible to calculatephysi-cal and mechanical product propertieswithout perform-ing expensive extrusion tests,the sponge rubberprocesswill bemorecontrollable in thefuture.

Conclusionand resultsThe describedinvestigationsshow that the processtem-peraturesshould be well known and shouldbe hold onconstantly, to avoid fluctuation in quality. Furthermore,itcanbe shownthat the rheologicalparameters, which are

measuredunder similar temperaturesconditionsasduringprocessing, allow a predictionof the product properties.Therefore, the calculation of the temperaturedevelop-ment in the curing unit, e.g., in the UHF unit, is reason-able.Under theseconditions, it will be possible to formregression modelsfor the description of the final profilepropertiesby the use of characteristic rheological para-meters.Thus,beforeproductionstarts,a predictionof theproductpropertieswil l bepossible,sothatthecostfor theproduction of sealingprofiles canbeeconomised.

NomenclatureADC = AzodicarbonamideCBS = Cyclohexylbenzothiazole-2-sulfenamideCR = ChoroprenerubberDPG = DiphenylguanidineEPDM = Ethylene-propylene-dienerubberHA = Hot airLCM = Liquid curing methodLSR = Liquid siliconerubberMBT = 2-MercaptobenzothiazoleMDR = Moving die rheometerNBR = Nitrile-butadienerubberNR = NaturalrubberSBR = Styrene-butadienerubberUHF = Ultra high frequencyZDEC = Zinc dibenzyldithiocarbamateZDBP = Zinc dibutyldithiophosphate

Dc = Temperature difference within the profile intheUHF unit [K]

Dw = Specificdissipation[J/m3]e0 = Electric field constant:8.85610–12 [As/Vm]er = Relativedielectric constantq = Densityof therubber compound [kg/m3]tand = Dissipation factor

A = Cross-sectionof theprofile [m2]cp = Specificheatof therubbercompound[J/kgK]E = Electric field strength of theUHF unit [V/m]f = Frequencyof theUHF unit [s–1]L = Lengthof theUHF unit [m]_m = Massthroughput of therubbercompound

[kg/s]Pmax = Maximumpressure in therheometer

(vulcameter) [Pa]Pw = Absorbed energy of thematerialin theUHF

unit [W/m3]_Q = Powerabsorbedby therubbercompound [W]S9 = Elastictorquein therheometer(vulcameter)

[Nm]S9deg = Torqueat themomentof increaseof the

pressure[Nm]

Fig. 10. Specific dissipation relating to the heating rate inUHF unit.

Analysisof theproductionof spongerubberprofiles 75

S9min = Minimum torque [Nm]t = Residencetime in theUHF unit [s]v = Velocity of theprofile in theUHF unit [m/s]

Acknowledgement:The investigations set out in this reportreceivedfinancial supportfrom the Ministry of EconomicsandTechnology (BMWi) by AiF e.V., to whomwe extendour thanks.Further thanksto the Bayer AG, Leverkusen, for the generoussupply of materials, test ranges and rheological measuringinstruments.

Received:August1, 2000

[1] J. W. M. Noordermeer, in: “RAPRA4th Cellular PolymerConference”, UK, 1997,p. 1.

[2] G. Stella, N. P. Cheremisinoff, in: “ International RubberConference”, Paris,1990, p. 43.

[3] A. Hill, Tech.Handel1992, 79, 292.[4] J. A. Sezna,H. Burhin, in: “146. Meeting of the Rubber

Division, American Chemical Society”, Pittsburgh, USA,1994,paperno.93.

[5] DIN 53529(1993), “Vulkametrie”.[6] H. Focht,Kautsch.Gummi Kunstst.1976, 29, 187.[7] W. Hoffmann, “Rubber TechnologyHandbook”, Hanser

Publishers,Munich,Vienna,New York 1989.[8] M. Fedtke,F. Schramm,Kautsch.Gummi Kunstst. 1998,

51, 201.[9] J. Ippen,“Mischungenfur diekontinuierlicheVulkanisation

im UHF-Feld”, unpublishedreport, BayerAG, Leverkusen1969.

[10] W. Wagner, “Warmeubertragung”, Vogelverlag,Wurzburg1984.

[11] DIN 53 504(1985),“Bestimmung von Reißfestigkeit,Zug-festigkeit, Reißdehnungund Spannungswertenim Zugver-such”.