Constitutive Characterization of Extruded Rubber …...Constitutive Characterization of Extruded...

Constitutive Characterization of ExtrudedRubber Blends by means of an Extrusion Rheometer

Herbert W. MÜLLNER*, André WIECZOREK**, Sabine SEILER**, Josef EBERHARDSTEINER*

** Semperit Technische Produkte Ges.m.b.H., Wimpassing im Schwarzatale, Austria

* Institute for Mechanics of Materials and Structures, Vienna University of Technology, Austria

Literature:

[1] S. Arrhenius: Über die Dissociationswärme und denEinfluss der Temperatur auf den Dissociationsgrad derElektrolyte. Zeitschrift für physikalische Chemie, 4 (1889):96-116.[2] M. Klüppel, R. Badura, R.H. Schuster: Thermodynamikund Rheologie weichgemachter Nitrilkautschuke –Kapillarrheologische Korrekturen. Kautschuk GummiKunststoffe 45 (1992): 614-619.[3] A. Limper, D. Schramm, M. Glabisch: PraktischeErprobung eines Extrusionsrheometers zur schnellenBeurteilung von Verarbeitungseigenschaften. KautschukGummi Kunststoffe 52 (1999): 644-651.[4] H.W. Müllner, J. Eberhardsteiner, P. Mackenzie-Helnwein: Constitutive Characterization of Rubber Blendsby Means of Capillary-Viscometry, Journal of Non-Newtonian Fluid Mechanics, 141 (2007): submitted.

The influence of an extrusion process on the viscosityfunction of rubber blends is identified. This knowledgeis required for performance of realistic numericalsimulations of injection heads and extrusion tools forrubber profile production. The experimental basis of thisinvestigation are tests with both, capillary and extrusionrheometer. The latter is a combination of an industrialextruder and a circular cross-sectional extrusion tool,allowing the application of material characterizationmethods used for capillary experiments. Thus,determination of the viscosity function of extruded rubberblends is possible. Additional capillary experiments allowan identification of the influence of extrusion on theviscosity of rubber blends. For the materialcharacterization a nonlinear iteration scheme is used,which is proven to be applicable for rubber compoundsand rubber blends. Successful verification of extrusionrheometer tests are performed with numerical back-calculation of the corresponding pressure measurements.

In order to identify theinfluence of an extrusionprocess on the viscositymore precisely, an extrusionrheometer is used [3]. Itconsists of a verticalextruder and a specifiedextrusion tool. In order toavoid the disadvantages ofcapillary viscometry, anextrusion tool with a circularcross-section has beenbuilt. Thus, determinationof the viscosity is possibleunder usage of character-ization methods, which arestandardly applicable forcapillary experiments withrubber blends.The influence of salt bathvulcanization on the profileshape is not relevant, there-fore is it not investigated.The diameter of the cylinderis 14 mm, therefore pos-itioning of pressure trans-ducers within the cylinder ispossible. The length of thecylinder is 133 mm. Thislength allows arrangementof three pressure trans-ducers and two temperaturesensors. The distances ofthe pressure transducersavoid significant flow inter-action.

top left: extrusion tool top right: dimensions of toolmiddle left: vertical extruder middle right: supply of raw materialbottom left: melt pressure bottom right: melt temperature

The determination of viscoelastic properties of rubber blends by capillary viscometry is characterized by different viscosity functionsdue to different geometries of capillary dies. According to Klüppel et al. [2] the occurrence of wall slippage is one possibility for thisphenomenon. Due to different diameters of capillary dies (D = 0.5, 1.0, 2.0 mm) and extrusion tool (D = 14 mm) the viscosity functionscannot be compared. The investigation of extruded materials under usage of capillary viscometry allows verification of interactionbetween extrusion process and viscosity as well as geometry dependence of viscosity functions. Because of usage of circulargeometries for both experiments, capillary and extrusion rheometer tests, the consistency factors of different experiments can becompared. For raw and extruded rubber blends a clear relation between viscosity and geometry of the extrusion tool is identified.

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melt velocity v [mm/s]melt velocity v [mm/s]

A numerical simulation of an extrusionrheometer test is performed using thecomputational fluid dynamics softwarePOLYFLOW Version 3.92 (by FluentInc., Belgium). A working point at atool temperature of 70 °C and a screwspeed of 20 rpm is considered.Therefore, a comparison betweenexperiment and simulation allows avalidation of the present materialcharacterization according to Müllneret al. [4].For the simulation wall adhesion isconsidered. The consideration of wallslippage in the framework of numericalsimulations is still in progress. Due tosymmetry reasons a quarter of theextrusion rheometer is considered.As material description the power lawwith parameters according to themeasurements of the extrusionrheometer is used (values of middlepressure transducer). The distri-butions of various state variables areshown. In addition, the positions ofthe pressure transducers are indi-cated. A verification of pressuremeasurements is possible, which iscollected in the table situated underthis text.The obtained numerical results showa good agreement with experimentaldata under consideration of energydissipation according to Arrhenius [1].Therefore, a satisfactory validation forthe material characterization methodaccording to Müllner et al. [4] has beenachieved.However, the influence of an extrusionprocess on the viscosity function hasnot been identified. For this task,additional capillary experiments withraw and extruded material arerequired.

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melt velocity v [mm/s]

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without correction

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shear strain rate γ̇ [s 1]

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η[P

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k = 179640 Pas0.145 n = 0.145

k = 186900 Pas0.132 n = 0.132

k = 173296 Pas0.160 n = 0.160

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D = 14.0 mm

extruded materialEPDM rubber blends

capillary viscometerD = 0.5/1.0/2.0 mm

capillary viscometerD = 0.5/1.0/2.0 mm

geometry dependenceinfluence of extrusion

EPDM rubber blendsraw material

extrusion rheometer

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40/220/210/240/120/110/15/110/0.55/0.52.5/0.5

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n̄ = 0.270

k̄ = 85107 Pas0.270

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k̄ = 73739 Pas0.273

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diameter of capillary and extrusion tool D [mm]

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Top left: consideration of energy dissipationTop right: viscosity of raw rubber blendBottom left: viscosity with extrusion rheometerBottum right: viscosity of extruded rubber blend

Numerical ValidationExperimental Investigations

Material Characterization of Extruded Rubber BlendsOverview