9101American Ceramic Society Bulletin, Vol. 86, No. 9

Cordierite (2MgO·2Al2O3·5SiO2) ceramic materials show interesting thermomechanical properties,including high thermal shock resistance, because of their low thermal expansion coefficient.1,2 Accordingly,cordierite-based materials have found favor as honeycomb supports for catalytic converters in automobiles,furniture for self-cleaning ovens and industrial heat exchangers for gas turbines.

Many processing routes, including slip casting and dry processing,2,3 result in devices based on these mate-rials. Extrusion long has been used to shape ceramic objects, mostly for traditional applications, such as brick,tile and pipe.4 Extrusion also is commonly used in other industrial sectors, including food, agriculture, chem-istry and pharmaceuticals.5

Benbow and Bridgwater5,6 have demonstrated that the extrusion of particulate pastes that are comprised offine particles suspended in a liquid continuous phase through dies with circular cross section and having asquare entry (see Fig. 1) can be described by

P = Pe + P1 = 2(σ0 + αVn) ln(D0/D) + (τ0 + βVm)4(L/D) (1)

where α is a velocity-dependent factor for the convergent flow, β the velocity-dependent factor for parallelflow, n and m exponents, σ0 the paste bulk yield value, τ0 the paste characteristic initial wall shear stress, D0

and D the diameters of the barrel and die, respectively, L the die-land length and V the extrudate velocity. Inthis equation, die-entry (Pe) and die-land (P1) pressures are separated.

Rod Extrusion of Cordierite-BasedPaste Containing Aluminum-RichAnodizing Sludge

M.J. Ribeiro and J.A. Labrincha

The plasticities of various aluminum-rich anodizing sludge batchformulations that contained a plasticizer and a lubricant were testedusing stress–deformation curves, and the effects of ram speed andpressure were evaluated using the Benbow–Bridgwater model.

Fig. 1 Schematic of extrusion through a square die in a ram extruder (P is total extru-sion pressure, Pe is die-entry pressure and P1 is die-land pressure of paste).

9102 American Ceramic Society Bulletin, Vol. 86, No. 9

Rod ExtrusionA coefficient of static friction for the

extrudate (µ) can be calculated by

µ = τ0/σ0 (2)

and can be considered an importantparameter for controlled extrusion.7

In this work, cordierite-based rodswere extruded using a ceramic paste thatcontained aluminum sludge (waste froman aluminum anodizing process),diatomite and talc. To adjust the plastic-ity level to allow defect-free extrusion,various amounts of plasticizing and lubri-cating agents were added. Plastic behav-ior was characterized by stress–deforma-tion curves and compared with those ofstandard industrially prepared pastes.8 Asludge-based formulation was used to testthe applicability of the Benbow–Bridgwater model in the extrusion of rods.

Materials and Methods

The cordierite paste (CP) was prepared from a premixedpowder that contained 25 wt% precalcined (at 1400°C)aluminum anodizing sludge (Extrusal SA, Aveiro,Portugal), 43 wt% talc (Luzenac, France) and 32 wt%diatomite (Anglo-Portuguese Society of Diatomite, Óbidos,Portugal). An alternative paste (CP-S) was prepared usingsand (Mibal-B, Barqueiros, Portugal) as a diatomite replace-ment. Details of preparation and characterization of thesludge have been given elsewhere.8,9 To adjust the plasticitylevel, commercial additives were incorporated in the testformulations: a plasticizer (Zusoplast PS1, Zschimmer andSchwarz, Germany) and a lubricant (Zusoplast O59,Zschimmer and Schwarz) (Table 1).

The yield value and plasticity level the pastes were obtained from stress–deformation tests conductedusing plastic compression (Model LR 30K, Lloyd Instruments) in special metal molds. A screw extruder wasused to preextrud the formulations (Table 1) through a cylindrical die (diameter of ~33.0 mm) to improvemixing and homogeneity. These rods then were cut into test billets (diameter of ~33 mm and length of~43.0 mm). A minimum of three specimens per composition were tested. Compressive tests were conduct-ed at a constant loading rate of 2.0 mm/min until a maximum deformation of ~70% or the ultimate limitof the load cell (500 N) was reached.8

Extrusion tests were performed in a ram extruder using a rod die with various ram velocities: 1, 2, 5, 10,20, 30, 60, 100 and 200 mm/min. Contributing pressure drops generated during the extrusion were record-ed (Fig. 2). The apparatus used has been described in a previous work.4 Values of total pressure (P) appliedthrough the ram and pressure at the die-entry (Pe) were measured using digital sensors.

Stress/Deformation Behavior

Plastic deformations of CP and CP-S, with or without lubrication and plasticizer additions, were obtainedusing plastic compression (Fig. 3). Paste without additives exhibited low levels of plasticity, as defined in aprevious work.8

Fig. 2 Detailed representation of the die used for ram extrusion of rods productionwith the indication of partial pressure drops in the die-entry region and in die (Peis die-entry pressure, D0 = 34.14 mm, D1 = 25.88 mm, D2 = 35.00 mm, D3 = 7.75mm, L2 = 13.50 mm, L3 = 8.60 mm, θ = 6° and Φ = 44°).

Table 1 Tested CP Batch Formulations

Zusoplast PS1 Zusoplast O59 Moisture

Specimen (wt%) (wt%) content (%)

CP/without additive 45.8H 45.8

CP/6P4L45.8H 6 4 45.8

CP/6P4L34.9H 6 4 34.9

CP/8P4L45.0H 8 4 45.0

CP-S/6P4L34.5H 6 2 34.5

9103American Ceramic Society Bulletin, Vol. 86, No. 9

The plastic deformation region was narrow and yield stress values were high. Paste formulations thatexhibited this curve form were predicted to fail in extrusion. This proved to be the case with several failedattempts to obtain simple rods, even when relatively high pressures were applied. An industrial earthenwarepaste also was tested. The determined plasticity level, assumed as normal to use in a common ceramic process(e.g., roller, plastic press and extrusion), was compared with the CP plastic behavior. This comparison showedthat CP/6P4L45.8H paste had plasticity most like that of industrial paste. Therefore, it was selected for theextrusion studies (despite its higher level of moisture, which obliged a careful drying process).

The CP-S tested only with plasticizer and lubricant addition (CP-S/6P4L34.5H) was more plastic whencompared with the CP with the same moisture level. For the same plastic behavior, the CP-S needed ~10%less water content. This result confirmed that the water sequestration effect, induced by diatomite particles,was well documented4 and might explain the difference between the two pastes. A previous work8 showedwater to be a major contributor tothe level of plastic behavior. Wherelubricant and/or plasticizer wereadded, the level of water requiredwas reversed, such that the CP for-mulations required more water andlubricant when compared with CP-S. This effect was attributed to thepresence of diatomite. Because of thehigher moisture content of the CPformulation (with additives), carefuldrying and firing operations werepredictable.

Yield stress values (Fig. 3) thenwere used as an input parameter forextrusion modeling.

Extrusion CharacterizationExtrusion was characterized by

application of the Benbow–Bridgwater equations used for model-ing the flow of pastes through dies withcomplex geometry.4–6 The total pres-sure drop for the current die design comprised several definable contributions, p0 to p3 (Fig. 2):

P = Pe + P1 = p0 + p1 + p2 + p3

= [2(σ0 + αVn + τ0 cotg θ) ln(D0/D1) + βVm cotg θ)] + [(σ0 + αVn) ln(A1/A2)]

+ [2(σ0 + αVn + τ0 cotg Φ) ln(D1/D3) + βVm cotg Φ] + [4(τ0 + βVm)(L3/D3)] (3)

where θ and Φ are die parameter angles (where θ is the angle ofdie-entry region) (Fig. 2), Ax the areas at locations x, Dx the diam-eters at locations x and Lx the die-land lengths at locations x.

The plastic behavior of the CPs is similar to those of industrialpastes.8 Therefore, the extrudability of the CP was studied onlywith CP/6P4L45.8H and the five Benbow parameters (α, n, τ0, βand m) for this paste, which were obtained in a previous work4 andreported here (Table 2). The yield value (σ0) was assumed toequate to the yield stress determined from stress–deformationcurves (Fig. 3).

The measured values of the total pressure loss as a function ofextrusion velocity were compared with fitting curves obtained

Rod Extrusion

Table 2 Benbow Extrusion Parameters

Parameter CP/6P4L45.8H

α (MPa(s∙m–1)n] 0.462

n 0.284

β [MPa(s∙m–1)m] 0.126

m 0.433

τ 0 (MPa) 0.00909

σ0 (MPa) 0.06

µ 0.152

Fig. 3 Stress–deformation test curves of CP containing aluminum sludge.

Stress

(MPa

)

Deformation

9104 American Ceramic Society Bulletin, Vol. 86, No. 9

Rod Extrusionusing Eq. (3) for theCP/6P4L45.8H paste (Table 3and Fig. 4). To investigate thedifferences (in percent)obtained between predictedand experimental work, valueswere estimated from the quo-tient (predicted – experimen-tal)/experimental. Predictedvalues according to the modelseemed to be correct, and dif-ferences to the measured val-ues were <6.4%. There weresignificant differences at lowextrusion rates (18.2% in thefirst point) because of difficul-ties in stabilizing pressure/flux-ing characteristics in such shortperiods of time.

The evolution of predictedresults for each partial pressure(p0 to p3 as indicated in Fig. 2)according to Eq. (3) was deter-mined (Fig. 5). It was obviousthat p0 and p2 were the two maincontributors to the pressure loss,which suggested the relevanceof the plastic flow in the die-entry and at the end of the die-land (final shaping) regions. Bycontrast, p1 results were nega-tive, because it corresponded toa transition from a narrow to awide region (A1 < A2 in Fig. 2).Finally, p3 values were less rele-vant, because the length of theend of the dye was relativelysmall (L3 = 8.6 mm). �

About the Authors

M.J. Ribeiro is a faculty member withUIDM, ESTG, Polytechnique Institute ofViana do Castelo, Viana do Castelo, Portugal.J.A. Labrincha is a faculty member with theCeramics and Glass Engineering Dept., CICE-CO, University of Aveiro, Aveiro, Portugal.Correspondence regarding this article shouldbe address to J.A. Labrincha via e-mail atjal@cv.ua.pt.

Fig. 4 Total pressure drop developed upon extrusion of rods for CP/6P4L45.8H, wheren = 0.284, m = 0.433, α = 0.462 MPa(s∙m–1)0.284, β = 0.126 MPa(s∙m–1)0.433 and τ0 =0.00909 MPa ((�) measured and (—) fitted results).

Pres

sure

(MPa

)

Extrusion velocity (m·s-1)

Fig. 5 Component pressures through the rod die as predicted by each part of Eq. (3) forpaste CP/6P4L45.8H.

Pres

sure

(MPa

)

Extrusion velocity (m·s-1)

Table 3 Pressures for CP/6P4L45.8HExtrusion

Extrusion Experimental total Calculated total Differencevelocity (m/s) pressure (MPa) pressure (MPa) (%)

0.000283 0.3511 0.4151 18.23

0.000583 0.4570 0.4597 0.59

0.001333 0.5174 0.5270 1.84

0.002750 0.5603 0.6046 7.91

0.005267 0.6519 0.6940 6.45

0.00750 0.7145 0.7522 5.27

0.0150 0.8629 0.8910 3.26

0.0260 0.9951 1.0296 3.46

0.050 1.2330 1.2361 0.25

9105American Ceramic Society Bulletin, Vol. 86, No. 9

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Rod Extrusion