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9-1-2009

Interface Slippage Study Between Polyamide 12and Ethylene Butene Copolymer Melt In CapillaryExtrusionJinhai YangUniversity of Southern Mississippi

James L. WhiteUniversity of Akron

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Recommended CitationYang, J., White, J. L. (2009). Interface Slippage Study Between Polyamide 12 and Ethylene Butene Copolymer Melt In CapillaryExtrusion. Journal of Rheology, 53(5), 1121-1132.Available at: http://aquila.usm.edu/fac_pubs/1181

Interface slippage study between polyamide 12 and ethylene butenecopolymer melt in capillary extrusionJinhai Yang and James L. White Citation: Journal of Rheology 53, 1121 (2009); doi: 10.1122/1.3198245 View online: http://dx.doi.org/10.1122/1.3198245 View Table of Contents: http://scitation.aip.org/content/sor/journal/jor2/53/5?ver=pdfcov Published by the The Society of Rheology Articles you may be interested in New capillary rheometer allowing for small-angle x-ray scattering experiments inside thedie. Application to the extrusion of block copolymers, their macroscopic defects, and theirstructure J. Rheol. 50, 803 (2006); 10.1122/1.2279527 Disentanglement of polymer melts flowing through porous medium before entering acapillary die J. Rheol. 46, 1307 (2002); 10.1122/1.1501926 Interfacial phenomena in the capillary extrusion of metallocene polyethylenes J. Rheol. 41, 1299 (1997); 10.1122/1.550836 On Shear Stress at Wall and Mean Normal Stress Difference in Capillary Flow of PolymerMelts: Authors' Reply J. Rheol. 25, 147 (1981); 10.1122/1.549641 Notes: On Shear Stress at Wall and Mean Normal Stress Difference in Capillary Flow ofPolymer Melts by Okubo and Hori J. Rheol. 25, 139 (1981); 10.1122/1.549639

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Interface slippage study between polyamide 12 andethylene butene copolymer melt in capillary extrusion

Jinhai Yanga)

Department of Polymer Science and High Performance Materials, University ofSouthern Mississippi, 118 College Drive, Hattiesburg, Mississippi 39406

James L. White

Department of Polymer Engineering, The University of Akron, 250 South Forge,Akron, Ohio 44325

(Received 4 January 2009; final revision received 17 July 2009�

Synopsis

Extrusion of a polyamide 12 �PA12� material through a capillary die coated with an ethylenebutene copolymer �EBM� was studied. The EBM coated die significantly increased the flow ratesof the PA12 melt compared to a clean die at the same extrusion pressure. Introducing a maleicanhydride grafted ethylene-octene copolymer �EOM-g-MAH� into the EBM suppressed the effect.This behavior seems only explained by significant interface slippage between PA12 and EBMmelts, which could be eliminated by introducing covalent chemical bonds across the interface. Amathematical analysis was carried out to calculate the interface slippage. The shear stress whereslippage began to occur was around 0.045 MPa and the slippage velocity was around 15 mm/s at0.1 MPa. Adding EOM-g-MAH could largely decrease the interfacial tension between EBM andPA12, thus largely decrease the interface slippage.© 2009 The Society of Rheology. �DOI: 10.1122/1.3198245�

I. INTRODUCTION

There have been many studies of the possibility of slippage between flowing fluids andsolid surfaces. Some of those date to a rather early period �Newton �1726�; Bernoulli�1968�; Stokes �1846, 1851��. These led to the conclusion of the no-slip boundary con-dition on which Newtonian fluid mechanics is based. Of a more recent date are consid-erations of the boundary condition between two flowing liquids �Bingham �1922�; Taylor�1932�; Tomotika �1935��, where for Newtonian liquids it was concluded again that therewas no slippage.

Investigation about suspensions, elastomers, and molten thermoplastics flows since the1930s has caused new attention for the issue of potential slippage at liquid-solid andliquid-liquid interfaces. This was first critically studied for the former case. For 30 years,Mooney �1931, 1958, 1959, 1962� almost alone investigated slippage over steel surfaces.In the 1960s, Benbow et al. �1961�, Benbow and Lamb �1963�, and Tordella �1963�called attention to slippage in high density polyethylene in unstable flow in dies.

a�Author to whom correspondence should be addressed; electronic mail: shinnningsea@gmail.com

© 2009 by The Society of Rheology, Inc.1121J. Rheol. 53�5�, 1121-1132 September/October �2009� 0148-6055/2009/53�5�/1121/12/$27.00

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Slippage between polymer melts first came into question with the discovery of slip-page between hydrocarbon polymers and fluoropolymers �Mooney �1959�; Nam �1987�;Lee and White �2001�; Migler et al. �2001��. The problem of interfacial slippage betweenother polymer melts was separately considered by Lin �1979�, Lyngaae-Joergensen et al.�1988�, and by Zhao and Macosko �2002�. This involves investigations on blend systems,such as polypropylene/polystyrene �PP/PS� and polymethyl methyacrylate/PS blends,which exhibit lower viscosity than expected in multilayer extrusion.

In this paper, we propose a different experimental and investigate interface slippagebetween polyamide 12 �PA12� and an ethylene butene copolymer �EBM� in the moltenstate. The system is chosen for which slippage, because of the high interfacial tensionbetween polyamides and polyolefins �Chen and White �1993��, is most likely to occur andthe EBM is soluble in various hydrocarbon solvents, which simplifies the experimentalprocedure. We also study binary systems, which contain a 0.5% maleated ethylene-octenecopolymer mixed into EBM �EBM/EOM-g-MAH� in contact with PA12 melts. Thisshould lead to chemical reactions of the blend with PA12 amine chain ends. This shouldinterfere with the interface slippage.

II. EXPERIMENT

A. Materials

The major materials chosen for this study were an EBM with 12.5 mol % butene anda PA12. The EBM was obtained from Exxon Mobil �Exact 4041� and the PA12 fromEMS-Grivory �Grilamid L25�. Some experiments were also made with a 0.5% maleatedEOM-g-MAH, which was supplied by Exxon Mobil �Exxelor VA 1803�.

B. Rheological measurements

The shear viscosities of these polymers were determined using both cone-plate andcapillary instruments at a temperature of 190 °C. The results are shown in Fig. 1 forEBM and PA12 and in Fig. 2 for EBM/EOM-g-MAH mixtures.

FIG. 1. Shear viscosity as a function of shear stress for the EBM copolymer and PA12.

1122 J. YANG AND J. L. WHITE

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C. Interfacial tension

An extending thread experiment was used to determine the interfacial tension betweenPA12 and EBM and its blends with EOM-g-MAH. This method was originally suggestedby Tomokita in 1935 for Newtonian fluid pairs. It was subsequently used to measure theinterfacial tension of polymer pairs �Elmendorp 1986; Demarquette �2003��. PA12 wasmelt spun on an Instron capillary rheometer into threads with diameters of 80–100 �m.The EBM and EBM/EOM-g-MAH sheets with dimensions of 10�10�0.5 mm3 wereprepared by a compression molding press. The PA12 threads were placed between twoEBM sheets. The composite was enclosed between two glass slides and placed on a hotstage at 190 °C under a Leitz Laborex optical microscope.

D. Coated capillary experiment

To study the interface slippage behavior between EBM and PA12, a capillary die �withdiameter of 1.59 mm, L /D ratio of 28.5, and entry angle of 90°� was coated by 3 wt %of EBM solution in heptane. Before every test, the die was burned using a propane torchto clean up any remaining material. The solution was dripped into the die hole. An air gunwas used to blow the solution to coat the die and evaporate the solvent. The die wasweighted by an electronic balance with a precision of 0.01 mg �AX-205, Mettler Toledo�to evaluate the thickness of the EBM coating film as

t =�w

�S, �1�

where t is the EBM film thickness, �w is the weight increase in the die, � is the densityof EBM, and S is the surface area inside the die.

PA12 melt was extruded through both the clean die and the die coated with EBM. Therelationship between flow rate and shear stress was obtained. By comparing the Q vs. �w

relationship at both the cases, the slippage speed of PA12 melt on EBM film was char-acterized.

FIG. 2. Shear viscosity as a function of shear stress for EBM/EOM-g-MAH mixtures.

1123SLIPPAGE BETWEEN PA 12 AND EBM

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E. Flow analysis of two layer flow through a capillary die

Figure 3 shows the cross-section of PA12 melt flowing through the EBM coated die,where the thickness of the EBM coating is �, and the radius of the die is R, PA12 ispolymer B, and EBM is polymer A. We can obtain the flow rate of the melt by using thefollowing equation:

Q = QA + QB = �0

�R−��

2�rvBdr + ��R−��

R

2�rvAdr . �2�

Integrating by parts yields

Q = ��vBr2�0�R−�� + vAr2��R−��

R − �0

�R−��

r2dvB − ��R−��

R

r2dvA� . �3�

Equation �3� can be rewritten

Q = ���R − ��2vs + R2vA�R� + �0

�R−��

r2−dvB

drdr + �

�R−��

R

r2−dvA

drdr� , �4�

where vs is the slippage speed of melt B on melt A; vA�R� is the slippage speed of meltA on the die surface.

We have the force balance for both melts A and B,

r =�

�wR , �5�

where � is the shear stress at the position with the distance of r from the die center, and�w is the shear stress at the die wall. Equation �4� can be rewritten as

Q = ���R − ��2vs + R2vA�R� +R3

�w3 �

0

��R−��

�2−dvB

drd� +

R3

�w3 �

��R−��

�w

�2−dvA

drd�� .

�6�

FIG. 3. Cross-section of EBM coated die with PA12 flowing through.

1124 J. YANG AND J. L. WHITE

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For the special case where there is no coaxial arrangement, but the die is filled withpolymer melt A alone, we can obtain the value of vA�R� based on Mooney’s equation�Mooney �1931��

vA�R� = � ��Q/�R3���1/R�

��w

. �7�

Using a capillary rheometer with dies of diameter 1.52, 1.94, and 2.28 mm and L /Dratios of 10, 15, and 30, we evaluated vA�R�. For the EBM, Q /�R3 is plotted vs 1 /R inFig. 4. It is seen that

vA�R���w= 0. �8�

Clearly, Eq. �6� may be simplified to

Q = ���R − ��2vs +R3

�w3 �

0

��R−��

�2−dvB

drd� +

R3

�w3 �

��R−��

�w

�2−dvA

drd�� . �9�

We have the force balance

�R − ��R

=��R−��

�w. �10�

Figure 4 indicates no slippage between the EBM melt and the die surface. It has beenreported that PA12 does not show slippage on steel die walls �Ahn and White �2003��.Equation �9� can be rewritten as

Q = ��R − ��2vs + R − �

R3

QB���R−��,,R+ QA��w,R − R − �

R3

QA���R−��,,R, �11a�

QB���R−��,,R= R

��R−��3�

0

��R−��

�2−dvB

drd� , �11b�

FIG. 4. Mooney plot of Q /�R3 versus 1 /R for EBM.

1125SLIPPAGE BETWEEN PA 12 AND EBM

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QA��w,R = R

�w3�

0

�w

�2−dvA

drd� , �11c�

QA���R−��,,R= R

��R−��3�

0

��R−��

�2−dvA

drd� , �11d�

where QB ���R−��,,Ris the flow rate of only melt B through the clean die at shear stress of

��R−��; QA ��w,R is the flow rate of only melt A through the clean die at shear stress of �w;QA ���R−��,,R

is the flow rate of only melt A through the clean die at shear stress of ��R−��.

Their values can be obtained through the flow rate Q vs shear stress �w curves of bothEBM and PA12 melts flowing through clean dies.

If ��R, Eqs. �11� can be simplified as

Q = �R2vs + QB��w,R. �12�

III. RESULTS

To test the occurrence of slippage between PA12 and EBM, we coated the capillary diewith EBM by the method we mentioned above. PA12 melt was extruded through the die.Table I shows the measured weights of the EBM coated layers based on which the EBMlayer thicknesses were calculated. The total coated surfaces include inside the capillarydie surface and the die entrance surface. The die had a diameter of 1.59 mm and L /Dratio of 28.5. This gave the inside capillary die surface of 226 mm2. The die entrance hada diameter around 8.8 mm, and the entrance angle was 90°. This gave the die entrancesurface region to be around 83 mm2.

We determined the flow rates and pressure drops through the series of EBM coateddies listed in Table I. The results were plotted as flow rate Q versus shear stress �w andare shown in Fig. 5. It is seen that as the coating thickness increases, the value of Qincreases for a die wall shear stress higher than about 45 000 Pa. This begins at an EBMlayer thickness of about 10 �m and increases up to a thickness of 15 �m �Fig. 5�.

In Fig. 6, we show a plot of Q versus EBM coating thickness at a value of 0.1 MPa.The rapid rise of Q when the coating thickness is 10–15 �m is apparent.

We may look at our results in a different manner. In Fig. 7, we compare the results ofa 15 �m EBM coated die with the results of equivalent experiments on EBM and PA12in a clean die. At low die wall shear stresses, the coated die data coincide with the resultsof the PA12 experiments in clean dies. As the shear stress increases above 45 000 Pa, Qgradually increases toward the EBM data.

TABLE I. Measured weights of the EBM coated layer.

Clean die weight�0.0001 g�

EBM coated die weight�0.0001 g�

EBM coating weight�0.1 mg�

EBM thickness��m�

31.6638 31.6652 1.4 5.231.6628 31.666 3.2 11.831.6637 31.6678 4.1 15.131.6634 31.6698 6.4 23.631.6631 31.6701 7.0 25.831.6638 31.6735 9.7 35.7

1126 J. YANG AND J. L. WHITE

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A second set of experiments involved using EBM/EOM-g-MAH mixtures of variouscompositions. Table II shows our measurements of the interfacial tension between PA12and EBM/EOM-g-MAH mixtures. It is very clear that adding EOM-g-MAH into EBMcan greatly decrease the interfacial tension between PA12 and EBM. It also places co-valently bonded polymer chains through the interface.

In these experiments, the EBM/EOM-g-MAH mixtures were first extruded through aclean die. Without subsequent cleaning, the PA12 was extruded through the blend filleddie. Plots of Q versus shear stress for this experiment are shown in Fig. 8. It can be seenthat the extrusion rate is reduced with increasing EOM-g-MAH content. The die filledwith neat EBM produces the highest throughput at shear stresses above about 45 000 Pa

FIG. 5. Flow rate Q through capillaries with various thicknesses of EBM coating of Table I.

FIG. 6. Flow rate Q versus EBM coating thickness at a shear stress of 0.1 MPa.

1127SLIPPAGE BETWEEN PA 12 AND EBM

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�Fig. 8�. We show in Fig. 9 that the EBM coated die flow rate result of our earlierexperiments �Fig. 5� is very close to the EBM filled die result of Fig. 8.

IV. INTERPRETATION

It seems clear that certain conclusions can be drawn from our experimental results.

�1� At shear stresses below 45 000 Pa �0.045 MPa�, there seems to be no slippagebetween PA12 and EBM in flow through an EBM coated capillary die.

�2� At higher shear stresses, interfacial slippage occurs and rises rapidly with increasingshear stress �Fig. 10�. Quantitative slippage velocities may be computed using Eqs.�11� by considering the EBM coating thickness and Eq. �12� by disregarding theEBM coating thickness.

�3� Introducing a maleated EOM-g-MAH into the EBM suppresses the slippage �Fig.11�. Quantitative slippage velocities have been computed using Eq. �12�.

In this study, EBM has a lower viscosity than PA12. An alternative explanation to theincreased PA12 flow rate might be suggested to be the lubricating effect of the lessviscous EBM. However, as shown in Fig. 10, this does not seem to be the case even whencalculations using Eqs. �11� are carried out.

FIG. 7. Plot of Q versus shear stress for the EBM coated die together with plots of PA12 and EBM throughclean dies.

TABLE II. Interfacial tension between PA12 and EBM/EOM-g-MAHmixtures.

Polymer pairsInterfacial tension

�dynes/cm�

PA12/EBM 12.4PA12/�EBM/EOM-g-MAH �90/10� 3.35PA12/�EBM/EOM-g-MAH �80/20� 0.84PA12/�EBM/EOM-g-MAH �60/40� 0.25

1128 J. YANG AND J. L. WHITE

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In our analysis, we simplified the capillary extrusion as a two layer co-extrusionassuming the coating layer thickness did not change. But the EBM coating thickness inthe extrusion decreased gradually. Thus, the actual coating thickness when we collecteddata was thinner than the original value. At a shear stress of 0.127 MPa, the remainingEBM coating layer thickness was 11.5 �m after 10 s, 3.7 �m after 30 s, and 1 �m after111 s. With a constant extrusion speed, the extrusion pressure maintained a constant valuefor more than 2 min before it increased to the same value as using a clean die when thecoating was purged out. A detailed analysis is available in the dissertation of Yang �2008�.This also indicates that the lubricating effect of the EBM coating layer had little effect onthe flow rate of PA12.

FIG. 8. Plot of Q of PA12 in various EBM/EOM-g-MAH blends filled die.

FIG. 9. Plot of Q of PA12 in EBM filled die and EBM coated die.

1129SLIPPAGE BETWEEN PA 12 AND EBM

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This may be interpreted further. In both the PA12 and EBM phases, there are co-valently bonded polymer chains, which are entangled. Few EBM or PA12 covalentlybonded chains pass through the interface between them. The interface region is muchmore subject to a shear failure than the homogeneous PA12 or EBM phases.

Introducing the EOM-g-MAH into EBM should result in chemical reactions with theamine end groups of the PA12. This can greatly decrease the interfacial tension as TableII indicated. These end groups result in covalent chemical bonds across the interfaces andincrease the resistance to slippage. We have also calculated the interface slippage for thecase of EBM/EOM-g-MAH blends. This is shown in Fig. 11.

FIG. 10. Calculated slip velocity of PA12 in an EBM coated dies.

FIG. 11. Calculated slip velocity of PA12 in an EBM/EOM-g-MAH filled dies.

1130 J. YANG AND J. L. WHITE

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We also tried to extrude EBM melt through a die full of PA12 melt at 190 °C. Wefound that the flow rate of EBM was also increased even though the viscosity of PA12was higher than EBM as shown in Fig. 1. Based on Eq. �12�, the slippage velocity wasalso calculated and shown in Fig. 12.

We have sought to compare our results with the earlier literature. Most papers usedviscosity-composition data exhibiting minima as justifying the hypothesis of slippage atinterfaces. The most thoughtful study of these phenomena was by Zhao and Macosko�2002�, whose views are very similar to our own. These authors used synthesized blockcopolymers with the PP-PS blends investigated and found that this greatly increased theapparent viscosities of their multilayer systems presumably by reducing the slip velocity.This is very similar to our experiment, where we introduced EOM-g-MAH into EBM.

Zhao and Macosko �2002� cited apparent viscosity negative deviation data of variousmelt pairs. Both their data and our data are clearly ordered in terms of the levels ofincompatibility of the melt pairs. High incompatibility results in high slippage velocity.This suggests correlations with interfacial tension and other solution interaction param-eters. In our study, the slippage between PA12 and EBM was around 15 mm/s at 0.1 MPa.The value reported by Zhao and Macosko �2002� for PP and PS pairs was around 1 mm/s.The difference presumably is because of the larger interfacial tension between PA12 andEBM of 12.4 dynes/cm compared with that between PP and PS of 4.5 dynes/cm �Palmerand Demarquette �2005��.

V. CONCLUSIONS

In this paper, we design an experimental study to determine possible slip velocitiesbetween immiscible polymers. The flow of a polymer melt through a capillary tube filledwith a second non-slipping polymer melt was investigated. The internal region of acapillary die was coated with an EBM and a PA12 was extruded through it. On the basisof observed high flow rates through the coated capillary as compared to the clean capil-lary, it was concluded that slippage was occurring at this polymer melt-polymer meltinterface. The slippage velocity was calculated as a function of local shear stress. Sig-nificant slippage was observed at shear stress above 0.045 MPa. It was found that if amaleated EOM-g-MAH copolymer is added to the EBM, the slippage velocity is greatly

FIG. 12. Plot of Q of EBM in clean and PA12 filled die and its slippage velocity.

1131SLIPPAGE BETWEEN PA 12 AND EBM

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reduced. This is presumably because the EOM-g-MAH reacts with the amine end groupsof the PA12 producing covalently bonded chains through the interface.

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1132 J. YANG AND J. L. WHITE

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