12th International Workshop for East Asian Young Rheologists, Chonburi, Thailand, 8-11 February 2017
Hosted by Faculty of Science at Sriracha, Kasetsart University, THAILAND.

Slip and Roughness at a ETFE/PS Interface during Coextrusion
Takuya Ito1, Sathish K. Sukuraman1 and Masataka Sugimoto1*1

Yamagata University, 4-3-16 Jonan,Yonezawa, Yamagata, 992-8510, Japan*sugimoto@yz.yamagata-u.ac.jp

Abstract: We investigated the relationship between interfacial slip and interfacial roughness during
the coextrusion of ethylene-tetrafluoroethylene (ETFE) and Polystyrene (PS) in a capillaryrheometer. The interfacial slip velocity was determined using the modified Mooney method and theinterfacial roughness was evaluated using a laser microscope. The interfacial slip velocity exhibiteda power-law dependence on the interfacial stress. At low shear stresses, the power-law exponent
was approximately 6. Beyond a critical interfacial stress, σc-i, the power-law exponent decreased tovalue of approximately 2. At around the same critical interfacial stress, σc-i, interfacial roughness
also begins to increase with the interfacial shear stress. This suggests that the interfacial slip andinterfacial roughness occur together. However, by comparing coextrusion flow in dies with andwithout a contraction, it appears that both interfacial slip and contraction flow are needed for theobservation of interfacial roughness.

Keywords: “Coextrusion” , “Interfacial slip” , “Interfacial roughness”, “ETFE”, “PS”.
1. Introduction
Coextrusion
processing
offersgreat
advantages including the ability to readily
adjust the number of layers, control thethickness of the layers etc. It can often be alower cost option than other techniques for
producing multilayers, such as lamination,coating etc.. However, multilayers producedby coextrusion often exhibit roughness at thepolymer-polymer interface. This roughness
significantly worsens the appearance of themultilayer and also has a deleterious effect on
physical properties. Past work suggests thatthe occurrence of interfacial roughness isstrongly correlated to the occurrence ofinterfacial slip.
Recently, Komuro et al.1 proposed for amodification of the Mooney method, typicallyused to investigate wall slip, to investigate
slip at a polymer-polymer interface. They
found that interfacial slip velocity varies with
the interfacial stress as a power-law, and thatthe power-law exponent changed at a criticalstress σi.2
In this study, we focused on the interfacialslip and the interfacial roughness during thecoextrusion of Ethylene-tetrafluoroethylene
copolymer (ETFE) and PS using a capillaryrheometer.
2. Materials and Method
2.1 Materials
We used ETFE (Asahi Glass, Tm=190)and PS (PS Japan, Mw=312,000). An ETFE
rod of length 150 mm length and diameter 9.2mm was prepared by extrusion. In order tocoat a thin layer of PS on ETFE, the ETFE
rod was first dipped in a 20wt%
PS/tetrahydrofuran (THF) solution. The
coated rod was then dried in a vacuum ovenat 80 for 1 week.
2.2
Measuring
thepolymer-polymer
interfacial slip velocity
The core-shell coated rod sample wascoextruded in a capillary rheometer(Capilograph, Toyoseiki). Capillary dies ofthree different diameters D=1.0, 1.5 and 2.0
were used. The die length to diameter ratio
L/D=20 and the entry angle was fixed at 60°.
The volume flow rate at different interfacial12th International Workshop for East Asian Young Rheologists, Chonburi, Thailand, 8-11 February 2017
Hosted by Faculty of Science at Sriracha, Kasetsart University, THAILAND.

stress values was measured. The modified
Mooney method of Komuro et al.1, 2 was used
to determine the slip velocity as a function of
the interfacial shear stress.
2.3 Interfacial stress calculation bynumericalsimulation(2dimensions,generalised Newtonian fluid)
Interfacial slip velocity depends on theinterfacial stress. In order to determine thecoextrusion conditions corresponding to theobservation of interfacial roughness, weperformed numerical simulations of a 2
dimensional generalised Newtonian fluid. The
chosen coextrusion conditions corresponded
to a range of interfacial shear stresses that
included the critical interfacial stress, σc-i.
2.4 Coextrusion of ETFE/PS multilayers
The coextrusion conditions determined using
numerical simulation was used to fabricate
the ETFE/PS multilayers. The existence or
non-existence of interfacial roughness at
different values of the interfacial stress wasdetermined.
3. Results and Discussion
3.1 Slip velocity and interfacial roughness bycapillary coextrusion.
The interfacial shear stress dependence ofthe slip velocity is shown in Fig. 1. As can beseen from Fig.1, the dependence of theinterfacial slip velocity on the interfacial shear
stress is a power-law.

Consistent with previous work on similar
polymers2,3, the power-law exponent wasapproximately 6 at low shear stresses anddecreased to approximately 2 at a criticalinterfacial shear stress, σc-i ≈ 4 .0×104 Pa.
In order to investigate the interfacialroughness, the PS layer was first dissolved
using THF. The interface was dried and thenscanned using a laser microscope.Fig.2
presents the relationship between interfacialroughness and the interfacial shear stress. Theinterfacial roughness is parametrised by thestandard deviation of the positive andnegative deviations normal to the interface, Rq.
As can be seen from Fig. 2, Rq is essentiallyconstant at low stresses and begins to increasebeyond a certain critical stress. Interestingly,
this critical stress is essentially identical to thecritical stress of the slip velocity, σc-i.

Fig.2 Interfacial slip velocity (Vs) and interfacialroughness at the ETFE/PS interface(T=260).

3.2 Calculating of interfacial stress by CAE
The die geometries used are indicated in
Fig.3. Using these die geometries thecalculated flow rates, pressure drop and thecorresponding
interfacialstresses
areindicated in Table 1(we showed only using
geometry (a) calculated in this abstract, andTest2,3,4 on Table 1 indicated over thecritical interfacial stress σc-i).

Fig.1 Slip velocity at the ETFE/PS interface at
T=260. For comparison, literature data from twoother polymer pairs are also shown.

12th International Workshop for East Asian Young Rheologists, Chonburi, Thailand, 8-11 February 2017
Hosted by Faculty of Science at Sriracha, Kasetsart University, THAILAND.

4. Conclusions
A brief summary of findings is as follows:

Fig.3 Geometries of the die used in this study.
Table.1 Calculated flow rates, pressure drops and thecorresponding interfacial stress for the coextrusion ofETFE/PS through the die indicated in Fig. 3 (a).
Test

QETFE:QPS

ΔP(MPa)σi(×104Pa)

1

1:6

1.49

3.0

2

1:9

1.74

4.0

3

1:12

2.00

4.7

4

1:14.7

2.20

5.3

3.3 Interfacial roughness during thecoextrusion of ETFE/PS.
ETFE and PS were coextruded at theconditions determined by the numericalsimulation. Experimental data in coextrusion
using die geometry (a) showed Table 2. At theconditions that corresponded to an interfacial
stress that was greater than the critical value,
the presence of interfacial roughness wasvisually confirmed. Visual confirmation iseasier using the corresponding movies, rather
than the presented photographs.
Table.1 Experimented data of the flow rates, pressure drops
and the corresponding interfacial stress for the coextrusionof ETFE/PS through the die indicated in Fig. 3 (a).

1. The dependence of the interfacial slip
velocity on the interfacial shear stressduring the coextrusion of ETFE/PS is a
power-law. The power-law exponent ofapproximately 6 at low shear stresses
decreases to approximately 2 at a criticalinterfacial stress, σc-i ≈ 4.0 ×104 Pa
2. Interfacial roughness was essentiallynegligible at low shear stresses and begins
to increase at the same critical stresscorresponding to the interfacial slip
velocity, σc-i. Therefore, we suggest thatinterfacial slip and interfacial roughness
are related.
3. We confirmed that interfacial roughness
was observed when the interfacial stress
was greater than the critical interfacial
stress σc-i, provided the die included acontraction.
References
1. R. Komuro S. K. Sukumaran, M.
Sugimoto and K. Kiyohito, Slip at theimmiscible polymer melts Ⅰ: method tomeasure slip. Rheol Acta, 53 (2014), 23–30.
2. R. Komuro, S. K. Sukumaran, M.
Sugimoto and K. Kiyohito, Slip at theimmiscible polymer melts Ⅱ: Capillary
Flow of Polymers with Unequal
Viscosities. Nihon Reoroji Gakkaishi, 42(2014),, 151-156.

3. P. C. Lee, H. E. Park, David C. Morseand Christopher W. Macosko. Polymerpolymer interfacial slip in multilayeredfilms, J. Rheol., 53 (2009), 893-915.