Flow Visualization of Blending of

Elastomers and Thermoplastics in an

Internal Mixer*

Kyonsuku Min Polymer Engineering Center, University of Akron, Akron, Ohio 44325

ABSTRACT

Flow visualization of blending elastomers and perature and the polymer. The jlow regimes domi- plastics in an internal mixer is described. This is nate the phase morphology of blends of based on observations from front and transverse glass ethylene-propylene terpolymer and polystyrene. windows placed on the mixer. This enabled us to Various rotor designs used in this study creates dq- investigate thejlow behavior ofpure polymers during ferent two phase morphology due to various flow the blending operation. The jlow behavior was cat- jields. egorized into four regimes which depend on tem-

INTRODUCTION

The mixing or blending of several polymers to pro- duce a homogeneous product is an important problem in engineering. The most important of mixing ma- chines is the batch internal mixer. The internal mixer has a long history of being used to blend polymers. There appears to be a strong relationship between the

*The author would like to thank Professor I . L. White for his encourage- ment during the course of the research described in this paper.

mixing behavior and the flow fields in an internal mixer. Flow visualization in internal mixer is the most effective way to study the flow fields dominating the behavior of the internal mixer. Freakley and Wan Idris' reported room temperature flow visualization studies of silicone rubber in an internal mixer with Banbury type rotors using a poly(methy1 methacry- late) window perpendicular to the rotor axes. Recently Min and White' have presented flow visualization studies of major elastomers and plastics at tempera- tures of 100-180°C in a model internal mixer with

Advances in Polymer Technology, Vol. 7, No. 3, 243-257 (1987) 0 1987 by John Wiley & Sons, Inc. CCC 0730-6679/87/030243- I5$04.00

BLENDING IN AN INTERNAL MIXING

glass windows. In later studies the characteristics of flow around rotors of different design have been in- ~estigated,~ and the compounding of carbon black and oil into elastomers re~earched.~

In this study, we present flow visualization studies of the blending of a typical elastomer, ethyl- mined with a scanning electron microscope.

ene-propyrene terpolymer (EPDM), and a thermo- plastic, polystyrene (PS). Products of this type are commercially used as thermoplastic elastomers. Var- ious rotor designs have been studied. The phase mor- phologies of the EPDM/PS blends have been deter-

FIGURE 1 The schematic view of the system for flow visualization in an internal mixer.

VISUALIZATION OF FLOW IN INTERNAL LIIXER

--I I- tmr

AKE INTERNAL

( Rhamlr 150

MIXER

I

VIDEO CAMERA

TV . . ,

VIDEO RECORDER

FIGURE 2 Photographs of the front and transverse windows for flow visualization apparatus.

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EXPERIMENTAL

Materials and Conditions

In this study we used ethylene-propylene terpoly- mer (EPDM Epsyn 70A) supplied by Copolymer Corp. and a polystyrene (PS Styron 678D) by Dow Chem- ical. The blending temperatures used ranges from 140 to 180°C in steps of 20°C. A 50150 blend composition and a fill factor of 0.7 were chosen.

FIGURE 3 The rotors designs used in this study.

Banbury type

1

4-0- 4 1 . 2 7 mm

Apparatus jq3J- 35.76 mm 19 .05 ,11111 - - -

A Haake-Buchler Record 750 laboratory mixer with a specially designed front piece with a glass windows and heaters were used. This is the same apparatus described in our earlier ape$-^ except for the intro- duction of transverse win do^.^ A video camera, re-

motions of materials during mixing. The system is shown schematically in Figure 1. The front piece and chamber with glass windows are shown in Figures 2(a) and (b). The capacity of the chamber is 120 cc without rotors. The ratio of the rotor speeds is 7 : 6. The other rotors used in this study are: (i) mill roll rotors which involve two simple rolls; (ii) tapered rotors which have different diameters between the front and back rolls; (iii) screw type rotors which have a single screw thread (there are two combinations of right-right hand screws and right-left hand screws); (iv) Banbury type rotors which have two wings (larger and smaller ones). These are shown schematically in Figure 3. The first three rotors have a gap of 2.34 mm between the tips and the chamber wall except the front of tapered roll rotors which have a gap of 4.67 mm. The Banbury type rotor has a gap of 1.17 mm.

Mill rolls

- - _ -

/ / ' ' I k / / /

corder, and monitor was used to record the dynamic 2 . 3 4 mm

RIR and RIL screws

Procedure FIGURE 4 The DrOCedure of

The experiments to observe flow patterns from the EPDM White markers or mounting two red pigment polymers in an

internal mixer. front windows were carried out as follows. The poly- mers were premixed with less than 1 wt % zinc oxide at the same temperature as the blending temperature. This was carried out at 40 rpm for 2 min to produce samples for good contrast. Markers pigmented with red color (less than 1 wt %) were also similarly pre-

small strips and mounted into zinc oxide compounded polymer one by one (Fig. 4).

pared and pressed into sheets. These were cut into ps ( Yellow markers )

ADVANCES IN POLYMER TECHNOLOGY 245

BLENDING IN AN IN7’ERNAL. MIXING

FIGURE 5 Shear viscosity-shear rate curves of EPDM and PS at various temperature.

FIGURE 6 The still photos of motions of EPDM in an internal mixer as a function of time: (a) 180°C; (b) 160°C.

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to the above. PS was melted and EPDM was inserted in the central portion (see Figure 4). The markers pigmented with white color for EPDM and yellow for PS were mounted in polymers, respectively.

In order to study longitudinal motions with the transverse window of the mixer chamber, a similar procedure has been done. Red pigment was used for EPDM and yellow pigment for PS. 30 rpm and a fill factor of 0.7 were chosen.

Characterization

The phase morphology of the blends was examined with a JEOL Scanning Electron Microscope (Model U-3) and with an IS1 SX40 Scanning Electron Mi- croscope.

The viscosities of EPDM and PS were measured using a Monsanto Pmcessibility Tester. These are shown in Figure 5. The shear viscosities of both polymers are about the same at 140°C. As the temperature is increased, the shear viscosity of polystyrene drops rapidly and that of EPDM decreases slowly. This re- flects the high activation energy of viscous flow of polystyrene and the low values of high ethylene poly- olefins.

FIGURE 7 The still photos of motions of EPDM in an internal mixer with Banbury rotor as a function of time at 180°C (transverse views),

FIGURE 8 The still photos of motions of PS in an internal mixer with Banbury rotor as a function of time (front views): (a) 140°C; (b) 160°C.

ADVANCES IN POLYMER TECHNOLOGY 247

BLENDING IN AN INTERNAL MIXING

RESULTS

Behavior of PS and EPDM

Flow visualization of the behavior of pure EPDM and PS have been carried out for different rotor de- signs. Figures 6(a)-(c) show still photos of EPDM at various temperatures for Banbury type rotors. One can see the stretching and tearing of material on the bridge when the wings of the rotors pass by and away from the inter-rotor region. A stagnant region is also ob- served clearly on the bridge under the ram. Decreasing the temperature from 160 to 140°C causes EPDM to become more brittle. The stagnant region was found at all temperatures. At 180°C a sheeting out on the wall of chamber was observed. These results are sim- ilar to previous results reported elsewhere.24 Figure 7 shows still photos of transverse views for EPDM at 160°C. It shows zigzagging (kneading) motions by the large wing which pushes forward and the small wing which pushes backward. High shearing of ma- terials between the surface of the wings and the cham- ber wall is observed. The EPDM was tom into chunk by the high shearing zone. Empty zones behind the wings have been observed. These seem due to tearing of EPDM.

The PS, on the other hand, tends to form a band on the rotors at 140°C [Figure 8(a)]. No tearing was observed. Increasing the temperature from 140 to 160 to 180°C changed the flow behavior dramatically from a tight band to a liquidlike behavior which shows sheeting out on the wall [Figure 8(b)]. These phe- nomena have been seen for various rotor designs.

Behavior Blending EPDM and PS with Banbury Type Rotors

Figures 9(a) and (b) show typical still photos of the blending of EPDM and PS as a function of time at 180 and 160"C, at 0.7 fill factor and 30 rpm. The PS makes a band around the rotors. EPDM is chopped up into pieces and taken up into the band of PS. It is expected that a two phase morphology will consist of the matrix of PS due to the tight band around rotors and dispersed EPDM islands due to tearing. A dra- matic change of color from transparency to white was observed 1 min after the start of mixing. This phe- nomenon indicates light scattering due to the differ- ences of refractive indices between the matrix and the dispersed phase.

Figure 9(c) shows still photos of blending EPDM and PS at 140°C. With decreasing temperature, PS forms an increasingly tighter band around the surface of the rotors as shown in Figure 8(a). The chopped EPDM pieces were not pushed into the tighter PS band due to its apparently higher viscosity. After 1 min, the color didn't change as well as at 160 and 180"C, indicating a slowing of the dispersing process. Typical two phase morphology of EPDM/PS blends at 180, 160, and 140°C by Banbury type rotors are shown in Figure 10. Figures 10(a) and (b) shows the two phase morphology changed from the cocontinuous phase at shorter mixing times to a dispersed phase at longer mixing time. The flow behavior was similar at 160 and 180°C.

The size of dispersed phase of EPDM seems to be stabilized at 5-10 Fm after 4 min mixing. Generally decreasing the temperature to 140°C deteriorated the mixing quality. Figure 5 shows that the structure is made up of alternate layers of EPDM and PS. The viscosity ratio of EPDM to PS approaches unity at 140°C.

OTHER ROTOR DESIGNS

Mill Rolls

We now consider the influence of rotor design on two phase morphology and flow behavior. The two roll rotors show uniform flow along the surface of the rotors [Figures 1 1 (a) and (b)]. We could see the shear- ing between the surface of rotors and the wall of mixer chamber from the deformation of red markers mounted in the premixed materials (Figure 12). The markers became thinner and seem to make layers. From the transverse view EPDM was stretched and tom up along the direction of rotation. There was no observation of intramtor flow. PS shows the same band around rotors as seen in Banbury type rotors. Figure 13 shows trans- verse views of blending EPDM and PS for Mi11 rolls. The flow along the direction of rotation creates rodlike dispersed phase of PS and EPDM as shown in Figure 14.

Tapered Rolls

The tapered roll rotors add longitudinal flow com- ponent along the rotor axis due to the difference of

248 VOL. 7, NO. 3

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FIGURE 9 The still photos of motions of

blending in an internal mixer with Banbury type rotors (front views): (a) 180°C; (b) 160°C; (c) 140°C.

EPDMIPS-50150

ADVANCES IN POLYMER TECHNOLOGY 249

BLENDING IN AN INTERNAL MIXING

FIGURE 10 SEM microphotographs

blends as a function of mixing time for Banbury rotor: (a) 180°C; (b) 160°C; (c) 140°C.

Of EPDM/PS-50/50

VOL. 7, NO. 3 250

BLENDING IN AN INTERNAL MIXING

FIGURE 10

(Continued)

FIGURE 11 The transverse views of motions at 160°C with mill rolls: (a) PS; (b) EPDM.

ADVANCES IN POLYMER TECHNOLOGY 25 1

BLENDING IN AN INTERNU MIXING

FIGURE 12 The front views of motions of EPDM at 160°C with mill rolls.

~ ~~~~

FIGURE 13 The transverse views of blending EPDM/PS-50/50 at 160°C with mill rolls.

thickness of the gaps. The front views of motions for two roll mill and tapered roll rotors are very similar. The materials are sheared between the rotors and the wall of the mixer chamber. Little tearing on the bridge is observed, but small stagnant region is observed. Observations from the transverse view indicate the material flows longitudinally forward. The tendency which the PS makes the band around the rotor as shown in Figure 15 surface is observed for both rotors. EPDM was tom up by high shearing. The develop- ment of longitudinal flow due to the gaps appeared. The phase morphology showed twisted disperse grains of EPDM in a matrix of PS. Figure 16 shows the two phase morphology of blended PSEPDM with tapered rolls at 160°C, 30 rpm, and 2 min mixing time.

Screw Rotors

A dramatic difference was observed in the case of single screw rotors so arranged that both screws pump the fluid in a forward direction. There is no transfer from the right to the left rotor as seen in Figure 17. The boundary was clearly observed even through more revolutions. Around 1 min mixing time, the color changed from original transparency to white. This indicates that significant blending has occurred. It sug- gests that the size of the dispersed phase of EPDM approaches 5-10 pm. Actually the domain size of EPDM after 2 min mixing time became similar to that prepared by the Banbury type rotors after 4 min mix- ing time (see Figure 11). The transverse window of chamber shows that there is a large backward leakage flow between the top of the flight of the screw and chamber wall shown in Figure 18. When the materials flow through this gap, EPDM tears. This backward leakage flow w$ create much finer domains of EPDM for the same mixing time.

When we changed the combination of right-left hand screws to right-right hand screws, the flow be- haviors indicate there is a clear transfer of material from right side to left side rotor. Figure 19 shows the still photos of blending of EPDM and PS with the right-right hand screws combination. Stretching and tearing were 'observed only at the edges of front of left screw and back of right screw. Leakage flow oc- curred much less than in left-right hand combination. The phase morphology of blends prepared from the combination of right-left hand screws shows finer than ones prepared from the combination of right-right hand screws [Figures 20(a) and (b)].

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FIGURE 14 SEM microphotographs of blend of EPDMIPS-50150 at 160°C with mill rolls ‘mixing time 2 min).

FIGURE 15 The transverse views of blending EPDMIPS-50150 at 160°C with tapered rolls.

ADVANCES IN POLYMER TECHNOLOGY 253

BLENDING IN AN INTERNAL. MIXING

FIGURE 16 SEM microphotographs

blend at 160°C with tapered rolls (mixing time 2 min).

of EPDMIPS-50/50 FD 160OC:

-- 2 min

4 EPDM

FIGURE 17 The front views of blending EPDMIPS-5060 at 160°C with right-left hand screws.

DISCUSSION more liquid like flow in comparison with tight band at low temperature. As we described in our previous paper,3 there exist four flow regimes which occur at various temperatures. Figure 21 shows the four re- gimes: (1) powdery; (2) stretching and tearing; (3) tight band; (4) sheeting out.

EPDM exhibits regime 1, 2, and 4 without passing

Both EPDM and PS show different flow behavior at various temperatures. With decreasing temperature EPDM became brittle and PS formed a tight band around rotors. At high temperature EPDM shows sheeting out on the wall of the chamber. PS exhibits

254 VOL. 7, NO. 3

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FIGURE 18 The transverse views of blending EPDM/PS-50/50 at 160°C with right-left hand screws.

FIGURE 19 The front views of blending

160°C with right-right hand screws.

EPDM/PS-50/50 at

ADVANCES IN POLYMER TECHNOLOGY 255

BLENDING IN AN INTERNAL, MIXING

FIGURE 20 SEM microphotographs of blends of

with right-lefl (a) and right-right (b) hand screws.

EPDMIPS-50150

FIGURE 21 Four flow regimes: (1) powdery; (2) stretching and tearing; (3) tight band; (4) sheeting ( 1 ) .

regime 3 with various temperature. PS shows only 3 and 4 regimes. Taking into consideration blending such different polymers which show various regimes, the two phase morphology can be dominated by the flow behaviors. Decreasing temperature to match the viscosity of the matrix and the dispersed phase intro- duced coarser dispersed phase due to the tight band of PS. To introduce the finer dispersed phase, we should consider the flow behavior and the bulk prop- erty related to tearing. .

Different rotor designs create different elongation or shearing fields for the flow of the materials. The deformation history of these materials will produce a type of two phase morphology and a distribution of dispersed phase associated with the flow direction. The kinematics will dominate the determination of the two phase morphology. We now make comparisons among the designs of rotors. Mill type rotors produce EPDM rods arranged along the direction of rotation. For tapered rolled rotors the type of morphology of the dispersed EPDM shows a “grain” twisted along the flow direction. In the case of single screw rotors

out. Powdery

( 2 ) .

S t r e t c h i n g

t e a r i n g and

( 3 ) .

T i g h t band

( 4 ) .

Sheeting o u t

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the dispersed islands of EPDM in the matrix of PS is very similar to those blended with Banbury type rotors

islands prepared with screw rotors for the same mixing

type rotors over 160°C.

REFERENCES

above 1600~. H ~ ~ ~ ~ ~ ~ , the size of dispersed EPDM K. Freakley and W. Y. Wan Idris, Rubber Chem Technol.. 49, 134

2. K . Min and J . L. White, Rubber Chem. Technol.. 58, 1024 (1985).

4. K . Min and J . L. White, Rubber Chem. Technol.. 60, 377 (1987). time (2 min) is finer than ones prepared with Banbury 3. K. Min. J . In!. po'Ym. process. (In Press).

ADVANCES IN POLYMER TECHNOLOGY 257

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