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Transcript of Flow visualization of blending of elastomers and ... Blending of Elastomers and Thermoplastics in an

  • 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.

    244 VOL. 7, NO. 3

  • BLENDING IN AN INTERNAL MIXING

    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.

    246 VOL. 7. NO. 3

  • BLENDING IN AN INTERNAL MIXING

    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 t