Continuous mixers melting

Fig. 10.6 The single-stage FCM. (a) Size 15 FCM with chamber opened and rotated hydraulically (b) top view of staged apex twin rotors and the axial zones for carrying out the solids feed handling and the melting and mixing elementary steps (c) cross-sectional view of two rotor orientations, tip-to-tip (cx) and tip-to-flat (c2). [Reprinted by permission from E. L. Canedo and L. N. Valsamis, Farrel Continuous Mixer Systems for Plastics Compounding, in Plastics Compounding—Equipment and Processing, D. B. Todd, Ed., Hanser, Munich, 1998.]...

Example 10.1 Elements of a Plausible Melting Mechanism in Continuous Mixers... 

Another difference between the extruders and continuous mixers, pointed out by Valsamis and Canedo, is that, in the former, channel solids and melt flow dominate, and flow over screw flights is a secondary effect (except for power calculations). In contrast, the circumferential flow in CMs (and internal mixers) over the wing tips is the major flow component. Thus, while wing tips and screw flights appear to be equivalent machine elements, their role and function are quite different. The wing tips provide high shear... 

Fig. 10.47 The effect of the Power Law index in the Carreau model, and the melt-pool size for a characteristic model wedge with e/h — 3 and ot= 15° on the non-Newtonian qp/qd parameters. [Reprinted hy permission from L. N. Valsamis and E. L. Canedo, Mixing in the Farrel Continuous Mixer in Mixing and Compounding of Polymers, I. Manas-Zloczower and Z. Tadmor, Eds., Hanser, Munich, 1994.]...

Fig. 10.51 Comparison between the calculated (average) and experimental temperatures at five axial positions. The entrance melt temperature was assumed to be 200°C. [Reprinted by permission from T. Ishikawa, S. Kihara, K. Funatsu, T. Amaiwa, and K. Yano, Numerical Simulation and Experimental Verification of Nonisothermal Flow in Counterrotating Nonintermeshing Continuous Mixers, Polym. Eng. Sci., 40, 365 (2000).]...

Recalling the profound differences in the melting mechanisms in SSEs and in corotating twin-screw extruders (Co-TSE) (Chapter 5), we see that the latter one creates all of the melt almost instantaneously, resulting in a very narrow melt age distribution, while in SSE the age distribution is very broad. Thus, Co-TSEs and twin rotor melting devices [e.g., continuous mixers (CMs)] are better suited to be reactors of polymer melts, as is reflected in the current industrial reactive polymer processing practice. 

Condensation reactions in the melt between the anhydride groups of a copolymer containing maleic anhydride links and hydroxylated oligo-styrenes were carried out in discontinuous or continuous mixers. 

Twin screw compounders or Buss Kneaders are used for fully mixed and melted compounds for injection molding because heat history has less effect on otyect molding as long as the process temperatures and times are adjusted to accormt for the use of lower molecular weight resins in the formulations. For flexible compounds, Banbury mixers, continuous mixers, or compounding extruders preceded by static mixers can be employed successfully. 

The rubber, pellets, and powders are fed to the continuous mixer at the feed throat. The plasticizer, extender oil, or any other liquid components are fed downstream, usually at the third or fourth barrel. Other solid additives could also be added to the mixer using a top feeder or a side feeder at a barrel located further downstream. If the additives have a low melting point, adding them from the top with an auger-type of feeder is a better choice. 

Compatibilizer preparations are needed for use with a polypropylene and NBR/ HNBR blend, the components of which are grossly incompatible [7]. This could be a one-step process if one starts with commercially available maleic anhydride (MA) modified polypropylene and amine-terminated nitrile rubber. Then one can melt mix the above fimctionalized polymers in a batch or a continuous mixer and prepare the compatibilizer, as shown in step 2 below. One can also modify the polypropylene as described below in step 1, and use it to prepare the compatibilizer. 

Polypropylene is melt mixed with an organic peroxide and maleic anhydride using a Brabender, Banbtuy, or twin-screw mixer to graft anhydride ftmctionality on the polymer. The components could be blended together and fed to either a batch mixer or continuous mixer or the polymer is melted first and then the maleic anhydride and peroxide are added in that order. The unreacted maleic anhydride is removed before use. 

The melt mixers are either batch or continuous. The former requires lower investment cost, but is more labor-intensive, has low output and poor batch-to-batch reproducibility. The continuous mixers are extruders, continuous shaft mixers and specialty machines. 

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