Extruder compound mixing

In later stages, the technology developed and rubbers were compounded, mixed in mills, calendered in calenders, extruded in extruders, molded in hydraulic presses, cured in autoclaves and many hand operations of forming and curing became possible. State-of the art techniques of compounding were developed to produce rubber products of any shape and dimensions for many requirements. 

P. G. Andersen, Mixing Practices in Co-rotating Twin Screw Extruders, in Mixing and Compounding of Polymers, I. Manas-Zloczower and Z. Tadmor, Eds., Hanser, Munich (1994). 

There are several publications dealing with the comparison of efficiency of compounding, mixing, or reactive extruding in different types of extmders. Most of these studies suffer from the same aspect — the evaluated machines were not operated at a comparable level of performance efficiency. A summary of the reported observations is given in Table 9.14. 

The reactive extrusion is a process demanding precise operation by well-trained personnel. The knowledge of the extruder, compounding and mixing principles, design of screw and temperature profile, local residence time distribution, etc., is required. For optimization of the reactive extrusion in- or on-line process monitoring, control, and data logging are recommended — this imposes further demands on the extruder operator. 

There are several reports comparing different types of extruders for their efficiency to compounding, mixing, or reactive blending. This and associated information on compounding can be found in Chapter 9 Compounding Polymer Blends of this Handbook. 

For extrusion compounding, if the components have different densities (e.g. polymer versus fillers) or different shapes (pellets versus regrind flakes) then prefer to use the extruder for mixing. That is, operate the extruder with starved feeding conditions with the components metered separately into the extruder, see Section 9.11. On the other hand, for the blending of polymer feedstock for extrusion, if the components have very similar properties, then use tumble, rotating drum or ribbon blenders or rotor-stator blenders of the feed upstream of the extruder and use flood feeding of the mixture to the extruder. 

Mixing in compounding plays an important role in the continuous dissolution of polymers, especially in their melting using plasticising extruders. Compounding usually involves two sets of variables ... 

After substantial use ofsingle-screw extruders with mixing elements for compounding, the need for effective devolatilization emerged. To handle this new requirement, multistage screw extruders were developed, making the extruder much longer. 

For melt-compounded nanocomposites, the screw rotation speed of an extruder has a large influence on the dispersion of carbon nanotubes in the polymer matrix. Chen et al employed an ultrahigh-shear extruder to mix pristine MWNTs with PVDF pellets at a screw speed of 1000 rpm (1470 [62]. For the purpose of comparison, the MWNT/PVDF... 

Compound mixing was performed on different compounding equipment. For EVA organoclay-based nanocomposites, a laboratory twin-roll mill and an internal mixer heated to 145 °C were used. A corotating twin screw extruder from Leistritz, Germany, with a 27-mm screw diameter and an aspect ration of 40 L/D was used to generate polyethylene nanocomposites. The mass temperature was 190 °C at the extruder die. 

Keywords batch, continuous, mixing, mill, extruder, compounding, masterbatch, co-rotating machines. 

As mentioned in Section 14.1, blends without compatibilisation may have poor mechanical properties. Consequently, in comparing twin and single screw extruders for compounding mixed polymer waste streams with real situations in mind, their relative performances should ideally include compatibilisation. This approach has been taken by both Kallel and co-workers [39], and Bertin and Robin [40], the latter using actual waste polymers. 

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