PROCESSING | REACTIVE COMPOUNDING
Right: TPVs, which are increasingly being used in automotive sealing applications, lend them- selves to reactive
compounding
produced using simple mechanical blending. “The ability to tailor residence time distribution (RTD) and the degree of mixing combined with self-wiping characteristics makes intermeshing and co-rotating twin screw extruders the perfect reactor for polymeri- sation, polymer modification and compatibilisation of blends,” he says. “The limitations for reactive processing on twin
screw extruders are mostly economic and not technical; the extruder can produce the desired functionality, however, the capital equipment cost may be prohibitive at the capacity required to obtain the desired properties,“ according to Dreiblatt. “The relatively small volume of twin screw
extruders becomes a limitation for reactions requiring average residence time more than a few minutes. The majority of current materials pro- duced via reactive compounding can operate within this range of residence times. The new polymer chemistries - yet to be developed - may require much longer residence time,“ he says. Dreiblatt says there are only two options for
increasing residence time beyond a few minutes and both dramatically impact on the production cost of such materials. The first is to increase free volume and this means either using a larger diameter extruder for a given throughput or to accept significantly reduced capacity. He says that for a given screw diameter, the extruder length (L/D) can be extended in some cases up to 100D or more to provide increased free volume along with increased diameter ratio. The second option is to arrange two extruders in
series. “With the two-machine approach, the reaction can be completed in the first machine which feeds directly into a second machine for devolatilisation of reaction by-products and/or residual monomer. This option provides the flexibility to run each machine at different speeds - each machine can be a different diameter - to
optimise the reaction separately from the devola- tilisation, but requires more floor space and significantly more capital,“ he says. Dreiblatt says there are many patents describing
reactive compounding using multiple extruders in series, as the concept is not new. However, he adds that specially equipped test facilities to trial new reactive extrusion chemistries using the two-ma- chine approach for increased residence time will be needed. “Such facilities must be capable of feeding solids and liquids at multiple points along the extruders, deep vacuum pressure with multiple vents and handling potentially flammable sol- vents,” he says. “The US company Orrex, for example, has pioneered the development and production of new products via reactive com- pounding using two machines in series.”
JSW says its TEX-αIII extruder provides the torque levels
required for reaction compounding 30 COMPOUNDING WORLD | September 2018
Within the ring Dreiblatt adds that CPM’s RingExtruder (developed within the German company Extricom that was acquired by CPM in 2017) offers the same desirable characteristics of twin screw extruders for reactive processing – including self-wiping, narrow RTD and flexible screw configuration for optimised mixing – but with significantly higher free volume for reactive systems requiring much longer residence time. The high free volume is achieved in the RingEx- truder design using twelve co-rotating and intermeshing screws arranged around a stationary central core. This geometry provides the equivalent residence time of two twin screw extruders in series with much less floor space and with a lower cost for an equivalent throughput. The geometry of the RingExtruder also provides the increased surface/volume (compared to twin-screw extrud- ers) required for the high levelvs of heat transfer necessary for exothermic reactions. The higher surface/volume of the RingExtruder also provides increased venting efficiency. “There are many polymers and materials that are manufactured using multiple-step batch processes simply because the residence times required are much longer than can be achieved in twin screw extruders. These reactions may be possible within the RingExtruder with extended L/D,“ says Dreiblatt. He also sees potential for the technology in the TPU sector. “Reactive polymerisation of TPU has
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PHOTO: JSW
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