ALTERNATIVE COMPOUNDERS | MACHINERY
The growing interest in new bio-based and recycled plastic compounds is creating significant opportunities for alternative extruder designs, writes Mark Holmes
Engineering an alternative approach
Twin-screw extruders are the workhorse of for the compounding industry but there are alternatives. Multi-screw extruders, as well as kneaders and planetary roller extruders, have long been used in special application areas where their specific performance attributes, such as low shear and high dispersion, can pay dividends. The growing interest in processing of bio-based and recycled plastics, as well as compounds with sensitive additives, is presenting further opportunities. Researchers at the Spanish technology centre
Aimplas report two main trends in alternative compounders in recent times. “Firstly, there is a breakdown of barriers and traditional fields of application. For example, compounding natural fibres in reciprocating extruders, as well as reactive extrusion and devulcanisation in planetary roll extruders,” says Luis Roca, Leader of Compounding at the institute. “Secondly, alternative compounders are consoli-
dating new niches that are appearing, such as multiple screw extruders for recycled plastic in food contact applications, and reciprocating extruders for glass fibre-filled applications, highly filled materials and technical compounds,” Roca adds. “Normally, these extruders feature alternative mixing methods compared to co-rotating twin- screw extruders. This includes less intensive mixing, which is more suitable for shear-sensitive materials
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where mixing takes place to extend the surface contact of the plastic with the metal parts of the extruder. This avoids possible degradation through high-temperature peaks due to shear,” he says. “In addition, PVC has been a major application for
reciprocating extruders and planetary roll extruders,” Roca says. “Halogen-free compounds are also made using reciprocating extruders due to their capacity to mix considerable amounts of mineral fillers. Due to their capacity, multi-screw extruders are also used to spread polymers in very thin films, which enhances devolatilisation. This is especially useful when recycling PET for food contact applications.” Roca also highlights the use of alternatives to
twin screw extruders in applications such as devulcanisation or reactive extrusion (REX), as well as in processing of bioplastics, where less aggres- sive mixing conditions are needed. Other potential applications include polymers with higher filler contents, natural fibre compounds and recycled polyolefins for food contact use. The main reasons for the selection of alternative compounding systems for such applications include lower shear peaks, longer residence times, improved control of heat exchange and good mixing capacities. Given the potential scale of future opportunities,
Roca says equipment developers are aiming to further improve their ability to handle shear-sensitive materials — both polymers and fillers — and to
October 2023 | COMPOUNDING WORLD 59
Main image: Novel com- pounding extruders such as CPM’s
RingExtruder RE offer specific benefits in processing sensitive materials such as bioplastics
IMAGE: CPM EXTRUSION GROUP
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