PEX | materials feature
Crosslinked polyethylene (PEX) has carved out a niche for itself as one of the toughest pipe materials – and now there’s even a way to recycle it. Lou Reade reports
The power of PEX
Crosslinking is a blessing in terms of polymer strength – but a curse where recycling is concerned. By forming extra bonds in the polymer matrix, it boosts mechanical strength and other important properties. However, the formation of these extra bonds makes traditional thermoplastic recycling very difficult Now, researchers at the University of Akron in the
US have devised a method of recycling crosslinked plastics using ultrasonic extrusion. Avraam Isayev and Keyuan Huang, of the department
of polymer engineering, presented a paper on the subject at this year’s Antec conference in the USA. They said that the presence of three-dimensional
networks in crosslinked plastics such as PEX prevents the ability to flow and shape them on heating and shearing. “We have developed ultrasonic de-crosslinking
technology for recycling XHDPE and XLDPE,” they said. They used an ultrasonically aided, single-screw
extruder at 20Hz with a twin-screw extruder at 40Hz, applying different levels of ultrasonic energy. Studies showed that the technique broke the crosslinks rather than the polymer chains, they said.
www.pipeandprofile.com “Significant reduction of the extruder torque, die and
barrel pressures was seen during de-crosslinking,” said the researchers. The specific ultrasonic energy decreased with the
flow rate, and increased with the ultrasonic amplitude – while die pressure increased with the flow rate and decreased with the ultrasonic amplitude. This meant that applying ultrasonic treatment during extrusion enabled an increase of productivity. “Gel fraction and crosslink density of the de-
crosslinked XHDPE and XLDPE were decreased with increasing flow rate and ultrasonic amplitude,” said the researchers. The found a unique relation between the normalised
gel fraction and the normalised crosslink density of de-crosslinked XHDPE and XLDPE – regardless of the type of extruders and processing conditions.
Crosslink comparison Comparing the de-crosslinking processes for HDPE and LDPE by twin-screw extruder at the same values of the gel fraction indicated a lower die pressure for XHDPE
November/December 2016 | PIPE & PROFILE EXTRUSION 31
Uponor won a patent battle
over the use of its PEX
technology
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