Recycling of PE-X
When PE is cross-linked three-dimensionally, the low-temperature impact strength and stress cracking resistance of the material increase. In addition, the cross-linked PE (PE-X) can withstand higher temperatures. Applications for PE-X include, among others, insulation for medium- and high-voltage cables as well as piping for hot water and underfloor heating. It is difficult, however, to recycle products made from PE-X, since the cross-linked material can no longer be melted. In the course of a research project conducted at the Department of Polymer Engineering at the University of Akron, a new method for recycling PE-X has now been developed. In this method, the HDPE that was previously cross-linked with peroxides is subjected to ultrasonics in a single-screw extruder to break apart the cross-links between the molecular chains.
Extrusion with the aid of ultrasonics has for some time been used to de-vulcanize various elastomers. Until now, however, the energy introduced in the form of ultrasonic waves has resulted in overheating during processing of the cross-linked HDPE, causing a worsening of material properties. With improved control of the ultrasonic system, it is now possible to prevent this effect. Tests were conducted on a single-screw extruder equipped with two water-cooled ultrasonic heads. The ultrasonic action during extrusion breaks apart the cross-links, allowing the material to become fluid.
When investigating the mechanical properties of the material, it was found that the failure stress and elongation at break had changed only slightly compared to the original material. From this, the researchers conclude that during exposure to the ultrasonics primarily the cross-links between the molecular chains break, while the main macromolecular chains remain intact. Ultrasonically assisted processing of cross-linked HDPE in a single-screw extruder thus yields a melt-processible material with good mechanical properties. Further investigations will test the method on larger systems.
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