Why is it necessary to scrape the pipes prior to electrofusion - Is there really an oxide skin?

 

Papers # 2016 Berlin

IT IS WELL KNOWN THAT IT IS NECESSARY TO SCRAPE OFF A 0.2 MM OUTER LAYER OF A POLYETHYLENE (PE) PIPE SURFACE AT THE LOCATION FOR THE ELECTROFUSION WELD PRIOR TO WELDING. THE REASON FOR THIS PROCEDURE IS SAID TO BE REMOVAL OF AN OXIDIZED LAYER WHICH CAN RESTRICT MOLECULAR DIFFUSION ACROSS THE WELDING INTERFACE LEADING TO POOR WELDABILITY. SWEREA KIMAB HOWEVER, QUESTIONS THE EXISTENCE OF AN OXIDE SKIN, PARTLY SINCE IF THE PE WAS THAT EASILY OXIDIZED THAT IT NEEDS TO BE RE-SCRAPED AFTER A DAY DUE TO OXIDATION, THEN THERE WOULD BE A REAL PROBLEM WITH THE SERVICE LIFE OF THIS MATERIAL. THE ACTUAL REASON BEHIND THIS BEHAVIOR HAS BEEN STUDIED AND IS BELIEVED TO BE DUE TO MIGRATION OF STEARATES ADDED AS A PROCESSING AID IN PIPE PRODUCTION RATHER THAN OXIDATION.

It is well known that it is necessary to scrape off a 0.2 mm outer layer of the PE pipe surface at the location for the electrofusion weld prior to welding. The reason for this procedure is said to be removal of an oxidized layer (caused by UV degradation and/or the extrusion process) which can restrict molecular diffusion across the welding interface. It is also generally recommended that the welding is performed directly after scraping. However, Swerea KIMAB has questioned the existence of an oxide skin, partly since if the polyethylene was that easily oxidized that it needs to be re-scraped after a day due to oxidation, then there would be a real problem with the service life of this material. The actual reason behind this behavior has been studied and is believed to be due to additive migration in the pipe rather than oxidation. Better understanding about this will lead to possibilities to improve weld procedures and weld performance. The study has comprised both mechanical testing of pipes with and without scraping. Different time intervals from scraping to welding have been investigated and a thorough study on the chemical and physical properties of the material has been performed. In addition, pipes with an outer PP layer have also been studied to investigate if the weldability of this pipe after the removal of the PP layer is the same as for a scraped pipe.

Related keywords : Electrofusion, scraping, oxide skin, weldability.
Please note that the whole article content is available on PPCA website onlySource : 2016 Berlin


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