Failure Analysis Of Cross-Linked Polyethylene Pipe In Residential Plumbing And Heating Systems

June 12, 2015
Publication: ANTEC 2015. Society of Plastics Engineers
Author(s): Flip Sharff Arthur Davies

A rash of failures in PEX-AL-PEX piping in residential domestic hot water and hydronic heating systems in the United States and Canada led to an investigation into the nature and cause of the failures.  Failures, which occurred after less than 10 years of service, were characterized by blistering and splitting of the exterior pipe wall, preceded by darkening and bulging of the outer PEX layer.  Laboratory examination of field samples revealed brittle microcracking of the inner PEX layer and corrosion holes in the aluminum layer precipitating hydrostatic rupture of the outer PEX layer.  Tests showed that the PEX material had low gel content (indicative of inadequate crosslinking), and early depletion of the antioxidant, thus lowering the resistance to oxidative degradation.  This study shows the importance of proper formulation and testing of the antioxidant package for ensuring satisfactory performance of thermoplastic pipe for the intended design life.