Carl F. Baker
Papers # 2016 Berlin
The performance of HDPE resins with regards to slow crack growth has increased dramatically over the past two decades. The most common test method for pipe products is ASTM F1473, Standard test Method for Notch Tensile Test to measure the resistance to Slow Crack Growth of Polyethylene Pipes and Resins (PENT). Results are reported as hours to failure. Failure time has increased from a few hundred hours to over 50,000 hours in some cases. Crosslinked polyethylene (PEX) is known to have very high slow crack growth resistance as a result of crosslinking. Unfortunately the methods used to measure the slow crack growth resistance of PEX are different than those used for standard polyethylene (PE) pipe grades. This study reports PENT data for PEX pipe resins, which allows a more direct comparison of the slow crack growth properties of crosslinked PE pipe grades with standard PE pipe grades.
Crosslinked Polyethylene (PEX) pipe and tube are being used globally for potable water plumbing systems, industrial piping systems, radiant heating systems, etc., successfully. PEX materials are generally believed to have very good slow crack growth properties and some would argue that they are immune to slow crack growth. In general, the three common methods of crosslinking, peroxide, silane grafted, and radiation, are treated equally in the standards with regard to physical and mechanical properties. Most PEX standards include some sort of notched pipe test to demonstrate adequate resistance to crack propagation when a defect (notch) is present. However, there is little slow crack growth data available for PEX materials in a form that can be directly related to other materials. This paper will present PENT data for PEX pipe resins. The methodology used to prepare the samples is critical to successful PENT testing of PEX materials and this will be discussed. The data presented will include multiple temperatures and stresses along with failure mode analysis for each resin.