Cross-linked Polyethylene (PEX or XLPE):

Cross-linked polyethylene is a medium- to high-density polyethylene containing cross-link bonds introduced into the polymer structure, changing the thermoplast into a thermoset.

The high-temperature properties of the polymer are improved, its flow is reduced and its chemical resistance is enhanced.

PEX is used in some potable-water plumbing systems because tubes made of the material can be expanded to fit over a metal nipple and it will slowly return to its original shape, forming a permanent, water-tight, connection.