Cross-linked polyethylene
Cross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, insulation for high tension (high voltage or high volt in short) electrical cables, and baby play mats.
Source: Wikipedia — Cross-linked polyethylene (CC BY-SA 4.0)