PEX is the common name for cross-linked high-density polyethylene pipe.  PEX is formed into tubing, and is used predominantly in domestic water supply, hydronic radiant heating systems.  It is also used for industrial applications such as special natural gas and offshore oil applications, chemical transportation, and such.  PEX is quickly becoming a popular alternative to copper piping systems in residential applications. 

PEX is a materials that starts as high density polyethylene (HDPE) that becomes linked together permanently by a process called crosslinking.  The process can be completed by using one of three method per ASTM F-876 standard;

PEX-A     Peroxide - Peroxides are heat-activated chemicals that generate free radicals for crosslinking.  This is referred to as the Engel process.

PEX-B     Moisture-cured Vinylsilane - This involves grafting a reactive silane molecule to the backbone of the polyethylene.  This is called the Silane process.

PEX-C     Beta Irradiation - This method involves subjecting a dose of high-energy electrons to the PE.  This is called the Radiation process.