PEX's long-term, high temperature strength depends largely upon the degree of cross-linking achieved during production.  If the cross-linking percentage is too high, the material may perform poorly at high temperatures and be brittle.  If the percentage is too low, the material will not fully develop the enhanced properties associated with PEX.  There is no simple standard for cross-linking percentages. These numbers vary, depending on the production methods being used. The important thing is to be sure that the proper degree of cross-linking is achieved for a given process. Currently, PEX can be manufactured using one of a number of chemical or physical processes, all of which have been proven to provide the necessary properties.
   The PEX manufacturing processes currently used in the U. S. include:

Peroxide cross-linking, Pex-a--There are several different processes which use decomposing peroxide to cross-link polyethylene.  The Engel process, the first peroxide method developed, sinters together a granulated blend of HDPE, peroxide and stabilizers under high pressure.  In this process, cross-linking occurs during extrusion through a long heated die.
   The newest process 
    The Pont a'Mousson process can be used to cross-link low- and medium-density polyethylene.  Here, polyethylene is mixed with peroxide, extruded into pipe and cross-linked in a salt bath at temperatures ranging from 250 degrees to 280 degrees centigrade. 
    In the Daoplas process, the pipe is extruded before being diffused with peroxide through high temperature and pressure.

Silane cross-linking, Pex-b, --- the silane process is generally a three step process.  The first step involves grafting vinyl silane to a medium or high-density polyethylene resin to form a grafted co-polymer.  Next, this grafted co-polymer is dry-blended with a catalyst and extruded into the final product.  The pipe is then exposed to hot water or steam, which activates the catalyst and completes the cross-linking process in less than 12 hours.   

   Irradiation cross-linking, Pex-c--Irradiation is the only physical process available for cross-linking.  In this method, pipe is extruded in a normal manner and then exposed to radiation through a high speed conveyance system.


   Although each tubing manufacturer claims that their tubing is better than any one else's, the truth is, that PEX is made to such high standards that there is very little difference between them as long as they have a ASTM F876/F877  certification by an independent testing lab. All the tubing that you purchase should have a ASTM F876/F877 (SDR 9) certificate as tested by NSF (14 & 61) and CSA, conform to International  Standard 9001, and have a 25 year warranty that includes property damage.