There are basically three systems for breaking the carbon-hydrogen bond at several points in the polyethylene chain and form new cross-links between the chains, in a process known as cross-linking: this can be done basically with three methods called 'a', 'b' and 'c'. a: chemical method, with addition of peroxide directly during the extrusion process and formation of the tube. Peroxide decomposes, forming free radicals strongly reactive, thus capable of stripping hydrogen atoms from the polyethylene and create at those points a chemical bond transverse to the polymer chains. b : chemical method, in two step: first by adding a silane compound, cross-linkable polyethylene is obtained, i.e. with chemically active sites along the polymer chains. The effective cross-linking, aided by the addition of a catalyst, starts during extrusion of the pipe and can be completed in the presence of hot water or steam in a few hours. Complete cross-linking also takes place in air at normal ambient conditions within a few months. c: physical method that consists of bombarding PE in special bunkers with high-energy radiation.