Apollo PEX tubing is cross-linked, high-density polyethylene. The silane method, also called the "moisture cure" method, results in PEX-B, where the cross-linking is performed in a secondary post-extrusion process, producing cross-links between a cross-linking agent. The process is accelerated with heat and moisture. The cross-linked bonds are formed through silanol condensation between two grafted (VTMS) units, connecting the polyethylene chains with C-C-Si-O-SiC-C bridges. PEX tubing is for use in hot and cold potable water distribution systems. PEX tubing can also be used in "continuously recirculating" plumbing systems up to 140 deg F while maintaining chlorine resistance. The tubing may be installed in concrete, gypsum-based lightweight concrete, sand, asphalt, in or under wood flooring or behind wallboard or plaster.