vinyltrimethoxysilane (VTMS) units, connecting the polyethylene chains with C-C-Si-O-Si-C-C bridges. PEX-C (PE-Xc, PEXc) PEX-C is produced through electron beam processing, in a "cold" cross-linking process (below the crystal melting point). It provides less uniform, lower-degree cross-linking than the Engel method, especially at tube diameters over one inch (2.5 cm). When the process is not controlled properly, the outer layer of the tube may become brittle. However, it is the cleanest, most environmentally friendly method of the three, since it does not involve other chemicals and uses only high-energy electrons to split the carbon-hydrogen bonds and facilitate cross-linking. Plumbing PEX tubing is widely used to replace copper in plumbing applications. One estimate from 2006 was that residential use of PEX for delivering drinking water to home faucets was increasing by 40% annually. In 2006, The Philadelphia Inquirer recommended that plumbing installers switch from copper pipes to PEX. In the early to mid 20th century, mass-produced plumbing pipes were made from galvanized steel. As users experienced problems with the internal build-up of rust, which reduced water volume, these were replaced by copper tubing in the late 1960s. Plastic pipes with fittings using glue were used as well in later decades. Initially PEX tubing was the most popular way to transport water in hydronic radiant heating systems, and it was used first in hydronic systems from the 1960s onwards. Hydronic systems circulate water from a boiler or heater to places in the house needing heat, such as baseboard heaters or radiators. PEX is suitable for recirculating hot water. Gradually, PEX became more accepted for more indoor plumbing uses, such as carrying pressurized water to fixtures throughout the house. Increasingly, in the 2000s, copper pipes as well as plastic PVC pipes are being replaced with PEX. PEX can be used for underground purposes, although one report suggested that appropriate "sleeves" be used for such applications. Benefits Benefits of using PEX in plumbing include: Flexibility. PEX is a popular solution for residential water plumbing in new construction due to its flexibility. PEX tubing can easily bend without buckling or cracking and is often stored in rolls. PEX tubing can be supported with a metal or hard plastic brace to accommodate shorter-radius turns and only requires an elbow joint for tight corners. By contrast, other common indoor plumbing materials — namely PVC, CPVC and copper — are not flexible and require elbow joints to accommodate turns