PEX

Crosslinked polyethylene (PEX) is a high-temperature, flexible, polymer pipe. Crosslinking technology was first developed in Europe in the late 1960s and has since come into use around the world for a variety of applications. PEX has a 50-year history of successful use around the world with extensive testing for durability and material performance.

In North America, PEX is primarily produced in "nominal tube sizes" (NTS), with outside diameters matching copper tubing of the same nominal diameters (i.e., copper tube size, or CTS). PEX tubing has a standard dimension ratio (SDR) of 9 and is available in nominal diameters ranging from 1/4 to 4. PEX is also available in so-called pipe diameters in both nominal pipe size (NPS, also known as IPS) and metric sizes (DN). SDR 9 PEX tubing has standard hydrostatic pressure ratings of 160 psi at 73°F (1105 kPa at 23°C) and 100 psi at 180°F (690 kPa at 82°C). For PEX pipe, the pressure rating depends on the wall thickness and dimension ratio. Consult the specific PEX manufacturer's literature and listings for appropriate pressure ratings. Most diameters of PEX tubing and pipe are sold in both coils and straight lengths, with larger diameters primarily available in straight lengths.

Thanks to its smooth inner wall and flexibility, which allows installers to avoid the use of most elbow fittings, PEX tubing in hot- and cold-water distribution systems can generally be installed in place of copper tubing on size-for size basis.

Definition

PEX is a polyethylene material which has undergone a change in molecular structure using a chemical or a physical process whereby the polymer chains are chemically linked. Crosslinking of the polymer chains of polyethylene (HDPE) into PEX for pipes results in improved properties such as elevated temperature strength and performance, flexibility, chemical resistance, environmental stress crack resistance (ESCR), resistance to slow crack growth (SCG), toughness, and abrasion. Crosslinking makes PEX a "semi-thermoset" polymer, providing excellent long-term stability.

Overview

Polyethylene can be crosslinked using several technologies. All methods induce links between the single strands of HDPE to form a dense network or matrix through radical reactions. The number of links between the HDPE molecules determines the crosslink density and is an important factor in determining the physical properties of the material.

The three most common methods of crosslinking polyethylene are as follows:

Peroxide - This method employs organic peroxides that, when heated, generate reactive free radicals that splice PE chains together as the HDPE compound is extruded. Sometimes referred to as the PEXa Process.

Silane - This method involves grafting a reactive silane molecule to the backbone of the polyethylene, which allows the HDPE compound to crosslink after extrusion as pipe when subjected to moisture and heat, such as with steam curing. Sometimes referred to as the PEXb Process.

Electron Beam - This method involves subjecting a dose of high-energy electrons to the extruded HDPE pipe. Sometimes referred to as the PEXc Process.

Other crosslinking methods are also in commercial use. Although each method of crosslinking may produce slightly different characteristics, PEX tubing produced by any of the approved methods must meet the same qualification and performance requirements as specified in industry standards (e. g., ASTM F876, AWWA C904, CSA B137.5). The letter designations are not related to any type of rating system.