What is PEX?
PEX is the Cross-linked polyethylene, which is a form of polyethylene with cross-links. It is formed into tubing, and is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, and insulation for high tension (high voltage) electrical cables. It is also used for natural gas and offshore oil applications, chemical transportation, and transportation of sewage and slurries. PEX has become a common alternative to PVC, CPVC or copper tubing for use as residential water pipes.
What are the properties of PEX?
Almost all PEX used for pipe and tubing is made from high-density polyethylene (HDPE). PEX contains cross-linked bonds in the polymerstructure, changing the thermoplastic to a thermoset. Cross-linking is accomplished during or after the extrusion of the tubing. The required degree of cross-linking, according to ASTM Standard F876, is between 65 and 89%. A higher degree of cross-linking could result in brittleness and stress cracking of the material, while a lower degree of cross-linking could result in product with poorer physical properties.
Crosslinking improves the elevated-temperature properties of the base polymer. Adequate strength to 120–150 °C is maintained by reducing creep, the tendency to flow. Chemical resistance is enhanced by resisting dissolution. Low temperature properties are improved. Impact and tensile strength, scratch resistance, and resistance to brittle fracture are enhanced.
Almost all cross-linkable polyethylene compounds (XLPE) for wire and cable applications are based on LDPE. XLPE-insulated cables have a rated maximum conductor temperature of 90 °C and an emergency rating up to 140 °C, depending on the standard used. They have a conductor short-circuit rating of 250 °C. XLPE has excellent dielectricproperties, making it useful for medium voltage—1 to 69 kV AC, and high voltage cables—up to 380 kV AC-voltage, and several hundred kV DC.
Numerous modifications in the basic polymer structure can be made to maximize productivity during the manufacturing process. For medium voltage applications, reactivity can be boosted significantly. This results in higher line speeds in cases where limitations in either the curing or cooling processes within the continuous vulcanization(CV) tubes used to cross-link the insulation. XLPE insulations can be modified to limit the amount of by-product gases generated during the cross-linking process. This is particularly useful for high voltage cable and extra-high voltage cable applications, where degassing requirements can significantly lengthen cable manufacturing time.
What are the differences of PEXa, PEXb and PEXc?
PEX-A (PE-Xa, PEXa)
PEX-A is produced by the peroxide (Engel) method. This method performs "hot" cross-linking, above the crystal melting point. However, the process takes slightly longer than the other two methods as the polymer has to be kept at high temperature and pressure for long periods during the extrusion process. The cross-linked bonds are between carbon atoms.
In China, the PEXa pipes are still produced by this old extrusion technology, using ram extruder. Because of that PEXa material sticky features, the production speed is very slow, about 4-5 meters per min for 16mm.
Because of this low production efficiency and also its little complex production process, this kind of pipes market has declined greatly.
PEX-B (PE-Xb, PEXb)
The silane method, also called the "moisture cure" method, results in PEX-B. In this method, 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 vinyltrimethoxysilane (VTMS) units, connecting the polyethylene chains with C-C-Si-O-Si-C-C bridges.
Because of raw material technology update, now the PEXb pipes can be extruded like normal PE granules.
The highest extruding speed can be up to 30-35m/min by using single screw extrusion machinery. The production efficiency has been increased greatly.
And PEXb raw materials are more easily to get from the market. Its market share has been raised greatly in recent years.
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.
What are the application and benefits of PEX pipes?
PEX tubing is widely used to replace copper in plumbing applications. One estimate is that residential use of PEX for delivering drinking water to home faucets has increased by 40% annually, and there is substantial evidence that PEX is or will soon become the dominant technology for carrying water in homes and businesses in the next decade or so.
Benefits of using PEX in plumbing include:
1) Flexibility. PEX has become a contender for use in residential water plumbing because of its flexibility. It can bend into a wide-radius turn if space permits, or accommodate turns by using elbow joints. In addition, it can handle short-radius turns, sometimes supported with a metal brace; in contrast, PVC, CPVC and copper all require elbow joints. A single length of PEX pipe cannot handle a sharp 90-degree turn, however, so in those situations, it is necessary to connect two PEX pipes with a 90-degree PEX elbow joint.
2) Direct routing of pipes. PEX can run straight from a distribution point to an outlet fixture without cutting or splicing the pipe. This reduces the need for potentially weak and costly joints and reduces the drop in pressure due to turbulence induced at transitions. Since PEX is flexible, it is often possible to install a supply line directly from the water source to an appliance using just one connection at each end.
3) Greater water pressure at fixtures. Since PEX pipes typically have fewer sharp turns, there is greater water pressure at the sinks and showers and toilets where it is needed.
4) Easier installation. Installing PEX is much less labor-intensive than copper pipes, since there is no need to use torches to solder pipes together, or to use glue to attach pipes to fittings. PEX connections can be made by pushing together two matching parts using a compression fitting, or by using an adjustable wrench or a special crimping tool. Generally, fewer connections and fittings are needed in a PEX installation.
5) Non-corrodible. PEX, unlike copper, is not subject to corrosion from minerals or moisture.
6) Longevity. The advantageous properties of PEX also make it a candidate for progressive replacement of metal and thermoplastic pipes, especially in long-life applications, because the expected lifetime of PEX pipes reaches 50 years.
7) Suitable for hot and cold pipes. A convenient arrangement is to use color-coding to lessen the possibility of confusion. Typically, red PEX tubing is used for hot water while blue PEX tubing is used for cold water.
Environmental benefits. One account suggested that PEX used in radiant heating was better for the environment than copper, although it noted that the pipes were based on petroleum products.
Pipe insulation possible. Conventional foam wrap insulation materials can be added to PEX piping to keep hot water hot, and cold water cold, and prevent freezing, if necessary.
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