Cross-linked polyethylene , commonly abbreviated PEX , XPE or XLPE , is a crosslinked polyethylene form. It is used primarily in building pipeline system services, heating hydro radiation and cooling systems, domestic water piping, and insulation for high voltage (high voltage) power cables. It is also used for natural gas and offshore oil applications, chemical transportation, and waste and pulp transportation. PEX is an alternative to polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) or copper tubing for use as a residential water pipe.
Video Cross-linked polyethylene
Properties
Low temperature impact strengths, abrasion resistance and environmental stress crack resistance can be significantly improved with crosslinking, while hardness and stiffness are somewhat reduced. PEX does not melt again (analogous to elastomer) and heat resistance (longer to 120 ° C, for short periods without electrical or mechanical load up to 250 ° C). With increasing cross-density it also increases the maximum shear modulus (even at higher temperatures). PEX has significantly improved properties compared to regular PE. Crosslinking increases the temperature properties of the base polymer. Sufficient strength for 120-150 ° C is maintained and chemical stability is enhanced by withstanding dissolution. Low temperature properties improved. Impact and tensile strength, scratch resistance, and brittle fracture resistance are enhanced.
Almost all PEX is used for pipes and tubes made of high density polyethylene (HDPE). PEX contains crosslinking bonds in the polymer structure, converting thermoplastics into thermosets. Cross-linking is done during or after tubing extrusion. The required cross linkage level, according to ASTM Standard F876, is between 65% and 89%. Higher levels of crosslinking can lead to fragility and cracking of material stress, while lower cross linkages can result in products with worse physical properties.
Almost all cross-linkable polyethylene compounds (XLPE) for wire and cable applications are based on LDPE. XLPE insulated cables have a maximum rating of 90 Â ° C conductor and emergency rating up to 140 Â ° C, depending on the standard used. They have a short circuit rated conductor of 250 Â ° C. XLPE has excellent dielectric properties, making it useful for medium voltage - 1 to 69 kV AC, and high voltage cable - up to 380 k AC voltage, and several hundred kV DC.
Many modifications in the basic polymer structure can be made to maximize productivity during the manufacturing process. For medium voltage applications, reactivity can be significantly improved. This results in a higher line speed in cases where the limitation of either curing or cooling processes in a continuous vulkanization tube (CV) is used to connect the insulating cross. XLPE isolation can be modified to limit the amount of sideline gas produced during the cross-link process. This is especially useful for high voltage cables and extra high voltage cable applications, where the removal requirements of gas can extend cable manufacturing time significantly.
Maps Cross-linked polyethylene
Preparation methods
Various methods can be used to prepare PEX of thermoplastic polyethylene (PE-LD, PE-LLD or PE-HD). The first PEX materials were prepared in the 1930s, by irradiating extrusion tubes with electron beams. Electron beam processing methods were made feasible in the 1970s, but still expensive. In the 1960s, Engel cross-linking was developed. In this method, a peroxide is mixed with HDPE prior to extrusion, the crosslinking takes place during the course of the melting polymer through the heated die. In 1968, the Sioplas process used silicon hydride (silane) was patented, followed by another silane-based process, Monosil, in 1974. A process of using vinylillilan was followed in 1986.
Cross crosslinking types
A basic difference is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), crosslinked electron beam (PE-Xc) and cross-crosszone (PE-Xd).
What is shown is peroxide, silane and irradiation crossing. In each method, radicals are produced in the polyethylene chain (top center), either by radiation ( ) or peroxide (R-O-O-R). Then, two radical chains can be directly opposite (lower left) or indirectly by the silane compound (lower right).
- Cross-peroxide (PE-Xa) : Crosslinking of polyethylene using peroxides (eg dicumyl peroxide or di-tert-butyl peroxide) is still very important. In the so-called Engel Process, a mixture of HDPE and 2% peroxide are initially mixed at low temperatures in the extruder and then cross at high temperatures (between 200 ° C and 250 ° C). Peroxides break down into peroxide radicals (ROOs), which abstract (eliminate) hydrogen atoms from polymer chains, leading to radicals. When this merges, a cross linked network is formed. The resulting polymer network is uniform, with low voltage and high flexibility, which is softer and harder than (irradiated) PE-Xc. The same process is used for LDPE as well, although the temperature may vary from 160 ° C to 220 ° C.
- cross-cross (PE-Xb) : In the presence of silanes (eg polyethylenail silsil), polyethylene can initially be functioned by irradiation or with little peroxide. Then the Si-OH group can be formed in a water bath by hydrolysis, which condenses and binds a PE cross with the formation of Si-O-Si bridge. [16] Catalysts such as dibutyltin dilaurate can speed up the reaction.
- Cross-linked irradiation (PE-Xc) : Polyethylene cross linking is also possible by a downstream source of radiation (usually an electron accelerator, sometimes an isotope radiator). The PE product is crosslinked below the melting point of the crystal by separating the hydrogen atoms. ? -Radiation has a penetration depth of 10 mm, -Radiation of 100 mm. Thus, certain interiors or areas may be exempted from such cross-links. However, due to high capital and operating costs, cross-radiation only plays a small role compared to peroxide crosslinks. Unlike peroxide crosslinking, this process is done in a solid state. Thus, the crosslinking occurs mainly in amorphous regions, whereas crystallinity remains largely intact.
- Cross-ethos (PE-Xd) : In the so-called process of lubonyl polyethylene is a previously azo-linked compound after extrusion in a hot salt bath.
Crosslinking rate
The low cross level leads initially only to the multiplication of the molecular weight. Individual macromolecules are disconnected and no covalent tissue is formed. Polyethylene composed of large molecules behaves similarly to ultra high molecular weight polyethylene (PE-UHMW), ie, such as thermoplastic elastomers.
After a further cross (a cross rate of about 80%), individual macromolecules eventually connect to the network. This crosslinked polyethylene (PE-X) is thermosetically visible, it indicates rubber elastic behavior above the melting point and can not be processed again.
The degree of crosslinking (and therefore the extent of change) differs in intensity depending on the process. According to DIN 16892 (quality requirements for pipes made of PE-X) at least the following cross levels should be achieved:
- in peroxide crosslinking (PE-Xa): 75%
- with crosslinking silane (PE-Xb): 65%
- with cross-linked electron beam (PE-Xc): 60%
- in azo cross (PE-Xd): 60%
Classification
North America
All PEX pipes are manufactured with design specifications listed directly on the pipe. These specifications are listed to describe many pipe standards as well as provide specific details about the manufacturer. The reason that all these specifications are given, is to make the installer aware if the product meets the standards for the local code required. Labeling ensures the user that tubing complies with all the standards listed.
The materials used in the PEX pipeline in North America are determined by the classification of cells described in the ASTM standard, the most common being ASTM F876. The classification of cells for PEX includes 0006, 0008, 1006, 1008, 3006, 3008, 5006 and 5008, the most common being 5006. Classifications 0306, 3306, 5206 and 5306 are also common, these materials contain ultraviolet and/or inhibitor blockers for limited. UV Resistance. In North America all PEX tubing products are manufactured according to ASTM, NSF and CSA product standards, among which are ASTM F876 and F877 standard, NSF International standard NSF 14 and NSF 61 ("NSF-pw"), and Canadian Standards Association standard B137.5, in which the pipe is tested, certified and registered. The list and certification fulfilled by each product appears on the pipeline or pipeline to ensure that the product is used in the right designed application.
Europe
In European standards. there are three classifications referred to as PEX-A, -B, and -C. Class is not associated with any type rating system. PEX-A_ (PE-Xa, _PEXa) "> PEX-A (PE-Xa, PEX-A_ (PE-Xa, _PEXa)"> PEX-A (PE-Xa, _PEXa) PEXa)
PEX-A is produced by peroxide (Engel) method. This method performs a "hot" cross link, above the melting point of the crystal. However, the process is slightly longer than the other two methods because the polymer must be maintained at high temperature and pressure for a long time during the extrusion process. The crosslinking bonds are between the carbon atoms. PEX-B_ (PE-Xb, _PEXb) "> PEX-B (PE-Xb, PEX-B_ (PE-Xb, _PEXb)"> PEX-B (PE-Xb, PEX-B_ (PEX-B_) PEXb)
The silane method, also called the "moisture cure" method, produces PEX-B. In this method, cross-linking is done in a secondary post-extrusion process, resulting in a cross-link between cross-linked agents. This process is accelerated with heat and moisture. The crosslinking bond is formed by silanol condensation between two grafted vinyltrimethoxysilane (VTMS) units, which connect the polyethylene chain with the C-C-Si-O-Si-C-C bridge. PEX-C_ (PE-Xc, _PEXc) "> PEX-C (PE-Xc, PEX-C_ (PE-Xc, _PEXc)"> PEX-C (PE-Xc, PEXc)
PEX-C is generated through electron beam processing, in a "cross-linking" process (below the melting point of the crystal). It provides less uniform, lower cross-level than the Engel method, especially in tube diameter over one inch (2.5 cm). When the process is not well controlled, the outer layer of the tube can become brittle. However, this is the cleanest, most environmentally friendly method of all three, since it does not involve other chemicals and uses only high-energy electrons to separate 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 the use of PEX housing to deliver drinking water to a home faucet increased by 40% annually. In 2006, The Philadelphia Inquirer recommended that the installation of pipes switch from copper pipes to PEX.
In the early to mid-20th century, mass production pipes were made of galvanized steel. Because users experience problems with internal rust buildup, which reduces the volume of water, this was replaced with copper pipes in the late 1960s. Plastic pipe with glue fittings is used also in decades later. Initially PEX pipes were the most popular way to transport water in a hydraulic radiation heating system, and it was first used in a hydronic system from the 1960s onwards. Hydrocycling systems drain water from boilers or heaters to places in homes that require heat, such as baseboard heaters or radiators. PEX is suitable for hot water circulation.
Gradually, PEX becomes more acceptable for the use of more indoor pipes, such as bringing pressurized water to the appliances throughout the home. Increasingly, in 2000, copper pipe and PVC plastic pipe were replaced with PEX. PEX can be used for underground purposes, although one report suggests that the right "arm" is used for such applications.
Benefits
PEX has been approved for use in all fifty states of the United States and Canada, including the state of California, which approved its use in 2009. California allows the use of PEX for domestic water systems on a case-by-case basis. only in 2007. This is largely due to corrosion of the manifold, not the tubing itself, and allowed in California when used in hydraulic radiation heating systems. In 2009, the Building Standards Commission approved plastic pipes and PEX pipes to the California Plumbing Code (CPC), which allowed its use in hospital, clinic, residential and commercial construction throughout the state. The formal adoption of PEX into the CPC occurred on August 1, 2009, allowing local jurisdictions to approve its use in general, despite additional problems, and new approval issued in 2010 with the revised terms of the 2007 law.
Alternate materials
Alternative pipeline options include:
- Aluminum composite plastic is a laminated aluminum tube on the interior and exterior with a plastic coating for protection.
- Corrugated stainless steel pipes , continuous flexible pipes made of stainless steel with PVC interiors and air-tested for leakage.
- Polypropylene Pipes , similar in applications to CPVC but inert chemicals that do not contain harmful substances and reduce harmful emissions when consumed by fire. It is mainly used in radiant floor systems but gained popularity as a domestic leachate free drinking water pipe, especially in commercial applications.
- Polybutylene (PB) pipes are plastic polymer forms used in the manufacture of drinking water piping from the late 70s to 1995. However, it was found that polyoxymethylene (POM or Acetal) connectors were originally used to connect tubes polybutylene is susceptible to chemical attacks that are increased pressure by the addition of hypochlorite (a common chemical used to clean water). Broken connectors can be cracked and leaked in a highly stressed crease area, causing damage to surrounding structures. Then the system containing copper fittings appears to have no problem with hypochlorite attacks, but polybutylene is still not favored due to costly structural damage caused by previous problems and is not accepted in Canada and the US.
PEX-AL-PEX
Pipe PEX-AL-PEX , or pipe AluPEX , or PEX/Aluminum/PEX , or Multilayer created layer aluminum flanked by two layers of PEX. The metal layer serves as an oxygen barrier, stopping oxygen diffusion through the polymer matrix, so it can not dissolve into the water in the tube and corrode the metal components of the system. The thin aluminum layer, usually 1 or 2 mm, and gives some stiffness to the tube so that when bent to maintain the shape formed (normal PEX tube will return to straight). The aluminum layer also provides additional structural rigidity so that the tube will be suitable for higher safe temperature and operating pressure.
PEX tool kit
The PEX tool kit includes a number of basic tools necessary to make fittings and connections with PEX tubes. In many cases, such devices are purchased at a local hardware store, a plumbing supply store or assembled by a homeowner or contractor. PEX tool tools range from less than $ 100 and can go up to $ 300. Typical PEX tool kits include crimp tools, expander tools to join, clamp tools, PEX cutters, rings, boards, and staplers.
Other uses
- Artificial joints . Highly crosslinked polyethylene is used in artificial joints as a wear-resistant material. Cross-polyethylene is preferred in hip replacement because of its resistance to abrasive wear. Knee replacements, however, require PEs made with different parameters because crosslinking can affect mechanical strength and there is greater stress-concentration in the knee joint due to lower geometrical congruence of the bearing surface. Manufacturers start with very high molecular weight polyethylene, and crosslink with electron beam or gamma ray irradiation.
- Dental app . Some PEX applications have also been seen in dental restorations as composite fillers.
- Shipboard . PEX is also used in many canoes and kayaks. PEX is registered under the name Ram-X, and other brand-specific names. Because of the properties of Cross-Linked Polyethylene, repair of damage to the stomach is rather difficult. Some adhesives, such as 3M DP-8005, can bind to PEX, while larger improvements require smelting and blending more Polyethylene into canoes to form strong bonds and fill damaged areas.
- Power cord insulation . Cross-linked polyethylene is widely used as electrical insulation on power cables of all voltage ranges but is particularly suitable for medium voltage applications. This is the most common polymer insulation material. XLPE acronyms are commonly used to indicate cross-linked polyethylene insulation.
- Auto ducts and homes . PEX also referred to as XLPE is widely used in the automotive aftermarket industry for cold air intake systems and filter housings. Its properties include high heat deflection temperature, good impact resistance, chemical resistance, low flexural modulus and good environmental stress crack resistance. This XLPE shape is most often used in rotational molding; XLPE resin comes in the form of 35 mesh (500 μm) resin powder.
See also
- High-density polyethylene (HDPE)
- Polyethylene low-density linear (LLDPE)
- Low density polyethylene (LDPE)
- Medium density medium (MDPE)
- Stretch wrap
- Very high molecular weight polyethylene (UHMWPE)
References
External links
- Why is Polybutylene pipe such a big problem? Alternate view on PB piping.
- PEX history on the page of the Plastic Pipes Association and its Equipment (PPFA)
- PEX FAQ on the homepage of the Plastic Pipes and Equipment Association (PPFA)
- As a viable Polybutulene strike option.
- Analytical technique for characterizing cross-polyethylene
Source of the article : Wikipedia
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