FACTORS THAT SET XHHW-2 APART FROM THHN
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Thermoplastic and thermoset are two types of cable insulations commonly used for single conductors. It&#;s important to understand their strengths and weaknesses when designing and building a new facility, machine, or appliance.
THERMOPLASTIC INSULATION
THHN/THWN-2 is a thermoplastic product. It uses a thinner PVC insulation, which can lead to current leakage and dielectric breakdown in demanding circuits or from chemical or environmental exposure. The PVC insulation in THHN/THWN-2 emits a toxic smoke when burned and is less flexible at cold temperatures. However, it is lighter in weight and less expensive to manufacture.
Thermoplastic compounds can be heated, then shaped and cooled to maintain their desired shape. This process can typically be repeated by applying heat to remelt the insulation, which can lead to potential danger in some applications.
Examples of thermoplastic compounds include:
THERMOSET INSULATION
XHHW-2 is a thermoset product. Crosslink products like XHHW-2 have XLPE (crosslinked polyethylene) insulation instead of PVC. XLPE is more resistant to chemicals, ozone, and abrasions. Because of the chemistries involved, modern XLPE is far less toxic than PVC in the event of fire. In more applications, thermosets are more flexible than thermoplastics and are more advantageous for projects where complex, close-spaced bends are required.
Thermoset compounds utilize a curing process which causes a chemical reaction, allowing the polymers to cross-link. Once cured, the thermoset compounds retain their shape and will not remelt when heat is applied.
Examples of thermoset compounds include:
"PEX" redirects here. For other uses, see Pex (disambiguation)
A cross-linked polyethylene (PEX) pipeCross-linked polyethylene, commonly abbreviated PEX, XPE or XLPE, is a form of polyethylene with cross-links. It is used predominantly in building services pipework systems, hydronic radiant heating and cooling systems, domestic water piping, insulation for high tension (high voltage) electrical cables, and baby play mats. It is also used for natural gas and offshore oil applications, chemical transportation, and transportation of sewage and slurries. PEX is an alternative to polyvinyl chloride (PVC), chlorinated polyvinyl chloride (CPVC) or copper tubing for use as residential water pipes.
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Low-temperature impact strength, abrasion resistance and environmental stress cracking resistance can be increased significantly by crosslinking, whereas hardness and rigidity are somewhat reduced. Compared to thermoplastic polyethylene, PEX does not melt (analogous to elastomers) and is thermally resistant (over longer periods of up to 120 °C, for short periods without electrical or mechanical load up to 250 °C). With increasing crosslinking density also the maximum shear modulus increases (even at higher temperatures).[1][2] PEX has significantly enhanced properties compared with ordinary PE.
Almost all PEX used for pipe and tubing is made from high-density polyethylene (HDPE). PEX contains cross-linked bonds in the polymer structure, 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.
PEX has significantly enhanced properties compared to ordinary PE.[3] This is due to the introduction of crosslinks in the system, which can significantly improve the chemical, thermal, and mechanical properties of the polymer.[4] While HDPE and PEX both display increases in the initial tangent modulus and yield stress under temperature or strain-rate increases when undergoing compression, HDPE tends to exhibit flow behavior after reaching a higher yield stress and PEX tends to exhibit strain-hardening after reaching its slightly lower yield stress.[5] The latter exhibits some flow behavior but only after reaching higher true strains. The behavior observed in PEX is also mimicked by the thermoplastic ultra-high molecular weight polyethylene (UHMWPE). However, PEX displays a stronger temperature and strain-rate dependence than UHMWPE. Additionally, PEX is notable for its high thermal stability. It displays improved creep behavior (i.e. resists creep deformation) and maintains high strength and hardness at very high temperatures compared to thermoplastic polyethylene.[6]
The type of initial polymer structure and amount of crosslinking can have a large impact on the resulting mechanical properties of PEX.[7] One study looked at the effect of crosslinking low-density polyethylene (LDPE) with different amounts of dicumyl peroxide (DCP).[8] It was found that increasing the weight percent of the peroxide crosslinker resulted in a lower degree of crystallinity, as observed via differential scanning calorimetry (DSC). The degree to which a polymer crystallizes and crosslinks can have a significant impact on its properties, and it was indeed found that the increase in crosslinking degree and corresponding decrease in crystallinity correlated to a lower elongation at break. It was suggested that this was due to the higher presence of chemical crosslinks (the peroxides) compared to the physical crosslinks (formed by the crystallites), as chemical crosslinks tend to inhibit the elongation behavior of polymers. Additionally, it was found that the maximum tensile strength tended to increase since the intermolecular forces between chains increases with additional crosslinks. Similar results have been found with the addition of silane crosslinkers. In another study, the amount of silane crosslinker added to linear low-density polyethylene (LLDPE) was varied.[9] The resulting Young&#;s modulus and maximum tensile strength increased with crosslinker concentration but the elongation at break decreased due to decreases in crystallinity. The presence of fillers can further strengthen PEX&#;s mechanical properties. In the same study, the researchers looked at the effect of adding a filler known as montmorillonite (MMT) nanoclay and observed even higher Young&#;s moduli and tensile strengths, indicating a strong interfacial interaction between the silane crosslinked LLDPE and the MMT.
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 dielectric properties, 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.[citation needed] This is particularly useful for high-voltage cable and extra-high voltage cable applications, where degassing requirements can significantly lengthen cable manufacturing time.
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Various methods can be used to prepare PEX from thermoplastic polyethylene (PE-LD, PE-LLD or PE-HD).[10] The first PEX material was prepared in the s, by irradiating the extruded tube with an electron beam. The electron beam processing method was made feasible in the s, but was still expensive. In the s, Engel cross-linking was developed. In this method, a peroxide is mixed with the HDPE before extruding.[11] In , the Sioplas process using silicon hydride (silane) was patented, followed by another silane-based process, Monosil, in . A process using vinylsilane followed in .[citation needed]
Raw material: XLPE powder used in rotational molding in a factory[
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A basic distinction is made between peroxide crosslinking (PE-Xa), silane crosslinking (PE-Xb), electron beam crosslinking (PE-Xc) and azo crosslinking (PE-Xd).[2]
Shown are the peroxide, the silane and irradiation crosslinking. In each method, a hydrogen atom is removed from the polyethylene chain (top center), either by radiation (hν) or by peroxides (R-O-O-R), forming a radical. Then, two radical chains can crosslink, either directly (bottom left) or indirectly via silane compounds (bottom right).
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A low degree of crosslinking leads initially only to a multiplication of the molecular weight. The individual macromolecules are not linked and no covalent network is formed yet. Polyethylene that consists of those large molecules behaves similar to polyethylene of ultra high molecular weight (PE-UHMW), i.e. like a thermoplastic elastomer.[13]
Upon further crosslinking (crosslinking degree about 80%),[14] the individual macromolecules are eventually connected to a network. This crosslinked polyethylene (PE-X) is chemically seen a thermoset, it shows above the melting point rubber-elastic behavior and cannot be processed in the melt anymore.[13]
The degree of crosslinking (and hence the extent of the change) is different in intensity depending on the process. According to DIN (a quality requirement for pipes made of PE-X) at least the following degree of crosslinking must be achieved:[14]
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All PEX pipe is manufactured with its design specifications listed directly on the pipe. These specifications are listed to explain the pipe's many standards as well as giving specific detailing about the manufacturer. The reason that all these specifications are given, are so that the installer is aware if the product is meeting standards for the necessary local codes. The labeling ensures the user that the tubing is up to all the standards listed.[15]
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Materials used in PEX pipes in North America are defined by cell classifications that are described in ASTM standards, the most common being ASTM F876. Cell classifications for PEX include , , , , , , and , the most common being . Classifications , , and are also common, these materials containing ultraviolet blockers and/or inhibitors for limited UV resistance. In North America all PEX tubing products are manufactured to ASTM, NSF and CSA product standards, among them the aforementioned ASTM standard F876 as well as F877, NSF International standards NSF 14 and NSF 61 ("NSF-pw"), and Canadian Standards Association standard B137.5, to which the pipes are tested, certified and listed. The listings and certifications met by each product appear on the printline of the pipe or tubing to ensure the product is used in the proper applications for which it was designed.
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In European standards. there are three classifications referred to as PEX-A, -B, and -C. The classes are not related to any type of rating system.
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.
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.
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.
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Radiant heating system manifold using PEX tubing This copper exterior valve has burst from freezing; several reports suggest that PEX takes longer to burst under freezing conditions. PEX's flexibility allows for fewer connections, better water flow, and faster, simpler, and less expensive installation than comparable materials. A PEX push fitting allows an installer to join copper and PEX pipes by simply pushing them together for a watertight fit. Brass crimp fittings, another popular type of fittings primarily used for connection PEX to PEX, PEX to Threaded pipes. 1.Drop Ear Elbows connect PEX and threaded pipe at a 90-degree 2.PEX to Copper Solder Adapter 3.PEX to Copper Threaded Adapter 4.PEX to Female Threaded Adapter 5.PEX Plug - terminate end of pipe 6.PEX to PEX Coupling 7.PEX to PEX 90-degree Elbow 8.PEX to Copper Adapter 9.PEX to Copper 90-degree Elbow 10. PEX x PEX x PEX 3-way PEX Tee. Tools and fittings used in a plumbing installation with PEX piping. (1) crimping tool to squeeze a metal band to join a pipe and a fitting (2) compression coupling joining two 1/2 inch pipes (copper or PEX) (3) "T-joint" to connect 3/4", 3/4", and 1/2" pipes (4) Copper-to-PEX 1/2" connection (requires soldering) (5 and 6) tools to undo PEX connections (7) crimp rings to squeeze metal band to connect PEX to a fixture (8) PEX tube cutter.PEX tubing is widely used to replace copper in plumbing applications. One estimate from was that residential use of PEX for delivering drinking water to home faucets was increasing by 40% annually.[16] In , The Philadelphia Inquirer recommended that plumbing installers switch from copper pipes to PEX.[17]
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 pipes in the late s.[18] 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 s onwards.[16] Hydronic systems circulate water from a boiler or heater to places in the house needing heat, such as baseboard heaters or radiators.[19] PEX is suitable for recirculating hot water.[20]
Gradually, PEX became more accepted for more indoor plumbing uses, such as carrying pressurized water to fixtures throughout the house. Increasingly, since the s, copper pipes as well as plastic PVC pipes are being replaced with PEX.[18] PEX can be used for underground purposes, although one report suggested that appropriate "sleeves" be used for such applications.[20]
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Benefits of using PEX in plumbing include:
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PEX has been approved for use in all fifty states of the United States as well as Canada,[15] including the state of California, which approved its use in .[20] California allowed the use of PEX for domestic water systems on a case-by-case basis only in .[39] This was due mostly to concerns about corrosion of the manifolds (rather than the tubing itself) and California allowed PEX to be used for hydronic radiant heating systems but not potable water. In , the Building Standards Commission approved PEX plastic pipe and tubing to the California Plumbing Code (CPC), allowing its use in hospitals, clinics, residences, and commercial construction throughout the state.[20] Formal adoption of PEX into the CPC occurred on August 1, , allowing local jurisdictions to approve its general use,[40] although there were additional issues, and new approvals were issued in with revised wordings to the act.[41]
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Alternative plumbing choices include
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PEX-AL-PEX pipes, or AluPEX, or PEX/Aluminum/PEX, or Multilayer pipes are made of a layer of aluminum sandwiched between two layers of PEX. The metal layer serves as an oxygen barrier, stopping the oxygen diffusion through the polymer matrix, so it cannot dissolve into the water in the tube and corrode the metal components of the system.[42] The aluminium layer is thin, typically 1 or 2 mm, and provides some rigidity to the tube such that when bent it retains the shape formed (normal PEX tube will spring back to straight). The aluminium layer also provides additional structural rigidity such that the tube will be suitable for higher safe operating temperatures and pressures.
The use of AluPex tubing has grown greatly since . It is easy to work and position. Curves may be easily formed by hand. Tube exists for use with both hot and cold water and also for gas.[citation needed]
This product in Canada has been discontinued due to water infiltrating between the Layers resulting in premature failures.
There are two types of fitting that may be used. Crimped or compressive. Crimped connectors are less expensive but require a specialised crimping tool. Compression fittings are tightened with normal spanners and are designed to allow sections of the system to be easily disassembled, they are also popular for small works, esp. DIY, avoiding the need for extra tools.
A PEX tool kit includes a number of basic tools required for making fittings and connections with PEX tubing. In most cases, such kits are either bought at a local hardware store, plumbing supply store or assembled by either a home owner or a contractor. PEX tools kits range from under $100 and can go up to $300+. A typical PEX tool kit includes crimp tools, an expander tool for joining, clamp tools, PEX cutters, rings, boards, and staplers.[further explanation needed]
Multilayer AluPex tube and connector[
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