Cross-linked polyethylene piping

Cross-Linking. A thermoset system is produced when a polymer is cross-linked under static conditions, as in a compression mold. This is the basis of the production of vulcanized articles or cross-linked polyethylene pipe and wire insulation. If the same polymer is lightly cross-linked while it is being sheared in the molten state, however, it will remain thermoplastic. If it is more heavily cross-linked during this process, the final product may contain significant quantities of gel particles, but the whole mass will still be tractable. 

Cross-linked polyethylene may include impurities and voids from which the major causes of premature failures (SCG in pipes, treeing in electric cables, and cracking in medical implants) can originate. Thus, nondestructive evaluation with effective techniques that can discover defects at the incipient stage, before the component is put in operation, is of vital importance. Conventional ultrasound (up to 10 MHz), which is the current technique for detection of flaws in metal piping and vessels, is limited by the attenuated nature of polymers. A scanning acoustic microscope with operating frequencies up to 100-150 MHz has been found to be more effective. 

Figure 13.5 These PPSF hot water fittings, designed by Wavin for use with cross-hnked polyethylene-aluminum-cross-linked polyethylene composite pipe, are used in both household plumbing and radiator heating...

In order to connect one of its wells to its facility in the Badin distriet of Pakistan, British Petroleum Pakistan planned to constmct 5000 ft (1524 m) of 4-inch diameter spoolable reinforced composite pipe (SRCP). Python series 1500 cross-linked polyethylene (PEX) spoolable pipe was seleeted, offering a maximum operating temperature of 180°F (80°C). Table 18.3 reports the physical properties of the spoolable reinforced composite pipe. 

Cross-Linked Polyethylene (PEX). PEX pipe and tubing is made from essentially the same materials and by the same extrusion process used to manufacture PE pipe. The difference is that during or directly after extrusion the polymer molecules are cross-Unked to form a thermosetting material. The cross-linking may be effected by the addition of cross-Unking compounds to the PE, or by subsequent electron or high-frequency radiation. 

There are several plumbing material types for a householder to choose from when deciding on a plumbing material to be installed in a house copper, plastic (CPVC and PEX), and stainless steel. According to Marshutz survey (2000), copper is used in nearly 90% of homes in the U.S. followed by PEX (cross linked polyethylene) with a 7% installation rate, and CPVC (chlorinated polyvinyl chloride) with a 2% installation rate. Telephone surveys of plumbers conducted in 2005 show an increased use of plastic pipes, due to easier handling in installation and lower material cost (Scardina et al., 2007). 

A pipe application that has shown strong growth in recent years is cross-linked polyethylene for residential plumbing systems. These systems offer lower installation cost compared with copper piping, and some unique advantages such as less wait-time for hot water to reach the point of use, which also lowers energy usage for the hot water system. 

Peroxide crosslinked ionomer mixtures made from the abovementioned raw materials may be used in contact with fatty foods under certain limited conditions, according to their raw materials and additives. Vinyl chloride and vinylidene chloride should not be used in materials having either ionic (ionomer) or physical (e.g. through electron beam irradiation) crosslinks. Crosslinked polyethylene can be used for the manufacture of food contact materials, e.g. for drinking water pipes and fittings. The cross-linking can be done using either peroxide or electron beam irradiation. 

A number of manufactured products comprising thermoplastics (polyethylene, polyvinylchloride, polyamides,. ..) or elastomers (natural rubber, nitrile rubber, ethylene propylene rubber, silicones) are processed by cross-linking to improve their performances in insulated electric wires and cables, multilayered films for cooking pouches, shape memory tubes, pressure resistant water pipes, expandable foams, automotive parts exposed to motor... 

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