Polyethylene cross-linked

Three main approaches are used for cross-linking polyethylene  

The third process for cross-linking is the Sioplas process developed by Dow. The first stage of this involves the grafting of an easily hydrolysable trialkoxyvinylsilane onto the polyethylene chain, the site activation having been achieved with the aid of a small amount of peroxide. The compound is then extruded onto the wire, which is collected on a drum. The drum is then exposed to hot water, or, more commonly, low-pressure steam. The water hydrolyses the alkoxy groups, which then condense to form a siloxane cross-link.  

The cross-linking stage is facilitated by the use of a cross-linking catalyst, which is typically an organo-tin compound. A number of variations of this process exist and in one of these compounding, grafting and extmsion onto wire are carried out in the same extruder. 

There has been interest, particularly in Japan, in the production of cross-linked low-density polyethylene foam. Some processes, such as the Furukawa process and the Hitachi process, use chemical cross-linking techniques whilst others, such as the Sekisui process, involve radiation cross-linking. 

These cross-linked cellular materials have been used in the automotive industry for carpeting, boot mats and sound deadening. They have also found use for pipe insulation and as flotation media for oil-carrying and dredging hose. 

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Miscellaneous. Flame-resistant cross-linked polyethylene can be made with a number of fluoroborates and antimony oxide. This self-extinguishing material may contain the fluoroborates of NH, Na", K", Ca ", Mg ", Sr ", or Ba " in amounts of 4—20% (76). Magnesium fluoroborate cataly2es the epoxy treatment of cotton fabrics for permanent-press finishes (77) (see Textiles). 

There are three basic types of polyethylene foams of importance (/) extmded foams from low density polyethylene (LPDE) (2) foam products from high density polyethylene (HDPE) and (J) cross-linked polyethylene foams. Other polyolefin foams have an insignificant volume as compared to polyethylene foams and most of their uses are as resia extenders. 

The Hst price of 35 kg/m polyethylene foam boards is about 0.58/BE ( 6.90/kg) and that of cross-linked polyethylene foam is about 0.33 to 0.88/kg more expensive than uncross-linked polyethylene foam depending on the density and thickness of the foam. 

Cross-linked polyethylene-based compounds that contain dame-retardant components and compounds based on PVC cross-linked by radiation have also received high temperature rating. They find use not only in appHance wires but also in manufacturing under-the-hood automotive wires. 

Improved heat resistance is the most important advantage of cross-linked polyethylene (XITK) over thermoplastic polyethylene. A power cable... 

Antioxidants. The 1,2-dihydroquinolines have been used in a variety of ways as antioxidants (qv). For example, l,2-dihydro-2,2,4-trimethylquinoline along with its 6-decyl [81045-48-9] and 6-ethoxy [91-53-2] derivatives have been used as antio2onants (qv) and stabilizers (68). A polymer [26780-96-1] of l,2-dihydro-2,2,4-trimethylquinoline is used in resins, copolymers, lubricant oils, and synthetic fibers (69). These same compounds react with aldehydes and the products are useful as food antioxidants (70). A cross-linked polyethylene prepared with peroxides and other monomers in the presence of l,2-dihydro-6-ethoxyquinoline produces polymers with a chemically bonded antioxidant (71). 

NEMA WC7/1991 (ICEA 5-66-524) Cross linked polyethylene insulated wire and cable for the transmission and distribution of electrical energy ... 

In the United States high molecular weight polyethylene is used, and in the United Kingdom, cross-linked polyethylene with polyvinylchloride is used. 

Vulcanised (cross-linked) polyethylene is being used for cable application where service temperatures up to 90°C are encountered. Typical cross-linking agents for this purpose are peroxides such as dicumyl peroxide. The use of such agents is significantly cheaper than irradiation processes for the cross-linking of the polymer. An alternative process involves the use of vinyl silanes (see Section 10.9). 

The excellent electrical insulation properties of polyethylene have led to extensive use in cable and other wire-covering applications. Spectacular early uses included undersea cables and airborne radar and the materials continue to be used in substantial quantities. One particular trend is the increasing use of cross-linked polyethylene for this area of use. Such materials have improved heat resistance and in addition have given generally better resistance to stress cracking. Cellular polyethylene is used as the insulator for television downlead aerials. 

High thermostabilizing efficiency of polyamine disulphides relative to chemically cross-linked polyethylene is conditioned by the ability to accept macroradicals at the disulphide bridge and imine group. Besides, the presence of paramagnetic centers causes the adherence of macroradicals providing for an extra stabilizing effect [49]. 

TR torque rheometer XLPE cross-linked polyethylene... 

The cross-linked polyethylene retains shape upto 140°C and is used for cable insulation. Polyethylene is the most widely used plastic due to its low cost and ease of processing. 

Properties of peroxide cross-linked polyethylene foams manufactured by a nitrogen solution process, were examined for thermal conductivity, cellular structure and matrix polymer morphology. Theoretical models were used to determine the relative contributions of each heat transfer mechanism to the total thermal conductivity. Thermal radiation was found to contribute some 22-34% of the total and this was related to the foam s mean cell structure and the presence of any carbon black filler. There was no clear trend of thermal conductivity with density, but mainly by cell size. 27 refs. 

Figure 23 The trapping probability (Ag/go) and variation in surface potential per incident charge AF/go, as functions of incident electron energy for the 115-mm-thick cross-linked polyethylene (XLPE) sample. (From Ref. 32.)...

When transferring fuel and fuel additives from storage to the pump and to its final point of use, different types of lines and hoses are utilized. Mild steel, stainless steel, and cross-linked polyethylene hose are the typical materials of construction used. These materials withstand the temperature and environmental extremes of outdoor installation and are compatible with fuels and most additives. 

The higher crystalline fraction of the radiation cross-linked polyethylene even after a melt-freeze cycle has great technological merit for the heat-shrink packaging and electrical connector products. ... 

Compared with chemical cross-linking of PE, radiation curing produces a different product in many respects. The chemical cross-linking is done at temperatures near 125°C (257°F), where the polymer is in the molten state. Consequently, the cross-link density in the chemically cross-linked polyethylene is almost uniformly distributed, while there are relatively few cross-links in the crystalline fraction of the radiation cross-linked PE. The crystalline fraction of the radiation-processed polyethylene is greater than that in the chemically cured product. ... 

Cross-linkable polymers used for wire and cable insulations are polyolefins, certain fluoropolymers, and elastomers. Among these, radiation cross-linked polyethylene is the most widely used. The radiation cross-linking process of PE has also been the most widely studied. ... 

Since polyethylene becomes infusible after irradiation, the material should be cross linked before molding. If this is done, easy moldability, one of the prime attractions of the plastic, is lost. However, if polyethylene is irradiated after molding it can be vacuum drawn and formed in very thick sections to extremely deep draws, and with very little temperature control. One must be careful about heating cross-linked polyethylene since (1) above 230° to 240° an article will distort if it is not free from molding strains and (2) above 220° oxidative degradation reactions set in unless, of course, antioxidants have been added. 

PBDEs are used in different resins, polymers, and substrates at levels ranging from 5 to 30% by weight (EU 2001). Plastic materials that utilize PBDEs as flame retardants include ABS polyacrylonitrile (PAN) polyamide(PA) polybutylene terephthalate (PBT) polyethylene (PE) cross-linked polyethylene (XPE) polyethylene terephthalate (PET) polypropylene (PP) polystyrene (PS) high-impact polystyrene (HIPS) polyvinyl chloride (PVC) polyurethane (PUR) and unsaturated polyester (UPE). These polymers and examples of their final products are summarized inTable 5-2 (Hardy 2002 WHO 1994a). 

ABS = Acrylonitrile Butadene Styrene PA = Polyamide PAN = Polyacrylonitrile PBT = Polybbutylene Terephthalate PE= Polyethylene PET = Polyethylene Terephthalate PP = Polypropylene PUR = Polyirethane PVC = Polyvinyl chloride UPE = Un saturated polyester XPE = Cross-linked polyethylene ... 

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