Cross-linked polyethylene plastic

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. 

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). 

Fuel hoses such as those used to dispense methanol into vehicles have been developed that are methanol compatible. Cross-linked polyethylene has been found to be a compatible methanol fuel material. Fuel hoses designed for gasoline will harden and crack while depositing significant amounts of plasticizer into the methanol. 

Fluidized bed temperature 700°C. CPE cross-linked polyethylene CPP carpet polypropylene waste, MPW mixed plastic waste... 

Since its accidental discovery in the early 1930s in Great Britain from the failure of a chemical reaction under pressure, researchers efforts have been driven toward obtaining a PE with specific chemical, mechanical, and thermal characteristics for the fabrication of complex-shaped tools, or for use in adverse environmental eonditions. The fundamental way to improve material properties such as impact strength, chemieal resistanee, and thermal characteristics is via cross-linking. Indeed, the introduction of cross-linked polyethylene (PEX) in the early 1970s was another milestone in the plastic era. From that date, PEX has eaptured a giant share of the market beeause of its superior eharaeteristics with respeet to other plasties. 

Tobolsky and Takahashi (7,8) showed that large concentrations of S8 can remain dissolved in a liquid condition in other polymers. In many cases these compositions appear completely stable, i.e., there is no tendency for the dissolved sulfur to crystallize out. The best example is cross-linked polyethylene tetrasulfide polymers which can retain 40% of dissolved sulfur in the form of liquid S8 over long periods of time. The sulfur was shown to be S8 by quantitatively extracting it with carbon disulfide. It was demonstrated that the specific volume of the dissolved sulfur plotted against temperature fits smoothly with the data of specific volume of molten sulfur vs. temperature and finally that the mechanical properties of the cross-linked polymers containing dissolved sulfur are just what would be expected from plasticized, cross-linked, amorphous polymers. Ellis (9) reported the use of resins made by the interaction of 2,3-xylenol and sulfur monochloride as sulfur additives. These resins were added to three times their weight of molten sulfur. There was no indication of sulfur crystallization in the resultant material, which also... 

However some non-rubber compounds are also called elastomers if they exhibit a nondeforming elastic property similar to rubber at room temperature, even if the compound is relatively hard. The two main groups of non-rubber elastomers are thermoplastics, e.g., polyvinyl chloride, polypropylene and thermosets, e.g., ethylene propylene rubber, cross-linked polyethylene. These two groups are also covered by the term plastic . 

Flexible plastic foams may be found in the form of very soft cushioning materials used in upholstery, clothing interlayers, automobile seats, vibration absorbers, etc. The most common flexible foam resins are polyurethanes, foamed vinyls, cellular polyethylene, cross-linked polyethylene, and silicone foam. Semirigid foams are used for floatation devices, marine bumpers, special electrical insulation on television cables, packaging, and a host of other applications. Rigid foams are used in the production of airplane parts, boats, electronic encapsulation, and many furniture applications where wood was formerly used. 

Figure 2 Expansion characteristics of peroxide-cross-linked polyethylene. (Courtesy of Society of Plastics Engineers, Inc.)...

EC member states have voted that there should be no further restriction on the marketing and use of cadmium pigments for plastics. The International Cadmium Association (ICA) believes that this should now be safeguarded for the next three years, when a more far-reaching report on cadmium and cadmium oxide is expected. A Directive (91/338/EEC) was adopted by the EC to harmonize restrictions on the use of cadmium-based pigments that had been introduced by different countries. It does not ban the use of these materials but limits their use. For example, cadmium-based pigments may not be used in plastics materials where there are other satisfactory substitutes. Polypropylene and polystyrene are specifically listed as polymers where non-cadmium pigments must be used where possible. Other polymers where there is a restriction include thermoplastic polyesters, poly(methyl methacrylate), cross-linked polyethylene and melamine, urea, and polyester resins. 

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). 

When covalent bonds form between carbon chains, a cross-linked polymer results and the physical properties change sharply (Fig 21.34[c]). Cross-linked polyethylene is used for plastic screw caps on soda bottles. This plastic is rigid enough to mold as a solid and has enough mechanical strength to hold the screw thread needed to tighten the cap on the bottle. 

In the past, there were fewer accidents because buried cables were better protected. Most of the supply authorities used paper-insulated, lead-sheathed, single-wire or steel-tape armoured and served cables and very often protected them with cable tiles. For economic reasons cable tiles are seldom used now and many of the mains laid in recent years are unarmoured CNE cables with plastic insulation and sheathing, such as PVC and cross-linked polyethylene (XLPE). The outer concentric conductor, which is the earthed neutral, is copper or aluminium tape and consequently more vulnerable to damage than a steel-wire-armoured cable. Every year more new mains cables of this type are laid so the resultant hazard to excavators is growing and more accidents can be expected to occur unless the construction industry becomes more effective at taking precautions against striking buried cables. 

Tempersture. An increase in temperature produces a marked decrease in the volume resistance of plastic. Measurements above 10 S2 cm and 200°C are difficult even with excellent equipment, and in such cases the values are open to question. The marked difference between materials is apparent. The volume resistivity of PTFE decreases only slightly, even as it approaches its crystalline melting temperature of 327°C, whereas PE shows a marked reduction in resistivity as its crystalline melting temperature is approached. Cross-linked polyethylene shows a similar but less pronounced reduction. An asbestos phenolic resin may have a relatively low resistance at room temperature, but the decrease with temperature is not as pronounced as for a polyamide resin, which may show an influence of absorbed moisture. It is apparent that the room temperature resistance values commonly given in the manufacturer s data sheets, are of little value for design purposes. 

Laboratory tests indicated that gamma radiation treatment and cross-linking using triaHylcyanurate or acetylene produced a flexible recycled plastic from mixtures of polyethylene, polypropylene, general-purpose polystyrene, and high impact grade PS (62). 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Corrosion. Aqueous solutions of citric acid are mildly corrosive toward carbon steels. At elevated temperatures, 304 stainless steel is corroded by citric acid, but 316 stainless steel is resistant to corrosion. Many aluminum, copper, and nickel alloys are mildly corroded by citric acid. In general, glass and plastics such as fiber glass reinforced polyester, polyethylene, polypropylene, poly(vinyl chloride), and cross-linked poly(vinyl chloride) are not corroded by citric acid. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

In terms of tonnage the bulk of plastics produced are thermoplastics, a group which includes polyethylene, polyvinyl chloride (p.v.c.), the nylons, polycarbonates and cellulose acetate. There is however a second class of materials, the thermosetting plastics. They are supplied by the manufacturer either as long-chain molecules, similar to a typical thermoplastic molecule or as rather small branched molecules. They are shaped and then subjected to either heat or chemical reaction, or both, in such a way that the molecules link one with another to form a cross-linked network (Fig. 18.6). As the molecules are now interconnected they can no longer slide extensively one past the other and the material has set, cured or cross linked. Plastics materials behaving in this way are spoken of as thermosetting plastics, a term which is now used to include those materials which can in fact cross link with suitable catalysts at room temperature. 

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