Properties cross-linked polyethylene

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. 

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. 

Radical decay associated with combination of radicd species can result in cross-linking. Such reactions are used commercially to cross-link polyethylene, following irradiation. Cross-linking results in an improvement in the mechanical properties of the material, as is the case in irradiated poiyeth3d ie. It mi t be expected, therefore, that specimens which have been tensile testol under conditions which produce... 

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 . 

Cross-linked Polyethylene Polyethylene thermoplastics partially photochemically or chemically cross-linked. Has improved tensile strength, dielectric properties, and impact strength at low and elevated temperatures. 

Both linear and side-branched polyethylenes are of technological interest. As the number of side branches per 100 carbon atoms is increased from zero there are pronounced changes in physical properties (see Chapter 4), but the linear and side-branched pofymers are identical in one respect, which is that they may be reversibly heated to melt and then cooled to crystallize time and time again. On melting they flow as does a liquid and are thus thermoplastic (or thermal-flow) polymers. In this they are distinguished from cross-linked polyethylene, which when heated will not flow. 

Atkinson J.R., and R.Z. Cicek. 1983. Silane cross-linked polyethylene for prosthetic applications. Part I. Certain physical and mechanical properties related to the nature of the material. Biomaterials 4 267-275. 

Rezanejad, S., Kokab, M. Shape memory and mechanical properties of cross-linked polyethylene/clay nanocomposites. Eur. Polym. J. 43, 2856-2865 (2007)... 

While thermoplastic polymers soften at Tg, and if semicrystalline, melt at T, cross-linked polymers do not melt and flow (Fig. 5.2c ). Lightly cross-linked polymers soften as the temperature exceeds Tg, but they remain rubbery solids until the polymer decomposes. Highly cross-linked polymers often do not even soften and retain a high modulus until reaching the decomposition temperature. Thermoset resins, like unsaturated polyester, epoxy, and polyurethanes, have varied levels of cross-linking. However, thermoplastic resins can be modified to contain few cross-links lightly cross-linked polyethylene (XLPE) often improves the mechanical properties of rotomolded parts. 

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. 

Nedjar [26] have reported on the measurement volume of resistivity cross-linked polyethylene grades used in high-voltage cables and the effect of thermal aging on the electrical properties of the cable. 

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. 

Polyethylene Properties of the different forms of polyethylene are reflected in their uses. Linear molecules of polyethylene can pack together very closely, as shown in the model of high-density polyethylene (HDPE). The branches of branched polyefhylene keep the molecules from packing fightly, as shown in the low-density polyethylene (LDPE) structure. The cross-links of cross-linked polyethylene (cPE) make it very strong. 

Modification of Type C. Has some properties similar to Type A stock Specially compounded cross-linked polyethylene (Gatron). 

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