Cross-linked polyethylene mechanical properties

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... 

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. 

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. 

Asano T, Akagi M, Qarke IC, Masuda S, Ishii T, Nakamura T. Dose effects of cross-linking polyethylene for total knee arthroplasty on wear performance and mechanical properties. J Biomed Mater Res B Appl Biomater 2007 November 83(2) 615-22. 

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

PVC, another widely used polymer for wire and cable insulation, crosslinks under irradiation in an inert atmosphere. When irradiated in air, scission predominates.To make cross-linking dominant, multifunctional monomers, such as trifunctional acrylates and methacrylates, must be added. Fluoropolymers, such as copol5miers of ethylene and tetrafluoroethylene (ETFE), or polyvinylidene fluoride (PVDF) and polyvinyl fluoride (PVF), are widely used in wire and cable insulations. They are relatively easy to process and have excellent chemical and thermal resistance, but tend to creep, crack, and possess low mechanical stress at temperatures near their melting points. Radiation has been found to improve their mechanical properties and crack resistance. Ethylene propylene rubber (EPR) has also been used for wire and cable insulation. When blended with thermoplastic polyefins, such as low density polyethylene (LDPE), its processibility improves significantly. The typical addition of LDPE is 10%. Ethylene propylene copolymers and terpolymers with high PE content can be cross-linked by irradiation. ... 

Muratoglu, O.K. Bragdon, C.R. O Connor, A.S. Jasty, M. Harris, W.H. A novel method of cross-linking ultra-high molecular-weight polyethylene to improve wear reduce oxidation, and retain mechanical properties. J. Arthoplasty 2001, 16, 149-160. 

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