Cross-linkable Polyethylene (XLPE)

Properties and Applications

Cross-linkable polyethylene (XLPE, XPE, PEX) is a chemically modified high density polyethylene which can be crosslinked by moisture, radiation, catalysts, or with chemicals. The crosslinking converts polyolefin resins into insoluble and infusible polymers which have significantly improved impact strength and heat distortion but reduced melt index and elongation at break. Crosslinking also greatly increases creep, abrasion, and environmental stress crack resistance. The density and tensile strength of polyethylene are not or only slightly affected by crosslinking.

The three main types of cross-linkable polyethylene (XLPE) are

Peroxide cross-linked polyethylene: Polyethylene granulate and peroxide (dicumyl peroxide or di-tert-butyl peroxide) are mixed at low temperature in an extruder and then crosslinked at elevated temperatures. The peroxide decomposes forming peroxide radicals which abstract hydrogen atoms from the polymer backbone. The resulting radicals on the polymer backbone combine and form thereby crosslinks.

Silane cross-linked polyethylene: A vinylsilane is grafted via its double bond on the polyethylene through a free-radical reaction. The silane grafting is typically carried out in the molten state in the presence of a small amout of peroxide by reactive extrusion. The extruded articles are then cross-linked via exposure to hot water or steam.

Irradiation cross-linked polyethylene: The formed polyethylene articles are exposed to high-energy radiation such as gamma rays or electrons which generate free radicals which, in turn, induce crosslinking. The process is typically carried out in an inert atmosphere to prevent oxidative degradation. Due to high capital and operating costs, radiation crosslinking is less widely used than peroxide or silane crosslinking.

XLPE is produced on a much smaller scale than regular high density polyethylene (HDPE). It is often an excellent choice for applications that require improved chemical and/or abrasion resistance at reasonable cost. Important applications include wire and cable insulation, pipes and fittings and injection molded components.

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