In cooperation with Hitachi Ltd., the National Institute of Advanced Industrial Science and Technology, and Japan Steel Works Ltd., Hitachi Cable recently developed a technology that makes it possible to recycle waste silane cross-linked polyethylene - commonly used as a power cable insulation material - by allowing it to react it with supercritical alcohol (Note 1). The resulting material can then be used once again as a power cable insulation material.

In recent years, various recycling laws have been implemented amid growing concerns for the global environment, including the Electric Household Appliance Recycling Law. A newly emerging obligation for business enterprises is the recycling of waste matter. Even in the areas of wires and cables, the recycling of wires and cables covering materials has become an increasingly significant concern. However, the difficulty (arising from cross-linked bonds between polyethylene molecules) of molten molding silane cross-linked polyethylene (as shown in Table 1), which is used in significant quantities as power cable insulation, has blocked efforts to advance material recycling in this area.

Hitachi Cable has examined various ways to return cross-linked polyethylene to a thermoplastic polyethylene capable of being subjected to molten molding for reuse as a wire covering material.

The result is a technology in which silane cross-linked polyethylene is transformed into thermoplastic polyethylene that can be molten-molded by placing the cross-linked polyethylene in alcohol under high temperature, high pressure, supercritical conditions.

Several mass production methods have been examined. As shown in Figure 1, continuous production of thermoplastic polyethylene was shown to be possible using a method that injects supercritical alcohol into an extruding machine. The polyethylene produced in this way can be extrusion molded once again, with properties that make it an adequate insulation material for low-voltage power cables.

As a method for economical continuous processing of solid raw materials with supercritical fluids, this mass-production technique is expected to have applications well beyond cross-linked polymer recycling. Major repercussions are expected across a wide range of industries.

Hitachi Cable plans to cooperate with Shizuoka University, the National Institute of Advanced Industrial Science and Technology, and Japan Steel Works Ltd. to manufacture a test apparatus in the Key Technology Research Promotion Program commissioned by NEDO (New Energy and Industrial Technology Development Organization), while undertaking research to advance the safety, stability, and practical applications of this technology.


Note 1 Supercritical alcohol: A supercritical state refers to the state wherein critical temperature and pressure (critical point) at which gas and liquid coexist are exceeded, where the gas and liquid densities become equal and the two phases cannot be differentiated. Because supercritical alcohol has both the diffusion properties of a gas and the material solubility properties of a liquid, it has numerous potential applications as a reaction solvent.

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Figure 2 External view of wires manufactured with reclaimed polyethylene
(conductive cross-sectional area 2 mm², insulator thickness 0.8mm)