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Crosslinking with Cables
In cables, crosslinked LDPE or EVA are used as an insulation layer. Crosslinking provides polyolefins with:
- Good electrical properties
- Heat resistance
- Mechanical resistance
Peroxides are used to cure the insulation layer. The processing temperature is generally between 150-170°C and crosslinking generally occurs between 150-250°C.
Crosslinked polyolefins are also used in pipes and tanks.
Process
The process can be divided in 3 steps:
- Absorption
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This mixing method is based on the ability of certain polymers, in granular form, to absorb small amounts of liquids in their pores. This is an important technology for XLPE cables, where liquid or molten peroxide is sprayed on LDPE granules prior to extrusion.
It is important to have a closed but not confined system to avoid safety issues with volatile peroxides.
- Extrusion
Soaked LDPE granules and other materials required in the recipe are fed into a hopper directly placed above the screw. The screw conveys the molten plastics versus the die that shapes the extrudate. It is important to fix both extrusion temperature and speed in a way to avoid premature crosslinking (Scorch).
- Continuous Vulcanization
It is a crosslinking technology which is applied for the curing of cable insulation materials and based on a jacked tube which may be 60 meters long and operating under internal steam or nitrogen pressure of 1.4 MPa.
Critical points with peroxides influencing the electrical performance of the insulators:
- Attain complete decomposition to remove polar compounds
- Degassing of volatiles to improve dielectric properties and to avoid inhomogeneities in the materials
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