Polyethylene
(PE) has been widely used as a jacket for underground cables since it became
commercially available in large quantities in about 1950. For use as a jacket,
polyethylene may be compounded with carbon black or coloring material, and with
stabilizers.
Carbon black
gives the material the necessary sunlight protection for outdoor use. Polyethylene
for jacketing is categorized under three different densities:
Low density 0.910
to 0.925 grams per cm3
Medium
density 0.926 to 0.940 grams
per cm3
High density 0.941 to 0.965 grams per
cm3
Density
generally affects the crystallinity, hardness, melting point, and general physical
strength of the jacketing material. In addition to density, molecular weight
distnibution is important since it influences the processing and properties of
the polymer.
Polyethylene
jackets are an excellent choice where moisture resistance is a prime design
criteria since it has the best moisture resistance of any non-metallic jacket
material. When polyethylene is used as a jacket material, it should be compounded
with enough carbon black to prevent ultra-violet degradation.
Linear, low
density, high molecular weight (LLDPE) is the most popular jacket material
since it has better stresscrack resistance that the high density materials. High
density provides the best mechanical properties, but may be very difficult to
remove from the cable.
In
evaluating fillers, both black and non-black, it has been found that although many
of these materials improve the aging characteristics, carbon black is by far the
best. It has also been found that the aging resistance increases with carbon black
loading from 2 to 5 percent. Normally, a 2.5 to 3.0 percent loading is used.
Although PE
has good moisture resistance and good aging properties in its temperature
limits, it has poor flame resistance. This discourages using it as a jacket in
many circumstances. Polyethylene jackets have good cold bend properties since
they will pass a cold bend test at about -55 "C.
They are extremely
diflicult to bend at low temperature because of their stiffness. Like PVC, PE is
a thermoplastic material and will melt at elevated temperatures. This temperature
will vary slightly with molecular weight and density, but melt occurs at about
105 "C.
High density
polyethylene (HDPE) has been used extensively as the second (outer) layer for
"ruggedized" thermoplastic in secondary and low voltage street light
cables because of its toughness.
While black
polyethylene for jacketing is frequently an insulating material, with higher
loadings of carbon-black it can be a semiconducting material. This material has
been used for over 30 years in direct-buried applications to improve the
grounding of the concentric neutral.
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