PVC is the
most widely used nonmetallic jacketing material in the wire and cable industry.
Starting in 1935, when it first became available, the use of PVC grew rapidly
because of its low cost, its easy processing, and its excellent combination of
overall properties including fire and chemical resistance.
PVC belongs
to a group of polymers referred to as vinyls. The unmodified polymer contains approximately
55 % chlorine. It is fairly linear in structure (few side chains) with approximately
5 to 10 % crystallinity.
The material
must be compounded with additives such as fillers, plasticizers, and
stabilizers to attain flexibility, heat resistance, and low temperature
properties. General purpose jacketing materials normally posses good physical
strength, moisture resistance, adequate oil resistance, good flame resistance
and excellent resistance to weathering and to soil environments.
Flame resistance and low temperature
flexibility can both be improved within limits by the use of additives. General
purpose PVC compounds are recommended for installation at temperatures above
-10 "C, but specially formulated compounds may be used as low as -40
"C.
One of the
limitations of PVC jacketed cable is its tendency to creep under continuous
pressure. For this reason, cables which are to be supported vertically with
grips should not have PVC jackets. Hypalon or neoprenes are recommended for
such use.
In the low
voltage field, PVC is widely used as a single layer of material where it
functions both as insulation and jacket. Since PVC is a thermoplastic material,
it cannot take high temperatures. Under high current fault conditions the insulation
can be permanently damaged by melting or can emit plasticizers and become stiff
and brittle.
For this
reason, it is not used as utility secondary network cable. Similarly, in
industries that handle large amounts of heated material, or where there is the
possibility of excessive heat, the use of PVC is avoided because of its
tendency to melt or deform when heated to a high temperature.
Under continuous dc voltage in wet locations,
as in battery operated control circuits, single-conductor PVC-insulated cables
have frequently failed due to electro-endosmosis (water vapor ingress created
by voltage stress).
The large
percentage of chlorine can be released during a fire. When combined with
moisture, hydrochloric acid may be produced. This situation highlights one of
the major problems that can result from the use of PVC.
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