SUNLIGHT-RESISTANCE
OF CABLE COVERINGS
The explosion of
interest in beautification has been felt in the wire
and cable industry. The interest in cables other than
those that are round and black grows with each passing
day. Power lines and auxiliary equipment that blend
into the surrounding terrain and sky are more than
a curiosity. The aesthetic qualities of a cable design
will become a significant factor in cable specifications.
The continuous exposure of cables
to weather is of major concern to a cable engineer.
It is a matter of record that all polymertype coverings
undergo degradation over a period of time. Environment,
manner of installation, and chemical composition of
the polymer are significant factors influencing cable
longevity. In a very broad sense, the weathering resistance
of a specific polymer is dependent on its ability
to resist chemical change brought about by exposure
to oxygen and sunlight. The attack of oxygen on wire
and cable coverings has been well established and
will not be dwelt upon.
Sunlight, the source of all earthly
life, can likewise be a serious and potent threat
to wire covering. Generally speaking, the ultraviolet
band of the sunlight acts as a promoting agent for
the oxidation reaction of polymeric material - resulting
in crazing, chalking, or cracking of the covering.
To cite an example, during the earlier part of the
1940's, unpigmented polyethylene-insulated wire encountered
severe weathering degradation after only a few months
of outdoor exposure. After culmination of a 10-year
investigation, Bell Telephone Laboratory found that
suitable protection can be provided by a 1-2% concentration
of channel black having particle diameters of about
25 millimicrons or less when well dispersed in an
appropriately selected polyethylene. The last 20 years'
field experience has further proven that this carbon
black polyethylene composition is completely satisfactory
for outdoor wire and cable. This practice also holds
true for other polymer systems. Certainly, most thermosetting
wire coverings contain carbon black for reinforcement
purposes and prolonged exposure to sunlight poses
no problem. With light-colored compounds, sunlight
effect is predominant. This is the existing reason
why "black" is a highly preferable color
for cable coverings.
Colored compounds containing certain
amounts of white inorganic filler show a diminished
rate of oxidation in the early stages, since much
of the sunlight is reflected by these high refractive
index materials. However, after certain periods of
light exposure, the oxidation reaction ultimately
breaks down the molecular chain of polymers involved,
which finally leads into chalking and cracking. Recently,
certain colorless organic compounds, having absorptive
characteristics for ultraviolet light, have been introduced
into the market as a light or ultraviolet absorber.
These are being used either singly or collectively
with some inorganic fillers. There has been some expression
of doubt that a complete protection of polymer covering
can be achieved in this way since not enough of the
additive can be used to effect complete absorption
and the short-wave visible light, which is still quite
effective in promoting the light reaction, is not
really absorbed at all.
The question will naturally arise
concerning the method of evaluating various polymers
containing these ultraviolet light absorbents for
outdoor weathering resistance. Various laboratory
devices, such as Weather-O-Matic, Fade-O-Meter, etc.,
have been frequently used to ascertain at a greatly
accelerated speed the trend of reactions, which will
occur in outdoor exposure. This type of equipment
serves as an invaluable aid in the development work
of screening new compositions to obtain comparative
data under uniform and reproducible exposure conditions.
Although trials are made in correlating
practical outdoor weathering with accelerated laboratory
test results as so many thousand-hours-exposure in
Weather-O-Matic being equal to so many years actual
outdoor weathering, this correlation remains true
only to a specific type of material and cannot always
be transferred to a different family of polymer. For
this reason final conclusions in the evaluation of
various polymers systems for sunlight stability or
outdoor weathering rely solely on actual outdoor test
results.
In summary, there is a definite interest
in colored cables that could develop into a trend.
Accelerated tests for evaluating weathering-resistance
are underway and will be valuable in screening the
most promising compounds. Only actual outdoor weathering
for an extended period of time will give a true evaluation.
Compounds with histories of outdoor exposure approaching
the life expectancy associated with black coverings
are not currently available. Both cable manufacturers
and consumers must be tolerant concerning this situation
and plan their activities and experimental installations
accordingly. Testing has been underway for several
years and only time can provide a tangible answer.
