"Jeff Liebermann" wrote in message
...
On Sun, 25 Jan 2009 20:46:28 -0500, "NoSPAM"
wrote:
For ultraviolet protection, a small amount of titanium dioxide may be
added
(1 to 5%), but the microballoons will do a fair job alone. Painting the
finished insulators with acrylic paint (KrylonŽ) will also provide good
UV
resistance.
I've never seen any white antenna insulators (except porcelain). Most
of the plastic antenna insulators I've seen are either black or
sometimes blue. I presume the filler and coloring used are something
something other than titanium white. Yet, everything made from marine
fiberglass is bright white, presumably doped with titanium white for
UV protection.
Titanium dioxide is a very bright white. It reflects the ultraviolet, as
well as visible and near-infrared. I am not familiar with its absorption
spectra in the far infrared. It is extensively used in marine fiberglass
resins, both for decorative and UV protection uses. Zinc oxide is also
white and it is used in sunscreens for the same reason.
The old Hy-Gain center insulators that were blue were likely polybutylene
terephthalate which is naturally UV resistant. The black insulators
contained carbon black, a UV absorber.
Is there any benefit to the white color over something like black
graphite stiffener and fill, which also offer UV protection? Duz the
color make any difference in preventing polymer chain breakage?
Carbon black is added as an absorber of ultraviolet. Graphite nanotubes
and graphite fibers are relatively recent developments. They are added to
produce a composite material which is quite strong and rigid in nature.
Being rather expensive compared to inexpensive carbon black, they are
generally added to plastics only for improved mechanical properties.
Polypropylene, Nylon, and Delrin absorb ultraviolet light and the energy
goes to breaking the polymer chain. Polybutylene and polyethylene
terephthalate absorb ultraviolet energy but fluoresce in the visible and
near infrared giving back the energy at a longer wavelength.
Polynaphthalene terephthalate is such a strong fluorescer in the visible
that it requires quenching additives in many applications such as food
packaging. Polyesters are used in marine ropes for their high strength and
UV resistance.
Drivel: I've noticed that the white PCV electrical tape I sometimes
use to wrap coax and connectors seems to last longer than the same
tape in black.
I would bet that red PVC tape lasts for only a short time in sunlight.
Most organic red pigments are destroyed rather quickly by ultraviolet
light, a real problem for automobile paint. There is a reason for Clear
Coat beyond its looks!
73, Dr. Barry L. Ornitz WA4VZQ
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