"Jeff Liebermann" wrote in message
...
On Mon, 12 Jan 2009 21:57:52 -0500, "NoSPAM"
wrote:

Also I would like to add that the Krylon sprayed over the vinyl tape is
very important. Being an acrylic, clear Krylon protects the vinyl from
ultraviolet damage.
73, Barry WA4VZQ

I've been waterproofing RF connectors using 0.5" wide PTFE tape to
embalm the connector. The PTFE will eventually cold flow, forming a
water tight seal. The PTFE also prevents capillary action from
sucking water into the connector through the gaps, such as along the
outer knurling in UHF connectors. On top of that, goes a layer of 3M
Scotch 33+ electrical tape, which is claimed to be UV resistant. The
PTFE tape does the waterproofing. The outer PVC tape just holds
everything in place.

I've done some crude life tests on my roof at 2 year and 5 year
intervals. I found that when unwrapped, the connectors are a shiny
as they were when new.

However, I didn't think that PVC tape was porous and suffers from UV
embrittlement. After about 5 years, the tape does tend to be somewhat
bittle. I'll try some Krylon and see what it does. I'm a bit
concerned about adhesion to the shiny PVC tape and what will happen if
the joint is flexed. I dunno about spraying Krylon when I'm on top of
a tower. I'll let you know in about 5 years.

The issue is a little more complicated than simple UV resistance and
porosity. To begin with, pure PVC has a very poor UV resistance. But more
importantly, it is brittle and it needs a plasticizer to make it flexible.
The most common plasticizer is DOP [dioctal phthalate, or more correctly
2-ethylhexyl phthalate]. but this can leach out of the PVC leaving it stiff
and more brittle.. To achieve better low temperature flexibility, DOA
{2-ethylhexyl adipate} is often used. Vinyl tape has to be very flexible
and stretchy even at low temperatures, so high levels of plasticizers are
needed. DOA is usually used, either alone or in combination with DOP. DOA
tends to leach out faster than DOP. It also has poorer UV resistance than
DOP which is poor to start with. There are far better performing
plasticizers than either of these (made by reacting adipic or phthalic acid
with polyols), but while they offer superior UV resistance and leach out
much more slowly, they are rather expensive. Loading the PVC with carbon
black, an excellent absorber of ultraviolet (and visible) light, helps too,
but the amount of carbon black that can be used on an electrical tape is
limited by the need for good dielectric strength. Spraying the tape with
Krylon® acrylic paints provides both UV resistance and a slowing of the
loss of plasticizer.

I would not worry about the PTFE (Teflont®) tape. While very resistant to
liquid water, water vapor can pass through the tape easily and condense on
cold metal parts of the connector. PTFE tape is porous, but the surface
tension of water prevents it from passing through the pores. {Water vapor,
too, is a smaller molecule than oxygen or nitrogen.} Several layers of
electrical tape, wound in opposite directions, and sprayed with Krylon
survived at least 15 years in one of my outdoor applications, and when
removed, the coaxial connectors were shiny too.

In case anyone is wondering about my expertise here, I worked in the
research Laboratories of Eastman Chemical Company, who made a number of
plasticizers, and W. L. Gore & Associates, who started by making PTFE tape.
The breathability properties of Gore-Tex while still shedding water are a
result of controlled pore sizes in PTFE.

73, Dr. Barry L. Ornitz WA4VZQ