"Your coating is doomed, its just a matter of time". Sounds like a guy I
would like.
Thank you everyone for your responses. I had figured that most of the
available information would fall into one of two camps - either the
effects of UV from sunlight, which would be a broad spectrum, or high
energy radiation, such as cosmic radiation in space.
I had done some research into the topic some 20-25 years ago where the
polymeric degradation was generally by polymeric chain scission, or
cutting of the polymer chains, as Bob's friend indicats. I had not
considered the UV attack on the bonds between coating and substrate. I
will have to think about that some.
But in this application, the UV radiation is lower in power and more
specific in wavelength. On the other hand, all of the coatings we use,
with the exception of Parylene, have a fluorescent dye in them which
specifically absorbs UV light. I wonder if the dye would act like a UV
inhibitor when it comes to polymeric degradation. I would expect the dye
to lose fluorescence with continued bombardment though.
I will let you all know how this develops, if I can.
Doug Pauls
Bob Landman <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
11/12/2010 12:54 PM
Please respond to
TechNet E-Mail Forum <[log in to unmask]>; Please respond to
Bob Landman <[log in to unmask]>
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Re: [TN] UV radiation and conformal coating
Doug,
I sent your posting to an old college friend who is a physicist with a
great deal if working knowledge on chemical finishes as his company makes
paints and protective coatings for the marine environment. He is teamed
with Bill Rollins at Raytheon Tucson on a grant from the MDA to develop a
whisker-tough conformal coating.
He replied:
Bob,
Thanks for forwarding this to me. I am not subscribed to the list, so
please post this for me.
All,
Varnish fails on wood of boats in one or a few years, due to the
ultraviolet of the sunlight eventually getting thru the varnish [even with
UV absorbers] and breaking down the cellulose of the wood below. Cellulose
has a lot of carbon-carbon and carbon-oxygen bonds.
Indeed; the loss of gloss of varnish is due to surface-oxidation by the
solar ultraviolet photons breaking the atomic bonds of the
material at the surface; those surface free radicals react almost
immediately with atmospheric oxygen, and are literally burned away.
A carbon-carbon bond has an energy of about 348 kilojoules per mole, and
will readily break with a barely-ultraviolet photon.
A carbon-oxygen bond has an energy of about 360 kilojoules per mole, and a
carbon-hydrogen bond about 413. For more details, see
http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html .
Ultraviolet and even blue-visible photons have enough energy to break
these bonds.
Broken atomic bonds create free radicals, and some of these will bond onto
neighboring polymer chains, causing increased modulus, reduced elongation,
and in fact the polymer gets smaller. the reason flakes of paint curl
outwards when they fail is that the outer surface has more tensile stress
than the material below, due to free-radical damage at the
sunlight-exposed surface. So, you see, it's going on all the time the sun
shines.
Your conformal coating (made of carbon, hydrogen, oxygen and nitrogen and
perhaps other atoms) is doomed; it's only a matter of time.
Steve Smith
Www.smithandcompany.com
On Nov 11, 2010, at 8:20 AM, Douglas Pauls <[log in to unmask]>
wrote:
> Good morning all,
>
> Does anyone know of any papers on the effects of UV light on conformal
> coating? Not talking the cosmic radiation hard space kind. I know it
> would be wavelength and intensity specific. Got a question from one of
> our engineers and I, of course, gave the very intelligent answer of
"Beats
> me". But I have this wonderful resourse called Technet...............
>
> Doug Pauls
>
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