Re: [ATM] Epoxy mirrors (again)

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Wayne,

Epoxies, and most resin systems, have very high CTE's.  They are usually 
higher than aluminum, let alone Pyrex.  Aluminum is roughly 13x10e-6 
in/in/F.  Many epoxies are 30-50x10e-6 in/in/F.  Because of this, and other 
variables & properties, they cause all kinds of problems for visual 
spectrum applications when they are used alone and/or in thick layers.

Here is a short list of variables that make a pure resin (polyester, 
vinyl-ester, epoxy, UV resin, etc.) surface a very difficult endeavor in 
visual spectrum:
- they usually shrink.
- they are normally exothermic, like concrete, creating heat as they 
cure.  The thicker the layer of resin, the more it will exotherm (the 
higher its temperature will go).  100 grams of epoxy sitting in a 2" 
diameter paper cup with no lining can theoretically have such a runaway 
exothermic reaction as to cause the cup to catch on fire.  I've never tried 
that...  I can attest to it heating up though.  Once it is out in a thin 
layer however, the amount of exotherm is greatly hampered.  Controlling the 
thickness of the layer of resin is therefore a large variable that needs to 
be controlled in order to get repeatable results from mirror to mirror (or 
part to part).
- as temps rise, normally the resin's viscosity lowers and it will flow 
easier.  This causes all kinds of side effects.
- they exhibit very high CTE's.
- they require a heat (controlled oven, not home oven) cycle to fully cure 
them.  This is true even for "room temp" resin systems.  After room temp, 
typically no matter how long, they are still only 90% fully cured.  Once 
they are exposed to even moderately higher temperatures after that, the 
surface can and often will, change.  This change is usually seen as fiber 
print through (in composites), from either the woven good or unidirectional 
below the surface and/or surface dimpling/sag where the temps were higher.
- most resin systems are UV sensitive, even UV resin systems over time.

What are you spin casting the epoxy on (what type of material)?  This is 
yet another variable.  Others that seem extremely obvious but should not be 
overlooked are: temp, humidity, air content, wind (furnace, AC, fan, 
etc..), how you mix the resin, time as it relates to each step you do, 
etc..  Try to keep really accurate records.  Like the old saying goes, 
"garbage in, garbage out."  Things that seem trivial now might be just the 
variable that is causing the headaches.

How thick is the resin layer you are putting down?

To my knowledge there are two companies in the US and one in the UK that 
have extensive carbon fiber/epoxy mirror experience.  I don't know that 
much about the UK company but I did speak with them a few years ago.  The 
US companies have both been in R&D with CFRP mirrors for 15 and 20+ years 
respectively.  The underlying factor that still seems to remain is that 
they seem to have problems with print through, in visual spectrum at 
least.  This is why so many are being produced for directed energy (laser), 
radio, mm, sub mm, far IR and mid IR.  The surface requirements for those 
spectrums are much, much lower.

My own research with this has allowed me to produce surfaces like this:
http://www.dreamscopes.com/pages/proj-05/reflections-01.htm

Although the above samples generally show a near perfect surface, visual 
spectrum is very demanding.  My surfaces are not good enough for visual 
spectrum.  Plus there are other factors that make any form of 
composite/epoxy mirror a difficult task.  Getting a perfectly smooth 
surface is a great start but once that is accomplished, the figure then 
rears its ugly head...  As the composite is cooked or it's resin system 
produces heat, how do the CTE's of all materials involved affect the figure...

Having said that though, take notice of Edison's work.  He didn't invent 
the light bulb.  He "merely" refined it.  It took ~2000 trials of different 
materials before he found just the right filament that would illuminate AND 
have longevity.  So you may yet discover how to make it work.  You're on 
the right path by trying to understand the materials better.

Thank you.

Sincerely,
Shane Santi - Owner
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833  

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