Re: [ATM] Epoxy mirrors (again)

["Wayne Young" <atm@ganimede.demon.co.uk>]

Hi Shanti

Thanks for the comprehensive reply. Some answers, and more questions :

I am aware of the CTE issue with epoxies. I have chosen what I think is a 
unique substrate that solves that problem. The layer of epoxy is currently 
around 500 um thick, though in the future I hope to get this down to around 
100 um. I figured that at that thickness, worst-case thermally induced 
variation in the thickness of the epoxy would amount to much less than the 
wavelength of light ( about 0.4% of 100 um ). I have found epoxies with 
excellent wetting properties that naturally spread under the influence of 
normal gravity and have no problem forming such thin films without resorting 
to mechanical spreading or centrifuges. As I said - as I watched the last 
lot of epoxy spread out, I thought I'd found "the One" since it formed a 
completely flat surface. No ripples, no large scale distortion, no warping. 
When I came back to it the next morning - very fine ripples that looked sort 
of like the convection cells on the surface of the Sun had formed, but the 
ripples were about 1 cm across, so I don't think it was actual convection 
causing the problem - after all, the epoxy was only a few hundred microns 
thick. There was also a noticeable macroscopic sag in the surface height 
around the periphery of the 15 cm test disk. I think this was caused by 
liquid epoxy continuing to migrate up the walls of the container after the 
bulk of the epoxy had started to cure, causing the volume of epoxy under the 
surface to decrease. This is why I think if I could make the epoxy dome up, 
with an inverted meniscus, I could stop it migrating once it had assumed 
it's flat shape since it would reach an equilibrium point. I have yet to try 
this.

Now for the questions :

I visited your web site - very impressive ! How did you achieve the quality 
of surface finish on your creations ? Was it by mandrel replication ? I have 
seen others use precise moulds to create carbon-fibre mirrors, so I am 
familiar with that process.

Again - thanks for the feedback

Wayne


----- Original Message ----- 
From: "Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>
To: "Wayne Young" <atm@ganimede.demon.co.uk>
Cc: <atm@atmlist.net>
Sent: Tuesday, January 10, 2006 9:29 AM
Subject: Re: [ATM] Epoxy mirrors (again)


> 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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