Re: [ATM] Epoxy mirrors (again)

["Thomas Janstrom" <thomas@moiler.com>]

Ok one thing I know about the amide cured types is that they both
generate heat and depend on heat to cure, I'd hazard that the centre is
actually curing fractionally faster than the cooler surface layer.
However this is just a guess.

Maybe using some kind of very controlled temp environment might give
better results?

Cheers, Thomas.

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf
Of Wayne Young
Sent: Tuesday, 10 January 2006 7:50 PM
To: atm@atmlist.net
Subject: [ATM] Epoxy mirrors (again)

Hi all

I am sure all who have been on this list long enough have seen many
posts about spin-casting epoxy mirrors.

I have been experimenting myself with near-success and wondered if
anyone out there who is a chemist / physicist can give me a categorical
answer as to whether I am wasting my time - and if so, why ?

I've come close. Ive tried a whole bunch of different epoxies from
different manufacturers. I've tried UV cured epoxies as well as the
normal amine cured varieties. The ones I've had most success with (if
you can call it that) have been the super-low viscosity variety with
long cure times (greater than a couple of hours). The frustrating thing
is - these epoxies create absolutely perfect looking flats (just letting
the stuff pool) right up to the point where they start to set up, then
the urface gets really tiny undulations in it. To the non-astronomer,
the surface looks like a wonderful glossy smooth surface like glass, but
if you look closely, it's not completely flat.

So - what I want to know is : What happens ? Why does the stuff level so
well and give you a false sense of acheivement, only to go wrinkly at
the end ? Is there any way of preventing this ?

Some thoughts I had .....

Is it to do with differential cure rates at the surface compared to
under the surface ?
Do epoxies expand when they cure ? This would explain why small waves
appear if the surface cures before the bulk of it.
Could the waves be prevented if the surface was kept under tension by
causing the surface to "dome" slightly ? All the experiments I've done
have had the meniscus at the edge turned up (rather than down) compared
to the centre of the "puddle", but I think I can fix that.

Sorry there are so many questions, but this problem has been driving me
nuts on-and-off for years and I've come so close, I can't believe there
isn't a solution.

Many thanks

Wayne
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