Re: [ATM] Epoxy mirrors (again)

["Terin Lyr D'Amico" <terin.damico@verizon.net>]

Wayne,

I think one problem you are having is that the epoxy curing involves a
chemical reaction that needs to come to completion, and does so a varying
rates, whereas the process your are trying to emulate that is done in the
Stewart Labs involves simply changing a state of the glass, through a very
controlled process of stepping down the temperature in a pyrometrically
controlled furnace.

Just a high-level thought of what you may be experiencing.

Regards,

Terin




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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