Re: [ATM] Epoxy mirrors (again)

[Mark Holm <mdholm@telerama.com>]

Sorry I didn't see this message before.  One thought is that the resin
is changing temperature, perhaps as a result of the exothermic curing
reaction.  As the resin sets, it might start puckering as a result of
the thermal expansion/contraction.  The surface presumably would tend to
be cooler than the inside.  Both are reacting, but the surface can
reject heat faster than the inside.  Organic materials tend to have
higher thermal expansion coefficients than glass.  I would also not be
surprised to learn that the resin contracts some on curing.  It is
making new chemical bonds, and that ought to hold the molecules a bit
closer together than they would be on average if free to wiggle around. 
Shrinkage or expansion ought to be noted in manufacturers literature,
though perhaps not down to the tight tolerance an optician would want.

Mark Holm
mdholm@telerama.com
>
> 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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